Changes In Vibrational Structure Research Articles

UV resonance Raman study of cation-π interactions in an indole crown ether.

UV resonance Raman (UVRR) spectroscopy is used to probe changes in vibrational structure associated with cation-π interactions for the most prevalent amino acid π -donor, tryptophan. The model compound studied here is a diaza crown ether with two indole substituents. In the presence of sodium or potassium sequestered in the crown ether, or a protonated diaza group on the compound, the indole moieties participate in a cation-π interaction in which the pyrrolo group acts as the primary π-donor. Systematic shifts in relative intensity in the 760-780 cm-1 region are observed upon formation of this cation-π interaction; we propose that these modifications reflect shifts of the delocalized, ring-breathing W18 and hydrogen-out-of-plane (HOOP) vibrational modes in this spectral region. The observed changes are attributed to perturbations of the π-electron density as well as of normal modes that involve large displacement of the hydrogen atom on the C2 position of the pyrrole ring. Modest variations in the UVRR spectra for the three complexes studied here are correlated to differences in cation-π strength. Specifically, the UVRR spectrum of the sodium-bound complex differs from those of the potassium-bound or protonated-diaza complexes, and may reflect the observation that the C2 hydrogen atom in the sodium-bound complex exhibits the greatest perturbation relative to the other species. Normal modes sensitive to hydrogen-bonding, such as the tryptophan W10, W9, and W8 modes, also undergo shifts in the presence of the salts. These shifts reflect the strength of interaction of the indole N-H group with the iodide or hexafluorophosphate counteranion. The current observation that the W18 and HOOP normal mode regions of the indole crown ether compound are sensitive to cation-pyrrolo π interactions suggests that this region may provide reliable spectroscopic evidence of these important interactions in proteins.

Nuclear motion driven by the Renner–Teller effect as observed in the resonant Auger decay to the X̃2Π electronic ground state of N2O+

High-resolution Auger spectroscopy applied under resonant Auger Raman conditions is shown to be a powerful tool for characterizing complex potential energy surfaces in core-excited systems. Using the example of Nt 1s−1π*→X̃2Π resonant Auger transition in nitrous oxide we emphasize the interplay between the nuclear motion and the electronic decay. We show how the choice of excitation energy allows selection of core-excited species of different geometries. The nuclear dynamics of these species are mapped by measuring the resonant Auger decay spectra. In addition to the changes in vibrational structure observed for the resonant Auger decay spectra, a strong influence of nuclear motion on the electronic decay is revealed, inducing the so-called “dynamical Auger emission.” The experimental results are supported by ab initio quantum chemical calculations restricted to a linear geometry of the core-excited state.

Continuum resonance in ethylene: Evidence from vibrationally resolved core photoionization

The carbon $1s$ photoelectron spectrum of gas-phase ethylene has been measured with high resolution in order to study changes in vibrational structure arising from core photoionization through a continuum resonance. By evaluating the changes in vibrational intensity distribution we are able to determine that the ionization is non-Franck-Condon for energies close to the maximum of the broad resonance in the ethylene absorption spectrum. From these measurements it is possible to investigate the specific effect on the vibrational intensity distribution arising from the resonance. We find that the intensity of the C-H stretch vibrational mode is constant as the photon energy is scanned through the resonance, while the intensity of vibrations associated with the C-C bond varies with photon energy. This shows the existence of a shape resonance which, according to the scattering model of the shape resonance, must be primarily associated with the C-C bond rather than the C-H bond.

Experimental and theoretical studies on vibrational structure of metal complexes withm-halogenobenzoic acids

The effects of alkaline metals and halogen substituent on the vibrational structure of m-halogenobenzoates have been investigated by Infrared (IR), Raman, and quantum chemical methods. A complete assignments of vibrational spectra are based on the previous literature data and present theoretical approach. A good correspondence between experimental and calculated (density functional theory) vibrational spectra has been observed. The changes in vibrational structure caused by halogen substituent indicate perturbation of the electronic charge distribution in the aromatic ring. The effect of alkaline metal is much weaker and hidden by overwhelming effect of halogen. A linear relationships between vibrational frequencies of selected bands and ionic potential as well as electronegativity of halogen substituent have been established. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 62: 385–392, 1997

Intervalence excitation of the Creutz-Taube ion. Resonance Raman and time-dependent scattering studies of Franck-Condon effects

Franck-Condon effects associated with intervalence excitation of the Creutz-Taube ion (a prototypical delocalized mixed-valency system) have been evaluated via a time-dependent analysis of resonance Raman scattering intensities and the intervalence absorption lineshape. Quantitative studies in the extended near infrared (1320 nm excitation) revealed that at least eight, and possibly nine, vibrational modes are coupled to the intervalence transition. Numerical assessment of vibrational structural changes and reorganizational energy contributions showed that the most important modes, in a Franck-Condon sense, are metal-ligand stretches along the metal-bridge-metal axis (i.e. RuN(pyrazine) and trans-RuNH 3). Also active, however, are several pyrazine localized vibrations.

Experimental assessment of dynamic structural parameters for homogeneous and interfacial charge-transfer reactions: case studies based on time-dependent Raman scattering methods

The application of time-dependent scattering methodologies to dynamic structural problems involving charge transfer reactions is described. We show experimentally that a time-dependent analysis of resonance-enhanced Raman scattering can lead to a complete mode-by-mode description of the vibrational structural changes accompanying charge transfer and, therefore, a complete description of the vibrational activation barrier to charge transfer. (In other words, all force constants, all mode displacements, all bond-length displacements, and all individual energy components of the barrier can be determined.) The strategy is illustrated with case studies of internal (metal-to-ligand) charge transfer, ligand-bridged metal-to-metal charge transfer, outer-sphere charge transfer, and interfacial charge transfer.

External heavy-atom effect in room-temperature phosphorescence

The effect of external heavy-atom perturbers on the room-temperature phosphorescence (RTP) spectrum and dibenzo-(f,h)quinoxaline (DBQ) adsorbed on Whatman 4 filter paper is reported. The totally symmetric vibrational bands are predominantly enhanced, leading to a spectral change in vibrational structure. Inhomogeneity effects of molecule-perturber interactions are investigated by time-resolved phosphorimetry and lifetime measurements.