Volume 220, Issue 7 - 7

The ‘quantum-mechanically derived reaction coordinates’ (QMRC) for the proton transfer in hydrogen bonds involving fluorine, oxygen and chlorine have been derived from earlier ab initio calculations of potential energy surfaces. A comparison is made between QMRC and the corresponding reaction coordinates (BORC) derived by applying the Pauling bond order concept together with the principle of conservation of bond order. Theoretical calculations have shown that the sum of the bond orders remains close to constant along the reaction coordinate in agreement with the Pauling postulate. The BORC correlation curves agree very well with theoretical results. The results indicate that the BORC curve gives a good representation of the reaction coordinates (proton transfer path) for any X-H---Y aggregate.

Using the embedded-atom method as developed by Voter and Chen in combination with the variable metric/quasi-Newton and our own Aufbau/Abbau methods, we have identified the three most stable isomers of Au N clusters with N up to 150. For the first time clusters with tetrahedral symmetry are found to form the ground states of Au 17 and Au 34 . The Au 54 icosahedron without a central atom and the Au 146 decahedron are found to be particularly stable, whereas the highly symmetric second and third Mackay icosahedra that could have been obtained for N = 55 and 147, respectively, do not correspond to the particularly stable structures. The three lowest-lying isomers of Au 55 and Au 147 are low-symmetrical structures. Various structural and energetic properties are analysed, such as stability function, occurrence of magic-sized clusters, construction of icosahedral and fcc shells, and cluster growth.

The development of focusing crystal spectrometers for Compton scattering studies from 1930s to present is reviewed. The design principles have stayed the same, but the efficiency has improved much owing to the availability of large perfect crystal analyzers and efficient detectors. The reflectivity and energy resolution of the crystal can be tailored to the needs of the experiment by the asymmetric cut, thickness and bending radius of the crystal, and the response function of the spectrometer can be calculated precisely. The spectrometers at synchrotron radiation beamlines achieve better than 0.1 a.u. momentum resolution, and several statistically accurate Compton spectra can be acquired in one day. The latest developments include spectrometers that operate in the 100 keV range, and count rates are enhanced by using wide energy bands and dispersion compensation. Results obtained by a scanning reflection-type spectrometer installed at a rotating-anode X-ray generator demonstrate that accurate Compton profiles of light-element compounds can be determined even with a laboratory instrument. A map of the reciprocal form factor of silicon is constructed from 10 directional Compton profiles, and the map reveals details of chemical bonding.

The effect of ion hydration on the Compton profiles of Na + and F - ions each coordinated by six water molecules has been studied using density-functional theory. The two ionic systems have model character, since bond lengths, coordination numbers and orientation of the water molecules have been fixed to available experimental data. The coordinating water molecules are successively brought from a ‘far’ distance of 8.0 Å to a ‘near’ interacting (hydrating) distance below 3.0 Å. We find a strong influence of the number of hydrating water molecules on the calculated isotropic Compton profiles.

The momentum-space origin of the Wigner function for electronic systems gives a convenient fingerprint of the bonding situation in a molecule. This is owing to its connection to the off-diagonal regions of the one-particle reduced density matrix which are known to map the covalent sharing of electrons in a bound system. Since the topology of this “Parity Function” follows similar patterns as the commonly employed bond orders based on population analysis, this article endeavors to correlate the values of the parity function at special points in real space with such bond orders. We also point out a relationship between the parity function and experimentally obtainable scattering cross-sections.

Hexa-acetate-tetra-aqua-trimanganese diglycine dihydrated retains the two-dimensional character of commercial manganese diacetate tetrahydrate. The manganese atoms assemble in short centrosymetric chains that are further bridged through the carboxylic group of the acetate ions agglomerating the chains into a two-dimensional pattern. The layers are bonded through H-bonds involving the zwitterionic glycine and the solvate water molecules. Susceptibility measurements show a ferrimagnetic behaviour of the sample.

