Photodissociation transition states characterized by chirped pulse millimeter wave spectroscopy.
basic_science · Level V
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- Record sourced from PubMed, PMID 31852828.
- Also identified by DOI 10.1073/pnas.1911326116 and PMC identifier 6955290.
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Abstract
The 193-nm photolysis of CH<sub>2</sub>CHCN illustrates the capability of chirped-pulse Fourier transform millimeter-wave spectroscopy to characterize transition states. We investigate the HCN, HNC photofragments in highly excited vibrational states using both frequency and intensity information. Measured relative intensities of <i>J</i> = 1-0 rotational transition lines yield vibrational-level population distributions (VPD). These VPDs encode the properties of the parent molecule transition state at which the fragment molecule was born. A Poisson distribution formalism, based on the generalized Franck-Condon principle, is proposed as a framework for extracting information about the transition-state structure from the observed VPD. We employ the isotopologue CH<sub>2</sub>CDCN to disentangle the unimolecular 3-center DCN elimination mechanism from other pathways to HCN. Our experimental results reveal a previously unknown transition state that we tentatively associate with the HCN eliminated via a secondary, bimolecular reaction.