Microwave Spectrum of Methyl Difluoramine : Structure, Conformation, Dipole Moment, Barrier to Internal Rotation, and Quadrupole Coupling Constants of the Molecule

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Abstract
The microwave spectra of the common and three isotopic species of methyl difluoramine have been investigated from 8 to 27 Gc/sec. From an analysis of the rotational constants the following structural parameters are obtained:DCN1.449 ÅCH1.091 ÅNF1.413 Å∠NCHs110∘22′∠FNF100∘59′∠NCHa106∘14′∠CNF104∘38′∠HsCHa111∘45′∠HaCHa110∘15′.The conformation of the methyl group is such that the in-plane CH bond (CHs) is staggered with respect to the NF bonds. The methyl group is nearly symmetric and the NCH angles indicate the methyl group and its symmetry axis to be tilted away from the CN bond axis and towards the unshared electron pair by about 2.8°. Quadrupole hyperfine structure was resolved and analyzed for each of the four species. The quadrupole coupling constants of the common species are: χAA=6.45 Mc/sec, χBB=0.46 Mc/sec, χCC=−6.91 Mc/sec. A crude estimate of χBC (2.8±0.6 Mc/sec) is obtained by comparing data for the common and symmetric monodeutero species. The rotation of principal axes in going from one species to the other is 2°11′. A similar comparison of Stark effect data for the same two species makes it possible to determine uniquely the orientation of the dipole moment. The dipole moment is 2.57±0.03 D and it lies at an angle of 12°54′ with respect to the CN bond. No tunnel-effect splittings were resolved in the ground-vibrational state; however, such splittings were found in the first-excited torsional state and they yield the threefold symmetric part of the barrier to internal rotation as V3=4170 cal/mole. Using parameters based on reasonable notions of bonding in CH3NF2, the Townes and Dailey theory of quadrupole coupling is found to account satisfactorily, though approximately, for the observed coupling constants. Similar results are obtained for CD3ND2 and HNF2. The relationship of methyl group ``tilt'' to the problem of barriers to internal rotation is discussed.