The static and high frequency dielectric tensors, Born effective charges, vibrational spectrum at the Γ point, TO-LO splitting and IR intensities of calcite CaCO 3 have been calculated with the periodic ab initio CRYSTAL program, with five different basis sets of increasing size and four different Hamiltonians (HF, LDA, PBE, B3LYP). B3LYP is shown to perform better than the other options, in particular of LDA and PBE that are often used for the calculation of the vibrational spectrum of crystalline solids. When comparing B3LYP and experimental frequencies, the mean absolute difference is as small as 8.5 cm -1 ; this number reduces to 4.8 cm -1 if the two lowest experimental frequencies, that we suspect to be affected by a relatively large error, are excluded from statistics. Static and high frequency dielectric tensors, as well as IR intensities computed with the same hybrid scheme (B3LYP) compare quite favourably with experiment. The full set of modes is characterized by various tools including isotopic substitution, “freezing” one of the two subunits (Ca 2+ or CO 3 2- ) and graphical representations. A general tool has been implemented, that permits the automatic generation of the animation of the full set of modes starting from the CRYSTAL output (available at www.crystal.unito.it/vibs/calcite).

It is argued that experimental determinations of the one-electron density matrix of crystalline systems, γ 1 ( r , r ´), based on directional Compton profiles and X-ray structure factors could be useful to assess the quality of computational techniques which take electron correlation effects into account. A scheme is here outlined for evaluating the correlation correction to the Hartree–Fock one-electron density, starting from the solution obtained for the periodic system at a perturbative MPn level. The case of crystalline lithium hydride is discussed. Preliminary MP2 results for that system are presented, obtained using a local-correlation approach.

The field-adapted adjustable density matrix assembler (FA-ADMA), developed for ab initio quality quantum chemistry computations of proteins and other macromolecules, divides a molecule (the target) into fuzzy fragments. For these fragments, conventional quantum chemical calculations are performed using moderate-sized “parent molecules”, which contain both the fragment and all the local interactions of the fuzzy fragment with its surroundings within a pre-selected distance. Effects of distant parts of a target macromolecule are approximated in the fragment by point charges in the quantum chemical calculations. A known problem of the FA-ADMA approach, as well as mixed QM/MM calculations, is the treatment of the border between the quantum region and the point charges. One possibility in circumventing this problem is to use generalized hybrid orbitals (GHO) on the boundary atom next to the QM region. In this paper we describe the introduction of this approach in the FA-ADMA method, which includes a re-parameterization of the partial charges of the boundary atom. Four small proteins were taken as test cases to compare the new GHO-FA-ADMA to the original hydrogen-capping-atom scheme. The promising results demonstrate that, after the extension to sp 2 -hybridized atoms, GHO could greatly increase the accuracy of FA-ADMA calculations.

The exotic hydrogen-antihydrogen molecule and its properties are investigated. The system is metastable due to matter-antimatter annihilation, but also due to the possible double autoionization process describing the decay into protonium and positronium. The latter decay channel is energetically favored at small internuclear distances where the protonium dipole cannot permenently bind positronium. The leptonic density and reduced one-particle density matrices are calculated and discussed for the ground and the first excited state.

The paper discusses some aspects of the electron lone-pairs in H-bonded structures: their role in determining the short-range structure and the effect of the environment on the electron density. In the water molecule the entire non-bonded region appears to be equally accessible for hydrogen bonding and the details of the hydrogen-bond arrangement are mainly determined by simple geometrical and topological requirements. Many examples may be taken to illustrate that it is important to take the whole electron and nuclear distribution into account when discussing the relative arrangement of interacting molecules. The resulting structure of one particular compound is determined by the net balance of many intermolecular interactions and not only by the hydrogen bonding, even if the resulting structure is consistent with hydrogen-bond directionality. From structural data it can be concluded that the immediate acceptor of a hydrogen bond is some negative charge accumulation, such as in a lone-pair region, but not specifically any individual lone pairs in the traditional, atomic sense.

Nuclear inelastic scattering provides the unique opportunity to probe vibrational densities of states projected onto a selected Mössbauer isotope. This property can be used to monitor local changes of bonding strengths for particular modes, e.g. across phase transitions. Moreover, the partial vibrational DOS can be simulated in a straightforward way using standard quantum chemical methods. We present a comparative study of the two ferrocene-based plastic crystals octamethyl ferrocene (OMF) and octamethyl ethinyl ferrocene (OMFA), with particular emphasis on the solid-solid order-disorder transition in OMFA at 240 K. Using nuclear inelastic scattering (NIS), significant changes were observed when approaching and crossing the phase transition. Molecular NIS spectra were calculated using standard hybrid density-functional methods. The relative importance of intra- vs. intermolecular dynamics for the orientational phase transition is discussed.