Vibrational characterization of α-aminophosphinic acid derivatives of pyridine: DFT, Raman and SERS spectroscopy studies

Accepted Manuscript Title: Vibrational characterization of ␣-aminophosphinic acid derivatives of pyridine: DFT, Raman and SERS spectroscopy studies Author: Ewa Pi˛eta Edyta Proniewicz Andrzej Kudelski Tomasz K. Olszewski Bogdan Boduszek PII: DOI: Reference:

S0924-2031(16)30016-9 http://dx.doi.org/doi:10.1016/j.vibspec.2016.02.001 VIBSPE 2497

To appear in:

VIBSPE

Received date: Revised date: Accepted date:

1-10-2015 19-1-2016 2-2-2016

Please cite this article as: Ewa Pi˛eta, Edyta Proniewicz, Andrzej Kudelski, Tomasz K.Olszewski, Bogdan Boduszek, Vibrational characterization of rmalphaaminophosphinic acid derivatives of pyridine: DFT, Raman and SERS spectroscopy studies, Vibrational Spectroscopy http://dx.doi.org/10.1016/j.vibspec.2016.02.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

Vibrational characterization of -aminophosphinic acid derivatives of pyridine: DFT, Raman and SERS spectroscopy studies

Ewa Pięta,1 Edyta Proniewicz,2* Andrzej Kudelski,3 Tomasz K. Olszewski,4 Bogdan Boduszek4

1

Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland

2

Faculty of Foundry Engineering, AGH University of Science and Technology, Reymonta 23,

30-059 Kraków, Poland 3

Department of Chemistry, University of Warsaw, L. Pasteura 1, 02-093, Warszawa, Poland

4

Department of Organic Chemistry, Faculty of Chemistry, Wroclaw University of

Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland

*Correspondence to: E.P. (e-mail: [email protected]; Phone: +48-12-617-34-96. Fax: +48-12-633-6348)

1

ABSTRACT This paper shows Fourier-transform Raman (FT−RS) and surface-enhanced Raman spectroscopy (SERS) studies of three −aminophosphinic acid derivatives of pyridine: [(butylamino)(pyridin-2-yl) methyl]phenylphosphinic acid (–PyNH), [(butylamino)(pyridin3-yl)methyl] phenylphosphinic acid (β–PyNH), and [(benzylamino)(pyridin-4-yl)methyl] phenylphosphinic acid (–PyNH) immobilized onto colloidal sol nanoparticles and electrochemically roughened surface of silver. The molecular geometries and vibrational wavenumbers were calculated based on density functional theory (DFT) at the B3LYP 6– 311G(df,p) level of theory. Based on the comparison of the FT−RS experimental and theoretical vibrational bands with the SERS results the orientation of the −aminophosphinic acid derivatives of pyridine onto two silver substrates was proposed. The changes in the adsorption process under the influence of the position of substituent in regard to the ring N atom (in α−, β−, and γ−positions, respectively) and modification of the substituent (replacement of the butyl chain by the phenyl ring of high affinity to silver (γ−PyNH)) were also discussed. KEYWORDS: −Aminophosphinic acid derivatives of pyridine; Fourier-transform Raman spectroscopy, FT−RS;

Density functional

theory, DFT;

Surface-enhanced Raman

spectroscopy, SERS; Silver colloid; Electrochemically roughened silver electrode

2

1. Introduction In recent years, there has been growing interest in the applications of phosphorous derivatives of pyridine because of their stability and durability in both neutral and basic conditions [1–5]. Interestingly, the combination of a heterocyclic moiety with a phosphorous– containing fragment makes these compounds very attractive in terms of biological and chemical applications [6,7]. This is because the C–P bond cleavage phenomenon of pyridine aminophosphonate and aminophosphinate, occurring in acidic aqueous media, play an essential role in the decomposition process of heterocyclic aminophosphonates, such as derivatives of imidazole, thiazole and chinoline [6-8], and the formed products may serve as useful reagents in organic synthetic applications. Pyridine-like compounds have a wide range of applications ranging from medicinal drugs to agriculture products, including: insecticides, fungicides, herbicides, and plant growth regulators [9–12] and enzyme inhibitors [13]. Pyridines are very promising class of compounds due to the properties of the pyridine ring (stable, 6-π-electron, π-deficient, and nitrogen-containing aromatic structure, in which the ring nitrogen is more electronegative than the ring carbons) [10]. Chemical reactivity of this group of compounds is associated with the ability to the polarization of the π-electrons, presence of a large permanent dipole moment, and tendency for electron donation due to the presence of the electron-deficient carbon atom. Pyridines also exhibit good miscibility with the most organic solvents [10]. Thus, this group of compounds is considered as promising serine protease inhibitors [13], especially as a potent inhibitors of aminopeptidase N [14] – a transmembrane protease located in various kind of human organs, cells, and tissues [15–18]. Inhibition of this ectopeptidase may shed some light on design of the anti-cancer and anti-inflammatory drugs [15]. This class of molecules also acts as very effective ligands of heavy metal ions, particularly Cu(II) [19]. Here, we used the surface-enhanced Raman spectroscopy technique to study the adsorption mechanism of three pyridine aminophosphinates, including: −aminophosphinic acid derivatives of pyridine ([(butylamino)(pyridin-2-yl) methyl]phenylphosphinic acid (– PyNH),

[(butylamino)(pyridin-3-yl)methyl]

[(benzylamino)

phenylphosphinic

(pyridin-4-yl)methyl]phenylphosphinic

acid

acid

(β–PyNH),

(–PyNH))

(Table

and 1)

immobilized onto colloidal and roughened in the oxidation-reduction cycles (ORC) silver substrates. This is important because SERS gives information about supramolecular architectures and adsorption phenomena occurring at solid/liquid interface, which may 3

provide new insights into in vivo behavior of these molecules in the physiological conditions. In this paper is focused mainly on the changes in the molecular orientation onto the silver substrates due to the different position of the substituent relative to the ring nitrogen atom (in

