Ferdinando Taddei

Exocyclic push–pull conjugated compounds. Part 2. The effect of donor and acceptor substituents on the rotational barrier of push–pull ethylenes

Abstract The energy of the rotational barriers and electronic structure of the transition state in substituted ethylenes are discussed in the light of the results obtained from different theoretical MO ab initio approaches. The 6-31G ∗ basis set at Hartree–Fock (HF) level and with second-order Moller–Plesset perturbation theory (MP2) was employed, critical points were localized through full geometry relaxation and characterized by vibrational analysis. A multiconfigurational approach (MCSCF) with different active spaces was also employed. For alkenes the correct rotational transition state is obtained only from the MCSCF approach, whereas for push–pull olefins the HF approach with correlation corrections at MP2 level provides correct answers for the internal rotation around C(sp 2 )–C(sp 2 ) bonds. The choice is more critical when only acceptor or donor groups are present, especially when change of hybridization occurs at the atoms at the edges of the C–C bond in the critical points.

Coupled and uncoupled Hartree-Fock calculations of ring currents and proton chemical shifts: the aromatic character of five-membered heterocycles

Coupled and uncoupled Hartree-Fock perturbation theories are used within the framework of the ring current model to discuss the proton chemical shift of heterocyclic molecules. An appropriate partition of shielding constants is suggested and the calculated values support the hypothesis of a limited degree of aromaticity of this class of compounds. In addition, the results indicate that the commonly accepted scale of aromatic character based on chemical criteria is less consistent than that obtained by spectroscopical methods.

Exocyclic push–pull conjugated compounds. Part 1. Theoretical study of the effect of ring size on the structure, electronic properties and rotational barriers of cyclic analogoues of 1,1-diamino-2,2-dicyanoethylene

Abstract An MO theoretical ab initio study was performed on 2-exo-methylene push–pull derivatives having as donor groups nitrogen atoms, components of a heterocyclic ring, and as acceptors CN and COOEt groups. Five-, six- and seven-membered ring derivatives were considered. Calculations were also performed on a number of push–pull ethylenes whose experimental properties were reported in the chemical literature in order to test the soundness of the conclusions from theoretical approaches. The physical properties calculated for the latter molecules were compared with known experimental values, in order to check the predictive ability of the theoretical approaches employed. Geometrical features and torsional barriers in solution are satisfactorily reproduced. Results were obtained with different basis sets, second-order Moller–Plesset theory, in order to perform comparisons at different theoretical levels with a view to carrying out calculations in larger molecular systems. The widest range of comparisons between calculated values for the different molecules were carried out at the HF/6-31G∗//HF/6-31G∗ level. The origin of the torsional barrier for isomer interconversion as a function of the electronic properties of these molecules is discussed, in particular by examining the polarized character of the exo double bond. The role of the lone pairs of the nitrogen atoms in the push–pull mechanism is investigated, also in competition with an unsaturated bond within the ring. The different conjugation patterns that can be exploited within these molecules is examined within the donor–acceptor model and Natural Bond Orbital (NBO) theory. Empirical correlations are proposed in order to estimate dipole moments and absorption wavelengths for the family of push–pull olefins.

Exocyclic push–pull conjugated compounds. Part 3. An experimental NMR and theoretical MO ab initio study of the structure, the electronic properties and barriers to rotation about the exocyclic partial double bond in 2-exo-methylene- and 2-cyanoimino-quinazolines and -benzodiazepines

Abstract The structure of a number of 2-exo-methylene substituted quinazolines and benzodiazepines, respectively, 1, 3a,b, 4 ( X=–CN, –COOEt ) and their 2-cyanoimino substituted analogues 2, 3c,d ( X=–CN, –SO 2 C 6 H 4 –Me (p) was completely assigned by the whole arsenal of 1D and 2D NMR spectroscopic methods. The E/Z isomerism at the exo-cyclic double bond was determined by both NMR spectroscopy and confirmed by ab initio quantum chemical calculations; the Z isomer is the preferred one, its amount proved dependent on steric hindrance. Due to the push–pull effect in this part of the molecules the restricted rotation about the partial C2,C11 and C2,N11 double bonds, could also be studied and the barrier to rotation measured by dynamic NMR spectroscopy. The free energies of activation of this dynamic process proved very similar along the compounds studied but being dependent on the polarity of the solvent. Quantum chemical calculations at the ab initio level were employed to prove the stereochemistry at the exo-cyclic partial double bonds of 1–4, to calculate the barriers to rotation but also to discuss in detail both the ground and the transition state of the latter dynamic process in order to better understand electronic, inter- and intramolecular effects on the barrier to rotation which could be determined experimentally. In the cyanoimino substituted compounds 2, 3c,d, the MO ab initio calculations evidence the isomer interconversion to be better described by the internal rotation process than by the lateral shift mechanism.

Anisotropy of the nuclear spin coupling in PH−2, PH3, and PH+4 molecules

The anisotropy of the nuclear spin–spin coupling tensor, relative to P, H and H, H interacting nuclei, has been investigated by means of extended ab initio calculations in the molecules PH−2, PH3, and PH+4. The coupled Hartree–Fock perturbed scheme has been employed retaining the nonrelativistic Ramsey Hamiltonians. The most important contributions to the anisotropy in the P, H coupling arise from the Fermi contact‐spin‐dipolar cross term, which is partially counterbalanced by the spin‐orbit effect. The discrepancies emerging between computed and experimental J(P,H) in phosphine and phosphonium are interpreted in terms of neglected electron correlation contributions. A dramatic sensitivity of the Fermi contact term of the molecular geometry has been found for PH2−. The calculations show that the P–H coupling is more anisotropic than N–H, which is seemingly due to the enhanced nonsphericity of electron distribution.

