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Non-empirical calculations using the outer valence Green function (OVGF) approach, which incorporates the main portion of electron correlation and reorganization effects, are reported for the vertical ionization potentials of tria-, penta- and hepta-fulvene, tria-, pentatria-, penta- and heptapenta-fulvalene, spiropentadiene, spiroheptatriene and spirononatetraene. The results provide an overall interpretation of the salient features in the photoelectron spectra.

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.

The spin distribution and g-factors have been calculated for a number of dithiin cation radicals. The s-spin densities on sulphur calculated by the open-shell unrestricted INDO method both with and without annihilation of the quartet component have been correlated satisfactorily with the observed 33S hyperfine splittings. The g-factors calculated according to the Stone theory by an open-shell restricted version of the INDO method are found in good agreement with observation; their deviation from the free-spin comes from the large spin density localized on the sulphur atoms and the small excitation energy from the ‘NB’ sulphur orbitals to the odd MO. Comparative calculations with the sp and spd basis sets demonstrate the modest effect of the 3d(S) orbitals on the considered physical constants of the dithiin cations.

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.

Comparative calculations by the self-consistent perturbed INDO method on thiophene and by the unrestricted HF INDO method on the 2-thenyl radical show that satisfactory reproduction of nuclear spin coupling constants and hyperfine coupling constants for sulphur heterocyclics can be obtained by considering the sp interaction only.

The low volatility and thermal instability made the photoelectron (PE), electron transmission (ET), and dissociative electron attachment (DEA) spectroscopy measurements on curcumin (a potent chemopreventive agent) unsuccessful. The filled and empty electronic structure of curcumin was therefore investigated by exploiting the PES, ETS, and DEAS results for representative fragment molecules and suitable quantum-mechanical calculations. On this basis, a reliable pattern of the vertical ionization energies and electron attachment energies of curcumin was proposed. The π frontier molecular orbitals (MOs) are characterized by sizable interaction between the two phenol rings transmitted through the dicarbonyl chain and are associated with a remarkably low ionization energy and a negative electron attachment energy (i.e., a largely positive electron affinity), diagnostic of a stable anion state not observable in ETS. The lowest-energy electronic transitions of half-curcumin and curcumin and their color change by alkalization were interpreted with time-dependent density functional theory (DFT) calculations. For curcumin, it is shown that a loss of a phenolic proton occurs in alkaline ethanolic solution.

The conformational preferences of capsaicin were investigated by using the hybrid meta density functional theory (DFT) method MPWB1K. Its flexible, pendant side chain allows for a multitude of conformations only slightly different in energy. The distinctive vibrational features of the most stable conformers were characterized. To elucidate the most favorable reaction sites of capsaicin for radical scavenging, various homolytic bond-dissociation energies were also calculated. Of the possible radical intermediates, the allyl and benzyl radicals are energetically preferred. The filled and empty electronic structures of capsaicin were investigated by exploiting the photoelectron and electron-transmission spectra also of reference molecules and suitable quantum-mechanical calculations. On this basis, a reliable pattern of the vertical ionization energies and electron-attachment energies of capsaicin was proposed. The frontier π molecular orbitals are concentrated over the vanillyl moiety, with a modest influence of the amidic-aliphatic chain. The (negative) first vertical electron affinity is predicted to be similar to that of benzene. The absorption spectrum of capsaicin and its change by conversion into a phenolic deprotonated anion (modest bathochromic displacement) or a phenoxyl neutral radical (from colorless to red) were interpreted with time-dependent DFT calculations. ESR measurements following chemical or electrochemical reduction of capsaicin did not lead to detection of the corresponding radical anion. The spectra show fragmentation of the original molecule and formation of a variety of radical species which are believed to have a semiquinonic structure.

The equilibrium structures of artemisinin and a selection of its derivatives (potent antimalarial drugs) have been studied with the density functional theory ansatz B3LYP. Of the five rings of the artemisinin framework, it is only the pyranose ring B that exhibits a marked conformational flexibility, especially on addition of a pendant side chain at C-10. For the derivatives, the β-isomer with the axial substituent group is found to be energetically more stable than the α-isomer with the equatorial group. The assignment of the vibrational fundamentals has been supported by calculations on related model molecules and a normal coordinate analysis. This allows for a reliable characterization of the normal modes, mainly involving the peroxide linkage, in the claimed fingerprint region of 1,2,4-trioxanes. The electronic structures have also been studied by measuring and calculating significant features of the NMR, photoelectron and electron transmission spectra. In particular, a representative set of NMR chemical shifts and nuclear spin-spin coupling constants, obtained with DFT formalisms, compares favourably with experiment and fits expectation in terms of stereoelectronic effects of the vicinal oxygen lone pairs. Based on ab initio outer valence Green's function calculations, a consistent interpretation of the uppermost bands in the photoelectron spectra of artemisinin and derivatives has been advanced. The top ionization energies reflect a complex interaction of the various oxygen lone pair orbitals. Electron transmission spectroscopy is applied for the first time to compounds containing the peroxide bond and elucidates the empty level electronic structure of artemisinin and derivatives in the 0-6 eV energy range, with the support of MO calculations and comparison with the spectra of reference molecules. Electron attachment to the lowest-lying empty σ* MO, mainly localized on the O-O bridge, occurs at an energy (1.7 eV) exceptionally low for compounds not containing third-row or heavier elements.

Ab initio many-body calculations according to Cederbaum's OVGF formalism have been carried out on the IEs of the outer valence orbitals of the cyclopropene analogues, borirene, azirene, oxirene, silirene, phosphirene and thiirene. The internal correlation diagrams and comparison of the theoretical predictions with the experimental values available for the parent single-bond compounds and open-chain dimethyl analogues indicate a complex non-uniform role exerted by the bridgehead heteroatom in this series of highly strained heterocyclopropenes.

Coupled Hartree—Fock and equations-of-motion calculations at ab initio level are presented for the indirect nuclear spin—spin coupling tensors of OH − , H 2 O, H 3 O + , NH 2 − , NH 3 , NH 4 + , CH 4 and C 2 H 6 considering the electron—nucleus Fermi-contact, paramagnetic orbital and spin—dipolar interaction operators. The relative importance of the electron correlation for the various terms of the J tensor is examined. The electron correlation is found to make the more important correction to the Fermi-contact term and this is larger for the geminal interproton than for the one-bond coupling tensor.