–, –, and – positions, respectively), the modification of the substituent (replacement of the butyl chain by the phenyl ring of high affinity to silver (γ−PyNH)), and due to the development of the silver surface. We also employed Fourier-transform Raman (FT−RS) spectroscopy to determine the vibrational properties of the investigated compounds. To make a reliable assignment of the experimental frequencies to the normal mode motions we performed theoretical analysis using density functional theory (DFT) calculations with the B3LYP method at the 6−311G(df,p) level of theory [20–22]. The SERS technique is a very useful way to analyze adsorption process at the molecular level [23–26]. In SERS, the unique Raman signal amplification (up to 1014) is observed for molecules adsorbed or being in close proximity to a surface of a metallic substrate [21]. The most common SERS-active substrate is silver used as chemically or electrochemically roughened surface or in a colloidal form. Depending on the structure, some of the molecular moieties interact stronger with a metal surface than another. It was proved that compounds, which contain a molecular fragment with the high ability to be chemically bind to the silver substrate, such as pyridine, exhibit the largest enhancement [22]. The SERS effect can be caused by two enhancement mechanisms, one due to the very large local enhancement of the electromagnetic field (strongly dependent on the surface orientation electromagnetic mechanism, EM) and the other due to so-called charge transfer effect (CT) [27]. Unfortunately, it is not possible to state unambiguously what is the contribution of these two mechanisms to the observed enhancement in the SERS signals [27,28]. Nevertheless, the EM mechanism is considered to play the predominant role in the total observed enhancement of the Raman signal [29]. Consequently, the analysis of the SERS parameters, such as: enhancement, broadness, and wavenumber resulting from the individual functional groups, is essential for comprehensive description of the adsorption phenomena. However, this analysis is quite complex and for correct interpretation, it is necessary to apply the surface selection rules, which predict that the molecular vibrations with the large tensor components oriented along the axis normal to the metal substrate are the most enhanced [30– 32]. On the other hand, the contribution of the CT mechanism to the enhancement of the Raman spectral features plays an essential role in the increasing of the SERS signals intensity 4

of the benzene–like compounds, including pyridine [33]. This effect is connected with large enhancement of the totally symmetric stretching ring vibration (8a, A1), which appears at ~1600 cm-1 as the most intensive signal in the SERS spectrum [28]. Arenas et al. [27] described the importance of the CT mechanism in the enhancement of the relative intensities of the SERS bands of 2–methylpyrazine adsorbed onto the silver electrode. 2. Experimental and theoretical methods 2.1.

Synthesis

−Aminophosphinic acid derivatives of pyridine were synthesized via addition of silylated phosphorus esters to appropriate pyridine imines [4]. The detailed synthetic procedure and spectroscopic characterization of these compounds was already published [5,34]. All the −aminophosphinic acid derivatives of pyridine used in this study were in the form of racemic mixtures. 2.2.

FT-RS measurements

The FT–RS spectra were recorded using a Nicolet spectrometer, model NXR 9650, equipped with a continuum–wave Nd+3:YAG laser (1064 nm, the output laser power: 200 mW) and a liquid–nitrogen–cooled germanium detector. Typically, 1000 scans with a resolution of 4 cm-1 were collected. 2.3.

SERS measurements 2.3.1. Silver Sol

Colloidal silver nanoparticles were prepared by borohydride reduction of silver nitrate. Briefly, AgNO3 and NaBH4 were purchased from Wako (Japan) and used without further purification. Three portions of silver sols were obtained by the borohydride reduction of silver nitrate [20]. 5 mg of AgNO3 dissolved in 50 mL of deionized water (18 MΩ cm) at 4°C was added dropwise to 150 ml of 2 mM NaBH4 immersed in an ice bath and stirred vigorously. The obtained dark−yellow solution was stirred continuously for about 1 hour. Aqueous solution of the studied molecules was prepared by dissolution of the proper compound in deionized water (18 MΩ cm). 20 μl of 10-4 M sample solution was mixed with 40 μl of the silver nanoparticles.

5

Figure 1 presents UV−VIS spectra of an aqueous colloidal silver solution used in this work (see Ref. [23] for transmission electron microscopy (TEM) image of the sample/silver nanoparticles system). The SERS spectra were collected one hour after mixing the sample with the colloid nanoparticle system. Typically, the spectra were recorded three times from different spots using an InVia Renishaw spectrometer (with the spectral resolution set at 4 cm-1) equipped with CCD detector and the confocal microscope Leica with a 50 x long distance magnification objective. The 785.0 nm line of a diode laser was used as the excitation source. The laser power at the output was set at 40 mW. The obtained spectra from the series were almost identical, except for small differences (up to 5%) in some band intensities. No spectral changes associated with the sample decomposition or desorption process were observed during the measurements. 2.3.2. ORC Silver Substrate The specially roughened in oxidation-reduction cycles (ORC) silver substrates were prepared according to the standard procedure (see Ref. [35] for details). The morphology of the substrate (roughens size ~50–150 nm) can be found elsewhere [36]. The SERS spectra were collected one hour after soaking of the roughened silver substrate in the solution of studied compounds. Raman measurements were carried out using a Horiba Jobin–Yvon Labram HR800 spectrometer equipped with a Peltier-cooled CCD detector (1024 × 256 pixel), 600 grooves/mm holographic grating, and an Olympus BX40 microscope with a long distance 50 objective. A He-Ne laser provided the excitation radiation with the wavelength of 632.8 nm. 2.4.

UV-VIS Measurements

The UV−VIS spectra of the silver colloid and sample/silver nanoparticle system (Figure 1) were recorded using a Thermo Scientific spectrometer (model EVO−60). 2.5.

Theoretical analysis

The Gaussian 03 software package (at the Academic Computer Center “Cyfronet” in Krakow) was used to optimize structures (see Table 1) and to calculate vibrational spectra of the –aminophosphinic acid derivatives of pyridine [37]. A DFT method with the B3LYP level of theory was used to optimize the molecular structure of the investigated molecules. 6– 311G(df,p) – the split–valence triple zeta basis set (6–311G) was used as the basis set [38,39]. 6

The most stable structure of the pyridine–α–hydroxymethyl biphenyl phosphine oxide isomers is a cyclic dimer created by a pair of intermolecular hydrogen bonds between the H atom of the α-hydroxyl group and the O atom of the phosphoryl group of the two monomeric structures [23]. On this basis, we performed calculations for monomers and various kind of dimers but in this paper we present the calculations for the most stable dimers formed by – aminophosphinic acid derivatives of 2–, 3–, and 4–pyridine (see Table 1). The Raman intensities were calculated using the Raint program [40]. The 1064 nm line was used as an excitation wavelength. Then, the spectra were generated by the freeware GaussSum 0.8 software package (a scaling factor: 0.981; a FWHM (full width at half maximum) at 10 cm-1; a 50%/50% Gaussian/Lorentzian band shape) [41]. The PED (potential energy distribution) was obtained with the freeware Gar2ped program [42] linked with a visualization script. No imaginary wavenumbers were calculated that prove that the optimized structures match to the energy minima on the potential energy surface for nuclear motion. 2.6.

Fitting procedure

Fitting procedure of the FT–RS and SERS spectra of the –aminophosphinic acid derivatives of pyridine was performed with a GRAMS/AI program (Galactic Industries Co., Salem, NH). 3. Results and discussion 3.1.

