THE HIGH-PRESSURE RANGE OF THE ADDITION OF OH TO C2H2 AND C2H4

The kinetics of the bimolecular reactions OH+C2H2+M double left right arrow C2H2OH+M (1) and OH+C2H4+M double left right arrow C2H4OH+M (2) have been investigated over an extended pressure (1-130 bar) and tempe rature (300-800 K) range. The OH radicals have been generated by laser flash photolysis of suited precursors and their decays have been meas ured by saturated laser-induced fluorescence (SLIF) under pseudo-first -order conditions. The pressure dependences have been analyzed by cons tructing falloff curves at fixed temperatures leading to reliable extr apolations towards the high pressure limiting rate constants k infinit y. In the given temperature range these rate constants are represented as k(1,infinity) = 3.8x10(-11) exp (-910 K/T) cm(3) molecule(-1) s(-1 ) and as k(2,infinity) = 1.0x10(-11) cm(3) molecule(-1) s(-1). At temp eratures above 700 K biexponential decay curves have been obtained. Th e chemical equilibria of reactions (1) and (2) could be determined. By a third law analysis the equilibrium constants have been evaluated wi th reaction enthalpies for the addition complex C2H2OH of Delta(r)H(1) (o) (0 K) = -(146+/-10)kJ/mol and for C2H4OH of Delta(r)H(2)(o) (0 K) = -(123+/-6)kJ/mol, respectively. The two equilibrium constants are gi ven by K-1,K-eq = (5.4+/-2.2)x10(-2) (T/K)(-1.7+/-0.2) exp ((17560+/-1 200) K/T) bar(-1) and K-2,K-eq = 2.1x10(-2) (T/K)(-0.95+/-0.1)x exp((1 4780+/-720) K/T) bar(-1), respectively.