Fourier Transform Microwave Spectrum of the Nitrogen Molecule–Ethylene Oxide Complex: Intracomplex Motions

The rotational spectra of the N₂–ethylene oxide (EO) complex were measured in the frequency region from 4 to 27 GHz by Fourier transform microwave spectroscopy, paying particular attention to intracomplex motions. The isotopologues with enriched ¹⁵N₂ or ¹⁵NN as a moiety were also investigated. We have observed spectra of a strong/weak pair for each of the ortho and para states of the ¹⁴N₂–EO and ¹⁵N₂–EO species, which indicated that the complex existed in four distinct states. We interpreted, on the basis of the observed relative intensities, that these states were generated primarily by the exchange of the nitrogen atoms of the N₂ moiety, followed by that of the two CH₂ groups in the EO molecule. The ¹⁵NN–EO species was found to consist of two isomers, one with the ¹⁵N in the inner expressed as N¹⁵N–EO and the other in the outer position designated as ¹⁵NN–EO, and the spectra of both isomers were accompanied by one weak set of satellites. The observed spectra were rotationally assigned by using sum rules and were analyzed by the asymmetric-rotor program of <i>S</i>-reduction, with the standard deviation of less than 10 kHz. We have found some of the molecular parameters like <i>A</i>, <i>D</i>_<i>JK</i>, and <i>D</i>_<i>K</i> to be correlated between the two pairs of the spectra, and also, to much less extent, between the strong and weak members. The differences in these molecular parameters between the four sets were explained by the first-order Coriolis interaction between the “ground” and “excited” states generated by a combination of the two internal motions corresponding to the exchanges of the equivalent atoms and/or groups in the N₂ and EO constituents of the complex. These internal motions were simulated by the 2-fold internal rotations of the two moieties. We have carried out ab initio molecular orbital calculations at the level of MP2 with basis sets 6-311++G­(d,p), aug-cc-pVDZ, and aug-cc-pVTZ, to complement the information on the intracomplex motions obtained from the observed rotational spectra