Myung Soo Kim

Fragmentation probability in collisional-activation mass spectrometry (CA/MS). I. Effects of sequential collisional excitation

Abstract A probability theory has been developed to account for the effects of sequential collisional excitation in collisional-activation mass spectrometry. With simplifying assumptions concerning the collisional-activation mechanisms, cross-sections and the collisional energy-transfer probability Pc(E), expressions have been derived for the parent-ion and total daughter-ion intensities in collisional-activation spectra. One of the indications of the theory is that when sequential collisional excitation becomes appreciable, the usual Beers law expression for precursor-ion attenuation holds only approximately. Also, it has been shown that some information concerning Pc(E) may be obtained from collisional-activation experiments. Applying the expressions to collisional-activation data for 8.0 keV methane molecular ions, the parent-ion and total daughter-ion intensities can be accounted for adequately using the present theory. Also, it appears that the effects of sequential collisional excitation are significant.

Adsorption of picolinic and nicotinic acids on a silver sol surface investigated by Raman spectroscopy

Abstract Adsorption of picolinic and nicotinic acids on a silver sol surface has been investigated over a wide range of solution pH by surface-enhanced Raman scattering. Conspicuous appearance of zwitterionic bands in the ordinary and surface-enhanced Raman spectra of picolinic acid has enabled reliable analysis of the spectra. The adsorption mechanism derived from the spectral analysis supports the previous model proposed for isonicotinic acid. Namely, all three isomers (2-, 3-, and 4-) of pyridinecarboxylic acid exist as the anions on the silver surface when the sol solution is in medium acidic to basic conditions. Their molecular planes lie flat on the surface and both the carboxylates and the nitrogen atoms interact directly with the surface. In highly acidic conditions, the nitrogen atoms are protonated and the resulting zwitterions adsorb via their oxygen lone pair electrons.

Raman spectroscopy of 4-(methylthio)benzoic acid adsorbed on silver surfaces

Abstract The surface-enhanced Raman scattering of 4-(methylthio)benzoic acid has been investigated in aqueous silver sol and at a silver electrode. The molecule was found to adsorb on the silver surface via its sulfur atom and carboxylate group after deprotonation. The electrochemically produced 4-sulfidobenzoate was similarly found to be adsorbed on the silver. The symmetric stretching mode of the carboxylate anion (COO − ) was found to be down-shifted by 6–10 cm −1 from that in the free state when adsorption occurs via the π system of the carboxylate group. The mode should red-shift further by lowering the surface potential. However, the peak position of the less intense δ(COO − ) mode seemed to be insensitive to variation in the surface potential. The asymmetric stretching mode of the carboxylate group appeared, on the contrary, to be inactive in this bonding scheme.

Surface-enhanced Raman scattering of ortho- and para-mercaptophenols in silver sol

Abstract Surface-enhanced Raman scattering (SERS) of ortho- and para-mercaptophenols has been investigated in aqueous silver sol. Regardless of the solution pH and concentration, both molecules were found to adsorb on the sol surface via their sulfur atoms after losing sulfhydryl protons. Under very basic and submonolayer coverage conditions, the ortho isomer appeared also to bind to silver via its oxygen atom as o-sulfidophenoxide. Otherwise, the benzene rings and the hydroxyl groups of both isomers seemed to be pendant to the silver surface. From the present work, it seemed possible to investigate the adsorption of weak ligands to metal by SERS with the aid of nearby stronger ligands.

Raman spectroscopy of isonicotinic acid adsorbed onto silver sol surface

Abstract Adsorption of isonicotinic acid onto a silver colloidal particle surface was investigated over a wide range of solution pH by surface-enhanced Raman scattering. The molecule was found to adsorb onto the surface as an anionic form with the carboxylate-to-metal coordination providing the major interaction for the adsorption. The nitrogen-to-metal coordination seemed to play a secondary role in the adsorption process. Unlike the cases in organometallic chemistry, the flat coordination of the carboxylate group to the silver surface via its π orbitals was observed for isonicotinate adsorbed onto the aqueous sol particle surface. The adsorbate structure was found, however, to be completely different from that of the silver isonicotinate salt. That is, in the salt state, the carboxyl group was not coordinated at all to silver in contrast with the isonicotinate which was adsorbed onto the silver surface. Instead, the silver cation was bound to two ring nitrogen atoms.

Adsorption of aromatic nitriles on silver investigated by Raman spectroscopy

Abstract Spectral shifts of ν(CN) bands occurring when aromatic nitriles adsorb on silver surfaces have been investigated by surface-enhanced Raman scattering (SERS). Attention has been focused on the SERS of 1,4-dicyanobenzene(1,4-DCB) and 4-cyanobenzoic acid (4-CBA), both of which possess two functional groups available for bonding, to establish spectral correlation with the adsorption mechanism. Even though the coordination in most metal—nitrile complexes involes the σ donation of the nitrogen lone-pair electrons, it is found that a majority of aromatic nitriles adsorb on the silver surface via their Cue5fcN π systems. A slight increase in the Cue5fcN stretching frequency upon surface adsorption, observable for conjugated aromatic nitriles, is attributed to the disruption of conjugation rather than to end-on coordination.

Surface-enhanced Raman scattering of 1-butanethiol in silver sol

Abstract The surface-enhanced Raman scattering (SERS) of 1-butanethiol was investigated in a silver sol. The molecule was found to be chemisorbed dissociatively on the silver surface by rupture of its Sue5f8H bond. It is concluded that conformers of 1-butanethiol adsorbed selectively on the silver surface, the trans conformer around the C (1)ue5f8C (2) axes being more likely adsorbed when the bulk concentration of the molecule is enough for the full monolayer coverage. The vibrational assignment of the molecule in liquid phase has also been refined by using the SERS data.

Surface-enhanced Raman study of organic sulfides adsorbed on silver

Abstract The surface-enhanced Raman scatterings of dimethyl sulfide, diethyl sulfide and dimethyl disulfide have been investigated in silver sol. The dimethyl disulfide molecule decomposes on silver to the corresponding mercaptide implying facile cleavage of its Sue5f8S bond. The Cue5f8S bond in dialkyl monosulfide appears not to cleave on silver. For diethyl sulfide, the C 2 conformation seems to be favorable on silver than other conformations.

Surface-enhanced Raman scattering of 2-butanethiol in silver sol

Abstract The surface-enhanced Raman scattering (SERS) of 2-butanethiol was investigated in a silver sol. As observed for the primary thiols, 1-propanethiol and 1-butanethiol, the molecule was chemisorbed dissociatively on the silver surface by rupture of its Sue5f8H bond. At the bulk concentration sufficient for full monolayer coverage, the gauche conformer around the C(2)ue5f8C(3) bond was found to be the most favorable form on the surface. The SERS information was also used in the refinement of the vibrational assignments of 2-butanethiol in the liquid phase.

Raman spectroscopic investigation of 4-aminobenzonitrile adsorbed on silver

Abstract Surface-enhanced Raman scattering of 4-aminobenzonitrile has been investigated in a silver sol and at a silver electrode. The molecule was found to adsorb on the silver surface via both the amino and nitrile groups, with the benzene ring lying flat with respect to the surface. The most important interaction between the nitrile group and the surface was found to occur via π coordination. The observations made in this work are entirely consistent with the previously proposed model that most conjugated aromatic nitriles adsorb on silver via the Cue5fcN π system, and that their ν CN frequencies can either decrease or increase depending on the relative importance of nitrile-to-metal π donation and conjugation breakup due to side-on coordination.