Π-electron Calculations Research Articles

π-Electron Calculations Using the Local Linear-Response Coupled-Cluster Singles and Doubles Theory

Calculations of electronic excitation energies, electron charge distribution, and optical parameters for π-conjugated oligomeric systems are carried out with a new local semiempirical π-electron variant of the coupled clusters (CC) theory involving single and double excitations. The results obtained in the calculations demonstrate adequate accuracy of the proposed linear-response generalization of the local semiempirical π-electron CC approach employing covalently nonbonded molecules of ethylene as the basis set in the calculation. Limit values corresponding to the infinite system size for the lowest excitation energies for polyenes and for condensed and noncondensed polyfulvenes are obtained.

Semiempirical π-electron models for the calculation of MCD B terms for systems with approximate alternant pairing symmetry. MCD of biphenylene

The recent success of the PPP model in predicting MCD B terms for π-electron transitions in conjugated molecules has not been complete. When applied to alternant hydrocarbons, the pairing symmetry contained in the model leads to a prediction of vanishing B terms for all transitions, in disagreement with experimental observations. In order to calculate the MCD for such “soft” chromophores, complete inclusion of overlap between the basis 2p orbitals in π-electron calculations was investigated. The resulting wave functions did not have the perfect pairing symmetry, and the calculated nonzero B terms of benzenoid hydrocarbons were generally in satisfactory agreement with experiment, provided the choice of parameters did not cause large deviation from perfect pairing. The results also provide an empirical justification for a simplified and, strictly speaking, inconsistent π-electron procedure in which non-neighbor matrix elements are kept for the linear momentum operator but set equal to zero for the core Hamiltonian. Experimental MCD and LD spectra of biphenylene, a 4N-electron perimeter, are presented and compared with the theoretical results.

π-electron calculations of polarizabilities in conjugated systems

Finite-field technique has been applied to the calculation of π molecular polarizabilities within the Pariser–Parr–Pople Hamiltonian. This formalism allows to analyze large oligomers containing up to 400 atoms, and asymptotic behaviors may be attained in some cases. We have investigated the role of the polymer size, the backbone geometries, the presence of neutral and charged defects (solitons, polarons, bipolarons), the chemical nature of the monomeric unit on the polarizabilities. Generally, the polarizabilities are not deeply modified by a change of the geometrical characteristics, and may lead to huge values for sufficiently large chains. Besides, the charge of the system is the leading factor which determines the values of this property. The evolution of the polarizability with the number N of π atomic centers, αu depends largely on the charge, and on the defect. For the neutral systems, the polarizability per monomeric unit αu increases smoothly, and then exhibits an asymptotic behavior with N. For polymers with a defect, this variation is different: αu first increases with N, reaches a maximum αu max for Nmax, and finally tends to an asymptotic value. The values of αu max for Nmax depend on the type of defect (soliton vs polaron) and are rather sensitive to the dependence of the first-neighbor one-electron interaction with the interatomic distance. A large number of results on polyacetylene and polyheterocycles shows that there exists a simple law between the polarizability and the electronic gap, independently of the type of the monomeric unit. But this relation shows a deviation from a simple proportionality behavior, as soon as α reaches large values (as for example in polymers with defects).

Comparison of synchronous and asynchronous hydrogen transfer mechanisms in free-base porphyrins

A theoretical study is reported of the rate of transfer of the two inner hydrogens in free-base porphyrins as a function of temperature and isotopic hydrogen mass. The geometry, ground state energy and vibrational force field of cis- and trans-porphine- h 2 and - d 2 and the transition states between them are calculated by semi-empirical methods at the MNDO and AM1 level. As in earlier π-electron calculations, the molecular symmetry is found to be lower than D 2h. The calculations yield hydrogen tunneling path lengths and barriers, and identify the vibrations that govern the rate of transfer. These quantum-chemical data are used to formulate a model for calculating the transfer rate in terms of the golden rule. Three vibrational modes are considered explicitly: NH stretching, in-plane NH bending and an “effective” skeletal mode. The coupling is parametrized and the resulting formula is heuristically modified for strong coupling. Two transfer mechanisms are considered: (A) two-step transfer in which the hydrogens move asynchronously and (B) a one-step mechanism in which they move synchronously. Only mechanism A fits the available kinetic data in the range 100–300 K. It accounts satisfactorily for the observed temperature and isotope effects for parameter values close to those calculated. When substituted in the expression for mechanism B, the corresponding parameter values yield rate constants that are too small to be measured by conventional methods for the low temperatures where mechanism B is expected to dominate.

Electronic spectra and triplet state properties of superphane

The emission and the zero-field splitting parameters D and E of the superphane 1 and the [2 5](1,2,3,4,5)-cyclophane 2 are investigated. The large number of six and five methylene bridges, respectively, leads to a more rigid molecular frame and to a smaller transanular distance as compared to other [2.2]cyclophanes. The experimental results exhibit characteristics features which are due to these unusual properties; they are discussed and compared with semi-empirical Π-electron calculations.

