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1H NMR linewidths of small organic guest molecules physisorbed on different mesoporous silicas
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H NMR linewidths of small organic guest molecules physisorbed on different mesoporous silicas Gábor Szalontai PII:
S0022-2860(19)31755-7
DOI:
https://doi.org/10.1016/j.molstruc.2019.127646
Reference:
MOLSTR 127646
To appear in:
Journal of Molecular Structure
Received Date: 25 October 2019 Revised Date:
18 December 2019
Accepted Date: 23 December 2019
1 Please cite this article as: Gá. Szalontai, H NMR linewidths of small organic guest molecules physisorbed on different mesoporous silicas, Journal of Molecular Structure (2020), doi: https:// doi.org/10.1016/j.molstruc.2019.127646. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Szalontai Gábor University of Pannónia
Graphical abstract
silica aerogel
physical mixture of HMB and MCM-41 HMB in MCM-41 HMB on aerogel
crystalline hexamethylbenzene (HMB)
static 400 MHz 1H NMR spectra
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H NMR linewidths of small organic guest molecules physisorbed on different mesoporous silicas Gábor Szalontai (Szalontai Gábor) [email protected]) University of Pannonia, NMR laboratory (H-8200 Veszprém, Egyetem utca 10. Hungary)
Abstract: The excessive linewidths of 1H NMR spectra of solids is a longstanding difficulty. Organic molecules (crystalline at room temperatures) of different chemical characters (camphor, menthol, HMB, HMTA, biphenyl, terphenyl, anthracene, perylene, etc.) have been confined or physisorbed in regular (MCM-41, SBA-15) and irregular (aerogel) silica mesopores. Unexpectedly, steadily decreasing proton NMR linewidths have been observed upon increasing pore sizes. Comparison of the static 1H NMR spectra of the confined and “free” bulk molecules showed an impressive resolution improvement (a factor of 5 to 10). Static 1H NMR and 2H magic angle spinning (MAS) spectra pointed out the increased molecular mobility as origin. In a qualitative approach the possible reasons of mobility have been thoroughly discussed. The increased mobility seems to be easiest achievable in aerogel and so, in the future, this extreme light mesoporous silica nanomaterial (MSN) may be considered as “active” filling material in solid state NMR spectroscopy. Since sensitivity is not an issue in 1H MAS measurements the proposal simultaneous dilution of the dense 1H–1H spin network and increasing the molecular mobility of the molecules of interest by confinement or physisorption in/on silica
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mesopores - may find applications, especially in laboratories where high magnetic fields and state of art ultra-fast MAS rotation are not available.
KEYWORDS NMR, 1H, 2H, ssNMR linewidth, solid guests, molecular mobility, mesoporous silica
Highlights Narrow 1H NMR linewidths of solid guests in mesoporous silicas in static spectra. The confirmed mobility scales up with pore size, so defies simple Kelvin effect. Processes including surface diffusion and adsorption-desorption are suggested. Use of the extra-light irregular silica host aerogel as filling material is foreseen.
1. Introduction It is known that in solids the prohibitively large 1H NMR linewidths are dominated by the 1H-1H homonuclear dipolar interactions [1, 2, 3]. These, unlike the inhomogeneous interactions such as the heteronuclear dipole–dipole couplings, cannot be effectively removed by moderate magic angle spinning (MAS) rotations. In the last decade several ingenious multipulse sequences have been proposed to overcome this problem [4, 5, 6]. For a comprehensive review on the subject see [7]. For the same purpose others proposed the decrease of proton spin density by deuteration of the sample molecules [8, 9, 10]. In the meantime simpler but expensive solutions such as the superfast rotation (~ 100 kHz) at ultrahigh magnetic fields (> 20 T) became available in several but not all laboratories [11, 12]. Under such conditions linewidths of about 0.2-0.4 ppm can be routinely achieved nowadays. Nevertheless simpler solutions are still of interest and wanted. The first reports on the extremely narrow “liquid-like” NMR linewidths of small organic molecules (liquids at room temperature) when adsorbed on silica or zeolite surfaces
appeared
in
the
late
nineties.
2
The
molecules
studied
included
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hexamethyltetrasilane (HMTS) [13], H2O and benzene [14], and olefins (pentene and butene) [15]. Note that such linewidths require molecular tumbling rates in the picosecond range (10-10-10-12 seconds). With the advent of mesoporous (pore sizes 20 – 500Å) silica nanomaterials (MSN) [16, 17] it has been soon noticed that small molecules, even if solids at room temperature, also show this “liquid-like” behaviour if confined in MSNs. Günther et al. reported on the narrow lines observed in
13
C and
19
F MAS spectra of
benzenoids and hexamethylbenzene (HMB) adsorbed in different pore size (22-100Å) silicas [18]. Later Babonneau et al. published several papers on the advanced NMR techniques used for the characterization of Sol-Gel derived materials [19, 20]. Because of the special interest in the drug delivery processes [21] the behaviour of several drug molecules (e.g. ibuprofen, ketoprofen, indomethacin, aspirin) confined in MSN-s has also been thoroughly studied [22, 23, 24]. It is well known that molecules embedded in porous materials possess appreciable mobility with thermal correlation times of the order of 10-8 – 10-12 s at room temperature. Thus it is generally anticipated that lower activation energy barriers are expected. However, the origin of the higher molecular mobility, which scales down the dipolar couplings responsible for the extreme 1H linewidths observed in solids is not yet comprehensively understood. Surprisingly narrow lines have been observed by Zorin et al. under special circumstances such as arrangement of the spins in a one-dimensional geometry, e.g. in d4-urea-decanoic acid [25]. The effect was explained by a geometrical factor and implicitly spin-dilution rather than by the confinement of the decanoic acid in the urea channels. It was hinted in the early papers that the increased mobility of the confined molecules is solely due to, or is a consequence of, the “confinement effect” of the pores [26, 27, 28]. In the 10- 100 Å pore size range behaviour of the confined molecules followed the prediction of the Gibbs-Thomson equation [29, 30]. Therefore, freezing and melting temperature depression which is inversely proportional with the host pore diameter, was credited for the phenomenon. However, it is a question, whether macroscopic parameters such as melting point depression of bulk materials can be used to properly (i.e. not phenomenologically) describe the situation where the bulk phase does not exist. Not much later, others clearly proved the absence 3
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of strong long lasting interactions between the guest and the silica host [31, 32]. It was suggested that other factors such as weak and dynamic van der Waals (vdw) interactions or, in case of ionic guests, the effect of pore wall or simple spatial separations of the interacting proton spins (loading effect) can also contribute [33]. It is also known that the surface coverage of the OH (silanol) groups determines the adsorption-desorption behaviour, and consequently, the surface reactivity [16]. Recently, the adsorption-desorption kinetics of a dipeptide at the inner surface of SBA15 has been investigated by 2H MAS under different hydration levels and temperatures and was explained by a water-assisted two-side exchange model [34]. All these suggest that adsorption-desorption phenomena can, at least partially, also be credited for the improved resolution of NMR spectra. Whatever the exact reason is, a beneficial consequence of the increased molecular mobility is that in many cases the more convenient liquid state 1D and 2D NMR methodologies can be used for solid samples. The goal of the present paper is to demonstrate and compare the effect of enhanced mobility observed in three different silica mesopores (Mobil Catalytic Material, MCM, Santa Barbara Amorphous, SBA and aerogel). We are trying to confirm, evaluate and at least qualitatively understand the possible contributions such as the “surface curvature or confinement effect” [29], the physisorption and/or the surface diffusion of guest molecules [28]. The effects of adsorbate properties, silica pore size, specific surface areas, loading values and procedures on the observed 1H linewidths will be discussed in a systematic way. Possible practical applications of the 1H line narrowing in ssNMR spectroscopy will also be mentioned briefly.
2. Theory:
2.1 NMR: As mentioned the principal reason of the extreme linewidths of 1H MAS spectra is the presence of strong homonuclear dipolar couplings between the protons [35]. One way to cancel the dipolar couplings is the simulation (reproduction) of random molecular motions by rotating the sample around a predefined angle (MAS) [1]. However, only extreme fast MAS spinning, not readily available in most of the
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laboratories, can remove this contribution. Furthermore, the actual appearance of the spectrum depends heavily also on the nature of all NMR interactions responsible for the line broadening [25]. The “inhomogeneous” interactions (e.g. chemical shielding anisotropy (CSA) and heteronuclear dipolar couplings) refocus during a rotation cycle, thus can be removed from the spectra already at relatively low rotation speeds. However, it is not the case with the “homogeneous” ones (e.g. homonuclear 1H-1H dipolar couplings). Consequently, in these cases the centerband and the spinning sidebands have substantial widths which decrease only slowly with increasing spin rate (~1/rotation). Another, definitely much simpler, way is to force or let the molecules to move or tumble randomly similarly they do it in liquids or gases. Such voluntary “mobilization” has been detected in different mesopores, i.e. in the 20-500 Å pore range.
2.2 Physisorption: The forces involved include both long range attractive (van der Waals, wdw) and short range repulsive interactions. Note that these interactions do not depend on the polar nature of the hosts or guests and are therefore non-specific [28]. Concerning the thermodynamics of entrapped molecules, several models have been proposed for the interpretation of the observed phenomenon. Some of them can be referred to as “surface curvature effect” described originally by Derouane at al. [29]. It is a semi quantitative model to account for the interaction of a guest molecule and the pore surface. By decreasing the pore diameter the surface curvature is obviously increasing, so one can reach a point where the attractive (sticking) vdw forces vanish and the molecule appears as floating in the pores, no doubt an ideal situation to achieve high mobility. The mentioned phenomenological melting point reduction of small crystalline molecules (∆Tm) can be predicted by the Gibbs-Thomson equation [30].
∆ T m = T mB − T m = [4 V m T mB (γ lw − γ cw
)]/ [r ∆ H m ]
TmB = bulk melting temperature Tm = observed melting temperature Vm = crystal molar volume γlw and γcw are the liquid wall and crystal-wall interaction energies ∆Hm = molar melting enthalpy 5
(1)
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r = pore diameter (spherical pores are assumed) It is clear, however, that above a certain pore size the “surface curvature effect” cannot be responsible for the observed mobility thus another possible reasons, such as the dynamics of physisorbed molecules, must also be taken into account. In case of heterogeneous surfaces the differences of the chemical potential between the occupied and free locations lead to surface diffusion (non-localized adsorption, also called Volmer diffusion [28]). If the kinetic energy (kT) of the guest molecules (adsorbates) is high enough to overcame their motional activation energy, especially at low loading values, “gas-like” behaviour is observed [28]. At higher loading values, however, the lateral interaction between the adsorbed molecules becomes non-negligible which may lead to condensation [36].
