An ab initio study of the ground and first excited state of HCN ↔ HNC isomerization and a calculation of the HNC A → X fluorescence spectrum

We report ab initio calculations of the minima and transition states of the HCN → HNC isomerization in the X(A′) and A(A″) electronic states, and the A-X transition moment in the neighborhood of the HNC isomer. The HNC(A) potential is fit to a fourth-order Simons-Parr-Finlan force field, and the nuclear coordinate dependence of the A-X transition moment is fit to a polynomial in displacement coordinates. The A-X fluorescence spectrum is calculated using a rigorously calculated wavefunction for the HNC(A), the fitted transition moment, and hundreds of previously calculated HCN/HNC(X) vibrational wavefunctions, which were obtained using a previous global ab initio potential.