Nonplanar heme deformations and excited state displacements in nickel porphyrins detected by Raman spectroscopy at soret excitation

We have correlated the Raman intensities of out-of-plane modes of nickel porphyrins with the nonplanar deformations of specific symmetries, i.e., static normal coordinate deformations (SNCDs) expressed in terms of irreducible representations of the unperturbed D(4h) point group. The model porphyrins Ni(II) octaethyltetraphenylporphyrin (NiOETPP), Ni(II) tetra(isopropyl)porphyrin (NiT((i)Pr)P), Ni(II) tetra(tert-butyl)porphyrin (NiT((t)Bu)P), and Ni(II) meso-tetraphenylporphyrin (NiTPP) were chosen because they exhibit significant out-of-plane deformations of different symmetries. At B-band excitation, the Raman scattering of out-of-plane modes becomes activated mostly via the Franck-Condon mechanism. Some characteristic bands from out-of-plane modes in the spectra were identified as reliable predictors of the type and magnitude of out-of-plane deformation. The gamma(10)-gamma(13) bands are indicators of ruffling (B(1u)) deformations for porphyrins, as confirmed by data for NiTPP, NiT((i)Pr)P, and NiT((t)Bu)P, where the Raman intensity increases with the magnitude of the ruffling deformation. The gamma(15)-gamma(17) bands are indicators of saddling (B(2u)) deformations, as shown by data for NiOETPP, which is highly saddled. By comparing the relative intensities of these prominent Raman bands we estimated the vibronic coupling parameters using a self-consistent analysis, and showed that they reproduce the respective B-band absorption profiles. We also identified the deformations along the lowest wavenumber normal coordinates as the predominant reason for the Raman activity of out-of-plane modes. Our results suggest that some of the normal coordinates (gamma(10) and gamma(13)) may be used as tools to quantitatively probe the nonplanar deformations of metalloporphyrins with alkyl substituents at the meso-positions. Out-of-plane deformations also increase the vibronic coupling strength of some low frequency in-plane Raman modes, namely, nu(7) and nu(8). Generally, the Raman data suggest that the excited B-state is substantially more nonplanar than the ground state. The overall larger vibronic coupling of ruffled porphyrins yields substantially larger dipole strengths for the vibronic sidebands associated with the B-state transition, so that the relative absorptivity of the B(v) band can be used as a convenient tool to probe the nonplanarity of the porphyrin macrocycle.     

Vibrational spectra and normal coordinate analysis of p-cresol and its deuterated analogs

I.r. and Raman spectra of p-cresol and its seven deuterated analogs were investigated in dilute solutions of hydrophobic solvents. Assignments of the observed i.r. and Raman bands were made on the basis of isotopic frequency shifts, Raman polarization properties, i.r. intensifies and normal coordinate calculations. The calculated normal frequencies are in good agreement with the experimental ones: the average error below 1700 cm⁻¹ is 3.8 cm⁻¹ for 164 in-plane vibrations and 3.3 cm⁻¹ for 59 out-of-plane vibrations. The calculated vibrational modes may be useful in analysing the vibrational spectra of tyrosine. It is suggested that several doublets due to Fermi resonance and a trio of Raman bands in the 1260-1160 cm⁻¹ region are potential probes for the micro-environments of tyrosine side chains in proteins.     

四-（4-吡啶基）卟啉二酸聚集体的共振拉曼光谱

研究了近激子吸收带激发下四-（4-吡啶基）卟啉二酸（H8TPyP^6＋）聚集体的共振拉曼光谱。测量了H8TPyP^6＋单体和聚集体的紫外可见吸收谱和共振光散射光谱．在氘代位移的基础上结合相关体系振动光谱研究,对测得的H8TPyP^6＋单体和聚集体的拉曼谱带进行了指认．聚集体的形成导致H8TPyP^6＋的卟啉环CC／CN面内伸缩振动向低波数方向位移2-6cm^-1,而卟啉环鞍形面外振动带向高波数方向位移12cm^-1．基于拉曼谱带的强度和频率变化分析了聚集引起的H8TPyP^6＋分了内结构变化和分子间氢键作用．     

Analysis of vibrational circular dichroism spectra of (s)-(+)-2-butanol by rotational strengths expressed in local symmetry coordinates

The vibrational circular dichroism (VCD) spectra of (S)-(+)-2-butanol have been observed in dilute CS(2) solutions. Two strong VCD bands are assigned mainly to the OH bending modes with the aid of quantum chemical calculations. The calculated VCD spectra corresponding to these bands are shown to depend on the conformation of the OH group. To understand this feature, we have calculated the contribution of each local vibrational mode to the rotational strengths and concluded that the coupling of the group vibrations between the in-plane and out-of-plane modes about the locally assumed symmetry planes play a significant role in VCD. This finding has provided a new scope of VCD in relation to molecular vibrations.     

