A. David Buckingham

Ab initio investigation of the sum-frequency hyperpolarizability of small chiral molecules

Using a sum-over-states procedure based on configuration interaction singles /6-311++G**, we have computed the sum-frequency hyperpolarizability βijk(−3ω;2ω,ω) of two small chiral molecules, R-monofluoro-oxirane and R-(+)-propylene oxide. Excitation energies were scaled to fit experimental UV-absorption data and checked with ab initio values from time-dependent density functional theory. The isotropic part of the computed hyperpolarizabilities, β(−3ω;2ω,ω), is much smaller than that reported previously from sum-frequency generation experiments on aqueous solutions of arabinose. Comparison is made with a single-centre chiral model.

Cooperative and diminutive hydrogen bonding in Y...HCN...HCN and NCH...Y...HCN trimers (Y=BF,CO,N2).

A computational study of the cooperative effect of hydrogen bonding in Y...HCN...HCN and its diminutive effect in NCH...Y...HCN (Y=BF,CO,N2) linear complexes relative to the Y...HCN dimer was undertaken at the MP2/6-311++G(2d,2p) level of theory. It was found that the additional hydrogen bond in Y...HCN...HCN leads to an enhanced Y...HCN dissociation energy, extended H-C bond length, and larger redshift of the H-C stretch relative to Y...HCN, while opposite features are observed in NCH...Y...HCN. The cooperativity is diminished as the hardness of the Y atom directly bonded to the HCN molecule increases. A particularly interesting result is that the small bond contraction and blueshift associated with the H-C bond in BF...HCN is converted to a small bond extension and redshift on the formation of the BF...HCN...HCN trimer.

Communication: Permanent dipoles contribute to electric polarization in chiral NMR spectra

Nuclear magnetic resonance spectroscopy is blind to chirality because the spectra of a molecule and its mirror image are identical unless the environment is chiral. However, precessing nuclear magnetic moments in chiral molecules in a strong magnetic field induce an electric polarization through the nuclear magnetic shielding polarizability. This effect is equal and opposite for a molecule and its mirror image but is small and has not yet been observed. It is shown that the permanent electric dipole moment of a chiral molecule is partially oriented through the antisymmetric part of the nuclear magnetic shielding tensor, causing the electric dipole to precess with the nuclear magnetic moment and producing a much larger temperature-dependent electric polarization with better prospects of detection.

Vibrational frequency shifts in NeHF and ArHF complexes

The hydrogen fluoride vibrational frequency shift in the NeHF and ArHF complexes in their linear equilibrium configurations has been calculated using a theoretical model derived from quantum mechanical perturbation theory and involving first and second derivatives of the inert gas interaction energy with respect to displacements of the HF internuclear distance from equilibrium in the free molecule. These derivatives were obtained from ab initio calculations at the self-consistent field (SCF) and Moller-Plesset (MP)2 levels of theory. The calculated shifts are compared with harmonic frequency shifts computed from ab initio analytic second derivatives at the SCF and MP2 levels of theory. The frequency shifts for these complexes obtained by these two theoretical approaches are in reasonable agreement for ArHF but differ for NeHF whose shift is nearly zero. The basis set dependence of the shift was investigated at the SCF level for NeHF; large basis sets are required to reproduce the correct sign of the shift...

Chiral discrimination in NMR spectroscopy: computation of the relevant molecular pseudoscalars

Nuclear magnetic resonance (NMR) is normally blind to chirality but it has been predicted that precessing nuclear spins in a strong magnetic field induce a rotating electric polarisation that is of opposite sign for enantiomers. The polarisation is determined by two pseudoscalars, and . The former arises from the distortion of the electronic structure by the nuclear magnetic moment in the presence of the strong magnetic field and is equivalent to the linear effect of an electric field on the nuclear shielding tensor. determines the temperature-dependent partial orientation of the permanent electric dipole moment of the molecule by the antisymmetric part of the nuclear shielding tensor. Computations of these two contributions are reported for the nuclei in the chiral molecules N-methyloxaziridine, 2-methyloxirane, 1,3-dimethylallene, 1-fluoroethanol, 2-fluoroazirine, 1,2-M-dioxin, 1,2-M-dithiin, 1,2-M-diselenin and 1,2-M-ditellurin. For strongly dipolar molecules, is typically two to three orders of magnitude greater than , raising hopes for the detection of chirality in NMR spectroscopy.

