Spectra and structure of small ring compounds. XXXI Microwave, Raman and infrared spectra, conformation, dipole moment, and quadrupole coupling constants of 2-oxazoline

Abstract

The microwave spectrum of 2-oxazoline has been recorded from 12.4–40.0 GHz. Both Q-branch and R-branch assignments have been made for the ground and five vibrationally excited states of the ring puckering mode for both A - and B-type transitions. The components of the diple moment were determined by the Stark effect to be μa = 1.14±0.01, μb = 1.35±0.01, μc = 0.00±0.00, and μt = 1.77±0.01 D. The quadrupole coupling constants were found to have the following values: χa a = −3.54, χb b = 2.03, and χc c = 1.51 MHz. From the inertial defect Δc and the dipole moment, it is concluded that the heavy atom skeleton is planar. From the relative intensity measurements, the first excited state of the ring puckering vibration was found to have a frequency of 79±8 cm−1 and from a series of difference bands in the infrared spectrum, the ν = 0→1, 1→2, 2→3, 3→4, and 4→5 transitions were found to have the following frequencies: 74, 95, 103, 110, and 117 cm−1, respectively. The potential function governing the ring puckering motion was found to be of the form V = 22.2 (Z4+1.31Z2), where Z is the reduced ring puckering coordinate. The Raman spectra of gaseous, liquid, and solid 2-oxazoline was investigated from 0–3500 cm−1 and the infrared spectra of the gas and solid were recorded from 33–3500 cm−1. The vibrational assignment of the fundamentals is proposed which is consistent with the Cs conformation. The assignment is based on the gas-phase band contours, the depolarization values, and group frequency correlations. The number of lattice modes indicate that there are at least two molecules per primitive cell. The results are compared with those obtained for other similar five-membered rings.