Vibrational analysis of the imidazolate ring

IR and Raman spectra have been investigated for imidazolate and 4-methylimidazolate including five and three deuterated analogs, respectively. Assignment of the observed IR and Raman bands has been made on the basis of isotopic frequency shifts, Raman polarization properties, and normal coordinate calculations. The calculated normal frequencies are in good agreement with experimental ones: the average error below 1600 cm⁻¹ is 4.5 cm⁻¹ for 104 in-plane vibrations and 3.8 cm⁻¹ for 43 out-of-plane vibrations. The calculated vibrational modes are useful in analyzing the Raman bands of histidine residues in proteins.     

Raman under nitrogen. The high-resolution Raman spectroscopy of crystalline uranocene, thorocene, and ferrocene

The utility of recording Raman spectroscopy under liquid nitrogen, a technique we call Raman under nitrogen (RUN), is demonstrated for ferrocene, uranocene, and thorocene. Using RUN, low-temperature (liquid nitrogen cooled) Raman spectra for these compounds exhibit higher resolution than previous studies, and new vibrational features are reported. The first Raman spectra of crystalline uranocene at 77 K are reported using excitation from argon (5145 A) and krypton (6764 A) ion lasers. The spectra obtained showed bands corresponding to vibrational transitions at 212, 236, 259, 379, 753, 897, 1500, and 3042 cm(-1), assigned to ring-metal-ring stretching, ring-metal tilting, out-of-plane CCC bending, in-plane CCC bending, ring-breathing, C-H bending, CC stretching and CH stretching, respectively. The assigned vibrational bands are compared to those of uranocene in THF, (COT)2-, and thorocene. All vibrational frequencies of the ligands, except the 259 cm(-1) out-of-plane CCC bending mode, were found to increase upon coordination. A broad, polarizable band centered about approximately 460 cm(-1) was also observed. The 460 cm(-1) band is greatly enhanced relative to the vibrational Raman transitions with excitations from the krypton ion laser, which is indicative of an electronic resonance Raman process as has been shown previously. The electronic resonance Raman band is observed to split into three distinct bands at 450, 461, and 474 cm(-1) with 6764 A excitation. Relativistic density functional theory is used to provide theoretical interpretations of the measured spectra.     

Density functional theory and Raman spectroscopy applied to structure and vibrational mode analysis of 1,1',3,3'-tetraethyl-5,5',6,6'-tetrachloro- benzimidazolocarbocyanine iodide and its aggregate

We have measured electronic and Raman scattering spectra of 1,1',3,3'-tetraethyl-5,5',6,6'-tetrachloro-benzimidazolocarbocyanine iodide (TTBC) in various environments, and we have calculated the ground state geometric and spectroscopic properties of the TTBC cation in the gas and solution phases (e.g., bond distances, bond angles, charge distributions, and Raman vibrational frequencies) using density functional theory. Our structure calculations have shown that the ground state equilibrium structure of a cis-conformer lies ～200 cm(-1) above that of a trans-conformer and both conformers have C(2) symmetry. Calculated electronic transitions indicate that the difference between the first transitions of the two conformers is about 130 cm(-1). Raman spectral assignments of monomeric- and aggregated-TTBC cations have been aided by density functional calculations at the same level of the theory. Vibrational mode analyses of the calculated Raman spectra reveal that the observed Raman bands above 700 cm(-1) are mainly associated with the in-plane deformation of the benzimidazolo moieties, while bands below 700 cm(-1) are associated with out-of-plane deformations of the benzimidazolo moieties. We have also found that for the nonresonance excited experimental Raman spectrum of aggregated-TTBC cation, the Raman bands in the higher-frequency region are enhanced compared with those in the nonresonance spectrum of the monomeric cation. For the experimental Raman spectrum of the aggregate under resonance excitation, however, we find new Raman features below 600 cm(-1), in addition to a significantly enhanced Raman peak at 671 cm(-1) that are associated with out-of-plane distortions. Also, time-dependent density functional theory calculations suggest that the experimentally observed electronic transition at ～515 nm (i.e., 2.41 eV) in the absorption spectrum of the monomeric-TTBC cation predominantly results from the π → π? transition. Calculations are further interpreted as indicating that the observed shoulder in the absorption spectrum of TTBC in methanol at 494 nm (i.e., 2.51 eV) likely results from the ν(") = 0 → ν' = 1 transition and is not due to another electronic transition of the trans-conformer-despite the fact that measured and calculated NMR results (not provided here) support the prospect that the shoulder might be attributable to the 0-0 band of the cis-conformer.     

