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 H 2 O ν 2 bend (1600 cm -1 ) and the D 2 O ν 1 /ν 3 stretches (2670-2808 cm -1 ), using tunable infrared sources to probe a pulsed supersonic slit jet expansion. In the H 2 O bend region, there is a puzzling absence of K a = 0 transitions for C 2 H 2 -H 2 O, while both K a = 0 and 1 are observed for C 2 D 2 -H 2 O. This continues a pattern of "missing" K a 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 C 2 D 2 -H 2 O with K a ' = 1, 0.8 ns for C 2 H 2 -H 2 O with K a ' = 1, and 0.8 ns for C 2 D 2 -H 2 O with K a ' = 0. The lifetime for C 2 H 2 -H 2 O K a ' = 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 D 2 O ν 3 region, clear sub-bands with K a = 0 ← 1, 1 ← 0, and 2 ← 1 are observed which enable a good determination of the A rotational constant of C 2 H 2 -D 2 O (7.313 ± 0.007 cm -1 ), a first for acetylene-water dimer and larger than previously expected.