Wen-mei Wei

Theoretical study of sum-frequency vibrational spectroscopy on limonene surface

By combining molecule dynamics (MD) simulation and quantum chemistry computation, we calculate the surface sum-frequency vibrational spectroscopy (SFVS) of R-limonene molecules at the gas-liquid interface for SSP, PPP, and SPS polarization combinations. The distributions of the Euler angles are obtained using MD simulation, the ψ-distribution is between isotropic and Gaussian. Instead of the MD distributions, different analytical distributions such as the δ-function, Gaussian and isotropic distributions are applied to simulate surface SFVS. We find that different distributions significantly affect the absolute SFVS intensity and also influence on relative SFVS intensity, and the δ-function distribution should be used with caution when the orientation distribution is broad. Furthermore, the reason that the SPS signal is weak in reflected arrangement is discussed.

Density functional theory study on sum-frequency vibrational spectroscopy of arabinose chiral solutions.

Using time-dependent density functional computations we calculate the doubly resonant IR-UV sum-frequency vibrational spectroscopy and sum-frequency vibrational spectroscopy off electronic resonance for D-arabinose solutions. In comparison with the experimental detection limit, the calculated doubly resonant IR-UV sum-frequency vibrational spectroscopy is strong enough to be detectable.

Theoretical study of the resonance Raman spectra for meso-tetrakis(3,5-di-tertiarybutylphenyl)-porphyrin.

Applying time-dependent density functional theory (TDDFT), we study the resonance Raman spectra for the Q and B bands of the meso-tetrakis(3,5-di-tertiarybutylphenyl)-porphyrin (H2TBPP) molecule including both Raman A term (Franck-Condon term) and Raman B term (Herzberg-Teller term) contributions. It is found that Raman B term can be one order of magnitude larger than Raman A term and dominates resonance Raman for the Q band resonance. In comparison with the recent experimental Raman spectra of H2TBPP with incident light frequency 532nm, we predict the absence of 1580cm(-1) band in the resonance Raman spectra which agrees well with the experimental results, whereas the previous theoretical calculation using non-resonance strategy failed to do so.

Theoretical study of vibrational energy transfer of free OH groups at the water-air interface

Recent experimental studies have shown that the vibrational dynamics of free OH groups at the water-air interface is significantly different from that in bulk water. In this work, by performing molecular dynamics simulations and mixed quantum/classical calculations, we investigate different vibrational energy transfer pathways of free OH groups at the water-air interface. The calculated intramolecular vibrational energy transfer rate constant and the free OH bond reorientation time scale agree well with the experiment. It is also found that, due to the small intermolecular vibrational couplings, the intermolecular vibrational energy transfer pathway that is very important in bulk water plays a much less significant role in the vibrational energy relaxation of the free OH groups at the water-air interface.

Theoretical study of doubly resonant IR-UV hyper-Raman scattering.

Theoretically we study the doubly resonant IR-UV hyper-Raman scattering where the IR light is resonant to the vibrational transition and the UV/visible light is resonant to the electronic transition between the ground and excited states. Based on the Taylor expansion of the electric transition dipole moments with respect to the normal coordinates, we have derived the expressions for the hyper-Raman A, B, and C terms. Using quantum chemistry calculations, we have estimated the magnitudes for all the three terms. Due to double resonance, contributions from all the three terms should be detectable in experiments.

Theoretical Study on Raman Spectra of Aqueous Peroxynitric Acid

Using density functional theory and polarizable continuum models, we study the Raman spectra of aqueous peroxynitric acid. The calculated results indicate that the solvent effect has significant influence on the electric dipole transition moments between the ground and excited electronic state and Raman polarizabilities. The theoretical Raman spectra agree well with the experimental results. From the experimental depolarization ratio, we can conclude that peroxynitric acid is not a plane molecule. We also find that the hydrogen bond can enhance IR intensity of hydroxyl group by several times.