Microwave, Infrared, and Raman Spectra, Conformational Stability, Structure, Dipole Moment, and Vibrational Assignment for Cyclopropyl Isocyanate

Article abstract of DOI:10.1021/ja00211a009

The microwave spectrum of cyclopropyl isocyanate, c-C3H5NCO, has been recorded from 18.5 to 40.0 GHz.Two sets of a-type, R-branch transitions have been observed and assigned, on the basis of the rigid rotor model, to the trans and the cis conformers, which have the isocyanate moiety oriented in an s-trans or s-cis fashion, respectively, relative to the three-membered ring.In addition to the ground vibrational state transitions, lines due to two excited states of the CNC bend and four excited states of the asymmetric torsion for the trans conformer, as well as one torsional excited state for the cis form, have been assigned.The B and C rotational constants for the ground vibrational state of the trans and cis conformers are the following: B=1784.303 +/- 0.003, C=1716.133 +/-0.003 and B=2186.797 +/-0.010, C=2106.242 +/- 0.010 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.From the relative intensity measurements in the microwave spectrum, the torsional frequency is estimated to be 24 cm^-1 for the trans conformer and 14 cm^-1 for the cis form.The dipol moment components determined from the Stark effect are /μ_a/=2.56 +/- 0.02, /μ_c/=0.71 +/- 0.03, and /μ_t/=2.65 +/- 0.02 D for the trans conformer and /μ_a/=2.720 +/-0.004, /μ_b/=0.17 +/-0.01, and /μ_t/=2.726 +/- 0.001 D for the cis conformer.With reasonably assumed sructural parameters for the cyclopropyl moiety and bond distances for the isocyanate group, estimates of the differences between the cis and trans conformers of the title molecule for the following parameters were made: r(C-N), -1) and Raman (3300 to 10 cm^-1) spectra have been recorded for the gas and the solid states.Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained.Based on the band contours, depolarization values, and group frequencies, the normal vibrational modes have been assigned.From a temperature study of the Raman lines of an assigned conformer pair in the liquid phase, the value of ΔH between conformers has been determined to be 39 +/- 5 cm^-1, with the trans form being more stable.However, from the Raman spectrum of the solid the cis conformer has been determined to be the preferred form in the crystal.These results are compared with the corresponding quantities in some similar molecules.