Microwave and Infrared Spectra of Sulfur Dicyanide: Molecular Structure, Dipole Moment, Quadrupole Coupling Constants, Centrifugal Distortion Constants, and Vibrational Assignment

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The microwave spectra of S(CN)2, SC13CN2, SC2N15N, and 34S(CN)2 were studied in the frequency region 8 to 30 Gc/sec. Isotope shifts yield the following structural parameters: CS=1.701 Å, ∠CSC=98°22′, CN=1.156 Å, 2φ=108°24′, where 2φ is the angle between the two C–N internuclear lines. Thus the SCN angle deviates from linearity by 5°. From Stark-effect measurements the dipole moment was determined to be 3.04±0.03 D. Quadrupole hyperfine structure partially resolved in several transitions of S(CN)2 and SC2N15N yields the following coupling constants (Mc/sec): S(CN)2 (χAA=−1.51, χBB=0.30, χCC=1.21) SC2N15N (χAA=−1.51, χBB=0.24, χCC=1.27). Centrifugal distortion constants and the inertial defect for a low-lying vibrationally excited state (v4=1) of S(CN)2 were of great value in making the following assignment of normal frequencies: ω1(A1) 2190 cm−1 (CN stretch),ω5(B1) 2180 cm−1 (CN stretch),ω2(A1) 672 cm−1 (CS stretch),ω6(B1) 690 cm−1 (CS stretch),ω3(A1) 378 cm−1 (SCN def.),ω7(B1) 328 cm−1 (SCN def.),ω4(A1) 135 cm−1 (CSC def.),ω8(B2) 372 cm−1 (SCN def.),ω9(A2) (376) cm−1 (SCN def.).Measurements of the infrared spectrum extended to 50 cm−1; ω9(A2) which is infrared inactive, is estimated from an approximate potential function. Vibration—rotation interaction parameters determined from a normal coordinate analysis give inertial defects of 0.4503, 0.4499, 0.4548, and 0.4564 amu Å2 for S(CN)2, SC13CN2, SC2N15N, and 34S(CN)2, respectively. The corresponding observed values are 0.4869, 0.4864, 0.4927, and 0.4920. For the excited state v4=1, the calculated and observed values of the inertial defect are 1.2726 and 1.3837 amu Å2, respectively.

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