Thole's modified dipole interaction model for constructing molecular polarizabilities from effective, isotropic atomic polarizabilities is reviewed and extended. We report effective atomic polarizabilities for H, C, N, O, S, and the halogen atoms, independent of their chemical environment. They are obtained by fitting the model both to experimental and calculated molecular polarizabilities, the latter to enable one to model ab initio polarizabilities for various basis sets.

Phosphate moieties bind frequently at N-termini of helices in proteins. It is shown that this corresponds with an optimal interaction of the helix dipole and the charged phosphate. This favourable arrangement may have been discovered several times during evolution. In some enzymes, the helix dipole might be used in catalysis.

Recently, it has become possible to measure the mobility of charges along isolated chains of conjugated polymers. The mobility of holes along poly(phenylenevinylene) and polythiophene backbones were reported to be 0.43 and 0.02 cm 2 V -1 s -1, respectively. The large difference between the mobility of holes on poly-(phenylenevinylene) and polythiophene chains can be attributed to deviations from the coplanar alignment of structural units in the polymer backbone. The effect of such torsional disorder on intramolecular hole transport is studied theoretically in this paper using a model based on the tight-binding approximation. The calculated ratio of hole mobilities along poly(phenylenevinylene) and polythiophene chains was found to be in agreement with experimental findings. For both polymers, estimated mobilities become consistent with the experimental values if polymerization defects and chain end effects are included in the calculations. This suggests that even higher mobilities than those reported here can be realized by improving the effective conjugation along the polymer chain.

ABSTRACT:A new charge analysis is presented that gives an accurate description of the electrostatic potential from the charge distribution in molecules. This is achieved in three steps: first, the total density is written as a sum of atomic densities; next, from these atomic densities a set of atomic multipoles is defined; finally, these atomic multipoles are reconstructed exactly by distributing charges over all atoms. The method is generally applicable to any method able to provide atomic multipole moments, but in this article we take advantage of the way the electrostatic potential is calculated within the Density Functional Theory framework. We investigated a set of 31 molecules as well as all amino acid residues to test the quality of the method, and found accurate results for the molecular multipole moments directly from the DFT calculations. The deviations from experimental values for the dipole/quadrupole moments are also small. Finally, our Multipole Derived Charges reproduce both the atomic and molecular multipole moments exactly.

The synthesis of [PhC(NSiMe3)2]2Y(μ-Cl)2Li·2THF (1) from YCl3·3.5THF and [PhC(NSiMe3)2]Li, which is easily transformed into [PhC(NSiMe3)2]2YCl·THF (2), provides a useful entry into the chemistry of several bis(N,N‘-bis(trimethylsilyl)benzamidinato)yttrium complexes. Those prepared from 2 by chloride metathesis include [PhC(NSiMe3)2]2YR (R = BH4·THF (3), N(SiMe3)2 (4), 2,6-(CMe3)2-4-MeOC6H2 (5), (μ-Me)2Li·TMEDA (6) (TMEDA = N,N,N‘,N‘-tetramethylethylenediamine), CH2Ph·THF (7), CH(SiMe3)2 (8)). Similar to 8, [p-MeOC6H4C(NSiMe3)2]2YCH(SiMe3)2 (8 OMe ) could be prepared starting from [p-MeOC6H4C(NSiMe3)2]2YCl·THF (2 OMe ). Hydrogenolysis (4 atm) of 8 and 8 OMe affords dimeric hydrides {[p-X-C6H4C(NSiMe3)2]2Y(μ-H)}2 (X = H (9), X = MeO (9 OMe )). The alkyl 8 OMe and the hydride 9 have been characterized by an X-ray diffraction structure determination. Sterically the bis(N,N‘-bis(trimethylsilyl)benzamidinate) ligand system resembles more the bis(pentamethylcyclopentadienyl) than the bis(cyclopentadienyl) ligand set. However, INDO/1 semi-empirical MO studies indicate that the electronic properties of [HC(NH)2]2YCH3 (used as a model for bis(benzamidinato)yttrium alkyl complexes) are rather different from [C5H5]2YCH3. The yttrium atom in [HC(NH)2]2YCH3 is considerably more positively charged than in [C5H5]2YCH3. The resulting strong ionic character of the bis(benzamidinate) system is held responsible for the absence of agostic interactions and H/D exchange and the low hydrogenolysis rate observed.

