O-Nitrosobis(trifluoromethyl)hydroxylamine: Unexpected conformational properties and an unusually long (CF3)2NO-NO bond. A combined study of the gaseous and solid states

Abstract

The conformational properties and the geometric structure of gaseous O-nitrosobis(trifluoromethyl)-hydroxylamine, (CF3)2N-O-N=O, have been studied by infrared spectroscopy, electron diffraction, and ab initio calculations. From the relative intensities and band contours of the two N=O vibrations, a mixture of 80(10)% trans-syn and 20(10)% trans-anti conformers (ΔG = G(syn) - G(anti) = -0.8(3) kcal/mol) is derived (trans describes the orientation of the O-N bond relative to the CNC bisector and syn/anti refers to the orientation of N=O relative to N-O). Such a mixture of conformera with planar NONO groups is consistent with the electron diffraction intensities. In addition, the compound has been subjected to X-ray crystal structure analysis. In the solid state, only the trans-syn conformer is present. The O-N single bond is unusually long and differs strongly in the two phases: 1.572(21) Å in the gaseous state and 1.669(3) Å in the solid state. The results of ab initio calculations depend strongly on the calculational method, especially for the relative energy of the two conformera and for the O-N bond length: ΔE = E(syn)-E(anti) =-1.8, +0.6, and -3.3 kcal/mol and O-N =1.527, 1.401, and 1.717 Å with HF/3-21G,HF/6-31G*, and MP2/6-31G*, respectively. None of these predictions agrees very well with the experimental results for the gas phase.