Mechanistic Investigation of Photochemistry and Chemiluminescence using Multi-configurational Quantum Chemistry

Abstract

The mechanistic description of excited state reactions involves unique challenges. Unlike ground state chemistry, where transition states control reactions, excited state molecular transformations are largely governed by crossings of potential energy surfaces called "conical intersections". Commonly used quantum chemical methods like Hartree-Fock and DFT, which work well for ground states are not suitable for describing conical intersections and regions of potential energy surfaces around them. We have used multiconfigurational quantum chemical approaches like CASSCF and its extensions to investigate three important chemical reactions, all of which involve excited electronic states. Multiconfigurational approaches are technically challenging and require adept chemical intuition for their correct application.

Ortho-nitrotoluene (oNT), on photoexcitation undergoes excited state intramolecular hydrogen transfer (ESIHT) as well as dissociation to various photo products. The ESIHT process is representative of the primary step in the deprotection of ortho-nitrobenzyl (oNB) derivatives that are widely used as photo-labile caged compounds. In the literature, an experimental study on oNT has reported two distinctly different time scales (1 and 1500 ps) for the ESIHT reaction. Our study explains the reason for these lifetimes and provides a detailed mechanistic picture of the photodecay. The photodissociation of oNT is also of major interest because it is a prototype for high-energy materials. Using 1-nitropropene as a model system for oNT, we have studied the photodissociation and proposed an unexplored excited singlet pathway for the formation of NO, which rationalizes its observed bimodal translational energy distribution. We have also investigated the origin of chemiluminescence in the NO + O3 gas phase reaction. We found that the chemiluminescence is due to emission from the NO2 vibronic states associated with the ground and first excited electronic states, which are populated in the nascent NO2 produced in the reaction. An analysis of the product energy distribution indicates that the major fraction of the reaction energy channeled into the vibrational modes of NO2, sufficient to populate the vibronic states of NO2. Besides obtaining new mechanistic insights on reactoins involving electronically excited states, we have also developed a method to find a crossing point between three states when these states have two different spin multiplicities. Taken together, our studies demonstrate that multiconfigurational quantum-chemical methods provide fundamental insights on complex excited state processes that are not obtainable by experiment or other theoretical methods.