Matrix-isolation IR and UV spectra of Si 3 H 8 , i-Si 4 H 10 , and the two conformers of n-Si 4 H 10 have been recorded. A quantitative separation of the IR spectrum of n-Si 4 H 10 into contributions from the anti and gauche forms was accomplished by a combination of matrix annealing and selective monochromatic photodestruction experiments. A qualitative separation of their UV spectra was achieved as well. The IR spectra of Si 3 H 8 , i-Si 4 H 10 , and the two conformers of n-Si 4 H 10 have been assigned by comparison with results of ab initio calculations, which reproduce the frequencies and even the relative intensities quite well. The calculations predict dihedral angles ω of 180°and 57°for the anti and the gauche conformer of n-Si 4 H 10 , respectively, and confirm earlier predictions of nearly equal stability for an isolated molecule. In the matrix, the anti conformer is more stable. The conformational effects on the UV spectrum of n-Si 4 H 10 are not those anticipated from simple models of the Sandorfy or ladder C type, in that it is primarily not the energy but the intensity of the low-energy excited singlet states that depends strongly on the SiSiSiSi dihedral angle ω. This result is interpreted in terms of data from 6-in-8 CASSCF 6-31G* calculations, which predict an avoided crossing between a strongly allowed σσ* B state and a very weakly allowed σπ* B state as ω changes, with the former lower in energy at 180°and the latter lower at 0°. Consequences for attempts to understand the effects of conformation on optical spectra of polysilanes are noted.

Based on the physico-chemical properties of water dimers, their near infrared and visible absorption of solar radiation in the earth's atmosphere is calculated. The calculation uses equilibrium constants determined by statistical mechanics, and a vibrational absorption spectrum determined by a coupled oscillator quantum mechanics model and ab inirio quantum chemistry. The resulting total atmospheric absorption was calculated using a line-by-line radiative-transfer model, and depends significantly on the dimer abundance, as well as on the frequency and line width given to vibrational transitions. The best estimate achieved for the possible range of total absorption from 400 nm to 5000 nm by water dimer in the tropics is 1.63.3 W m-2. In a global-warming scenario, the increased temperature and water vapour partial pressure result in a nonlinear increase in the absorption of solar radiation by water clusters. Most of the energy from water dimer absorption is deposited in the lower troposphere, particularly in the tropics, tending to make it more convective.

Fourier Transform infrared spectrometry has been used to study the vibrational spectroscopy of water clusters prepared in a supersonic expansion. The clusters are assigned as being composed of 10−100 waters based on consideration of the free OH and bound OH vibrations and the experimental conditions used. Four vibrational features were observed simultaneously including the bend, free OH stretch, bound OH stretch, and an unresolved group of intermolecular vibrations. No absorption features were observed between 4000 and 8000 cm-1. This is the first observation of the bending vibration for any size gas-phase water cluster.