Minimum energy path , intramolecular

In order to define how the nuclei move as a reaction progresses from reactants to transition structure to products, one must choose a definition of how a reaction occurs. There are two such definitions in common use. One definition is the minimum energy path (MEP), which defines a reaction coordinate in which the absolute minimum amount of energy is necessary to reach each point on the coordinate. A second definition is a dynamical description of how molecules undergo intramolecular vibrational redistribution until the vibrational motion occurs in a direction that leads to a reaction. The MEP definition is an intuitive description of the reaction steps. The dynamical description more closely describes the true behavior molecules as seen with femtosecond spectroscopy. 

The surface in Fig. 12 demonstrates that there is little coupling between the C—F translation coordinate and the bending coordinate of the complex. Stated another way, the time scale for intramolecular vibrational redistribution between these coordinates is slow compared to the time scale for breaking the C—F bond. These conclusions are not obvious upon examination of the minimum energy path shown in Fig. 11, and indeed such diagrams, while generally instructive, can lead to improper conclusions because they hide the multidimensional nature of the true PFS. A central assumption of statistical product distribution theories... 

Andres, J., Arnau, A., Silla, E., Bertran, J., Tapia, O. Atheoretical study of the intramolecular solvolytic mechanism of the Meyer-Schuster reaction. MINDO/3 and CNDO/2 calculations of minimum energy paths. THEOCHEM1983, 14,49-54. 

BEBO = bond-energy-bond-order CID = collision-induced dissociation DC = dynamical correlation DIM = diatom-ics-in-molecules DMBE = double many-body expansion EHF = extended Hartree-Fock FFT = fast Fourier transform IVR = intramolecular energy distribution LEPS = London-Eyring-Polanyi-Sato MBE = many-body expansion MEP = minimum energy path PES = potential energy surface TST = transition-state theory. 

More recently Barone and Adamo [27] have theoretically re-investigated the PE functions of 2PY along the CR path for tautomerization. Instead of the third local minimum on the excited A (n7r ) PE surface found in Ref.[36] they have obtained only an energy plateau in the central part of the CR path. The difference in the slope of PE functions reported in both papers can partially result from the definition of the CR path defined in Cartesian and intramolecular coordinates, respectively. The Authors of Ref. [27] claim, however, that they do not see any convicing evidence of dissociation of the mobile hydrogen atom on the A (n7r ) PE surface. This simply results from their method of calculation of the excited-state energies (CIS). One cannot properly describe dissociation at the HF level of approximation. A multireference wave function (of a CASSCF type) is needed for proper description of this process. Let us mention that a quite similar behavior with respect to ESIPT reaction has recently been reported for formamide [37]. 

Figure 6.4b shows the potential energy along the most probable reaction path of Figure 6.4a. The potential has a shallow minimum corresponding to the physisor-bed state, at a relatively large distance R from the surface, where the intramolecular distance r is close to the equilibrium value of the molecule in the gas phase. The forces responsible for physical adsorption are the weak van der Waals forces. These dispersive forces are proportional to the polarizability of a molecule and result from electromagnetic interactions between the adsorbing molecule and the surface.

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