Electronic Spectra of Polyatomic Molecules

The anomalously large Ham effect observed in pyrene has been studied in various solvents at room temperature by measuring the fluorescence and absorption spectra. As in the case of benzene, the Ham bands of pyrene were assigned to the 0–0 and ag-vibration bands. This was confirmed by fluorescence polarization measurement. The correlation of the intensities of the fluorescence and absorption bands to solvent polarities is discussed.

Solvent Effect on the Vibrational Structures of the Fluorescence and Absorption Spectra of Pyrene

https://iopscience.iop.org/article/10.1088/0034-4885/8/1/312/pdf

Molecular electronic spectra, dispersion and polarization: The theoretical interpretation and computation of oscillator strengths and intensities

An analysis is made of the changes that occur in the 2600, 2050, and 1850 A transitions of benzene on chemical substitution. Formulas derived by first‐ and second‐order perturbation theory, with coefficients evaluated by molecular orbital theory, are presented and applied to a large collection of intensity‐change and frequency‐shift data obtained from the literature. Most of these data pertain to the 2600 A transition. The first‐order intensity formula derived for the 2600 A transition (taking the excited state to be 1B2u) is found to hold quite well for substituents with monoshift (frequency shift on monosubstitution) less than 1500 cm‐1; the first‐ and second‐order frequency‐shift formula, for substituents with monoshift less than 2000 cm‐1. No attempt is made to analyze the intensity changes observed in the 2050 and 1850 A transitions as these changes are too small, relative to the initial intensity, to be reliably measured for weakly perturbing substituents from existing spectra. The frequency shifts,...

Changes in the Electronic Transitions of Aromatic Hydrocarbons on Chemical Substitution. II. Application of Perturbation Theory to Substituted‐Benzene Spectra

The infra‐red spectrum of crystalline benzene has been studied at −12°C, −65°C and −170°C as well as the liquid at 28°C. The lines are extremely sharp, the mean line width for fundamentals being 7 cm−1 in the crystal. In three cases, it is less than the spectral slit width. All out‐of‐plane degenerate modes are split by about 10 cm−1. Selection rules are obeyed and agree with the reported x‐ray structure. All ungerade fundamentals are observed directly, some for the first time. Fifty‐two combination bands are observed. The frequency assignments of Ingold et al. are confirmed except for the B2u species. Evidence is cited for re‐assigning ν14 from 1648 cm−1 to 1310 cm−1 and ν15 from 1110 cm−1 to 1150 cm−1. Ingold's isotopic data does not conflict with this change. Combination frequencies involving torsional lattice modes are apparently observed at −170°C.

The Vibrational Spectra of Molecules and Complex Ions in Crystals. II. Benzene

The absorption spectrum of benzene at 2800–2200A has been analyzed. The vibrational structure of the system has been found to be in agreement with the selection rules for a forbidden transition (1A1g→1B2u). The transition becomes possible when vibrations of type Eg+ distort the molecule. Only carbon frequencies have been found to be effective. The interpreted data are collected in series on page 211. The analysis is corroborated by comparison with the absorption of solid benzene at —259°C, with the absorption spectrum of heavy benzene and with the fluorescence spectra of both benzenes.

A. L. Sklar

Papers

Analysis of the Near Ultraviolet Electronic Transition of Benzene