Spectra Of Pyridine Research Articles

Adsorption of Pyridine on Au(110) as Measured by Reflection Anisotropy Spectroscopy

The reflection anisotropy spectra of pyridine adsorbed onto a Au(110) surface in an electrochemical cell demonstrates that this system forms an ordered structure. The reflection anisotropy spectroscopy (RAS) of pyridine/Au(110) is attributed to transitions which are shifted from their position in gas-phase spectra by the effects of the interaction of the N lone pair orbitals with the Au(110) surface. The time dependence of the reflection anisotropy spectrum of the pyridine/Au(110) system shows that after the initial onset of pyridine adsorption it takes for the system to yield the spectrum of the equilibrium ordered structure. A similar time is required for the RAS signal to return to that of the Au(110) surface following the desorption of pyridine. © 2003 The Electrochemical Society. All rights reserved.

Surface-enhanced Raman scattering of pyridine on platinum and nickel electrodes in nonaqueous solutions

Surface-enhanced Raman scattering (SERS) spectra of pyridine adsorbed onto bare platinum and nickel electrodes in nonaqueous solutions are reported in this Letter. There are similarities and differences between the SERS from aqueous and nonaqueous solutions. The surface enhancement factor for platinum in acetonitrile solution has been calculated to decrease by a factor of ca. 10 compared with that in the aqueous media. The double-band character for the ring breathing mode is observed at 1009 and 1019 cm −1. Two adsorption modes of pyridine on the platinum surface were assumed. Part of the pyridine molecules may be chemisorbed onto the surface, with the ring plane oriented vertical to the surface; other pyridine molecules may co-adsorb with lithium cations onto the surface.

Local characterisation of inhomogeneous Cu surfaces by surface-enhanced Raman scattering

An attempt was made to compare surface-enhanced Raman spectra (SERS) of the probe molecules adsorbed on highly dispersed Cu surfaces of both real catalyst and various model systems produced from pure Cu. In particular, we searched for the Cu clusters with adsorption properties significantly differing from the properties of the clusters produced from pure copper on the surface of highly active copper-containing catalysts. The SERS spectra of pyridine used as a probe molecule were collected with a Raman spectrometer connected to a microscope having a focal radius of less than 10 −6 m. It was found, for example, that for the very active catalyst produced from 50Cu–50Zr amorphous precursor by high-pressure hydrogen charging, the relative intensities of pyridine bands are considerably different at some places to those observed for copper. At other areas on the sample, however, the spectrum is similar to that observed for pure Cu. This indicates that the local SERS technique has the potential to investigate adsorbate properties that are usually hidden by averaging if the signal is collected from a larger surface area.

Transformation of Pyridine to α-Pyridyl on W(110) As Probed by Vibrational Spectroscopy: Experiments and Calculations

Using infrared spectroscopy and ab initio electronic structure calculations, we can make a conclusive assignment of the vibrational spectrum of pyridine adsorbed on W(110) and monitor its transformation to α-pyridyl. The assignments are further corroborated using isotopic 15N vibrational shifts. We find that the pyridine molecule is already tilted edgewise prior to the transformation, and both the pyridine and the α-pyridyl species have their molecular planes perpendicular to the surface.

In situ FTIR spectra of pyridine adsorbed on SiO 2–Al 2O 3, TiO 2, ZrO 2 and CeO 2: general considerations for the identification of acid sites on surfaces of finely divided metal oxides

Exposure of strong Lewis (coordinatively unsaturated metal atoms) and Bronsted (proton donor OH-groups) acid sites on solid surfaces is a prime demand for potential adsorptive and catalytic applications. In situ FTIR spectroscopy of small adsorbed base molecules, often NH 3, pyridine, CH 3CN, NO or CO molecules, has been well established as a powerful surface analytical technique for characterization of nature, strength and concentration of acid sites. Pyridine (Py) has been preferred as an IR probe molecule of finely divided metal oxide surfaces at room (RT) and higher temperature regimes, since it is (i) more selective and stable than NH 3; (ii) much more strongly adsorbed than CO and CH 3CN; and (iii) relatively more sensitive to the strength of Lewis acid sites than NO. In the present work, in situ IR spectra of Py adsorbed at ≥RT on characterized alumina, silica, silica–alumina, titania, zirconia and ceria were measured, and compared with RT-spectra of liquid and gas phase Py obtained under identical spectroscopic conditions, in order to characterize spectral consequences of mutual Py–Py interactions in the adsorbed phase. It has been concluded that the availability of Lewis acid sites can be unequivocally monitored by formation of coordinated Py molecules giving rise to IR-absorption(s) due to the ν 8a mode of νCCN vibrations at 1630–1600 cm −1, where the higher the frequency assumed, the stronger the acidity of the site. Formation of pyridinium surface species (PyH +) is identifiable by (i) an ν 8a-absorption at ≥1630 cm −1; (ii) an ν 19b-absorption at 1550–1530 cm −1; as well as (iii) νN +H and δN +H absorptions occurring, respectively, near 2450 and 1580 cm −1, and, thus, the availability of Bronsted acid sites. Moreover, products and IR-characteristics of Py surface reactions at >RT have been identified, and used to imply nature of surface base sites (OH −and O 2−) involved in formation of acid–base site pairs.