3. Materials and Methods 3.1 Sample preparation Several loading procedures, including wet impregnation (adsorption from solvent) and dry methods (adsorption by ball mill grinding and simple mixing), have already been tested and published [18]. Normally, our samples were prepared by two hours of magnetic stirring in an appropriate volatile solvent (typically in CHCl3), followed by 2-3 hours of air drying to let the solvent residues to evaporate. In case of physical mixtures we grinded the samples gently for a few minutes by hand in an achate mortar. None of these methods are expected to change the pore structures. The loading factors (f) were calculated from the guest / host ratios (mg/mg). 1H MAS spectra of the MSNs indicated the presence of water, its quantity corresponded to the room temperature equilibrium value (~ 15 w %). Since the water content of the silica pores may have role in the adsorption- desorption kinetics it has not been removed beforehand. In case of apolar solvents (CHCl3) used in the wet method the water content of the silicas did not change significantly (see Fig. S1 in the SI material), however in case of polar solvent such as ethanol the water withdrawal can be severe.
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3.2 Solid state NMR Spectroscopy (ssNMR) All NMR spectra were recorded on a Bruker Avance II 400 spectrometer, with proton frequency of 400.13 MHz (9.38 T) equipped with a 2.5 mm and a 4 mm CP/MAS probes. 8-12 mg of polycrystalline samples was sufficient to fill up the 2.5 mm Zirconia rotors (thin wall tube with short Vespel caps). The applied rotation speeds were between 5000-25000 Hz. The cross-polarization contact time was varied between 1.5-3.0 ms. As external references for the 13C measurement signal of the carbonyl group of the alphaglycine (176.5 ppm) was used. The 13C NMR spectra of non-deuterated complexes were recorded under high-power proton decoupling. Since proton decoupling did not result in resolution improvement, no decoupling was applied in case of the 2H MAS NMR spectra. The sample temperature was not regulated (~300 +/- 1 K); note, however, that above 20 kHz rotation the actual sample temperature could be 15-35 K higher due to the frictional heat of the rotation. For the measurements (13C, 1H and 2H) and simulations of spectra we have used the standard NMR methods (Bruker Corporation Topspin 2.1.3.): single-pulse MAS (SP), Hahn-echo [37], CP/MAS [38], and high-power protondecoupled (Bruker zg, vacp.av, hpdec.av sequences) experiments. Number of scans used was 1k-2k for the 13C spectra and a few dozens and a few hundreds scans were sufficient for the 1H and 2H spectra, respectively. The weighting function was exponential broadening (5-50 Hz).
3.3 Host MSN-s materials As hosts we have used two ordered MSN-s namely MCM-41 (pore size ~ 27-40 Å, surface area (sa) 1000-1200 m2/g, ordered non-intersecting hexagonal 1D channels of controlled size) [39] and SBA-15 (pore size ~ 50-60 Å, 600-800 m2/g, ordered tubular 1D channels) [14], and one disordered native silica (SiO2)n called aerogel [40]. The exact pore size (ps) distribution, specific surface areas and other parameters of the MCM-41 and SBA-15 hosts we used have already been published by Szegedi et al.
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elsewhere [41]. The specific surface areas were evaluated by the BrunauerEmmett-Teller (BET) and/or Barret-Joyner-Halenda (BJH) methods using N2. The disordered silica aerogels are, in general, characterized by ultra-low density (~0.07 gr/cm3) and high surface area (~ 600 – 1000 m2/g), for non-aggregated system the pore volume depends on the particle-packing geometry [42, 16]. Their pore diameters lie in the mesoporous range (20-500 Å) [40, 43], furthermore the pore structure allows a mass transport which approaches the gas-phase diffusion limits [44]. It is also known that the surface coverage of the silanol groups (Si-OH) influences the adsorption behaviour of the gel [16]. 29Si MAS and CPMAS spectra have been recorded for all silica, the observed relatively large linewidths (650-750 Hz) confirmed the amorphous nature of the pore walls. The native aerogel [45], which we used in dry form, was thoroughly characterized in water suspended state [46]. The measurements indicated a specific surface area of 633 m2/g and near spherical pores with mean pore size of about ~ 140 Å (BET) or 180-200 Å (BJH). Note that these pore size values represent the central value of a wider Gaussian
distribution.
3.4 Solid guest molecules studied For the demonstration, in an attempt to prove the general nature of the phenomenon, we have used small neutral organic molecules of different geometric and chemical characters. Note that at room temperature all of them are crystalline. They include rigid planar (HMB, polyaromatics with anisotropic shapes), conformationally flexible with polar functional groups (menthol, simple amino acids), and near spherical (HMTA) molecules. Some of them may interact with the silanol groups of the surfaces and/or
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each other by hydrogen bonds, while in case of others, this possibility does not exist. The size of the selected molecules was obviously limited by the size of the available pores.
3.4.1 Waxy (extremely low and low melting point crystals) and rigid solids: L-menthol (MW=156.2, Tm=36-38
o
C), camphor (MW=152.2, Tm=174-175
hexamethyl-benzene (HMB, MW=152.2, Tm=164-166
o
C),
o
C), and hexamethylene-
tetramine (HMTA, MW=140.2, Tm=280oC).
menthol
camphor
HMB
HMTA
Scheme 1.
3.4.2 Perdeuterated polyaromatics (melting point range 71-220 oC):
The selected compounds (biphenyl-d10, terphenyl-d14, anthracene-d10, phenanthrene-d10, chrysene-d12, and perylene-d12) are planar with gradually increasing molecular weight. (Scheme 2)
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terphenyl-d14 biphenyl-d10
phenanthrene-d10
anthracene-d10
perylene-d12 chrysene-d12
Scheme 2.
4. Results
First we have tried guest molecules with low to medium melting points (L-menthol and camphor, see Scheme 1). Of them the case of the camphor proved to be particularly interesting and instructive (see Figs. 1, 2 and 3). We used this molecule to demonstrate the effects of the confinement (by static experiments) and the MAS rotation on the linewidths.
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4.1 Static linewidths in bulk and confinement, MAS linewidths vs. spinning speeds
Static 10 kHz 20 kHz 30 kHz
Camphor in MCM-41
Static 10 kHz 20 kHz 30 kHz
Crystalline camphor
Figure 1. Comparison of 1H spectra of free crystalline camphor (bottom) to the relevant spectra of the same camphor confined in MCM-41 (27 Å pores, f =0.55) (top). Static (black line) and single pulse (SP) MAS (10, 20, 30 kHz) spectra. Sample preparation method: wet impregnation. The scale is given in ppm.
In fact the camphor is a somewhat special case since it sublimes appreciably at room temperature and atmospheric pressure. For that reason it can serve as model compound for studying low-energy processes such as the effect of the room temperature molecular mobility on the solid 1H NMR spectra. While linewidth of the static free crystalline camphor is huge, for the confined molecule it is smaller by at least a factor of ten (compare the black lines in Fig.1 (bottom) and (top), respectively). As expected the MAS rotation brings further substantial resolution improvement (see the spectra recorded at 10, 20, and 30 kHz). It is interesting to note that the resolution of spectra of the suspected confined/physisorbed molecules (upper traces) are lower relative to that of the free crystalline ligands recorded at the same rotation speeds. A possible indication
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that in MCM-41 at room temperature the camphor molecules exist in two different environments or forms, e.g. we are facing with a mixture of confined and physisorbed forms. However, an exchange between the two can neither be excluded.
4.2 Comparisons of linewidths (lw) vs. pore sizes (ps) and specific surface areas (sa) Since the calculated volume of a camphor molecule is about 258 Å3, 100 mg MCM-41 offers ~ 1250 Å3 space/molecule (calculated from the porosity data). In case of uniform loading of 10 mg of camphor, i.e. if f ~ 0.1 only about one fifth of the available space is occupied by the guest molecules. The same number is about ~ 590 Å3 /molecule and ~ 225 Å3 /molecule for f ~ 0.23 and f ~ 0.5, respectively, thus with a loading factor of 0.5 we are close to the full loading of the pores, however, there is no guarantee for a homogeneous loading.
camphor in SBA-15 lw = ∆ν1/2 ~ 1550 Hz
camphor in aerogel lw = ∆ν1/2 ~ 500 Hz
camphor (crystalline) lw = ∆ν1/2 ~ 5-7000 Hz
camphor in MCM-41 lw = ∆ν1/2 ~ 2030 Hz
Figure 2. Static 400 MHz 1H spectra of free crystalline camphor, camphor confined in MCM-41 (ps=27 Å, sa=1175 m2/g), SBA-15 (ps=68 Å, sa=749 m2/g), and silica aerogel (ps=140 Å, sa=633 m2/g). Sample preparation method: wet impregnation. The loading factor was ~ 0.5 in each case. The broad hump at about 16 ppm is partly due to the background 1H signal of the 4 mm probe.
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From Fig. 2 it is clear that the larger the averaged pore size the narrower the lines are, and, at least formally, the smaller the surface areas the smaller the linewidths are. The reversed relationship observed between the mobility and the available surface area (sa) of our mesoporous silicas (saMCM-41 > saSBA-15 > saaerogel) is perhaps due to the fact that the guest molecules are not (cannot be) anchored to the silica matrix for long. In case of aerogel relatively free the mass transport of the guest molecules was pointed out earlier [44]. It is important to mention that similar correlations have been found for all molecules studied.
4.3 The effect of sample preparation and loading on resolution
As mentioned already in case of molecules soluble in easily evaporating solvents such as CHCl3 or ethanol the wet impregnation was the method of choice. This method, depending of the actual loading value, may result in formation of monomolecular layers or sparingly located guest molecules on the silica surface. Surprisingly, similar, though inferior, resolution could be achieved by preparing simple physical mixtures (see section 3.1) of the guest and host molecules. The interpretation of the phenomenon is not straightforward. Under such conditions still a significant fraction of differently oriented guest crystallites must exist, the confinement, if at all, can only be partial. Nevertheless this is a clear indication that even non-confined molecules may possess increased mobility (see Fig.4.)