THE STATIC POLARIZABILITIES AND HYPERPOLARIZABILITIES OF LI2

The electronic contribution to the components of the static polarizability and second hyperpolarizability of Li2 is studied using SCF, MP2 and MP4[SDTQ] theories with several large basis sets. The computed polarizability is within the experimental error limits. The static vibrational contributions to these properties are also determined. The nuclear motion has a considerable effect on the second hyperpolarizability but not on the polarizability.

Using monomer properties to obtain integrated intensities for vibrational transitions of van der Waals complexes

The integrated intensities of vibrational transitions depend on the magnitude of the derivatives of the dipole with respect to nuclear motion. Normally, the only reliable way to compute such derivatives is by tedious and expensive ab initio calculations. In this paper, we present a simplification for weakly bound complexes based on distributed schemes for describing the charge densities and polarizabilities of the monomers. Formulations based on both Cartesian and spherical harmonics are presented. The results for both these schemes agree exactly with each other, and qualitatively with full ab initio calculations for the hydrogen fluoride dimer, (HF)2.

Vibrational frequency shifts in OC⋯HF and N2⋯HF

The shift in the harmonic vibrational frequency of hydrogen fluoride on forming the weakly bound complexes OC⋯HF and N2⋯HF has been obtained by ab initio quantum chemical techniques. We used two approaches: (i) through analytic second derivatives at the SCF and MP2 levels of theory, (ii) through a model based on perturbation theory and involving first and second derivatives of the interaction energy with respect to displacements of the HF internuclear distance from equilibrium in the free molecule. These derivatives were obtained from ab initio calculations at the SCF and MP2 levels. The red shifts obtained for these complexes by these theoretical approaches are in agreement, but are larger than those observed spectroscopically. The computed red-shifts are reduced by the large-amplitude zero-point bending vibrations of HF in the complexes.

Isotope effects on the stability of the nitrogen—acetylene van der Waals dimer

Abstract Harmonic vibrational frequencies for various isotopomers of the nitrogen—acetylene van der Waals dimer are calculated at the SCF and MP2 ab initio levels of theory, using large basis sets. The relative stabilities of the dimers N 2 …HCCD and N 2 …DCCH are determined from differences in the zero-point energies. It is found that the D-bonded species are energetically favoured over the H-bonded species by 8.9 cm −1 with our largest basis set at MP2. We also computed a zero-point energy difference of about 0.6 cm −1 favouring N 2 …H 13 CCH over N 2 …HC 13 CH and 14 N 15 N…HCCH over 15 N 14 N…HCCH. These calculations are consistent with most, although not all, recent experimental observations of Legon, Wallwork and Fowler

A theory of vibrational frequency shifts revisited: application to dimers of LiH with the inert gases He, Ne, Ar and Kr

The objective of this paper is to contribute towards an understanding of the anomalous blue vibrational shifts that have been observed on forming some hydrogen bonds. It is shown that linear complexes of the LiH molecule with an inert gas atom Rg exhibit red or blue shifts of the LiH vibrational frequency depending upon whether Rg is attached to the Li or the H atom. The shifts in the frequency of the Li–H vibration on forming the weakly bound linear complexes Li–H…Rg and H–Li…Rg (Rg = He, Ne, Ar, Kr) were determined by ab initio computations at the MP2/6-311++G(2d, 2p) level of theory. These frequency shifts were found to be in good agreement with predictions from a model based on perturbation theory and involving first and second derivatives U′ and U′′ of the interaction energy with respect to displacement of the Li–H bond length from its equilibrium value in the isolated molecule. Concentration of the Li–H vibrational motion in the light H atom causes U′ and U′′ to be dominated by repulsion in Li–H…Rg and by attraction in H–Li…Rg, producing blue and red shifts, respectively. The bond length changes on complexation are well predicted by U′.The objective of this paper is to contribute towards an understanding of the anomalous blue vibrational shifts that have been observed on forming some hydrogen bonds. It is shown that linear complexes of the LiH molecule with an inert gas atom Rg exhibit red or blue shifts of the LiH vibrational frequency depending upon whether Rg is attached to the Li or the H atom. The shifts in the frequency of the Li–H vibration on forming the weakly bound linear complexes Li–H…Rg and H–Li…Rg (Rg = He, Ne, Ar, Kr) were determined by ab initio computations at the MP2/6-311++G(2d, 2p) level of theory. These frequency shifts were found to be in good agreement with predictions from a model based on perturbation theory and involving first and second derivatives U′ and U′′ of the interaction energy with respect to displacement of the Li–H bond length from its equilibrium value in the isolated molecule. Concentration of the Li–H vibrational motion in the light H atom causes U′ and U′′ to be dominated by repulsion in Li–H…Rg ...