New Infrared Spectra of Acetylene-Water Dimers: First Determination of the Rotational Constant and Another -Dependent Anomaly.

Infrared spectra of acetylene-water complexes are studied in the regions of the HO ν bend (1600 cm) and the DO ν/ν stretches (2670-2808 cm), using tunable infrared sources to probe a pulsed supersonic slit jet expansion. In the HO bend region, there is a puzzling absence of = 0 transitions for CH-HO, while both = 0 and 1 are observed for CD-HO. This continues a pattern of "missing" states noted in previous infrared studies of acetylene-water. Noticeable line broadening gives estimates of upper state predissociation lifetimes of about 1.6 ns for CD-HO with = 1, 0.8 ns for CH-HO with = 1, and 0.8 ns for CD-HO with = 0. The lifetime for CH-HO = 0 must be much shorter (<0.05 ns) since rapid predissociation seems to be the only explanation for its absence in the spectrum. In the DO ν region, clear sub-bands with = 0 ← 1, 1 ← 0, and 2 ← 1 are observed which enable a good determination of the rotational constant of CH-DO (7.313 ± 0.007 cm), a first for acetylene-water dimer and larger than previously expected.