DFT study on the geometric, electronic structure and Raman spectra of 5,15-diphenylporphine

The ground state geometric, electronic structure and Raman spectra of 5,15-diphenylporphine (H(2)DPP) have been studied using B3LYP/6-31G(d) method and compared with that of well-studied free base porphine (H(2)P) and meso-tetraphenylporphine (H(2)TPP). Calculation shows that 5,15-substitution causes remarkable in-plane distortion, whereas the resulting out-of-plane distortion is negligible. The calculated electronic structure of H(2)DPP is consistent with the absorption spectra compared with H(2)P and H(2)TPP. The calculated vibrational frequencies of H(2)DPP scaled with a single factor of 0.971 agree well with experimental data (the rms error is 8.0 cm(-1)). The assignment of experimental Raman bands of H(2)DPP was discussed on the basis of theoretical calculation and the comparison with that of H(2)P and H(2)TPP. The splitting of some vibrational modes involving the motion of C(m) atom, such as nu(1), nu(8), and nu(10), was observed and was attributed to the diversification of the environment around C(m) atoms. As the shift of absorption peaks, the shift of some structure-sensitive Raman bands of H(2)DPP form that of H(2)TPP and H(2)P was attributed to the in-plane nuclear reorganization (IPNR) induced by phenyl-substitution, though the contribution of nonplanarity mechanism could not be excluded completely.     

Resonance Raman spectroscopy reveals new insight into the electronic structure of beta-hematin and malaria pigment

Resonance Raman spectra of beta-hematin and hemin are reported for a range of excitation wavelengths including 406, 488, 514, 568, 633, 780, 830, and 1064 nm. Dramatic enhancement of A(1g) modes (1570, 1371, 795, 677, and 344 cm(-1)), ring breathing modes (850-650 cm(-1)), and out-of-plane modes including iron-ligand modes (400-200 cm(-1)) were observed when irradiating with 780- and 830-nm laser excitation wavelengths for beta-hematin and to a lesser extent hemin. Absorbance spectra recorded during the transformation of hemin to beta-hematin showed a red-shift of the Soret and Q (0-1) bands, which has been interpreted as excitonic coupling resulting from porphyrin aggregation. A small broad electronic transition observed at 867 nm was assigned to a z-polarized charge-transfer transition d(xy) --> e(g)(pi). The extraordinary band enhancement observed when exciting with near-infrared excitation wavelengths in beta-hematin when compared to hemin is explained in terms of an aggregated enhanced Raman scattering hypothesis based on the intermolecular excitonic interactions between porphyrinic units. This study provides new insight into the electronic structure of beta-hematin and therefore hemozoin (malaria pigment). The results have important implications in the design and testing of new anti-malaria drugs that specifically interfere with hemozoin formation.     