Geometry

The previously obtained results for three pyridine-α-hydroxymethyl biphenyl phosphine oxide isomers proved that the cyclic dimer created by two intermolecular hydrogen bonds between the H atom of the α-hydroxyl group of the first monomer and the O atom of the phosphoryl group the second monomer is the most stable structure [23]. Similar findings were obtained for the compounds studied in this paper. The most stable structures are cyclic dimers formed by two intermolecular hydrogen bonds between the H atom of the phosphinic group of the first monomer and the O atom of the same group from the second monomer. The calculated stabilization energies for α–PyNH, β–PyNH, and γ–PyNH dimers are 15, 22 and 21 kcal/mol, respectively (correction for the basis set superposition error (BSSE) was taken into account). According to the Jeffrey’s system, these hydrogen bonds could be classified as the strong hydrogen bonds [43]. 7

The bond lengths and bond angles are listed in Table 2, whereas the atom numbering scheme for the studied compounds is given in Table 1. The optimized C–C and C–N bond lengths of the Py and Ph rings (Table 2) in α–PyNH, β–PyNH, and γ–PyNH are in good agreement with those found in the X-ray (Py: C–C = 1.379 Å and C–N = 1.337 Å; Ph: C–C = 1.380 Å) [44] and DFT studies (Py: C–C = 1.397 Å and C–N = 1.334 Å; Ph: C–C = 1.380 Å) [45] of mono-substituted pyridines and phenyls. The C–C bonds in the Ph ring are slightly longer than those observed in the Py moiety. The calculated C7–C2 distance for –PyNH (1.525 Å) is slightly longer than that in –PyNH and –PyNH (1.509 and 1.515 Å, respectively), implying direct impact of NPy of the Py ring on the former bond. Additionally, the C2–C7–N17 angle in –PyNH (115.2o) is slightly wider than that in β–PyNH (112.7o) and

–PyNH (113.3o). The calculated P9=O10 bond length for all the studied molecules (~1.510 Å) are in the range of distances characteristic for this class of molecules [46]. Furthermore, the calculated C7–N17 (~1.45 Å), C7–P8 (~1.87Å), and P8–C11 (~1.81Å) bond lengths are almost equal for all the analogues. The detailed information about the remaining bond lengths and angles together with dihedral angles and hydrogen bridges are summarized in Table 2. 3.2.

FT–RS and DFT studies

The experimental (black solid line) and theoretical (red dashes line) FT–RS spectra of three −aminophosphinic acid derivatives of pyridine, –PyNH, –PyNH, and –PyNH, in the solid state are presented in Figure 2. As can be seen from this figure, the predictions of the calculated data (wavenumbers and intensities) are in good agreement with the experimental results. The previous study proved that there are no changes in the spectral patterns between the spectra of molecules in the solid state and in an aqueous solution [23]. Therefore, we presented the FT–RS spectra of the investigated compounds in the solid state. The theoretical and experimental wavenumbers for the studied molecules together with the potential energy distribution (PED, %) are summarized in Table S1 (Supplementary Material). For proper analysis of the presented spectra, the aforementioned PED analysis (DFT, B3LYP 6–311G(df,p)), literature data on the phenyl-substituted molecules [47–49], methylpyridines [50], monosubstituted pyridines [44,51–58], phenyls [59], and our previous results [23,24] have been considered. As expected, the FT–RS spectra of all the studied compounds (Figure 2) are dominated by bands attributed to the phenyl (Ph) and pyridine (Py) rings vibrations (Table 3). Briefly, the modes due to the Ph ring occur at ~3055 [ν2 – ν(CH); according to the Wilson 8

numbering scheme [60]], ~1590 [ν8a], ~1573 [ν8b], ~1487 [ρr(CC(H)C), ν(CC)], ~1445 [ν19b], 1289 [ν(CC)], ~1160 [ρr(CC(H)C)], ~1070 [ν(CC)], ~1028 [ν18a], 999 [ν12], 762 – 761 [δp(Ph)], and ~618 cm-1 [ν6b]. The ν12 mode is the strongest band in the spectra of all the investigated molecules and its relative intensity is almost the same for all the isomers. Whereas the strength of the ν18a vibration significantly increases for –PyNH only. The corresponding vibrational modes of Py appear at 1487 – 1483 [ν19a], 1217 – 1183 [ν9a], 1068 – 1065 [ν18a], 1056 – 1055 [ν18b], 1035 [ν12], 1004 – 986 [ν1], 809 – 805 [δp(Py)], 763 – 729 [ν4], 718 – 712 [ν11], and 649 – 645 cm-1 [ν6b]. The Py ν8a and ν8b modes are enhanced in the same spectral regions as for Ph (~1590 and 1573 cm-1, respectively). In the case of the – PyNH isomer, an additional Raman signal at 1639 cm-1 due to ν1+ν6b mode (Table S1, Supplementary Material) is observed [55]. Other spectral feature that shows similar intensity among the spectra of all the studied molecules appears at 1135 – 1130 cm-1 and is due to the ρr(NC(H,H)C), ν(POH), γ(CN(H)C), ρr(C(H,H)C(H,H)CH2), ν(CαN), ρr(CC(H,H)C), ρr(CC(H,H)C), and ν(CC) vibrations (Table S1, Supplementary Material). The oscillations of the benzylamino– and butylamino– fragments are seen in the Raman spectra at ~1317 [ρw(CC(H,H)C), ρt(CC(H,H)C) and/or ρt(NC(H,H)C)], 1237 – 1232 [ρt(CC(H,H)C) and/or ρt(NC(H,H)C)], 847 – 846 [δ(CH), ρr(CCH3) and/or δoop(CC(H)C)Ph], ~730 [ρr(CC(H,H)C)], and 591 – 550 cm-1 [ρs(CH2)]. The detailed list of the calculated vibrational modes is given in Table S1 (Supplementary Material). 3.3.

