The ground state harmonic frequencies of gas phase H/DSi35Cl and H/DSi79Br have been determined by exciting single vibronic bands of the Ã 1A″–X̃ 1A′ electronic transition and recording the dispersed fluorescence. The jet-cooled radicals were produced in a pulsed discharge jet using H/DSiX3 (X=Cl or Br) precursors. The emission data were fitted to an anharmonic model and a normal coordinate analysis of the harmonic frequencies allowed the determination of five of the six force constants of each molecule. Using previously obtained v″=0 rotational constants and the improved force fields, average (rz) and estimated equilibrium (rez) structures were calculated for both monohalosilylenes. The validity of the force constants was evaluated by comparing calculated and observed zero-point inertial defects and by simulating the Franck–Condon profiles of the observed emission spectra in the harmonic approximation.

Molecular dynamics simulations and fluorescence anisotropy decay measurements are used to investigate the rotational diffusion of anthracene in two organic solvents-cyclohexane and 2-propanol-at several temperatures. Molecular dynamics simulations of 1 ns length were performed for anthracene in cyclohexane ͑at 280, 296, and 310 K͒ and in 2-propanol ͑at 296 K͒. The calculated time constants for reorientation of the short in-plane axis were 7-9 and 11-16 ps at 296 K in cyclohexane and 2-propanol, respectively, in excellent agreement with corresponding fluorescence depolarization measurements of 8 and 14 ps. The measured rotational reorientation times and the calculated average rotational diffusion coefficients varied in accord with DebyeStokes-Einstein theory. Their magnitudes were close to values predicted for an ellipsoid of shape and size equivalent to an anthracene molecule, and exhibited predictable variation with external conditions-increasing with temperature and decreasing with solvent viscosity. However, analysis of the calculated rotational diffusion coefficients for the individual molecular axes gave a more complex picture. The diffusion was highly anisotropic and changes in temperature and solvent type led to nonuniform variation of the diffusion coefficients. The nature of these changes was rationalized based on analysis of variation of solvation patterns with temperature and solvent.

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Objectives-This study proposes the use of a new parameter of vocal aerodynamics, phonation threshold flow (PTF). The sensitivity of PTF and phonation threshold pressure (PTP) were quantitatively compared to the percent of vocal fold elongation from physiologic length.Methods-Ten excised canine larynges were mounted on a bench apparatus capable of controlling vocal fold elongation. Subglottal airflow was gradually increased until the onset of phonation. Elongation of the vocal folds was varied from +0% (physiologic length) to +15%, and PTF and PTP were measured. Results-MeanPTFs at physiologic vocal fold length ranged from 101 mL/s and 217 mL/s. No statistically significant relationship was found to exist between the size of the larynx and the measured PTF values (p=.404). The average percentage change of PTF compared to the magnitude of elongation was found to be statistically significant (p< 0.001). Data indicated that PTF was proportional to the percent of vocal fold elongation. Conclusion-PTF