Role of multiply excited configurations in π-electron calculations of electronic states

Quantum chemical calculations of correlation effects in π-electronic spectra have been performed for model molecular systems. In evaluation of singlet, triplet, and quintet states, as well as in the relevant intensities of electronic transitions, multiply excited configurations were taken into account. The results point out the essential role of electron correlation, for triplet and quintet manifolds in particular.

Magnetic circular dichroism of cyclic π-Electron systems. 24. Polarizations and assignments in some pentacyclic and hexacyclic benzenoid hydrocarbons

Measurements of magnetic circular dichroism and of linear dichroism in stretched polyethylene permitted the assignment of a large number electronic transitions in dibenz[a,c]anthracene, dibenza[a,h]anthracene, dibenz[a,j]anthracene, picene, and benzo[rst]pentaphene. The results are in satisfactory agreement with π-electron calculations. Implications for the analytical chemistry of these hydrocarbons and their simple derivatives are discussed.

Modelling magnetic circular dichroism with application to imidazoles

The magnetic circular dichroism of imidazole and its derivatives are calculated using the π-electron approximation, where the resonance integral is computed by using the formula ▪ plus higher order terms. The magnetic circular dichroism (MCD) spectra for the molecules are also presented. As a complementary procedure to the π-electron calculation, Michl's perimeter model [1] for predicting the signs of the MCD is employed.

MCD spectra of the triphenylene positive ion. A test of the pairing theorem for highly symmetrical hydrocarbons

The magnetic circular dichroism (MCD) spectra of the monovalent positive ion of triphenylene have been measured at different temperatures and compared with those of the corresponding negative ion of the same molecule. The results confirm the predicted pairing relations for the A and C parameters of MCD spectra of highly symmetrical hydrocarbons. The spectroscopic assignment of the bands, made with the aid of a π-electron calculation (PPP approximation), supports an earlier assignment for the triphenylene negative ion.

MCD and absorption spectra of the monovalent anion of tribenzo [12] annulene

The magnetic circular dichroism (MCD) and absorption spectra of the monovalent anion of tribenzo [12] annulene have been measured in the visible and the near-infrared region. The absence of a temperature dependence of the MCD proves a non-degenerate ground state. An assignment of the spectra has been given with the aid of a semi-empirical π-electron calculation (PPP approximation). Influence of the counter ion on optical and ESR spectra was tested by the use of different solvents. No divalent anions could be obtained.

MCD of the triphenylene anion radical. Influence of ion-pairing and Jahn-Teller effect

The magnetic circular dichroism and absorption spectra of the ion-pairs of (triphenylene −/Na +) have been measured in a temperature range from 20° to −150°C. The spectra clearly show A and C terms, however the interaction with the cation makes the zeroth moment of MCD a non-linear function of the reciprocal of the temperature. This is interpreted in terms of a model, in which the effect of the cation on the ground state is introduced as a parameter. For the ion-pair the difference between the experimental value of C/D and the value, calculated by a semi-empirical π-electron calculation (PPP approximation) may be an indication for the presence of a dynamic Jahn-Teller effect. Contrary to the intensity and energy changes, the presence of the cation hardly affects the groundstate magnetic moment of the anion. For one of the bands the experimental value of C/D makes a change of a previous assignment necessary. The MCD and absorption spectra were measured simultaneously, between 3500 and 9000 Å, on a specially built apparatus.

Preparation, nucleophilic reactivity, nmr, uv and fluorescence spectroscopy, and matrix photoionization of a li-tmeda salt of the benzocyclobut[a]acena

Deprotonation of 6b,10b-dihydrobenzo[j]cyclobut[a]acenaphthylene 1a produces a monoanion 2 or a condensed cyclobutadiene dianion 3 depending on reaction conditions. A crystalline salt of 3 was isolated and analyzed for 2Li + -2TMEDA- 3 . Both 2 and 3 are deuterated and methylated at the bridgehead positions. The NMR spectrum of 2 suggests exclusive delocalization of charge into the naphthalene subunit, that of 3 shows some interesting differences in charge distribution in comparison to the dianions of acenaphthylene and fluoranthene. Matrix photoionization of the dianion 3 leads reversibly to the condensed cyclobutadiene radical anion 4, photoejection of a second electron destroys 4 irreversibly. The UV-visible spectra of 3 and 4 are compared and interpreted; 3 shows intense red fluorescence, unusual for a hydrocarbon dianion. The charge distribution in 3 and the UV-visible spectra of 3 and 4 are discussed in terms of π-electron calculations.

Excited singlet states of pyrene. Polarization directions and magnetic circular dichroism of azapyrenes

Linear dichroism in polyethylene, polarization of fluorescence excitation, and magnetic circular dichroism of pyrene and three of its aza derivatives have been measured and used to uncover new electronic transitions and to determine polarization directions. The MCD signs are sensitive to structure. The results, including absolute MCD signs, compare well with π-electron calculations. The effects of systematic incorporation of doubly excited configurations in the CI procedure are investigated. Finally, it is shown that the absolute MCD signs, and in particular, their variation with the location of the aza nitrogen, are readily understood in qualitative terms upon inspection of Hückel MO's, permitting facile predictions for other isomers with-out any computations.

