A new method for constructing empirical valence bond potential energy surfaces for reactions is presented. Building on the generalized Gaussian approach of Chang-Miller, V12(2)(q) is represented by a Gaussian times a polynomial at the transition state and generalized to handle any number of data points on the potential energy surface. The method is applied to two model surfaces and the HCN isomerization reaction. The applications demonstrate that the present method overcomes the divergence problems encountered in some other approaches. The use of Cartesian versus internal or redundant internal coordinates is discussed.
Transition structures and reaction paths for the BH m Cl n system have been characterized at the MP2(full)/6-31G(d) level of theory; heats of reaction and barriers have been computed at the G2 level of theory. Calculations show that the insertion reactions of BH m Cl n into H 2 and HCl (m + n ) 0, 1, 2) occur by highly distorted, non-least motion transition states, with barriers that increase with chlorine substitution on boron. Hydrogen abstraction from BH and BH 3-n Cl n by chlorine proceeds with little or no barrier by a linear transition state. By contrast, BH 3-n Cl n + H f BH 2-n Cl n + H 2 occurs by an addition-elimination path via a tetracoordinate intermediate. Likewise, BH 2-n Cl n + X f BH 1-n Cl n + HX or BH 2-n Cl n-1 + XCl (X ) H, Cl) are also addition-elimination processes. Abstraction of Cl from BCl and BH 3-n Cl n by H is endothermic and occurs by a bent transition state.
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