Microwave spectrum, conformation, intramolecular hydrogen bonding and ab initio calculations for 2-nitroethanol

The microwave spectrum of 2-nitroethanol has been investigated in the 26.5-39.0 GHz spectral region at room temperature. The spectra of the ground and of six excited states of the torsional vibration of the nitro group of one rotamer were assigned and could be fitted to the ordinary Watson Hamiltonian to within the experimental accuracy. The nitro group is not rotating freely but has a low torsional frequency determined to be 34(10) cm(-1) by relative intensity measurements. The identified conformer has a gauche arrangement for the O-C-C-N chain of atoms and is stabilized by a weak six-membered intramolecular O-H ... O hydrogen bond formed between the hydrogen atom of the hydroxyl group and one of the oxygen atoms of the nitro group. The N-O bond (where the oxygen atom is involved in intramolecular hydrogen bonding), and the C-C bond are about 17 degrees from being completely eclipsed. This rotamer is at least 4 kJ mol(-1) more stable than any other rotameric form of the molecule. The microwave work has been assisted by ab initio computations made at the MP2/6-31 + + G** level of theory.