The rate constant for Cl + NH 3 f HCl + NH 2 has been measured over 290-570 K by the time-resolved resonance fluorescence technique. Ground-state Cl atoms were generated by 193 nm excimer laser photolysis of CCl 4 and reacted under pseudo-first-order conditions with excess NH 3 . The forward rate constant was fit by the expression k 1 ) (1.08 ( 0.05) × 10 -11 exp(-11.47 ( 0.16 kJ mol, where the uncertainties in the Arrhenius parameters are (1 σ and the 95% confidence limits for k 1 are (11%. To rationalize the activation energy, which is 7.4 kJ mol -1 below the endothermicity in the middle of the 1/T range, the potential energy surface was characterized with MPWB1K/6-31++G(2df,2p) theory. The products NH 2 + HCl form a hydrogen-bonded adduct, separated from Cl + NH 3 by a transition state lower in energy than the products. The rate constant for the reverse process k -1 was derived via modified transition state theory, and the computed k -1 exhibits a negative activation energy, which in combination with the experimental equilibrium constant yields k 1 in fair accord with experiment.

The laser flash photolysis resonance fluorescence technique was used to monitor atomic Cl kinetics. Loss of Cl following photolysis of CCl 4 and NaCl was used to determine k(Cl + C 6 H 6 ) ) 6.4 × 10 -12 exp(-18.1 kJ mol -1 /RT) cm 3 molecule -1 s -1 over 578-922 K and k(Cl + C 6 D 6 ) ) 6.2 × 10 -12 exp(-22.8 kJ mol -1 /RT) cm 3 molecule -1 s -1 over 635-922 K. Inclusion of literature data at room temperature leads to a recommendation of k(Cl + C 6 H 6 ) ) 6.1 × 10 -11 exp(-31.6 kJ mol -1 /RT) cm 3 molecule -1 s -1 for 296-922 K. Monitoring growth of Cl during the reaction of phenyl with HCl led to k(C 6 H 5 + HCl) ) 1.14 × 10 -12 exp(+5.2 kJ mol -1 /RT) cm 3 molecule -1 s -1 over 294-748 K, k(C 6 H 5 + DCl) ) 7.7 × 10 -13 exp(+4.9 kJ mol -1 /RT) cm 3 molecule -1 s -1 over 292-546 K, an approximate k(C 6 H 5 + C 6 H 5 I) ) 2 × 10 -11 cm 3 molecule -1 s -1 over 300-750 K, and an upper limit k(Cl + C 6 H 5 I) e 5.3 × 10 -12 exp(+2.8 kJ mol -1 /RT) cm 3 molecule -1 s -1 over 300-750 K. Confidence limits are discussed in the text. Third-law analysis of the equilibrium constant yields the bond dissociation enthalpy D 298 (C 6 H 5 -H) ) 472.1 ( 2.5 kJ mol -1 and thus the enthalpy of formation ∆ f H 298 (C 6 H 5 ) ) 337.0 ( 2.5 kJ mol -1 .