Fluorobenzene and p -difluorobenzene microsolvated by methanol: An infrared spectroscopic and ab initio theoretical investigation

Laser spectroscopy, i.e., resonant two-photon ionization (R2PI), IR/R2PI ion depletion and hole burning spectroscopy have been applied in an experimental study of heterogenous clusters consisting of fluorobenzene (FB) or p-difluorobenzene (pDFB), respectively, microsolvated by up to three methanol molecules. Their infrared ion depletion spectra were taken in the region of the OH and CH stretches of methanol. In these complexes the methanol molecules form subclusters, which are weakly hydrogen bonded to either the aromatic π-system (πOH) or to the fluorine substituent (σF) and a CH group (σCH). In FB⋅(MeOH)1, pDFB⋅(MeOH)1, pDFB⋅(MeOH)2 and one isomer of FB⋅(MeOH)2 the methanol subunits exclusively exhibit σF and σCH H-bonds. A further isomer of FB⋅(MeOH)2 exhibits a πOH type interaction. For FB⋅(MeOH)3 and pDFB⋅(MeOH)3 (1:3) complexes the methanol subcluster may take on either a chainlike or a ringlike conformation. In the chainlike isomer of FB⋅(MeOH)3 the methanol trimer interacts with the chromophore via a πOH H-bond while with the corresponding pDFB cluster a σF H-bond is formed. High level ab initio calculations, still feasible for molecular clusters of this size, have also been carried out to supplement the experimental investigations. These calculations include complete geometry optimizations of FB⋅(MeOH)n (n=1–2) and pDFB⋅(MeOH)1 at the MP2/6-31+G* and B3LYP/6-31+G* levels of theory. The minimum energy structures of the microsolvates together with their harmonic vibrational spectra allow a reliable assignment of the experimentally observed spectra and the determination of structure of the complexes.