Molecular rotation and isomerisation in hydrogen isocyanide

Chan, Wai‐To; Shen, DeLin; Pritchard, H. O.

doi:10.1039/ft9959101717

Cited by 11 publications

(6 citation statements)

References 16 publications

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“…As we have noted previously, 3,4,12 the calculated rate constant for the isomerisation of CH 3 NC is about a factor of 20 below the observed reaction rate, and a similar discrepancy of a factor of 20 was found by Stimac and Barker 13 during the simulation of the CH 3 -O-ON-O radical thermal dissociation into CH 3 O + NO 2 , using a similarly constructed potential energy surface. 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.…”

Section: Discussionmentioning

confidence: 94%

Defective modelling of chaotic motions on empirical potential energy surfaces

Pritchard

2013

RSC Adv.

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The progression from regular to chaotic behaviour with increasing vibrational energy is examined for two unimolecular reactions, CH 3 NC = CH 3 CN and NCNC = NCCN. The potential energy surfaces used are, respectively, a piecewise empirical construction and an ab initio numerical surface. It is found that in the former case, the motions never become chaotic in the appropriate energy range, but in the latter, they seem to be approaching that ideal condition. The reasons for this difference are subject to speculation at the present time, but there seems to be a strong impediment to randomisation of energy in one case that is not present in the other. An attempt is made to formulate a semi-quantitative measure of chaotic behaviour in these reactions. Until this problem with synthetic potential energy surfaces can be resolved, these results have important consequences for the numerical modelling of larger polyatomic systems, up to and including such problems as protein folding.

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Section: Discussionmentioning

confidence: 94%

Defective modelling of chaotic motions on empirical potential energy surfaces

Pritchard

2013

RSC Adv.

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“…The adjustments needed in this case could also give clues for the improvement of other similar empirical surfaces where ab initio data are not available, for example the dissociation of the peroxy radical CH 3 -O-O-N-O / CH 3 O + NO 2 studied by Stimac and Barker: 30 they also found the rate a factor of 20 too small but, as well, the ensemble decay resembled that of a triatomic molecule. 31 Of course, this factor of 20 could just be a coincidence as the potential energy surface is far more tangled, and there might be signicant complications from "roaming" effects, such are found in the dissociation of other nitrocompounds. 32 We should note that for a dissociation reaction, the comparison of forward and reverse reactions is more difficult than for a simple isomerisation process.…”

Section: Discussionmentioning

confidence: 99%

Validating potential energy surfaces for classical trajectory calculations

Pritchard

2015

RSC Adv.

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“…It is a matter of fact that a “stable occupation” of vibrational or rotational levels “across to the TS” can change the geometry of the TS (and, of course, it rises the absolute height of the barrier). For example, HCN, a calculation64 results in a movement of the TS by 2° along the TS valley, if the rotational levels increase from j = 0 to j = 70. Thus, the “ground”‐TS, the col of the PES where no further states are occupied, is only the calculable point in configuration space, which we use for the classical TST.…”

Section: Discussionmentioning

confidence: 99%

Transition state theory with Tsallis statistics

Quapp

Zech

2009

J Comput Chem

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We discuss the rate of an elementary chemical reaction. We use the reaction path and especially its saddle point on the potential energy surface. The reaction path connects reactant and product of a reaction over the transition state (TS). Usually, the TS is assumed near or at the single saddle point of the reaction path. By means of comparison of the statistics of states at the reactant and at the TS, one can estimate the reaction rate by the Eyring theory. We propose to use the Tsallis statistics at the TS, a statistics of seldom accidents. Thus, we propose to generalize the well-known Boltzmann-Gibbs statistics, which is the limiting case of the Tsallis statistics. We use features of this nonextensive thermostatistics. The basic properties of the statistics are used to derive (approximated) partition functions, and they are applied on reaction rates. The approximation includes a factorization of the partition functions. The theory is applied to HCN isomerization to HNC, and to the reaction H(2) + CN --> H + HCN. It allows an accordance with experimental estimations of the reaction rates.

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Molecular rotation and isomerisation in hydrogen isocyanide

Cited by 11 publications

References 16 publications

Defective modelling of chaotic motions on empirical potential energy surfaces

Defective modelling of chaotic motions on empirical potential energy surfaces

Validating potential energy surfaces for classical trajectory calculations

Transition state theory with Tsallis statistics

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