Raman and infrared spectroscopy of selected vanadates

Raman and infrared spectroscopy has been used to study the structure of selected vanadates including pascoite, huemulite, barnesite, hewettite, metahewettite, hummerite. Pascoite, rauvite and huemulite are examples of simple salts involving the decavanadates anion (V10O28)6-. Decavanadate consists of four distinct VO6 units which are reflected in Raman bands at the higher wavenumbers. The Raman spectra of these minerals are characterised by two intense bands at 991 and 965 cm(-1). Four pascoite Raman bands are observed at 991, 965, 958 and 905 cm(-1) and originate from four distinct VO6 sites. The other minerals namely barnesite, hewettite, metahewettite and hummerite have similar layered structures to the decavanadates but are based upon (V5O14)3- units. Barnesite is characterised by a single Raman band at 1010 cm(-1), whilst hummerite has Raman bands at 999 and 962 cm(-1). The absence of four distinct bands indicates the overlap of the vibrational modes from two of the VO6 sites. Metarossite is characterised by a strong band at 953 cm(-1). These bands are assigned to nu1 symmetric stretching modes of (V6O16)2- units and terminal VO3 units. In the infrared spectra of these minerals, bands are observed in the 837-860 cm(-1) and in the 803-833 cm(-1) region. In some of the Raman spectra bands are observed for pascoite, hummerite and metahewettite in similar positions. These bands are assigned to nu3 antisymmetric stretching of (V10O28)6- units or (V5O14)3- units. Because of the complexity of the spectra in the low wavenumber region assignment of bands is difficult. Bands are observed in the 404-458 cm(-1) region and are assigned to the nu2 bending modes of (V10O28)6- units or (V5O14)3- units. Raman bands are observed in the 530-620 cm(-1) region and are assigned to the nu4 bending modes of (V10O28)6- units or (V5O14)3- units. The Raman spectra of the vanadates in the low wavenumber region are complex with multiple overlapping bands which are probably due to VO subunits and MO bonds.     

Vibrational assignment of aluminum(III) tris-acetylacetone

The geometry, frequency and intensity of the vibrational bands of aluminum(III) Tris-acetylacetone Al(AA)3 and its 1,3,5-(13)C derivative were obtained by the Hartree-Fock (HF) and Density Functional Theory (DFT) with the B3LYP, B1LYP, and G96LYP functionals and using the 6-31G* basis set. The calculated frequencies are compared with the solid IR and Raman spectra. All of the measured IR and Raman bands were interpreted in terms of the calculated vibrational modes. Most computed bands are predicted to be at higher wavenumbers than the experimental bands. The calculated bond lengths and bond angles are in good agreement with the experimental results. Analysis of the vibrational spectra indicates a strong coupling between the chelated ring modes. Four bands in the 500-390 cm(-1) frequency range are assigned to the vibrations of metal-ligand bonds.     

Infrared spectroscopy of matrix-isolated polycyclic aromatic hydrocarbon cations. 3. The polyacenes anthracene, tetracene, and pentacene

Gaseous, ionized polycyclic aromatic hydrocarbons (PAHs) are thought to be responsible for a very common family of interstellar infrared emission bands. Unfortunately, very little infrared spectroscopic data are available on ionized PAHs. Here we present the near- and mid-infrared spectra of the polyacene cations anthracene, tetracene, and pentacene. We also report the vibrational frequencies and relative intensities of the pentacene anion. The cation bands corresponding to the CC modes are typically about 10-20 times more intense than those of the CH out-of-plane bending vibrations. For the cations the CC stretching and CH in-plane bending modes give rise to bands which are an order of magnitude stronger than for the neutral species, and the CH out-of-plane bends produce bands which are 3-20 times weaker than in the neutral species. This behavior is similar to that found for most other PAH cations. The most intense PAH cation bands fall within the envelopes of the most intense interstellar features. The strongest absorptions in the polyacenes anthracene, tetracene, and pentacene tend to group around 1400 cm-1 (between about 1340 and 1500 cm-1) and near 1180 cm-1, regions of only moderate interstellar emission. These very strong polyacene bands tend to fall in gaps in the spectra of the other PAH cations studied to date suggesting that while PAHs with polyacene structures may contribute to specific regions of the interstellar emission spectra, they are not dominant members of the interstellar PAH family.     

Assignment and anharmonicity analysis of overtone and combination bands observed in the resonance Raman spectra of carotenoids

The resonance Raman spectra of all-trans carotenoids have been observed in the region of 5000-500 cm⁻¹ for samples in glassy solution at 77 K and in the in vivo state at room temperature. Prominent bands in the wavenumber region higher than 2000 cm⁻¹ are assigned to either overtones or combinations of three modes due to skeletal stretches and the CH₃ in-plane rock. From the wavenumbers of the observed Raman bands, anharmonicity constants for these three modes (including cross-term constants) are obtained. It is found that, for each carotenoid studied, the cross-term anharmonicity constant between the CC and CC stretches is significantly larger than the other anharmonicity constants.     

