H. H. Jaffé

Use of the CNDO Method in Spectroscopy. I. Benzene, Pyridine, and the Diazines

The CNDO method has been modified by substitution of semiempirical Coulomb integrals similar to those used in the Pariser‐Parr‐Pople method, and by the introduction of a new empirical parameter κ to differentiate resonance integrals between σ orbitals from those between π orbitals. The CNDO method with this change in parameterization is extended to the calculation of electronic spectra and applied to the isoelectronic compounds benzene, pyridine, pyridazine, pyrimidine, and pyrazine. The results obtained were refined by a limited CI calculation and compared with the best available experimental data. It was found that the agreement was quite satisfactory for both n→π* and π→π* singlet‐singlet transitions. The relative energies of the pi and lone‐pair orbitals in pyridine and the diazines are compared and an explanation proposed for the observed orders. Also, the nature of the “lone pairs” in these compounds is discussed.

Use of the CNDO Method in Spectroscopy. II. Five‐Membered Rings

The modified CNDO method previously reported has been used to calculate the electronic spectra of cyclopentadiene, the cyclopentadienide ion, pyrrole, furan, pyrazole, imidazole, 2‐pyrrolecarboxaldehyde, and furfural. In general, the results obtained agree quite well with experimental data. Because the CNDO method treats explicitly all σ and π valence electrons of a molecule, the results of the calculations are used to discuss some of the σ–π interactions which previously could not be treated. The calculations are successful in reflecting changes in the electronic spectra of compounds as a result of extending conjugation or addition of a substituent.

Use of the CNDO Method in Spectroscopy. III. Monosubstituted Benzenes and Pyridines

The modified CNDO method previously reported has been applied to the calculation of a series of monosubstituted benzenes and pyridines, namely, aniline, anilinium ion, benzonitrile, nitrosobenzene, phenol, phenoxide ion, toluene, pyridinium ion, pyridine‐N‐oxide, 1‐hydroxy‐pyridinium ion, the aminopyridines, and the cyanopyridines. The calculated values of the transition energies and corresponding oscillator strengths are in good agreement with available experimental data on the electronic spectra of these compounds. An analysis of the lower‐energy excited states by means of the calculated results suggests that there is an astonishingly small amount of charge‐transfer character associated with the observed ultraviolet bands. The calculations indicate that the changes in charge densities in going from the ground state to the lowest energy π → π* excited state can be related to the Hammett σs for the substituents.

Use of the CNDO Method in Spectroscopy. IV. Small Molecules: Spectra and Ground‐State Properties

The modified CNDO method has been applied to the study of the electronic‐absroption spectra of several small molecules; namely, formaldehyde, formic acid, formamide, allene, ketene, and diazomethane, with quite satisfactory results. The lowest energy excited state of the last three compounds has been assigned as the state which arises from a π → π′* transition. A study has also been made of the calculation of ground‐state properties from the spectroscopic parameterization. It has been concluded that although this method can give reasonable estimates of these properties, it is most reliable when used to evaluate ground‐state properties which depend upon calculated eigenvalues.

Use of the CNDO method in spectroscopy. VIII. Molecules containing boron, fluorine, and chlorine

An extension of CNDO/S calculations to compounds of B, F, and Cl is reported. Except for some small molecules, good results are obtained for transition energies and intensities.

SLATER-CONDON PARAMETERS FROM SPECTRAL DATA

Slater—Condon parameters have been fitted by least‐squares methods, as far as spectroscopic data are available, for all the neutral atoms of the first three periods of the periodic system, and for their isoelectronic mono‐positive ions, and for the isoelectronic di‐ and tri‐positive ions of the first two periods. Separate evaluations were performed for each configuration, and for pooled data from all configurations. It is concluded that the approximations involved in the Slater—Condon method are at least as serious as the further approximations introduced by pooling of data from several configurations.

Use of the CNDO Method in Spectroscopy. V. Spin—Orbit Coupling

We report here the use of CNDO/S wavefunctions in the calculation of oscillator strengths of singlet—triplet transitions and the corresponding triplet radiative lifetimes induced by spin—orbit coupling. The starting functions for the perturbation calculations are ground and excited state functions including limited configuration interaction. Numerical calculations are carried through for formaldehyde and azulene. The importance of the calculated σ→π* states is discussed.

Nuclear Magnetic Resonance Spectra of Some Dialkylmercury Compounds

The Hg199 and H1 nuclear magnetic resonance spectra of a series of dialkylmercury compounds have been obtained. The Hg199 results indicate that the shielding of the mercury increases along the series Me<n—Pr< Et<i—Pr. This increased shielding seems to be connected with an interaction of Hg199 and the β protons, as evidenced by the H1 resonance peaks and the Hg199–H1 spin‐spin coupling constants.

The use of CNDO in spectroscopy. XV. Two photon absorption

Two−photon absorptivities have been calculated within the CNDO/S‐CI molecular orbital framework of Del Bene and Jaffe utilizing the second order time dependent perturbation equations of Goppert–Mayer and polarization methods of McClain. Good agreement is found between this theory and experiment for transition energies, symmetries, and two‐photon absorptivities for the following molecules: biphenyl, terphenyl, 2,2′‐difluorobiphenyl, 2,2′‐bipyridyl, phenanthrene, and the isoelectronic series: fluorene, carbazole, dibenzofuran.

Excited‐State Variational Calculations with Orthogonality Constraint

Open‐shell pi‐electron calculations have been performed on several singlet excited states of pyridine, pyrazine, pyrimidine, pyridazine, and naphthalene using a recently developed variational technique. Spectral results obtained via the new method are compared with results based on virtual orbitals and experiment. Configuration interaction was found to be necessary for adequate state representation, and the results of the new method are generally better than virtual orbital results, but the CI calculation over separately minimized configurations is very laborious. This new method has some computational advantages in certain applications, but it cannot be recommended for spectral calculations since acceptable spectral results can be obtained more easily using the Pariser–Parr–Pople scheme.