Microwave spectrum, conformational equilibrium, 14N quadrupole coupling constants, dipole moment, vibrational frequencies and quantum chemical calculations for acrylamide

The microwave spectrum of acrylamide (CH2=CHCONH2) has been investigated in the 20.0-60.5 GHz spectral region at room temperature. Two conformers were assigned. The most stable rotamer is planar with the carbonyl and vinyl groups in the syn conformation. Five vibrationally excited states of the C-C torsional motion and six other vibrationally excited states have been assigned for this conformer, as have three deuterated species (amide group). The frequencies of most of these excited states have been determined by relative intensity measurements. The dipole moment of this rotamer is (in units of Debye) mu(a) = 0.269(3), mu(b) = 3.42(2), mu(c), = 0.12(24) and mu(tot) = 3.44(5). The N-14 quadrupole coupling constants were found to be chi(aa) = 1.87(26) and chi(bb) = 2.73(16) MHz, respectively. The second, less stable skew form is non-planar with the C=C-C=O chain of atoms approximately 155 degrees from syn. The ground vibrational state and two excited states were assigned. The syn form was found to be 6.5(6) kJ mol(-1) more stable than the skew conformer by relative intensity measurements. Quantum chemical calculations at HF, MP2 and DFT levels of theory using the rather large basis sets 6-311+G**, 6-311++G** and cc-pVTZ have been made to assist the experimental work. (C) 2000 Elsevier Science B.V. All rights reserved.