Heteronuclear NOE in the solid state

Journal of Molecular Structure 602-603 (2002) 505±510

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Heteronuclear NOE in the solid state q Julia S. Higgins a, Andy H. Hodgson a,b, Robert V. Law b,* b

a Department of Chemical Engineering and Chemical Technology, Imperial College, Prince Consort Road, London SW7 2BY, UK Department of Chemistry, Imperial College of Science and Technology, Exhibition Road, South Kensington London SW7 2AY, UK

Received 3 April 2001; accepted 16 April 2001

Abstract A series of small molecules and polymers was examined in order to investigate the heteronuclear NOE effect in solids. Contrary to a previously reported interpretation it was found that caution has to be used in interpreting NOE rates purely in terms of correlation times. The NOE rate of a methyl group was not solely dependent upon correlation times but also the distance (r 26) from the nearest methyl neighbour. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Magic angle spinning; Nuclear Overhauser effect; Polymers; Distances; Heteronuclear

1. Introduction he nuclear Overhauser effect (NOE) has been extensively used in solution state NMR spectroscopy [1] to investigate structure and conformation of molecules. In the solid state, however, there have been relatively few applications [2±8] principally because of the dif®culties in obtaining high resolution 1H spectra to carry out the homonuclear NOE experiments [9] and the dominance of 1H± 1H spin diffusion. However as it is possible to obtain high resolution solid state 13C NMR spectra using magic angle spinning (MAS), it is increasingly being realised that there is often suf®cient molecular motion in the solid state to allow the NOE to be effectively utilised in a heteronuclear sense. Though not as often used as the homonuclear type experiment, heteronuclear NOE (in this case 1H± 13C) can still be a useful guide to

different types of molecular motion present [10]. Here local dipolar ¯uctuations from the 1H affect the neighbouring 13C and increase the intensity of the signal. In a previous study, it was demonstrated that both the initial NOE build up rate and the ®nal equilibrium (steady state) value are of use in characterising the molecular motion of solid polymers. In order to allow systems to NOE there has to be suf®ciently short correlation times present. In solids, therefore, this is essentially restricted to the rotation about the C3 axis in methyl groups other groups. Other motions e.g. phenyl ring ¯ips are too slow to allow NOE to occur. From heteronuclear NOE, we have established another possible application viz. to measure internuclear distances between methyl groups present in a molecule. The approach that we take in this paper is carried out by relating the distances between methyl groups present in the molecule to the NOE build up rates.

q

Dedicated to Professor Graham A. Webb on the occasion of his 65th birthday. * Corresponding author. Tel.: 144-20-594-5860; fax: 144-20594-5804. E-mail address: [email protected] (R.V. Law).

2. Experimental Samples atactic poly(methymethacrylate)(PMMA),

0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0022-286 0(01)00731-1

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Fig. 1. Tetramethylbisphenol-A (TMBPA).

isotactic poly(propylene)(iPP), poly(phenyleneoxide)(PPO), poly(isobutylene)(PiB) and tetramethylbisphenol-A(TMBPA) (Fig. 1) were and adamantane obtained from Aldrich, hexamethylbenzene(HMB) from Lancaster, poly(carbonate) from PolySciences and the poly(tetramethylcarbonate)(TMPC) (Fig. 2) was a gift from Bayer. All polymers were used without any further puri®cation. Samples were analysed using a Bruker 200 DSX solid-state NMR spectrometer. The 1H and 13C Larmor frequency were 200.14 and 50.33 MHz, respectively. The samples were packed into 4 mm zirconia rotors and sealed with a Kel-F endcap. The samples were then were spun at the magic angle (54.748) at a frequency of 3.0±8.0 kHz (^5 Hz) and held at 300.0 K (^0.1 K) using a Bruker double bearing probehead. All samples were externally referenced to the up®eld peak of adamantane (29.5 ppm relative to tetramethylsilane). Typically 100±1000 scans were accumulated for each spectrum. The proton NOE `pulse-comb' consisted of a series of 5.0 ms, 908 1H pulses separated by a 1 ms delay, as shown in Fig. 3 [11]. The number of pulses in the `comb' ranged from 1 to 15000. A 908 pulse length of 4.0 ms was used to excite the carbons. The delay

