Brian E. Applegate

The formation of cyclic water complexes by sequential ring insertion: Experiment and theory

The growth of water clusters in liquid helium droplets results in the formation of cyclic structures up to and including the hexamer. In view of the sequential nature of the molecular pick-up process, the formation of water rings involves the insertion of water monomers into preformed cyclic water clusters. The implication of this observation is that the barriers to the ring insertion process are low enough to be overcome during the experiment. This paper presents a combined experimental and theoretical effort to explore the insertion process in detail. Our results provide important new insights into the dynamics of hydrogen-bonded networks. We map out the cluster potential energy surfaces and visualize them using disconnectivity graphs. Nonequilibrium walks on these surfaces show that ring water clusters can be formed during sequential addition of water molecules by surmounting small barriers that are thermally accessible even at the low temperature of the experiment. We find that the effects of zero-poi...

Calculation of the Jahn-Teller effect in benzene cation: Application to spectral analysis

Ab initio calculations have been performed for the cations of benzene, C6H6, and its fluorinated analogs, C6F6 and C6H3F3. Calculated molecular parameters characterizing the Jahn-Teller potential energy surface (PES) are very consistent with those derived from the spectra of C6F6+ and C6H3F3+. However the calculated Jahn-Teller stabilization energy for the benzene cation is roughly three times greater than that previously reported experimentally. With the aid of the calculated values, a more complete analysis of the available spectral data for C6H6+ and C6D6+ is performed, with an emphasis on the data from ZEKE experiments and IR spectra of the Ar⋅C6H6+, Ne⋅C6H6+, and Ar⋅C6D6+ complexes. The comprehensive analysis reveals Jahn-Teller activity in 3 e2g modes for C6(H/D)6+ and provides values for their vibrational frequencies, linear and quadratic Jahn-Teller coupling constants, as well as quadratic coupling constants for several other degenerate modes. These new molecular parameters are generally in good a...

The Jahn–Teller and related effects in the cyclopentadienyl radical. I. The ab initio calculation of spectroscopically observable parameters

Ab initio calculations are performed for the X 2E1″ and A 2A2″ states of the cyclopentadienyl radical. An important goal of these calculations is to guide the analysis of the experimentally observed A 2A2″– X 2E1″ electronic spectrum. Vibrational frequencies for both the X and A state are reported. Large changes in frequency between the states for out-of-plane vibrations are found, leading to the expectation that overtones of these modes will appear strongly in the spectrum. Additionally, spectroscopically obtainable parameters describing the Jahn–Teller effect are calculated for the X state. Using all this information the X−A electronic spectrum is predicted for both C5H5 and C5D5.

The Jahn–Teller and related effects in the cyclopentadienyl radical. II. Vibrational analysis of the à 2A2″–X̃ 2E1″ electronic transition

The laser excited, jet-cooled A 2A2″–X 2E1″ electronic spectrum of the cyclopentadienyl radical yields detailed information about the vibronic structure of both its A and X states. A straightforward assignment of the A vibronic structure is presented. The X state vibronic structure reveals a comprehensive picture of the Jahn–Teller distortion of its potential energy surface. The molecular parameters characterizing the Jahn–Teller interaction provide the stabilization energy and distorted geometry, which are compared to previous experimental and ab initio results.

Explorations of conical intersections and their ramifications for chemistry through the Jahn–Teller effect

Much recent progress has been made theoretically and computationally towards understanding the importance of conical intersections for chemical reactions. Nonetheless, experimental characterization of conical intersections has proven extremely difficult with one striking exception: the Jahn-Teller conical intersection. This article overviews the fundamental similarity of a variety of conical intersections and demonstrates how the spectroscopy of Jahn-Teller active molecules can be used to characterize them. Specific results are reviewed for four representative Jahn-Teller active molecules, C5H5, C6H6+, Ag3 and CH3O.

Photofragmentation dynamics of the thiomethoxy radical

The radiative and nonradiative decay of the A 2E electronic state of the CH3S radical has been investigated using a variety of experimental techniques. Lifetimes have been measured for a number of vibrational levels; these data have been analyzed along with similar results previously obtained for other methoxy radical family members, CH3O, CD3O, and CF3S. It is concluded that the totality of the data is best described by a model which postulates mode-selective fragmentation into a methyl (or fluoromethyl) radical and an O or S atom. For CH3S there appears a second nonradiative decay channel, possibly producing H+CH2S.

Competition between radiation and photofragmentation in the à 2Σ+ state of the SH/D rare gas complexes

The natural lifetimes of a large number of the vibrational levels of the excited A 2Σ+ electronic state of the family of rare gas complexes, R⋅SH (R=Ne, Ar, and Kr) and their deuterides, are reported. It is well known that the natural lifetime of the A 2Σ+ state of isolated SH/D is markedly shortened by a photofragmentation process. Our results for the complexes show that the rare gas atom plays an important role in inhibiting this process. From a classical model of the molecular system we are able to explain the trends observed in our lifetime data. The data from the R⋅SD complexes where for some vibrational levels the deuterium atom appears to be trapped between the rare gas and sulfur atoms allows us to establish a radiative lifetime for these complexes and the SH/D monomer.