Theoretical evaluation of Raman spectra and enhancement factors for a molecule adsorbed on a complex-shaped metal particle

We present a methodology for the calculation of surface enhanced Raman scattering (SERS) spectra of molecules adsorbed on metal particles possibly immersed in a solvent. The molecule is treated at the ab initio level whereas the metal particle and the solvent are described through their dielectric properties. Harmonic vibrational frequencies are calculated analytically also including metal and solvent effects. Raman activities are obtained by differentiating with respect to normal coordinates the effective polarisability of the metal–molecule system. Enhancement factors and Raman spectra for pyridine adsorbed on complex-shaped silver particles are reported and discussed.

1H NMR study of pyridine-type ligands coordinated to the paramagnetic polyoxometalates, [Co IIW 11MO 39] n− (M=Co II, Ni II, Cu II, or Fe III)

1H NMR spectra of pyridine, 3-, and 4-methylpyridine coordinated to the paramagnetic polyoxometalates, [Co IIW 11Co IIO 39] 8− (CoCo), [Co IIW 11Ni IIO 39] 8− (CoNi), [Co IIW 11Cu IIO 39] 8− (CoCu), and [Co IIW 11Fe IIIO 39] 7− (CoFe) are reported. For these complexes the ligand exchange is slow on the NMR time-scale, and separate resonance lines have been observed for the complexes and the free ligands. Since no NMR signal is observed for [SiW 11Cu II(ptl)O 39] 6− and [SiW 11Fe III(ptl)O 39] 5− (ptl=pyridine-type ligand), appearance of good NMR spectra for the CoCu and CoFe complexes indicates that magnetic coupling with the fast relaxing Co II ion shortens the electronic relaxation times of the Cu II and Fe III ions by one or two orders of magnitude. The isotropic shifts for the CoNi and CoCu complexes parallel those of the [SiW 11Ni IIO 39] 6− (SiNi) complexes which come mainly from the contact shifts. CoNi and CoCu complexes, which exhibit sharper NMR lines than SiNi complexes, may be useful in measuring the contact shifts for complex systems. The isotropic shifts for the CoCo and CoFe complexes contain both contact and pseudocontact shifts. Analysis of the isotropic shifts for the CoCo complex of pyridine shows that exchange coupling between the two Co II ions reduces the contact shifts and reverses the signs of the pseudocontact shifts.

On the occurrence of the central line (∼1025 cm −1) in the SERS spectra of pyridine adsorbed on silver hydrosols

The occurrence of a central line at ∼1025 cm −1 between the surface-enhanced Raman scattering (SERS) bands of pyridine at 1008 and 1036 cm −1 has been first detected in silver hydrosols. This band, which has no counterpart in the Raman spectrum of the free ligand, is observed in acidic aqueous suspension as corresponding to that observed in an electrochemical cell and attributed to adsorption of pyridinium cation. When pyridine is adsorbed on an aged colloid in an alkaline medium two different species are detected. A central band at about the same wave number occurs, attributable to pyridine bound to silver ion cluster on the metal surface, oxidised by ambient air.

Surface Raman spectroscopic studies on the adsorption of pyridine at bare iron electrodes

Surface-enhanced Raman spectroscopic (SERS) studies have been extended successfully to iron electrodes by using a highly sensitive confocal microprobe Raman system. Several surface roughening procedures were attempted to obtain high-quality surface Raman spectra of pyridine on iron electrodes. Results show that proper surface roughness factor can not only increase the surface area, but also induce a weak SERS effect. Potential-dependent surface Raman spectra of pyridine in neutral solution suggest that the pyridine molecule may be adsorbed onto the iron electrode through the nitrogen atom by a coordination bond of FeN in an end-on configuration (or slightly tilting). Decreasing the solution pH value to ca. 2.0 did not affect the adsorption of pyridine and the pyridinium ion may dissociate before its adsorption onto the iron surface.

