Control of conformers combining cooling by supersonic expansion of seeded molecular beams with hexapole selection and alignment: experiment and theory on 2-butanol.

Selection and alignment of rotamers and, more in general, of conformers in the gas phase is a challenge that we tackle experimentally by supersonic expansion of seeded molecular beams and hexapolar electrostatic fields with quadrupole mass detection. The studied system involves the nine conformers of the asymmetric-top molecule 2-butanol, which coexist because of nearly free rotations around a CC and a CO bond. From the measured time-of-flight of a 2-butanol supersonic molecular beam seeded in either He or Ar, the corresponding velocity distributions are obtained. The different nature and masses of the seeding gas decrease selectively the vibrational temperature and determine the population of the conformers, which is assessed on the basis of their statistical distribution, derived from high level accompanying quantum mechanical calculations. The use of a hexapolar electrostatic field permits us to induce a variation of the population distribution as a function of the applied voltage and of the selective focusing and alignment of the conformers. A technique, recently developed for treating asymmetric tops and involving extensive trajectory simulations, is applied to obtain the link between the focusing curves, i.e. the dependence of the beam intensity on the hexapole voltage, and the conformers' populations and alignment. Perspectives are provided for photo- and stereo-dynamics experiments, particularly appealing also on account that 2-butanol is the simplest chiral alcohol.

Quantum chemistry of C(3)H(6)O molecules: structure and stability, isomerization pathways, and chirality changing mechanisms.

Electronic structure calculations were carried out to study the various isomers of formula C(3)H(6)O, as a part of our current quantum chemical and dynamical approaches to intra- and intermolecular kinetics for the C(n)H(2n)O (n = 1, 2, 3) molecules. The usefulness of the GRRM (global reaction route mapping) program developed by Ohno and Maeda in predicting the structure of all isomers and of the transition states connecting them is fully exploited. All the isomers are identified as local minima on the MP2/CC-PVDZ potential energy surface. Acetone is the most stable isomer. In increasing order of stability the others are propanal, 2-propenol, 1-propenol, allyl alcohol, methyl vinyl ether, cyclopropanol, propylene oxide, and oxetane. Various isomerization paths connecting them are identified. All the transition states are fully characterized using intrinsic reaction coordinate calculations. The isomerization reactions may proceed through a single step or involve an intermediate species which is either a carbene or a diradical. Special attention is devoted to propylene oxide, a favorite molecule in current photochemical and stereodynamical studies because of its chiral nature. It is a rigid molecule, and chirality switching is found to be supported by its isomers. Two different chirality switching mechanisms which are assisted by propanal and allyl alcohol are presented.

Intramolecular dynamics of RS-SR' systems (R, R' = H, F, Cl, CH3, C2 H5): torsional potentials, energy levels, partition functions.

The equilibrium and cis and trans geometric structures of a series of disulfides RS-SR' (HSSH, HSSF, HSSCl, HSSCH3, HSSC2H5, FSSF, ClSSCl, ClSSF, CH3SSCH3, C2H5SSC2H5) have been studied using both density functional theory (B3LYP/6-311++G(3df, 3pd)) and second-order Møller-Plesset perturbation theory (MP2(FU)/aug-cc-pVTZ). The effects of internal rotation on the structural parameters have been analyzed and the torsional potentials around the S-S bond have been compared with experimental and theoretical information, when available, as well as with the analogues of the peroxydic family. Torsional levels were calculated, and their distribution as a function of temperature was determined, obtaining partition functions. This information is of interest for statistical approaches to equilibrium properties and to rates of processes in which torsional anharmonicity is relevant. It is also required for recent atmospheric modeling studies and also for prototypical chiral separation experiments, in view of a possible dynamic mechanism for chirality exchange by molecular collisions. In general, barriers for such processes are higher than for the corresponding peroxides, and accordingly, rates for chirality change are estimated to be consistently smaller.

Level distributions, partition functions, and rates of chirality changing processes for the torsional mode around O-O bonds.

