Π-electron Charge Research Articles

Metamagnetic Transition and Its Related Magnetocapacitance Effect in Phthalocyanine-Molecular Conductor Exhibiting Giant Magnetoresistance

The magnetization and transport properties of iron-phthalocyanine molecular conductors are investigated under pulsed high magnetic fields up to 55 T. The metamagnetic transition is observed at approximately 14 T, where conductivity is enhanced. Below this transition, the tan δ (=e′′/e′) in the dielectric constants shows a monotonic decrease as the magnetic field strength increases, indicating that the magnetic field stabilizes the charge order. On the verge of this transition, the dielectric constants show a hysteresis below the weak-ferromagnetic transition temperature (6 K), suggesting that the π-electron charge order contributes to the weak ferromagnetism.

Modified π-SCF MO CI calculations for anionic forms of pyrimidinic nucleic acid bases

The π-scf mo ci method (within the Pariser-Parr-Pople scheme) is modifed to interpret the ultraviolet spectra of anionic forms of molecules and to obtain the π-electronic charge distribution in the ground state of the anions similar to the charges calculated within the cndo/2 (or indo) procedure. The calculated positions of the electronic absorption bands for anions of uracil, its thioanalogs, cytosine, and isocytosine are in good agreement with the experimental data. In particular, the calculations predict that the monoanions of uracil and thiouracils with the proton at N3 absorb at longer wavelengths than the corresponding monoanions with the proton at N1.

Taking into account universal intermolecular interactions by the Hückel molecular orbital method

We show that interactions between molecules according to a universal mechanism can be interpreted as changes in the Coulomb integrals in their Hamiltonians. Supplementing the ? technique with the assumption that there is a proportionality between the relative change in the π-electronic charge on the atom and the interaction energy causing these changes allows us to calculate the electronic spectra for solvated dye molecules and their van der Waals complexes. We show that in the molecular orbital calculation, a universal interaction function can act as a semiempirical parameter.

Charge and bonding redistribution in octatetraene driven by a strong laser field: Time-dependent Hartree–Fock simulation

The time evolution of the reduced single electron density matrix for the π electrons in conjugated molecules in an intense laser field is calculated by numerically integrating the time-dependent Hartree–Fock equations for the Pariser–Parr–Pople model Hamiltonian. We study the field frequency and intensity dependence of the π-electron response, the charge fluctuations with respect to the ground state, and the molecular polarization in unsubstituted and acceptor–donor substituted octatetraene, where we also examine the interaction between a permanent electric dipole moment and the laser field. Our results demonstrate the origin of large nonperturbative changes in the π-electronic charges and bond orders (diagonal and off-diagonal density matrix elements, respectively) induced by a strong laser field.

Studies of fluorescent dyes: part 1. An investigation of the electronic spectral properties of substituted coumarins

Coumarin derivatives represent one of the most important chemical classes of organic fluorescent materials. The results of PPP molecular orbital calculations, after refinement by parameter optimisation, were found to provide a good account of the electronic absorption spectra for a series of substituted coumarins. The electronic structures of several of the derivatives and the nature of the electronic excitation process are discussed in terms of the calculated π-electron charge densities. ©

Solvent effects within the CS INDO method. Geometrical distortion and solvatochromism of merocyanine dyes

This work represents the first step of a theoretical study aiming at explicitly including solvation in the CS INDO CI calculations of ground and excited state properties of organic dyes. The reported theoretical treatment falls within the electrostatic continuum theories and introduces solute–solvent interaction in the SCF calculation according to Klopman's solvaton model. The procedure was tested on two merocyanines characterized by opposite solvatochromic behaviours. It was found that solvaton systems most suitable for π-conjugated donor–acceptor dyes, like merocyanines, can be built from the net π-electron charges. The results show that the combined solvaton/CS INDO scheme is able to describe rather well the modifications of both geometries and absorption spectra of merocyanines as a function of the medium polarity. The advantages of the proposed approach with respect to other semi-empirical procedures are discussed.

