Abstract

A new and convenient calculation method based on Rice-Ramsperger-Kassel-Marcus (RRKM) theory assuming an extremely loose transition state (LTS) has been attempted to predict the branching ratio in photodissociation. This method enables estimation of the branching ratios without detailed structural information on the transition state which is indispensable in conventional RRKM calculations. To evaluate our simple method through comparison to the experimental results, photodissociation of 1-bromo-3-chlorobenzene cation (3BCB+) was chosen as a model unimolecular reaction system which has two distinct photodissociation channels in ultraviolet region: 3BCB+ → Br-dissociated daughter ion (ClBz+) + Br and 3BCB+ → Cl-dissociated daughter ion (BrBz+) + Cl. The branching ratio was monitored with decreasing the internal energy using a linear tandem time-of-flight mass spectrometer, which clearly showed decreased branching ratios of 3BCB+ → ClBz+ + Br over 3BCB+ → BrBz+ + Cl in reasonable agreement with the calculation results employing the new method. Although there was some discrepancy in internal energy between the experimental and calculation results, the new calculation method is worth to be extended to other diverse systems considering its intuitive and simple nature.

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