Ab initio treatment of the Renner-Teller effect for the X 2 B 1 and A 2 A 1 electronic states of NH2

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Abstract
A large AO basis including atom-centred f- and s- and p-type bond functions is employed to obtain potential curves for the X 2 B 1 and A 2 A 1 states of NH2 at close to the full CI level of treatment. Two different matrix methods are then applied to describe the Renner-Teller coupling between the bending vibrational states of the two electronic species; excellent agreement is found between the two sets of theoretical results. Use of a simple kinetic energy operator with a constant reduced mass leads to a term value spectrum which agrees to within 100 cm-1 for the measured locations of all low lying vibronic levels in this system and to within 300 cm-1 of the highest lying such species observed to date. Similarly good agreement is found for the ND2 and NHD isotopic species, whereby in these cases the amount of vibronic coupling is, not surprisingly, found to be markedly less than in NH2 itself. Equilibrium bond lengths and angles of this system are calculated to agree to within 0·01 Å and 1° of their observed values and computed vibrational frequencies are found to agree to within 3 per cent of their measured counterparts. The ab initio T e value for the A 2 A 1-X 2 B 1 transition is 11 350 cm-1, in very good accord with the value recently reported by Jungen, Hallin and Merer of 11 294 cm-1 based on an empirical fitting procedure to the observed results, but the calculated barrier height of 980 cm-1 in the A 2 A 1 state is 250 cm-1 greater than the corresponding value obtained from the JHM empirical fit. Furthermore such discrepancies between ab initio CI and empirically fitted potential data become much greater for small bond angles beyond ϑ = 90°, whereby it is argued that uncertainties in the form of the bending kinetic energy operator for large amplitude motion as well as failure to consider a low-energy 2 B 2 state in NH2 which undergoes a Jahn-Teller intersection with 2 A 1 in the neighbourhood of ϑ = 70° make it difficult for empirical fitting procedures to obtain high accuracy for the potential curves in the small angle bending region. Finally the computed intensity distributions for the A 2 A 1-X 2 B 1 transition in this system are found to be in almost quantitative agreement both with what has been measured and also with what is obtained in the JHM fitting procedure.

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