Quasiclassical direct dynamics trajectories

Quasiclassical direct dynamics trajectories at the various levels of theory were later calculated to study the central barrier dynamics for the C1 I CH3C1, Cr + C2H5C1, C1- + CH3I, F +CH3C1, OH +CH3C1, and other Sn2 reactions.31,32,47,97 108 The effect of initial reaction conditions, such as energy injection, substrate orientations, and the mode of collision, on the fate of the reaction, product, and energy distribution, was analyzed. Some of these trajectory calculations required serious modification in RRKM and TST for... 

Ab initio direct dynamics trajectory calculations were performed at the B3LYP/6-311+G(d,p) level of theory.39 The trajectories were initiated at the separated reactants, TS (HOOCH3- F ), and the CII 2( 011)0 H- F region on IRC, with quasiclassical sampling including ZPE. The trajectories from the reactants were started at 15 A separation of the two species with small attractive potentials of -0.21 or -0.13 kcalmol-1. The collision impact parameter was chosen randomly between zero and maximum. The initial CH3OOH vibrational and rotational degrees of freedom were selected from their 300 K Boltzmann distributions. 

Direct dynamics calculations were carried out with quasiclassical normalmode sampling from a canonical ensemble at 923 K (the experimental reaction temperature). Simulations initiated at the vicinity of TS for rearrangement of carbene 13 to 14 via oxirene 12, and 300 trajectories were obtained at DFT methods. The preliminary results reported in the manuscript showed that preferred formation of 15a over 15b by the ratio of 1.8 7.6 depends on the method used. The results were qualitatively consistent with the value of 2.5 deduced from the experiment. The non-unity ratio likely arises from the situation that two methyl groups in 14 are dynamically unequal on the carbene formation process. 

The ring-opening reaction of cyclopropyl radical [Equation (7)] was shown to occur at 174°C to give ally radical, but the product stereochemistry was unclear. Ab initio direct dynamics study was carried out to clarify the stereochemical course of the reaction.40 Trajectories were initiated at the ringopening TS obtained at CASSCF(3,3)/6-31G(d), with quasiclassical normal sampling at the experimental temperature of 174°C. ZPE was included, and thermal vibrational energy was sampled from the normal-mode Boltzmann distribution. A rotational energy of RT/2 was added toward the allyl radical product. 

Doubleday, C. Nendel, M. Houk, K. N. Thweatt, D. Page, M. Direct dynamics quasiclassical trajectory study of the stereochemistry of the vinylcyclopropane-cyclopentene rearrangement, J. Am. Chem. Soc. 1999,121,4720-4721. 

Doubleday, C. Bolton, K. Hase, W. L. Direct dynamics quasiclassical trajectory study of the thermal stereomutations of cyclopropane, J. Phys. Chem. A 1998, 102,... 

H(2s). a third ( A ) surface is close to the first two in the entrance channel of the potential, but correlates with the OH(2r ") product. Nonadiabatic behaviour may occur at the resulting hyperconical intersections and the dynamics of the trajectories which switch potential energy surfaces are quite different from those which remain on the lowest surface for the entire reaction. In a recent quasiclassical surface-hopping trajectory calculation, using a multisurface DIM representation of the potential [52], we found that trajectories which begin on the first excited A surface, and switch to the lower surface in the exit channel, for example, usually avoid the potential minimum completely, and have fairly direct dynamics resembling those of a simple H atom abstraction. 

Dynamics Simulation of the Stereomutation of Cyclopropane C. Doubleday, Jr., K. Bolton, and W. L. Hase, J. Phys. Chem. A, 102, 3648 (1998). Direct Dynamics Quasiclassical Trajectory Study of the Thermal Stereomutations of Cyclopropane. 

The phenomenon of such bimodal lifetime distribution proposed for reaction 1 on the basis of direct quasiclassical trajectory calculations were tested experimentally with the reaction of diaza-[2.2.1]bicycloheptane to [2.1. Ojbicyclopentane [Equation (2)].6 8 Experimental study on reaction 2 showed that the exo isomer 5x is formed favorably over the endo isomer 5n by about 3 1 in the gas phase. One explanation for the preferential formation of 5x invokes a competitive concerted and stepwise mechanism the concerted pathway directly from 4 to 5 gives 5x with the inversion of configuration at the carbon from which N2 is departing, whereas the stepwise pathway goes through the radical intermediate and leads to both 5x and 5n in equal amount. Alternatively, the product stereochemistry can be rationalized by dynamic matching between the entrance channel to the cyclopentane-1,3-diyl radical intermediate and the exit channel to bicyclo[2.1. Ojpentane as was assumed for reaction 2. 

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