Search

Absorption spectra are reported for cis‐ and trans‐1,3,5‐hexatriene in the region between 47 000 and 69 000 cm−1. The observed transitions are compared to those predicted from Pople‐Pariser‐Parr pi‐electron calculations using configuration interaction including both single and double excitations. The ordering of states predicted from calculations based on single excitations appears to be more plausible than that of the more extensive single‐plus‐double excitation calculations. Vibrational progressions associated with the higher energy ππ* transitions of the trans isomer are almost identical to those reported previously for the lowest energy allowed transition (39 000–47 000 cm−1). The linewidths of the trans isomer transitions are considerably smaller than those for the cis isomer transitions throughout the entire vacuum ultraviolet region. An ionization petential of 8.27 eV is obtained for the trans isomer from a Rydberg series extrapolation. This value compares favorably with the value (8.29 eV) obtained recently from photoelectron spectroscopy. Strong bands which could be assigned unambiguously as Rydberg transitions were not found for cis‐hexatriene. However, the shape of the absorption curve in the region 62 000–68 000 cm−1 implies that the ionization potential for cis‐hexatriene is probably within 0.1 eV of that for trans‐hexatriene.

Absorption and emission spectra of all‐trans 1, 3, 5, 7‐octatetraene are presented along with fluorescence quantum yields and lifetimes. In solution, a gap of about 3000 cm−1 is found between the first band of the 1 1Ag→11Bu transition and the onset of the emission spectrum. Excitation spectra of concentrated solutions at 77 K show low‐lying bands in this gap, the lowest energy band being almost coincident with the highest energy fluorescence band. On the other hand, gas phase fluorescence spectra show no gap between the lowest energy 11Ag→11Bu absorption band and the first fluorescence band. The radiative lifetime in hexane is 220 ns at room temperature and 190 ns at 77 K. The radiative lifetime for the gas phase fluorescence is estimated to be longer than 150 ns. The solvent dependence of the absorption and emission spectra, the fluorescence lifetimes, and the vibrational frequencies observed in solution imply support for the conjecture of Karplus et al. that the lowest excited singlet state is of 1Ag s...

Radial distribution functions of electrons with respect to the other electrons in neon and argon atoms are derived from Laurila's experimental x‐ray intensities. The resulting distribution functions are used to calculate experimental electron—electron potential‐energy values. In qualitative features the distributions compare favorably with those calculated from existing wavefunctions. Uncertainties in the distribution functions due to the scatter of experimental data points and to the restricted angular range of the data were large enough to obscure effects of electron correlation.

The electron—electron and electron—nuclear radial distribution functions P(r12) and D(r) have been calculated for ground states of heliumlike systems (Z=2 to 8). Computations were based on the correlated and uncorrelated wavefunctions published by Roothaan et al. Elastic and inelastic scattering factors for calculating the intensities I([open phi]) of x rays scattered by two‐electron systems were determined from the distribution functions. The correction functions ΔP(r12) and ΔI([open phi]) representing the differences between correlated and uncorrelated results were found to follow a simple dependency on atomic number. It was observed that universal functions could be established which closely approximated each of the functions D(r)/Z*, P(r12)/Z*, I([open phi]), ΔP(r12), and Z*ΔI([open phi]) for all of the systems studied if the functions were expressed in terms of the reduced variables r′=Z*r and [open phi]′=2 arcsin[Z*−1 sin([open phi]/2)]. The appropriate Z* was very nearly the Slater effective nuclea...

A new least‐squares procedure has been devised for interpreting electron‐diffraction patterns by a comparison of experimental total intensities with calculated intensities. It differs from all procedures published previously in its direct use of observed quantities in place of the customary ``reduced intensities'' derived subjectively from manually smoothed data. The computer is programmed to select the atomic background function needed to establish the molecular intensity and to refine the background as it refines the desired structural parameters. Operating characteristics of the new procedure are discussed. Results of its application in the structure determination of gaseous B(CH3)3 are presented and compared with results determined by the radial distribution method. Although the new procedure requires somewhat more accurate trial values of molecular parameters for convergence than does the radial distribution analysis, it is intrinsically a faster, and, in some ways, a more powerful tool for deducing optimum molecular parameters.

The absorption spectra of cis‐ and trans‐ 1,3,5‐hexatriene are reported for the region of the first allowed singlet‐singlet electronic transition, 2600 to 2100 A. A least‐squares procedure in which a Lorentzian line shape is assumed for each band was used to fit the spectra. Progressions are found of 408, 1262, and 1626 cm−1 for the cis isomer and of 350, 1232, and 1628 cm−1 for the trans isomer. The 0–0 transition for the cis compound, 39668 cm−1, lies about 130 cm−1 lower than that for the trans, 39800 cm−1. The fact that the 0–0 transition energy for cis is less than that for trans is interpreted as evidence that the cis isomer has a non‐planar geometry in the electronic ground state. The extinction coefficient for the 0–0 band of the trans isomer is about 1.5 times that for the cis.

Distributions of fundamental vibrational frequencies for linear polyenes are estimated from a transferable set of force constants. Particular attention is focused on reproducing observed experimental spectra in the carbon—carbon double‐bond stretching region, 1400–1650 cm−1. Stretching force constants for carbon—carbon bonds are deduced from fractional pi‐bond orders, whereas interaction constants for these stretching motions are inferred from the mutual polarizabilities of the bonds. Calculated distributions for vibrational frequencies are in agreement with experimental data only if long‐range interaction constants are employed. The results obtained in the present study are compared to results obtained in previous studies where the long‐range interaction constants were assumed to be zero. Predictions of the vibrational spectra of the first excited electronic states of linear polyenes are provided along with a discussion of the types of vibrational frequencies which are expected to undergo significant cha...

A simple scheme is proposed for predicting effects of electron correlation on intra‐atomic electron—electron radial distribution functions and on intensities of x rays scattered by gas atoms. It makes use of a relationship connecting the Coulomb hole function for an electron pair with the corresponding correlation energy. The method is applied to the beryllium atom in its ground state. Results compare favorably with results calculated directly from correlated and Hartree—Fock wavefunctions.

The distribution of internuclear distances in gaseous XeF6 exhibits unusually diffuse XeF6 bonded and F–F geminal nonbonded peaks, the latter of which is severely skewed. The distribution proves the molecule cannot be a regular octahedron vibrating in independent normal modes. The instantaneous molecular configurations encountered by the incident electrons are predominantly in the broad vicinity of C3υ structures conveniently described as distorted octahedra in which the xenon lone pair avoids the bonding pairs. In these distorted structures the XeF bond lengths are distributed over a range of approximately 0.08 A with the longer bonds tending to be those adjacent to the avoided region of the coordination sphere. Fluorines suffer angular displacements from octahedral sites which range up to 5° or 10° in the vicinity of the avoided region.Alternative interpretations of the diffraction data are developed in detail, ranging from models of statically deformed molecules to those of dynamically inverting molecu...