Ab initio and DFT calculations of structure and vibrational spectra of pyridine and its isotopomers

The molecular parameters and vibrational spectra of pyridine, pyridine-d5 and partially deuterated pyridines have been computed using the MP2, BLYP, BP86, BVWN, SVWN and B3LYP methodology and the cc-pVDZ, cc-pVTZ and 6-311++G(d,p) basis sets. The results are compared with the available experimental data in the gas phase. Bond distances, bond angles, dipole moment and harmonic frequencies computed by the MP2/cc-pVTZ and B3LYP/cc-pVTZ are in good agreement with the available experimental data. The calculations are found to be valuable in verifying a number of less certain experimental vibrational assignments. The results suggest that the DFT methods incorporate some anharmonic contributions, which improve the agreement with the experimental frequencies.

Selection rules for the charge transfer enhancement mechanism in SERS: dependence of the intensities on the L-matrix

Given the similarity between the charge transfer mechanism in the SERS effect and the resonance Raman (RR) process, the strongest enhanced bands in the SERS spectra can be explained through the Franck–Condon factors (A-term in RR). These factors are numerically related to the ΔQ vector, which represents the displacement of the potential energy minimums of the two states involved in the resonant process along each normal coordinate. ΔQ is calculated through the transformation ΔQ=L−1ΔR and related to the intensity of the band by Peticolas’ equation. The dependence of the ΔQ displacements and therefore of the calculated intensities on the L-matrix has been studied for the SERS spectra of pyridine.

Characterization of Beta/MCM-41 composite molecular sieve compared with the mechanical mixture

A Beta/MCM-41 composite was prepared for the first time through a two-step crystallization process combining a low crystallized zeolite Beta synthesis gel with a surfactant cetyltrimethylammonium bromide (CTAB) solution. The composite has been investigated and compared with a mechanical mixture of Beta and MCM-41 for their structure, acidity and catalytic activity by means of XRD, SEM, 129Xe NMR, DTG, N2 adsorption, IR spectra of pyridine and catalytic cracking reaction. The XRD result shows that the Beta/MCM-41 composite possesses a well-ordered mesoporous MCM-41 phase and a zeolite Beta phase. The result of N2 adsorption indicates that the composite contains a bimodal mesopore system and the microporous structure of zeolite Beta. Moreover, the results of 129Xe NMR and DTG suggest that the mesopore wall of the composite is constructed by a lot of secondary building units characteristic of zeolite Beta, which is quite different from the amorphous character of MCM-41 in the mechanical mixture. Although the total number of acid sites of the composite is comparable to that of the mechanical mixture, the composite possesses more strong Brønsted acid sites due to the initially crystallized mesostructure and shows higher catalytic activity for n-heptane cracking compared with the mechanical mixture.

Ecology as a social and economic issue

A Beta/MCM-41 composite was prepared for the first time through a two-step crystallization process combining a low crystallized zeolite Beta synthesis gel with a surfactant cetyltrimethylammonium bromide (CTAB) solution. The composite has been investigated and compared with a mechanical mixture of Beta and MCM-41 for their structure, acidity and catalytic activity by means of XRD, SEM, 129Xe NMR, DTG, N2 adsorption, IR spectra of pyridine and catalytic cracking reaction. The XRD result shows that the Beta/MCM-41 composite possesses a well-ordered mesoporous MCM-41 phase and a zeolite Beta phase. The result of N2 adsorption indicates that the composite contains a bimodal mesopore system and the microporous structure of zeolite Beta. Moreover, the results of 129Xe NMR and DTG suggest that the mesopore wall of the composite is constructed by a lot of secondary building units characteristic of zeolite Beta, which is quite different from the amorphous character of MCM-41 in the mechanical mixture. Although the total number of acid sites of the composite is comparable to that of the mechanical mixture, the composite possesses more strong Brønsted acid sites due to the initially crystallized mesostructure and shows higher catalytic activity for n-heptane cracking compared with the mechanical mixture.

Vibrational analysis of the inelastic neutron scattering spectrum of pyridine

A vibrational analysis of the pyridine molecule has been performed by combining inelastic neutron scattering (INS) data and quantum mechanical calculations at the RHF, MP2 and B3LYP levels, with the 6-31G** and 6-311G** basis sets. Firstly, in order to test which level of theory is the best in reproducing the INS profile, this was calculated from the atomic displacement matrix of the pyridine molecule. DFT proved to be the most suitable option reproducing the above-mentioned spectrum, for both wave numbers and intensities. Secondly, in order to approximate the calculated spectrum to the observed one, the initial force constants matrix calculated at the B3LYP/6-311G** level was symmetrized using a set of independent symmetry coordinates (C 2v). All the diagonal and some off-diagonal force constants were fitted until the difference between observed and calculated spectra was minimized. Good agreement between both calculated and experimental INS spectra supports the validity of our ‘empirical’ (or effective) force field.