The conjugation and effective charges of the atoms at the triple bond in germylacetylenes

The integrated extinction coefficients (A) of the C≡C stretching modes in the IR spectra of 12 germylacetylenes Me3GeC≡CR are determined by the resonance interactions of substituents with the triple bond. TheA1/2 values change linearly with change in the difference between the effective π-electron charges on the atoms at the triple bond and σ0R constants of organic substituents R. The average value of the σ0R constant of the Me3Ge substituent in the compounds studied is +0.06. The resonance acceptor effect of the Me3Ge substituent toward the triple bond (d,π-conjugation) is stronger than the donor effect (σ,π-conjugation).

An investigation of the electronic spectral properties of the coloured photoproducts derived from some photochromic naphtho[2,1- b]pyrans

Some chromenes are of current interest as photochromic dyes. The results of PPP molecular orbital calculations, after refinement by parameter optimisation, were found to provide a reasonable account of the experimental λ max values for the ring-opened photoproducts formed from a range of substituted naphtho[2,1- b]pyrans. The agreement between experimental and calculated values was less good for two compounds, but this may be explained on the basis of steric constraints which are likely to reduce molecular planarity. The electronic structure of one of the photoproducts and the nature of the electronic excitation process are discussed in terms of the calculated π-electron charge densities.

Ab initio MO Optimization of Molecular Structures of Fluoro- and Chloro-Substituted Dibenzo-p-dioxines and the Effect of Halogen Substitution at the 2,3,7,8-Positions on Metabolic Attack

Full ab initio optimizations were performed on the molecular structures of 24 fluorinated and chlorinated dibenzodioxines (PFDD/PCDD ) and dibenzofurans (PFDF/PCDF). Reasonable agreement was found by comparing the geometries of four calculated structures with known X-ray data from the literature. For the fluorine substituent, calculated electron densities (Mulliken total charges and π-electron charges) clearly demonstrate the opposite influence of the inductive (I) and mesomeric (M) effect. The changes in π-densities at carbons in ortho-, meta- and para-position are constant for each fluorine substituent (independent of degree, pattern, and position of substitution). It is thus possible to calculate the π-densities of the substituted dioxines by increments starting from dibenzodioxine. π-Charges from quantum mechanical calculations and the increment system show good agreement even for OctaFDD (O8FDD ), where eight substituent effects are acting additively. Compared with fluorine, the chlorine substituent exercises a smaller -I-effect and a clearly weaker +M-effect. The HOMO coefficients of the unsubstituted dibenzodioxine and dibenzofuran, extracted from ab initio calculations, yield a good explanation for the observed regioselective metabolic attack at the 2,3,7,8-positions. The squares of the HOMO-coefficients of the 2,3,7,8-positions in dibenzodioxine (DD ) are about ten times greater than those of the 1,4,6,9-positions. These HOMO coefficients are practically unaffected by halide substitution. But halogen substitution reduces strongly the electron density at the halogen-bound carbon, which, however, is a necessary prerequisite for the electrophilic oxygen transfer during metabolism. One would therefore expect halogen substitution of dibenzodioxine and dibenzofuran (DF) at the 2,3,7,8-position to hinder metabolism, as is indeed found. This provides a plausible explanation for the highly selective tissue retention of 2,3,7,8-substituted PCDDs and PCDFs. Our ab initio calculations of five tetra CDDs (T4CDDs) confirm the postulate of Kobayashi et al. [1 ] who, using semiempirical calculations, found a correlation between the toxicity of a dioxine congener and its absolute molecular hardness. The 2,3,7,8-T4CDD also exhibits the smallest absolute hardness (derived from the HOMO-LUMO energy gap) in our calculations.

