Bond dissociation transition state theory

Several attempts to relate the rate for bond scission (kc) with the molecular stress ( jr) have been reported over the years, most of them could be formally traced back to de Boer s model of a stressed bond [92] and Eyring s formulation of the transition state theory [94]. Yew and Davidson [99], in their shearing experiment with DNA, considered the hydrodynamic drag contribution to the tensile force exerted on the bond when the reactant molecule enters the activated state. If this force is exerted along the reaction coordinate over a distance 81, the activation energy for bond dissociation would be reduced by the amount ... 

Phase space theory (PST) has been widely used for estimation of rates and energy partitioning for ion dissociations. It can be considered within the framework of transition-state theory as the limiting case of a loose transition state, in which all product degrees of freedom are statistically fully accessible at the transition state. As such, it is expected to give an upper limit for dissociation rates and to be best suited to barrierless dissociations involving reaction coordinates with simple bond cleavage character. 

In addition to this type of empirical approach, there are several other approaches that are related more directly to specific properties of the organic, such as the C-H bond dissociation enthalpies (Heicklen, 1981 Jolly et a.L, 1985), ionization energy (Gaffney and Levine, 1979), or NMR shifts (Hodson, 1988). In addition, molecular orbital calculations (Klamt, 1993) and transition state theory (Cohen and Benson, 1987) have been applied. 

In many bimolecular reactions the maximum occurs at about 30% of the dissociation energy required to break the original bonds (Hinshelwood, 1941). The essence of the transition state theory (Eyring, 1935) lies in the recognition... 

Many statistical models have been applied to reaction (3.1), and it might be considered a test case for theoretical treatments of the rate constant. The process inverse to (3.1), the dissociation of ethane, has also been extensively studied experimentally25,26 and theoretically.116,2 2,27 The theoretical predictions for the rate of dissociation are, of course, quite sensitive to the value of the bond dissociation energy. On the other hand, recombination rates depend only weakly on that quantity. In the present review, attention is focused on the prediction of the recombination rate using the transition state theory outlined in Section IIC. First, the high-pressure limit of kr, denoted by kK, is considered, particularly its temperature dependence. This is followed by a brief description of some results for the pressure dependence of kr and for the dissociation of a vibrationally excited C2H6 molecule. 

The latter essentially estimates the M-L bond energy in the complex prior to dissociation. This cannot be measured directly since, in practice, the dissociation process will always be accompanied by relaxation. Thus, if the relaxation of the products lowers their energies, then the in-complex value will be higher than experimentally observed, and vice versa. However, the concept of an in-complex bond energy is useful in Transition State theory. For example, in a dissociative process, the initial slope of the reaction profile will be dominated by the energy of the bond being broken which would be better represented by the in-complex value than by the (dissociated) bond energy. 

Gas phase studies of the reaction (II) have given clear but indirect evidence for the role of the Bril intermediate. By studying the reaction in a cluster, one could assign a zero of time to when the fast photolysis laser broke the HBr bond and thereby demonstrate the time delay before the appearance of products. The delay is not long and one can wonder if the three-atom system had time to sample all of its available phase space, as is typically assumed in some of the most widely used theories of unimolecular dissociation. (These fall into the category called transition state theories .)... 

Variational transition state theory (VTST) is useful when no TS can be explicitly identified (for Morse-like potential, e.g., direct bond dissociation), where there is no TS. It is based on the idea that there is a bottleneck in the phase space during the dissociatioa This can be explained by the fact that during the dissociation process the molecule needs to reach at a certain point a very specific conformation, without which it cannot go further to disassociate. The Arrhenius equation can be written in terms of exponential of Gibbs free energy and exponential of entropy, which characterize the nmnber of distinct states reachable with that amount of energy. 

Barriers to rotation around the Cca —N bonds have been determined experimentally for diaminocarbenes (3) and (4) and their protonated and lithiated counterparts the possible involvement of lithium or a proton in the dimerization of these acyclic diaminocarbenes was also reported. A computational study of the dimerization of diaminocarbenes has been performed via rate constant calculations using general transition-state theory calculations. Such a dimerization has been shown to be a rapid equilibrium between the carbenes and the tetra-A-alkyl-substituted enetetramines (5), by characterization of metathesis products when two different tetramines were mixed. The thermodynamic parameters of this Wanzlick equilibrium have been determined for the A-ethyl-substituted compound the enthalpy of dissociation has been evaluated at 13.7kcalmol and the entropy at 30.4calmor K . Complex-ation of diaminocarbenes by alkali metals has been clearly established by a shift of the C NMR signal from the carbene carbon of more than 5 ppm. ... 

Decomposition of methyl nitrite by concerted elimination (CH2O and HNO formation) and O-N bond dissociation (CH3O and NO formation) has been investigated by classical trajectories and statistical variational efficient microcanonical sampling-transition state theory. The dissociation, which exhibits an inverse deuterium isotope effect, is markedly faster than the four-centre elimination process. The UHF/AMl MO method has been used to study thermolyses of aroyl nitrites. ... 

From a chemical viewpoint, bond scission under stress is a particular case of a un-imolecular dissociation reaction whose rate is enhanced by mechanical stress. As such, it could be treated with Eyring s transition-state theory [Eq. (37)], which permits one to bring the treatment of rate processes within the scope of thermodynamic arguments. By combining de Boer s thermodynamic formulation and the transition-state theory, Tobolsky and Eyring in 1943 developed the rate theory for thermally activated fracture of polymeric threads. When put into an Arrhenius-... 

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. 

Hartree-Fock theory is very useful for providing initial, first-level predictions for many systems. It is also reasonably good at computing the structures and vibrational frequencies of stable molecules and some transition states. As such, it is a good base-level theory. However, its neglect of electron correlation makes it unsuitable for some purposes. For example, it is insufficient for accurate modeling of the energetics of reactions and bond dissociation. 

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