Riccardo Zanasi

Computational approach to molecular magnetic properties by continuous transformation of the origin of the current density

Abstract The continuous set of gauge transformations method (CSGT) for calculating magnetic susceptibilities and nuclear magnetic shieldings reported by Keith and Bader is analyzed. To avoid numerical integration, analytical expressions are obtained for direct computation of the magnetic properties, which are written in closed form as a sum of the conventional paramagnetic terms and cotributions which, in the limit of exact eigenfunctions to a model Hamiltonian, reduce to the conventional diamagnetic terms. The latter can also be expressed as the expectation value of certain commutators over the reference electronic state. It is shown that the CSGT method is related to the Geertsen procedure. The magnetic shielding within the CSGT method is origin independent, but the CSGT susceptibility depends on the origin of the vector potential, whereas the Geertsen average susceptibility is independent of the origin of the vector potential, but depends on the origin of the reference frame.

On CHF calculations of second-order magnetic properties using the method of continuous transformation of origin of the current density

SummaryA fully analytical formulation is outlined for computing molecular magnetic susceptibilities and nuclear magnetic shieldings via a continuous change of origin of the electronic current density induced by an external magnetic field. The change of origin is described in terms of a (continuous) arbitrary shift functiond(r). Coupled Hartree-Fock second-order magnetic properties of CH4 and CO2 molecules have been computed, using the special choiced(r)=r as generating function. A detailed analysis of results obtained with a variety of basis sets reveals that such a method is not as good as previously suggested. Large basis sets must be used to obtain accurate magnetic properties. On the other hand, all the components of theoretical nuclear magnetic shielding evaluated via this approach are independent of the origin of the vector potential. In general, theoretical magnetic susceptibilities depend linearly on the distance between different coordinate frames, but are origin independent for centre-symmetric molecules.

Coupled Hartree–Fock calculations of molecular magnetic properties annihilating the transverse paramagnetic current density

The reliability of the continuous transformations of origin of the current density method, which makes the transverse paramagnetic current vanish (CTOCD‐PZ), for the prediction of nearly gauge‐origin independent molecular magnetic susceptibility and gauge‐origin independent nuclear magnetic shielding, is proved on the basis of a fairly large number of calculations. It is shown that, within the computational scheme provided by the coupled Hartree–Fock perturbation theory (CHF), convergence towards the presumed Hartree–Fock limit, for magnetic susceptibility and proton magnetic shielding, is systematically reached using basis sets which are smaller than those required by conventional common origin and CTOCD‐DZ techniques. For second‐row nuclear magnetic shieldings a variant of the CTOCD‐PZ method, which shifts the origin of the current towards the nearest nucleus for points close to nuclei, as suggested originally by Keith and Bader with the CSDGT method [T. A. Keith and R. F. W. Bader, Chem. Phys. Lett. 21...

Theoretical determination of the magnetic properties of HCl, H2S, PH3, and SiH4 molecules

Coupled Hartree–Fock perturbation theory has been applied to evaluate second‐order magnetic properties of HCl, H2S, PH3, and SiH4 molecules. An efficient procedure for processing only symmetry distinct two‐electron integrals in the coupled HF algorithm is outlined, which permits the use of very large basis sets of expansion with limited computer efforts. Several near Hartree–Fock wave functions are examined, particular attention being devoted to the role of polarization functions for the second‐row atoms. Comparison between computed and available experimental results is gratifying and demonstrates the practicability of perturbed HF calculations based on the group‐theoretical procedure sketched in this paper. An analysis of the gauge dependence affecting the calculated values is performed, which yields a severe test on the reliability of the present calculations.

Structure and properties of C70

Abstract Electric and magnetic properties are calculated for C 70 at the optimised geometry in the STO-3G basis. The calculation gives a pictorial interpretation of the NMR spectrum of C 70 ; upper and lower region of the molecule have bond alternation and average chemical shift within 4 ppm of C 60 but the five central rings are benzenoid and the equatorial carbons lie 2 ppm downfield of benzene.

