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 symmetry. The solution data imply that the low-lying state is about 6400 cm−1 below the 11Bu level. On the other hand, the lack of a gap between absorption and emission and the long lifetime found for the gas phase are not compatible with this model.
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.
The distribution of internuclear distances ingaseous XeF. exhibits unusually diffuse XeF. 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 C ao 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 molecules. In all cases it is necessary to assume that tl u bending amplitudes are enormous and correlated in a certain way with substantial ~ deformations. Notwithstanding the small fraction of time that XeF. spends near 0. symmetry, it is possible to construct a molecular potential-energy function mort' or less compatiable with the diffraction data in which the minimum energy occurs at 0. symmerty. The most notable feature of this model is the almost vanishing restoring force for small tlu bending distortions. Indeed, the mean curvature of the potential surface for this model corresponds to a v, force constant F" of 1(JS mdyn/ A or less. Various rapidly inverting non-O. structures embodying particular combinations of t,. and tlu deformations from 0. symmetry give slightly better radial distribution functions, however. In the region of molecular configuration where the gas molecules spend most of their time, the form of the potential-energy function required to represent the data does not distinguish between a Jahn-Teller first-order term or a cubic V «6 term as the agent responsible for introducing the k. deformation. The Jahn-Teller term is consistent with Goodman's interpretation of the molecule. On the other hand, the cubic term is found to be exactly analogous to that for other molecules with stereochemically active lone pairs (e.g., SF" CIF,). Therefore, the question as to why the XeFe molecule is distorted remains open. The reported absence of any observable gas-phase paramagnetism weighs against the Jahn-Teller interpretation.The qualitative success but quantitative failure of the valence-shell-electron-pair-repulsion theory is discussed and the relevance of the "pseudo-Jahn-Teller" formalism of Longuet-Higgins et at. is pointed out.Brief comparisons are made with isoelectronic ions.
A gas-phase electron-diffraction investigation of xenon hexafluoride has been carried out in an effort to obtain structural information which might shed light on the curious properties of the compound. Elaborate precautions were taken to prevent decomposition or contamination of the sample (99.8 mole % pure). Several innovations were introduced into the structure analysis to minimize difficulties encountered in conventional analyses of molecules containing both heavy and light atoms. Two different sets of analyses (I and II) employing two different levels of approximation in electron scattering theory were conducted to test the adequacy of the expressions usually adopted. Analysis I was based on Hartree–Fock x-ray elastic scattering factors, Heisenberg–Bewilogua inelastic scattering factors, and (modified) Born phase-shift corrections for Thomas–Fermi atomic fields. Improvements in Analysis II included the new electron elastic scattering factors and Born phase-shift corrections calculated by the partial wave method by Cox and Bonham, and Hartree–Fock inelastic scattering factors. The Hartree–Fock phase-shift correction was not in complete agreement with experiment but was markedly better than the Thomas–Fermi correction. The effect of ionic character on phase shift was investigated theoretically and shown to be significant. A mean Xe–F bond length of 1.890 ± 0.005 Å was found, but the radial distribution function for Xe–F bonds corresponded to that of a composite for nonequivalent bonds. Amplitudes of bending oscillations are notably large. The diffraction data are not compatible with a regular octahedral XeF6 molecule vibrating in independent normal modes. A more detailed exposition of alternative structures and internal motion is presented in Paper II.
The new zerovalent metal phosphite complex, W[P(OMe),],, has been synthesized and shown to be photochemically very reactive as illustrated in the synthesis of a series of new seven-and eight-co-ordinate tungsten hydrides.
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 changes on electronic excitation.
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