An ab initio study of the effect of the 3d orbitals of phosphorus on the properties of the phosphate group

A large number of properties of the dimethylphosphate anion (DMP^?), model system for the phosphate group of the nucleic acids and the phospholipid components of membranes, has been investigated by the SCF ab initio procedure including the 3d orbitals of the phosphorus atom in the gaussian basis set and the results of the computations were compared with similar results obtained previously without taking these orbitals into account. The properties investigated include the conformational states of DMP with respect to the torsion about the P-O_ester bonds, the distribution of the electronic charges, the molecular electrostatic potentials generated by DMP^?, the characteristics of the molecular orbitals, particularly the electronic isodensity maps, the hydration scheme. Qualitatively, the introduction of the 3d orbitals modifies little the general aspects of the results obtained for most of the properties studied. On the quantitative level significant, although generally not great, modifications may be noticed with respect to some features of these properties. One of the strongest influences of the introduction of the 3d orbitals concerns the decrease of the net electronic charges in DMP^?.     

Quantum mechanical studies of environmental effects on biomolecules

LCAO SCF calculations with a (7s, 3p/3s) atomic basis are performed on urea and hydrated urea in order to determine the preferred sites of water fixation using the supermolecule approach. It appears that up to five water molecules can be directly bound to urea in the first hydration shell.     

Anab initio SCF molecular orbital study of acetylcholine

A standard ab initio (STO 3 G) study indicates, in distinction to theab initio molecular fragment results, that the preferred conformation of acetylcholin is gauche.     

Cation π interaction between acetylcholine and the benzene ring

 Ab initio self-consistent-field second-order M?ller–Plesset perturbation theory computations including basis set superposition error and zero-point vibrational energy corrections have been performed on the complexation of benzene with the polar head of acetylcholine (ACh). The ACh–benzene complex is about 0.5 kcal/mol less stable than the corresponding tetramethylammonium (TMA)–benzene complex, with a structure a little distorted with respect to the latter. The electronic structure of ACh is little modified by the ligand. Overall, the replacement of one methyl group of TMA by the acetyl tail of ACh does not affect strongly the complexation to benzene, as far as the main interaction is concerned. Received: 1 April 1999 / Accepted: 19 October 1999 / Published online: 14 March 2000     

The interaction of alkali metal cations with oxygen-containing ligands

Ab initio SCF calculations on the interaction of Li⁺ cation with H₂O and H₂CO using two basis sets are presented. Partitioning of SCF energies of interaction into Coulomb-, exchange- and delocalization energies has been performed. Coulomb- and delocalization energies are compared with classical electrostatic and polarization energies. A detailed analysis of the calculated wave functions demonstrates that in the complexes investigated here, charge transfer is of minor importance only. Polarization of the molecules in the strong inhomogeneous field of the cation leads to complicated electron density rearrangements which can be interpreted most easily in terms of polarization of individual localized MO's.     

Electrostatic molecular potentials in hydrogen-bonded systems

A study is made of the modifications of the electrostatic molecular potential brought about by hydrogen bonding both in the hydrogen-bond region itself and in the external regions of the proton-acceptor and proton-donor molecules. Systems up to four successive units in a chain of donor-acceptors are considered. The possibility of obtaining a satisfactory picture of the global potential by a simple superposition of the potentials of the individual units is evaluated.     

On the molecular electrostatic potentials obtained from CNDO wave functions

The various approximations proposed for computing molecular potentials with CNDO wave functions are tested on the case of guanine and shown to be unable to reproduce correctly the essential fine features of theab initio potential.     

On the molecular electrostatic potentials obtained with CNDO and INDO wave functions

Molecular electrostatic potentials computed with CNDO/2 and INDO wave functions are shown to present systematic differences with respect to ab initio potentials in the case of out-of-plane potentials and in-plane vicinal hetero atoms in planar hetero molecules.