Infrared Study of Substituted Acetophenones Adsorbed on Silica

Infrared spectra are reported of silica preheated at 873 K and exposed at 295 K to the vapors of four acetophenones (H, 4-Me, 4-OMe, and 4-NO2). Surface–adsorbate interactions involved hydrogen bonding between isolated silanol groups and carbonyl groups, the strength of the hydrogen bonds being related to the electronic effects of the acetophenone ring substituents. The 4-OMe compound also generated SiOH···OMe hydrogen bonds probably via a bridging structure involving two silanol groups perturbed simultaneously by each adsorbed molecule. The mode of adsorption of substituted benzene derivatives on silica is strongly dependent on the electronic effects of the substituents on each other and on the aromatic π-electron charge density.

Molecular and electronic structure of 1,2-disilacyclobutabenzenes. Ab initio molecular orbital and density functional study

The geometric and electronic structure of 1,2-disilacyclobutabenzene and its polyannelated analogues is examined by the MP2(fc)/6-31G * and B3LYP/6-31G * methods. It is found that the bond distances alternate in the benzene fragment, the ipso bond being the longest. This finding is rationalised by the rehybridization at the carbon junction atoms. The average C C distances are larger than in the free benzene, reflecting an interesting blow-up phenomenon. The latter is interpreted by the π-electron charge transfer interaction between the highest occupied MOs of the benzene ring and the φ * (SiH 2) antibonding fragment orbitals of the SiH 2 groups. The destabilisation energy of these systems was examined by the homodesmic reactions. It is shown that the total strain energy is additive to a good approximation, depending on the number of annelated four-membered rings.

Fast calculation of π-electron charge densities in organic molecules: Non-condensed compounds

A novel approach to fast computation of π-electron charges in non-condensed conjugated systems is described. The conjugated parts of the molecular structure are regarded as formed of simple rings (annulenes) and chains (polyenes) and their molecular orbital (MO) energies and eigenvectors are given with simple analytical expressions. The nature of the different atoms within the structure are further characterized by employing the perturbation MO theory. The reliability of the charges is tested by correlations with 13C nuclear magnetic resonance chemical shifts. Keywords: Phenyls, substituted; Molecular orbitals; Charge densities; Fast calculation; Finite differences theory; Nuclear magnetic resonance; Perturbation theory

Is dicyclopropylacetylene freely rotating? A combined ab initio and gas-phase electron diffraction investigation of the molecular structure and conformations of dicyclopropylacetylene

The molecular structure of dicylopropylacetylene (DCPA) has been studied by gas-phase electron diffraction and ab initio calculations using the basis sets 3–21G∗, 4–21G∗, 6–31G∗, 631-G∗∗, and MP2 6–31 G∗ . The diffraction data are consistent with the gauche rotamer ( C 2 symmetry) which has the bisectors of the cyclopropyl groups rotated 91.7(7.5)° away from one another. However, a model with a low torsional barrier in which the cyclopropyl rings are considered to be essentially freely rotating cannot be ruled out. Ab initio calculations utilizing the RHF 6–31 G∗ (and MP2 6–31 G∗ ) basis sets are consistent with the gauche rotamer lying 383.8 cal mol −1 (362.8 cal mol −1) and 418.5 cal mol −1 (376.4 cal mol −1) lower in energy than the trans and cis rotamers, respectively. Large amplitude treatment has been undertaken applying a dynamical model consisting of a mixture of numerous pseudo-conformers. The force field provided by the ab initio calculations was utilized to calculate the vibrational mean amplitudes and to determine the r α structure. The major bond distances ( r a ) obtained from least-squares analysis (uncertainties in parentheses are 3σ) are: r( CH) = 1.091(3) A ̊ , r( CC>) = 1.197(3) A ̊ , r( CC) = 1.438(4) A ̊ , r( C 1C 2) = 1.525(2) A ̊ , and r( C 2C 3) = 1.511 A ̊ (assumed). The torsional angle around the CCCC axis τ = 91.7(7.5)°. The noticeable shortening of the carbon-carbon single bonds between the electron-rich systems, the triple bond and the cyclopropyl rings is discussed invoking the concept of π-electronic charge distribution of the carbon-carbon triple bond and other approaches. In order to obtain more pertinent details about the origin of the potential barrier in DCPA, the structures and the potential barriers for internal rotation of the dihalogenated derivatives of DCPA, bis-(1-fluorocyclopropyl)acetylene, (1-fluorocyclopropyl)(1-chlorocyclopropyl)acetylene, and bis-(1-chlorocyclopropyl)acetylene have been determined by ab initio calculations using the basis sets 4–21G∗ and 6–31G∗. The values of the most interesting parameter in this context, the dihedral angle τ, were found to be 98.65°, 102.11°, and 107.08°, respectively. The structural results of DCPA and its dihalogenated derivatives are compared and discussed.

