Structure, energetics, and reactions of alkali tetramers.

Electronic structure calculations have been carried out for all possible alkali tetramers that can be formed from X(2) + X(2) → X(2)X(2), X(2) + Y(2) → X(2)Y(2), and XY + XY → X(2)Y(2) alkali dimer association reactions. Vibrationally stable rhombic (D(2h)) and planar (C(s)) structures are found for all possible tetramers formed from the alkali metals, Li to Cs. All tetramer formation reactions (from ground state singlet homonuclear or heteronuclear dimers) are found to be exothermic with binding energies ranging from 6282 cm(-1) for Li(2)Li(2) to 1985 cm(-1) for Cs(2)Cs(2). Extensive calculations, carried out at long-range for several reactant pairs, indicate that there are barrier-less pathways for the formation of tetramers from dimer association reactions. At low temperatures, direct formation of tetramers is unlikely, owing to the large exothermicity associated with these association reactions, but atom exchange reactions (X(2) + Y(2) ↔ XY + XY) are possible for some species.

Long range intermolecular interactions between the alkali diatomics Na(2), K(2), and NaK.

Long range interactions between the ground state alkali diatomics Na(2)-Na(2), K(2)-K(2), Na(2)-K(2), and NaK-NaK are examined. Interaction energies are first determined from ab initio calculations at the coupled-cluster with singles, doubles, and perturbative triples [CCSD(T)] level of theory, including counterpoise corrections. Long range energies calculated from diatomic molecular properties (polarizabilities and dipole and quadrupole moments) are then compared with the ab initio energies. A simple asymptotic model potential E(LR)=E(elec)+E(disp)+E(ind) is shown to accurately represent the intermolecular interactions for these systems at long range.

Rotational spectrum, tunneling motions, and potential barriers of benzyl alcohol.

The rotational spectra of benzyl alcohol and of its OD isotopologue have been assigned and measured in a supersonic expansion, either with pulsed-jet Fourier transform microwave or free jet absorption millimeter wave spectroscopy. The spectrum is consistent with a gauche conformation of the oxygen atom, characterized by a theta (OC(7)-C(1)C(2)) dihedral angle of approximately 55 degrees. Such a configuration is 4-fold degenerate, corresponding to minima with theta approximately +/-60 degrees, +/-120 degrees. The four equivalent minima are separated by two kinds of barrier, corresponding to theta = +/-90 degrees, and 0 or 180 degrees. Only the theta = +/-90 degrees barriers are low enough to generate a tunneling splitting, which has been measured in a spectrum strongly perturbed by tunneling interactions. The observed splittings diminish considerably upon deuterium substitution. The tunneling splittings are consistent with a barrier about 280 cm(-1) and high level ab initio calculations predicting a 320 cm(-1) barrier.

Two conformers observed and characterized in isobutylbenzene.

The microwave spectrum of isobutylbenzene (2-methyl-1-phenylpropane) reveals the presence of two conformers that are characterized by their microwave spectra and by quantum chemical calculations. The more stable conformer has a gauche configuration of the C(phenyl)-C(1)-C(2)-H chain coupled with a approximately 80 degrees dihedral angle between the phenyl group and the C(phenyl)-C(1)-C(2) plane with C(1) symmetry. The less stable conformer has a plane of symmetry, C(s), with an anti configuration of the C(phenyl)-C(1)-C(2)-H chain coupled with a 90 degrees dihedral angle between the phenyl group and the C(phenyl)-C(1)-C(2) plane. The rotational constant values are 3070.9273(4) MHz, 736.01980(6) MHz, and 680.92889(6) MHz for the C(1) species and 2500.780(8) MHz, 885.72743(10) MHz, and 770.42036(10) MHz for the C(s) species. Quantum chemical calculations are in agreement with these structures and predict a relative energy between those two conformers of 0.4 kcal/mol.