Microwave, infrared, and Raman spectra, structures, quadrupole coupling constants, barrier to internal rotation, and vibrational assignment of 1-chloro-2-methylpropene

Abstract

The microwave spectra of (CH3)2CCH35Cl and (CH3)2CCH37Cl have been recorded from 12.5 and 39.0 GHz. a-type transitions were observed and R-branch assignments have been made for the ground vibrational states for both isotopes. The rotational constants were found to have the following values: for (CH3)2CCH35Cl, A = 8400.77±1.18, B = 2258.18±0.02, C = 1818.72±0.02 MHz; for (CH3)2CCH37Cl, A = 8399.31±0.66, B = 2199.93±0.02, C = 1780.73±0.02 MHz. From a diagnostic least-squares adjustment to fit the six rotational constants and with reasonable assumed carbon–hydrogen distances and angles, the following heavy atom structural parameters were obtained: r(C–C) = 1.507±0.013 Å, r(C = C) = 1.355±0.019 Å, r(C–C1) = 1.750±0.019 Å, ∢C–C = C(cis) = 122.20°±4.50°, ∢C–C = C(trans) = 119.39°±1.84°, and ∢CCCl = 124.24°±2.79°. The quadrupole coupling constants for the (CH3)2CCH35Cl molecule were found to have the following values: χaa = −49.2, χbb = 19.5, and χcc = 29.7 MHz. From an analysis of the internal rotational splittings, the threefold barrier for the cis methyl group was found to be 288±5 cm−1 (0.824 kcal/mole). The infrared (3500–50 cm−1) and Raman spectra (3500–10 cm−1) have been recorded for both the gas and solid states. Additionally, the Raman spectrum of the liquid was recorded and qualitative depolarization values were obtained. All of the normal modes have been assigned based on band contours, depolarization values, and group frequencies. Several torsional transitions for the trans methyl group have been observed in the far infrared spectrum of the vapor, and from these transitions a barrier of 745±60 cm−1 (2.14 kcal/mole) has been calculated for the internal rotation of the trans methyl group. These results are compared to the corresponding quantities in similar molecules.