David C. Clary

Definition of the hydrogen bond (IUPAC Recommendations 2011)

A novel definition for the hydrogen bond is recommended here. It takes into account the theoretical and experimental knowledge acquired over the past century. This definition insists on some evidence. Six criteria are listed that could be used as evidence for the presence of a hydrogen bond.

Defining the hydrogen bond: An account (IUPAC Technical Report)

The term “hydrogen bond” has been used in the literature for nearly a century now. While its importance has been realized by physicists, chemists, biologists, and material scientists, there has been a continual debate about what this term means. This debate has intensified following some important experimental results, especially in the last decade, which questioned the basis of the traditional view on hydrogen bonding. Most important among them are the direct experimental evidence for a partial covalent nature and the observation of a blue-shift in stretching frequency following X–H···Y hydrogen bond formation (XH being the hydrogen bond donor and Y being the hydrogen bond acceptor). Considering the recent experimental and theoretical advances, we have proposed a new definition of the hydrogen bond, which emphasizes the need for evidence. A list of criteria has been provided, and these can be used as evidence for the hydrogen bond formation. This list is followed by some characteristics that are observed in typical hydrogen-bonding environments.

Potential optimized discrete variable representation

Abstract A new discrete variable representation (DVR) is proposed where the eigenstates of one-dimensional reference Hamiltonians are used to obtain the DVR localized basis functions. The method is applied to two model vibrational problems, the two- and three-dimensional Henon—Heiles potentials, for which it is found to be much more efficient than the usual DVR and distributed Gaussian basis methods.

Quantum reactive scattering of four‐atom reactions with nonlinear geometry: OH+H2→H2O+H

A quantum mechanical method is described for calculating state‐selected cross sections and rate constants for four‐atom reactions of the general form AB+CD→ABC+D with nonlinear geometry. The method involves using hyperspherical coordinates to describe the BC and CD bonds, accounting for both the rotation of the AB molecule and the bending mode of the ABC molecule with a spherical harmonic basis set, holding the AB spectator bond length fixed and applying a version of the bending‐corrected‐rotating line approximation to treat the rotation of the CD molecule. The method is applied to the OH(j)+H2(v)→H2O(n,m)+H reaction, and its reverse reaction, where v and j are vibrational and rotational quantum numbers, and n and m label bending and local OH‐stretching vibrational states of the H2O molecule. A modified potential energy surface based on a fit to ab initio data is used. Comparisons of the calculated cross sections are made with quasiclassical trajectory calculations. The effect of the bending and stretchin...

Rates of chemical reactions dominated by long-range intermolecular forces

Some simple theories are described for calculating rate constants for neutral chemical reactions in the gas phase which are dominated by long-range forces and have no barriers in the potential energy surface. The methods involve combining an adiabatic capture approach with the quantum mechanical sudden approximation of non-reactive energy transfer. A version of the technique is shown to work well in calculations on the O + OH reaction. The methods are applied to the diatom-diatom reactions SO + OH, NO + BrO, NO + ClO, CN + O2 and CH + O2. The results provide new information on the origin of the negative temperature dependence of the rate constants for reactions such as these.

Calculations of rate constants for ion-molecule reactions using a combined capture and centrifugal sudden approximation

Rate constant calculations are reported for the gas-phase reactions of HCN with H-, D- and H+ 3 ions. The technique used in the computations involves a combination of adiabatic capture and centrifugal sudden approximations. The convergence of the calculated cross sections, with respect to rotational basis functions, is improved by use of localized functions of the form exp (A cos θ), where θ is the atom-molecule orientation angle. These functions localize the hindered rotational wavefunctions about the collinear configuration, which is appropriate for the collisions of ions with molecules possessing dipole moments. Excellent agreement with experimental room temperature rate constants is obtained. The theory predicts sharply increasing rate constants as the temperature decreases, and this is related to the strong sensitivity of rotationally selected rate constants to the initial rotational state.

Quantum scattering calculations on the OH+H2(v=0,1), OH+D2, and OD+H2 reactions

Quantum reactive scattering calculations are reported for the four‐atom reactions OH+H2(v=0,1)→H2O+H, OH+D2→HOD+D, and OD+H2→DOH+H, and their reverse reactions. The method involves using hyperspherical coordinates to describe the H2 vibration and one local OH stretching vibration of H2O, accounting for both the rotation of the OH and the bending mode of H2O with a spherical harmonic basis set, and applying a version of the bending‐corrected rotating‐line approximation to treat the rotation of H2 and the vibration of initial OH. The method gives cross sections and rate coefficients for these reactions which are state selected in the initial OH(j) rotational and H2(v) vibrational states and in the H2O(n,m) product states where n and m label bending and local OH‐stretching vibrational states of H2O. A modified potential‐energy surface based on a fit to ab initio data is used. The calculated rate coefficients for both the OH+H2(v=0) and OH+D2(v=0) reactions agree very well with experiment over the whole tempe...

Application of hyperspherical coordinates to four‐atom reactive scattering: H2+CN→H+HCN

We develop the use of Delves’ hyperspherical coordinates to study the reactive scattering of four‐atom systems within the collinear approximation. We present quantum mechanical calculations of reaction probabilities for the collinear exothermic reaction H2+CN →H+HCN. We use a potential energy surface which reproduces the essential characteristics of the reaction. The effect of freezing the CN bondlength to its equilibrium value during the reaction is also investigated and is found to be a good approximation. It is found that HCN product vibrational states with the C–H stretch excited are produced preferentially in the reaction.

Calculations of the tunneling splittings in water dimer and trimer using diffusion Monte Carlo

The diffusion Monte Carlo (DMC) method is used to calculate rovibrational bound states of the water dimer and trimer. The rigid body form of DMC is employed, together with correlated sampling of energy differences between states of different symmetry. This allows calculation of the tunneling splittings in (H2O)2 and (H2O)3. The results for (H2O)2 are in quite good agreement with those obtained using a basis set method, and also agree well with experiment. In addition, we have made predictions for similar splittings in (D2O)3 and several water dimer isotopomers. In all the calculations, we have used the potential energy surface due to Millot and Stone which is known to give quite good agreement with experiment for the tunneling splittings in (H2O)2.

QUANTUM THEORY OF PLANAR FOUR-ATOM REACTIONS

An exact quantum mechanical theory is developed to treat four‐atom reactions of the type AB+CD↔(BCD+A, ACD+B), where the atoms are constrained to move in a plane. The theory makes use of an unbiased set of hyperspherical coordinates. A method is proposed for implementing the theory that exploits the potential optimized discrete variable representation. Application is made to the calculation of rovibrational state‐to‐state reaction probabilities for the reaction H2+OH↔H2O+H, in which the length of the OH spectator bond is held fixed. The results show that a rotating bond approximation, in which the H2 molecule is not allowed to rotate, gives good results for vibrationally selected reaction probabilities. The effect of reactant rotation and vibration on the reactivity and product distributions is discussed for the reactions H2+OH→H2O+H and H2O+H→H2+OH.