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 effect of strong electric and magnetic fields on the depolarization ratios of gases

The influence of a strong electric field F on the polarization of light scattered elastically by small gaseous molecules is investigated. Two effects are found:

• (i) The field distorts the molecules, thereby changing their polarizabilities. If they are isotropically polarizable when F = 0, and hence capable of scattering only polarized light from a parallel beam, this distortion may lead to depolarization. For inert gas atoms, this depolarization is proportional to F ⁴, and hence normally very small, but for tetrahedral molecules it is proportional to β ² F ², where β is the first hyperpolarizability of the molecule.

• (ii) F tends to orientate anisotropic molecules, thereby affecting the polarization of the scattered light; this effect is related to the anisotropy in the molecular polarizability, and to the dipole moment, but is not likely to lead to information that is not obtainable by simpler means.

The effect of a strong magnetic field, in place of F, is also discussed.     

Cross-correlation in band-limited ocean ambient noise fields

Observations of ambient noise in the ocean are generally band limited, because of the natural spectral shape of the noise or the restricted bandwidth of the detection system. Either way, the noise may be regarded as white noise to which a band-limiting filter has been applied. An analysis of the two-point cross-correlation function of such filtered noise is presented for two cases, isotropic and surface-generated noise. The most pronounced effects occur with high-pass and bandpass filters when the low-frequency cut-off falls well above the first few zeros in the coherence function. In this situation, the sensor separation is very many times the longest acoustic wavelength (associated with the lowest frequency) in the passband. The filtering then produces sharp pulses at correlation delays equal to the numerical value of the acoustic travel time between the sensors. Although these pulses are narrow, they have a finite width, within which a fine structure appears in the form of multiple rapid oscillations, due to the differentiating action of the filter. The number of such oscillations increases as the low-frequency roll-off of the filter becomes steeper. This fine structure is evident in several recently published experimental determinations of the cross-correlation function of band-limited ocean ambient noise.     

Shift of proton magnetic shielding in ClH ··· Y dimers (Y = N2, CO,BF)

The change in the proton magnetic shielding constant of ClH on the formation of the linear hydrogen-bonded ClH?·?·?·?Y (Y?=?N₂,?CO,?BF) complexes was determined by GIAO ab initio computations at the B3LYP/aug-cc-pVQZ level of theory. The characteristic downfield shift of the isotropic proton magnetic resonance in the vibrationally red-shifted complexes (ClH?·?·?·?N₂, ClH?·?·?·?CO and ClH?·?·?·?BF) is significantly larger than in the blue-shifted complexes (ClH?·?·?·?OC and ClH?·?·?·?FB). These results are rationalized by considering the changes in the magnetic and electric contributions to the proton shielding in ClH.     

Relation of the electric field at a nucleus to other molecular properties

A series of response tensors relating the electric field at a nucleus in a molecule to external fields and field gradients is defined. Each new tensor is a derivative of a molecular multipole moment or polarizability with respect to motion of that nucleus. Connections with infrared and Raman intensities and with changes in molecular geometry induced by external fields are discussed. Sum rules and symmetry conditions on the response tensors are given. Numerical examples are presented for some small molecules.     

On the correlation between bond-length change and vibrational frequency shift in hydrogen-bonded complexes: a computational study of Y...HCl dimers (Y = N2, CO, BF)

The H-Cl bond-length change and the harmonic vibrational frequency shift of the H-Cl stretch on formation of the linear isoelectronic Y...H-Cl complexes (Y = N(2), CO, BF) have been determined by ab initio computations at different levels of theory. These shifts are in agreement with predictions from a model based on perturbation theory and involving the first and second derivatives of the interaction energy with respect to displacement of the H-Cl bond length from its equilibrium value in the isolated monomer. At the highest level of theory, blue shifts were obtained for BF...HCl and CO...HCl, while red shifts were obtained for FB...HCl, OC...HCl, and N(2)...HCl. These vibrational characteristics are rationalized by considering the balance between the interaction energy derivatives obtained from the perturbative model. The widely believed correlation between the bond-length change and the sign of the frequency shift obtained on complexation is discussed and found to be unreliable.     

Polarizabilities of interacting polar molecules

Collision-induced polarizability in the electro-optical Kerr effect is shown to be the dominant contributor to the second Kerr virial coefficient B_K of dipolar gases. It gives a positive B_K and may be orders of magnitude larger than the contribution due to the intrinsic anisotropy in the polarizability of the free molecules, thus resolving a long-standing discrepancy between experiment and theory. The collision-induced contribution provides a reasonable fit to the observed B_K for the fluoromethanes if a simple Stockmayer-type potential is used.     

A7DB: a relational database for mutational,physiological and pharmacological data related to the α7 nicotinic acetylcholine receptor

Background  

Nicotinic acetylcholine receptors (nAChRs) are pentameric proteins that are important drug targets for a variety of diseases including Alzheimer's, schizophrenia and various forms of epilepsy. One of the most intensively studied nAChR subunits in recent years has been α7. This subunit can form functional homomeric pentamers (α7)₅, which can make interpretation of physiological and structural data much simpler. The growing amount of structural, pharmacological and physiological data for these receptors indicates the need for a dedicated and accurate database to provide a means to access this information in a coherent manner.