Structures, charge distributions, and dynamical properties of weakly bound complexes of aromatic molecules in their ground and electronically excited states (original) (raw)
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Berichte der Bunsengesellschaft/Physical Chemistry Chemical Physics
The method previously presented for mass-selective ground-state vibrational spectroscopy of aromatic van der Waals complexes by populating ground-state levels via a pumpldump laser pulse sequence, followed by selective resonant two-photon ionization (R2PI) of the vibrationally relaxed complexes [T. Burgi et al., Chem. Phys. Lett., 225, 351 (1994)l is here extended to the detection of the aromatic molecular product from the So state vibrational predissociation (VP) process. Schemes and results are presented which allow (a) extended measurements of intraand intermolecular vibrational frequencies of carbazole.Ar in the So state, (b) detection of vibrationally hot, but rotationally cold carbazole VP product, and (c) vibrationally and rotationally hot carbazole VP products. A much improved determination of the van der Waals dissociation energy was made by combining results from various experiments, as Do(S,) = 530k1.5 cm-'.
Structural Characterization of Aromatic−Aromatic Complexes by Rotational Coherence Spectroscopy
The Journal of Physical Chemistry A, 1998
Rotational coherence spectroscopy (RCS) has been applied in structural studies of (a) aromatic-aromatic van der Waals complexes of the form M-X, where M ) perylene or fluorene and X ) benzene or toluene, and (b) aromatic-aliphatic hydrocarbon dimers of the form M-Y, where M is as above and Y ) cyclohexane or methylcyclohexane. For all of the perylene complexes the experimentally determined rotational constants are found to be consistent with centrally bound, parallel-stacked structures in which the monomer planes are separated by a distance of 3.5-4.3 A. Analogous geometries also characterize the fluorene-aliphatic species. However, the two fluorene-aromatic complexes have structures that depart from the parallel-stacked form. Both species have slipped geometries in which the monomer planes are not directly over one another. And, in the case of fluorene-benzene, the two aromatic planes are tilted with respect to one another. These differences are attributed to the significant contribution of electrostatic forces in determining the fluorenearomatic geometries. † Present address:
2005
Rotational spectroscopic studies on weakly bound complexes have provided accurate structural information on these complexes. This review summarizes rotational spectra of weakly bound H 2 O and H 2 S complexes, with C 6 H 6 , C 2 H 4 and Ar m (m=2,3) and the structural information obtained from the spectra. The equilibrium structures of these complexes are strikingly similar, with the O-H or S-H interacting with the p cloud in C 6 H 6 /C 2 H 4 or with Ar m . The Ar 3 -H 2 X and C 6 H 6 -H 2 X are both symmetric tops, despite H 2 X (X = O,S) being asymmetric tops. Both C 2 H 4 -H 2 X and Ar 2 H 2 X are asymmetric tops, mainly due to the lower symmetry in C 2 H 4 /Ar 2 compared to C 6 H 6 /Ar 3 . In all these complexes, H 2 X does not contribute to the moment of inertia along the axis perpendicular to its partner’s molecular plane (for C 6 H 6 , C 2 H 4 and Ar 3 ) or axis (for Ar 2 ). Ab initio electronic structure theory calculations at the state-of-the-art level show that the structu...
The Journal of Physical Chemistry B, 2001
Rotationally resolved S 1 rS 0 fluorescence excitation spectra of three methylindoles and their single atom van der Waals complexes with argon have been obtained. Each spectrum is extensively perturbed by the hindered internal rotation of the methyl group. Analyses of these perturbations show that the barriers to such motion are substantially increased by complex formation. Barriers of this type are primarily electronic in origin. Thus, even at the relatively large distances (3.5 Å) found in van der Waals complexes, the attachment of a weakly bound argon atom has a significant effect on the electron distribution in the indole ring.
Journal of Physical Chemistry A, 1997
The S 2 -S 0 ( 1 L a ) fluorescence excitation and emission spectra of the van der Waals complexes of three azulene (Az) derivatives, 2-chloroazulene (ClAz), 2-methylazulene (MAz), and 1,3-dimethylazulene (DMAz), with the rare gases, Ar, Kr, and Xe, have been measured under jet-cooled conditions. The microscopic solvent shifts, δν j, of the origin bands in the S 0 -S 2 spectra associated with complexation of the chromophores with one and two rare gas atoms increase with increasing polarizability of the adatom(s), consistent with the dominance of dispersion in the binding. Although there are substantial variations in the relative values of δν j among the Az derivatives examined, all of the δν j values are relatively small and are similar to those of the 1 L b (S 0 -S 1 ) transitions in the rare gas complexes of naphthalene and its methyl-substituted derivatives. The theory of microscopic solvent shifts of Jortner et al. has been used to analyze the solvent shift data. Comparisons of the sources of the oscillator strengths and van der Waals binding interactions in the azuleneand naphthalene-rare gas systems are revealing and suggest that the variations in δν j with substitution pattern are primarily electronic in their origin and arise from variations in excited state configuration interactions, the magnitude of which depend on the S 2 -S n energy spacings. These spacings can be varied by placing substituents either along the long axis (2-position) or parallel to the short axis( 1,3-positions) so that they selectively perturb, respectively, the long axis polarized and the short axis polarized transitions. The structures and binding energies of the complexes of these derivatives have also been modeled using Lennard-Jones type calculations and have been compared with those of Az itself. The observed progressions in the low-frequency intermolecular vibrations in each case are assigned to that excited state bending mode which is parallel to the long axis of the chromophore, in agreement with model calculations using one-dimensional Morse and Taylor's series potential functions.
Atom—atom potential parameters for van der Waals complexes of aromatics and rare-gas atoms
Chemical Physics Letters, 1990
The structure of the dispersion terms in the interaction for van der Waals complexes of aromatics and we-gas atoms is examined and found to contain important three-body terms. Using calculations for benzene with a sequence of rare gases, the errors in approximating the three-body lr-electron dispersion energy by atom-atom terms are found to be generally s&o&ant, but small for certain symmetrical positions (including the equilibrium position). For these positions, the total interaction coefficients are compared with empixically determined values using a Lennard-Jones model. Agreement is good after waling to allow for higherorder terms.
The Journal of Chemical Physics, 1995
An expression for the interaction potential between two anisotropic molecules is derived. This expression is suitable for describing the van der Waals interaction between two chromophores within a bichromophoric molecular cluster. For the anthracene-naphthalene cluster the calculation predicts the existence of two isomers, in agreement with experimental observations. The model is also successfully applied to other clusters yielding better results than those obtained by alternative methods which do not take into account the anisotropy of molecular polarizability.
2016
The dissociation energy (D 0) of an isolated and cold molecular complex in the gas-phase is a fundamental measure of the strength of the intermolecular interactions between its constituent moieties. Accurate D 0 values are important for the understanding of intermolecular bonding, for benchmarking high-level theoretical calculations and for the parametrization of force-field models used in fields ranging from crystallography to biochemistry. We review experimental and theoretical methods for determining gas-phase D 0 values of M•S complexes, where M is a (hetero)aromatic molecule and S is a closed-shell "solvent" atom or molecule. The experimental methods discussed involve M-centered (S 0 → S 1) electronic excitation, which is often followed by ionization to the M + •S ion. The D 0 is measured by depositing a defined amount of vibrational energy in the neutral ground state, giving M ̸ = •S, the neutral