SERS studies 3.3.1. ORC Silver Substrate

Figure 3 compares the SERS spectra of –PyNH, –PyNH, and –PyNH adsorbed -1

onto the ORC silver substrate in the spectral range of 1800 – 350 cm , while Table 3 summarizes the wavenumbers and full width at half maximum (fwhm) of the selected aromatic ring modes enhanced in these spectra. As expected, the SERS spectra of all the investigated molecules are rich in bands due to the Ph and Py rings vibrations (Table 3). It should be noticed that the adsorption geometry of these species is strongly influenced by the substituent position (in the α (2–), β (3–), and  (4–) positions). For example, for α–PyNH, there are observed significant changes in the position and relative intensity of some bands between the FT–RS and SERS spectra. The 1010 cm-1 [ν1 of Py] SERS signal (the most pronounced one) gains the intensity and shifts to the higher wavenumber (Δῡ = 19 cm-1) in 9

comparison to the FT–RS spectrum; however, it is only slightly broadened (fwhmFT-RS = 10 cm-1 and fwhmSERS = 14 cm-1). Another very intensive SERS signal at 1046 cm-1 [ν18b of Py] is considerably broadened (Δfwhm = 8 cm-1) and shifted to the lower wavenumber (Δῡ = –9 cm-1). In addition, broad (fwhm = 28 cm-1) and very strong 1622 cm-1 and broad and irregular 824 cm-1 SERS signals due to ν1 + ν6b of Py and δp(Py), respectively, appeared. Remaining slightly strengthened and broadened bands assigned to the Py vibrations appear at 1591 [ν8a, Δfwhm = 14 cm-1], 1561 [ν8b, Δfwhm = 4 cm-1], and 657 cm-1 [ν6b, Δfwhm = 6 cm-1]. All the aforementioned observations indicate that Py accepts a vertical orientation onto the ORC silver substrate (–bonding through the nitrogen atom). This statement is consistent with the literature concerning pyridine immobilized onto a metal substrate, which reports two different Py ring orientations; i.e., edge-on and flat-on, with regard to a metal surface [61]. In the first case (–bonding via the nitrogen atom), the Py in-plane vibrations (1, 2, 6a, 8a, 9a, 12, 18a, and 19a; the A1 point symmetry group) are enhanced, whereas the Py out–of–plane modes are not observed or slightly strengthened only. For this orientation, the bands due to the ring breathing modes of adsorbate typically enhance upon adsorption and band wavenumbers usually red-shift, excluding the ν1 mode, for which significant blue-shift is observed [62]. Consequently, for the flat–on mode (–bonding through the ring), the ν1 vibration is blue– shifted by about 10 cm-1. In addition, the broad and asymmetric 824 cm-1 band with the shoulder at 804 cm-1 due to δp(Py) is seen. It should be emphasized that unlike to the FT–RS spectrum, in the SERS spectrum of α–PyNH the spectral features due to Ph are either not present or weakly enhanced; i.e. 12 (at 999 cm-1) and 18a (at 1034 cm-1). This implies that the phenyl ring is situated a relatively long way from the ORC silver surface. It is also noteworthy that unlike to the α–PyNH FT–RS spectrum, in the SERS spectrum of α–PyNH a very strong 1467 cm-1 band due to δas(CH3) of the butylamino group is observed. Additionally, a set of the SERS signals (at 1327 [ρt], 1301 [ρt], 1220 cm-1 [ν(CC), ρr(CαC(H)N), and ν(P=O)]), involving the vibrations of this molecular fragment, is seen. Therefore, it could be stated that the butylamino group takes part in the adsorption process of this isomer onto the ORC silver substrate. The considerably enhancement of the bands due to the C–P–O (at 546 cm-1) and C–N (at 1243, 1153, and 1100 cm-1) moieties also suggests that the N–C–P–O molecular fragment interacts with the ORC silver substrate.

10

As is apparent from the comparison of Figures 2 and 3, the SERS spectral pattern of

–PyNH onto the ORC silver substrate is similar to that of the FT–RS spectrum. Neither a substantial downshift nor significant Ph bands broadening, in comparison with those of the FT–RS spectrum (Table 3), is observed. These observations imply that the Ph ring is oriented vertically in respect to the ORC silver surface. On the other hand, it seems that Py is away the ORC silver substrate and does not play a significant role in the adsorption process of –PyNH onto the ORC silver surface. This is because only a few weak bands due to the Py ring vibrations (i.e., 6b at 645 cm-1, 18b at 1048 cm-1, 19b at 1448 cm-1) are enhanced. A set of the weak SERS signals assigned to (P=O) (at 1238 cm-1) and (P–O) (at 1137 and 763 cm-1) is also observed. Thus, the phosphinic group should be located in some proximity to the ORC silver surface. In the SERS spectrum of –PyNH deposited onto the ORC silver substrate, the relative intensity of the ν12 mode (cat 1006 m-1) due to Ph decreases in comparison with that in the corresponding FT–RS spectrum. The noticeably shift to the higher wavenumber (Δῡ = 7 cm-1) and width broadening (Δfwhm = 7 cm-1) of this spectral feature is also observed. These observations suggest a rather tilted orientation of the phenyl ring with regard to the ORC silver substrate. Similarly, the ν1 mode due to Py decreases in the relative intensity upon adsorption, negligibly broadens (Δfwhm = 6 cm-1), and blue-shifts to 1010 cm-1 (Δῡ = 6 cm-1). Based on these findings, a tilted orientation of the Py ring with regard to substrate surface is proposed. Furthermore, the strong enhancement of the 1206 [ν(CαCPy)] (see Table 1 for the Cα and CPy labelling scheme) and 1064 cm-1 [ν(CC), ν(CαN), ν(CC)Py, and ν(CN)Py] SERS signals is noticed. Slightly strengthened are also the bands assigned to the ν(CN)/ν(CC) vibrations (at 1085, 883, and 864 cm-1). Moreover, the significant broadening (Δfwhm = 17 cm1

) and irregular shape of the ~1636 cm-1 spectral feature indicate that this band is complex and

consists of two overlapped SERS signals at 1640 and 1633 cm-1 (see Figure 3, inset). According to the literature the 1640 cm-1 band could be due to ν1 + ν6b mode [55,63–65]. However, the PED analysis (DFT, B3LYP/6-311) for bis-[(hydroxyl-piridin-3-yl-methyl)]phosphinic acid have shown that this band could be alternatively assigned to the (CC)Py + (CN)Py + (CH)Py mode [55]. The latter band is due to the δas(NH) mode. Tus, it is suggested that the NH group is located in the close proximity to the ORC silver surface or interacts with it.

