Photochemistry of aldehyde clusters: cross-molecular versus unimolecular reaction dynamics

Abstract

The unimolecular photochemistry of aldehydes has been extensively studied, both experimentally and computationally. However, less is known about the role of cross-molecular photochemical processes in the condensed-phase photolysis of aldehydes. The triplet-state photochemistry of pentanal in its pentameric (n = 5) cluster was investigated as a model for photochemical reactions of aliphatic aldehydes in atmospheric aerosols. This study employs ''on the fly'' dynamics simulations using a semi-empirical MRCI electronic code for the singlet and triplet states involved. Previous studies have shown that the triplet-state photochemistry of an isolated pentanal molecule is dominated by Norrish I and II reactions. The main findings for the cluster are: (1) 55% of the trajectories lead to a unimolecular or cross-molecular reaction within a timescale of 100 ps; (2) cross-molecular reactions occur in over 70% of the reactive trajectories; (3) the main cross-molecular processes involve an H-atom transfer from the CHO group of the excited pentanal to an O atom of a nearby pentanal; and (4) the unimolecular Norrish II reaction is suppressed by the cluster environment. The predictions are qualitatively supported by experimental results on the condensed-phase photolysis of an aliphatic aldehyde, undecanal. The computational approach should be useful for predicting the mechanisms of other condensed-phase organic photochemical reactions. These results demonstrate a major role of cross-molecular processes in the condensed-phase photolysis of carbonyls. The cross-molecular reactions discussed in this work are relevant to photolysis-driven processes in atmospheric organic aerosols. It is expected that the condensed-phase environment of an organic aerosol particle should support a multitude of similar cross-molecular photochemical processes.

Stereoselective Excited-State Isomerization and Decay Paths in cis-Cyclobiazobenzene

“…85 Further technical details are given in previous publications. [86][87][88][89][90][91][92][93][94][95]…”

Structural Causes of Singlet/triplet Preferences of Norrish Type II Reactions in Carbonyls

“…8,12,35,94,102 More theoretical studies of the NTII biradical 42 are required, and ab initio dynamics has only been performed for the NTII reaction in pentanal. 43,44 Furthermore, in order to assess the competition between NTII and other photolysis reactions on S 1 , T 1 , and S 0 kinetic modelling is required. The reaction energies, TS structures, and S 1 /S 0 MECIs reported in this paper can be used as the basis for master equation calculations for NTII reactions across many carbonyl species.…”

“…Theoretical work on other mechanistic aspects of the NTII reaction, such as the dynamics of the biradical, are available in other studies, but only for a few carbonyl species. [42][43][44] An overview of the previous modelling work for the NTII reaction in carbonyls is presented below to give context to this current study.…”

Untitled

“…Shemesh used a semi-empirical quantum-chemical potential surface for the excited states in MD simulations for the photochemical process. An important result is that cross-molecular hydrogen transfer reactions play a larger role than do unimolecular reactions (134). 3 .…”