Rotational Spectra, Molecular Structure, and Electric Dipole Moment of Propanethial S-Oxide Jennifer Z. Gillies* and Elizabeth Cotter Department of Chemistry, Siena College, LoudonVille, New York 12211 Charles W. Gillies and Hugh E. Warner Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12180 Eric Block Department of Chemistry, State UniVersity of New York at Albany, Albany, New York 12222 ReceiVed: January 21, 1999 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, ∆ JK )-62.69(2) kHz, ∆ 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, ∆ JK ) -12.5(5) kHz, ∆ 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(CdS) ) 1.585(6) Å, r(SsO) ) 1.473(2) Å, r(dCHsCH 2 ) ) 1.513(7) Å, r(CH 3 sCH 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. I. Introduction Thial S-oxides have the structure RCHdS + sO - and are the sulfur analogues of carbonyl oxides, RCHdO + sO - . The high reactivity of this class of organosulfur compounds makes study difficult. They have been investigated spectroscopically in the gas phase by pumping an appropriate sulfoxide precursor through a hot zone and then into the absorption cell of the spectrometer. 1 The first report of the parent species, methanethial S-oxide (CH 2 SO), used microwave spectroscopy to characterize the compound and determine that it has a planar gas-phase geometry. 1 Propanethial S-oxide (CH 3 CH 2 CHSO) plays an important role in Allium chemistry. There is considerable evidence that indicates it is the lachrymatory factor responsible for tearing when an onion is cut. 2 Recently, propanethial S-oxide was detected in the vapor above chopped onions by a direct in situ spectroscopic technique. 3 Freshly chopped onions were placed in a tube and the volatile gases carried in Ne/He to a modified solenoid gas valve which sampled the gas mixture for analysis in the Fabry-Perot cavity of a Fourier transform microwave spectrometer. 3 The high resolution and molecular isotopic specificity of the Fourier transform microwave tech- nique provided an unambiguous way to identify the site of deuterium incorporation in propanethial S-oxide obtained from an onion-D 2 O homogenate. This isotopic study was used to elucidate the mechanism of the formation of propanethial S-oxide from the process of cutting onions. 3 To use the in situ method described above analytically, it was necessary to observe and assign the rotational spectrum of propanethial S-oxide. As shown in Figure 1, (Z) and (E) configurations of propanethial S-oxide, 1a, 1b, arise from geometrical isomerization about the CdS bond. The spectro- scopic problem is further complicated by the possibility of rotational isomerism of the ethyl group relative to CHSO. This paper reports the microwave spectroscopic data and assignments which support the chemical results discussed above for (Z)- and (E)-propanethial S-oxide. The rotational spectra were recorded by producing propanethial S-oxide in a Ne/He flow by pyrolysis of 2-methyl-2-propyl-1′-propenyl sulfoxide, 2. A similar pyro- lytic method has been used in an analogous study of the rotational spectra of (Z)- and (E)-ethanethial S-oxide, 3a, 3b. 4 Spectra of six isotopomers of the (Z) configuration were assigned and a heavy atom structure was obtained from the effective moments of inertia. In addition, a weak spectrum was assigned to the (E) configuration, which is formed in much lower Figure 1. (Z) 1a and (E) 1b geometrical isomers of propanethial S-oxide shown for the skew conformations. 4948 J. Phys. Chem. A 1999, 103, 4948-4954 10.1021/jp9902506 CCC: $18.00 © 1999 American Chemical Society Published on Web 06/06/1999