Methyl isocyanide is an RRKM molecule after all

Abstract

Classical trajectory calculations on the methyl isocyanide molecule at energies above 25,000 cm-l confirm that the rate of reaction to methyl cyanide is bimodal, with a very fast rate before 0.1 ps, and a slower rate from then on. We conclude that before 0.1 ps, the reacting molecules are unrandomized, but thereafter, they are essentially randomized, with decay to products being, to a good approximation, pure exponential. We estimate that the time for randomization is roughly 0

Validating potential energy surfaces for classical trajectory calculations

“…As we, 21 and others, 22 found, there is a sharp drop within the rst ps in the number of molecules remaining, due to some possessing excess energy in the critical directions reacting before equilibration could occur. Then (at a similar energy), a long period, up to over 200 ps (instead of the original 3 ps) 21 when the logarithm of the number remaining is closely linear with respect to time; aer that, the points become rather scattered, as those few of the original 1000 now le no longer comprise an acceptable statistical sample.…”

“…From the manner in which the initial energy is allocated to the molecules, very crudely, about two-thirds of the total momentum will reside in the CH 3 moiety. During the rst ps, 21,22 every molecule assigned sufficient N]C angular motion will react, leaving a sub-ensemble of molecules with mainly C-N]C vibration and a modicum of C-N]C bending motions which are isolated from the motions inside the CH 3 group. The excess energy in the methyl group then leaks rather slowly (on a roughly $100 ps time scale) by a rst order process whence the N]C group attains sufficient angular motion to invert almost instantly.…”

Defective modelling of chaotic motions on empirical potential energy surfaces

“…This raises the question how much chaos is needed for there to be a first-order decay? Both CH 3 NC and NCNC exhibit first-order ensemble decay, 8,15 but in the first case, the distribution of states is far from chaotic, whereas in the second it seems to be approaching total chaos: thus, first-order decay does not imply a proper chaotic distribution among the reactant states. On the other hand, microscopic reversibility, which fails badly in the first case, but not in the second, seems to be a more reliable indicator.…”

“…In addition, their ensemble decay resembles that of a small molecule like HNC, 14 instead of a first-order process like that for CH 3 NC. 15 That these deficits are due to insufficient mixing has been demonstrated by adding more coupling between the vibrations: this was done by either using coupling constants published by Sumpter and Thompson, 7 or by an artificial coupling procedure in which some momenta were reversed every 0.1 ps 12 mimicking, crudely, collisions at very high pressure, or black-body radiation 3 in a collisionless environment. However, there is no way of knowing, beforehand, how much coupling to add, as the reaction rate can be driven too high, especially just above threshold, as shown in Fig.…”

“…For example, in the light of Fig. 4(b), why does the decay of an ensemble of 1000 trajectories behave in a perfectly logarithmic fashion 15 when there are two isolated quasi-chaotic regions of phase space? This could only be true if molecules trapped on either side of the blockage decayed with the same intrinsic rate.…”