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SOS‐CI calculations at ab initio level according to Nakatsuji’s formalism, considering all the singly excited configurations and at the same time introducing doubly excited configurations in a restricted way, are presented for the isotropic and anisotropic components of the indirect nuclear spin–spin coupling tensors of directly bonded nuclei in N2H4, P2H4, PH2NH2, B2H4, BH2NH2, BH2PH2, B2H6, BH3NH3, and BH3PH3. Full tensor matrices were calculated taking into account all the spin interaction operators, including the usually neglected ‘‘diamagnetic’’ orbital term. The relative importance of the various interaction mechanisms for J and ΔJ as well as the effects of dihedral angle rotation around the NN, PP, and PN bonds and of the pyramidization of nitrogen in PH2NH2 were examined.

The equations‐of‐motion (EOM) method, developed by Rowe and by Shibuya and McKoy as an approach to calculating excitation energies and transition amplitudes relative to a correlated ground state directly, has been extended to the calculation of the indirect nuclear spin–spin coupling tensors. The theoretical values, calculated at ab initio level taking into account all the four electron–nucleus interactions, of a series of saturated and unsaturated molecules, CH4, C2H6, C2H4, C2H2, CN−, HCN, HNC, and HCNH+, correlate satisfactorily with the available experimental data and with corresponding CHF–CI results. The EOM formalism may be therefore a convenient alternative to the time‐consuming CHF–CI procedures in order to get reasonable reproduction of the J tensors, even for molecules which suffer from UHF triplet instability in single configuration CHF calculations.

The two‐photon absorption properties of trans‐1,3‐butadiene have been calculated at an ab initio level employing RPA vertical excitation energies and transition moments and taking the random molecular orientation into account. For single‐beam two‐photon absorption with every possible combination of photon polarization the transition probability coefficients and polarization ratios for the low‐lying 1Ag and 1Bg final states are reported. The strongest two‐photon absorptivity is predicted under excitation from linearly polarized photons for 1Ag states and from circularly polarized photons for 1Bg states. The polarization ratios take sizably different values for the various 1Ag states. The case of concerted absorption of two photons with various polarizations from two laser beams has also been investigated exhaustively as a function of the photon frequencies.

Coupled‐Hartree–Fock and equations‐of‐motion calculations at ab initio level for the boron–boron and boron–proton indirect nuclear spin–spin coupling constants in a representative series of polyboranes (B2H6, B2H4, B2H−7, B3H−8, B4H10, and B5H9) are presented. The boron–hydrogen–boron coupling constant is very sensitive to molecular structure and shows neither a unique sign nor a constancy of absolute value. It is found to be negative in B2H6. On the other hand, the coupling constant for a two‐center two‐electron boron–boron bond is predicted to be invariably positive in sign. Results show that the positive sign must also be attributed to 1J(BH), irrespective of the bridged or normal nature of the boron–hydrogen bond.

In this paper we document the possible performances of the equations‐of‐motion method for predicting second‐order magnetic properties (indirect nuclear spin–spin coupling tensors, nuclear screening tensors, and magnetic susceptibilities). Nonempirical calculations for a series of molecules containing conjugated or cumulated multiple bonds and most of them suffering from UHF triplet instability are reported. In spite of the limitation of the 631G** basis set used, the results fit reasonably well with the main features of the experimental data in all cases but some geminal J’s and the σ’s in diacetylene. Inclusion of the (2p‐2h) interactions brings the theoretical results to a closer agreement with observation. Effects of electron correlation at (1p‐1h) and (1p‐1h) +(2p‐2h) levels of approximation are analyzed by comparison with corresponding results obtained by standard CHF procedures. They are found to be of remarkable importance for the paramagnetic component of σ and χ and for the Fermi‐contact contribu...

The diamagnetic spin‐orbit contribution to the nuclear spin–spin coupling constants of water has been calculated employing SCF wave functions of increasing accuracy. The results demonstrate that this term is small in the case of direct O–H average coupling constants. On the other hand, a significant contribution (≂−7 Hz) to geminal H–H coupling constant is found. The theoretical results are slightly affected by the characteristics of the basis set employed in the calculations.