Microwave spectrum, conformation, and low frequency vibrational spectrum of cyclopropylisothiocyanate

Abstract

The microwave spectrum of cyclopropylisothiocyanate, c-C3H5NCS, has been investigated in the region from 12.5 to 40.0 GHz. Two sets of a-type R-branch transitions were observed and assigned, on the basis of the rigid rotor model, to the trans and cis conformers. One excited vibrational state for each conformer was also identified and tentatively assigned as the asymmetric NCS torsion. The B and C rotational constants in the ground vibrational state for the trans and cis conformers of c-C3H5NCS were found to be: B=1122.689±0.002, C=1095.619±0.002 MHz, and B=1346.859±0.003, C=1309.166±0.002 MHz, respectively. The values of the A rotational constants for both conformers were not well determined because the measured transitions are not very sensitive to these constants. The dipole moment components were determined for the trans conformer from the Stark effect to be ‖μa‖=3.11±0.02, ‖μc‖=0.62±0.01, and ‖μt‖=3.17±0.01 D and for the cis conformer to be ‖μa‖=3.34±0.01, ‖μb‖=0.27±0.07, and ‖μt‖=3.35±0.01 D. With reasonably assumed structural parameters for the cyclopropyl moiety and bond distances for the isothiocyanate group, the other structural parameters were calculated from the rotational constants by the diagnostic least squares method to be: r(C–N)=1.390±0.016 Å,∢CNC=143.8±4.2 °, and ∢CCN=118.7±0.1 ° for the trans conformer and r(C–N)=1.414±0.017 Å, ∢CNC=144.4±1.1, and ∢CCN=118.6±2.3 ° for the cis conformer. The far infrared spectrum of the vapor was also recorded and the C–N=C bend was observed at ∼104 cm−1 but little evidence could be found for the second conformer in the vibrational spectrum. These results are compared to the corresponding ones in some related molecules.