MCD and absorption spectra of tribenzo[12] annulene

The magnetic circular dichroism (MCD) and absorption spectra of tribenzo [12] annulene (tribenzo [a,e,i] cyclododecene-5,6,11,12,17,18-hexadehydro) have been measured and interpreted in terms of a semiemperical π-electron calculation (PPP approximation). Below 40 000 cm −1 four transitions could be detected. Two of them are symmetry allowed, but only one of these shows a large absorption intensity, which is explained with the aid of pairing rules. The different behaviour of the bands in absorption and MCD measurements, at room and lower temperatures is discussed.

Linear dichroism and magnetic circular dichroism of acenaphth[1,2-a]acenaphthylene

Ultraviolet-visible polarized absorption spectra of acenaphth[1,2-a]acenaphthylene have been determined from low-temperature measurements of linear dichroism in stretched polyethylene. The magnetic circular dichroic spectrum has been measured in solution. Transition energies, polarizations, and relative intensities, as well as the absolute signs of B terms of low-energy transitions are in good agreement with results of π-electron calculations and with simple qualitative arguments, which also account for the close similarity to the known spectra of acenaphthylene.

ChemInform Abstract: MEASUREMENT AND SCF‐CI PPP CALCULATION OF POLARIZED ABSORPTION AND MAGNETIC CIRCULAR DICHROISM OF AZULENO(5,6,7‐CD)PHENALENE

U.v.-visible polarized absorption spectra of azuleno[5, 6, 7-cd]phenalene have been determined from measurements of linear dichroism in stretched polymer sheets at 77 K. The magnetic circular dichroic (m.c.d.) spectrum has been measured in solution. π-Electron calculations of transition energies, oscillator strengths, polarization directions and m.c.d. B terms were performed and the results are in good agreement with the experimental data for several lower energy transitions. The energy level scheme and transition polarizations as well as signs of B terms can be understood easily on the basis of the HMO orbitals of azulene and m-divinylbenzene. At higher energies, numerous transitions overlap and a detailed analysis is impossible.

Excited states of dicyclohepta[ cd,gh]pentalene: Linear dichroism in a stretched polymer, magnetic circular dichroism, and calculations

Ordinary and linear dichroic uv-visible absorption spectra of dicyclohepta[ cd,gh]pentalene have been determined from measurements on solutions in 3-methylpentane and in stretched sheets of ethylene-vinyl acetate copolymer at 77 K. Magnetic circular dichroic spectrum was measured at room temperature. The spectra reveal the presence of numerous separate electronic transitions and permit assignment of state symmetries and of relative direction and sense of some magnetic dipole transition moments between excited states with respect to direction and sense of electric dipole transition moments from ground to these excited states. π-electron calculations account well for the results, including the absolute signs of the peaks in the MCD spectrum (with a single exception).

ChemInform Abstract: EXCITED STATES OF DINAPHTH(DE‐1,2,3/D′E′‐5,6,7)AZULENE, POLARIZED ABSORPTION IN STRETCHED POLYMERS AND MAGNETIC CIRCULAR DICHROISM

Chemischer InformationsdienstVolume 5, Issue 34 Physical Organic Chemistry ChemInform Abstract: EXCITED STATES OF DINAPHTH(DE-1,2,3/D′E′-5,6,7)AZULENE, POLARIZED ABSORPTION IN STRETCHED POLYMERS AND MAGNETIC CIRCULAR DICHROISM JAROSLAV KOLC, JAROSLAV KOLCSearch for more papers by this authorJOSEF MICHL, JOSEF MICHLSearch for more papers by this author JAROSLAV KOLC, JAROSLAV KOLCSearch for more papers by this authorJOSEF MICHL, JOSEF MICHLSearch for more papers by this author First published: August 27, 1974 https://doi.org/10.1002/chin.197434066AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume5, Issue34August 27, 1974 RelatedInformation

Excited states of dinaphth[ de-1, 2, 3; d′e′-5, 6, 7]azulene: Polarized absorption in stretched polymers and magnetic circular dichroism

UV-visible polarized absorption spectra of dinaphth[ de-1,2,3; d′e′-5,6,7]azulene I have been determined from measurements of linear dichroism in stretched polymer sheets. Magnetic circular dichroic (MCD) spectra have been measured in solution and help to distinguish individual electronic transitions. π-Electron calculations of transition energies, oscillator strengths, and polarization directions are in good agreement with the experimental data and permit a straightforward correlation of the low-energy part of the spectrum of I with that of the parent pleiadiene II.

ChemInform Abstract: ELECTRONIC SPECTRA OF METAL CORROLE ANIONS

The electronic spectra of copper(II) and nickel(II) corrole anions are described and compared with the corresponding metal porphyrin spectra. The spectra are interpreted with the aid of self-consistent field Pariser–Parr–Pople π-electron calculations, and comparison with similar calculations for the porphin ring allows the origin of the differences between the two types of spectrum to be traced.