Size-Specific Infrared Spectra of Benzene-(H 2 O) n Clusters ( n = 1 through 7): Evidence for Noncyclic (H 2 O) n Structures

Resonant ion-dip infrared spectroscopy has been used to record size-specific infrared spectra of C 6 H 6 -(H 2 O) n clusters with n = 1 through 7 in the O—H stretch region. The O—H stretch spectra show a dramatic dependence on cluster size. For the n = 3 to 5 clusters, the transitions can be divided into three types—attributable to free, π hydrogen-bonded, and single donor water-water O—H stretches—consistent with a C 6 H 6 -(H 2 O) n structure in which benzene is on the surface of a cyclic (H 2 O) n cluster. In n = 6 and 7 clusters, the spectra show distinct new transitions in the 3500 to 3600 wave number region. After comparison of these results with the predictions of ab initio calculations on (H 2 O) n clusters, these new transitions have been assigned to double donor O—H stretches associated with the formation of a more compact, noncyclic structure beginning with (H 2 O) 6 . This is the same size cluster for which ab initio calculations predict that a changeover to noncyclic (H 2 O) n structures will occur.