  The general approach to classical unimolecular reaction rates due to Thiele
is revisited in light of recent advances in the phase space formulation of
transition state theory for multidimensional systems. We analyze in detail the
gap time distribution and associated reactant lifetime distribution for the
isomerization reaction HCN $\rightleftharpoons$ CNH. Both algebraic (power law)
and exponential decay regimes have been identified. Statistical estimates of
the isomerization rate are compared with the numerically determined decay rate.
Examination of the decay properties of subsensembles of trajectories that exit
the HCN well through either of 2 available symmetry related product channels
shows that the complete trajectory ensemble effectively attains the full
symmetry of the system phase space on a short timescale $t \lesssim 0.5$ ps,
after which the product branching ratio is 1:1, the "statistical" value. At
intermediate times, this statistical product ratio is accompanied by
nonexponential (nonstatistical) decay. We point out close parallels between the
dynamical behavior inferred from the gap time distribution for HCN and
nonstatistical behavior recently identified in reactions of some organic
molecules.