An investigation of the electronic spectral properties of the merocyanines derived from photochromic spiroindolinonaphth[2,1- b][1,4]oxazines

Spiroindolinonaphth[2,1- b][1,4]oxazines are of considerable current interest as photochromic dyes with good colouring power and fatigue resistance. The results of PPP molecular orbital calculations, after refinement by parameter optimisation, were found to provide a reasonable account of the experimental λ max values for the ring-opened merocyanine forms of a range of spirooxazines, although precise interpretation is complicated by the possible presence of a number of geometrical isomers. The electronic structure of the photomerocyanine derived from the parent compound and the nature of the electronic excitation process are discussed in terms of the calculated π-electron charge densities.

Colour and constitution relationships in organic pigments: Part 3—phthalocyanines

Phthalocyanines provide the most important classical organic pigments in the blue and green shade area and some derivatives are of interest as near infrared absorbers. The results of PPP molecular orbital calculations after refinement by parameter optimisation were found to provide good agreement with experimental λ max values for a range of metal-free phthalocyanines and their complexes. The nature of the electronic excitation processes are discussed in terms of the calculated changes in π-electron charge density.

π-Electron “ring currents” in dipleiadiene

Calculations are effected of the “London” π-electron magnetic properties of dipleiadiene, the synthesis of which was reported some years ago by Vogel et al. who, at the same time, made deductions about the π-electron “ring currents” in this molecule on the basis of its NMR spectra. Six different methods of calculation are invoked; these range in sophistication from the method of McWeeny, based on a simple Hückel molecular orbital, to the method of Coulson et al. (which makes minimal appeal to the London integral-approximation), based on a Pariser-Parr-Pople self-consistent-field wavefunction in which resonance integrals are made iteratively self-consistent with respect to the corresponding calculated bond orders. Discussion is included on trends in the predicted magnetic properties observed when the sophistication of the method of calculation is increased. It is concluded that at least the seven-membered ring of dipleiadiene appears to bear a paramagnetic π-electron “ring current”, though the overall “London” contribution to its magnetic susceptibility perpendicular to the molecular plane is probably weakly diamagnetic. It is thought that the very high field shifts of the peripheral attached protons in dipleiadiene (observed by Vogel et al. in the 1H NMR spectrum of the molecule) could as easily be due to non-uniform π-electron charge density effects in this non-alternant hydrocarbon as to the presence of strongly paramagnetic π-electron “ring currents”. The dianion of dipleiadiene is calculated to be strongly diamagnetic in its “London” π-electron magnetic properties (of the same order, in fact, as the neutral condensed benzenoid hydrocarbons of similar size) and the extent of this diamagnetism is found to be remarkably insensitive to the sophistication of the method used to calculate it.

