Gabriella Poggi

Vibronic coupling in the benzyl radical

Abstract The vibronic perturbations among the lowest excited states of benzyl, computed by a CNDO/S program in the floating-orbital scheme, are presented. The ν8b (ν18b) modes are the vibrations that most strongly (weakly) couple the quasidegenerate 1A2 and 2B2 states. The results are relevant to the interpretation of benzyl emission and absorption spectra.

Effect of the nature of the aromatic groups on the lowest excited states of trans-1,2-diarylethenes

The photophysical and photochemical behaviour of a series of diarylethenes has been investigated experimentally. For the same compounds the potential-energy curves of trans→cis photoisomerisations and the ethenic bond orders in the S1 and T1 states have been calculated. The purpose of the study is to assess which properties of the aryls govern the photophysical behaviour of the diarylethenes. It is found that the relative energies of the La and Lb states of the larger aryl with respect to the ethene Bu state determine to a large extent the photophysical and photochemical properties of the diarylethenes.

A photophysical and theoretical study of styrylanthracenes

The photophysical properties of the three isomeric trans-n-styrylanthracenes (n-StAs, with n= 1, 2, 9), whose lowest excited states are expected to display a prevalent aromatic character, have been studied in different solvents. Fluorescence quantum yields and lifetimes have been measured as a function of temperature in a wide temperature range. All trans compounds, including the two rotamers of 2-StA, are relatively highly fluorescent and do not photoisomerize to the cis compounds. Semiempirical MO + Cl Calculations have been performed to assess the shape of the potential-energy surfaces in the lowest excited states of the compounds investigated. These calculations indicate that S1 and T1 in all the n-StAs have a sizeable barrier on the torsional coordinate from trans to cis. The barrier for the cis- to trans- process is expected to be lower. The combined results of the experimental and theoretical studies offer a clear description of the excited-state behaviour of trans StAs.

The generation and spectral characterization of oligothiophenes radical cations. A pulse radiolysis investigation

Abstract Conjugated polymers and oligomers of thiophene have been employed in a number of electronic devices due to the change in their conductivity by several orders of magnitude when oxidized to their cationic states. The radical cations of oligothiophenes (nT), with number of rings n =1–6, have been produced by pulse radiolysis for the first time and their spectra have been characterized. Electronic structure and transition energies are briefly discussed in the light of semiempirical theoretical calculations.

On the stereoselectivity of the Paternò-Büchi reaction between carbonyl compounds and 2-furylmethanol derivatives. The case of aliphatic aldehydes and ketones

Abstract The Paterno–Buchi reaction between 2-furylmethanol derivatives and aliphatic aldehydes and ketones induced by irradiation through Vycor at 8°C shows high regioselectivity but no stereoselectivity. This behaviour can be rationalised by assuming that this type of compound reacts through both singlet and triplet excited states. Ab initio calculations are in agreement with the formation of the 1,4-biradical. The more stable biradical accounts for the observed regioselectivity. The lack of stereoselectivity was discussed on the basis of two hypotheses. The allylic strain proposed by Adam does not account for the observed results. On the contrary, hydrogen bond interaction between (triplet excited) carbonyl oxygen and hydroxy group is able to describe the observed behaviour.

An ab initio study of hydrogen abstraction by fluorine, chlorine and bromine atoms from ethane and propane

Abstract The H-abstraction reaction by F, Cl and Br radicals from ethane and propane is studied using ab initio UHF, UHF/ MP2 and UHF/MP4 computational methods with the 6–31G ∗ basis set. It is found that in all the cases examined the reaction proceeds through a transition state where the halogen atom, the abstracted H atom and the C atom are almost collinear. The geometries of the various transition states are not significantly modified by the dynamic correlation corrections included with the MP2 treatment. However the correlation energy contributions on reactants and transition states are essential to obtain values of activation energy in reasonable agreement with experimental results. Also the trend of reactivity, which decreases in the direction F → Cl → Br, and the selectivity toward primary and secondary hydrogens are in good agreement with experiment. Another interesting point concerns the performance of the effective potential basis set LAN1DZ, which is found to yield reliable geometries for the transition states. So, more accurate activation energies can be obtained cheaply by single-point computations with the 6–31G ∗ basis set on LANL1DZ optimized geometries. A simple diabatic model is used to rationalize the trend of reactivity along the series F → Cl → Br and the greater reactivity of secondary versus primary hydrogens.