Infrared and Raman spectra of 3,5-diamino-6-(o-C6H4X)-1,2,4-triazines [X = F,Cl, Br,CH3

Raman and infrared spectra of four substituted 3,5-diamino-6-(ortho-substituted phenyl)-1,2,4-triazines, having ortho-fluoro, -chloro, -bromo and -methyl groups on the phenyl ring, are reported and discussed. Bands due to substituent sensitive phenyl vibrations are observed in both the Raman and infrared spectra. The Raman spectra of all four compounds have strong bands near 770 and 1330 cm(-1) which are assigned to the ring breathing vibration of the 1,2,4-triazine ring and an asymmetric triazine C-NH2 stretching vibration, respectively. A medium/strong band near 800 cm(-1) in the infrared spectra is attributed to an out-of-plane bending vibration of the substituted 1,2,4-triazine ring.     

The infrared and Raman spectra of trans- and cis-tetrachloro-tetramethylbicyclopropylidene

The title compounds trans- and cis-2,2,2',2'-tetrachloro-3,3,3',3'-tetramethyl-bicyclopopylidene were synthesized, and their infrared and Raman spectra were recorded. Non-coincidence between the IR and Raman bands of the trans compound suggested C(2h) symmetry and a planar ring system. In the cis compound most of the IR and Raman bands coincided and a C(2v) symmetry seems likely. The exocyclic CC double bond gave rise to a medium/weak Raman band at 1,847 cm(-1) in the trans compound. In the cis derivative IR and Raman bands both at 1,825 cm(-1) were observed. From similarities with related molecules, the ring breathing, the antisymmetric ring stretch, the CCl(2) out-of-phase and in-phase stretch and the out-of-plane ring bending modes have been tentatively assigned for the trans and cis compounds.     

Raman spectroscopic study of pascoite Ca3V10O(28)·17H2O

Raman spectroscopy has been used to study the molecular structure of the vanadate mineral pascoite. Pascoite, rauvite and huemulite are examples of simple salts involving the decavanadate anion (V10O28)6-. Decavanadate consists of four distinct VO6 units which are reflected in Raman bands occurring at higher wavenumbers. The Raman spectrum of pascoite is characterised by two intense bands at 991 and 965 cm(-1). Raman bands are observed at 991, 965, 958 and 905 cm(-1) and originate from four distinct VO6 sites in the mineral structure. In the infrared spectra of pascoite, two wavenumber regions are observed between: (1) 837 and 860, and (2) between 803 and 833 cm(-1). These bands are assigned to ν3 antisymmetric stretching modes of (V10O28)6- or (V5O14)3- units. The spectrum is highly complex in the lower wavenumber region, and therefore the assignment of bands is difficult. Bands observed in the 404 to 458 cm(-1) region are assigned to the ν2 bending modes of (V10O28)6- or (V5O14)3- units. Raman bands observed in the 530-620 cm(-1) region are assigned to the ν4 bending modes of (V10O28)6- or (V5O14)3- units. The Raman spectra of the vanadates in the low wavenumber region are complex with multiple overlapping bands which are probably due to VO subunits and MO bonds.     

Density functional theory studies on the Raman and IR spectra of meso-tetraphenylporphyrin diacid

The vibrational spectra of meso-tetraphenylporphyrin diacid (H4TPP2+) have been studied with the density functional theory. Raman and IR spectra of H4TPP2+ and its N-deuterated analogue (D4TPP2+) are measured and compared with the computational results. Complete assignments of observed IR and Raman bands were proposed on the bases of calculation results. The DFT calculations reproduce 140 observed fundamentals with the RMS 8.6 cm-1. The computational as well as the experimental results reveal that the saddle-distortion of porphyrin macrocycle for the diacid leads to a significant effect on its vibrational spectra. Especially, several out-of-plane skeletal modes, which were either unobserved or very weak in the Raman spectra of CuTPP and H2TPP, are activated in the Raman spectra of the diacids. In addition, enhancement for the Raman bands of phenyl CC stretching modes were observed and attributed to the conjugation effect of pi-systems of the phenyl and the porphyrinato macrocycles.     

