Bibliothek

Notizen: The techniques of resonant two-photon ionization (R2PI) and pulsed field ionization (PFI) were used to measure absorption spectra of ortho-chlorotoluene (S1, A˜ 1A1) and of ortho-chlorotoluene+ (D0, X˜ 2A1; the cation ground state) for internally cold molecules in a pulsed nozzle expansion. The adiabatic ionization potential is 71 318±5 cm−1=8.8423±0.0006 eV. Most of the observed low lying torsion–vibrational structure in both S1 and D0 can be assigned using a one-dimensional torsional model plus low frequency vibrational modes whose identity is corroborated by the ab initio normal modes of D0. The intensities of certain weak, forbidden torsion–vibration combination bands in the S1–S0 spectrum are well predicted by a nuclear coordinate dependence of the electric dipole transition moment. The threefold methyl torsional barrier is V3=144.2±2.0 cm−1 in S1 and V3=456.5±2.0 cm−1 in D0. Ab initio calculations at the HF/6-31G* level find V3=481 cm−1 in S0 with the minimum energy conformation pseudo-trans, i.e., with one CH bond lying in the plane of the ring on the opposite side of the chlorine substituent. Spectral band intensities show that the minimum is pseudo-trans in S1 and D0 as well.In both S1 and D0, excitation of either of the two lowest frequency out-of-plane bending modes, ν38 or ν37, leaves the methyl torsional potential essentially unchanged. In S0, S1, and D0, the barrier is substantially larger for ortho-chlorotoluene than for ortho-fluorotoluene, consistent with greater steric repulsion between the 3p chlorine lone pair and the CH bond pairs. The effects of π excitation and π ionization on the barrier are similar in ortho-chlorotoluene and ortho-fluorotoluene. Apparently both chlorine and fluorine are weak π donors that have similar effects on the crucial ring CC bond orders closest to methyl. Both the S1 and D0 spectra reveal several examples of torsion–vibrational coupling which perturbs torsional state energies and produces extra bands. Simple zeroth-order models of energy levels and coupling strengths fit experimental frequencies and band intensities well. Deperturbation yields coupling matrix elements between torsion and out-of-plane bending vibrations that fall in the narrow range 6–15 cm−1 in all cases. For three different molecules with widely varying methyl rotor barriers from 10 to 450 cm−1, the magnitude of such coupling matrix elements is similar, always in the range 3–15 cm−1. This provides guidance for theoretical models of intramolecular vibrational energy redistribution. © 1996 American Institute of Physics.

Notizen: In toluenelike molecules with sixfold internal rotation symmetry, three weak, Franck–Condon forbidden, pure torsional transitions m21, m30, and m41 invariably appear in S1–S0 absorption spectra. The intensities are typically 1%–5% of the allowed bands, m00 and m11. Determination of the preferred conformer in S1 as staggered or eclipsed relies on proper assignment of the upper state of the m30 band as 3a‘1 or 3a‘2 under the molecular symmetry group G12. In addition, inferences of the preferred conformer in S0 and D0 (ground state cation) from band intensities of fluorescence and threshold photoionization spectra have also relied on the same assignment. For a set of six molecules having sixfold rotor potentials, including both –CH3 and –CD3 rotors, we present experimental relative intensities for the three forbidden S1–S0 absorption bands. Within an adiabatic electronic representation, we show how a Fourier expansion of the three components of the S1–S0 electric dipole transition moment predicts relative forbidden band intensities in quantitative agreement with experiment. This fixes the assignment of the upper state of the key m30 transition as 3a‘1, establishes the preferred S1 conformer as staggered for all six molecules, and places earlier inferences of the preferred conformers in S0 and D0 on solid ground. © 1995 American Institute of Physics.

Notizen: In substituted toluenes, the potential energy barrier to internal methyl rotation and the preferred methyl conformation depend on the position of the fluorine, amino, or methyl substituents and also on the electronic state, either S0, S1, or ground state cation. We present a unified picture of the electronic factors controlling these effects. In S0 and cation, ab initio electronic structure calculations of modest scale produce rotor potentials in good agreement with experiment. The methyl group provides a sensitive probe of local ring geometry. When the geometry of the ring in the vicinity of the rotor has good local C2v symmetry, the barrier is invariably small. In S0 ortho-substituted toluenes, we use natural steric analysis to show that repulsive steric interactions between the halogen lone pair and the methyl CH bonds dominate over attractive donor–acceptor interactions to favor the pseudo-trans conformation. When steric interactions are unimportant, the key determinant of rotor barrier height is the difference in π-bond order between the two ring CC bonds nearest methyl.The barrier height is proportional to the calculated bond order difference, with slope of 950 cm−1 per bond. Attractive donor–acceptor interactions favor the conformation that places the rotor CH bond cis to the ring CC bond of higher order, analogous to the localized case of 2-methylpropene. In toluene cations, π-ionization creates a pattern of long and short ring CC bonds. Simple molecular orbital theory readily explains the coarse bond-order patterns calculated for ortho- and meta-substituted toluene cations. A localized picture of π-bonding from natural resonance theory explains more subtle details of the distribution of CC bond orders about the ring. When π-ionization places the methyl group between ring CC bonds of quite different order, a substantial barrier results. This explains the strong preference of m-fluorotoluene+ for the pseudo-cis conformation and contributes to the preference of o-fluorotoluene+ for the pseudo-trans conformation. Finally, we speculate that a similar molecular orbital argument applied to S1 might explain the observed characteristic changes in barrier height on S1–S0 excitation of ortho- and meta-substituted toluenes. © 1995 American Institute of Physics.

Notizen: We investigate the nature of low-power nonlinear effects in high-temperature superconducting microwave devices by measuring third harmonic generation at 76 K in coplanar waveguide transmission lines of different geometries fabricated from YBa2Cu3O7−δ thin films. The measured power in the third-harmonic signal changes systematically with film thickness, center conductor linewidth, and line length. We analyze these results using a simple model for a transmission line with a nonlinear inductance arising from a current-dependent superconducting penetration depth. This analysis describes quantitatively the observed differences in harmonic generation for transmission lines of different dimensions, and yields a single geometry-independent parameter (the nonlinear scaling current density J0) to quantify the observed nonlinear behavior. For the thin film samples studied here J0=3.0×107 A/cm2 at 76 K for all geometries investigated. These results provide the means to establish a lower limit for the expected nonlinear response of superconducting components of arbitrary geometry at microwave frequencies. © 1999 American Institute of Physics.