Rotational Spectra, Molecular Structure, and Electric Dipole Moment of Propanethial S-Oxide

Pulsed-beam Fourier transform microwave spectroscopy was used to study propanethial S -oxide ( 1 ), an unstable species generated in a ( Z )/( E ) ratio of 98/2 by pyrolysis of 2-methyl-2-propyl-1‘-propenyl vinyl sulfoxide at 350 °C in Ar or Ne/He gas flows. A fit of 25 transition frequencies to a Watson “ A ” reduced Hamiltonian gave the following rotational constants and centrifugal distortion constants for the normal isotopomer of ( Z )- 1a: A = 10 182.2558(3) MHz, B = 2209.5000(9) MHz, C = 1997.1734(8) MHz, Δ JK = −62.69(2) kHz, Δ J = 4.165(3) kHz, δ J = 0.3336(3) kHz and δ K = −31.7(4) kHz. Twelve transitions were fit in the same manner for ( E )- 1b to give A = 16231(86) MHz, B = 1823.6154(7) MHz, C = 1785.7215(7) MHz, Δ JK = −12.5(5) kHz, Δ J = 0.48(1) kHz, δ J = −0.060(8) kHz. Microwave spectra of six ( Z ) isotopomers were assigned and a partial substitution structure was derived: r (C S) = 1.585(6) Å, r (S O) = 1.473(2) Å, r ( CH CH 2 ) = 1.513(7) Å, r (CH 3 CH 2 ) = 1.536(3) Å, θ(CSO) = 113.8(2)°, θ(CCS) = 126.7(3)°, and φ(CCCS) = 118.4°. The electric dipole moment components of the ( Z ) isomer along the a, b, and c principal axes were measured to be μ a = 2.59(2) D, μ b = 2.11(2) D, and μ c = 0.33(6) D, respectively, which gives a total electric dipole moment of 3.35(2) D.