Short-time photodissociation dynamics of iodoalkanes: refinement using resonance Raman intensities of isotopic derivatives

Abstract

Resonance Raman spectra of and iodide in cyclohexane solution were 1,1,1,3,3,3-d 6 -2-iodopropane d 9 -tert-butyl obtained at excitation wavelengths of 266 and 217.8 nm. The 266 nm spectra, resonant with the directly dissociative A-state absorption, display long overtone progressions in the nominal C-I stretch (ca. 500 cm-1) and some intensity in fundamentals, overtones and combination bands of bending and C-C stretching modes. The 266 nm resonance Raman intensities and the A-band absorption spectrum are simultaneously modelled using wavepacket propagation on a multidimensional locally harmonic potential surface. The dimensionless normal coordinate wavepacket motions determined from the simulations are converted into internal coordinate motions by making use of previous results on the isotopically unsubstituted compounds to remove some of the indeterminacy in the signs of the normal mode displacements. In the Ðrst 10 fs the CÈI bond lengthens less in these higher alkanes than in iodoethane, suggesting that more of the available energy goes toward CÈI bond cleavage in iodoethane.

Resonance Raman study of the A-band short-time photodissociation dynamics of axial and equatorial conformers of iodocyclopentane

“…There is reasonable agreement between most of the calculated and experimental Raman features but several bands with contributions from fundamental peaks exhibit poor agreement presumably due to preresonantresonant interference which has been observed for fundamental bands in other iodoalkane A-band resonance Raman spectra. 29,33,67,73,76 Table II͒ the calculated fit to the experimental absorption band and/or resonance Raman cross sections is noticeably poorer. Our overall agreement between the experiment and calculated absorption and resonance Raman cross sections seems reasonable bearing in mind the simple model and approximations used in the calculations.…”

“…The Franck-Condon region C-I bond length changes associated with the A-band photodissociation of iodoethane, 2-iodopropane, and 2-methyl-2-iodopropane show a reasonable correlation with the amount of internal energy and the widths of their internal energy distributions found for the radical photoproducts as determined by time-of-flight photofragment spectroscopy experiments. 1,4,6,8,54,67,76,82 This suggests that the equatorial conformer of iodocyclopentane may produce cyclopentyl radicals with less internal excitation than the axial conformer in as much as the Franck-Condon region C-I bond length changes correlate with the internal energy distributions of the cyclopentyl radical photofragments. This would be noticeably different than the case for iodocyclohexane which found that the cyclohexyl radicals formed from A-band photodissociation had more internal excitation for the equatorial conformer parent than the axial conformer parent 81 and may suggest that the energy partitioning of iodocyclopentane is significantly different than that of iodocyclohexane for their axial and equatorial conformations.…”

“…Sample concentrations ranging from 0.10 to 0.15 M iodocyclopentane in cyclohexane solvent were used in the Raman experiments. The methods and apparatus used for the resonance Raman spectroscopy experiments have been previously described [69][70][71][72][73][74][75][76][77][78][79] and only a short description will be given here. The harmonics of a nanosecond Nd:YAG laser ͑Spectra-Physics GCR-150-10͒ and their hydrogen Raman shifted lines gave the excitation wavelengths for the resonance Raman experiments.…”

“…However, we would expect that any inhomogeneous broadening effects on the A-band iodocyclopentane spectra are relatively small since many previous resonance Raman intensity analysis investigations of A-band iodoalkanes found little evidence or need to include substantial inhomogeneous broadening to simultaneously model the resonance Raman partial excitation profiles, absolute Raman cross sections, and absorption band. 29,[31][32][33]67,[72][73][74][75][76][77][78][79]82 Possible moderate perturbations of the relative intensities may become discernible when resonance Raman excitation profiles with many data points over the A-band absorption become available.…”

“…This intensity pattern is similar to that observed in other iodoalkane A-band resonance Raman spectra. [27][28][29][30][31][32][33][34][35][36][65][66][67][68]76,78,79 The multidimensional character of the excited state potential energy surface and/or mixing of the internal coordinates in the ground-state normal mode descriptions complicates the extraction of the short-time photodissociation dynamics from the resonance Raman intensities. To obtain the relative contributions of these two effects and extract the short-time photodissociation dynamics associated with the axial and equatorial conformers of iodocyclopentane we have done a semiquantitative resonance Raman intensity analysis that makes use of the normal-mode descriptions of the Franck-Condon active vibrational modes.…”