Zero kinetic energy photoelectron spectroscopy of jet cooled benzo[a]pyrene from resonantly enhanced multiphoton ionization

We report zero kinetic energy (ZEKE) photoelectron spectroscopy of benzo[a]pyrene (BaP) via resonantly enhanced multiphoton ionization (REMPI). Our analysis concentrates on the vibrational modes of the first excited state (S(1)) and those of the ground cationic state (D(0)). Similar to pyrene, another peri-condensed polycyclic aromatic hydrocarbon we have investigated, the first two electronically excited states of BaP exhibit extensive configuration interactions. However, the two electronic states are of the same symmetry, hence vibronic coupling does not introduce any out-of-plane modes in the REMPI spectrum, and Franck-Condon analysis is qualitatively satisfactory. The ZEKE spectra from the in-plane modes observed in the REMPI spectrum demonstrate strong propensity in preserving the vibrational excitation of the intermediate state. Although several additional bands in combination with the vibrational mode of the intermediate state are identifiable, they are much lower in intensity. This observation implies that the molecular structure of BaP has a tremendous capability to accommodate changes in charge density. All observed bands of the cation are IR active, establishing the role of ZEKE spectroscopy in mapping out far infrared bands for astrophysical applications.     

2-pyridone: The role of out-of-plane vibrations on the S1<-->S0 spectra and S1 state reactivity

The S(1)<-->S(0) vibronic spectra of supersonic jet-cooled 2-pyridone [pyridin-2-one (2PY)] and its N-H deuterated isotopomer (d-2PY) have been recorded by two-color resonant two-photon ionization, laser-induced fluorescence and emission, and fluorescence depletion spectroscopies. By combining these methods, the B origin of 2PY at 0(0) (0)+98 cm(-1) and the bands at +218 and +252 cm(-1) are identified as overtones of the S(1) state out-of-plane vibrations nu(1) (') and nu(2) ('), as are the analogous bands of d-2PY. Anharmonic double-minimum potentials are derived for the respective out-of-plane coordinates that predict further nu(1) (') and nu(2) (') overtones and combinations, reproducing approximately 80% of the vibronic bands up to 600 cm(-1) above the 0(0) (0) band. The fluorescence spectra excited at the electronic origins and the nu(1) (') and nu(2) (') out-of-plane overtone levels confirm these assignments. The S(1) nonplanar minima and S(1)<--S(0) out-of-plane progressions are in agreement with the determination of nonplanar vibrationally averaged geometries for the 0(0) (0) and 0(0) (0)+98 cm(-1) upper states by Held et al. [J. Chem. Phys. 95, 8732 (1991)]. The fluorescence lifetimes of the S(1) state vibrations show strong mode dependence: Those of the out-of-plane levels decrease rapidly above 200 cm(-1) excess vibrational energy, while the in-plane vibrations nu(5) ('), nu(8) ('), and nu(9) (') have longer lifetimes, although they are above or interspersed with the "dark" out-of-plane states. This is interpreted in terms of an S(1) (') state reaction with a low barrier towards a conical intersection with a prefulvenic geometry. Out-of-plane vibrational states can directly surmount this barrier, whereas in-plane vibrations are much less efficient in this respect. Analysis of the fluorescence spectra allows to identify nine in-plane S(0) (') state fundamentals, overtones of the S(0) state nu(1) (") and nu(2) (") out-of-plane vibrations, and >30 other overtones and combination bands. The B3LYP6-311++G(d,p) calculated anharmonic wave numbers are in very good agreement with the observed fundamentals, overtones, and combinations, with a deviation Delta(rms)=1.3%.     

Surface enhanced Raman spectra of isomeric acetylpyridines adsorbed on silver sol

Raman spectra of 2-, 3- and 4-acetylpyridines (AP) are obtained in bulk phase, in aqueous solution and in the adsorbed state on colloidal silver particles. Addition of the acetylpyridines on the Ag-sol results in aggregation of the silver particles showing characteristic charge transfer (CT) bands. Significant surface enhancement of the Raman bands are observed. Both the estimated enhancement factor and the absorption maxima of the CT bands are in inverse parallel relationship with the electron density on the nitrogen atom as reflected by the Hammett σ values of the substituents. It is inferred that the charge transfer interactions between the adsorbates and the metal particles contribute to the enhancement mechanism. This is further substantiated by the concentration dependence of enhancement. A classical electromagnetic contribution is demonstrated by the Raman excitation frequency dependence of SERS. The results further show that the molecules are adsorbed on the metal surface through the nitrogen atom. Appearance of some out-of-plane modes in the SERS spectra suggests that the pyridyl ring planes are not perpendicular to the metal surface, but are tilted.