11

The presence of the weak 968 cm-1 (due to the out–of–plane modes: δoop(NC(H)C)Py, δoop(CC(H)C)Py, δoop(CC(H)N)Py, and δp(Py)] and ~402 cm-1 [as(Py) and δoop(CC(H)C)Py] SERS signals proves the above statement that Py adopts the tilted orientation with regard to the ORC silver surface. On the other hand, the lack of the spectral features due to the phosphinic moiety oscillations indicates that this group is away from the ORC silver substrate and does not take part in the adsorption process of –PyNH onto this surface. 3.3.2. Colloidal Ag substrate Figure 4 presents the SERS spectra of –PyNH, –PyNH, and –PyNH adsorbed onto the colloidal silver surface in the spectral range of 1800 – 350 cm-1, whereas Table 3 lists the wavenumbers and fwhm of the selected aromatic ring modes enhanced in these spectra. As is apparent, the SERS spectra of –PyNH and –PyNH adsorbed on the silver sol are similar to the corresponding SERS spectra of these molecules deposited onto the ORC silver surface. Therefore, Py in –PyNH is distanced from the colloidal silver surface, whereas the Ph ring adopts somehow the tilted orientation with regard to the colloidal silver nanoparticles surface (based on the weakening of the ν12 band of Ph). Another interesting observation is the fact that the adsorption geometry of –PyNH onto the colloidal silver surface is similar like that onto the ORC silver substrate. The intensity of the ν12 band due to Ph is comparable to this observed in the corresponding FT–RS spectrum. In addition, the noteworthy blue–shift (Δῡ = 4 cm-1) and broadening (Δfwhm = 9 cm1

) of this spectral feature is observed. These findings indicate that Ph adopts rather the vertical

orientation onto the colloidal silver. Additionally, the comparable relative SERS intensity on both silver substrates (colloidal and ORC), but lower than that in the FT–RS spectrum, of the Py ν1 mode (1011 cm-1) implies that the Py ring accepts tilted orientation in respect to the colloidal silver nanoparticles surface. Consequently, the same bands arising from the CαCPy, CαN, CCPy, CNPy, and NH moieties oscillations are enhanced. These observations confirmed that the –PyNH isomer adopts similar adsorption geometry onto both studied silver substrates. For α–PyNH, there are the noticeable spectral changes between the spectra measured for sample non adsorbed and adsorbed onto the ORC and colloidal silver substrates. The Py ν1 mode substantially blue–shifts to 1013 cm-1 (Δῡ = 21 cm-1), broadens (Δfwhm = 5 cm-1), and strengthens in the silver sol spectrum in comparison to those in the corresponding FT–RS 12

spectrum. Also, the Py ν18b vibration red–shifts to 1048 cm-1 (Δῡ = –7 cm-1), broadens (Δfwhm = 13 cm-1), and substantially increases in the relative intensity. Thus, the band due to the Py ring breathing vibrations increase in the relative intensity upon adsorption onto the colloidal silver substrate. Additionally, the wavenumbers of the most of Py bands decrease, whereas the ν1 mode significantly shifts to the higher frequencies. Based on these observations it can be suggested that Py accepts the edge–on adsorption mode (–bonding via the nitrogen atom). In contrast, the intensity of the Ph ν12 mode negligibly weakens together with slight band broadening (Δfwhm = 7 cm-1) and blue–shifting in wavenumbers (Δῡ = 2 cm-1). These findings indicate that Ph adopts the tilted arrangement onto the colloidal silver nanoparticles. Also, a set of bands (at ~1467, 760, and 730 cm-1) due to the butylamino chain oscillations is enhanced. These SERS signals together with the fact that in the SERS spectrum of α–PyNH in the silver sol (Fig. 4, the top trace) the ~1225 cm-1 band, due to the P=O unit vibrations, is also enhance prove that both fragments (butyl amino and P=O) participate in the interaction with the colloidal silver surface. The proposed manner of binding to the ORC and colloidal silver surfaces of the investigated compounds is given in Figure 5.

4. Conclusions In this paper we discussed the mode of adsorption of –PyNH, –PyNH, and –PyNH immobilized onto the electrochemically roughened in the oxidation–reduction cycles (ORC; ~50–150 nm roughens size) and onto colloidal (~15–20 nm roughens size) silver substrates. The prediction of the adsorption geometry of these molecules was possible thanks to the analysis of the changes in the wavenumber, width, and enhancement of the proper bands between the SERS and FT–RS spectra. The experimental FT–RS results were supported by the theoretical calculations (DFT, B3LYP 6–311G(df,p)). We demonstrated that the substituent position in the pyridine ring and modification of the substituent (replacement of the butyl chain by the phenyl ring of high affinity to silver (γ−PyNH)) have significant impact on the molecules’ geometry onto both studied silver substrates. For instance, Py in –PyNH adopts the vertical arrangement onto the ORC silver surface (–bonding via the nitrogen atom), whereas Ph is rather away from this substrate. Also, it can be assumed that the CN and butylamino groups assist in the adsorption process. In contrast, Ph in –PyNH is vertically oriented with regard to the ORC silver substrate, whereas Py is distanced from this surface and does not influence the adsorption process. In 13

the case of γ−PyNH, both the Ph and Py rings accept the tilted orientation with regard to the ORC silver substrate. The NH group is situated in the close proximity to this surface or interacts with it. It is noteworthy that the adsorption geometry of γ−PyNH is similar onto both studied silver substrates. Briefly, Ph accepts the vertical orientation, whereas Py prefers the tilted arrangement with regard to the colloidal silver surface and the NH molecular fragment is strongly involved in the adsorption process. In the case of –PyNH, Py is situated relatively long way from the colloidal silver nanoparticles, whereas Ph adopts the tilted orientation with regard to this surface. On the other hand, Py of –PyNH prefers the edge−on adsorption onto both the discussed silver surfaces. Ph of –PyNH accepts the tilted orientation onto the colloidal silver nanoparticles and is away from the ORC silver surface. The butylamino group is situated in the close proximity to both the silver surfaces. According to the PED analysis (Supplementary Materials) we expect a few Raman bands involving influence from the phosphinic moiety vibrations (deformation of CPhP(O,O)Cα: 400 – 420, 496 – 566, and 640 – 680 cm-1; P–O stretches: 809 – 866 cm-1; P– OH bending vibrations: 981 – 989 cm-1; P–OH stretches: 1143 – 1182 cm-1; and P=O stretches: 1218 – 1270 cm-1. The aforementioned bands are the complex vibrations. In generally, in the SERS spectra of the investigated isomers in the silver sol and onto the ORC silver surface these bands are relatively weak and overlap with the bands due to the vibrations of the other molecular fragments of the investigated molecules. Thus, it is very risky to draw general conclusions on the participation of this group in the molecule adsorption process. However, in the SERS spectra of –PyNH and –PyNH in the silver sol and –PyNH onto the ORC silver substrates there are enough intensive and separated bands (–PyNH: 1220 cm1

((P=O)) and 546 cm-1 (C–P–O vibrations); –PyNH: 1238 cm-1 ((P=O)), 1137 cm-1

((P–O)), and 763 cm-1 (P–O stretches); ––PyNH: 1225 cm-1 ((P=O))), which allow to suggest the participation of the phosphonic acid moiety in the interaction with silver. Therefore, it seems that the C−P−O fragment of –PyNH and the P=O and P−O moieties of

–PyNH are located in the close proximity to the ORC silver surface. Whereas in the colloidal silver the P=O unit of –PyNH only is situated near this surface.