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∆ν1/2 ~ 180 Hz f = 1.31 ∆ν1/2 ~ 150 Hz f = 0.55 ∆ν1/2 ~ 150 Hz f = 0.23 ∆ν1/2 ~ 150 Hz f = 0.10
Figure 3. The effect of loading (f): 1H SP MAS spectra of camphor confined in MCM41 recorded at 20 kHz. Strong signal at 4.7 ppm: water. Sample preparation technique: wet impregnation (CHCl3).
The decreasing load (less guest molecule / surface or free volume) results in somewhat better resolution (Fig. 3). However, below a loading factor of about 0.3 this effect becomes insignificant.
4.4 Spectra of a non-spherical rigid solid: free and confined/physisorbed hexamethylbenzene (HMB)
The
1H
MAS linewidth dependence of the free crystalline HMB on
inhomogeneous contributions such as anisotropy of the bulk magnetic susceptibility (ABMS), inhomogeneous bulk magnetic fields, etc. has been reported by Zorin et al. [25] in the 5000-20000 Hz rotation range. Here we report the dependence of the 1H static linewidth of HMB on the pore size and the loading procedures used (wet adsorption vs. gentle dry grinding).
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physisorbed on aerogel ∆ν ~ 600 Hz, f = 0.5 1/2
static free crystalline HMB ∆ν ~ 15000 Hz 1/2
physisorbed on MCM-41 ∆ν ~ 750 Hz, f = 0.09 1/2
„physical mixture” of HMB and MCM-41, ∆ν ~ 1200 Hz, f = 0.60 1/2
Figure 4. Static 1H spectra of free crystalline HMB, HMB confined in MCM-41 (Sample preparation method: wet impregnation, low loading), physical mixture of HMB and MCM-41 (medium loading), and HMB physisorbed on aerogel (medium loading). Sample preparation method: gentle grinding. The broad hump between 16 -18 ppm is partly due to the background signal of the probe.
It is clear that the confinement/physisorption of HMB causes enormous resolution improvement (Fig.4). It is also worthwhile to mention that the linewidth of the physical mixture is larger only by a factor of two. Static HMB spectra obtained by the Hahnecho sequence and by single-pulse (SP) experiment produced similar linewidths which do not indicate substantial susceptibility (ABMS) contribution [26]. The spectra suggest two 1H environments both in aerogel and MCM-41 hosts (~ 4.7 (H2O) and 2.5 (-CH3) ppm) whose linewidths are substantially different (~ 1300 and ~ 600-750 Hz, respectively). Note, however, that the methyl rotation, inherent in solids too, scales the linewidths of the methyl groups.
The confinement suspected and achieved by the different loading methods in different silicas has also been checked by 13C CPMAS and MAS spectra. Here again the HMB
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served as a convenient test molecule. Our results (see SI material) agree well with those of Günther et al. [18] who reported on the 13C MAS spectra of HMB adsorbed on (or confined in) an irregular silica (Silica-60). Spectra and data obtained for the camphorMCM-41 pair are also found in the supporting material (SI).
4.5 Spectra of a near spherical rigid solid: 1H NMR of hexamethyltetramine (HMTA) confined/physisorbed in MCM-41 and SBA-15
It was of interest to compare the behaviour of HMB to a more spherical guest, namely HMTA which is presumably more mobile already in the bulk phase. Indeed, 1H static linewidth of the HMTA is only about 750 – 800 Hz if confined in MCM-41 which is reduced further to about 150 Hz when rotated at 30 kHz. The spectrum (Fig. 5) displays two resolved chemical sites (water and N-CH2-N signals). The 13C CPMAS spectra (not shown), like the HMB-MCM-41 case, exhibit extremely low sensitivity. For spectra of other compounds studied and further details see Figs. S5-S13 of the Supporting Information material (SI).
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Static 10 kHz 20 kHz 30 kHz ~ 750-800 Hz
Figure 5. Comparison of static and MAS 1H (SP) spectra of HMTA confined in MCM41. Static (black line) and MAS (10, 20, 30 kHz) spectra. Sample preparation method: wet impregnation. Loading (f) = 0.44, the scale is given in ppm.
4.6 Confinement and mobility check by 2H MAS spectroscopy
It is well known that solid-state 2H NMR spectroscopy is the method of choice for studying molecular motions in solids [47, 3]. The line shapes of 2H static and MAS spectra are sensitive to molecular motions with correlation times of the order 10-4-10-6 Hz. The effective quadrupole-coupling values (CQ), normally obtained from the simulation of the spinning sideband manifold [3], are used for this purpose. The method has already been used for the study of simple organic groups covalently attached to [48], or physisorbed [35] on ordered mesoporous silicas, and is equally suitable for studying adsorption-desorption kinetics of confined molecules. E.g. using a highly water soluble dipeptide as probe, a water-cluster mediated exchange between a “frozen”
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conformation and an isotropic-like motional state was confirmed in hydrated SBA-15 [35]. To further demonstrate that the observed mobility is not restricted to water soluble molecules and to the presence of excess water in the pores neutral perdeuterated aromatic molecules with low water solubility, gradually increasing molecular weight, different melting temperatures and anisotropic shapes (see Scheme 2) have been selected, as guests, for our purposes. Note that in these molecules, due to the small chemical shift differences, resolution of the chemically different 2H sites is not expected. Like previous cases the confinment have been achieved by the wet impregnation method whereas the physical mixtures by gentle dry grinding. In solutions, their tumbling, though anisotropic, is fast and results in singlets free of the quadrupole effect. If confined in pores, depending on the actual pore size and loading, but also on the volume of the guest molecules, often only limited motional freedom is expected which may results in the observation of the quadrupolar line shape.
4.6.1 Selected 2H labelled examples (see Table): In aerogel the low melting point, conformationally instable, rod-like biphenyl molecules showed a singlet at the isotropic chemical shift without spinning side-bands (SSB) manifold (see Fig. 6). The lack of SSB-s clearly indicates that in these pores even the huge (~ 170 kHz) quadrupole coupling constant is averaged to near zero due to the extreme mobility of the molecules. For the longer terphenyl molecules, having much higher melting point (212 oC), the situation is different. While in MCM-41 we see only a broadened singlet (linewidth ~ 800 Hz), in aerogel the typical SSB pattern of rigid crystalline molecules is seen (Fig. 7). The observed effective quadrupolar coupling value, CQ is practically identical with that of the free crystalline molecule (see Table below). The different behaviour of the biphenyl and terphenyl molecules, which exists in aerogel, clearly indicates significant mobility difference among them. Considering the melting point (∆tm = 140 oC) and size difference of the two molecules this is not unexpected. This, combined with the large pores of the aerogel (the diameters lie in the 20-500 Å range), makes possible even the crystallization of the guest.
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physisorbed on aerogel
free crystalline biphenyl-d10
Figure 6. 2H MAS spectra, (bottom) free crystalline biphenyl-d10 (melting point 72 oC, rotation 10.5 kHz), (top) biphenyl-d10 physisorbed on aerogel (f= 0.45, rotation 10.7 kHz).
physisorbed on aerogel
lw = ∆ν1/2 ~ 150 Hz
confined in MCM-41
lw = ∆ν1/2 ~ 800 Hz
crystalline terphenyl-d14
lw = ∆ν1/2 ~ 250 Hz
mv = 365 Å3
Figure 7. Room temperature 2H MAS spectra: (bottom) free crystalline terphenyl-d14 (melting point 212 oC, molecular volume (mv) 365 Å3, rotation 12.4 k Hz), (middle) terphenyl-d14 confined in MCM-41 (f= 0.13, rotation 10 kHz), and (top) terphenyl-d14 physisorbed on aerogel (f= 0.63, rotation 10.6 kHz)
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In case of anthracene (Scheme 2), a rigid elongated planar polyaromatic molecule with melting point of 215 oC and molecular volume of 267.4 Å3, we compare the 2H MAS spectra obtained for samples confined in MCM-41 and aerogel (see Fig. 8). While in MCM-41 the spectrum (middle) suggests an adsorption – desorption equilibrium in the intermediate range (like the cited dipeptide case [35]) since we see an exchange broadened SSB pattern with linewidths of about 800-900 Hz. However, in aerogel the spectrum (top) is dominated by the rigid crystalline phase (the CQ and linewidths values are practically identical with those of the crystalline compound). Like the terphenyl case, this can readily be understood in terms of a depressed melting temperature in MCM-41 and an “unchanged” melting temperature in the 3-5 times larger pores available in the aerogel which seem to be sufficient even for crystal growing.
physisorbed on aerogel
loaded in MCM-41
free crystalline anthracene-d10
Figure 8. 2H MAS spectra of free crystalline anthracene-d10 (melting point of 215 oC, lw = ∆ν1/2 ~ 350 Hz, rotation 20 kHz) (bottom), anthracene-d10 loaded in MCM-41 (f=
20
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0.32, (lw = ∆ν1/2 ~ 800-900 Hz, rotation 20 kHz) (middle), and in aerogel (lw = ∆ν1/2 ~ 350 Hz, rotation 16 kHz) (top).
Phenanthrene (see Scheme 2): Also planar with the same molecular weight as anthracene but possesses less elongated shape and much lower melting point (101 oC). Adsorption behaviour of this molecule on Silica-60 has already been reported [16]. Based on the measured specific surface area values obtained upon gradually increased loadings they suspected non-homogeneous distribution of the sample in the pores (blocking the pores). To our surprise, when loaded into aerogel (f ~ 0.32) only a sharp singlet was observed in the spectrum (see SI, Fig. S6). When compared to anthracene, the significantly higher mobility of phenanthrene observed in aerogel can be explained, similarly to the case of biphenyl, by the lower melting point of it. In the cases of even larger guest molecules such as the four-ringed chrysene and perylene, though the averaged aerogel pore size would allow it, the spectra do not show the isotropic singlets but display quadrupolar SSB manifold characteristic for crystalline phases (see SI, Fig. S7). This is also in line with their high melting temperatures (Table).
