Interlocking Triplet Electronic States of Isocyanic Acid: Sources of Nonadiabatic Photofragmentation Dynamics

Abstract

The triplet electronic states of isocyanic acid have been systematically investigated by means of state-of-the-art electronic structure methods, including various correlation techniques based on the coupled-cluster ansatz [CCSD, EOM-CCSD, CCSD(T), and BD(TQ)], second- through fifth-order Møller−Plesset perturbation theory (MP2-MP5), and the complete active space self-consistent field approach. The one-particle [(C,N,O)/H] basis sets for these studies ranged in quality from [4s2p1d/2s1p] to [7s6p5d4f3g2h1i/6s5p4d3f2g1h]. Vertical excitation energies were determined for the lowest 13 triplet states (5 valence, 8 Rydberg), and potential energy curves for bending to and from linearity were generated for 10 of these states, revealing intricate state interactions and numerous actual and avoided crossings. An extensive mapping was then executed for the interlocking ã3A‘ ‘ and b̃3A‘ surfaces, which produced geometric structures, relative energies, harmonic vibrational frequencies, and selected large-amplitude vibrational eigenstates, for torsional conformers, inversion barriers, fragmentation barriers, dissociation products, and ionization limits, in addition to identifying intermingled conical intersections. The lowest-energy conformer on the ã3A‘ ‘ surface is actually a skewed (C1) structure with a torsion angle of 143°, a barrier to planarity of only 74 cm-1, an adiabatic excitation energy near T0 = 30 056 cm-1, and an exit barrier for 3NH + CO fragmentation of only about = 3252 cm-1. It is discovered that there are actually no legitimate minima (removed from conical intersections) on the b̃3A‘ surface, because in-plane optimizations bring associated structures below the companion 3A‘ ‘ state and subsequently connect them to the lowest triplet surface via torsional excursions along imaginary-frequency normal modes.