Imaging the Dynamics of Reactions between Cl Atoms and the Cyclic Ethers Oxirane and Oxetane

Abstract

Direct current slice velocity map ion images of the HCl(nu' = 0, J') products from the photoinitiated reactions of ground state Cl atoms with ethane, oxirane (c-C2H4O), and oxetane (c-C3H6O), at respective mean collision energies of 5.5, 6.5, and 7.3 kcal mol-1(-1), were analyzed using a Legendre moment fitting procedure. The experimental method and the fitting technique were tested by comparing the derived center-of-mass (CM) frame angular scattering distribution for the HCl(v' = 0, J' = 1) products from the reaction of Cl + C2H6 with those determined by Suits and co-workers from a crossed molecular beam experiment. For the Cl + c-C2H4O reaction, a broad, forward, and backward peaking CM frame angular distribution of HCl(nu' = 0, J' = 2) products was determined, with an average fraction of the available energy released as product translational energy of f t, equal to 0.52 +/- 0.18. The HCl consumes only 1% of the available energy, and conservation arguments dictate that the radical coproduct is significantly internally excited, corresponding to an average fraction of the available energy of f int(c-C2H3O), equal to 0.47 +/- 0.18. For the reaction of oxetane with Cl atoms, abstraction of H atoms is possible from carbon atoms from positions either alpha or beta to the O atom. The contributions to the reaction from these two H-atom abstraction channels were estimated to be 63 and 37%, consistent with an unbiased propensity for removal of alpha- and beta-H atoms that are present in 2:1 abundance. The angular scatter of products in the CM frame is also broad and forward-backward peaking and is reminiscent of the products of the Cl + CH3OH and CH3OCH3 reactions. The derived mean fraction of the available energy channelled into product translation is f t = 0.54 +/- 0.12 for each of the two abstraction pathways. With only a small amount of energy in the rotation of the HCl(nu' = 0), the remainder is accounted for by excitation of the radical coproduct internal modes, with f int(c-C3H5O) = 0.42 +/- 0.12 for both alpha- and beta-H abstraction. The broad product scattering in the CM frame observed for both reactions of Cl atoms with the cyclic ethers is consistent with reactive collisions over a wide range of impact parameters, as might be expected for barrierless reactions with loose transition states.

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

“…The velocity information was initially derived from analysis of time-of-flight (TOF) profiles obtained in a TOF mass spectrometer, [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] but velocity map imaging (VMI) 19,20 has since become the method of choice. [21][22][23][24][25][26][27][28][29][30][31][32] The experimental measurements and dynamical calculations for H-atom abstraction reactions involving simple alkanes illustrate a wealth of dynamical behaviour. 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]…”

Adiabatic and nonadiabatic dynamics in the CH3(CD3)+HCl reaction

“…14,33 Reactions were initiated by photodissociation of CH 3 I ͑Ald-rich, 99.5%͒ or CD 3 I ͑CK Gas, 98%, 99.5% of D atoms͒, which was diluted to 10% in He or Ar and coexpanded through a late mixing nozzle with a 15% mixture of HCl ͑Aldrich, 99+ %͒ in He or Ar. The flows of the dilute CH 3 I ͑or CD 3 I͒ and HCl gas mixtures were regulated by two separate pulsed nozzles ͑General Valve series 9͒ mounted at 90°t o one another on a triangular polytetrafluoroethylene ͑PTFE͒ block that contained a Y-shaped channel with internal diameter of 0.8 mm to merge the two gas pulses into a single flow just prior to expansion into high vacuum.…”

“…A separable expansion in Legendre polynomials was made of the distributions in cos , the c.m.-frame scattering angle, and f t Ј =2f t − 1, where f t is the fraction of the available energy becoming the total kinetic energy release ͑TKER͒ of the products. [33][34][35][36] The distribution functions of these c.m.-frame parameters are denoted as P͑cos ͒ and P͑f t Ј͒ ͓although results will be presented as the related P͑f t ͔͒, with defined as the angle between the relative velocities of CH 4 and CH 3 or, equivalently, HCl and Cl/ Cl * . The expanded functions have moments that are determined in the fitting procedure and that describe the forms of P͑cos ͒ and P͑f t ͒.…”

“…The outcomes were best-fit c.m.-frame distributions for P͑cos ͒ and P͑f t ͒ and plots of the associated laboratory-frame Legendre moments for direct comparison with the experimental M L expt ͑ Cl ͒. The modified fitting program was previously tested 33 by comparing the analysis of velocity map imaging data for the reaction of Cl with ethane with the outcomes of crossed molecular beam scattering experiments conducted by Huang et al 38 The results of these fits to the L = 0 moments of velocity map images for Cl atoms from reaction ͑1͒ showed almost no difference with the numbers of Legendre polynomials ͑in the range 3-5͒ used in the expansion of the c.m.-frame distributions, while for the second Legendre moment ͑L =2͒ of the experimental image, best fits were obtained with four Legendre polynomial functions in cos and f t Ј, which are denoted as ͑4,4͒ ͓the notation ͑n , m͒ indicates n and m Legendre polynomials in the respective expansions of P͑cos ͒ and P͑f t Ј͔͒. The results of these fits are superimposed on the experimental data in Fig.…”

Velocity map imaging of the dynamics of bimolecular chemical reactions

“…[80][81][82][83][84][85][86][87][88][89] Our prior work demonstrated rich dynamics of these compounds in H-atom abstraction reactions with Cl atoms associated with the low barriers to reaction, and weakly bound complexes on the reagent and product sides of the transition state region. In addition, the low energy release and resultant slow separation of products, allows reorientation by rotational motion to occur as the products separate and long range interactions play a part in the partitioning of the available energy into rotational degrees of freedom.…”