Calculated “London” π-electron magnetic properties of some “perturbed” annulenes, their dications and dianions

The “periphery model” for planar polycyclic conjugated systems is tested by applying the method of Coulson et al. for calculating π-electron magnetic properties of such species to a selection of polycyclic neutral molecules, and their respective dianions and dications, all of which appear to have the attributes necessary for them to be considered “perturbed” annulenes. Because some of the species investigated have the potential to be strongly paramagnetic in their calculated π-electron magnetic properties, it was considered appropriate to base this application of the approach of Coulson et al. on a Pariser-Parr-Pople wavefunction that made resonance integrals iteratively self-consistent with respect to calculated bond orders. With one exception (which is discussed) these computations broadly support an analogy, in the context of conjugated systems that may be regarded as “perturbed annulenes”, of Sondheimer's observation that the dianions (and, by implication, the dications) of genuine (mono-cyclic) [4 n]-annulenes are diamagnetic, whereas doubly charged [4 n + 2]-annulenes are paramagnetic, in their predicted “London” (“ring current”) π-electron magnetic properties. These conclusions are not inconsistent with the available experimental data on 1H NMR chemical shifts; it is, however, pointed out that the latter are not necessarily directly comparable with the present calculations because, in the case of the non-alternant and/or multiply-charged species included in this study, other factors such as non-uniform π-electron charge densities are thought likely to make as significant a contribution to the experimentally observed 1H NMR chemical shifts in these systems as the “ring current” effects that are actually being considered here.

Mechanism of stereospecific chlorination of polyene via prolonged chemical doping

Stereospecific chlorination of one-dimensional conjugated polyacetylene, (CH) x , through chemical doping with FeCl 3 was investigated. Based on the step-wise chlorination mechanism proposed, molecular orbital calculations were carried out using finite polyene models. The central region in the polyene segment of (CH) x is confirmed to be the more preferable doping site compared with the terminal region. The cation centre of the positively charged polyene segment affords the first chlorination site for the chloride anion, Cl −, dissociated from the dopant species as FeCl 4 −. The π-electron charge polarization on the chlorinated polyene enables the 1,2 addition of Cl 2 to occur repeatedly The steric hindrance, mainly due to the nuclear repulsion between the chlorine atoms, determines the energetically favourable trans-cis structure as a repeating unit of C 4H 4Cl 4, resulting in a stereoregular chlorinated polyacetylene, (CHCl) x .

Interaction of Caffeine with L-Tryptophan: Study by 1H Nuclear Magnetic Resonance Spectroscopy

The interaction of caffeine with L-tryptophan was investigated in aqueous solution adjusted to an ionic strength of 0.20, at pH 6.2, by using 1H NMR spectroscopy. Caffeine interacted with L-tryptophan at a molar ratio of 1:1 by parallel stacking. The thermodynamic parameters obtained for the complexation were as follows: ΔGo = −1.70 kcal/mol at 25 oC(K = 17.6M−1), ΔHo = −3.7 kcal/mol, ΔSo = −6.7 eu. It was found that the driving force for parallel stacking is a combination of electrostatic, polarization, and dispersion interactions, and the contribution of hydrophobic bonding is not large. A possible stacking mode is postulated based on the complexation shifts and π-electron charge distributions of caffeine and L-tryptophan.

Synthesis and Characterization of Some New Organophosphonates and Their Adducts with Some Metal Salts

Abstract The synthesis of the new monobasic organophosphonates (C2H5O)RPO(OH), R=benzyl, benzoyl, or butanoyl group, by the hydrolysis of the neutral esters (C2H5O)2RPO is reported. The IR spectra of the new ligands showed that they were existed in solutions as dimers. The pKa values in 50% aqueous ethanol found to be 4.46, 3.98, and 3.48 for the benzyl, butanoyl, and benzoyl derivatives, respectively. These values were consistent with the calculated π-electron charge density. Solid complexes of the benzoylphosphonate with Pr, Eu, Er, and Co metal ions as well as the benzyl–Eu complex were isolated. The complex ratios were found to be 1:2 metal to phosphonate dimer. The octahedral arrangements were completed by the presence of a coordinated bidentate NO3 group. Ionic properties of these complexes were ruled out from conductometric measurements. The IR spectra as well as the calculated π-electron charge density for the benzoyl complexes showed that the preference site for complex formation was from the carbonyl oxygen in contrast with the benzyl one where complexation through the phosphoryl group in addition to the hydroxyl oxygen.