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Table. Collection of 2H quadrupolar tensor data (effective quadrupole coupling, CQ and quadrupolar tensor asymmetry, ηQ) of the perdeuterated polyaromatic compounds physisorbed on MCM-41 and/or in aerogel (the data are results of simulations of the experimental spectra carried out by the Bruker TopSpin 2.1.3. software).
biphenyl-d10
Free crystalline guest
Guest molecules confined in
molecules
different mesopores
Quadrupole
Q tensor
couplings,
asymmetry,
CQ, [kHz]
ηQ
170
0.03
CQ [kHz] / ηQ
in aerogel
o
(mp 72 C) terphenyl-d14
CQ ~ 0 170
0.08
(mp 212 oC) anthracene-d10
181
0.0
o
(mp 215 C) phenanthrene-d10
181
0.0
(mp 101 oC) perylene-d12
in aerogel
~0
~165 /0.07
in MCM-41
in aerogel
144 /0.5
179/0.17
in aerogel CQ ~ 0
178
0.0
(mp 277 oC) chrysene-d12
in MCM-41
in aerogel CQ ~ 167/0.1
180
0.1
(mp 254 oC)
in aerogel CQ ~147/0.3
5. Discussions
By comparing the behaviour of the same molecule in different pore size silicas a clear trend emerges, namely, the larger pores the smaller the observed 1H MAS line widths (inversely proportional with the mobility of the molecules) are. This contradicts with the expectations based solely on “surface curvature effects” of the Gibbs-Thomson model [30] or on the “supermobility” of guest molecules caused by the dynamic equilibrium of
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attractive vdw forces [49]. Most probably already in the regular pores of SBA-15 with pore diameter of ~ 50-60 Å, not to mention the ball of string-like aerogel (~140 – 180 Å), we are out of the effective region of this model.
From this perspective it is constructive to compare the behaviour of the low melting point pair of biphenyl and phenanthrene with their high melting point “analogues” (terphenyl and anthracene) (see Table and Figs. 6, 7, and 8). While in the smaller pores of MCM-41 the high melting point terphenyl shows extreme mobility, it is rigid in aerogel (pore size increment factor ~ 5) where the surface curvature effect can practically be ignored. At the same time the low melting point biphenyl and phenanthrene show liquid-like mobility in the aerogel where mainly the kinetic/thermal energy of the molecules can contribute to it. Finally, like the terphenyl, the high melting point anthracene is also rigid in aerogel (Fig. 8) because, lacking significant “surface curvature” contribution, the available kinetic/thermal energy alone is perhaps not sufficient to mobilize its molecules, i.e. the equilibrium is shifted toward the adsorbed (condensed) state. In case of wet loading procedure and at loading factors high enough to allow for mono- or bilayer formation on the surfaces the liquid-like behaviour of the guest molecules in the large pores can only be understood in terms of the surface diffusion caused by the different chemical potential of different locations of the heterogeneous surface [28, 36]. If the guest molecules are rare on the silica surface the intermolecular attractive forces among them can be ignored. They are physisorbed to the surface by weak interactions (van der Waals, hydrogen bonds, etc.) thus the same thermal energy available at room temperature can give them larger mobility relative to what exists in the crystal lattice or in the bulk of liquids where the molecular motions are strongly limited by the packing forces and liquid viscosity, respectively. At high loadings (total guest volume > available porosity) the co-existence of two phases can be anticipated, namely a highly mobile surface layer and, provided the available space allows it, and a rigid (bulk: crystalline or amorphous) solid. It seems also that, at least above a certain pore size where the adsorption-desorption process is not influenced by steric factors, the regular or irregular nature of the pores does not play important role.
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Concerning the loading procedures tested, it is worthwhile to remember that the dry grinding method producing “physical mixtures” turns out to be surprisingly effective from the point of line narrowing. This deserves further comments. It is known that effective grinding results in reduced particle sizes and increased surface areas [50] and thus higher guest mobility. However, the gentle grinding alone we applied hardly can cause such substantial impact. It seems that the “dilution“ effect, the size and distance of 1H-1H dipole-dipole tensors sitting in different neighbouring molecules or rather clusters, is equally important. In this respect it is noteworthy that in case of HMB using a non-porous matrix (alfa-quartz) also resulted in a 50 % line width reduction (see supporting material). Concerning the specific interactions, the silanol groups of the surface and/or the adsorbed water molecules can also play an active role in the equilibrium. In this respect the relatively free mass transport [44] could explain the extreme linewidths observed in the irregular aerogel, where the pores are large, interconnected, and nearly spherical [46]. Finally, it is clear that the smaller pore hosts (MCM-41 and SBA-15), while can confine a single guest molecule or a small cluster of them, do not make possible the crystal formation for steric reasons.
6. Conclusions Organic molecules of different characters (camphor, menthol, HMB, HMTA, biphenyl, terphenyl, anthracene, perylene, etc., all crystalline at room temperatures.) confined and/or physisorbed in regular (MCM-41, SBA-15) and irregular (aerogel) mesopores were studied. Upon gradually increasing pore size we observed steadily decreasing proton linewidth in static spectra which was further reduced by MAS rotation. 13
C CPMAS and MAS spectra (see SI) were used to confirm the confined/adsorbed or
non-confined/desorbed state of the molecules. 2H MAS spectra point to molecular mobility as main reason. Comparison of the static spectra of the physisorbed and “free” bulk molecules shows an impressive resolution improvement (a factor of 5 to 10 was observed in each case looked at so far). When combined with fast MAS rotation 1H line widths of 100-200 Hz could be achieved. Concerning the origin of the mobility there seems to be synergism between the confinement effects (surface curvature) and an 24
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adsorption-desorption equilibrium and/or surface diffusion of the guest molecules caused by the kinetic-thermal energy available at room temperature. The outcome depends primarily on the pore size, the available specific surface area, and surface heterogeneity of the hosts but also on physicochemical properties of the guest molecules. Significant line narrowing can be expected as long as formation of the condensed “bulk phase” is prohibited either for steric reasons or due to the large freely available surface area. The increased mobility seems to be easiest achievable in aerogel and so in the future this material may find routine applications also in NMR spectroscopy. The proposal - simultaneous dilution of the dense 1H–1H spin network and increasing the molecular mobility by confinement or adsorption in silica mesopores - can find interesting applications, especially in laboratories where the ultra-fast MAS rotation [51] is not available.
Acknowledgements: The mesoporous silicas were generous gifts from Dr. Ágnes Szegedi (MCM-41 and SBA-15, TTK MTA, Budapest) and Dr. István Lázár (native aerogel, University of Debrecen), who are gratefully acknowledged. Drs. Tamás Kristóf and Géza Horváth are thanked for valuable discussions on the thermodynamics of entrapped molecules. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
7. References
[1] M. Mehring, High Resolution NMR Spectroscopy in Solids, Springer-Verlag, Berlin, 1978. [2] K. Schmidt-Rohr and H.W. Spiess, Multidimensional Solid-Sate NMR and Polymers, Academic Press, London, 1994.
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[3] M.J. Duer, Introduction to Solid-State NMR Spectroscopy, Blackwell Publishing, Oxford, 2004. [4] I. Schnell and H.W. Spiess, J. Magn. Reson. 2001, 151, 153-227. [5] E. Vinogradov, P.H. Madhu, S. Vega, New Techniques in Solid-State NMR, Springer-Verlag, Berlin, 2005, V. 246, p. 33. [6] S.P. Brown, Solid State Nuclear Magnetic Resonance 2012, 41, 1. [7] P. Hodgkinson, Ann. Reports on NMR Spectroscopy, Elsevier, Amsterdam, 2011, V.72, pp.185-223. [8] B. Reif, R.G. Griffin, J. Magn. Reson. 2003, 160, 78. [9] V. Chevelkov, B.J. van Rossum, F. Castellani, K. Rehbein, A. Diehl, M. Hohwy, S. Steuernagel, F. Engelke, H. Oschkinat, B. Reif,. J. Am. Chem. Soc. 2003, 125, 7788. [10] E.K. Paulson, C.R. Morcombe, V. Gaponenko, B. Dancheck, R.A. Byrd, K.W. Zilm, J. Am. Chem. Soc. 2003, 125, 15831. [11] U. Sternberg, R. Witter, I. Kuprov, J.M. Lamley, A. Oss, J.R. Lewandowski. A. Samoson, J. Magn. Res. 2018, 291, 32-39. [12] M.K. Pandey, R. Zhang, K. Hashi, S. Ohki, G. Nishijima, S. Matsumoto, T.Noguchi, K. Deguchi, A. Goto, T.Shimizu, H. Maeda, M. Takahashi, Y. Yanagisawa, T. Yamazaki, S. Iguchi, R. Tanaka, T. Nemoto, T. Miyamoto, H. Suematsu, K.Saito, T. Miki, A. Ramamoorthy, Y. Nishiyama, J. Magn.Reson. 2015, 261, 1.
[13] G.W. Aksnes, L. Kimtys, Mag. Res. Chem. 1998, 36, 747. [14]: S. Ayyappan, N. Supyaprakash, K.V. Ramanazhan, and C.N.R. Rao, J. Porous Materials, 1999, 6, 5. [15] W. Böhlmann, D. Michel, J. Roland, Mag. Res. Chem. 1999, 37, S126-134.
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[16] C. J. Brinker, G.W. Scherer, Sol-Gel Science, The Physics and Chemistry of Sol– Gel Processing, Elsevier, Amsterdam, 1990. [17] J. Trébosc, J.W. Wiench, S. Hu, V.S.Y. Lin, M. Pruski, J. Am. Chem. Soc. 2005, 127, 3057. [18] H. Günther, S. Oepen, M. Ebener, V. Francke, Mag. Res. Chem. 1999, 37, S142S146. [19] C. Bonhomme, C. Coelho, N. Baccile, C. Gervais, T. Azais, F. Babonneau, Acc. Chem. Res. 2007, 40, 738. [20] F. Babonneau, C. Bonhomme, C. Gervais, J. Maquet, J. Sol-Gel Science and Techn. 2004, 31, 9. [21] S. Wang, Microporous and Mesoporous Materials 2009, 117, 1. [22] E. Skorupska, A. Jezierna, S. Kazmierski, M.J. Potrzebowski, Solid Sate Nuclear Magnetic Resonance 2014, 57-58, 2. [23] T. Azais, C. Tourné-Péteilh, F. Aussenac, N. Baccile, C. Coelho, J.M. Devoisselle, F. Babonneau, Chem. Mater. 2006, 18, 6382. [24] W. Zeng, X.F. Qian, J. Yin, Z-K. Zhu, Materials Chemistry and Physics 2006, 97, 437. [25] V.E. Zorin, S.P. Brown, P. Hodgkinson, J.Chem.Phys. 2006, 125, 144508. [26] M.M. Maricq, J.S. Waugh, J. Chem. Phys. 1979, 70, 3300. [27] G.W. Aksnes, L. Kimtys, Mag. Res. Chem. 1998, 36, 747. [28] F. Rouquerol, J. Rouquerol, K. Sing, Adsorption by powders & porous solids Academic Press, London, 1999. [29] E.G. Derouane, J.M, Andre, A.A. Lucas, J.Cat. 1988, 110, 58. [30] M. Alcoutalbi, G.B. Mckenna, J. Phys. Condens. Matter 2005, 17, R461.
