Ultraviolet photodissociation of vinyl iodide: understanding the halogen dependence of photodissociation mechanisms in vinyl halides

Abstract

The photodissociation of vinyl iodide has been investigated at several wavelengths between 193 and 266 nm using three techniques: time-resolved Fourier transform emission spectroscopy, multiple pass laser absorption spectroscopy, and velocity-mapped ion imaging. The only dissociation channel observed is C-I bond cleavage to produce C2H3 (nu, N) + I (2P(J)) at all wavelengths investigated. Unlike photodissociation of other vinyl halides (C2H3X, X = F, Cl, Br), in which the HX product channel is significant, no HI elimination is observed. The angular and translational energy distributions of I atoms indicate that atomic products arise solely from dissociation on excited states with negligible contribution from internal conversion to the ground state. We derive an upper limit on the C-I bond strength of D0(C2H3-I) < or = 65 kcal mol(-1). The ground-state potential-energy surface of vinyl iodide is explored by ab initio calculations. We present a model in which the highest occupied molecular orbital in vinyl halides has increasing X(np) non-bonding character with increasing halogen mass. This change leads to reduced torsional force around the C-C bond in the excited state. Because the ground-state energy is highest when the CH2 plane is perpendicular to the CHX plane, a reduced torsional force in the excited state correlates with a lower rate for internal conversion compared to excited-state C-X bond fission. This model explains the gradual change in photodissociation mechanisms of vinyl halides from the dominance of internal conversion in vinyl fluoride to the dominance of excited-state dissociation in vinyl iodide.

Facile identification of photocleavable reactive metabolites and oxidative stress biomarkers in proteins via mass spectrometry

“…The peptide RYLPT is modified by a single bromine and a single chlorine atom. Photodissociation of this peptide produces Loss of bromine is the dominant fragment, which holds well with previous observations of the general trends in dissociation for halogenated species [43]. Loss of chlorine is also observed, and also seems to be enhanced by the presence of the bromine relative to the loss of chlorine in peptides which do not contain bromine.…”

Time-resolved broadband cavity-enhanced absorption spectroscopy for chemical kinetics.

“…A 266-nm laser pulse produced C 2 H 3 radicals by photolysis of a dilute mixture of 0.9% vinyl iodide [21] in 10 Torr N 2 bath gas. Using photolysis laser fluence of 18 mJ/cm 2 , the initial vinyl radical concentration was 1•10 13 cm -3 .…”

Transition state region in the A-Band photodissociation of allyl iodide—A femtosecond extreme ultraviolet transient absorption study

“…Thus, it possibly originates from the 3 Q 0 surface, in line with the high I*/I branching ratio (signifying high population and consequently higher probability of detection) in allyl iodide. This initial assignment could potentially be tested by characterizing the transient peak intensities in other alkyl iodides with systematically varying I*/(I + I * ) branching fractions at the same excitation wavelength (0.67 in C 2 H 5 I, 71 0.6 in C 3 H 7 I, 72 0.32 in C 6 H 5 I, 73 0.27 in C 2 H 3 I, 74 and 0.22 in C 6 H 11 I). 75 However, it must be mentioned that more thorough assignment of the transient species as a general feature of A-band photodissociation in halogenated hydrocarbons necessitates the availability of more quantitative information on the exact oscillator strengths of both the valence-excited and core-excited states, their slopes in the TS region, as well as the adiabaticity of the valence-excited states.…”