In this paper a combined experimental and quantum chemical study of the geometry and opto-electronic properties of unsubstituted and dialkoxy-sustituted phenylene-vinylene oligomers ͑PV's͒ is presented. The optical absorption spectra for PV cations with different chain lengths and substitution patterns were measured using pulse radiolysis with time-resolved spectrophotometric detection from 1380 to 500 nm ͑0.9 to 2.5 eV͒. The geometries of the PV's studied were optimized using density functional theory ͑DFT͒ for both the neutral and singly charged molecule. The spectra for the PV radical cations were then calculated using singly excited configuration interaction with an intermediate neglect of differential overlap reference wave function method together with the DFT geometry. The agreement between experimental and theoretical absorption energies is excellent; most of the calculated radical cation absorption energies are within 0.15 eV of the experimental values. The pattern of dialkoxy-substitution is found to have a large effect on the optical absorption spectrum of the cation. Using the calculated charge distribution it is shown that the degree of delocalization of the charge correlates with the energy of the lowest absorption band. If alkoxy side chains are present on some of the rings the positive charge tends to localize at those sites.

In this work we present theory and implementation for a discrete reaction field model within Density Functional Theory ͑DFT͒ for studying solvent effects on molecules. The model combines a quantum mechanical ͑QM͒ description of the solute and a classical description of the solvent molecules ͑MM͒. The solvent molecules are modeled by point charges representing the permanent electronic charge distribution, and distributed polarizabilities for describing the solvent polarization arising from many-body interactions. The QM/MM interactions are introduced into the Kohn-Sham equations, thereby allowing for the solute to be polarized by the solvent and vice versa. Here we present some initial results for water in aqueous solution. It is found that the inclusion of solvent polarization is essential for an accurate description of dipole and quadrupole moments in the liquid phase. We find a very good agreement between the liquid phase dipole and quadrupole moments obtained using the Local Density Approximation and results obtained with a similar model at the Coupled Cluster Singles and Doubles level of theory using the same water cluster structure. The influence of basis set and exchange correlation functional on the liquid phase properties was investigated and indicates that for an accurate description of the liquid phase properties using DFT a good description of the gas phase dipole moment and molecular polarizability are also needed.

A discrete solvent reaction field model for calculating frequency-dependent molecular linear response properties of molecules in solution is presented. The model combines a time-dependent density functional theory ͑QM͒ description of the solute molecule with a classical ͑MM͒ description of the discrete solvent molecules. The classical solvent molecules are represented using distributed atomic charges and polarizabilities. All the atomic parameters have been chosen so as to describe molecular gas phase properties of the solvent molecule, i.e., the atomic charges reproduce the molecular dipole moment and the atomic polarizabilities reproduce the molecular polarizability tensor using a modified dipole interaction model. The QM/MM interactions are introduced into the Kohn-Sham equations and all interactions are solved self-consistently, thereby allowing for the solute to be polarized by the solvent. Furthermore, the inclusion of polarizabilities in the MM part allows for the solvent molecules to be polarized by the solute and by interactions with other solvent molecules. Initial applications of the model to calculate the vertical electronic excitation energies and frequency-dependent molecular polarizability of a water molecule in a cluster of 127 classical water molecules are presented. The effect of using different exchange correlation ͑xc͒-potentials is investigated and the results are compared with results from wave function methods combined with a similar solvent model both at the correlated and uncorrelated level of theory. It is shown that accurate results in agreement with correlated wave function results can be obtained using xc-potentials with the correct asymptotic behavior.