Resonant ion-dip infrared spectroscopy of hydrogen bonding molecular clusters containing benzene

Resonant ion-dip infrared spectroscopy (RIDIRS) has been employed to record ground state, cluster-size specific spectra of $\rm C\sb6H\sb6$-$\rm (H\sb2O)\sb{n}$ with n = 1-7 and $\rm C\sb6H\sb6$-$\rm (CH\sb3OH)\sb{m}$ with m = 1-6 in the OH stretch fundamental region. This region (3000 to 3800 cm$\sp{-1}$) was covered by an optical parametric oscillator. By crossing a molecular beam with the IR radiation prior to resonant two-photon ionizing a cluster, size-specific information was attained by measuring the attenuation in ion signal detected by time-of-flight mass spectroscopy. The OH stretch spectra show a dramatic dependence on cluster size, and indicate that both water and methanol molecules tend to form single subclusters when complexed with benzene. In $\rm C\sb6H\sb6$-$\rm (H\sb2O)\sb1$ the OH stretch spectrum is complicated by additional structure due to internal rotation and torsion of H$\sb2$O in the complex. This complexity is not encountered in any other clusters. The spectrum of $\rm C\sb6H\sb6$-$\rm (H\sb2O)\sb2$ closely resembles that of free water dimer. For the n = 3-5 clusters, the spectra can be divided into three categories, due to free OH (3715 cm$\sp{-1}$), $\pi$ H-bonded OH (3650 cm$\sp{-1}$) and single donor OH stretches, consistent with $\rm C\sb6H\sb6$-$\rm (H\sb2O)\sb{n}$ clusters composed of cyclic water subclusters where one of the free OH\u27s is used to form a $\pi$ H-bond with benzene. At n = 6 and 7, the spectra show distinct new transitions in the 3500-3600 cm$\sp{-1}$ region. By comparing with the predictions of ab initio calculations on (H$\sb2$O)$\sb{\rm n}$ (n = 6 and 7), these new transitions are assigned to double donor OH stretches associated with the formation of more compact, three-dimensional structures. In $\rm C\sb6H\sb6$-$\rm (CH\sb3PH)\sb{m}$, no free OH stretches are ever observed, only $\pi$ H-bonded OH stretches and single donor OH stretches. When m = 1-3, methanol molecules form chain-like structures $\pi$ H-bonded to benzene by the terminal free OH; in the chain, the other OH(s) are single donors forming links between methanol molecules. When m = 4-6, methanol forms cyclic structures which are unable to form $\pi$ H-bonds and exhibit only single donor absorptions. RIDIRS is also used in combination with IR/UV holeburning techniques to spectroscopically identify and label two isomers of $\rm (C\sb6H\sb6)\sb2$-$\rm (CH\sb3OH)\sb3$