Pyridine Raman spectrum

The extraordinary enhancement of the Raman spectrum of pyridine adsorbed on silver has resulted in the investigation of pyridine as a candidate for possible enhancement on other metals. More recently, SERS has been used to probe the adsorption of molecules on metals that are of catalytic importance. For example, SERS spectra have been collected of pyridine adsorbed on metallic rhodium in the first demonstration of surface-enhancement from metallic rhodium. While the enhancement of pyridine on silver is thought to arise from an electrodynamic enhancement, the enhancement of the pyridine Raman spectrum by rhodium is thought to arise from a chemical effect since rhodium, unlike silver, is not a free electron metal. 

It was first reported in 1974 that the Raman spectrum of pyridine is enhanced by many orders of magnitude when... 

The first report of the SERS spectrum of a species adsorbed at the electrode/ electrolyte interface was by Fleischman et al (1974) and concerned pyridine on silver. The Raman spectrum of the adsorbed pyridine was only observed after repeated oxidation/reduction cycles of the silver electrode, which resulted in a roughened surface. Initially, it was thought that the 106-fold enhancement in emission intensity arose as a result of the substantially increased surface area of the Ag and thus depended simply on the amount of adsorbate. However, Jeanmarie and Van Duync (1977) and Albrecht and Creighton (1977), independently reported that only a single oxidation/reduction cycle was required to produce an intense Raman spectrum and calculations showed that the increase in surface area could not possibly be sufficient to give the observed enhancement. 

The ground state spectrum in Figure 5 exhibits the typical features of the Raman spectrum of a bipyridine complex (40,51,52). Seven relatively intense peaks dominate the spectrum. These may be approximately described as the seven symmetric C-C and C-N stretches expected of bipyridine in any point group wherein the two pyridine rings are related by a symmetry element. 

Characterization of Surface Species by SERS. Before presenting the results obtained with 1, the spectral features which have proven to be useful in identifying surface species will be reviewed. Both in solution and by SERS, pyridines show a ring mode in the Raman spectrum near 1600 cm-. When the ring nitrogen is protonated, this band disappears and is replaced by a band near 1640 cm-1. The... 

Hg(SAr )2 is a monomeric complex and forms colorless crystals which dissolve readily in chloroform or toluene but are less soluble in light petroleum. The IR spectrum shows a strong Hg-S stretching band at 378 cm-1 (v sym). while vsym occurs at 332 cm-1 in the Raman spectrum. The 199Hg NMR resonance is found at 5 = — 1274 ppm (relative to neat HgMe2). The compound sublimes above 170°C/0.01 mmHg and decomposes at 270°C.10 It forms a crystalline T-shaped 1 1 adduct with pyridine.11 H NMR (CDC13) 5 1.52 (s, 9H), 1.80 (s, 18H), 7.40 (s, 2H). 

Figure 3. Raman spectrum of pyridine in the region of the v, (991—1016 cm1) and vie (1030-1036 cm 1) fundamentals (A) liquid pyridine, (B) pyridine adsorbed on a NaX zeolite, and (C) pyridine adsorbed on a Zri exchanged (78% exchange) NaX zeolite. The vertical bar represents (A) 2000 Hz, (B) 100 Hz, (C) 50 Hz.

The vibrational spectrum of 4-pyridine-carboxylic acid on alumina in Fig. 4d is equivalent to an infrared or Raman spectrum and can provide a great deal of information about the structure and bonding characteristics of the molecular layer on the oxide surface. For example, the absence of the characteristic > q mode at 1680 cm 1 and the presence of the symmetric and anti-symmetric O-C-O stretching frequencies at 1380 and 1550 cm indicate that 4-pyridine-carboxylic acid loses a proton and bonds to the aluminum oxide as a carboxylate ion. 

The IR spectra of LMC12, [L2Cu(H20)2]2+ and [L2M]2+, where M = Co or Cu, L = 2-acetylpyridine(5,6-diphenyl-[l,2,4]-triazin-3-yl)hydrazone, show that L is tridentate via triazine N, azomethine N and pyridine N atoms245 The resonance Raman spectrum of RhCl(CO)(iPr2Ph-bian), where Pr2Ph-bian = l,2-bis[(2,6-diisopropylphenyl)imino)acenaphthene, revealed the predominantly Rh - bian (MLCT) character of the visible electronic absorption.246... 

These considerations, however, cannot exclude the possibility that a vibration of a pyridine-halide-metal (atom or surface) complex is responsible for the debated Raman feature. This would explain the shift of the frequency from that of a metal-halide frequency, the stability to cathodic potentials" (and, perhaps, the relative insensitivity to the metal itself). One should mention in this context that Krasser et reported a band at 240 cm in the Raman spectrum of pyridine-silver cluster complexes, which they associate with a pyridine-Ag mode. 

Replacement of the dinitrogen ligand in [Os(N2)(NH3)5] by pyridine under aerobic conditions or of the weakly bound triflate ligand in [0s(0S02CF3)(NH3)5] + by pyridine yields [Os(py)(NH3)j] cyclic voltammetric studies show reversible reduction to [Os(py)(NH3)5f" 62.90,91 and the surface-enhanced Raman spectrum (SERS) of the complex (both normal and deuteriated) shows Os-N, ammine and internal pyridine vibrational modes. Infrared spectra were also measured. ... 

J1 The Raman spectrum of Mo(CO)g in pyridine has bands at 2119 and 2015 cm Microwave irradiation of this solution yielded three products with the following Raman bands ... 

Over the past decades, smface enhanced Raman scattering (SERS) has became a valuable spectroscopic technique as a powerful smface diagnostic tool. In 1974 Fleischmann, Hendra, and McQuillan performed the first measurement of a surface Raman spectrum from pyridine adsorbed on an electrochemically roughened silver electrode. It has been explained that some vibrational bands of pyridine are selectively enhanced a million times. This increases the sensitivity of... 

As a first example for illustrating the application of Raman spectroscopy in characteri2ing the orientation of surface species, we consider pyridine adsorption on an Ag surface [84], for several reasons. The first SERS experiment was carried out using pyridine as the adsorbed species. Secondly, pyridine has a large Raman cross section, relatively simple molecular structure, and a good assignment of bands appearing in its normal Raman spectrum and SER spectrum. Thirdly, pyridine is an excellent model molecule for surface coordination studies. Eourthly, interactions of the pyridine molecule with the metal surface involve both the it and lone-pair electrons. 

Figure 6-24. Raman spectrum of pyridine (approximately 0.005M) adsorbed from aqueous solution onto a) silver and b) gold hydrosols. Intensities of the two prominent bands near 1000 cm" are enhanced by factors of up to 100 over those for an aqueous pyridine solution. (Reproduced by permission from ref. [209],)...

The vibrational wavenumbers of the Ni(CN)4 group vibrations of the M-Ni-bipy complexes and M-Ni-bipy-G clathrates [G=Toluene, aniline or JV,A -dimethyl-aniline] are given in Table I. The v(CN) and 6(NiCN) vibrational wave-numbers are found to be similar to those of Hofmann type clathrates [6] and the pyridine [7] complex, showing that the M-Ni(CN)4 layers have been preserved. Since we observed only one v(CN) (E ) band in the IR spectrum and the other two v(CN) (4ig and 5i ) bands in the Raman spectrum of the Cd-Ni-bipy complex, we propose a square planar environment around the tetracyanonickelate ion. 

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