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The performance of the newly proposed 6-31G(##) basis set for calculating the equilibrium structure and vibrational frequencies of transition metal carbonyl complexes has been studied at the HF and DFT levels of theory. The 6-31G(##) basis set has been constructed by augmentation of the 6-31G basis set by diffuse and polarization functions, which are generated from the corresponding 6-31G basis AOs response functions obtained in the frame of propagator approach. The predicted values of bond distances and vibrational frequencies for the title compounds are in good agreement with the experimental data. The relative energies and HOMO-LUMO gaps were also estimated for the series of MCO complexes.

The results of theoretical and experimental investigation of 17 O NMR chemical shifts for a number of epoxidic compounds are reported. The calculations were performed for the MP2/6-311G(d) level reference geometries using the GIAO and CSGT methods within the coupled Hartree-Fock perturbation theory. Various basis sets were applied in calculations of the chemical shifts. The performance of recently developed magnetically consistent basis sets and their advantages over the standard ones are discussed. The obtained results allow one to assign NMR signals for epoxides for which experimental data were obtained for the mixtures of stereoisomers.

Two methods of physically justified improvement of the STO and GTO basis set quality are suggested for ab initio calculations of molecular magnetic properties. They are based on the analysis of analytical expressions for the first-order correction (response) functions to the unperturbed basis AO's. The response functions have been obtained by solution of the inhomogeneous Schrödinger equation for the model problem "a oneelectron atom in an external uniform field", by the closed representation of the Green's function. It has been shown that unlike the London orbitals for magnetic field the Green's function method enables us to get the general solutions of the inhomogeneous Schrödinger equation. The methods elaborated have been applied in test calculations of magnetic susceptibilities and nuclear shielding constants of the first-and second-row hydrides and diatomics in STO-NG, split-valence CGTO basis sets, and extended ones constructed on their base. Analysis of results obtained has allowed us to determine the field of applicability for the suggested methods of basis set construction adapted for the magnetic properties calculations.

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a b s t r a c tSecond-order electric and magnetic properties calculated using an approach based upon the simultaneous analytical dependence of the bond order matrix and basis set functions on the corresponding perturbation parameters have been obtained and analyzed for a series of organic molecules. Various methods of selection of basis set quality for different atoms in investigated molecules were examined in conjunction with calculations of 1 H NMR chemical shifts. Comparison of the results obtained at different levels of theory (HF, DFT, MP2) demonstrates small correlation effects for polarizability and magnetic susceptibility while the electron correlation effects play crucial role for calculations of nuclear magnetic shielding (chemical shifts).