Salvatore Millefiori

Electronic properties of neuroleptics: ionization energies of benzodiazepines.

AbstractVertical ionization energies (VIEs) of medazepam, nordazepam and their molecular subunits have been calculated using the electron propagator method in the P3/CEP-31G* approximation. Vertical electron affinities (VEAs) have been obtained with a ∆SCF procedure at the DFT-B3LYP/6-31+G* level of theory. Excellent correlations have been achieved between IEcalc and IEexp, allowing reliable assignment of the ionization processes. Our proposed assignment differs in many instances from that previously reported in the literature. The electronic structure of the frontier Dyson orbitals shows that the IE and EA values of the benzodiazepines can be modulated by substitution at the benzene rings. Hardness values, evaluated as (IE − EA)/2, follow the trend of the experimental singlet transition energies. Medazepam is a less hard (i.e., less stable) compound than nordazepam. FigureIEcalcvs IEexp for medazepam and nordazepam

Theoretical study of the structure and torsional potential of pyrrole oligomers

The molecular structure and conformational behaviour of 2,2′-bipyrrole (α-BPy), 2,2′:5′,2″-terpyrrole (α-TPy) and 2,2′:5′,2″:5″,2‴-quaterpyrrole (α-QPy) have been determined by abinitio HF, MP2 and density functional theory methods, using the 6-31G* basis set. The syn↔anti interconversion process through internal rotation about the C–C inter-ring bond generates in all calculations a three-barrier, four-fold potential. Minima are found in the anti-gauche and syn-gauche regions, maxima for the planar and perpendicular conformations, the anti-gauche structure being the global minimum. The energetics and the location of these critical points significantly depend on the theoretical method, the electron correlation and zero-point vibrational energies beng important factors. The relaxed rotor approximation must be used to obtain quantitative results, especially in the syn region where strong NH–NH dipole interactions are present which induce some loss of planarity of the pyrrole ring and tilting of the NH bond with respect to the ring plane. HF and B3LYP potential-energy curves are rather flat, particularly around the planar anti conformation, suggesting conformational flexibility. By contrast MP2 calculations strongly favour the anti-gauche form, indicating hindered rotation. The energetics and conformational behaviour of α-oligopyrroles are closely related to the torsional potential of the parent α-BPy. The minimum-energy conformations of α-TPy and α-QPy were all found to be anti-gauche (helix-like) structures. There is evidence that the π-electron system strengthens, the geometrical parameters of the pyrrole ring rapidly converge and the torsional potential around the planar anti conformation decreases as the α-oligomerization increases, suggesting that in the polymer limit planar, very conformationally flexible structures are highly probable, in agreement with the X-ray results in the solid. The potential-energy curves were analyzed in terms of conjugative and nonbonding interactions through a Fourier decomposition procedure. This highlights the predominant role of nonbonding interactions over conjugative ones and the peculiar behaviour of the MP2 method which favours hyperconjugative and probably NH–π hydrogen-bonding interactions which stabilize the perpendicular conformation.

(Hyper)polarizability of chalcogenophenes C4H4X (X = O, S, Se, Te) Conventional ab initio and density functional theory study

Abstract Equilibrium geometries, APT and NPA charge densities, dipole moments, and linear and quadratic polarizabilities of the five-membered chalcogenophenes C 4 H 4 X ( X = O , S , Se , Te ) were calculated by means of the all-electron conventional ab initio and density functional theory methods (S-null, B3-null, S-VWN and B3-LYP) using the specifically optimized Dougherty-Spackman and Sadlej basis sets. Very good agreement is found between theoretical and available experimental data. The results highlight the heteroatom and electron correlation effects on the molecular properties. Both linear and quadratic dipole polarizabilities increase steadily as the atomic number increases, the trend along the series being dominated by the heteroatom property and by the stability of the molecule. Possible correlations between dipole (hyper)polarizability and aromaticity, hardness, and electronic structure were investigated and briefly discussed. The exaltation polarizability parameter τ, a possible new aromaticity index, together with atomization energy evaluations, suggest that, in addition to molecular size effects, thermodynamic stability factors determine the average dipole polarizability values.

Theoretical determination of the vibrational and electronic (hyper)polarizabilities of C4H4X (X=O, S, Se, Te) heterocycles

Electronic, pure vibrational and dispersion contributions to the linear polarizability, α, and first hyperpolarizability, β, of the group 16 chalcogenides C4H4X (X=O, S, Se, Te) have been studied by ab initio calculations using the 6-31G*, the Sadlej POL, correlation consistent Dunning and aug-CEP-31G basis sets. Electron correlation was introduced through the MP2 theory. Vibrational (hyper)polarizabilities, evaluated within the double harmonic oscillator approximation, were analyzed in terms of single normal mode contributions. They were found to be a fraction of the electronic counterpart. βv does not show a clear dependence on the heavy atom, contrary to βe, which steadily increases along the series. The βv/βe ratio is very dependent on the basis set, decreasing and also changing sign in the presence of diffuse functions. Frequency-dependent electronic (hyper)polarizabilities were evaluated in the 0.02–0.16 a.u. frequency range. In the low-frequency range they can be fitted into a power series expansion in ω. In accord with the theory, the second-order coefficients of the β expansion are the same for both the SHG and EOPE non-linear optical processes, while in the α expansion they can be related to the lowest electronic excitation energy.

