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Density-functional theory has been applied to analyze the resonance Raman spectrum of the carotenoid all-trans-spheroidene. Besides natural-abundance spheroidene, 19 ²H- or ¹³C-containing isotopomers have been taken into consideration. Calculated frequencies are found to be in excellent agreement with experiment. In particular, the observed variation of vibrational frequencies with isotopic composition is nicely reproduced, which proves the assignments. A simple estimate of the resonance Raman intensities correctly predicts which vibrational transitions are resonance enhanced. Density-functional theory is well suited to describe the normal-mode structure of a molecule as large as a carotenoid.

A detailed comparison between three-dimensional classical surface hopping calculations and quantum mechanical calculations is presented for the photodissociation of water in the &Btilde; band. Accurate coupled diabatic potential energy surfaces are used in these calculations. Tully's “fewest switches” method using an adiabatic representation for the electronic states is applied for the surface hopping procedure. Studied are the energy dependence of the branching ratios for the possible fragmentation channels, including electronically nonadiabatic channels, and the probabilities for particular vibrational or rotational product states of the electronically excited OH(A) fragment. Although the classical results generally agree well with the quantum results, some serious errors in the classical calculations were detected. First, it is found that the calculated fractions for the O(¹D) + H2 and O(³P) + H + H fragments are too large. Second, the absence of quantization of the vibrational energy in classical mechanics has consequences for the details of the rotational product state distribution of the OH(A,v=0) fragments. This is important for the “single N phenomenon”, an experimentally observed strong preference for populating the highest rotational product state for which the rotational barrier energy is lower than the available energy (S. A. Harich, X. F. Yang, R. van Harrevelt, and M. C. van Hemert, Phys. Rev. Lett., 2001). For a two-dimensional model, where the above-mentioned problems of classical trajectory calculations do not occur, excellent agreement between classical and quantum results is found. Classical trajectories were followed to explain the single N phenomenon and the origin of the experimentally observed vibrational excitation of OH(A) fragments.

Rotational and vibrational state distributions have been measured for the OH/OD(A²Σ⁺) fragment produced by the photodissociation of H2O/D2O at a set of wavelengths in the 133−119 nm range that lead to excitation of the &Btilde;(¹A1) state, either directly or through particular absorption features of the &Ctilde;¹B1 ← &Xtilde;¹A1 and &Dtilde;¹A1 ← &Xtilde;¹A1 Rydberg transitions. The experimental distributions are obtained from the A²Σ⁺−X²Π fluorescence spectra using a truncated single value decomposition method (TSVD). They are compared to distributions calculated in a complete three-dimensional quantum mechanical treatment by using two sets of diabatic potential energy surfaces obtained by Dobbyn and Knowles (DK) and by van Harrevelt and van Hemert (Leiden). The measured vibrational branching ratios seem to be better accounted for by the former and the rotational distributions by the latter. However, a more detailed analysis indicates that the DK surface would presumably account for the experiment if the room-temperature rotational distribution of the parent water molecule were introduced in the calculation. The experimental results are therefore interpreted as a confirmation of the more direct and shorter-lived trajectories favored by the DK surface.

Molecular dynamics studies of liquids consisting of either 1,4-dioxane or 1,3-dioxane molecules are performed with the aim to understand why and how liquid 1,4-dioxane may act as a polar solvent in solute−solvent interactions, despite the lack of a permanent dipole moment of the solvent molecules. An intermolecular pair potential is designed such that static quantities as density and radial distribution function, as well as dynamic quantities as diffusion coefficient and rotation correlation functions, are correctly recovered from the simulations, both for liquid 1,4-dioxane and for liquid 1,3-dioxane. An additional test is the comparison of the dielectric constant of liquid 1,3-dioxane from experiment (εexp = 13.6) with the value derived from the MD simulated reaction field and liquid density (εcalc = 12.8). In these simulations, a 1,3-dioxane molecule is selected as a dipolar probe, whose dipole moment (μprobe) is then changed artificially by varying its site charges. The reaction field F at the center of the dipolar probe, arising from orientation polarization of the solvent molecules, is calculated. This yields the dielectric constant by applying Onsager's expression for the reaction field. When the dipolar 1,3-dioxane is embedded in liquid 1,4-dioxane, a simulated field F is obtained, whose magnitude is of the same order of magnitude as Onsager's reaction field in polar organic solvents. In both liquids the simulated field F arises nearly entirely from the local dipoles of the solvent molecules encountered in the first shell around the dipolar probe. An effective dielectric constant of liquid 1,4-dioxane of about 7 is obtained, which may be used to calculate solvatochromic shifts of dipolar solutes in this solvent on the basis of Onsager's reaction field, provided that no specific solute solvent interactions are involved.

We calculate isotope shifts and perform a normal-coordinate analysis to explain the observed systematic variation with isotopic composition of the intersystem crossing for deuterated pentacene in p-terphenyl. The tuning of a near resonance of the first excited singlet state S1 and vibrational levels of T3 by S1 ← T3 isotope shifts is analyzed. The physics of isotope shifts is discussed, and an explanation is given for the phenomenological sum rule dictating additivity of isotope shifts upon multiple substitution. We discuss mechanisms through which the host matrix interactions influence isotope shifts and increase inhomogeneous broadening.