Pople Method Research Articles

The resonance Raman spectra of Orange II and Para Red: molecular structure and vibrational assignment

Resonance Raman spectra are reported for Orange II and Para Red under both neutral and basic conditions. The properties of the various tautomeric forms of the dyes were calculated by the Pariser—Parr—Pople method. In neutral solution, both dyes are present in the hydrazone form while in basic solution the naphtholate form of the dyes has an azo structure. Comparison of the spectra of the two dyes in neutral and basic solutions, the changes in the spectra on deuteration and correlation with the calculated bond orders enabled a vibrational assignment to be made. The naphthalene ring vibrations were found to make a major contribution to the spectra, with intense bands between 1600 and 1480 cm −1 apparently being characteristic of the ortho-quinonoid form of the ring.

Colour and Constitution of 1,4‐Naphthoquinonoid Dyes ‐ Modified PPP MO Calculations and Substituent Effects

Calculation of the n‐electron excitation energies of naphthoquinonoid dyes has been carried out by means of the Pariser—Parr—Pople (PPP) method using the variable, β,γ‐approximation. A good linear correlation between the observed and calculated values for the first π‐π* excitation was found for a new series of naphthoquinonoid dyes. The effect of substituents on the colour of different dyes was studied on the basis of these calculated results, i.e. HOMO—LUMO energy levels and π‐electron‐density changes accompanying the excitation. The intramolecular charge‐transfer character of the π‐π* transition was shown to be the same as occurs in some anthraquinonoid dyes.

A theoretical study of shake up structure of benzene and furan homologues

The intensities of the shake up lines of the benzene and furan homologues have been computed by the Pariser—Parr—Pople method including single excitation CI in the final state. The overall agreement is quite satisfactory both for the energy position and intensity. The method seems quite adequate to treat core valence excitations in molecules with a π electron system.

A theoretical study of shake up structure of benzene and furan homologues

The intensities of the shake up lines of the benzene and furan homologues have been computed by the Pariser—Parr—Pople method including single exciation CI in the final state. The overall agreement is quite satisfactory both for the energy position and intensity. The method seems quite adequate to treat core valence excitations in molecules with a π electron system.

Photoconductivité électrique des composés aromatiques nitrés halogénés en solutions

Reversible long lifetime (10 3s) photoconductivity has already been found for many halogeno and nitro organic compounds dissolved in some ethers. In this work an attempt was therefore made to investigate the photoconductivity of some chloronitrobenzenes and bromonitrobenzenes for compounds possessing two functional groups. The irradiation at 254 nm of both halogenobenzenes and halogenonitrobenzenes resulted in the formation of halide ions. The quantum yields of formation of these ions were found to be much lower for halogenonitrobenzenes than for halogenobenzenes. These results might be explained quantitatively by the differences in the electronic structures (S 0, S 1, T 1) of the compounds. The electron density functions for molecules of the compounds investigated, obtained from quantum calculations by the Pariser—Parr—Pople method, indicate that the NO 2 group plays a predominant role in the reactivity of these compounds. The UV irradiation of halogenonitrobenzenes dissolved in ethyl ether or tetrahydrofuran gives rise to two effects: (a) an increase in conductivity during the exposure and (b) relaxation of the conductivity after the irradiation is stopped. Both effects were investigated in a constant electric field. It was found that the structure of the compound, its concentration, the nature of the solvent, the temperature, the time of irradiation and the light intensity had an influence on the effects. An analysis of the kinetics of both processes indicated the complicated nature of the increase in the conductivity during irradiation whereas the relaxation of the conductivity appeared to obey first-order kinetics. A preliminary reaction scheme is proposed which is in accord with the experimental observations as well as with our knowledge of the photochemistry of halogenonitroarenes. From our results it may be also seen that the photochemical behaviour of halogenonitrobenzenes (probably also halogenonitroarenes) is mainly determined by the nitro group.

Elastic scattering of slow electrons by CH4and H2O using a local exchange potential and new polarisation potential

A parameter-free polarisation potential is proposed for the elastic scattering of electrons by polyatomic molecules, based on the method of Pople and Schofield and the work of Temkin. Cross sections are obtained for CH4 and H2O using this polarisation potential and a Hara local exchange potential. Agreement with experiment is good.