14

Acknowledgments The authors acknowledges the Academic Computer Center “Cyfronet” in Krakow for the opportunity to carry out calculations. This work was supported by the Polish National Science Center (Grant No. N N204 354840 to E. Proniewicz). E. Pięta wishes to thank Lesser Poland Scholarship Fund for PhD students “Doctus” co-financed by European Social Fund for the financial support. E. Pięta are grateful to the Marian Smoluchowski Krakow Research Consortium: “Matter−Energy−Future” (KNOW) for financial support in the form of a scholarship. B.B. and T.K.O. acknowledge funding from a statutory activity subsidy from the Polish Ministry of Science and Higher Education for the Faculty of Chemistry, Wroclaw University of Technology.

Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version, at doi: ... It contains the calculated wavenumbers and potential energy distribution (PED, %) for the FT−RS spectra of the investigated −aminophosphinic acid derivatives of pyridine (Table S1).

15

References [1]

W. Goldeman, T.K. Olszewski, B. Boduszek, W. Sawka-Dobrowolska, Tetrahedron 62 (2006) 4506−4518.

[2]

E.D. Naydenova, P.T. Todorov, K.D. Troev, Amino Acids 38 (2010) 23−30.

[3]

G.X. He, J.P. Krise, R. Oliyai, in: V.J. Stella, R.T. Borchardt, M.J. Hageman, R. Oliyai, H. Maah, J. Tilley, Prodrugs: Challenges and Rewards, AAPS Press, USA, 2007, pp. 225−226.

[4]

B. Boduszek, Polish J. Chem. 75 (2001) 663−672.

[5]

B. Boduszek, T.K. Olszewski, W. Goldeman, M. Konieczna, Phosphorus Sulfur 181 (2006) 787−795.

[6]

B. Boduszek, Tetrahedron 52 (1996) 12483−12494.

[7]

T.K. Olszewski, B. Boduszek, Synthesis 3 (2011) 0437−0442.

[8]

T.K. Olszewski, B. Boduszek, Tetrahedron 66 (2010) 8661−8666.

[9]

J. Michalska, B. Boduszek, T.K. Olszewski, Heteroatom Chem. 22 (2011) 617−624.

[10]

E.F.V. Scriven, R. Murugan, Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc. Hoboken, NJ, 2005.

[11]

A. Mucha, P. Kafarski, Ł. Berlicki, J. Med. Chem. 54 (2011) 5955−5980.

[12]

Z.H. Kudzin, D.K. Gralak, G. Andrijewski, J. Drabowicz, J. Łuczak, J. Chromatogr. A. 998 (2003) 183−199.

[13]

J.C. Powers, S. Odake, J. Oleksyszyn, H. Hori, T. Ueda, B. Boduszek, C. Kam, Agents Actions Supplements 42 (1993) 3−18.

[14]

J. Habdas, B. Boduszek, J. Pept. Sci. 15 (2009) 305−311.

[15]

B. Bauvois, D. Dauzonne, Med. Res. Rev. 26 (2006) 88−130.

[16]

H. Chen, B.P. Roques, Bioorg. Med. Chem. Lett. 9 (1999) 1511−151.

[17]

C. Antczak, I. De Meester, B. Bauvois, Bioessays 23 (2001) 251−260.

[18]

C. Antczak, I. De Meester, B. Bauvois, J. Biol. Regul. Homeostasis Agents 15 (2001) 130−139.

[19]

R. Lipiński, L. Chruściński, P. Młynarz, B. Boduszek, H. Kozłowski, Inorg. Chim. Acta 322 (2001) 157−161.

[20]

P.C. Lee, D. Meisel, J. Phys. Chem. 86 (1982) 3391–3395.

[21]

K. Kneipp, H. Kneipp, I. Itzkan, R.R. Dasari, M.S. Feld, J. Phys. Condens. Matter 14 (2002) R597–R624.

16

[22]

R.L. Birke, V. Znamenskiy, J.R. Lombardi, J. Chem. Phys. 132 (2010) 214707-1– 214707-15.

[23]

E. Proniewicz, E. Pięta, K. Zborowski, A. Kudelski, B. Boduszek, T.K. Olszewski, Y. Kim, L.M. Proniewicz, J. Phys. Chem. A 118 (2014) 5614–5625.

[24]

E. Pięta, E. Proniewicz, B. Boduszek, T.K. Olszewski, Y. Kim, L.M. Proniewicz, Spectr.-Int. J. (2014) Article ID 894749, DOI: 10.1155/2014/894749.

[25]

E. Pięta, E. Proniewicz, Y. Kim, L.M. Proniewicz, Spectrochim. Acta A 121 (2014) 121−128.

[26]

E. Proniewicz, E. Pięta, A. Kudelski, N. Piergies, D. Skołuba, Y. Kim, L.M. Proniewicz, J. Phys. Chem. A 117 (2013) 4963−4972.

[27]

J.F. Arenas, I.L. Tocón, J.C. Otero, J.I. Marcos, Vib. Spectrosc. 19 (1999) 212−221.

[28]

J.F. Arenas, I.L. Tocón, J.C. Otero, J.I. Marcos, J. Phys. Chem. 100 (1996) 9254−9261.

[29]

M. Moskovits, J. Raman Spectrosc. 36 (2005) 485−496.

[30]

M. Moskovits, J. Chem. Phys. 77 (1982) 4408−4416.

[31]

M. Moskovits, Rev. Mod. Phys. 57 (1985) 783−826.

[32]

X. Gao, J.P. Davies, M.J. Weaver, J. Phys. Chem. 94 (1990) 6858−6865.

[33]

S.P. Centeno, I. López-Tocón, J.F. Arenas, J. Soto, J.C. Otero, J. Phys. Chem. B 110 (2006) 14916–14922.

[34]

B. Boduszek, T.K. Olszewski, Polish J. Chem. 76 (2002) 1619–1623.

[35]

E. Podstawka, A. Kudelski, M. Drąg, J. Oleksyszyn, L.M. Proniewicz, J. Raman Spectrosc. 40 (2009) 1578−1584.

[36]

E. Podstawka-Proniewicz, A. Kudelski, Y. Kim, L.M. Proniewicz, J. Phys. Chem. B 115 (2011) 7097−7108.

[37]

M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery, T.Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. 17

Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian03, Revision A.1, Gaussian Inc., Pittsburgh, PA, 2003. [38]

F. Jensen, Introduction to Computational Chemistry, John Willey and Sons Ltd, Chichester, 2007.

[39]

M.J. Frisch, J.A. Pople, J.S. Binkley, J. Chem. Phys. 80 (1984) 3265−3269.

