Correlated ab initio molecular orbital (MP3, MP4) and density functional (PW91, MPW91) studies on the conformations of 1,2-diphenylethane

Isomerization kinetics of flexible molecules in the gas phase: Atomistic versus coarse-grained sampling

Isomerization kinetics of molecules in the gas phase naturally falls on the microcanonical ensemble of statistical mechanics, which for small systems might significantly differ from the more traditional canonical ensemble. In this work, we explore the examples of cis-trans isomerization in butane and bibenzyl and to what extent the fully atomistic rate constants in isolated molecules can be reproduced by coarse-graining the system into a lower dimensional potential of mean force (PMF) along a reaction coordinate of interest, the orthogonal degrees of freedom acting as a canonical bath in a Langevin description. Time independent microcanonical rate constants can be properly defined from appropriate state residence time correlation functions; however, the resulting rate constants acquire some time dependence upon canonical averaging of initial conditions. Stationary rate constants are recovered once the molecule is placed into a real condensed environment pertaining to the canonical ensemble. The effective one-dimensional kinetics along the PMF, based on appropriately chosen inertia and damping parameters, quantitatively reproduces the atomistic rate constants at short times but deviates systematically over long times owing to the neglect of some couplings between the system and the bath that are all intrinsically present in the atomistic treatment. In bibenzyl, where stronger temperature effects are noted than in butane, the effective Langevin dynamics along the PMF still performs well at short times, indicating the potential interest of this extremely simplified approach for sampling high-dimensional energy surfaces and evaluating reaction rate constants.

Dimerization of the Benzyl Radical in a High-Temperature Pyrolysis Reactor Investigated by IR/UV Ion Dip Spectroscopy

We investigate the self-reaction of benzyl, C₇ H₇ , in a high-temperature pyrolysis reactor. The work is motivated by the observation that resonance-stabilized benzyl radicals can accumulate in reactive environments and contribute to the formation of polycyclic aromatic hydrocarbons (PAHs) and soot. Reaction products are detected by IR/UV ion dip spectroscopy, using infrared radiation from the free electron laser FELIX, and are identified by comparison with computed spectra. Among the reaction products identified by their IR absorption are several PAHs linked to toluene combustion such as bibenzyl, phenanthrene, diphenylmethane, and fluorene. The identification of 9,10-dihydrophenanthrene provides evidence for a mechanism of phenanthrene formation from bibenzyl that proceeds by initial cyclization rather than an initial hydrogen loss to stilbene.

Computational Chemistry Meets Experiments for Explaining the Behavior of Bibenzyl: A Thermochemical and Spectroscopic (Infrared, Raman, and NMR) Investigation

The structure, conformational behavior, and spectroscopic parameters of bibenzyl have been investigated by a computational protocol including proper treatment of anharmonic and hindered rotor contributions. Conventional hybrid functionals overstabilize the anti conformer while low-order post-Hartree-Fock (MP2) approaches strongly favor the gauche conformer. However, inclusion of semiempirical dispersion effects in density functionals or coupled cluster post-Hartree-Fock models agree in forecasting the simultaneous presence of both conformers in the gas phase with a slightly larger stability (0.7 kcal·mol(-1)) of the gauche conformer. Addition of thermal and entropic effects finally leads to very close Gibbs free energies for both conformers and, thus, to a slight preference for the gauche form due to statistical factors (2 vs 1). The situation remains essentially the same in solution. On these grounds, perturbative vibrational computations including both electrical and mechanical anharmonicities lead to IR and Raman spectra in remarkable agreement with experiment. Full assignment of the IR spectra explains the presence of peaks from gauche or anti conformers. Comparison between computed and experimental Raman spectra confirms that both conformers are present in liquid phase, whereas the anti conformer seems to be preponderant in the solid state. Also computed NMR parameters are in good agreement with experiment.

Low-energy vibrational spectra of flexible diphenyl molecules: biphenyl, diphenylmethane, bibenzyl and 2-, 3- and 4-phenyltoluene

Gas phase spectra of six bi-phenyl molecules are reported allowing an accurate determination of their active low-frequency vibrational modes.

Internal rotation in 1,2-di-(p-XC6H4)ethanes (X=H, Br, NO2): infrared spectra and compensation effect

Infrared absorption spectra and internal rotation of 1,2-di-(p-XC(6)H(4))ethanes (X=H, Br, NO(2)) in crystalline phase, liquid and solutions at various temperatures have been investigated. Band fitting was applied to conformationally sensitive regions of the spectra, and assignment of the peaks to trans and gauche conformations was performed. Enthalpy and entropy differences of the conformers (deltaH(0) and deltaS(0)) were found to be solvent-dependent, and it is interpreted in terms of previously discovered compensation effect. The values deltaH(0) and deltaS(0) for 1,2-di-(p-NO(2)C(6)H(4))ethane obtained are unusually large.