Jan Almlöf

AB initio calculations on the equilibruim geometry and rotation barriers in biphenyl

Abstract The geometry of the biphenyl molecule has been studied with the MO LCAO method using a gausian basis set of “double zeta” quality. For various values of the twist angle between the phenyl ring planes the positions of the hydrogen atoms in the 2-position and the distance between the two rings have been simultaneously optimized. The lowest energy has been obtained for a non-planar conformation having a twist angle of 32°. The rotation barriers are 1.2 and 4.5 kcal/mol for the planar perpendicular cases, respectively.

Correlation effects on hydrogen-bond potentials. SCF Cl calculations for the systems HF−2 and H3O−2

Abstract The effects of neglecting electron correlation in the Hartree-Fock approximation have been investigated for selected properties of two strongly hydrogen-bonded systems. Accounting for correlation will substantially lower and, in some cases, remove barriers for proton transfer which have been obtained using the Hartree-Fock approximation. As a consequence, the asymmetric hydrogen-bond stretching frequencies in HF−2 are found to increase when correlation is included in the calculations, contrary to the case for more regular stretching frequencies. The hydrogen-bond energies of the two systems are found to be rather insensitive to correlation effects.

AB initio SCF LCAO MO Calculations on styrene

Abstract Minimal basis set ab initio SCF LCAO MO calculations with gaussian-type have been performed for different conformations of the styrene molecule. The computations show the molecule to be planar and the rotational barrier of the vinyl group is estimated to 3.9 kcal/mole.

AB initio MO LCAO UHF calculations of magnetic hyperfine interactions in π-radicals. Isotropic and anisotropic couplings of ·CH3, ·NH3+, ·C2H5 and ·N2H4+

Abstract A series of π-electron radicals have been investigated with ab initio methods, using the unrestricted Hartree-Fock approximation. The geometries have been partially optimized, and magnetic hyperfine coupling constants have been evaluated. For the dipolar hyperfine couplings, the results obtained are in fair agreement with experiment, whereas the isotropic coupling constants show greater deviations. These values are also shown to be sensitive to annihilation of the quartet contamination of the wavefunction.

A comparative study of electrostatic potential maps obtained with different basis sets

Abstract Electrostatic potential maps are plotted using five different basis sets for the ethylene molecule. Basis sets of at least double zeta quality ought to be used in order to obtain reliable details in the potential around a molecule.

A conformational study of ethylene glycol

Abstract Ab initio calculations were performed for some different conformers of 1,2-ethanediol in order to reveal their relative energies. The equilibrium conformation is of gauche type with a comparatively weak intramolecular hydrogen bond. The energy of the all- trans conformer is 3 kcal/mol above the minimum.

Theoretical studies of electrophilic substitutions in fluorobenzene

Abstract Results from an ab initio MO LCAO SCF study of the electronic structure of fluorobenzene are reported. A minimal basis set of contracted gaussian functions has been used to calculate the electrostatic molecular potential. The obtained isopotential maps indicate that fluorine has a para-directing effect in accordance with chemical evidence.

Ab initio MO LCAO UHF calculations of magnetic hyperfine interactions in σ radicals. Isotropic and anisotropic couplings of NO2 and CO2

Abstract The magnetic hyperfine coupling constants in NO 2 and CO 2 − have been computed by an initio methods. Spin annihilation is found to be essential in order to obtain useful results for the dipolar couplings, but has much less influence on the isotropic couplings. The electric quadrupole coupling constants have also been evaluated, and are in good agreement with available experimental data.

Ab initio MO LCAO UHF calculations of the stability in dimeric ethylene radical ions

Abstract The stability of ethylene dimer ions has been computed by ab initio methods. The positively charged dimer is stable (0.6–0.7 eV) while the neutral and the negatively charged dimers are unstable with respect to decomposition into monomers. The magnetic hyperfine coupling constants have also been evaluated, hyperfine splittings are half those in the monomer in agreement with experimental data.

Conformational study of meta-halogenostyrenes

A conformational study of meta-halogeno substituted styrenes was performed using minimal basis ab initio calculations on the two planar conformers of m-fluorostyrene and a low-resolution microwave spectrum of m-iodostyrene. The results obtained from the calculations indicate that the energy difference between the two conformers is very small. Furthermore, from the microwave spectrum of m-iodostyrene, experimental evidence was obtained for the existence of two planar or near-planar conformers in roughly equal amounts.