The molecular electron density distribution meeting place of X-ray diffraction and quantum chemistry intermediate - between theory and experiment

Quantum chemistry and the concepts used daily in chemistry are increasingly growing apart. Among the concepts that are able to bridge the gap between theory and experimental practice, electron density distribution has an important place. The study of this distribution has led to new developments in theory, including Hellmann-Feynman theory and the density functional theory. The possibilities and limitations of these methods are discussed. Various ways of analysing the electron density distribution are presented and discussed. X-ray diffraction enables us to ¿observe¿ the electron density distribution and electrostatic properties. Experimental results are compared with the results of quantum chemical calculations. It is shown that even intermolecular interaction is observable with this method. Problems in determining ionic charges are seen to be inherent in the method

The shielding of external electric fields in atoms revisited

An atom, placed in an external homogeneous field, will show a complex charge distribution. The pattern of the polarization density distribution, obtained by subtracting the original electron from the one of the polarized atom, can easily be explained by considering the various orbitals. Poisson's equation relates the induced field to polarization density distribution

The electron density distribution in the hydrogen bond. A quantum chemical and crystallographic study

With the help of Hartree—Fock—Slater calculations in which very large basis sets are employed, the polarisation of the water molecule by an electric field is explored. The various features in the electron density distribution are encountered again in the long hydrogen bond in the water dimer, showing that polarisation is the main effect. In short hydrogen bonds, exchange repulsion is shown to be equally important.\ud \ud The quality of the computational method is tested by comparing the results of the calculation of the electron density distribution in the crystal of α-oxalic acid dihydrate with the results of accurate X-ray diffraction measurements. By using models in which subsequently covalent bonding, hydrogen bonding and the electrostatic crystalline field are included, the effects of the various components are explored. Only the full theoretical model gives excellent agreement with the experiment, showing the quality of the model and the sensitivity of the experiment

The crystal structure of urea nitrate

The structure of urea nitrate has been solved, by the use of three-dimensional X-ray data. Data were collected using Cu Ke and Mo K0~ radiations. The structure consists of layers with urea and nitrate groups held together by hydrogen bonds. The positions of all hydrogen atoms were found. The final R values for Cu and Mo measurements are 4.8% and 6.2% respectively. The agreement between the two sets of data is good

Molecular separation by thermosensitive hydrogelmembranes

A new method for separation of molecules of different size is presented. The method is a useful addition to conventional separation methods which depend mainly on gel permeation chromatography using size exclusion. In the new method, hydrogel membranes are used which swelling level can be thermally controlled. In this study, a crosslinked poly(N-isopropylacrylamide¿co-butylmethacrylate 95:5mol%) membrane is used and three solutes of distinct molecular size: two dextrans with molecular weights of 150,000 and 4,400 g/mol respectively and uranine with a molecular weight of 376 g/mol. The swelling of the membranes as function of temperature was measured as well as the influence of the swelling level on the permeability of the three solutes. the influence of the swelling level and the solute size on the permeability was as expected from the free-volume theory. Based on these permeability phenomena, separation was performed in a continuous way by varying the membrane swelling at the appropriate time. A linear relationship between inverse membrane hydration and solute diffusion was found for uranine and dextran (MW=4,400), indicating the validity of the free-volume theory

Accuracy of various approximations to exchange and correlation for the electron density distribution in atoms and small molecules

The general usefulness of various local and non-local approximations to the exchange-correlation potential in density functional theory is studied by comparing resulting electron density distributions to essentially exact results for light atoms. The correlation contribution to the electron density in CO and H2O is compared with CI results. It is concluded that density functional theory provides a viable alternative to HF and CI approaches for the calculation of deformation densities, although the response of the electron density to the correlation potential is only moderately accurate

Electrostatic Molecular Interaction from X-ray Diffraction Data. II. Test on Theoretical Pyrazine Data

In a previous paper [Moss & Feil (1981). Acta Cryst. A37, 414-421] a method was reported to calculate the electrostatic potential and the electrostatic interaction energy from single-crystal X-ray diffraction data. The method was applied to experimental pyrazine data; however, owing to the relatively low quality of the data, the results were inconclusive. In the present paper the results are presented of a model study in which the method has been applied to the analysis of ideal error-free diffraction data calculated from a theoretical wavefunction. The molecular quadrupole moments and the electrostatic interaction energies of two pyrazine molecules thus obtained are in very good agreement with the corresponding results derived directly from the wavefunction. Thus the proposed method may be used to determine the long-range electrostatic component of molecular interactions from highly accurate X-ray diffraction data

A modified plane wave model for calculating UV photo-ionization cross-sections

Photoionization cross-sections are calculated for a number of molecules, using a plane wave method. Agreement with experimental data is considerably improved with respect to common plane wave results if the energy of the photoelectron is assumed to equal the incident photon energy

The electron density distribution in CN−, LiCN and LiNC. The use of minimal and extended basis set SCF calculations

Electron density maps are reported for the CN−ion and the LiCN and LiNC molecules, calculated from molecular wave-functions near the Hartree-Fock limit. The electron density distribution derived from CNDO/ 2 wavefunctions does not resemble the ab initio results. The ultimate ability of a minimal basis set to represent the electron density near the Hartree-Fock limit, has been tested. The requirement of N-representability of the trial electron density has been satisfied. It is found that the molecular valence density cannot be reproduced to a satisfactory extent by a minimal set of Slater orbitals, even when the exponents of the basis orbitals are optimized

Uncoupled Hartree-Fock calculations of the polarizability and hyperpolarizabilities of nitrophenols

The polarizability and hyperpolarizabilities of nitrophenols as model compounds for studying nonlinear optics have been investigated at the Hartree-Fock level of approximation by means of the Dalgarno Uncoupled Hartree-Fock (DUHF) or Sum Over Orbitals (SOO) method. The additive character and the charge transfer effects in α,β,γ and have been analyzed in terms of the δ and π molecular orbital contributions, the contribution of the individual π molecular orbitals, and the contribution of the highest occupied and the lowest unoccupied\ud molecular orbitals. Within the SOO approach, the reliability of the Two-Level Model has been tested and the influence of the rotation of the nitro group and of the presence of the intramolecular hydrogen bonding in ortho-nitrophenol have been studied. The results show that the present method is a reliable and efficient tool for the prediction of trends in the molecular polarizability and hyperpolarizabilities of large molecule