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.

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It cannot be simpler! This applies, however, to the structure of the title compound rather than to the experimental conditions under which it is prepared. This prototype for complexes with side‐on H2 bonding was formed by cocondensation of monomeric CuCl with H2 in an argon matrix and unambiguously characterized by IR spectroscopy. In such simple systems the ab initio calculations can almost take the place of experiment—they also clearly favor the side‐on coordination of the H2 ligand and predict the surprisingly high energy of formation (93 kJ mol−1).

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