[40]

D. Michalska, R. Wysokiński, Chem. Phys. Lett. 403 (2005) 211−217.

[41]

N.M. O’Boyle, J.G. Vos, GaussSum 0.8, Dublin City University, Dublin, 2004. Available at http://gausssum.sourceforge.net.

[42]

J.L.M. Martin, C. Van Alsenoy, Gar2ped, University of Antwerp, Antwerp, 1995.

[43]

T. Steiner, Angew. Chem., Int. Ed. 41 (2002) 48−76.

[44]

D.-Y. Wu, B. Ren, Y.-X. Jiang, X. Xu, Z.-Q. Tian, J. Phys. Chem. A 106 (2002) 9042−9052.

[45]

F.H. Allen, O. Kennard, D.G. Watson, J. Chem. Soc. II (1987) S1−S18.

[46]

A.C. Durrell, H.B. Gray, N. Hazari, C.D. Incarvito, J. Liu, E.C.Y. Yan, Cryst. Growth Des. 10 (2010) 1482−1485.

[47]

P. Sett, S. Chattopadhyay, P.K. Mallick, Spectrochim. Acta, Part A 56 (2000) 855−875.

[48]

A.C. Testa, S. Schneider, B. Brem, J. Photochem. Photobiol. A 107 (1997) 147−152.

[49]

G. Buntinx, R. Naskrecki, C. Didierjean, O. Poizat, J. Phys. Chem. A 101 (1997) 8768−8777.

[50]

I. López Tocón, M.S. Woolley, J.C. Otero, J.I. Marcos, J. Mol. Struct. 470 (1998) 241−246.

[51]

D.H. Dressler, Y. Mastai, M. Rosenbluh, Y. Fleger, J. Mol. Struct. 935 (2009) 92−96.

[52]

D.Y. Wu, X.M. Liu, S. Duan, X. Xu, B. Ren, S.H. Lin, Z.Q. Tian, J. Phys. Chem. C 112 (2008) 4195−4204.

[53]

W.H. Li, X.Y. Li, N.T. Yu, Chem. Phys. Lett. 305 (1999) 303−310.

[54]

W.H. Tsai, F.J. Boerio, S.J. Clarson, E.E. Parsonage, M. Tirrell, Macromolecules 24 (1991) 2538−2545.

18

[55]

E. Podstawka, A. Kudelski, T.K. Olszewski, B. Boduszek, J. Phys. Chem. B 113 (2009) 10035−10042.

[56]

G. Mizutani, S. Ushioda, J. Chem. Phys. 91 (1989) 598−602.

[57]

H. Chang, K.C. Hwang, J. Am. Chem. Soc. 106 (1984) 6586−6592.

[58]

L.J. Cao, A.Y. Li, H.B. Ji, L. Xu, Y. Zhang, J. Mol. Struct. 959 (2010) 80−86.

[59]

G. Varsanyi, Assignments for Vibrational Spectra of Seven Hundred Benzene Derivatives, Academic Kiado, Budapest, 1973.

[60]

E.B., Jr. Wilson, Phys. Rev. 45 (1934) 706−714.

[61]

J.A. Creighton, Surf. Sci. 124 (1983) 209−219.

[62]

A.G. Brolo, D.E. Irish, J. Lipkowski, J. Phys. Chem. B 101 (1997) 3906−3909.

[63]

M.H. Palmer, J. Mol. Struct. 834–836 (2007) 113-128.

[64]

E.G. Robertson J. Mol. Spectrosc. 231 (2005) 50-56.

[65]

J.W. Anthonsen, D.H. Christensen, J.T. Nielsen, O.F. Nielsen, Spectrochim. Acta 32A (1976) 971-982.

19

FIGURE CAPTIONS Figure 1. Excitation spectrum (UV–VIS) of a clear Ag colloid used in this study and sample/Ag nanoparticles system.

20

Figure 2. Experimental (black solid lines) and theoretical (red dashes lines) FT-RS spectra of

–PyNH, –PyNH, and –PyNH in the spectral ranges of 3300 – 2750 and 1800 – 350 cm-1, and the FT-RS spectrum of –PyNH in aqueous solution (inset).

21

Figure 3. SERS spectra of –PyNH, –PyNH, and –PyNH adsorbed onto Ag ORC substrate in the spectral range of 1800 – 350 cm-1.

s

22

Figure 4. SERS spectra of –PyNH, –PyNH, and –PyNH adsorbed onto a colloidal silver surface in the spectral range of 1800 – 350 cm-1.

23

Figure 5. The proposed adsorption geometry of the investigated compounds onto ORC and colloidal Ag surfaces.

24

Table 1. Molecular structure of the dimeric structure of the investigated –aminophosphinic acid derivatives of pyridine together with the atom numbering scheme.

molecular structure

abbreviation

name of monomer

–PyNH

[(pyridino-2-yl)-N-

24 25 6 37

36

35

5 23 4 3 22

18 C 1 Py 19 20 347 2 Cα 33 21 17 9 38 42 26 27 39 8 32 10 11 16 12 28 40

41

31 15 14 30

(butyl)amino]-phenylmethylphosphinic acid

13 29 42

41 36 21 40 20 1 33 37 25 6 222 17 19 39 18 CPy 34 35 38 24 5 3 7 4 10 27 26 23 8 Cα 32 9 16 11 28 15 12 31 14 13 29 30

33 34 14 15 28 1 35 13 16 6 22532 11 12 5 326 27 10 4 31 CPy 7 8 29 9 17 36 30 43 Cα 24 19 18 37 42 23 22 38 41 21 20 39 40

–PyNH

[(pyridino-3-yl)-N(butyl)amino]-phenylmethylphosphinic acid

–PyNH

[(pyridino-4-yl)-N(benzyl)amino]-phenylmethylphosphinic acid

25

Table 2. Selected calculated bond lengths and angles of −aminophosphinic acid derivatives of pyridine. bond N1–C2 C2–C3 C3–C4 C4–C5 C5–C6 N1–C6 C2/C3/C4–C7 C7–N17 C7–P8 P8–O9 P8–O10 P8–C11 C11–C12 C12–C13 C13–C14 C14–C15 C15–C16 C11–C16 N17–C18 C18–C19 C19–C20 C20–C21 C21–C22 C22–C23 C23–C24 C19–C24 dihedral angle N1–C2–C7–N17 C2–C3–C7–N17 C3–C4–C7–N17 N1–C2–C7–P8 C2–C3–C7–P8 C3–C4–C7–P8 N17–C7–P8–O10

α-PyNH β-PyNH length [Å]

