Electrochromic properties of poly(alkoxy-terthiophenes): an experimental and theoretical investigation

Much effort has been made to synthesize novel compounds and enhance the optical properties of poly(terthiophenes). The electrochromic properties of poly(4,4′′-dimetoxy-3′-methyl-2′-5′,2′′-terthiophene) (PDMMT) and poly(4,4′′-dipentoxy-3′-methyl-2′-5′,2′′-terthiophene) (PDPMT) have been studied focusing on the differences in the alkoxy-group length. Theoretical calculations were employed to elucidate the structural and thermodynamic stability of the monomers and dimmers. The results showed that in this type of thiophenes large alkoxy chains assist positive charge dispersion through hyperconjugative effect. Thus, PDPMT is thermodynamically more stable than PDMMT in the oxidized state, leading to better electrochromic stability and optical memory.     

Molecular structure and electronic properties of a series of oligoalkylthiophenes: A theoretical investigation

Molecular geometries and electronic properties of 3-alkylthiophenes (ATs) and their oligomers (OATs) are studied by the density functional theory (DFT). Calculations are performed on the oligomers formed by n repeating units, where n ranges from 1 to 6, using the B3LYP/6-31G** level of theory. The results obtained show that the doped oligomers have more satisfactory structural and electronic characteristics for the conducting polymers. The conjugated system in the doped oligomers has more aromaticity, with expanded and planar chains. The calculated energy gap values between the frontal orbitals and also the ionization potential values for the oligomers indicate that with increase in the oligomer chain length, the conductive band gap decreases. Furthermore, our calculations suggest that an electron-donating alkyl substituent at position 3 of the thiophene ring plays an important role in the structural and electronic properties of the polymers.     

An efficient synthesis of quinazolines: a theoretical and experimental study on the photochemistry of oxime derivatives

A new photochemical method for the preparation of quinazolines (2) through irradiation of [2-(methyleneamino)phenyl]methanone oximes (1) is reported. The photoreaction has been studied in depth by experimental, theoretical and photochemical methods. A six-electron electrocyclic ring closure mechanism, followed by water loss, is proposed and rationalized to explain the formation of quinazolines (2).     

Influence of 5-chloro and 5-methyl benzotriazole on the corrosion of copper in acid solution: an experimental and a theoretical approach

The inhibition of copper corrosion in aerated 0.1 mol l⁻¹ hydrochloric acid solutions was studied using electrochemical polarization in the presence of different concentrations of benzotriazole and its two derivatives, 5-chloro and 5-methyl benzotriazole. The inhibition efficiencies obtained from cathodic Tafel plots increased markedly with increase in the additive concentration. Benzotriazole and 5-methyl-benzotriazole were found to be cathodic type corrosion inhibitors for concentrations higher than 10⁻⁴ mol l⁻¹ . However, the 5-chloro-benzotriazole was found to be a mixed inhibitor for concentrations up to 10⁻³ mol l⁻¹, above this concentration the inhibitor behaves as an anodic type inhibitor. The inhibitors are physisorbed on the copper surface following a Langmuir’s isotherm. The inhibition efficiencies depended on the inhibitor concentration and follows the order 5-chloro-benzotriazole > 5-methyl-benzotriazole > 1-H-benzotriazole. From the theoretical calculations, the change in the inhibition mechanism observed for 5-chloro-benzotriazole at concentrations higher than 10⁻³ mol l⁻¹ is associated with the electronic acceptor characteristic of chloro, which increases the benzotriazole acidity allowing the formation of CuBTA.     

