We present the theoretical foundation of a novel technique for the orientation of ∑-molecules and symmetric tops with permanent electric dipole moment. The method bases on the adiabatic transformation of free rotational states into those of librational oscillations taking place during the passage of a molecule into a strong electric field. Several examples of calculated spatial distributions of the molecular axis are given. The computational results demonstrate that in connection with highly relaxed supersonic nozzle beams only moderately strong electric fields are required to generate a marked orientation of the axis. In a first application of this technique we studied steric effects of the reaction K+CH3I→KI+CH3 at two elevated collision energies (0.79 and 1.24 eV). All observed steric effects could be rationalized in terms of a simple impulsive reaction model. We find that backward scattered products are more likely formed if the approaching atoms face the I-end of CH3I—in agreement with low energy results—while forward scattered products are favorably formed if the CH3-end is encountered.
The concept of directional axis distributions and orientation-dependent reaction cross sections is used to describe the effect of the mutual orientation of reagents on the outcome of reactive beam collisions. The axis distributions and cross sections are expanded in series of Legendre polynomials and real spherical harmonics, respectively, and characterized by the expansion coefficients (moments). The interrelations between the moments of the cross sections and the directionally dependent experimental data (steric effects) on the one hand and the anisotropic properties of the potential energy surfaces on the other hand are presented. Recent progress in the field of dynamical stereochemistry results from the prep- aration of molecular orientation and alignment by using the novel brute force technique and an optical method, respectively. The theories of both methods are summarized, and typical experimental arrangements are presented. All experimental results based on these techniques are reviewed. Among the most important ones are the first orientation effects observed in a reaction with a (1)Σ: molecule (K + ICl) and the alignment effects found in the bench mark reaction Li + HF → LiF + H.
Results of a crossed beam study on the state specific reaction Li+HF (v=1, j=1, m=0)→LiF+H at a translational collision energy of Etr=0.42 eV are reported. Angular distributions of LiF have been measured for three different distributions of the internuclear axis of HF, namely an isotropic one, one where the axis is aligned with the relative velocity of reagents V and one where the axis is aligned perpendicular to both V and the scattering plane. We find a marked influence of these collision geometries (steric effects) on (i) the angular distributions; (ii) the partition of available energy; and (iii) the integral reaction cross sections. The ratio of the latter for preferred side-on and end-on collisions with HF amounts to 1.76. From the angular distributions of products, double-differential cross sections in the center-of-mass frame are determined which exhibit in all three cases preferred backward scattering of LiF. They provide three out of four accessible moments of the orientation-dependent double-differential reaction cross section. The results are compared to quasiclassical trajectory calculations based on the potential energy surface of Chen and Schaefer [J. Chem. Phys. 72, 4376 (1980)] and to predictions of a modified direct interaction with product repulsion (DIPR) model. The latter suggests that the stringent correlation between the electric dipole moment d (the synonym for molecular axis) and the direction in which the products are ejected is relaxed and both a reorientation of the molecular axis during the approach of reagents and an interaction between the products during separation play an important role. These conclusions are supported by trajectory calculations.
The versatility of the brute force orientation of polar asymmetric top molecules in a molecular beam has been investigated. In symmetric top molecules the electric field only mixes free rotor basis functions with different J but equal K and M values, but in asymmetric top molecules the mixing includes K, because of the asymmetry, in addition to field-induced J-mixing. This distinction is important with respect to the orientation behavior. For asymmetric top molecules all Stark curves for different J-states and different K-values, but equal M, in the corresponding symmetric eigenbasis, feature avoided crossings. Dependent on the velocity with which the molecules pass through the orientation field, these avoided crossings will be traversed adiabatically or nonadiabatically. For near-symmetric top molecules, such as iodobenzene, the crossings will in general be nonadiabatic, and, as expected, the behavior is similar to that of the corresponding symmetric top. If the crossings are adiabatic, the orientation behavior can be drastically different from the behavior of the corresponding symmetric top molecule. A strong asymmetry need not always be prohibitive in attaining a perceptible degree of orientation, as is demonstrated by the case of water.
Laboratory angular distributions (LAB ADs) have been measured for the Li ] HF (v \ 1, j \ 1) reaction in a crossed molecular beam experiment at the collision energies 0.231 eV and 0.416 eV and compared with the results of extensive quasi-classical trajectory (QCT) calculations performed on the most recent ab initio potential energy surface (PES) for this system. The calculations also include the collision energy dependence of the integral and di †erential cross sections in the range 0.025È0.5 eV (2.4È48.2 kJ mol~1). In particular, the total QCT integral reactive cross sections have been found to be in very good agreement with recent quantum mechanical (QM) calculations carried out on the same PES by Lara et al. (J. Chem. Phys., 1998, 109, 9391). In addition, the triple scattering angleÈrecoil velocity di †erential cross section has been calculated in order to simulate the experimental LAB AD. An excellent concordance between both sets of data has been found, indicating that the reaction of Li with HF in v \ 1 can be very well described by QCT calculations on the mentioned PES.
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