Ultrafast molecular reaction dynamics in real-time: progress over a decade

One of the goals of researchers in the field of reaction dynamics is to develop an understanding of the elementary steps involved in a chemical reaction on a molecular level (see e.g. Ref. 1). The century-old Arrhenius rate law, a phenomenological description of the temperature dependence of rates of reactions in bulk, has been used extensively to deduce activation energies and frequency factors. The activated complex theory (also referred to as absolute rate theory or transition-state theory, see e.g. Refs. 2, 3) postulated more than 50 years ago, provides a useful interpretation of the Arrhenius rate parameters in terms of molecular properties. These parameters contain practical information about rates, but they do not express the molecular details of a reaction. At this juncture, two types of questions can be raised--one concerning the effects of the environment on rates in condensed media, and the other, the purely molecular aspects of reactions in the absence of an environment, i.e. in an isolated molecular system. We restrict our attention to the latter case for the purposes of this review

Unimolecular reaction rates in solution and in the isolated molecule: Comparison of diphenyl butadiene nonradiative decay in solutions and supersonic jets

The recent study of diphenyl butadiene (DPB) in supersonic jets and in solution by Shepanski et al.(1) and by Courtney and Felming(2), respectively, provides an opportunity to compare the isomerization rates measured in the isolated molecule (jet) with those measured at very low viscosity in solution. These comparisons should shed light on the vibrational energy flows between “optical” and “reactive” modes in the isolated molecule and on the connection between activated, friction dependent, models of barrier crossing in solution,(3-5) and statistical RRK (or RRKM) theories of gas phase unimolecular reactions(6)

Exciton and vibronic effects in the spectroscopy of bianthracene in supersonic beams

Excitation and dispersed fluorescence spectra of 9,9’‐bianthracene in a supersonic expansion are reported. The spectra are anthracene‐like, indicating that the rings are weakly coupled. Exciton effects are considered in the interpretation of the spectra. The torsional potential in S_1 is modeled as a double‐well (Gaussian perturbation on a one‐dimensional harmonic oscillator) with barriers to perpendicularity and planarity of ∼30 and ∼1100 cm^(−1), respectively. The S_0 torsional potential shows negative anharmonicity which is modeled as a quartic perturbation. Anthracenic modes in S_1 and S_0 are also assigned. Finally, measurements of S_1 fluorescence lifetimes up to ∼6000 cm^(−1) excess energy in the excited state show no evidence of charge transfer

Picosecond photofragment spectroscopy. I. Microcanonical state-to-state rates of the reaction NCNO→CN+NO

This paper, the first in a series of three papers, gives a detailed account of our studies on picosecond photofragment spectroscopy. The unimolecular reaction NCNO→CN+NO is examined in detail here. Microcanonical state‐to‐state rates are measured in molecular beams at different energies in the reagent NCNO using pump–probe techniques: one picosecond pulse initiates the reaction from an initial (v,J) state and a second pulse, delayed in time, monitors the CN radical product in a specific rovibrational state, or the reagent NCNO (transient absorption). The threshold energy for reaction is determined to be 17 083 cm^(−1) (bond energy=48.8 kcal/mol). Measured rates are found to be sharply dependent on the total energy of the reagent, but independent of the rotational quantum state of product CN. Results of transient absorption measurements are used to argue that the ground statepotential energy surface dominates the reaction in the range of excess energies studied. The energy dependence of the rates, k_(MC)(E), is compared with that predicted by statistical theories. Both standard RRKM (tight transition state) and phase space theory (loose transition state) fail to reproduce the data over the full range of energies studied, even though nascent product state distributions are known to be in accord with PST at these energies. Furthermore, k_(MC)(E) is not a strictly monotonically increasing function of energy but exhibits some structure which cannot be explained by simple statistical theories. We advance some explanations for this structure and deviations from statistical theories

Real-time picosecond clocking of the collision complex in a bimolecular reaction: The birth of OH from H+CO_2

Picosecond (and femtosecond) photofragment spectroscopy has recently provided time-resolved, state-to-state dynamics of molecular photofragmentation. The focus of these experiments was on unimolecular reactions, where two main issues are fundamental to the dynamics: the nature of the "half-collision" and the degree to which statistical theories account for the time evolution of product state distributions (PSDs)

Application of unimolecular reaction rate theory for highly flexible transition states to the dissociation of NCNO into NC and NO

A recently described method for implementing RRKM theory for unimolecular reactions with highly flexible transition states is applied to the calculation of energy and angular momentum resolved rate constants and rotational–vibrational energy distributions for the reaction NCNO-->h nu NCNO*-->NCNO(vib. hot)-->NC+NO. The dissociation rate results are compared to the recent experimental results of Khundkar et al., and the vibrational and rotational distribution results are compared to the experimental values of Nadler et al. Comparison is also made with phase space theory calculations. The calculated rotational distributions at energies below the vibrational threshold of the products are the same as those of PST. At energies (2348, 2875 cm^−1) above this threshold energy the rovibrational distribution is in better agreement with the data than is that of PST. The need for obtaining more accurate ab initio potential energy surfaces is noted, particularly for treating reactions at still higher energies