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A typical “chelate” effect is demonstrated by means of all-electron ab initio LCGO SCF calculations on the [Li(HCONH 2 ) 2 ] + complex in different geometrical arrangements. Energetic factors and the electronic structure of the complexes are discussed, showing the chelate effect to be connected with striking changes in bonding, density distribution and electronic energy.

A small gaussian lobe basis is given for MO SCF calculations on iodine compounds within reasonable computing times. The reliability of the calculations is tested by a comparison of the calculated with the experimental X-ray fluorescence spectrum of iodine. The agreement of both, as well as the correspondence of calculated and experimental physical properties of diatomic compounds, indicate the basis to be suitable for a quantum chemical treatment of larger molecules containing this element.

A comparison ofab initio calculations employing different basis sets with corresponding CNDO/2 results for the Li+/HCONH2 complex shows that these methods lead to completely different energy surfaces for this system. Reduction of the basis set, even to the minimal size, does not bring about serious changes in the results of theab initio calculations, whereas in the semiempirical treatment some methodical errors seem to occur. When using however, theab initio minimum geometry the CNDO/2 calculations also give a qualitatively correct picture for the influence of the cation on the amide modecule.

Two possible monomers of IO2F3 (C2v, trigonal-bipyramidal and Cs, tetragonal-pyramidal) have been calculated by theLCGO−MO−SCF-method. The calculations indicate the C2v conformation to be strongly favoured by its total energy. The electronic structure of the compound is discussed on the basis of population analysis and electron density maps. A comparison of the19F-NMR spectrum of (IO2F3)n, with these calculated data supports the results indicated by the energy values.

CNDO/2 calculations have been performed for the compounds CF3NCO, H3SiNCO, and F2PNCO. The calculated minimum geometries are found to be in agreement with spectroscopic results. The NCO group, however, turns out to be nonlinear in CF3NCO and H3SiNCO, as in other compounds of this series. Only F2PNCO seems to have a NCO angle of 180 degrees. Charge density distribution and the character of the bonds are discussed for the molecules under investigation.

In dimeric hydrogen bonded aggregates of acetic acid and its halogenated homologues, stabilization energies, geometries and charge distributions depend on the acidity and basicity of the monomers. For stabilization and bond lengths, the effects of basicity predominate; e.g., CH 3 COOH.CH 3 COOH is more stable than CF 3 COOH.CH 3 COOH. Calculated charge densities are in good agreement with 1 H NMR chemical-shift measurements. A further correlation is found between the stabilization energy of the adducts investigated, and the pK-values of the corresponding acids in the solvent acetic acid.

The interaction of Li+ with a formamide molecule has been studied systematically. The energy surface for the formation of the Li(HCONH2) complex is discussed, in connection with a SCF-MO-LCGO treatment of the corresponding adiabatic reaction pathway. The energetic situation for a bonding of the cation to nitrogen and the nonplanar conformation formed thereby has been calculated and compared to the more stable planar arrangement. The description of the ion-amide-interaction is completed by density maps and localized molecular orbitals.

The reaction pathways for the solvation of Li+ and Cl– in formic acid have been investigated by means of CNDO/2 MO SCF calculations. The energetically most favoured pathways for these reactions have been determined.The method describes satisfactorily the complete reactions, for both of which a nearly simultaneous change is found in all geometric parameters within short distances between the ions and formic acid. For the cation, long range interaction is observed only with carbonyl oxygen, while at the equilibrium position, interaction also exists with carbon and CH hydrogen. The polarization of the hydrogen bond in the anionic solvate and its function in charge transfer are discussed in detail.

CNDO/2 calculations have been performed for the mono-, di- and tetra-solvates of Li + , Na + and Cl − with formic acid. The most stable position for the cations is found to be between the CH hydrogen and carbonyl oxygen, confirming similar conclusions from experimental results. The calculated changes in electron densities agree well with observations in 1 H-NMR spectra. Calculated solvation energies are found to show the right relative order for the cations, although absolute values are too high. For the anion, also the absolute value is in reasonable agreement with experiment. CNDO minimum geometries, charge distributions and bond indices are given for all solvates and discussed in respect to possible methodical errors.

The nonlinearity of the NCO group, as to be calculated after the CNDO method and in agreement with some experimental data, has been investigated systematically. The energy values, calculated for different angles, show the nuclear repulsion energy to be the determining factor for the bent (trans-) configuration of these compounds. The extent of this angle cannot be explained with classical chemical characteristics. A significant correlation is found, however, with the electronic situation at the C-atom, whose net charge and covalent bonding are likewise influenced by the NCO angle.

A chemical interpretation of the adiabatic reaction pathway for the formation of the (Li-HCOOH) + complex is given in terms of localized INDO molecular orbitals. Bonding and polarization effects in the solvation reaction are discussed. The localization procedure proves to be a valuable tool for the interpretation of chemical reactions.

Calculations on some isocyanates have been performed by means of semi-empirical MO methods (CNDO/2). The NCO group is found to be nonlinear, in agreement with recent experimental result. Energies, charge densities and bond indices are discussed in relation to the stability of these compounds.

CNDO/2 calculations have been performed for the compounds CF3NCO, H3SiNCO, and F2PNCO. The calculated minimum geometries are found to be in agreement with spectroscopic results. The NCO group, however, turns out to be nonlinear in CF3NCO and H3SiNCO, as in other compounds of this series. Only F2PNCO seems to have a NCO angle of 180 degrees. Charge density distribution and the character of the bonds are discussed for the molecules under investigation.

The bonding in the F5SO--ion is discussed by comparing the results of CNDO MO calculations on this and some related compounds with data obtained from IR- and RAMAN-spectra. Theoretical and experimental results are found to be in good agreement. The-yet not prepared -acid F5SOH is discussed with respect to its stabilization and acidity.

By means of ab initio LCGO-MO-SCF calculations on a series of iodine fluorides and oxyfluorides. an attempt is made to obtain some quantum chemical background for the chemistry of these molecules, The agreement of the calculated data with various experimental results is quite good, despite the small basis set and neglect of relativistic and correlation effects. Electronic structure and bonding of the compounds are discussed, leading to the conclusion, that the chemistry of iodine is determined almost exclusively by p-electrons. Some physical quantities and reaction energies are discussed in relation to the experimentally observed chemical behaviour of the molecules investigated, such as dissociation, disproportionation, association tendency and 19F chemical shifts.

The influence of Li+ and Be2+ on the barrier to internal rotation in dimethylformamide has been investigated by means of 1H magnetic resonance line shape analysis. The changes in the activation energy at various ion concentrations are discussed in connection with nonempirical LCGO-MO-SCF calculations for the 1 : 1 complexes of Li+ and Be2+ with formamide and dimethylformamide and the 2 : 1 complex of Li+ with formamide. Good agreement could be established between the experimental data and the quantum chemical model calculations concerning the relative changes of the activation energy for internal rotation.