ELECTRONIC EXCITATION OF ORGANOSULFUR RADICALS

Abstract Transition energies of neutral and ionic sulfur containing heterocyclic radicals have been calculated by the method of Longuet-Higgins and Pople and compared with experimental excitation energies obtained by UV/VIS absorption and photoelectron spectroscopy. In most cases the calculated transition energies satisfactorily match the ones found experimentally. A few compounds behave exceptionally. Discrepancies between the two sets of experimental values are attributed to different molecular geometries of the radical species. According to the calculations, the UV/VIS absorption spectra of recently synthesized sulfur and nitrogen containing radicals are compatible with the 1,2,3-dithiazolyl structure.

Magnetic circular dichroism spectrum of 9,9′-spirobifluorene

The m.c.d. spectrum of the 9,9′-spirobifluorene molecule is presented and discussed and the lowest excited state is shown to be doubly degenerate. CI wavefunctions are computed by the Pariser–Parr–Pople method with the inclusion of the σ–π interactions at the spiro carbon atom, and the values of the m.c.d. A and B constants are evaluated for the various molecular excited states. To evaluate the m.c.d. B constants we propose a procedure based on the direct solution, by a CI expansion method, of the perturbative equations, which provide the first-order corrections to the ground- and excited-state wavefunctions in the presence of a static magnetic field. It is shown that the σ–π interactions, by breaking the pairing symmetry of alternant hydrocarbons, provide a large contribution to the intensity and to the m.c.d. A constant of the lowest electronic transition. An assignment of the u.v. spectrum of spirobifluorene is proposed, consistent with the spectral data available from absorption, linear dichroism and magnetic circular dichroism measurements.

ChemInform Abstract: POLARIZABILITY CALCULATIONS WITH THE SCF METHOD. IV. VARIOUS CONJUGATED HYDROCARBONS

We calculate the third‐order nonlinear electrical polarizabilities of a group of 16 conjugated hydrocarbon molecules with 8 to 14 carbon atoms by making use of the Pariser–Parr–Pople method. We find that the results are quite different from values that were derived previously from the Huckel method. The new values of the space‐averaged third‐order susceptibilities γ are all positive. The largest γ values are predicted for naphthoquinodimethane‐2,6 and for the conjugated hydrocarbon chains, in particular, for tetradecaheptaene.

Zur Interpretation des UV/Vis‐Spektralverhaltens substituierter Benzendiazoniumionen

Interpretation of the UV/vis Spectral Behaviour of Substituted Benzenediazonium IonsIn order to synthesize new, potent, wide‐range photosensitive diazonium compounds, an explanation of the various influences on the position of the longest wavelength absorption band (λmax) of the diazonium ions should be given.For this purpose, SCF‐LCI calculations according to the Pariser, Parr, Pople method were made. The absorption behaviour of the diazonium ions can be explained on the basis of simple perturbation‐theoretical MO considerations. According to the regularities mentioned, it is possible to consider 4‐aminosubstituted benzenediazonium ions as meropolymethines. With the perturbation theory it is also possible to define the limit of λmax attainable with benzenediazonium compounds.

On the π–π* spectra of 1-phenylbutadiene and 1-phenylhexatriene. A theoretical study including electron correlation

The π→π* spectra of 1-phenylbutadiene, 1-phenylhexatriene and, for comparison, styrene are calculated using the Pariser–Parr–Pople method with doubly and triply excited configurations included. The results are consistent with the available experimental data. In each case, the state corresponding to the 2 1Ag state of polyenes is calculated to be the third lowest excited state and to lie just above the state responsible for the “conjugation band”.

Polarizability calculations with the SCF method. IV. Various conjugated hydrocarbons

We calculate the third-order nonlinear electrical polarizabilities of a group of 16 conjugated hydrocarbon molecules with 8 to 14 carbon atoms by making use of the Pariser–Parr–Pople method. We find that the results are quite different from values that were derived previously from the Hückel method. The new values of the space-averaged third-order susceptibilities γ are all positive. The largest γ values are predicted for naphthoquinodimethane-2,6 and for the conjugated hydrocarbon chains, in particular, for tetradecaheptaene.

Colour and Constitution of Anthraquinonoid Dyes. Part II–Modified PPP Calculations and Annelation Effects

The Pariser–Parr–Pople (PPP) method with the variable (3, yapproximation has been used to calculate the ^–electronic excitation energy of 1, 4–disubstituted benzo– and dibenzo–anthraquinone dyes. A good linear correlation between the observed and calculated values for the first –* excitation was found for a series of benzo AQ dyes. The effect of annelation on the colour of benzo AQ dyes was studied on the basis of the calculated results. It was concluded that 2, 3–annelation differs from 5, 6– and 6, 7–annelation because the 2, 3–benzo ring is also included in the donor moiety.

