Combining electronic and nuclear structures of gas-phase species: theoretical and experimental approaches to understanding novel materials

Gas-phase multiphoton spectroscopy has been used in conjunction with theoretical calculations to examine the geometry changes that occur in two molecules, anethole and benzocaine, upon excitation to their S1 electronic state, and then once more upon the removal of an electron. Anethole was found to have two rotational conformers, anti and syn, differentiated by the relative direction the para-substituted ring substituents faced. The syn conformer was found to change geometry more substantially upon excitation than did the anti conformer, and this was interpreted using the RICC2 calculated electronic configuration. Ionisation energies of 60615 cm-1 and 60624 cm-1 were found for the syn and anti conformers, respectively.
The complexation of anethole with argon was further investigated, showing the stabilising effect the argon atom has on the cation, seen in a lowering of ionisation energy by 69 cm-1 for the syn conformer and 58 cm-1 for the anti. The argon atom was found to attach to anethole in a π-bonding arrangement above the ring, and binding energy was found to increase upon excitation, and then again upon ionisation.
Benzocaine was shown to undergo a dramatic geometry change upon excitation to the S1 state. Significant distortion along the NH2 inversion coordinate was found, but an exact geometry of the excited state remains elusive. ZEKE spectra measured from multiple intermediate S1 vibronic states showed complex vibrational structure. Of particular interest is one spectrum showed a splitting of peaks due to quantum tunnelling of the hydrogen atoms through the NH2 inversion coordinate. Elsewhere, many vibrations were significantly shifted in frequency as the electronic configuration changed.