Acetylenic C–H and methyl C–D bond fission in photodissociation of vibrationally excited propyne-d3

Abstract

Vibrational preexcitation of a state containing three quanta of C–H stretch (3ν1) results in C–H acetylenic and C–D methyl bond rupture in the ∼243.1 nm photolysis of CD3C≡CH, in contrast to previous observations of the almost isoenergetic 193 nm photodissociation of propynes. The C–D bond fission is the dominant pathway with a D/H branching ratio of 2.0±0.5 at a combined energy of ∼50 830 cm−1. The average translational energies of D and H atoms are nearly identical, although the C–H acetylenic and C–D methyl bond energies differ quite extensively, pointing to different dynamics on the involved potential energy surfaces.

Photofragment translational spectroscopy of allene, propyne, and propyne-d3 at 193 nm

“…3, 203.3, 209.0, and 213.3 nm were identical to those from allene, suggesting production of the propargyl radical from each C 3 H 4 isomer. Furthermore, Chen et al [11] investigated CD 3 CCH by vibrationally exciting the molecule with three quanta of the C-H stretch prior to dissociation at 243.1 nm. Even though the C-H bond was pre-excited, C-D bond fission was found to be the dominant channel, with a reported branching ratio for D loss/H loss of 2.0 AE 0.5.…”

“…The UV and VUV photodissociation of allene and propyne has been investigated by several other experimental groups [4][5][6][7][8][9][10][11][12][13] while numerous theoretical studies [14][15][16][17][18][19][20][21] have been conducted to characterize the ground and excited states of both isomers. The photodissociation dynamics of allene at 193 nm were studied by Jackson et al [5] with molecular beam photofragment translational spectroscopy (PTS), using electron impact (EI) ionization of the scattered photoproducts.…”

Spectroscopy of the Acetylenic CH Stretch of Propyne-d3 in the Region of the Second Overtone

“…These extra peaks are related to the Q branches of two hot bands arising from the 3ν 1 + ν 10 − ν 10 and the 3ν 1 + 2ν 10 − 2ν 10 transitions, where ν 10 is the degenerate C-C≡C bend with a fundamental frequency of 306.4 cm −1 (19). The identification of these hot bands is assured by their elimination in the action spectra of the H and D photofragments resulting from the ∼ 243.1 nm photodissociation of vibrationally excited jet-cooled samples of CD 3 C≡CH (17).…”

“…The propyne-d 3 isotopomer was chosen for this study because of our interest in testing the effect of initial vibrational excitation on its photodissociation outcome (17) and the lack of information concerning its overtones. Its structure clarifies an inherently complex photodissociation process, in which both the C-D methyl and C-H acetylenic bonds are cleaved.…”

HIGH-RESOLUTION INFRARED SPECTRUM OF THE Ν3+ν8 COMBINATION BAND OF JET-COOLED PROPYNE

“…As a prolate symmetric top the aliphatic (CH 3 ) and acetylenic (CH) stretches are suitably decoupled from each other that the strong acetylenic CH stretch mode (m 1 ) is not strongly perturbed [10]. Studies of spectra that are perturbed through weak near-resonant couplings to background vibrational states, as seen in other transitions of propyne, make it of interest for studying intramolecular vibrational relaxation (IVR) [11][12][13]10,[14][15][16][17]. Moreover, comparison between high-resolution measurements as presented here for propyne and ab initio methods offers a good test of the accuracy of the Hamiltonians used to describe the involved molecular energy levels.…”