Theoretical analysis of the Norrish reaction mechanism in aliphatic polyamide

The Norrish reaction mechanism responsible for the chain scission of aliphatic polyamides (<i>i.e.</i>, nylon) was investigated using time-dependent density functional theory and a simplified model, <i>N</i>-ethylacetamide (NEA). The low-lying excited states (ESs) of NEA were characterized in terms of their molecular orbital properties, and the transition state for the Norrish reaction in the singlet and triplet ES was also identified. Our previous study revealed that a direct photodissociation mechanism contributing to the C-N bond cleavage within the peptide moiety (C_CO-N) initiates the primary photodegradation path due to its barrierless nature and high oscillator strength [J. Sang, Y. Orimoto and Y. Aoki, <i>J. Phys. Chem. A</i>, 2024, <b>128</b>, 8865-8877]. In this work, based on the lower barriers for the Norrish type II mechanism (activation energies always less than 15 kcal mol^-1) than those for the Norrish type I reaction of NEA (exceeds 20 kcal mol^-1), the Norrish type II mechanism mainly constitutes the secondary photodegradation path, causing the N-C bond disruption adjacent to the carbonyl group. Furthermore, it was clarified that both the C_CO-N bond photodissociation process and Norrish reaction mechanisms arise from the same singlet ES. These novel quantum chemical insights are proposed for the first time and are helpful in designing robust polyamide fibers with improved resistance against the photolytic process.