The spectral characterization of thiophene radical cation generated by pulse radiolysis

Pulse radiolysis matched with kinetic spectrometry has been employed to produce and characterize the radical cations of the series of unsubstituted oligothiophenes from one to six rings in dichloromethane dilute solutions. The concentration of radicals has been tuned to low values so as to slow down dimerization processes. The spectra obtained for the radical cations with 2 to 6 rings agree with previous literature reports. The novel isolation of the radical cation of thiophene itself and its spectral characterization, both experimental and theoretical, stresses its close relationship to the electronic structure of oligothiophenes. This fact strongly recommends that the charge carrier properties of oligothiophenes be interpreted on the basis of the molecular orbital theory rather than of the polaron model. The state responsible for the main UV absorption band is found to be described by a mixture of configurations where the HOMO→LUMO transition is present with different spin couplings.

An ab initio study of the competing reaction channels in the reaction of HOCO radicals with NO and O2.

The reaction between HOCO and NO, and that between HOCO and O(2), have been examined using the quadratic configuration interaction method to locate and optimize the critical points on the potential energy surfaces. Analysis of the critical points provides new insight into new intermediates and pathways by which these reactions occur and help explain recent experimental results. In the HOCO+O(2) reaction, the symmetry-allowed products, CO(2)+HO(2), can be obtained both via direct hydrogen abstraction by O(2) on the HCO radicals, as well as through an adduct, HOC(O)O(2), which can proceed to give the same products. The less-than-unity yield of CO(2) observed in the experimental studies of the HOCO+NO reaction, as well as the lack of CO, can be explained by the formation of a stable HOC(O)NO adduct.

Ab initio SCF–M.O.–CI description of the lowest excited states of planar and twisted styrene by an extended π orbital basis

The results of two different ab initio CI descriptions of planar and twisted styrene have been compared, the distinction lying in the basis used for π orbitals, which was minimal in one case and extended in the other, while minimal basis orbitals were used throughout for the σ skeleton. The need for a more accurate description seems to arise in the vicinity of the perpendicular geometry, where the zwitterionic states relevant in that region are strongly stabilized with respect to the diradical-type states by the presence of diffuse orbitals. In particular, the state corresponding to electron migration from methylene to benzyl becomes the lowest excited state at the twisted geometry in the extended basis description. The theoretical results are discussed in the light of current experimental knowledge of the system.

An ab initio study of H abstraction in halogen-substituted methanes by the -OH radical

Abstract The H-abstraction reaction by the OH radical from methane and various halomethanes (CH 3 Cl, CH 3 F, CH 2 Cl 2 , CH 2 ClF, CH 2 F 2 , CHCl 2 F and CHF 3 ) was studied using ab initio unrestricted Hartree-Fock (UHF) and multiconfigurational self-consistent field (MCSCF) computational methods. It was found that the structure of the transition state determined at the UHF and MCSCF levels is very similar and that the topology of these reaction surfaces is satisfactorily described using a UHF wavefunction. It is pointed out that to obtain values of the activation energies in good agreement with experiment it is essential to take into account the correlation energy contribution on reactants and transition states. The MP2/6-31G ∗ //3-21 G ∗ approach (6–31 G ∗ single-point second-order Moller-Plesset (MP2) computations on the 3-21G s* optimized geometries) is proposed as the minimal computational level needed to obtain a description of these reactions in reasonable agreement with the experimental evidence.