Ab initio and density functional theory calculations of the dipole polarizabilities of ethene, benzene and naphthalene

Geometries and static dipole polarizabilities α and polarizability anisotropy Δα of ethene, benzene and naphthalene have been investigated by ab initio and density functional theory (DFT) methods using a variety of basis sets. In DFT calculations contributions to the exchange energy were estimated by the Slater (S), Becke (B) and hybrid Becke (B3) functionals. Exchange correlation effects were estimated by combining these functionals with the Vosko, Wilk and Nusair (VWN) and Lee, Young and Parr (LYP) correlation functionals. Excellent agreement is found between the DFT results and the available experimental and high level correlated ab initio data, particularly when the B3LYP functional is used in conjunction with the aug-cc-pVDZ basis in molecular structure calculations and with the specifically designed HUZ-SV( + sd + sp) basis in polarizability calculations CC bond lengths being predicted with Δr rms = 0.007 A and mean dipole polarizability within 1–2% of the experimental value. The relative performance of the functionals increases in the order: S-null < B-null < B3-null < BLYP < SVWN < B3LYP. The exchange part of the functional appears to be dominant and tends to increase 〈α〉 and Δα values, the correlation part acting in balancing it. The molecular geometry plays a crucial role. Proper calculations carried out in ethene show that a great part of the correlation effects on 〈α〉 and Δα are encompassed in Hartree-Fock (HF) calculations which use the correlated geometry.

Second hyperpolarisability of furan homologues C4H4X (X=O, S, Se, Te): ab initio HF and DFT study

Abstract Static and dynamic second hyperpolarisabilities of the furan homologues C 4 H 4 X (X=O, S, Se, Te), were investigated at ab initio HF and DFT levels using the POL Sadlej basis sets. The results show a monotonic increase of the property along the series with 〈 γ 〉 C 4 H 4 X essentially determined by 〈 γ 〉 X , whereas dispersion contributions are almost constant. By comparison with CCSD(T) results, electron correlation contributions are well computed by the B3LYP method. Experimental 〈 γ 〉 values are reproduced within ca. 30% except for tellurophene, where it is underestimated by 85%.

Hydrogen bonding and tautomerism in 3-substituted β-thioxoketones: an ab initio molecular orbital study

The molecular geometry of some 3-substituted β-thioxoketones (R = CN, OH, F, CH3) and of 3-methyl-4-mercaptopent-3-en-2-one (6) has been fully optimised by ab initio molecular orbital calculations using the 3-21G basis set. The energetics of the intramolecular hydrogen bonding and tautomerism in these compounds have been studied using 6-31G** and MP2/6-31G** basis sets. The 3-substitution does not change the geometry of the hydrogen bridge except in the sterically hindered compound 6. Accordingly, the hydrogen-bonding strength (EH B) is little influenced by the substitutent, except in 6, where it increases by ca. 58% with respect to the parent compound, reaching 90 kJ mol–1. EH B does not follow Hammett-type behaviour in parallel with the observed variation in the chelate proton chemical shift of the related β-diketones. This is supported by the molecular orbital and population analysis which, inter alia, shows that the β-thioxoketone skeleton behaves as a strong σ-electron donor towards the 3-substituent, including the OH group. Very strong hydrogen bonds in this class of compounds can be achieved only by steric effects, electronic effects of the substituent being damped by conjugative effects and charge redistributions outside the H–bridge. The tautomeric equilibrium is markedly shifted towards the enolic form in the CN— and CH3— derivatives, whilst in the OH— derivative the enethiolic structure is favoured. The interconversion (Z)-enol ⇌(Z)-enethiol process cannot be regarded as an internal acid–base reaction. The tautomeric energy, relative to the parent compound, rather reflects differential conjugative effects. On the basis of the calculated dipole moment values, the stable gas-phase tautomer is expected to be relatively even more stable in solution.

Electronic spectra of nitrobenzene derivatives

Abstract The gas and solution phase u.v. spectra of some nitrobenzene derivatives with substituents of different electron-donating power were analysed in terms of locally excited states and intramolecular charge transfer configurations in the MIM and PPP approximations. The spectral behaviour of the examined compounds is defined by the different electron-donating power of the substituents and by the degree of mixing of the excited states, following the substitution position.

Theoretical investigation of the structure and conformational behaviour of small selenophene oligomers

Abstract The molecular structure and the conformational behaviour of selenophene, 2,2′-biselenophene (α-2Se), 2,2′:5′,2″-terselenophene (α-3Se) and 2,2′:5′,2″:5″,2″′-quaterselenophene (α-4Se) were determined through conventional ab initio and density functional calculations using a polarized valence double zeta basis set. Hartree—Fock (HF) calculations predict very flat four-fold torsional potentials where the minimum energy conformations correspond to anti—gauche structures and the less stable conformations to the syn form. The planar and perpendicular conformations are transition states. Zero point vibrational energy corrections have negligible effects. B3LYP calculations favour π-electron interactions suggesting that the planar anti form is the ground state of α-oligoselenophenes. Torsional potentials were analysed by a Fourier procedure in terms of non-bonding and conjugative interactions and were compared with corresponding data in sulfur analogues.

Tautomerism and intramolecular hydrogen bonding in β-thioxoketones. An ab initio molecular orbital study on monothiomalondialdehyde

The energetics of tautomerism and intramolecular hydrogen bonding of monothiomalondialdehyde have been studied by fully optimizing with the 3–21G basis set the molecular geometry of several conformers followed by single-point calculations at a higher level of theory. At MP2/6-31G** level the z-enol tautomer is predicted to be 2.09 kJ mol–1 more stable than the z-enethiol tautomer, in agreement with experimental results, while the inclusion of zero-point vibrational energy pushes the equilibrium considerably towards the z-enethiol structure. The OHS hydrogen bond is ca. five times stronger than the SHO bond and appears to be a decisive factor in the stabilization of the enolic structure. The complete set of Δ(SCF6-31G**) ionization potentials (valence and core) is reported and the results are briefly discussed by reference to the effects of the hydrogen bonding on the relative stabilities of the tautomers in the ionized states.