Conformational analysis of 2-halocyclopentanones by NMR and IR spectroscopies and theoretical calculations

The conformational isomerism of 2-chlorocyclopentanone and 2-bromocyclopentanone has been determined through the solvent dependence of the ¹H NMR ³J_HH coupling constants, theoretical calculations and infrared data, using the solvation theory for the treatment of NMR data. In 2-chlorocyclopentanone, the energy difference (E_Ψ-e − E_Ψ-a), in the isolated molecule at B3LYP level of theory, between the pseudo-equatorial (Ψ-e) and pseudo-axial (Ψ-a) conformers is 0.42 kcal mol⁻¹, which decreases in CCl₄ and in acetonitrile solutions, in good agreement with infrared data (ν_CO), despite the uncertainties of the latter method. The conformational equilibrium for 2-bromocyclopentanone is also between the Ψ-e and Ψ-a conformations, with an energy difference (E_Ψ-e − E_Ψ-a), in the isolated molecule at B3LYP level of theory, is 0.85 kcal mol⁻¹ which decreases in CCl₄ and in acetonitrile solutions, also in good agreement with infrared data.     

Interaction in trans-2-halocyclohexanols — an infrared and theoretical study

The O–H and C–O stretching frequencies of trans-2-halocyclohexanols in CCl₄ solutions have been measured and theoretical calculations have been performed to elucidate the main interactions, which are responsible for the conformational equilibria in these systems. It can be concluded that hydrogen bonding is predominant for trans-2-fluorocyclohexanol, leading to a stabilization of the eq–eq conformation, while for the chlorine, bromine and iodine derivatives, besides hydrogen bonding, gauche and steric interactions are also present.     

Reference molecules for nonlinear optics: a joint experimental and theoretical investigation

Hyper-Rayleigh scattering (HRS) experiments and quantum chemical calculations are combined to investigate the second-order nonlinear optical responses of a series of reference molecules, namely, carbon tetrachloride, chloroform, trichloroacetonitrile, acetonitrile, and dichloromethane. The multipolar decomposition of the first hyperpolarizability tensor through the use of the spherical harmonics formalism is employed to highlight the impact of the symmetry of the molecular scatterers on their nonlinear optical responses. It is demonstrated that HRS is a technique of choice to probe the molecular symmetry of the compounds. Coupled-cluster calculations performed at the coupled-cluster level with singles, doubles, and perturbative triples in combination with highly extended basis sets and including environment effects by using the polarizable continuum model qualitatively reproduce the molecular first hyperpolarizabilities and depolarization ratios of the molecular scatterers.     

The molecular structure and conformation of trichloronitromethane as determined by gas-phase electron diffraction and theoretical calculations

The molecular structure of trichloronitromethane has been studied in the gas phase using electron diffraction data. The molecules are found to undergo low barrier rotation about the CN bond with a planar CNO₂ moiety in agreement with HF/MP2/B3LYP/6-311G(d,p) calculations. The experimental data are consistent with a dynamic model using a potential function for the torsion of V = (V₆/2)(1 − cos 6τ). The major geometrical parameters (r_g and ) for the eclipsed form, obtained from least squares analysis of the data are as follows: r(NO₃) = r(NO₄) = 1.213(2) Å, r(CN) = 1.592(6) Å, r(CCl)_av = 1.749(1) Å, Cl₅CN/Cl₆CN = 109. 6°/106.3°(2), O₃NC/O₄NC = 117. 6°/114.1°(4), τCl₅C₁N₂O₃ = 0.0°, and V₆ = 0.20(25) kcal/mol.     

Principle and mechanism of direct porphyrin metalation: joint experimental and theoretical investigation

The direct metalation of tetraphenylporphyrin with bare metal atoms (Co and Zn) was studied with X-ray photoelectron spectroscopy, scanning tunneling microscopy, and temperature-programmed reaction measurements on ordered monolayer films of the molecules adsorbed on a Ag(111) surface. The mechanism of this novel type of surface reaction was investigated using density functional theory (DFT) calculations for the related gas-phase reactions of the unsubstituted porphyrin with the metals Fe, Co, Ni, Cu, and Zn. The reaction starts with the formation of an initial complex, in which the metal atom is coordinated by the intact unreduced porphyrin. This complex resembles the sitting-atop complex proposed for porphyrin metalation with metal ions in solution. In two subsequent steps, the pyrrolic hydrogen atoms are transferred to the metal atom, forming H2, which is eventually released. The activation barriers of the H-transfer steps vary for the different metal atoms. DFT calculations suggest that metalations with Fe, Co, and Ni show two-state reactivity, while those with Cu and Zn proceed on a single potential energy surface. For metalation with Zn, we calculated a barrier of the first hydrogen transfer step of 32.6 kcal mol(-1), in good agreement with the overall experimental activation energy of 31 kcal mol(-1).     

