The dynamics of formation of HCl products from the reaction of Cl atoms with methanol, ethanol, and dimethyl ether

Abstract

The dynamics of ground state Cl(2P3/2) atom reactions with methanol, methanol-d1, ethanol, and dimethyl ether have been studied both experimentally and theoretically. The reactions were photoinitiated by 355 nm photolysis of Cl2 to produce monoenergetic Cl(2P3/2) atoms that react with ground electronic state organic molecules under single collision conditions. The rotational quantum state population distributions of the nascent HCl(ν′) products were probed by 2+1 resonance-enhanced multiphoton ionization in a time-of-flight mass spectrometer. Nascent HCl(ν′=0) products from reaction of Cl atoms with methanol, methanol-d1 (CH3OD), ethanol, and dimethyl ether, at mean collision energies in the range of 5.6–6.7 kcal/mol, exhibit distributions of population over rotational levels that all peak at J′=3–5. The average rotational energies of the HCl(ν′=0) products for the respective reactions are 〈Erot〉=330±29, 300±24, 340±24, and 256±17 cm−1 (1σ uncertainties). Ab initio calculations were performed in order to examine the mechanisms of Cl atom abstraction of hydrogen from the alcohols and ether. Optimized geometrical structures and vibrational frequencies of molecular complexes and transition states on the reaction pathways were obtained at the MP2/6-311G(d,p) level and their energies were further refined at the G2 level of theory. Comparisons are drawn between the mechanisms and energetic pathways of the various reactions. The degree of rotational excitation of the HCl, which is significantly greater than for Cl atom abstraction of an H atom from alkanes, is attributed to a dipole–dipole interaction between the HCl and RCHOR′ (R, R′=H or CH3) moieties in the products’ region of the potential energy surface.

Vibrationally resolved dynamics of the reaction of Cl atoms with 2,3-dimethylbut-2-ene in chlorinated solvents

“…1, 2 Direct abstraction of an H atom is observed for reactions of Cl atoms with alkanes, [3][4][5] whereas reactions with alkenes show competition between direct abstraction and addition-elimination pathways. [6][7][8] Reactions of Cl atoms with simple alcohols, amines, haloalkanes and ethers exhibit similar abstraction dynamics, [9][10][11][12][13][14][15] with further interaction of the separating products after the transition state (TS) also influencing the scattering. 1,2,16 An extensive body of experimental and computational work has connected the observed kinetics and dynamics with the topology of the potential energy surfaces for representative reactions.…”

Velocity map imaging the dynamics of the reactions of Cl atoms with neopentane and tetramethylsilane

“…For example, the shape of the transition state, with near linear Cl-H-C moiety, is reflected in the low rotational excitation of HCl products; scattering angles are largely determined by impact parameter and their distributions can vary with product rotational and vibrational quantum states; 1 and the reactions of Cl atoms with methane and partially deuterated isotopologues exhibit reagent vibrational mode specificity, [4][5][6][7][9][10][11][12]14,15,[33][34][35][36][37][38][39][40] electronically non-adiabatic pathways, 29,31,41,42 and evidence for scattering resonances. 43 Reactions of functionalized organic molecules (RH = alcohols, 24,44,45 amines, 46 alkyl halides 23,47 and linear and cyclic ethers 24,44,48,49 ) reveal the effects of weakly bound complexes, molecular reorientation, and molecular shape on the chemical dynamics. 50 The gas phase studies also serve as benchmarks for comparison with recent investigations of the kinetics and dynamics of reactions of the type summarized by (1) (with a variety of organic molecules RH) in liquid solutions using ultrafast transient absorption measurements of rates of loss of reagents and formation of products.…”

Nonadiabatic dynamics in the CH3+HCl→CH4+Cl(PJ2) reaction

“…Under otherwise similar experimental conditions, nascent HCl rotational distributions from H-atom abstraction reactions between Cl and various polyatomic organic molecules showed no evidence of secondary collisional relaxation up to 200 ns after photoinitiation, and very fast secondary reactions of the product radicals were not observed. 20 Furthermore, the relative Cl and Cl * signals would be expected to show different dependences on the time delay between the photolysis and probe laser pulses, were quenching collisions between Cl and I * to be responsible for Cl * ; the rise times for both Cl and Cl * signals were, however, identical, and ⌫ was thus independent of time delay.…”

“…The collision energy distribution is broadened by the nascent CH 3 internal state distributions and thermal motion of the reagents in the expansion. The rotational temperature of the expansion was found to be ͑77± 9͒ K by recording 2 + 1 REMPI spectra of a region of the R and S branches of the HCl͑f 3 ⌬ 2 − X 1 ⌺ + ͒ ͑0,0͒ transition, as described previously, 20 and, since translational cooling is more efficient than rotational relaxation, this provides an upper limit to the spread in the collisional energy distribution. Using the standard formulas for photoinitiated reactions, 21 the FWHM of the collision energy distributions for individual CH 3 channels are within the range 560-790 cm −1 ͑1.6-2.3 kcal mol −1 ͒.…”