Early-time photodissociation dynamics of chloroiodomethane in the A-band absorption from resonance Raman intensity analysis

Abstract

We have obtained resonance Raman spectra and absolute Raman cross sections for h 2-chloroiodomethane ͑fourteen excitation wavelengths between 200 nm and 355 nm͒ and d 2-chloroiodomethane ͑for 282.4 nm excitation͒ in cyclohexane solution. Most of the intensity in the A-band resonance Raman spectra appears in the nominal C-I stretch overtones progression and combination bands of the nominal C-I stretch overtones with the fundamentals of the CH 2 wag, CH 2 scissor, and the Cl-C-I bend or C-Cl stretch fundamentals. The A-band absorption and absolute resonance Raman intensities were simulated using a simple model which included preresonant contributions to the fundamental Raman peaks and time-dependent wave packet calculations. The motion of the wave packet on the excited state surface was converted from dimensionless normal coordinates into internal coordinates using the results of normal coordinate calculations. The A-band short-time photodissociation dynamics of chloroiodomethane shows that the C-I bond lengthens, the I-C-Cl and H-C-I angles become smaller, and the H-C-Cl angles become larger. These internal coordinate motions which are associated with relatively low frequency modes are consistent with a simple impulsive ''soft'' radical model of the photodissociation and the CH 2 Cl group changing to a more planar structure. However, the C-H bond length does not change much and the H-C-H angle ͑associated with higher frequency modes͒ becomes slightly smaller which is inconsistent with the ''soft'' radical model and the CH 2 Cl group changing to a more planar structure. This suggests that an impulsive ''semirigid'' radical model may be more appropriate than the ''soft'' radical model to qualitatively describe the chloroiodomethane photodissociation. An ambiguity in the assignment of the 724 cm Ϫ1 Raman peak and its associated combination bands to combination bands of the nominal C-I stretch overtones with the fundamentals of the Cl-C-I bend or C-Cl stretch fundamentals limits what we are able to determine about the C-Cl bond length changes during the initial stages of the photodissociation.

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

“…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.…”

Resonance Raman Investigation of the MLCT Transition in [Pt(dppm)2(PhC⋮C)2] and the MMLCT Transition in [Pt2(μ-dppm)2(μ-PhC⋮C)(PhC⋮C)2]+

“…The resonance Raman experimental apparatus and methods have been described previously. [47][48][49][50][51][52] Excitation frequencies for the resonance Raman experiments were generated by hydrogen Raman shifting the second, third, and fourth harmonics of a Spectra-Physics GCR-150-10 Nd:YAG laser. The flowing liquid jet sample or stirred cell sample was excited by a lightly focused laser beam (with 20-50 µJ per pulse in ∼1 mm diameter on the sample).…”

Efficient dehalogenation of polyhalomethanes and production of strong acids in aqueous environments: Water-catalyzed O–H-insertion and HI-elimination reactions of isodiiodomethane (CH2I–I) with water

“…44,[47][48][49][50][51] Gas-and solutionphase resonance Raman experiments for several polyhalomethanes showed that photodissociation typically had a noticeable multidimensional character and Franck-Condon region dynamics qualitatively consistent with a semirigid radical description of the dissociation process. [59][60][61][62][63][64][65][66][67][68][69] Ultraviolet excitation of polyhalomethanes in condensed phases leads to photoproducts with characteristic transient absorption bands in the ultraviolet and visible regions which were assigned to a number of different species. [70][71][72][73][74][75][76][77][78][79][80][81][82] Femtosecond transient absorption measurements showed that these photoproducts are mainly formed by geminate recombination of the photofragments within the solvent cage.…”