Effect of noncovalent interactions on conformers of the n-butylbenzene monomer studied by mass analyzed threshold ionization spectroscopy and basis-set convergent ab initio computations

Two conformational isomers of the aromatic hydrocarbon n-butylbenzene have been studied using two-color MATI (mass analyzed threshold ionization) spectroscopy to explore the effect of conformation on ionization dynamics. Cationic states of gauche-conformer III and anti- conformers IV were selectively produced by two-color excitation via the respective S1 origins. Adiabatic ionization potentials of the gauche- and anticonformations were determined to be 70146 and 69872 ± 5 cm-1 respectively. Spectral features and vibrational modes are interpreted with the aid of MP2/cc-pVDZ ab initio calculations, and ionization-induced changes in the molecular conformations are discussed. Complete basis set (CBS) ab initio studies at MP2 level reveal reliable energetics for all four n-butylbenzene conformers observed in earlier two-color REMPI (resonance enhanced multiphoton ionization) spectra. For the S0 state, the energies of conformer III, IV and V are above conformer I by 130, 289, 73 cm-1, respectively. Furthermore, the combination of the CBS calculations with the measured REMPI, MATI spectra allowed the determination of the energetics of all four conformers in the S1 and D0 states. © 2008 American Chemical Society.