γ-PyNH

1.334 1.396 1.386 1.390 1.388 1.333 1.525 1.452 1.862 1.580 1.512 1.809 1.397 1.391 1.391 1.392 1.389 1.397 1.466 1.528 1.532 1.529

1.334 1.390 1.393 1.394 1.389 1.334 1.515 1.459 1.866 1.588 1.511 1.810 1.397 1.389 1.392 1.392 1.390 1.398 1.466 1.516 1.394 1.392 1.390 1.392 1.389 1.396

1.332 1.401 1.393 1.388 1.390 1.334 1.509 1.465 1.871 1.582 1.506 1.812 1.394 1.390 1.392 1.391 1.390 1.397 1.465 1.527 1.531 1.530

angle [º] -21.8 -46.4 -31.9 100.6 73.5 179.0

69.4

90.8 51.0

N17–C7–P8–O9

52.6

-57.4

-71.9

N17–C7–P8–C11

-59.7

-168.0

174.4

C2–C7–P8–O10 C3–C7–P8–O10 C4–C7–P8–O10 C2–C7–P8–O9 C3–C7–P8–O9

52.4 -53.8 -74.3 -74.0 179.3

C4–C7–P8–O9 C3–C2–C7–P8 C4–C3–C7–P8

162.7 -77.4 -106.6

α-PyNH bond N1–C2–C3 C2–C3–C4 C3–C4–C5 C4–C5–C6 C5–C6–N1 C3–C2–C7 N1–C2–C7 C4–C3–C7 C2/C3/C4–C7–N17 C2/C3/C4–C7–P8 C7–P8–O9 C7–P8–O10 C7–P8–C11 P8–C11–C12 C11–C12–C13 C12–C13–C14 C13–C14–C15 C14–C15–C16 C15–C16–C11 C7–N17–C18 N17–C18–C19 C18–C19–C20 C19–C20–C21 C20–C21–C22 C21–C22–C23 dihedral angle C5–C4–C7–P8 C3–C2–C7–N17 C4–C3–C7–N17 C5–C4–C7–N17 O10–P8–O9–H27 O10–P8–C11–C12 C7–N17–C18–C19 N17–C18–C19– C20 C18–C19–C20– C21 hydrogen bridge O9–H27···O10' P8–O10···H27' P8–O9–H27···O10' C7–P8–O10···H27'

122.4 118.7 119.0 118.1 123.3 123.9 115.8 115.2 110.1 106.2 111.0 107.5 118.1 120.1 120.1 120.1 120.2 119.8 114.1 111.8 114.6 112.8

γ-PyNH

121.4 112.7 110.7 105.2 111.5 107.1 119.1 120.0 120.0 120.1 120.1 120.0 115.2 111.9 114.7 112.8

121.8 113.3 111.1 102.2 109.8 111.2 119.6 120.0 120.1 120.2 120.1 120.0 116.5 112.6 120.2 120.8 120.0 119.6

123.9 119.0 117.3 119.2 123.7

angle [º] -88.6 160.1 133.5 -24.2 16.0 -178.1

-23.2 -154.7 -170.2

148.7 33.6 27.3 71.2

-65.7

-62.5

-139.8

-175.9

-176.0

-177.7

173.1 130.2 20.8 -90.3

hydrogen bridge H27···O10' O10···H27'

β-PyNH angle [º] 123.8 117.3 119.4 118.4 123.2 121.3

1.561 1.562

angle [º] 175.7 128.2 6.16 -94.8 length [Å] 1.545 1.541

171.9 129.1 -20.2 -119.4

1.582 1.582

26

Table 3. Wavenumber and full width at half maximum of selected aromatic ring modes of the -aminophosphinic acid derivatives of pyridine.a

Mode α–PyNH ν6b [Ph] ν6b [Py] ν4 [Py] ν12 [Ph] ν1 [Py] ν18a [Ph] ν18b [Py] ν19b [Py] ν8b [Py/Ph] ν8a [Py/Ph] ν1+ν6b [Py] β–PyNH ν6b [Ph] ν6b [Py] ν4 [Py] ν12 [Ph] ν1 [Py] ν18a [Ph] ν18b [Py] ν19b [Py] ν8b [Py/Ph] ν8a [Py/Ph] ν1+ν6b [Py] –PyNH

Theoretical calculation ῡ [cm−1] IR [a. u.]

FT-RS ῡ [cm−1]

fwhm [cm−1]

IR [a. u.]

625 640 681 999 996 1030 1057 1445 1585 1602

34 19 39 178 158 42 87 9 22 58

618 649

5 5

2.2 0.6

999 992 1028 1055 1446 1573 1590

8 10 9 9 16 12 8

10.1 4.2 1.8 3.8 1.9 1.7 5.6

616 643

61 82

615 645

7 6

1.8 0.9

999 994 1031 1053 1444 1580 1601

232 12 58 87 42 28 166

997 985 1027 1056 1446 1575 1591

5 8 6 8 10 5 6

10.1 1.1 7.1 2.1 1.8 2.2 6.5

Ag colloid ῡ [cm−1] fwhm [cm−1]

SERS Ag ORC ῡ [cm−1]

fwhm [cm−1]

617 649 695 1001 1013 1029 1048 1440 1573 1603 1633

9 19 4 15 15 16 22 24 10 12 16

620 657

13 11

999 1010 1034 1046 1440 1561 1591 1622

13 14 13 17 18 16 22 28

617 649 694 996

9 11 18 12

616 645

6 10

998

7

1030 1048 1439 1575 1590 1633

12 15 26 19 19 28

1029 1048 1448 1576 1594

9 14 12 8 9

27

624 62 618 6 2.3 618 10 618 9 ν6b [Ph] 637 45 649 6 1.3 649 14 651 15 ν6b [Py] 685 5 694 6 2.9 695 16 ν4 [Py] 1002 170 999 8 10.0 1003 17 1006 15 ν12 [Ph] 1004 11 8.1 1011 11 1010 17 ν1 [Py] 1028 56 1033 10 2.3 1026 18 1028 18 ν18a [Ph] 1077 12 1045 8 1.8 1044 9 1047 15 ν18b [Py] 1445 2 1450 7 1.0 1440 25 1448 16 ν19b [Py] 6 1571 10 1.0 1588 20 ν8b [Py/Ph] 1587 71 1589 8 2.8 1604 13 1605 16 ν8a [Py/Ph] 1604 1639 11 4.1 1634 20 1636 28 ν1+ν6b [Py] a Abbreviations: Py, the pyridine ring; Ph, the phenyl ring; ν, stretching vibrations; ῡ, wavenumber; IR, Raman intensity; fwhm, full width at half maximum.

28