Timothy W. Robinson

High level ab initio studies of the excited states of sulfuric acid and sulfur trioxide

We have calculated the vertical excitation energies and oscillator strengths of the lowest energy electronic transitions in sulfuric acid (H2SO4) and sulfur trioxide (SO3) with a range of ab initio methods. We have found that the highest level calculations with the complete active space self-consistent field and multireference configuration interaction (MRCI) methods predict transition energies much lower than those previously calculated with the simpler configuration interaction-singles method. The MRCI calculated electronic transitions for SO3 are in good agreement with the experimental results, whereas electronic transitions in vapor phase H2SO4 have yet to be observed. Our MRCI results suggest that the lowest lying electronic excitation in H2SO4 occurs around 144 nm and that the cross section in the actinic region is very small.

High level ab initio studies of the low-lying excited states in the H2O⋅O2 complex

The lowest energy electronic transitions in the weakly bound van der Waals complex of water and oxygen (H2O⋅O2) are studied using ab initio methods. The vertical excitation energies for the two low-lying singlet states are calculated with the complete active space self-consistent field and multireference configuration interaction (MRCI) methods, and are compared to those calculated in the oxygen molecule. The MRCI calculations predict blueshifts of about 150 and 250 cm−1 for the transition frequencies on formation of the complex. These calculated shifts can provide assistance towards the spectroscopic identification of H2O⋅O2.

The effect of NH2-inversion tunneling splitting on the NH-stretching overtone spectra of aniline vapour

The room temperature vibrational overtone spectrum of aniline vapour has been recorded by intracavity laser photoacoustic spectroscopy in the 9700 cm−1 to 16 600 cm−1 region. The NH-stretching overtone bands exhibit spectral structure due to the tunneling splitting of the amino group inversion states. We find that this tunneling splitting steadily decreases with increasing NH-stretching vibrational excitation, and at the fourth NH-stretching overtone the splitting is negligible. Wave numbers and intensities of the NH- and CH-stretching vibrational overtones were calculated with a harmonically coupled anharmonic oscillator local mode model to facilitate spectral assignment. The conventional liquid phase overtone spectra of aniline were recorded and the measured absolute intensities are in good agreement with the calculated intensities.

Chapter 8 – Atmospheric Photolysis of Sulfuric Acid

Abstract We describe theoretical methods for the calculation of vibrational and electronic transitions in sulfuric acid, from which absorption cross sections can be obtained in the infrared through to the vacuum ultraviolet region. In the absence of experimental cross sections these calculations provide invaluable input for the assessment of the atmospheric photolysis of sulfuric acid. The vibrational model is based on a local mode model that includes the OH-stretching and SOH-bending vibrations, while the electronic transitions are calculated with coupled cluster response theory. These approaches are sufficient to describe the dominant vibrational transitions in the near infrared and visible regions, the lowest lying electronic transitions in the ultraviolet region and the higher energy electronic transitions in the region of Lyman- α radiation. We highlight the influence quantum mechanical calculations have had in the recent discussion of the atmospheric photolysis of sulfuric acid, and show that theoretical calculations can provide absorption cross sections of an accuracy that is useful in atmospheric science.