Microwave spectra, molecular structure, and electric dipole moment of 1,2-dithiin

A pulsed-beam Fourier transform microwave spectrometer was used to investigate the rotational spectrum of 1,2-dithia-3,5-cyclohexadiene in the range 8 to 18 GHz. The observed a-type transitions were fit to a Watson asymmetric top Hamiltonian, giving the following rotational constants for the normal isotopomer: A = 3326.362(2) MHz, B = 3102.842(2) MHz, C = 1756.0026(7) MHz. An electric dipole moment of mu(total) = mu(a) = 1.850(1) D was determined from Stark effect measurements. Observed nuclear spin statistical weights, an inertial defect of Delta = I-c - I-a - I-b = -27.00685 u Angstrom(2), and zero mu(b) and mu(c) electric dipole components show the molecule has C-2 symmetry which corresponds to a twisted conformation about the S-S bond. Spectral assignments were obtained for the singly substituted S-34 and two C-13 isotopomers in natural abundance. The moments of inertia of the four isotopomers were used to determine the following r(s) structural ring parameters: r(S-S) = 2.051(3) Angstrom, r(C-S) = 1.759(4) Angstrom, r(C=C) = 1.353(3) Angstrom, r(C-C) = 1.451(1) Angstrom, theta(SSC) = 98.7(2)degrees, theta(SCC) = 121.4(2)degrees, theta(CCC) = 124.2(2)degrees, phi(SSCC)= 41.2 degrees, phi(SSCC)= 41.2 degrees, phi(SCCC) = -0.3 degrees, phi(CCCC) = 29.0 degrees, and phi(CSSC) = 53.9 degrees. (C) 1996 Academic Press, Inc.