between the beginning of each pulse-comb was varied to give a constant recycle delay for the carbons of D ˆ d 1 t: D was selected for each sample such that D . 5p T1 ; where T1 is the spin lattice relaxation time, to avoid any saturation effects. The 13C T1 were measured for each sample so that a correct delay (5 times 13C T1) in the Bloch decay or single pulse excitation (SPE) spectrum could be chosen. The relative intensities of each peak for the equilibrium NOE values, h , were compared to the SPE spectrum to determine the enhancement factor. The same number of scans for each spectra (NOE and SPE) was collected. The NOE build up data and equilibrium values were treated in a similar manner to that reported previously [10]. 3. Results and discussion The origin of NOE has been extensively discussed elsewhere and only a very brief summary will be given here. The NOE gives rise to a signal enhancement whenever double irradiation is used, it can occur as a homo- or heteronuclear effect and originates from a direct result of a normally forbidden cross-relaxation process. The cross-relaxation processes are zero (W0) and/or double (W2) quantum transitions (ZQT, DQT) that occur in order to restore the now perturbed non-equilibrium state back towards equilibrium (Fig. 4). The ZQT or DQT pathways are strongly dependent upon the type of molecular motion present in the system with higher NOE equilibrium values indicative of the DQT pathway. For the heteronuclear case, 1H± 13C, the degree of enhancement (h ) will vary from 0.15 to a maximum value of 1.99 (or 199%) at the extreme narrowing limit and will not fall below zero because gH =gC . 2:38 [1].

Fig. 2. Poly(tetramethylcarbonate) (TMPC).

J.S. Higgins et al. / Journal of Molecular Structure 602-603 (2002) 505±510

507

Fig. 3. NOE pulse sequence. n ˆ number of 1H 908 pulses. d ˆ delay before 1H 908 pulses. t ˆ delay during 1H 908 pulses.

Throughout the series of samples examined, the high equilibrium values of the methyl groups (Table 1) for the NOE indicate that the DQ term predominates and that they tend towards the extreme narrowing limit. Although it is the methyl carbons that are the focus of this work, NOE data for nonmethyl carbons were also recorded. For the carbons, other than methyls (not listed), the NOE equilibrium values appear to correspond with the idea reported previously that carbon atoms close to the methyl groups have a higher equilibrium value because the nearby methyl group acts as a strong source of dipolar relaxation. Other groups that are far from a methyl group show either minimum (0.15) or only a small enhancement [2]. For the methyl groups in the series of samples the NOE rates and equilibrium values are given in Table 1. From the results it is possible to divide the compounds into two classes: those with ®xed distances between the methyl groups dictated by molecular geometry e.g. Figs. 5 and 6 and those

Fig. 4. Cross-relaxation scheme for a CH pair. a and b refer to the relative spin state of each nucleus. W1H, W1C ˆ spin lattice relaxation times for 1H, 13C, respectively. W1C0, W1C zero quantum (ZQT) and double quantum transitions (DQT).

with a less restricted geometry due to freely rotating bonds. In geometries ®xed due to the molecular con®guration, it is evident from our data that there is a strong relationship between the NOE build up rate and distance between the methyl groups. Previously it has been reported [10] that the NOE build up rate was dependent upon the correlation time of the methyl. However, our data suggests that caution has to be used in interpreting NOE build up rates purely in terms of correlation times. The NOE rate of a methyl is not solely dependent upon correlation times but also upon the distance from the nearest methyl neighbour. Those geometries with two methyl groups attached to the same carbon (Fig. 5) as in poly(carbonate) (PC) or TMPC show an extremely fast NOE build up rate. Ê ) intranuclear This is due to the relatively short (2.5 A distance between the carbon centres which allows each methyl to act as a source of ¯uctuating dipolar ®elds for the other. This can also be seen in the monomer where due to crystallographic inequivalence there are now two resolved signals for the aliphatic methyls (in both BPA and TMBPA). The two different rates for aliphatic methyls in the monomer may be due to the closeness of additional methyls in the crystalline unit cell. In BPA the nearest neighbour intermolecular distance (from carbon to carbon centre) for each methyl is different, varying Ê [11]. This is also true for from 4.00 to 5.19 A TMBPA where the distance for each methyl is 4.16 Ê , respectively [12]. It is also interesting to and 4.63 A note that in PC it has been reported that NOE rate was seen to be an average of the two crystallographically inequivalent methyls found in the monomer BPA. For the amorphous polymer, this is an average of these values. This would represent a distribution of chemical shifts available to the methyls in the

