Toward an Intimate Understanding of the Structural Properties and Conformational Preference of Oxoesters and Thioesters:  Gas and Crystal Structure and Conformational Analysis of Dimethyl Monothiocarbonate, CH₃OC(O)SCH₃

The molecular structure and conformational properties of dimethyl monothiocarbonate, CH₃OC(O)SCH₃, have been studied in the gas phase by gas electron diffraction (GED) and vibrational spectroscopy and in the solid state by X-ray crystallography. The experimental investigations were supplemented by quantum chemical calculations at the B3LYP/6-311++G(3df,2p) and MP2/6-311++G(2df,p) levels of approximation. The gaseous molecule exhibits only one conformation having C_s symmetry with synperiplanar orientation of both the CS and the CO single bonds relative to the CO double bond. The following skeletal geometric parameters were derived from the GED analysis (r_hl values with 3σ uncertainties):  CO = 1.203(4) Å, C(sp²)O = 1.335(5) Å, C(sp³)O = 1.437(5) Å, C(sp²)S = 1.763(5) Å, and C(sp³)S = 1.803(5) Å; OCO = 125.9(8)°, OCS = 125.7(7)°, OCS = 108.4(9)°, and COC = 113.4(15)°. The structure of a single crystal, grown by a miniature zone-melting procedure, was determined by X-ray diffraction analysis at a low temperature. The crystalline solid [monoclinic, P2₁/n, a = 12.6409(9) Å, b = 4.1678(3) Å, and c = 19.940(1) Å, β = 98.164(1)°] exists exclusively as molecules in the synperiplanar conformation and with geometrical parameters that agree with those of the molecule in the gas phase. The results are discussed in terms of anomeric and mesomeric effects and in terms of a natural bond orbital analysis.