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[31] G. Dosseh, Y. Xia, C. Alba-Simonesco, J. Phys. Chem. B 2003, 107, 6445. [32] T. Azais, G. Hartmeyer, S. Quignard, G. Laurent, and F. Babonneau, J. Phys. Chem. C. 2010, 114, 8884. [33] N. Baccile, F. Babonneau, Microporous and Mesoporous Materials 2008, 110, 534. [34] W.Y. Huang, J.I. Zink, J. Phys. Chem. C. 2016, 120, 23780. [35] S. Jayanthi, S. Kababya, A. Schmidt, S. Vega, J. Phys. Chem. C. 2016, 120, 2797. [36] J-G. Choi, D.D. Do, H.D. Do, Ind. Eng. Chem. Res. 2001, 40, 4005. [37] E.L. Hahn, Phys. Rev. 1950, 80, 580. [38] A. Pines, M.G. Gibby, J.S. Waugh, J. Chem. Phys. 1973, 59, 569. [39] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Smith, C.T. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker J. Am. Chem. Soc. 1992, 114, 10834. [40] Z. Ma, B.C. Dunn, G.C. Turpin, E.M. Eyring, R.R. Ernst, R.J. Pugmire, Fuel Processing Technology 2007, 88, 29. [41] A. Szegedi, M. Popova, I. Goshev, J. Mihály, J. Solid State Chemistry 2011, 184, 1201. [42] N. Hüsing, U. Shubert, Angew. Chem. Int. Ed. 1998, 37, 22-45. [43] J. Fricke, T. Tillotson, Thin Solid Films 1997, 297, 312. [44] N. Leventis, I.A. Elder, D.R. Rolison, M.L. Anderson, C.I. Merbacher, Chem. Mater. 1999, 11, 2837. [45] I. Lázár, I. Fábián, Gels 2016, 2, 26. [46] J. Kalmár, M. Kéri, Zs. Erdei, I. Bányai, I. Lázár, G. Lente, and I. Fábián, RSC Advances 2015, 5, 107237.
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[47] M.S. Greenfield, A.D. Ronemus, R.L. Vold, R.R. Vold, P.D. Ellis, T.E. Raidy J. Magn. Res. 1987, 72, 89. [48] Q. Wang, E. Jordan, D.F. Shantz J. Phys. Chem. C. 2009, 113, 18142. [49] C. Kempball, Adv. Catal. 1950, 2, 239. [50] X. Lou, M. Shen, C. Li, Q. Chen, B. Hu Solid State Nuclear Magnetic Resonance, 2018, 94, 20. [51] S. Laage, J. R. Sachleben, S. Steuernagel, R. Pierattelli, G. Pintacuda, L. Emsley J. Magn. Reson. 2009, 196, 133.
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
H NMR linewidths of small organic guest molecules physisorbed on different mesoporous silicas Gábor Szalontai PII:
S0022-2860(19)31755-7
DOI:
https://doi.org/10.1016/j.molstruc.2019.127646
Reference:
MOLSTR 127646
To appear in:
Journal of Molecular Structure
Received Date: 25 October 2019 Revised Date:
18 December 2019
Accepted Date: 23 December 2019
1 Please cite this article as: Gá. Szalontai, H NMR linewidths of small organic guest molecules physisorbed on different mesoporous silicas, Journal of Molecular Structure (2020), doi: https:// doi.org/10.1016/j.molstruc.2019.127646. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Szalontai Gábor University of Pannónia
Graphical abstract
silica aerogel
physical mixture of HMB and MCM-41 HMB in MCM-41 HMB on aerogel
crystalline hexamethylbenzene (HMB)
static 400 MHz 1H NMR spectra
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H NMR linewidths of small organic guest molecules physisorbed on different mesoporous silicas Gábor Szalontai (Szalontai Gábor) [email protected]) University of Pannonia, NMR laboratory (H-8200 Veszprém, Egyetem utca 10. Hungary)
Abstract: The excessive linewidths of 1H NMR spectra of solids is a longstanding difficulty. Organic molecules (crystalline at room temperatures) of different chemical characters (camphor, menthol, HMB, HMTA, biphenyl, terphenyl, anthracene, perylene, etc.) have been confined or physisorbed in regular (MCM-41, SBA-15) and irregular (aerogel) silica mesopores. Unexpectedly, steadily decreasing proton NMR linewidths have been observed upon increasing pore sizes. Comparison of the static 1H NMR spectra of the confined and “free” bulk molecules showed an impressive resolution improvement (a factor of 5 to 10). Static 1H NMR and 2H magic angle spinning (MAS) spectra pointed out the increased molecular mobility as origin. In a qualitative approach the possible reasons of mobility have been thoroughly discussed. The increased mobility seems to be easiest achievable in aerogel and so, in the future, this extreme light mesoporous silica nanomaterial (MSN) may be considered as “active” filling material in solid state NMR spectroscopy. Since sensitivity is not an issue in 1H MAS measurements the proposal simultaneous dilution of the dense 1H–1H spin network and increasing the molecular mobility of the molecules of interest by confinement or physisorption in/on silica
1
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mesopores - may find applications, especially in laboratories where high magnetic fields and state of art ultra-fast MAS rotation are not available.
KEYWORDS NMR, 1H, 2H, ssNMR linewidth, solid guests, molecular mobility, mesoporous silica
Highlights Narrow 1H NMR linewidths of solid guests in mesoporous silicas in static spectra. The confirmed mobility scales up with pore size, so defies simple Kelvin effect. Processes including surface diffusion and adsorption-desorption are suggested. Use of the extra-light irregular silica host aerogel as filling material is foreseen.
1. Introduction It is known that in solids the prohibitively large 1H NMR linewidths are dominated by the 1H-1H homonuclear dipolar interactions [1, 2, 3]. These, unlike the inhomogeneous interactions such as the heteronuclear dipole–dipole couplings, cannot be effectively removed by moderate magic angle spinning (MAS) rotations. In the last decade several ingenious multipulse sequences have been proposed to overcome this problem [4, 5, 6]. For a comprehensive review on the subject see [7]. For the same purpose others proposed the decrease of proton spin density by deuteration of the sample molecules [8, 9, 10]. In the meantime simpler but expensive solutions such as the superfast rotation (~ 100 kHz) at ultrahigh magnetic fields (> 20 T) became available in several but not all laboratories [11, 12]. Under such conditions linewidths of about 0.2-0.4 ppm can be routinely achieved nowadays. Nevertheless simpler solutions are still of interest and wanted. The first reports on the extremely narrow “liquid-like” NMR linewidths of small organic molecules (liquids at room temperature) when adsorbed on silica or zeolite surfaces
appeared
in
the
late
nineties.
2
The
molecules
studied
included
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hexamethyltetrasilane (HMTS) [13], H2O and benzene [14], and olefins (pentene and butene) [15]. Note that such linewidths require molecular tumbling rates in the picosecond range (10-10-10-12 seconds). With the advent of mesoporous (pore sizes 20 – 500Å) silica nanomaterials (MSN) [16, 17] it has been soon noticed that small molecules, even if solids at room temperature, also show this “liquid-like” behaviour if confined in MSNs. Günther et al. reported on the narrow lines observed in
13
C and
19
F MAS spectra of
benzenoids and hexamethylbenzene (HMB) adsorbed in different pore size (22-100Å) silicas [18]. Later Babonneau et al. published several papers on the advanced NMR techniques used for the characterization of Sol-Gel derived materials [19, 20]. Because of the special interest in the drug delivery processes [21] the behaviour of several drug molecules (e.g. ibuprofen, ketoprofen, indomethacin, aspirin) confined in MSN-s has also been thoroughly studied [22, 23, 24]. It is well known that molecules embedded in porous materials possess appreciable mobility with thermal correlation times of the order of 10-8 – 10-12 s at room temperature. Thus it is generally anticipated that lower activation energy barriers are expected. However, the origin of the higher molecular mobility, which scales down the dipolar couplings responsible for the extreme 1H linewidths observed in solids is not yet comprehensively understood. Surprisingly narrow lines have been observed by Zorin et al. under special circumstances such as arrangement of the spins in a one-dimensional geometry, e.g. in d4-urea-decanoic acid [25]. The effect was explained by a geometrical factor and implicitly spin-dilution rather than by the confinement of the decanoic acid in the urea channels. It was hinted in the early papers that the increased mobility of the confined molecules is solely due to, or is a consequence of, the “confinement effect” of the pores [26, 27, 28]. In the 10- 100 Å pore size range behaviour of the confined molecules followed the prediction of the Gibbs-Thomson equation [29, 30]. Therefore, freezing and melting temperature depression which is inversely proportional with the host pore diameter, was credited for the phenomenon. However, it is a question, whether macroscopic parameters such as melting point depression of bulk materials can be used to properly (i.e. not phenomenologically) describe the situation where the bulk phase does not exist. Not much later, others clearly proved the absence 3
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of strong long lasting interactions between the guest and the silica host [31, 32]. It was suggested that other factors such as weak and dynamic van der Waals (vdw) interactions or, in case of ionic guests, the effect of pore wall or simple spatial separations of the interacting proton spins (loading effect) can also contribute [33]. It is also known that the surface coverage of the OH (silanol) groups determines the adsorption-desorption behaviour, and consequently, the surface reactivity [16]. Recently, the adsorption-desorption kinetics of a dipeptide at the inner surface of SBA15 has been investigated by 2H MAS under different hydration levels and temperatures and was explained by a water-assisted two-side exchange model [34]. All these suggest that adsorption-desorption phenomena can, at least partially, also be credited for the improved resolution of NMR spectra. Whatever the exact reason is, a beneficial consequence of the increased molecular mobility is that in many cases the more convenient liquid state 1D and 2D NMR methodologies can be used for solid samples. The goal of the present paper is to demonstrate and compare the effect of enhanced mobility observed in three different silica mesopores (Mobil Catalytic Material, MCM, Santa Barbara Amorphous, SBA and aerogel). We are trying to confirm, evaluate and at least qualitatively understand the possible contributions such as the “surface curvature or confinement effect” [29], the physisorption and/or the surface diffusion of guest molecules [28]. The effects of adsorbate properties, silica pore size, specific surface areas, loading values and procedures on the observed 1H linewidths will be discussed in a systematic way. Possible practical applications of the 1H line narrowing in ssNMR spectroscopy will also be mentioned briefly.
