Knowledge Bank

Several infrared bands of $C2H2$ dimer have been recorded in the monomer $\nu 5$ band region by pulsed free-Jet direct absorption spectroscopy with a frequency tunable diode laser. More than 100 transitions, located on the $\sim cm-1$ higher-frequency side of the monomer band origin, have been assigned to the atype and transitions of a prolate asymmetric top. Each transition splits into three tunneling components, and the splitting between the lowest and highest components is $0.02-0.03cm-1$. The distance between the centers of mass of the monomer subunits is determined to be 4.4 A from the B and C constants. This distance is too large for a slipped parallel geometry of $C2h$ symmetry but is consistent with a hydrogen-bonded T-shaped geometry of $C2v$ symmetry where the bonded H and the adjacent C atoms are close to the van der Waals contact. The tunneling splittings and the intensity alternation due to the nuclear spin statistics are well accounted for in terms of the simultaneous internal rotation of two monomer subunits among four equivalent equilibrium T-shaped configurations.