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

Infrared spectra of acetylene–water complexes are studied in the regions of the H2O ν2 bend (1600 cm–1) and the D2O ν1/ν3 stretches (2670–2808 cm–1), using tunable infrared sources to probe a pulsed supersonic slit jet expansion. In the H2O bend region, there is a puzzling absence of Ka = 0 transitions for C2H2–H2O, while both Ka = 0 and 1 are observed for C2D2–H2O. This continues a pattern of "missing" Ka 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 C2D2–H2O with Ka' = 1, 0.8 ns for C2H2–H2O with Ka' = 1, and 0.8 ns for C2D2–H2O with Ka' = 0. The lifetime for C2H2–H2O Ka' = 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 D2O ν3 region, clear sub-bands with Ka = 0 ← 1, 1 ← 0, and 2 ← 1 are observed which enable a good determination of the A rotational constant of C2H2–D2O (7.313 ± 0.007 cm–1), a first for acetylene-water dimer and larger than previously expected.