Ground state multiplicity of acylnitrenes: computational and experimental studies

The singlet—triplet energy splitting (ΔE _ST = E _S − E _T ) was calculated for formylnitrene (5) and for the syn- and anti-rotamers of carboxynitrene HOC(O)N (6) by the CCSD(T) method. Extrapolation of ΔE _ST to a complete basis set was calculated to be negative for 5 and strongly positive for 6. Similar results were obtained by the G2 procedure. The reason for the dramatic stabilization of the singlet state appeared to be a special bonding interaction between the nitrogen and oxygen atoms, which results in the structure intermediate between those of nitrene and oxazirene. It was found that the B3LYP/6-31G(d) method overestimates ΔE _ST by ∼9 kcal mol⁻¹ for 5 and by ∼7 kcal mol⁻¹ for 6. Taking into account this overestimation and the results of DFT calculations, it was concluded that benzoylnitrene has a singlet ground state. It was proved experimentally using photolysis of benzoyl azide in an argon matrix at 12 K that benzoylnitrene has a singlet ground state and its structure is similar to that of oxazirene. Nevertheless, these singlet intermediates have low barrier to the aziridine formation, which is traditionally considered to be a typical singlet nitrene reaction.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 519–526, March, 2005.     

Gas-phase chemistry of ionized and protonated GeF4: a joint experimental and theoretical study

The gas-phase ion chemistry of GeF(4) and of its mixtures with water, ammonia and hydrocarbons was investigated by ion trap mass spectrometry (ITMS) and ab initio calculations. Under ITMS conditions, the only fragment detected from ionized GeF(4) is GeF(3)(+). This cation is a strong Lewis acid, able to react with H(2)O, NH(3) and the unsaturated C(2)H(2), C(2)H(4) and C(6)H(6) by addition-HF elimination reactions to form F(2)Ge(XH)(+), FGe(XH)(2)(+), Ge(XH)(3)(+) (X = OH or NH(2)), F(2)GeC(2)H(+), F(2)GeC(2)H(3)(+) and F(2)GeC(6)H(5)(+). The structure, stability and thermochemistry of these products and the mechanistic aspects of the exemplary reactions of GeF(3)(+) with H(2)O, NH(3) and C(6)H(6) were investigated by MP2 and coupled cluster calculations. The experimental proton affinity (PA) and gas basicity (GB) of GeF(4) were estimated as 121.5 ± 6.0 and 117.1 ± 6.0 kcal mol(-1), respectively, and GeF(4)H(+) was theoretically characterized as an ion-dipole complex between GeF(3)(+) and HF. Consistently, it reacts with simple inorganic and organic molecules to form GeF(3)(+)-L complexes (L = H(2)O, NH(3), C(2)H(2), C(2)H(4), C(6)H(6), CO(2), SO(2) and GeF(4)). The theoretical investigation of the stability of these ions with respect to GeF(3)(+) and L disclosed nearly linear correlations between their dissociation enthalpies and free energies and the PA and GB of L. Comparing the behavior of GeF(3)(+) with the previously investigated CF(3)(+) and SiF(3)(+) revealed a periodically reversed order of reactivity CF(3)(+) < GeF(3)(+) < SiF(3)(+). This parallels the order of the Lewis acidities of the three cations.     