The Low-Lying Excited States of Pyridine

We assign the observed phosphorescence and high-resolution singlet-to-triplet absorption spectra of pyridine, performing large model vibronic coupling calculations involving six active modes and three near-degenerate electronic states, 3A1, (2)3A1, and 3B1. Vibronic coupling primarily through ν8a between 3A1 and (2)3A1 results in the breakdown of the pseudo-parity selection rule: the lower surface has a double minimum, with each well corresponding to a diabatic single-determinant (π,π*) excitation. These states then strongly vibronically couple to 3B1 primarily through ν16b, resulting in a very complex 3A‘ manifold. In addition, for the singlet manifold, we detail a low-lying conical intersection between 1B1 and 1A2 and suggest that this intersection should have observable consequences for excited-state dynamics. These conclusions are obtained through the examination of all states below 5 eV in energy [1B1 (S1), 1B2 (S2), 1A2 (S3), 3A1, (2)3A1, 3B1, 3A2, and 3B2], performing CIS, CASSCF, CASPT2, CCSD, CC...

Infrared Study of Pyridine Adsorbed on Unpromoted and Promoted Sulfated Zirconia

Infrared spectra of pyridine adsorbed on zirconia and Pt–zirconia exhibited bands consistent with Lewis acid sites. Sulfated zirconia, after heating at 400°C, exhibited bands indicating about equal amounts of Lewis (L) and Brønsted (B) acid sites; the B/L ratio decreased with increasing reactivation temperature to become essentially zero after reactivation at 600°C. In contrast, both Pt and Fe/Mn promoted sulfated zirconia exhibited B/L ratios that increased slightly with increasing reactivation temperature. It is proposed that pyridine reacts with a Lewis acid site to produce an adsorbed “pyridone-type” surface species with the concurrent generation of a proton. The superior performance of the promoted sulfated zirconia catalyst is believed to arise from its ability to generate Brønsted acid sites.

Charge transfer in SERS: spectra of 3,5-dimethylpyridine at a silver electrode

The SERS spectra of 3,5-dimethylpyridine recorded at different electrode potentials were analysed. Their most characteristic feature is the enhancement of the band assigned to mode 8a as the electrode potential becomes more negative. This behaviour is different to that observed in the SERS spectra of pyridine where all four modes 6a, 1, 9a and 8a are enhanced in spite of the fact that both molecules belong to the same molecular point group. These differences were explained on the basis of a resonant charge transfer mechanism (CT). The band assigned to the fundamental νs(CH3) was used to estimate the electromagnetic mechanism contributions to SERS intensities, which allowed the direct correlation of the theoretical SERS-CT intensities calculated on the basis of Peticolas’ equation with experimental results. © 1998 John Wiley & Sons, Ltd.

In situ FTIR study of pyridine adsorbed on a polycrystalline gold electrode

In situ IR spectra of pyridine adsorbed on a polycrystalline gold electrode in 1 M NaF have been measured. An in-plane ring symmetrical vibration due to vertically adsorbed pyridine was observed at around 1605 cm−1 accompanying a decrease of the same mode band at 1596 cm−1 due to dissolved pyridine. The position and shift of the bands due to vertically adsorbed pyridine were independent of the concentration of pyridine, leading us to deduce that it is caused by a weak Stark effect and/or an effect of the electrode potential on the nonbonding orbital of the nitrogen atom. The vertically adsorbed pyridine was observed at 0.7 V (vs Ag/AgCl), at which potential detection of adsorbed pyridine by an electrochemical method has not been reported. A decrease in an asymmetric band at 1446 cm−1 due to dissolved pyridine was also observed at more positive potentials.

Surface‐enhanced Raman scattering (SERS) at Copper(I) oxide

Surface-enhanced Raman scattering spectra of pyridine at a copper(I) oxide hydrosol and at Cu2O-covered copper electrode produced by ‘oscillating reaction roughening’ are reported. It is shown that on such surfaces pyridine molecules adsorb as pyridinium cations. The ‘oscillating reaction’ pretreatment yields a passivated Cu surface which does not undergo any reduction during the cathodic scan, producing a SERS spectrum of pyridinium cations even after decreasing the electrode potential to strongly negative values. © 1998 John Wiley & Sons, Ltd.

Surface-enhanced Raman scattering (SERS) at Copper(I) oxide

Surface-enhanced Raman scattering spectra of pyridine at a copper(I) oxide hydrosol and at Cu2O-covered copper electrode produced by ‘oscillating reaction roughening’ are reported. It is shown that on such surfaces pyridine molecules adsorb as pyridinium cations. The ‘oscillating reaction’ pretreatment yields a passivated Cu surface which does not undergo any reduction during the cathodic scan, producing a SERS spectrum of pyridinium cations even after decreasing the electrode potential to strongly negative values. © 1998 John Wiley & Sons, Ltd.