New spectra of (HCl)3 and (HCl)4: Is the tetramer planar?

Spectra of HCl trimer and tetramer are studied in the H-Cl fundamental stretch region using a rapid-scan optical parametric oscillator source to probe a pulsed supersonic slit jet expansion. Compared to previous work on these weakly bound clusters, the effective rotational temperature in the jet (≈2.3 K) is much lower, resulting in reduced spectral congestion. For the trimer, this enables detection and assignment of all six isotopologue components of the degenerate perpendicular band around 2809 cm-1, and these new data result in significant changes in the resonant vibrational coupling parameters used to model isotopic effects. Very weak trimer "hot" bands are also observed. For the tetramer, a weak parallel band around 2790 cm-1 is observed and analyzed. Its presence suggests that the tetramer structure is (slightly) nonplanar, as predicted by modern ab initio calculations, and the validity of this conclusion is discussed. The much stronger perpendicular band of the tetramer around 2776 cm-1 has a considerable amount of partly resolved rotational structure, but it is lifetime-broadened (≈0.01 cm-1) and too congested to permit detailed analysis. Applying an expanded resonant vibrational coupling model, with input from the parallel band results, the overall shape of the perpendicular band is simulated with reasonable accuracy for two rotational temperatures, 2.3 and 13 K.