An integrated experimental and theoretical investigation of the vibrational modes and molecular structure of a new Cu(II)–phosphocreatine complex

A new complex of Cu(II) of stoichiometry [Cu(PCr)(H₂O)] was prepared from aqueous solution at pH 6. Its synthesis, characterization, thermogravimetric, vibrational spectroscopy, and electron paramagnetic resonance analyses were described and suggest that phosphocreatine (PCr) in solid state is acting as a tridentate ligand (the nitrogen atom of the guanidine group and the oxygen atoms of the phosphate group and the carboxylate group being the donor atoms). The fourth position is occupied by a water molecule. These results were confirmed through computational calculations (DFT/B3LYP:6-311G theoretical procedure). The tetra coordination of Cu(II) arranged in a quadratic planar geometry was found to be more stable in the DFT calculations. The calculated vibrational spectrum agrees well with the FT-IR experimental spectrum.     

Mechanistic study of the collision-induced dissociation of sodium-cationized polylactide oligomers: A joint experimental and theoretical investigation

The low-kinetic energy collision-induced dissociation (CID) behavior of different sodiumcationized polylactide (PLA) oligomers was thoroughly investigated to shed some light on the analytical potentialities of CID experiments in the context of polymer characterization. Indeed, investigation of several end-groups modified PLA reveals that, in addition to the expected end-group specific dissociations, collisionally-excited PLA.Na⁺ suffer from a backbone cleavage. The so-obtained sodium-bound dimer cations consecutively undergo the loss of a monomeric residue that corresponds to neutral acrylic acid. The experimental observations, performed on a hybrid Q-ToF instrument, were totally corroborated by a theoretical study involving DFT calculations, molecular mechanics, and molecular dynamics calculations.     

Infrared spectra of two sexithiophenes in neutral and doped states: a theoretical and spectroscopic study

The FT-infrared spectra of two sexithiophenes having their end ,′-positions substituted by n-hexyl or -thiohexyl groups, in neutral and doped states, are studied with the main aim of deriving information about the π-electrons delocalization and about the electronic structure of the charged defects created upon doping with iodine. The analysis of the experimental data is aided by Density Functional Theory calculations. The modifications in the electronic structure of the sexithiophene backbone induced by the n-thiohexyl encapsulation are discussed from the point of view of single molecule interactions in thiol-terminated π-conjugated oligomers bound to metallic or cluster electrodes.     

Chemical effects in SERS of pyrazine adsorbed on Au-Pd bimetallic nanoparticles: a theoretical investigation

Chemical enhancement in surface-enhanced Raman scattering (SERS) of pyrazine adsorbed on Au-Pd nanoclusters is investigated by using density functional theory. Changing Pd content in the bimetallic clusters enables modulation of the direct chemical interactions between the pyrazine and the clusters. The magnitude of chemical enhancement is correlated well with the induced polarizability for the complexes with low Pd content, which fails for the complexes with high Pd content. Furthermore, the dependence of chemical enhancement on cluster size and coupling is also described by the induced polarizability. Additionally, the chemical enhancement in the cluster-molecule-cluster junction is found to account for as much as 10(3), which suggests that a chemical mechanism might be more important than previously believed, in particular for Au-Pd bimetallic nanoparticle aggregates.     

Structural study of 8-azole derivatives of protoberberine alkaloids: experimental and quantum chemical approach

New derivatives of protoberberine alkaloids were prepared by nucleophilic addition of some azoles (differing in bulkiness) to the iminium functionality of the quaternary protoberberine alkaloids. Compounds were structurally characterized mainly by ¹H and ¹³C NMR spectroscopy, and the structure of 8-carbazol-1-yl-7,8-dihydroberberine was determined using single-crystal X-ray diffraction. Additionally, conformational behaviors of five derivatives varying in bulkiness of the azole moiety have been investigated by low temperature NMR spectroscopy and quantum chemical calculations. Ring current effects of pyrrole and carbazole moieties on selected ¹H NMR resonances have been characterized, visualized, and discussed.