In view of the particular attention recently devoted to hindered rotations, we have tested reduced kinetic energy operators to study the torsional mode around the O-O bond for H(2)O(2) and for a series of its derivatives (HOOCl, HOOCN, HOOF, HOONO, HOOMe, HOOEt, MeOOMe, ClOOCl, FOOCl, FOOF, and FOONO), for which we had previously determined potential energy profiles along the dihedral ROOR(') angle [R,R(')=H,F,Cl,CN,NO,Me (=CH(3)), Et (=C(2)H(5))]. We have calculated level distributions as a function of temperature and partition functions for all systems. Specifically, for the H(2)O(2) system we have used two procedures for the reduction in the kinetic energy operator to that of a rigid-rotor-like one and the calculated partition functions are compared with previous work. Quantum partition functions are evaluated both by quantum level state sums and by simple classical approximations. A semiclassical approach, using a linear approximation of the classical path and a quadratic Feynman-Hibbs approximation of Feynman path integral, introduced in previous work and here applied to the torsional mode, is shown to greatly improve the classical approximations. Further improvement is obtained by the explicit introduction of the dependence of the moment of inertia from the torsional angle. These results permit one to discuss the characteristic time for chirality changes for the investigated molecules either by quantum mechanical tunneling (dominating at low temperatures) or by transition state theory (expected to provide an estimate of racemization rates in the high energy limit).

A quantum chemical study of H2S2: Intramolecular torsional mode and intermolecular interactions with rare gases.

The structural and energetic properties of the H(2)S(2) molecule have been studied using density functional theory, second-order Moller-Plesset method, and coupled cluster theory with several basis sets. In order to extend previous work on intra- and intermolecular dynamics of the chirality changing modes for H(2)O(2) and its derivatives, our focus has been on the torsion around the S-S bond, along with an extensive characterization of the intermolecular potentials of H(2)S(2) with the rare gases (He, Ne, Ar, and Kr). Use is made of previously defined coordinates and expansion formulas for the potentials which allow for a faithful representation of geometrical and symmetry properties of these systems that involve the interaction of an atom with a floppy molecule. The potential energy surfaces obtained in this work are useful for classical and quantum mechanical simulations of molecular collisions responsible for chirality changing processes of possible interest in the modeling of prebiotic phenomena.

Quantum study of peroxidic bonds and torsional levels for ROOR' molecules (R, R'=H, F, Cl, NO, CN).

We present here a systematic study by quantum mechanical methods of a series of molecules (HOOF, HOOCl, HOONO, HOOCN, FOOF, ClOOF, ClOOCl, and FOONO), corresponding to substitutions of one or both hydrogens in hydrogen peroxide. The emphasis is on the structural and energetic properties and on the features of the internal modes, in particular, the torsion around the O-O bond, which leads to the chirality changing isomerization. The cis and trans barriers appear to vary remarkably upon substitution by halogen groups. They are compared with experimental and theoretical information, when available, and analyzed by reference to a previous systematic analysis of the effects of alkyl substitutions. Torsional levels were calculated, and their distribution as a function of temperature was determined. This information is of interest for statistical approaches to equilibrium properties and to rates of processes where torsional anharmonicity is relevant, as required for recent atmospheric modeling studies and also for prototypical chiral separation experiments, in view of a possible dynamic mechanism for chirality exchange by molecular collisions. Dipole moments are also presented.

The hydrogen peroxide-rare gas systems: quantum chemical calculations and hyperspherical harmonic representation of the potential energy surface for atom-floppy molecule interactions.

A quantum chemical exploration is reported on the interaction potentials of H2O2 with the rare gases, He, Ne, Ar, Kr, and Xe. Hydrogen peroxide (the simplest example of chiral molecule in its equilibrium geometry) is modeled as rigid except for the torsional mode around the O-O bond. However, on the basis of previous work (Maciel, G. S.; et al. Chem. Phys. Lett. 2006 432, 383), the internal mode description is based, rather than on the vectors of the usual valence picture, on the orthogonal local representation, which was demonstrated useful for molecular dynamics simulations, because the torsion around the vector joining the center-of-mass of the two OH radicals mimics accurately the adiabatic reaction path for chirality changing isomerization, following the torsional potential energy profile from equilibrium through the barriers for the trans and cis geometries. The basic motivation of this work is the determination of potential energy surfaces for the interactions to be used in classical and quantum simulations of molecular collisions, specifically those leading to chirality changes of possible relevance in the modeling of prebiotic phenomena. Particular attention is devoted to the definition of coordinates and expansion formulas for the potentials, allowing for a faithful representation of geometrical and symmetry properties of these systems, prototypical of the interaction of an atom with a floppy molecule.