A b i n i t i o calculations of the barrier for internal rotation in nitrous acid

A b initio calculations of the trans–cis energy difference and the barrier for internal rotation in nitrous acid were made using a double zeta quality basis set and MCSCF+CI methods. The geometry parameters used were the optimized ones obtained by Skaarup and Boggs. The trans–gauche barrier and trans–cis energy difference were calculated here to be 11.4 and 2.5 kcal/mole in good agreement with the experimental values of 11.57±0.2 and 0.4±1.0 kcal/mole, respectively. It was found that exclusion of only one configuration involving excitation of a lone pair on the NOH oxygen from the MCSCF and CI lists reduced the trans–gauche barrier to 6.4 kcal/mole. This shows the importance of lone pair electrons on the oxygen, and not on the nitrogen as previously suggested, to the high barrier.