Infrared spectrum and stability of a π-type hydrogen-bonded complex between the OH and C2H2 reactants

Abstract

A hydrogen-bonded complex between the hydroxyl radical and acetylene has been stabilized in the reactant channel well leading to the addition reaction and characterized by infrared action spectroscopy in the OH overtone region. Analysis of the rotational band structure associated with the a-type transition observed at 6885.53(1) cm(-1) (origin) reveals a T-shaped structure with a 3.327(5) A separation between the centers of mass of the monomer constituents. The OH (v = 1) product states populated following vibrational predissociation show that dissociation proceeds by two mechanisms: intramolecular vibrational to rotational energy transfer and intermolecular vibrational energy transfer. The highest observed OH product state establishes an upper limit of 956 cm(-1) for the stability of the pi-type hydrogen-bonded complex. The experimental results are in good accord with the intermolecular distance and well depth at the T-shaped minimum energy configuration obtained from complementary ab initio calculations, which were carried out at the restricted coupled cluster singles, doubles, noniterative triples level of theory with extrapolation to the complete basis set limit.

Pulsed Laval nozzle study of the kinetics of OH with unsaturated hydrocarbons at very low temperatures

“…A very recent study from Leeds (McKee et al 46 ) measured the temperature and 1015 pressure dependence of the rate coefficient in He over the range 210 -373 K and 5-760 Torr, and also in N 2 and SF 6 at room temperature. A full summary of previous determinations and calculations of k 3 in the literature is given in McKee et al 46 The results yielded the following expression for k 3 A weakly-bound T-shaped OH-C 2 H 2 van der Waals complex has been observed within a supersonic free-jet expansion for which the binding energy was measured to be 11.4 kJ mol -1 , 48 and confirmed by ab initio calculations. 48 The van der Waals complex is likely to be formed without a 1035 activation barrier, and at the low temperatures of the pulsed Laval expansion (down to 69 K in the present work) formation and stabilisation of the complex might be expected to lead to a measurable loss of OH radicals.…”

“…However, this was not observed, and so it is concluded that under our conditions the 1040 complex is not formed to a great extent. Lower temperatures (as produced in a free-jet expansion 48 ) or consierably higher densities may be required to stabilise the complex and enable it to be inferred via loss of signal from the OH reagent.…”

An Experimental Study of the Kinetics of OH/OD(v = 1,2,3) + SO2: The Limiting High-Pressure Rate Coefficients as a Function of Temperature

“…This temperature dependence is subtle compared to OH(v=1,2) + C2H2 where a distinct minimum was observed at ~ 300 K, with ki increasing much more strongly with temperature. This higher temperature minimum for OH(D)/SO2 implies that its vdW complex is slightly more strongly bound than that between OH/C2H2, ~ 10 kJ mol -1 , [41][42] based on the observation that the probability for VET in HCl and HF was seen to go through a minimum at ca. 350 and 1000 K, respectively, where the heats of dimerization are 9…”

Infrared rovibrational spectroscopy of OH–C2H2 in 4He nanodroplets: Parity splitting due to partially quenched electronic angular momentum

“…[1][2][3] Along the entrance channel to this reaction, a dipole-quadrupole interaction stabilizes a T-shaped, hydrogen bonded complex (OH-C 2 H 2 ), whose zero-point level lies ≈2.7 kcal mol −1 below the separated reactants. 4 Moreover, this entrance channel complex is located behind a barrier ≈1 kcal mol −1 above the reactant asymptote. [4][5][6] The orbital degeneracy of the OH moiety is lifted upon complex formation, and the orbital angular momentum is quenched upon C-O bond formation.…”

“…4 Moreover, this entrance channel complex is located behind a barrier ≈1 kcal mol −1 above the reactant asymptote. [4][5][6] The orbital degeneracy of the OH moiety is lifted upon complex formation, and the orbital angular momentum is quenched upon C-O bond formation. Nuclear motion carrying the system from reactants to products, therefore, occurs on two coupled potential surfaces (A ′ and A ′′ ), which are degenerate at long range and distinguished by the orientation of the OH singly occupied pπ orbital.…”