Conformational properties and homolytic bond cleavage of organic peroxides. I: an empirical approach based upon molecular mechanics and ab initio calculations

The conformational features of a large number of hydroperoxides ROOH and peroxides ROOR′, where R and R′ are alkyl groups of different and increasing size and phenyl rings, including ortho substituted derivatives, were obtained from molecular mechanics calculations by employing a standard package. For the molecules of small molecular size, comparison was carried out with the results of ab initio calculations. Heats of formation were also obtained from molecular mechanics for hydroperoxides and peroxides: The values are, in general, overestimated. For the molecules containing the CF3 group, the calculated values are subject to large errors and heats of formation were obtained from ab initio total energies in the “atom equivalents” scheme. To estimate the homolytic dissociation energies of the different bonds in the peroxide molecules, heats of formation of R·, ·OR, and ·OOR radicals were employed and several of them had to be calculated. Different approaches were employed—molecular mechanics calculations, ab initio energies within the atom equivalent and isodesmic reaction schemes, and Bensons group additivity rule; values consistent within the different calculation methods were chosen for estimating dissociation energies. The bond dissociation energies indicate different trends in these molecules as a function of the nature of the R and R′ groups and the possible electronic effects operating in these molecules are discussed.

Homolytic bond-dissociation in peroxides, peroxyacids, peroxyesters and related radicals: ab-initio MO calculations.

Abstract Homolytic dissociation energies for different cleavage paths in a number of peroxides peroxyacids and peroxyesters and in the radicals formed from these molecules, were calculated from the total MO ab-initio molecular energies of the chemical species involved in the bond cleavage reactions and compared with experimental values, where known.

The dissociative nature of the radical anion of benzyl chloride. A theoretical MO ab initio approach

Abstract The potential energy profile of the benzyl chloride radical anion as a function of the CCl bond distance calculated with ab initio MO theory, 6–31G∗ level and molecular relaxation shows a purely dissociative behavior, provided that constraints are dictated to the direction of the departing chloride anion. If the departing anion is allowed complete freedom, a minimum in the energy profile is found corresponding to a molecular complex between the benzyl radical and the chloride anion. This behavior differs from that of the chlorobenzene radical anion, which shows a change of the symmetry of the electronic ground state from 2B1 to 2A1 on elongating the CCl bond distance.

Ground-state molecular stabilization of substituted ethylenes. A theoretical mo ab-initio thermochemical study

Abstract The origin of the rotational barrier around the partial C C double bond in substituted ethylenes is discussed with reference to the stabilization of the conformational minimum (GS) and of the rotational transition state (TS). Molecules with different polar character of the double bond were chosen, ranging from ethylene to olefins with strong push–pull character. The enthalpies of hydrogenation, Δ H hydr. , and of formation, Δ H f 0 , of these molecules were employed to obtain the stability of GS; these thermochemical properties were calculated with MO ab-initio theory at HF/6-311G ∗∗ //HF/6-311G ∗∗ and MP2/6-311G ∗∗ //HF/6-311G ∗∗ levels and with CBS-4M model chemistry. The stabilization of TS was derived from the torsional potential for rotation around the C C bond. The lowering of the energy content of GS of substituted ethylenes, referring to ethylene, is accompanied by an even greater stabilization of TS, thus a lowering of the rotational barrier with respect to ethylene is generally found in these molecules.

A THEORETICAL MO AB INITIO APPROACH TO THE CONFORMATIONAL PROPERTIES AND HOMOLYTIC BOND CLEAVAGE IN ARYL DISULPHIDES

Abstract The structural and conformational properties of disulphides R1S-SR2, with R1 = phenil ring (Ph) and R2 = H, Me and Ph and, for a comparison, those of the disulphides with R1,R2 = H,Me were determined with MO ab initio calculations. The level of theory chosen was the 3–21G∗ with full geometry relaxation and electron correlation corrections at a second order Moller-Plesset perturbation scheme (MP2/3-21G∗//MP2/3-21G∗). This choice allowed a comparison of the properties calculated for the molecules containing alkyl and phenyl groups at the same level of theory. All the disulphides examined showed patterns of the potential energy for internal rotation with a minimum conformation of skew type and two maxima, one of cis and one of trans type, with the former representing the higher transition state for internal rotation. The effect of the R group is higher on the cis barrier and the effect of Me and Ph is respectively to increase and decrease the barrier, with respect to RH. The bond energy required for homolitically breaking the bonds (BDE) in these molecules and in the radicals RSS was estimated both from calculated total molecular energies, and from heats of formation of the species involved in the dissociation processes. The BDEs from the ab initio energies were found to have been largely underestimated, which applies to a different extent to the SS, SH and CS bonds. For the SS bond, the BDEs calculated at the level of theory adopted here are proportional to values from experiments or from higher levels of theoretical approaches. For the disulphides with R1,R2 = H,Me, calculations were performed with the sophisticated Gaussian-2 (G2) scheme and the BDEs obtained are very close to the most accurate determinations, showing that a high level of theory is necessary to obtain these quantities at a convenient level of confidence. The origin of the barriers for internal rotation and of the different bond strengths in the molecules and radicals containing the SS bond was analyzed within the natural bond orbital (NBO) description of donor-acceptor interactions.