2. Theory:
2.1 NMR: As mentioned the principal reason of the extreme linewidths of 1H MAS spectra is the presence of strong homonuclear dipolar couplings between the protons [35]. One way to cancel the dipolar couplings is the simulation (reproduction) of random molecular motions by rotating the sample around a predefined angle (MAS) [1]. However, only extreme fast MAS spinning, not readily available in most of the
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laboratories, can remove this contribution. Furthermore, the actual appearance of the spectrum depends heavily also on the nature of all NMR interactions responsible for the line broadening [25]. The “inhomogeneous” interactions (e.g. chemical shielding anisotropy (CSA) and heteronuclear dipolar couplings) refocus during a rotation cycle, thus can be removed from the spectra already at relatively low rotation speeds. However, it is not the case with the “homogeneous” ones (e.g. homonuclear 1H-1H dipolar couplings). Consequently, in these cases the centerband and the spinning sidebands have substantial widths which decrease only slowly with increasing spin rate (~1/rotation). Another, definitely much simpler, way is to force or let the molecules to move or tumble randomly similarly they do it in liquids or gases. Such voluntary “mobilization” has been detected in different mesopores, i.e. in the 20-500 Å pore range.
2.2 Physisorption: The forces involved include both long range attractive (van der Waals, wdw) and short range repulsive interactions. Note that these interactions do not depend on the polar nature of the hosts or guests and are therefore non-specific [28]. Concerning the thermodynamics of entrapped molecules, several models have been proposed for the interpretation of the observed phenomenon. Some of them can be referred to as “surface curvature effect” described originally by Derouane at al. [29]. It is a semi quantitative model to account for the interaction of a guest molecule and the pore surface. By decreasing the pore diameter the surface curvature is obviously increasing, so one can reach a point where the attractive (sticking) vdw forces vanish and the molecule appears as floating in the pores, no doubt an ideal situation to achieve high mobility. The mentioned phenomenological melting point reduction of small crystalline molecules (∆Tm) can be predicted by the Gibbs-Thomson equation [30].
∆ T m = T mB − T m = [4 V m T mB (γ lw − γ cw
)]/ [r ∆ H m ]
TmB = bulk melting temperature Tm = observed melting temperature Vm = crystal molar volume γlw and γcw are the liquid wall and crystal-wall interaction energies ∆Hm = molar melting enthalpy 5
(1)
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r = pore diameter (spherical pores are assumed) It is clear, however, that above a certain pore size the “surface curvature effect” cannot be responsible for the observed mobility thus another possible reasons, such as the dynamics of physisorbed molecules, must also be taken into account. In case of heterogeneous surfaces the differences of the chemical potential between the occupied and free locations lead to surface diffusion (non-localized adsorption, also called Volmer diffusion [28]). If the kinetic energy (kT) of the guest molecules (adsorbates) is high enough to overcame their motional activation energy, especially at low loading values, “gas-like” behaviour is observed [28]. At higher loading values, however, the lateral interaction between the adsorbed molecules becomes non-negligible which may lead to condensation [36].
3. Materials and Methods 3.1 Sample preparation Several loading procedures, including wet impregnation (adsorption from solvent) and dry methods (adsorption by ball mill grinding and simple mixing), have already been tested and published [18]. Normally, our samples were prepared by two hours of magnetic stirring in an appropriate volatile solvent (typically in CHCl3), followed by 2-3 hours of air drying to let the solvent residues to evaporate. In case of physical mixtures we grinded the samples gently for a few minutes by hand in an achate mortar. None of these methods are expected to change the pore structures. The loading factors (f) were calculated from the guest / host ratios (mg/mg). 1H MAS spectra of the MSNs indicated the presence of water, its quantity corresponded to the room temperature equilibrium value (~ 15 w %). Since the water content of the silica pores may have role in the adsorption- desorption kinetics it has not been removed beforehand. In case of apolar solvents (CHCl3) used in the wet method the water content of the silicas did not change significantly (see Fig. S1 in the SI material), however in case of polar solvent such as ethanol the water withdrawal can be severe.
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3.2 Solid state NMR Spectroscopy (ssNMR) All NMR spectra were recorded on a Bruker Avance II 400 spectrometer, with proton frequency of 400.13 MHz (9.38 T) equipped with a 2.5 mm and a 4 mm CP/MAS probes. 8-12 mg of polycrystalline samples was sufficient to fill up the 2.5 mm Zirconia rotors (thin wall tube with short Vespel caps). The applied rotation speeds were between 5000-25000 Hz. The cross-polarization contact time was varied between 1.5-3.0 ms. As external references for the 13C measurement signal of the carbonyl group of the alphaglycine (176.5 ppm) was used. The 13C NMR spectra of non-deuterated complexes were recorded under high-power proton decoupling. Since proton decoupling did not result in resolution improvement, no decoupling was applied in case of the 2H MAS NMR spectra. The sample temperature was not regulated (~300 +/- 1 K); note, however, that above 20 kHz rotation the actual sample temperature could be 15-35 K higher due to the frictional heat of the rotation. For the measurements (13C, 1H and 2H) and simulations of spectra we have used the standard NMR methods (Bruker Corporation Topspin 2.1.3.): single-pulse MAS (SP), Hahn-echo [37], CP/MAS [38], and high-power protondecoupled (Bruker zg, vacp.av, hpdec.av sequences) experiments. Number of scans used was 1k-2k for the 13C spectra and a few dozens and a few hundreds scans were sufficient for the 1H and 2H spectra, respectively. The weighting function was exponential broadening (5-50 Hz).
3.3 Host MSN-s materials As hosts we have used two ordered MSN-s namely MCM-41 (pore size ~ 27-40 Å, surface area (sa) 1000-1200 m2/g, ordered non-intersecting hexagonal 1D channels of controlled size) [39] and SBA-15 (pore size ~ 50-60 Å, 600-800 m2/g, ordered tubular 1D channels) [14], and one disordered native silica (SiO2)n called aerogel [40]. The exact pore size (ps) distribution, specific surface areas and other parameters of the MCM-41 and SBA-15 hosts we used have already been published by Szegedi et al.
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elsewhere [41]. The specific surface areas were evaluated by the BrunauerEmmett-Teller (BET) and/or Barret-Joyner-Halenda (BJH) methods using N2. The disordered silica aerogels are, in general, characterized by ultra-low density (~0.07 gr/cm3) and high surface area (~ 600 – 1000 m2/g), for non-aggregated system the pore volume depends on the particle-packing geometry [42, 16]. Their pore diameters lie in the mesoporous range (20-500 Å) [40, 43], furthermore the pore structure allows a mass transport which approaches the gas-phase diffusion limits [44]. It is also known that the surface coverage of the silanol groups (Si-OH) influences the adsorption behaviour of the gel [16]. 29Si MAS and CPMAS spectra have been recorded for all silica, the observed relatively large linewidths (650-750 Hz) confirmed the amorphous nature of the pore walls. The native aerogel [45], which we used in dry form, was thoroughly characterized in water suspended state [46]. The measurements indicated a specific surface area of 633 m2/g and near spherical pores with mean pore size of about ~ 140 Å (BET) or 180-200 Å (BJH). Note that these pore size values represent the central value of a wider Gaussian
distribution.
3.4 Solid guest molecules studied For the demonstration, in an attempt to prove the general nature of the phenomenon, we have used small neutral organic molecules of different geometric and chemical characters. Note that at room temperature all of them are crystalline. They include rigid planar (HMB, polyaromatics with anisotropic shapes), conformationally flexible with polar functional groups (menthol, simple amino acids), and near spherical (HMTA) molecules. Some of them may interact with the silanol groups of the surfaces and/or
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each other by hydrogen bonds, while in case of others, this possibility does not exist. The size of the selected molecules was obviously limited by the size of the available pores.
3.4.1 Waxy (extremely low and low melting point crystals) and rigid solids: L-menthol (MW=156.2, Tm=36-38
o
C), camphor (MW=152.2, Tm=174-175
hexamethyl-benzene (HMB, MW=152.2, Tm=164-166
o
C),
o
C), and hexamethylene-
tetramine (HMTA, MW=140.2, Tm=280oC).
menthol
camphor
HMB
HMTA
Scheme 1.
3.4.2 Perdeuterated polyaromatics (melting point range 71-220 oC):
The selected compounds (biphenyl-d10, terphenyl-d14, anthracene-d10, phenanthrene-d10, chrysene-d12, and perylene-d12) are planar with gradually increasing molecular weight. (Scheme 2)
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terphenyl-d14 biphenyl-d10
phenanthrene-d10
anthracene-d10
perylene-d12 chrysene-d12
Scheme 2.
4. Results
First we have tried guest molecules with low to medium melting points (L-menthol and camphor, see Scheme 1). Of them the case of the camphor proved to be particularly interesting and instructive (see Figs. 1, 2 and 3). We used this molecule to demonstrate the effects of the confinement (by static experiments) and the MAS rotation on the linewidths.
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4.1 Static linewidths in bulk and confinement, MAS linewidths vs. spinning speeds
Static 10 kHz 20 kHz 30 kHz
Camphor in MCM-41
Static 10 kHz 20 kHz 30 kHz
Crystalline camphor
Figure 1. Comparison of 1H spectra of free crystalline camphor (bottom) to the relevant spectra of the same camphor confined in MCM-41 (27 Å pores, f =0.55) (top). Static (black line) and single pulse (SP) MAS (10, 20, 30 kHz) spectra. Sample preparation method: wet impregnation. The scale is given in ppm.