On the theoretical determination of molecular first hyperpolarizability

Coupled Hartree–Fock perturbation theory has been applied to evaluate the first hyperpolarizability in the molecules H2O, NH3, CH4, and CO. A group‐theoretical approach has been employed, which makes it possible for a reduced file of symmetry distinct two‐electron integrals to be processed at each iteration with reduced computer efforts. Several very large basis sets, including up to 4f functions on the heavy atom and 3d on hydrogen, have been retained, which yield SCF energies, dipole moments, polarizabilities, and first hyperpolarizabilities very near to the corresponding HF quantities. Except in the case of CO, large discrepancies with respect to experimental data have been found.

On the calculation of parity-violating energies in hydrogen peroxide and hydrogen disulphide molecules within the random-phase approximation

Abstract It is shown via extended numerical tests that the ab initio calculation of a parity-violating energy term, at the random-phase approximation (or coupled Hartree-Fock perturbation theory) level of accuracy, gives results which are more than one order of magnitude larger than those usually obtained by means of less accurate methods employed so far. These findings make more plausible the hypothesis of electroweak selection of natural enantiomers.

Chiral Triazole Fungicide Difenoconazole: Absolute Stereochemistry, Stereoselective Bioactivity, Aquatic Toxicity, and Environmental Behavior in Vegetables and Soil

In this study, the systemic assessments of the stereoisomers of triazole fungicide difenoconazole are reported for the first time, including absolute stereochemistry, stereoselective bioactivity toward pathogens (Alternaria sonali, Fulvia fulva, Botrytis cinerea, and Rhizoctonia solani), and toxicity toward aquatic organisms (Scenedesmus obliquus, Daphnia magna, and Danio rerio). Moreover, the stereoselective degradation of difenoconazole in vegetables (cucumber, Cucumis sativus and tomato, Lycopersicon esculentum) under field conditions and in soil under laboratory-controlled conditions (aerobic and anaerobic) was investigated. There were 1.33-24.2-fold and 1.04-6.78-fold differences in bioactivity and toxicity, respectively. Investigations on the stereoselective degradation of difenoconazole in vegetables showed that the highest-toxic and lowest-bioactive (2S,4S)-stereoisomer displays a different enrichment behavior in different plant species. Under aerobic or anaerobic conditions, (2R,4R)- and (2R,4S)-difenoconazole were preferentially degraded in the soil. Moreover, difenoconazole was configurationally stable in the test soil matrices. On the basis of biological activity, ecotoxicity, and environmental behavior, it is likely that the use of pure (2R,4S)-difenoconazole instead of the commercial stereoisomer mix may help to increase the bioactivity and reduce environmental pollution.

Connection between the nuclear electric shielding tensor and the infrared intensity

Abstract The integrated IR intensity can be directly related to the electric shielding tensor of the nuclei in a molecule. This allows us to measure, by IR spectroscopy, the effective electric field at the nuclei of a molecule immersed in an external electric field. Relations exist with the dynamic polarizability and the magnetizability, which implies the possibility of a unified treatment of electric and magnetic second-order properties.

Theoretical studies of the benzene molecule: Magnetic susceptibility and nuclear shielding constants

Coupled Hartree–Fock perturbation theory is employed to evaluate the magnetic susceptibility, proton and carbon shielding constants, and their gauge dependence in benzene within the framework of an LCAO approach retaining 198 CGTO’s as the basis set. The magnetic properties are rationalized by analyzing the orbital contributions: the theoretical results indicate that an important role is played by π electrons, which largely contribute to the determination of the bulk of magnetic susceptibility and shieldings. The basic soundness of the ring current model for benzene seems to have been confirmed, but our findings demonstrate that also local electron circulations provide significant deshielding effects on the proton chemical shift and a relevant contribution to the in‐plane susceptibility.