Microwave Spectrum, Structure, Dipole Moment, and Quadrupole Coupling Constants of 1,2,5-Oxadiazole

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Abstract
The microwave spectra of the normal species (C2H2N2O) and the C13CH2N2O, C2H2N15NO, C2D2N2O, and C2HDN2O isotopic species of 1,2,5-oxadiazole have been assigned. The value of the inertial defect indicates a planar structure for the molecule. The structural parameters obtained from the isotopic changes in the moments of inertia plus the center-of-mass condition, Σimibi=0, are: O–N=1.380±0.003 Å, C=N=1.300±0.003 Å, C–C=1.421±0.003 Å, C–H=1.076±0.003 Å,∠NON=110∘24′±20′,∠ONC=105∘49′±20′,∠CCN=108∘59′±20′,∠CCH=130∘10′±35′,∠NCH=120∘51′±35′.Stark-effect measurements gave a dipole moment of 3.38±0.04 D. The nitrogen quadrupole coupling constants of the normal species were determined from an analysis of the hyperfine splittings of four low J Q-branch lines to be: χaa=−0.69±0.09 Mc/sec, χbb=−0.46±0.13 Mc/sec, χcc=1.15±0.04 Mc/sec. These constants correctly predict the shape and splitting of an additional low J R-branch line. The absence of a low-lying vibrationally excited state, the absence of an out-of-plane component of the dipole moment, and the relative intensities of the quadrupole components of the lines of C2H2N2O and C2D2N2O are further evidence for the planarity and C2v symmetry of the molecule.