In fact the camphor is a somewhat special case since it sublimes appreciably at room temperature and atmospheric pressure. For that reason it can serve as model compound for studying low-energy processes such as the effect of the room temperature molecular mobility on the solid 1H NMR spectra. While linewidth of the static free crystalline camphor is huge, for the confined molecule it is smaller by at least a factor of ten (compare the black lines in Fig.1 (bottom) and (top), respectively). As expected the MAS rotation brings further substantial resolution improvement (see the spectra recorded at 10, 20, and 30 kHz). It is interesting to note that the resolution of spectra of the suspected confined/physisorbed molecules (upper traces) are lower relative to that of the free crystalline ligands recorded at the same rotation speeds. A possible indication
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that in MCM-41 at room temperature the camphor molecules exist in two different environments or forms, e.g. we are facing with a mixture of confined and physisorbed forms. However, an exchange between the two can neither be excluded.
4.2 Comparisons of linewidths (lw) vs. pore sizes (ps) and specific surface areas (sa) Since the calculated volume of a camphor molecule is about 258 Å3, 100 mg MCM-41 offers ~ 1250 Å3 space/molecule (calculated from the porosity data). In case of uniform loading of 10 mg of camphor, i.e. if f ~ 0.1 only about one fifth of the available space is occupied by the guest molecules. The same number is about ~ 590 Å3 /molecule and ~ 225 Å3 /molecule for f ~ 0.23 and f ~ 0.5, respectively, thus with a loading factor of 0.5 we are close to the full loading of the pores, however, there is no guarantee for a homogeneous loading.
camphor in SBA-15 lw = ∆ν1/2 ~ 1550 Hz
camphor in aerogel lw = ∆ν1/2 ~ 500 Hz
camphor (crystalline) lw = ∆ν1/2 ~ 5-7000 Hz
camphor in MCM-41 lw = ∆ν1/2 ~ 2030 Hz
Figure 2. Static 400 MHz 1H spectra of free crystalline camphor, camphor confined in MCM-41 (ps=27 Å, sa=1175 m2/g), SBA-15 (ps=68 Å, sa=749 m2/g), and silica aerogel (ps=140 Å, sa=633 m2/g). Sample preparation method: wet impregnation. The loading factor was ~ 0.5 in each case. The broad hump at about 16 ppm is partly due to the background 1H signal of the 4 mm probe.
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From Fig. 2 it is clear that the larger the averaged pore size the narrower the lines are, and, at least formally, the smaller the surface areas the smaller the linewidths are. The reversed relationship observed between the mobility and the available surface area (sa) of our mesoporous silicas (saMCM-41 > saSBA-15 > saaerogel) is perhaps due to the fact that the guest molecules are not (cannot be) anchored to the silica matrix for long. In case of aerogel relatively free the mass transport of the guest molecules was pointed out earlier [44]. It is important to mention that similar correlations have been found for all molecules studied.
4.3 The effect of sample preparation and loading on resolution
As mentioned already in case of molecules soluble in easily evaporating solvents such as CHCl3 or ethanol the wet impregnation was the method of choice. This method, depending of the actual loading value, may result in formation of monomolecular layers or sparingly located guest molecules on the silica surface. Surprisingly, similar, though inferior, resolution could be achieved by preparing simple physical mixtures (see section 3.1) of the guest and host molecules. The interpretation of the phenomenon is not straightforward. Under such conditions still a significant fraction of differently oriented guest crystallites must exist, the confinement, if at all, can only be partial. Nevertheless this is a clear indication that even non-confined molecules may possess increased mobility (see Fig.4.)
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∆ν1/2 ~ 180 Hz f = 1.31 ∆ν1/2 ~ 150 Hz f = 0.55 ∆ν1/2 ~ 150 Hz f = 0.23 ∆ν1/2 ~ 150 Hz f = 0.10
Figure 3. The effect of loading (f): 1H SP MAS spectra of camphor confined in MCM41 recorded at 20 kHz. Strong signal at 4.7 ppm: water. Sample preparation technique: wet impregnation (CHCl3).
The decreasing load (less guest molecule / surface or free volume) results in somewhat better resolution (Fig. 3). However, below a loading factor of about 0.3 this effect becomes insignificant.
4.4 Spectra of a non-spherical rigid solid: free and confined/physisorbed hexamethylbenzene (HMB)
The
1H
MAS linewidth dependence of the free crystalline HMB on
inhomogeneous contributions such as anisotropy of the bulk magnetic susceptibility (ABMS), inhomogeneous bulk magnetic fields, etc. has been reported by Zorin et al. [25] in the 5000-20000 Hz rotation range. Here we report the dependence of the 1H static linewidth of HMB on the pore size and the loading procedures used (wet adsorption vs. gentle dry grinding).
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physisorbed on aerogel ∆ν ~ 600 Hz, f = 0.5 1/2
static free crystalline HMB ∆ν ~ 15000 Hz 1/2
physisorbed on MCM-41 ∆ν ~ 750 Hz, f = 0.09 1/2
„physical mixture” of HMB and MCM-41, ∆ν ~ 1200 Hz, f = 0.60 1/2
Figure 4. Static 1H spectra of free crystalline HMB, HMB confined in MCM-41 (Sample preparation method: wet impregnation, low loading), physical mixture of HMB and MCM-41 (medium loading), and HMB physisorbed on aerogel (medium loading). Sample preparation method: gentle grinding. The broad hump between 16 -18 ppm is partly due to the background signal of the probe.
It is clear that the confinement/physisorption of HMB causes enormous resolution improvement (Fig.4). It is also worthwhile to mention that the linewidth of the physical mixture is larger only by a factor of two. Static HMB spectra obtained by the Hahnecho sequence and by single-pulse (SP) experiment produced similar linewidths which do not indicate substantial susceptibility (ABMS) contribution [26]. The spectra suggest two 1H environments both in aerogel and MCM-41 hosts (~ 4.7 (H2O) and 2.5 (-CH3) ppm) whose linewidths are substantially different (~ 1300 and ~ 600-750 Hz, respectively). Note, however, that the methyl rotation, inherent in solids too, scales the linewidths of the methyl groups.
The confinement suspected and achieved by the different loading methods in different silicas has also been checked by 13C CPMAS and MAS spectra. Here again the HMB
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served as a convenient test molecule. Our results (see SI material) agree well with those of Günther et al. [18] who reported on the 13C MAS spectra of HMB adsorbed on (or confined in) an irregular silica (Silica-60). Spectra and data obtained for the camphorMCM-41 pair are also found in the supporting material (SI).
4.5 Spectra of a near spherical rigid solid: 1H NMR of hexamethyltetramine (HMTA) confined/physisorbed in MCM-41 and SBA-15
It was of interest to compare the behaviour of HMB to a more spherical guest, namely HMTA which is presumably more mobile already in the bulk phase. Indeed, 1H static linewidth of the HMTA is only about 750 – 800 Hz if confined in MCM-41 which is reduced further to about 150 Hz when rotated at 30 kHz. The spectrum (Fig. 5) displays two resolved chemical sites (water and N-CH2-N signals). The 13C CPMAS spectra (not shown), like the HMB-MCM-41 case, exhibit extremely low sensitivity. For spectra of other compounds studied and further details see Figs. S5-S13 of the Supporting Information material (SI).
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Static 10 kHz 20 kHz 30 kHz ~ 750-800 Hz
Figure 5. Comparison of static and MAS 1H (SP) spectra of HMTA confined in MCM41. Static (black line) and MAS (10, 20, 30 kHz) spectra. Sample preparation method: wet impregnation. Loading (f) = 0.44, the scale is given in ppm.
4.6 Confinement and mobility check by 2H MAS spectroscopy
It is well known that solid-state 2H NMR spectroscopy is the method of choice for studying molecular motions in solids [47, 3]. The line shapes of 2H static and MAS spectra are sensitive to molecular motions with correlation times of the order 10-4-10-6 Hz. The effective quadrupole-coupling values (CQ), normally obtained from the simulation of the spinning sideband manifold [3], are used for this purpose. The method has already been used for the study of simple organic groups covalently attached to [48], or physisorbed [35] on ordered mesoporous silicas, and is equally suitable for studying adsorption-desorption kinetics of confined molecules. E.g. using a highly water soluble dipeptide as probe, a water-cluster mediated exchange between a “frozen”
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conformation and an isotropic-like motional state was confirmed in hydrated SBA-15 [35]. To further demonstrate that the observed mobility is not restricted to water soluble molecules and to the presence of excess water in the pores neutral perdeuterated aromatic molecules with low water solubility, gradually increasing molecular weight, different melting temperatures and anisotropic shapes (see Scheme 2) have been selected, as guests, for our purposes. Note that in these molecules, due to the small chemical shift differences, resolution of the chemically different 2H sites is not expected. Like previous cases the confinment have been achieved by the wet impregnation method whereas the physical mixtures by gentle dry grinding. In solutions, their tumbling, though anisotropic, is fast and results in singlets free of the quadrupole effect. If confined in pores, depending on the actual pore size and loading, but also on the volume of the guest molecules, often only limited motional freedom is expected which may results in the observation of the quadrupolar line shape.
4.6.1 Selected 2H labelled examples (see Table): In aerogel the low melting point, conformationally instable, rod-like biphenyl molecules showed a singlet at the isotropic chemical shift without spinning side-bands (SSB) manifold (see Fig. 6). The lack of SSB-s clearly indicates that in these pores even the huge (~ 170 kHz) quadrupole coupling constant is averaged to near zero due to the extreme mobility of the molecules. For the longer terphenyl molecules, having much higher melting point (212 oC), the situation is different. While in MCM-41 we see only a broadened singlet (linewidth ~ 800 Hz), in aerogel the typical SSB pattern of rigid crystalline molecules is seen (Fig. 7). The observed effective quadrupolar coupling value, CQ is practically identical with that of the free crystalline molecule (see Table below). The different behaviour of the biphenyl and terphenyl molecules, which exists in aerogel, clearly indicates significant mobility difference among them. Considering the melting point (∆tm = 140 oC) and size difference of the two molecules this is not unexpected. This, combined with the large pores of the aerogel (the diameters lie in the 20-500 Å range), makes possible even the crystallization of the guest.
