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

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: $$\begin{array}{cc}{\omega }_1{\mathrm{(A}}_1\text{)\hspace{0.17em}2190\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\mathrm{\hspace{0.17em}(}\mathrm{CN\; stretch}\mathrm{),}& {\omega }_5{\mathrm{(B}}_1\text{)\hspace{0.17em}2180\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\mathrm{\hspace{0.17em}(}\mathrm{CN\; stretch}\mathrm{),}\\ {\omega }_2{\mathrm{(A}}_1\text{)\hspace{0.17em}\hspace{0.17em}672\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\mathrm{\hspace{0.17em}(}\mathrm{CS\; stretch}\mathrm{),}& {\omega }_6{\mathrm{(B}}_1\text{)\hspace{0.17em}\hspace{0.17em}690\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\mathrm{\hspace{0.17em}(}\mathrm{CS\; stretch}\mathrm{),}\\ {\omega }_3{\mathrm{(A}}_1\text{)\hspace{0.17em}\hspace{0.17em}378\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\mathrm{\hspace{0.17em}(}\mathrm{SCN\; def}\mathrm{.),}& {\omega }_7{\mathrm{(B}}_1\text{)\hspace{0.17em}\hspace{0.17em}328\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\mathrm{\hspace{0.17em}(}\mathrm{SCN\; def}\mathrm{.),}\\ {\omega }_4{\mathrm{(A}}_1\text{)\hspace{0.17em}\hspace{0.17em}135\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\mathrm{\hspace{0.17em}(}\mathrm{CSC\; def}\mathrm{.),}& {\omega }_8{\mathrm{(B}}_2\text{)\hspace{0.17em}\hspace{0.17em}372\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\mathrm{\hspace{0.17em}(}\mathrm{SCN\; def}\mathrm{.),}\\ & {\omega }_9{\mathrm{(A}}_2\text{)\hspace{0.17em}(376)\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\mathrm{\hspace{0.17em}(}\mathrm{SCN\; def}\mathrm{.).}\end{array}$$ 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 Ų 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 Ų, respectively.