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J.S. Higgins et al. / Journal of Molecular Structure 602-603 (2002) 505±510

Table 1 Initial NOE rates and equilibrium values for the materials studied. All curves ®tted to h ˆ …s=r†…1 2 exp…2rt†† by least squares ®t. Where r is the cross-relaxation rate and s is the total spin±lattice relaxation rate Samples

Initial/cross relaxation, s a

Total spin±lattice, r b

Equ. NOE c

Nearest Me neighbour d,e

No. of Me

aPMMA aMe aPMMA esterMe iPP PiB PC TMPC Al TMPC Ph TMBPA Al TMBPA Al2 f TMBPA Ph TMBPA Ph2 f PPO HMB Adamantane CH Adamantane CH2

8.23 0.20 3.12 14.6 16.3 20.0 0.15 14.6 7.47 0.24 0.17 0.12 1.61 0.56 1.04

9.92 0.23 1.79 11.6 11.7 11.4 0.24 7.84 14.6 0.16 0.19 0.51 1.47 0.31 0.54

0.83 0.88 1.74 1.26 1.40 1.75 0.64 1.86 0.53 1.48 0.91 0.23 1.09 1.89 1.93

ca. 3.6 ca. 5.0 ca. 4.4 2.50 2.50 2.50 4.60 2.50 2.50 4.60 4.60 5.07 2.93 N/A N/A

2 2 2 1 1 1 1 1 1 1 1 1 2 N/A N/A

a b c d e f

s ˆ W 2 2 W0 ; s 21, error ^ 0.1. r ˆ W2 1 2W1 1 W0 ; s 21. s /r , no units, error ^ 0.05. Ê. A For iPP and aPMMA these were approximated distances because of the free rotation about the bonds between the methyls. In TMBPA there are two distinct crystallographic sites.

amorphous solid. However, for the TMPC and its monomer TMBPA this is not the case. For the crystalline monomer the NOE rates are (7.5 and 14.6 s 21) whereas that of the polymer (20.0 s 21) is not the average. This may imply that there may be addition methyl±methyl interactions in the polymer e.g. between the aromatic phenyl methyl and aliphatic methyl which are not possible in the crystal structure because of the restricted number of conformations present. Also induction times previously observed were not seen, this may be due to the higher ®eld used giving an alternative source of relaxation viz. chemical shift anisotropy. The intuitive explanation that nearby methyl groups contribute signi®cantly to NOE build up rates on

Fig. 5. Common structural moiety present in poly(carbonate), poly(tetramethylcarbonate) (including their monomer) and polyisobutylene.

methyls helps explain the discrepancy in TMPC and the monomer (TMBPA) between the aliphatic and aromatic methyls. From the data it appears that the aliphatic methyl groups have a much faster NOE build up rate that the aromatic. This would appear counterintuitive as the aliphatic methyl groups are in a more sterically hindered position than the aromatic groups and would therefore have a longer correlation time. However, if we consider the nearest neighbour methyl for the aromatic methyl carbon (in TMPC across the Ê distant and the carbonate group) is about 4.7 A Ê aliphatic methyl is 2.5 A, this would explainable if

Fig. 6. Common structural moiety present in poly(tetramethylcarbonate) (including the monomer) and polyphenylene oxide.

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Fig. 7. Graph of distance verses initial NOE rates with a ®t of y < ar26 to the data. Where a is a constant and r is the methyl±methyl distance.

NOE build up rate is dependent upon nearest methyl distance. This is also borne out by quasi-elastic neutron scattering studies of methyl rotation where aromatic methyls have a lower activation energy (and hence shorter correlation time) than aliphatic methyls [13]. To test this hypothesis further we looked at other systems where a similar geometry (with the Ê apart) existed poly(isobutylene). Here methyls 2.5 A the NOE rate is also extremely fast. Furthermore, similar geometries (Fig. 6) were examined for the aromatic methyl groups ªe.g. PPOº that showed similar behaviour to TMPC. This may also explain the results reported previously that PaMS has a much faster NOE rate than P4MS. The most likely explanation is that on average the methyl groups in Pa MS are closer together, along the backbone, than in P4MS where they sit away from the backbone. To substantiate this further we also looked at a system that had methyl groups away from the backbone and also along it, PMMA. Here the a-methyl (8.2 s 21) has faster NOE build up rate than the methyl ester (0.2 s 21) for the same reasons given previously. Methyl rotation in PMMA has been studied [14] using quasi-elastic neutron scattering and it was found that at room temperature, the a-methyl rotates at less than 10 9 Hz while the ester methyl moves at 10 11 Hz. From this data we have been able to plot MeMe distance verses NOE rate (Fig. 7). It can be

seen from this data that it is possible to ®t a curve to this data. This gives y < ar 26 relationship indication that the NOE build up rate is strongly dipolar in nature. No comment has yet been made on the possibility of spin-rotation being a source of relaxation [15,16]. Though this has been considered to be a small contribution to the overall relaxation rate [16]. Further investigation is being carried out to con®rm this. 4. Conclusions Although short correlation times …tc , 10210 s† are required for heteronuclear NOE caution is required when interpreting the cross-relaxation rate, s , purely in these terms. From the data it is possible to discern a y < ar 26 relationship indicating that the relaxation rate is dipolar in nature. From this it is possible to determine distances between methyl groups using heteronuclear NOE in crystalline and amorphous solids. Acknowledgements We wish to acknowledge the EPSRC for the studentship for AWH. We would also like to thank

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JoaÄo Cabral for interesting discussions on the molecular motions of polymers. References [1] D. Neuhaus, M.P. Williamson, The Nuclear Overhauser Effect in Structural and Conformational Analysis, VCH Publishers, Weinheim, 1989. [2] J.L. White, J.F. Haw, Journal of the American Chemical Society 112 (1990) 5896. [3] J.L. White, P. Mirau, Macromolecules 6 (1993) 3049. [4] R.V. Law, G.D. Love, C.E. Snape, Energy and Fuels 7 (1993) 1148. [5] R.V. Law, D.C. Sherrington, C.E. Snape, I. Ando, H. Korosu, Industrial and Engineering Chemical Research 34 (1995) 2740. [6] R.V. Law, D.C. Sherrington, C.E. Snape, I. Ando, H. Korosu, Macromolecules 29 (1996) 6284. [7] R.V. Law, D.C. Sherrington, C.E. Snape, Macromolecules 30 (1997) 2868.

[8] R.V. Law, D.C. Sherrington, in: I. Ando, T. Asakura (Eds.), Crosslinked Polymers, Solid State NMR of Polymers, Elsevier, Amsterdam, 1998 chapter 15. [9] P. Caravatti, P. Neuenschwandler, R.R. Ernst, Macromolecules 19 (1986) 1889. [10] J.L. White, Solid Sate Nuclear Magnetic Resonance Spectroscopy 10 (1997) 79. [11] A. Findlay, R.K. Harris, Journal of Magnetic Resonance 87 (1990) 605. [12] V.K. Belskii, N.Y. Chernikova, V.K. Rotaru, M.M. Kruchinin, Krystallogra®ya 28 (1983) 685. [13] J.S. Higgins, A.W. Hodgson, R.V. Law, Acta Crystallographica C, submitted for publication. [14] J.S. Higgins, H.C. Benoit, Polymers and Neutron Scattering, Oxford University Press 1994, Ch. 9, p.317. [15] J.R. Lyerla, D.M. Grant, R.K. Harris, Journal of Physical Chemistry 75 (1971) 585. [16] C.F. Schmidt, S.I. Chan, Journal of Magnetic Resonance 5 (1971) 151.