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physisorbed on aerogel
free crystalline biphenyl-d10
Figure 6. 2H MAS spectra, (bottom) free crystalline biphenyl-d10 (melting point 72 oC, rotation 10.5 kHz), (top) biphenyl-d10 physisorbed on aerogel (f= 0.45, rotation 10.7 kHz).
physisorbed on aerogel
lw = ∆ν1/2 ~ 150 Hz
confined in MCM-41
lw = ∆ν1/2 ~ 800 Hz
crystalline terphenyl-d14
lw = ∆ν1/2 ~ 250 Hz
mv = 365 Å3
Figure 7. Room temperature 2H MAS spectra: (bottom) free crystalline terphenyl-d14 (melting point 212 oC, molecular volume (mv) 365 Å3, rotation 12.4 k Hz), (middle) terphenyl-d14 confined in MCM-41 (f= 0.13, rotation 10 kHz), and (top) terphenyl-d14 physisorbed on aerogel (f= 0.63, rotation 10.6 kHz)
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In case of anthracene (Scheme 2), a rigid elongated planar polyaromatic molecule with melting point of 215 oC and molecular volume of 267.4 Å3, we compare the 2H MAS spectra obtained for samples confined in MCM-41 and aerogel (see Fig. 8). While in MCM-41 the spectrum (middle) suggests an adsorption – desorption equilibrium in the intermediate range (like the cited dipeptide case [35]) since we see an exchange broadened SSB pattern with linewidths of about 800-900 Hz. However, in aerogel the spectrum (top) is dominated by the rigid crystalline phase (the CQ and linewidths values are practically identical with those of the crystalline compound). Like the terphenyl case, this can readily be understood in terms of a depressed melting temperature in MCM-41 and an “unchanged” melting temperature in the 3-5 times larger pores available in the aerogel which seem to be sufficient even for crystal growing.
physisorbed on aerogel
loaded in MCM-41
free crystalline anthracene-d10
Figure 8. 2H MAS spectra of free crystalline anthracene-d10 (melting point of 215 oC, lw = ∆ν1/2 ~ 350 Hz, rotation 20 kHz) (bottom), anthracene-d10 loaded in MCM-41 (f=
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0.32, (lw = ∆ν1/2 ~ 800-900 Hz, rotation 20 kHz) (middle), and in aerogel (lw = ∆ν1/2 ~ 350 Hz, rotation 16 kHz) (top).
Phenanthrene (see Scheme 2): Also planar with the same molecular weight as anthracene but possesses less elongated shape and much lower melting point (101 oC). Adsorption behaviour of this molecule on Silica-60 has already been reported [16]. Based on the measured specific surface area values obtained upon gradually increased loadings they suspected non-homogeneous distribution of the sample in the pores (blocking the pores). To our surprise, when loaded into aerogel (f ~ 0.32) only a sharp singlet was observed in the spectrum (see SI, Fig. S6). When compared to anthracene, the significantly higher mobility of phenanthrene observed in aerogel can be explained, similarly to the case of biphenyl, by the lower melting point of it. In the cases of even larger guest molecules such as the four-ringed chrysene and perylene, though the averaged aerogel pore size would allow it, the spectra do not show the isotropic singlets but display quadrupolar SSB manifold characteristic for crystalline phases (see SI, Fig. S7). This is also in line with their high melting temperatures (Table).
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Table. Collection of 2H quadrupolar tensor data (effective quadrupole coupling, CQ and quadrupolar tensor asymmetry, ηQ) of the perdeuterated polyaromatic compounds physisorbed on MCM-41 and/or in aerogel (the data are results of simulations of the experimental spectra carried out by the Bruker TopSpin 2.1.3. software).
biphenyl-d10
Free crystalline guest
Guest molecules confined in
molecules
different mesopores
Quadrupole
Q tensor
couplings,
asymmetry,
CQ, [kHz]
ηQ
170
0.03
CQ [kHz] / ηQ
in aerogel
o
(mp 72 C) terphenyl-d14
CQ ~ 0 170
0.08
(mp 212 oC) anthracene-d10
181
0.0
o
(mp 215 C) phenanthrene-d10
181
0.0
(mp 101 oC) perylene-d12
in aerogel
~0
~165 /0.07
in MCM-41
in aerogel
144 /0.5
179/0.17
in aerogel CQ ~ 0
178
0.0
(mp 277 oC) chrysene-d12
in MCM-41
in aerogel CQ ~ 167/0.1
180
0.1
(mp 254 oC)
in aerogel CQ ~147/0.3
5. Discussions
By comparing the behaviour of the same molecule in different pore size silicas a clear trend emerges, namely, the larger pores the smaller the observed 1H MAS line widths (inversely proportional with the mobility of the molecules) are. This contradicts with the expectations based solely on “surface curvature effects” of the Gibbs-Thomson model [30] or on the “supermobility” of guest molecules caused by the dynamic equilibrium of
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attractive vdw forces [49]. Most probably already in the regular pores of SBA-15 with pore diameter of ~ 50-60 Å, not to mention the ball of string-like aerogel (~140 – 180 Å), we are out of the effective region of this model.
From this perspective it is constructive to compare the behaviour of the low melting point pair of biphenyl and phenanthrene with their high melting point “analogues” (terphenyl and anthracene) (see Table and Figs. 6, 7, and 8). While in the smaller pores of MCM-41 the high melting point terphenyl shows extreme mobility, it is rigid in aerogel (pore size increment factor ~ 5) where the surface curvature effect can practically be ignored. At the same time the low melting point biphenyl and phenanthrene show liquid-like mobility in the aerogel where mainly the kinetic/thermal energy of the molecules can contribute to it. Finally, like the terphenyl, the high melting point anthracene is also rigid in aerogel (Fig. 8) because, lacking significant “surface curvature” contribution, the available kinetic/thermal energy alone is perhaps not sufficient to mobilize its molecules, i.e. the equilibrium is shifted toward the adsorbed (condensed) state. In case of wet loading procedure and at loading factors high enough to allow for mono- or bilayer formation on the surfaces the liquid-like behaviour of the guest molecules in the large pores can only be understood in terms of the surface diffusion caused by the different chemical potential of different locations of the heterogeneous surface [28, 36]. If the guest molecules are rare on the silica surface the intermolecular attractive forces among them can be ignored. They are physisorbed to the surface by weak interactions (van der Waals, hydrogen bonds, etc.) thus the same thermal energy available at room temperature can give them larger mobility relative to what exists in the crystal lattice or in the bulk of liquids where the molecular motions are strongly limited by the packing forces and liquid viscosity, respectively. At high loadings (total guest volume > available porosity) the co-existence of two phases can be anticipated, namely a highly mobile surface layer and, provided the available space allows it, and a rigid (bulk: crystalline or amorphous) solid. It seems also that, at least above a certain pore size where the adsorption-desorption process is not influenced by steric factors, the regular or irregular nature of the pores does not play important role.
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Concerning the loading procedures tested, it is worthwhile to remember that the dry grinding method producing “physical mixtures” turns out to be surprisingly effective from the point of line narrowing. This deserves further comments. It is known that effective grinding results in reduced particle sizes and increased surface areas [50] and thus higher guest mobility. However, the gentle grinding alone we applied hardly can cause such substantial impact. It seems that the “dilution“ effect, the size and distance of 1H-1H dipole-dipole tensors sitting in different neighbouring molecules or rather clusters, is equally important. In this respect it is noteworthy that in case of HMB using a non-porous matrix (alfa-quartz) also resulted in a 50 % line width reduction (see supporting material). Concerning the specific interactions, the silanol groups of the surface and/or the adsorbed water molecules can also play an active role in the equilibrium. In this respect the relatively free mass transport [44] could explain the extreme linewidths observed in the irregular aerogel, where the pores are large, interconnected, and nearly spherical [46]. Finally, it is clear that the smaller pore hosts (MCM-41 and SBA-15), while can confine a single guest molecule or a small cluster of them, do not make possible the crystal formation for steric reasons.
6. Conclusions Organic molecules of different characters (camphor, menthol, HMB, HMTA, biphenyl, terphenyl, anthracene, perylene, etc., all crystalline at room temperatures.) confined and/or physisorbed in regular (MCM-41, SBA-15) and irregular (aerogel) mesopores were studied. Upon gradually increasing pore size we observed steadily decreasing proton linewidth in static spectra which was further reduced by MAS rotation. 13
C CPMAS and MAS spectra (see SI) were used to confirm the confined/adsorbed or
non-confined/desorbed state of the molecules. 2H MAS spectra point to molecular mobility as main reason. Comparison of the static spectra of the physisorbed and “free” bulk molecules shows an impressive resolution improvement (a factor of 5 to 10 was observed in each case looked at so far). When combined with fast MAS rotation 1H line widths of 100-200 Hz could be achieved. Concerning the origin of the mobility there seems to be synergism between the confinement effects (surface curvature) and an 24
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adsorption-desorption equilibrium and/or surface diffusion of the guest molecules caused by the kinetic-thermal energy available at room temperature. The outcome depends primarily on the pore size, the available specific surface area, and surface heterogeneity of the hosts but also on physicochemical properties of the guest molecules. Significant line narrowing can be expected as long as formation of the condensed “bulk phase” is prohibited either for steric reasons or due to the large freely available surface area. The increased mobility seems to be easiest achievable in aerogel and so in the future this material may find routine applications also in NMR spectroscopy. The proposal - simultaneous dilution of the dense 1H–1H spin network and increasing the molecular mobility by confinement or adsorption in silica mesopores - can find interesting applications, especially in laboratories where the ultra-fast MAS rotation [51] is not available.
Acknowledgements: The mesoporous silicas were generous gifts from Dr. Ágnes Szegedi (MCM-41 and SBA-15, TTK MTA, Budapest) and Dr. István Lázár (native aerogel, University of Debrecen), who are gratefully acknowledged. Drs. Tamás Kristóf and Géza Horváth are thanked for valuable discussions on the thermodynamics of entrapped molecules. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
7. References
[1] M. Mehring, High Resolution NMR Spectroscopy in Solids, Springer-Verlag, Berlin, 1978. [2] K. Schmidt-Rohr and H.W. Spiess, Multidimensional Solid-Sate NMR and Polymers, Academic Press, London, 1994.
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: