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

Laser spectroscopy, i.e., resonant two-photon ionization (R2PI), IR/R2PI io n depletion and hole burning spectroscopy have been applied in an experimen tal study of heterogenous clusters consisting of fluorobenzene (FB) or p-di fluorobenzene (pDFB), respectively, microsolvated by up to three methanol m olecules. Their infrared ion depletion spectra were taken in the region of the OH and CH stretches of methanol. In these complexes the methanol molecu les form subclusters, which are weakly hydrogen bonded to either the aromat ic pi-system (pi(OH)) or to the fluorine substituent (sigma(F)) and a CH gr oup (sigma(CH)). In FB .(MeOH)(1), pDFB .(MeOH)(1), pDFB .(MeOH)(2) and one isomer of FB .(MeOH)(2) the methanol subunits exclusively exhibit sigma(F) and sigma(CH) H-bonds. A further isomer of FB .(MeOH)(2) exhibits a pi(OH) type interaction. For FB .(MeOH)(3) and pDFB .(MeOH)(3) (1:3) complexes th e methanol subcluster may take on either a chainlike or a ringlike conforma tion. In the chainlike isomer of FB .(MeOH)(3) the methanol trimer interact s with the chromophore via a pi(OH) H-bond while with the corresponding pDF B cluster a sigma(F) H-bond is formed. High level ab initio calculations, s till feasible for molecular clusters of this size, have also been carried o ut to supplement the experimental investigations. These calculations includ e complete geometry optimizations of FB .(MeOH)(n) (n = 1-2) and pDFB .(MeO H)(1) at the MP2/6-31 + G* and B3LYP/6-31 + G* levels of theory. The minimu m energy structures of the microsolvates together with their harmonic vibra tional spectra allow a reliable assignment of the experimentally observed s pectra and the determination of structure of the complexes. (C) 2000 Americ an Institute of Physics. [S0021-9606(00)01804-3].