Paolo Lazzeretti

Assessment of aromaticity via molecular response properties

The possibility of founding a quantitative theory of aromaticity on the basis of measurable response properties is discussed. The reasons suggesting that nonmeasurable parameters are unsuitable for quantitative evaluation of aromaticity are analyzed.

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.

Electric and magnetic properties of the aromatic sixty-carbon cage

Abstract We report the use of large supercomputer resources to perform the first ab initio calculations on the polarisability and magnetisability of C 60 and its smaller analogue C 20 2+ . It is found that both molecules have the high polarisability characteristic of π systems and the diamagnetic magnetisability typical of aromatic molecules. Insofar as it is possible to extrapolate to the basis-set limit, the calculations predict net diamagnetism for the clusters, and a diamagnetic effect on the NMR chemical shift of an encapsulated atom.

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.

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.

MOLECULAR MAGNETIC PROPERTIES WITHIN CONTINUOUS TRANSFORMATIONS OF ORIGIN OF THE CURRENT DENSITY

A new method for the calculation of molecular magnetic susceptibility and nuclear magnetic shielding is presented. It is based on continuous transformations of origin of the current density which make the transverse part of the paramagnetic current vanish. For any molecule all the components of the nuclear magnetic shielding tensor provided by the new method are independent on the origin of the gauge, whereas the components of the magnetizability tensor are translationally invariant only for center‐symmetric molecules (they, in general, show a linear dependence on the shift of origin). This method, termed CTOCD‐PZ, has been implemented for the theoretical determination of molecular magnetic properties via numerical integration techniques and, from preliminar results obtained for methane and carbon dioxide molecules, reveals reliable.

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.

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.