Spectroscopic characterization of isomerization transition states

Abstract

Shaking out details of transition states Chemists liken reaction energetics to a landscape with hills and valleys. In this context, the transition state represents the highest barrier that reagents must pass over en route to forming products. Baraban et al. introduce a framework for extracting details about the transition state of rearrangement reactions directly from vibrational spectral data. They identified a characteristic pattern in the spacing between vibrational energy levels near the transition state, which revealed its energy as well as the specific motions involved in surmounting the barrier. Science , this issue p. 1338

Probing cis-trans isomerization in the S1 state of C2H2 via H-atom action and hot band-pumped IR-UV double resonance spectroscopies

“…At the saddle point, the local bending vibrational intervals must go through a minimum, in similar fashion to the "Dixon dip" 27 of the large amplitude bending intervals in triatomic molecules. Recent calculations of the level structure near the barrier 25,28,29 support this conclusion.…”

“…In addition to providing information on the extent of isomerization tunneling and cis-trans interactions, this measurement is necessary to deperturb the input data for effective frequency dip analyses, which can provide a detailed characterization of the isomerization transition state. 29 It is interesting to compare this staggering to that of other trans levels in the ν 6 progression. 3 4 (ℓ ′′ = 1) level; the change in angular momentum upon excitation is denoted by the Greek symbols in the upper left of the figure.…”

Reduced dimension rovibrational variational calculations of the S1 state of C2H2. II. The S1 rovibrational manifold and the effects of isomerization

“…Such difficulties have been noted before, for example, in Jacobson and Child's semiempirical, semiclassical inversion model of large amplitude motion in HCP, 36 and in Barnes and Kellman's approach to the isomerization in HO 2 . 37,38 Though new empirical methods 39 have been proposed to model the multi-mode vibrational level structure resulting from the existence of a saddle point, an ab initio calculation that predicts the global rovibrational energy level structure and the associated wavefunctions is particularly useful for understanding both the physical nature and spectroscopic details of the emergent patterns caused by the isomerization. A further consequence of isomerization in the S 1 state is that cis vibrational levels appear weakly in theà −X spectra via tunneling into the trans well.…”

Calculation of Transition State Energies in the HCN–HNC Isomerization with an Algebraic Model

“…However, the experimental study of transition states is hindered by the saddle structure of the phase space region they inhabit. In recent works, Baraban et al 3 and Mellau et al 4 presented an interesting approach that allows for the characterization of the transition state in isomerization reactions using as an input, spectroscopic data in the frequency domain. The approach in both works is based on a particular spectroscopic pattern: the appearance of a dip in the spacing of adjacent quantum levels for overtone series associated with degrees of freedom that are connected with the reaction coordinate.…”

“…Baraban et al quantify this dip making use of the effective frequency; a quantity defined for quantum systems as w ef f (n) = ∂E(n) ∂n = ∆E ∆n , i.e. the discrete derivative of the system energy with respect to the principal quantum number n. 3,4 They suggest a simple formula 3 to parameterize the dependence of the effective frequency on the midpoint vibrational energy E…”