N. O. Lipari

Electron–vibration interactions in benzene and deuterobenzene

An exactly solvable model of the linear interaction of molecular vibrations with the electronic states of molecules is utilized to describe the intensity distributions in band systems observed in the ultraviolet photoelectron and ultraviolet absorption spectra of C6H6 and C6D6. From this analysis, the linear coupling constants describing the vibration‐induced energy shifts of the 1e1g(π), 3e2g(σ), and 3a1g(σ) one‐electron orbitals and of the 1B2u excited state are determined. These coupling constants also are calculated using the spectroscopic CNDO/S2 molecular‐orbital model of Lipari and Duke and the vibrational normal modes of Whiffen. The sensitivity of the calculated coupling constants to the precise form of the molecular orbital model is shown to be substantial. The CNDO/S2 model provides a semiquantitative (±50%) description of the measured coupling constants, whereas prior CNDO models (e.g., CNDO/2) predict values for these quantities deviating by as much as an order of magnitude from the measured ...

Intramolecular vibrational stabilization of the charge density wave state in organic metals

Abstract On the basis of a tight-binding molecular orbital model of a conducting chain of large planar organic molecules we conclude that the charge density wave state in the organic metal TTF-TCNQ is stabilized predominantly by a set of small amplitude intra molecular distortions while the condensate effective mass characterizing its dynamical properties is dominated by large amplitude inter molecular distortions.

The electronic structure of dialkylbenzenes

A new CNDO model parametrization, differing from earlier ones primarily by its embodiment of large orbital exponents for conjugated carbon atoms, is proposed for the description of the electronic spectra of aromatic hydrocarbons. In contrast to its predecessors, the new model is shown to describe both the ultraviolet absorption and the valence electron eigenvalue spectra of methyl substituted benzenes. The predictions of this model are compared with those of earlier molecular‐orbital calculations (Hartree–Fock, CNDO/2, and SPINDO) and with the ultraviolet‐photoemission and ultraviolet‐absorption spectra of benzene, p‐xylene, m‐xylene, and 1,4‐bis(trifluoromethyl) benzene. The model also correctly predicts the symmetry of the radical anion ESR spectra of those compounds for which they have been measured.

The electronic structure of 1,2‐di(p‐tolyl)ethane and of pure and substituted [2.2]paracyclophane

Two new ’’spectroscopic’’ CNDO model parametrizations, constructed to describe the electronic excitation spectra of conjugated hydrocarbons, are utilized to analyze these spectra for the double‐benzene‐ring molecules 1,2‐di(p‐tolyl)ethane, [2.2]paracyclophane, pseudopara‐dibromo[2.2]paracyclophane, pseudopara‐dicyano[2.2]paracyclophane, and 1,1,2,2,9,9,10,10 octafluoro[2.2]paracyclophane. Our CNDO/S3 model predicts orbital eigenvalue spectra which are in quantitative (ΔE∼0.1 eV) correspondence with the ultraviolet photoemission spectra of those molecules for which they have been measured. Its predictions of the ultraviolet absorption spectra of 1,2‐di(p‐tolyl)ethane are equally quantitative, although those for the absorption spectra of the [2.2]paracyclophanes are only qualitative (ΔE≲0.3 eV). A major success of the model is its correct prediction of the observed symmetry of the radical anion ESR spectra of 1,2‐di(p‐tolyl)ethane and [2.2]‐paracyclophane: the latter result having eluded prior molecular‐orb...

The electronic structure of polyacenes: Naphthalene through pentacene

The new ’’spectroscopic’’ CNDO/S2 model parametrization, constructed to describe the electronic excitation spectra of conjugated hydrocarbons, is applied to determine the electronic structure of naphthalene, anthracene, naphthacene (tetracene), and pentacene. The occupied and virtual orbital eigenvalue spectra are evaluated. Comparison of the former with observed ultraviolet photoemisiion (UPS) spectra reveals that the predicted energies of the highest‐energy two or three filled orbitals are in quantitative (ΔE≲0.1 eV) correspondence with these spectra. The ordering of the π‐electron levels agrees with earlier assignments, and the deeper levels (both π and σ) are in qualitative (ΔE≲0.5 eV) correspondence with the UPS spectra. The energies and nature of both singlet and triplet transition states are evaluated using configuration‐interaction analyses of appropriate dimensionality. All three singlet levels for each molecule agree with observed optical spectra to within ΔE?0.1 eV. The ordering and splittings ...

The interaction of electrons with the molecular vibrations of benzene

Abstract The linear coupling constants describing the interactions with lattice vibrations of π-electrons in benzene are extracted from ultraviolet absorption and photoemission spectra. They are predicted adequately by molecular-orbital theory which further predicts large quadratic-coupling terms.

Electron—molecular-vibration coupling in 7,7,8,8-tetracyano-p-Quinodimethane (TCNQ)

Abstract The linear coupling constants describing the interaction of the one-electron molecular orbitals with the molecular vibrations in TCNQ are calculated using a semi-empirical spectroscopically parameterized CNDO molecular orbital model. The cartesian displacements for the normal mode of vibrations are computed on the basis of a modified valence force field (MVFF). The calculated vibrational progressions associated with the highest occupied orbital are in excellent agreement with the observed high resolution photoemission spectrum. The present investigation also confirms the importance of the intramolecular coupling for the description of metal—insulator transitions in TTF—TCNQ.

The electronic structure of bond‐alternating and nonalternant conjugated hydrocarbons: Diphenylpolyenes and azulene

The electronic structures of the first four diphyenylpolyenes (stilbene, diphenylbutadiene, diphenylhexatriene, and diphenyloctatetraene) and of azulene are evaluated using the new ’’spectroscopic’’ SCF CNDO/S2 molecular orbital model. In the case of the diphenylpolyenes the higher‐energy π and σ ionization potentials and the strong allowed 1Bu optical transitions (in the vicinity of hν=3–4 eV) are predicted to within about 0.1 eV by this model. In addition, the inclusion in our analysis of σ as well as π electronic orbitals permits the detailed interpretation of the electronic origin of all prominent maxima in their ultraviolet photoemission spectra. In the case of azulene, all major peaks in the photoemission and absorption spectra are predicted to within ΔE?0.4 eV. The predicted dipole moment is 2.1 D, as compared with a Pariser–Parr–Pople value of 1.9 D. We conclude that the CNDO/S2 model can describe the electronic structure of nonalternant and bond‐alternating alternant hydrocarbons, as well as that...

Orbital identification via analyses of molecular-vibration-assisted photoemission: C6H6 and C6D6

Abstract Differing electronic charge densities in the various orbitals of molecules lead to distinctive intensity patterns of vibration-assisted photoemission. Analysis of this phenomenon in C 6 H 6 and C 6 D 6 reveals that the 11.49 eV ionization is associated with the 3e 2g (σ) orbital.

A new method in the theory of indirect excitons in semiconductors

Abstract A new method for the analysis of indirect excitons in semiconductors is introduced. This approach yields a physical interpretation of the various terms present in the Hamiltonian, and allows an accurate evaluation of the energies of the exciton levels. For Ge, a simpler but equally accurate “axial model” is introduced, which is very suitable for investigating the exciton dispersion, optical lineshapes, and the effect of exciton—exciton interactions and external fields.