Millimeter-Wave Rotational Spectrum of HSSH and DSSD. I. Q Branches

The Q-branch rotational lines of H2S2 have been measured in the frequency range 80–200 Gc/sec, those of D2S2, in the range from 60 to 220 Gc/sec. For HSSH, measurements were made on the torsional vibrational state υt = 1, and the S–S bond-stretching vibrational state υs = 1 as well as on the ground state. The molecules are the most nearly accidentally symmetric tops of any so far reported. Wang's asymmetry parameter bp = (C − B) / (2A − B − C) for the ground vibrational state of H32S32SH is found to have the value bp = − 1.10 × 10−5; for H32S34SH, bp = − 1.07 × 10−5; for D32S32SD, bp = − 2.10 × 10−6. The asymmetry increases markedly with the torsional oscillation: for H32S32SH when υt = 1, bp = − 4.055 × 10−5; it decreases with the S–S stretching: for H32S32SH when υs = 1, bp = − 7.72 × 10−6. The molecule is found to have the nonplanar chain structure HSSH, with the structural parameters having the values: dSH = 1.327 Å, dSS = 2.055 Å, ∠HSS = 91.32°, and the dihedral angle η = 90°36′. The barrier to relative internal rotation of the two SH groups is evidently very high, much higher than in H2O2. The internal rotational splitting of the ground vibrational state is too small to be resolved. In the torsional vibrational state υt = 1 it was found to be 2Δν = 15.99 Mc/sec. The near orthogonality of the bond angles, the S–S length, and the high barrier to internal rotation indicate significant π bonding through the mechanism of hyperconjugation.