Molecular beam electric resonance spectroscopy of the nitric oxide dimer

Abstract

Microwave and radiofrequency spectra have been observed for (NO)₂ using the technique of molecular beam electric resonance spectroscopy. Precise values have been determined for rotational, nuclear quadrupole coupling and spin-rotation constants of the ground electronic state treated as a singlet state. The values obtained are A = 25829·4803(20) MHz, B = 5614·3093(4) MHz, C = 4605·4396(12) MHz x^aa = -4·0652(2) MHz, x^bb - x^cc = -8·5498(6) MHz c_aa = 10·4(5) kHz, c_bb = 13·8(4) kHz, c_cc = 0·8(4) kHz. The electric dipole moment has also been determined: μ = 0·17120(2) D. From these data (NO)₂ is interpreted to have a symmetric cis-planar structure with the structural parameters r _NN = 2·33(12) Å, r _NO = 1·15(1) Å, ∠NNO = 95(5), 1/2(r _NN + r _NO) = 2·444(8) Å. The quadrupole coupling constants of the nitrogen nuclei and the dipole moment demonstrate that little electron rearrangement occurs on dimer formation. In addition x^aa and x^bb - x^cc indicate that the unpaired π electrons lie in the plane of the dimer implying an N-N bond of σ symmetry. The spin-rotation constants of the dimer are larger than expected; this fact is rationalized in terms of low lying electronic states.