Contributed by Robert W. Field, October 31, 2019 (sent for review July 5, 2019; reviewed by Brooks H. Pate and Richard N. Zare)
Transition states control results of molecular reactions. However, it is difficult to extract details about the transition state solely from the chemical products born from the transition state. This inability to experimentally characterize transition state limits our ability to predict and steer the outcomes of chemical processes. Here we describe the use of a recently developed, simultaneously broadband and high-resolution technique, chirped-pulse millimeter-wave spectroscopy, to measure the nascent product state distribution that results from the photolysis of vinyl cyanide and, from it, to infer properties of the transition state(s) involved. Insights gleaned from studies such as these are crucial to a detailed understanding of molecular quantum dynamics at a level that promises to allow us to calculate and manipulate chemistry.
The 193-nm photolysis of CH2CHCN 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 J = 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 CH2CDCN 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.
Author contributions: K.P., J.H.B., G.B.P., and R.W.F. designed research; K.P., J.H.B., P.B.C., R.G.S., J.S.M., S.J.K., and R.W.F. performed research; K.P. contributed new reagents/analytic tools; K.P., V.Y.C., J.H.B., P.B.C., S.J.K., and R.W.F. analyzed data; and K.P., J.H.B., S.J.K., and R.W.F. wrote the paper.
Reviewers: B.H.P., University of Virginia; and R.N.Z., Stanford University.
The authors declare no competing interest.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1911326116/-/DCSupplemental.
Published under the PNAS license.