Colour and Constitution of Anthraquinonoid Dyes. Part 1‐Modified PPP Calculations and Substituent Effects

Calculation of the –electronic excitation energies of anthraquinonoid dyes has been carried out by means of the Pariser–Parr–Pople (PPP) method with the variable β, γ approximation. A good linear correlation between the observed and calculated values for the first – * excitation was found for a series of amino– and hydroxy–anthraquinone dyes. The effect of substituents on the colour of different dyes was studied on the basis of these calculated results. The intramolecular charge–transfer character of the first – * transition was supported. The parameters used in this calculation can be adapted to estimate the colour of any anthraquinonoid dye.

Convergence acceleration of iterative sequences. the case of scf iteration

Based on a recent method of Pople et al for the solution of large systems of linear equations, a procedure is given for accelerating the convergence of slowly converging quasi-Newton— Raphson type algorithms. This procedure is particularly advantageous if the number of parameters is so large that the calculation and storage of the hessian is no longer practical. Application to the SCF problem is treated in detail.

ChemInform Abstract: POLARIZABILITY CALCULATIONS WITH THE SCF METHOD. II. THE BENZENE MOLECULE

We evaluate the total third order electric polarizabilities of the benzene molecule by means of the extended Huckel method and we calculate the π electron contribution to the third order electric polarizabilities by means of the Pariser–Parr–Pople (PPP) method. The extended Huckel method leads to a value of γ=0.8578×10−36 esu and our most precise PPP treatment gives γπ=0.9782×10−36 esu. By combining the various results we obtain a theoretical value γ=1.1332×10−36 esu which is consistent with the experimental results. We also report calculations of the linear static and dynamic polarizabilities.

Polarizability calculations with the SCF method. III. Ethylene, butadiene, and hexatriene

We evaluate the nonlinear third-order electric susceptibilities of ethylene, trans-butadiene, and trans-hexatriene by combining the Pariser–Parr–Pople method and the extended Hückel method. It follows from the nature of the calculations that the theoretical results are quite sensitive to approximations since the final results are obtained from various positive and negative contributions. If we allow for a possible 10% error in each contribution, then our result and the corresponding error margins are γ=−0.03±0.03 for ethylene, γ=1.35±0.75 for trans-butadiene, and γ=9.92 ±5 for trans-hexatriene, all values are expressed in terms of 10−36 esu. We feel that these values are consistent with experimental results obtained by Ward and Elliott.

A theoretical study of solvent effects on the 13C chemical shifts of some polar molecules

AbstractCalculations of the dependence of 13C chemical shifts on the dielectric constant, ϵ, of the medium are reported using the INDO/S parameterized version of Pople's method together with the solvaton model. Satisfactory agreement with the available experimental data is obtained. The rather sensitive dependence of chemical shift upon ϵ suggests that polar solute—solvent interactions in aprotic media should be taken into account when considering 13C chemical shifts.

Polarizability calculations with the SCF method. II. The benzene molecule

We evaluate the total third order electric polarizabilities of the benzene molecule by means of the extended Hückel method and we calculate the π electron contribution to the third order electric polarizabilities by means of the Pariser–Parr–Pople (PPP) method. The extended Hückel method leads to a value of γ=0.8578×10−36 esu and our most precise PPP treatment gives γπ=0.9782×10−36 esu. By combining the various results we obtain a theoretical value γ=1.1332×10−36 esu which is consistent with the experimental results. We also report calculations of the linear static and dynamic polarizabilities.

ChemInform Abstract: POLARIZABILITY CALCULATIONS WITH THE SCF METHOD. I. LINEAR AND DYNAMIC POLARIZABILITIES OF CONJUGATED HYDROCARBONS AND AROMATICS

We calculate the static and dynamic linear electric polarizabilities of a group of aromatic and conjugated hydrocarbon molecules by means of a modified Pariser–Parr–Pople method. One purpose of the calculation is a comparison between various PPP results and previously obtained corresponding Huckel results. We calculate linear static polarizabilities and a contribution, the C term, to the third‐order nonlinear susceptibilities. The PPP values and Huckel values are not too different from each other except for those molecules that have unusually large susceptibilities. For the latter molecules the PPP results are significantly smaller than the Huckel results.