Transition State Optimization

Gaussian includes a facility for automatically generating a starting structure for a transition state optimization based upon the reactants and products that the transition structure connects, known as the STQN method. This feature is requested with the QST2 option to the Opt keyword. Input files using this option will include two title and molecule specification sections. The facility generates a guess for the transition structure which is midway between the reactants and products, in terms of redundant internal coordinates. 

Here is the input file for an optimization of the transition structure for the reaction H3CO —t H2COH (a simple 1,2 hydrogen shift reaction). We specify a UHF calculation (open shell) since the molecular system is a doublet  

The STQN facility requires that corresponding atoms appear in the same order within the two molecule specifications (although it does not matter whether the reactants 01 the products appear first). The bonding in the two structures does not need to be the same, however. 

Exploring Chemistry with Electronic Structure Methods 

In the predicted transition structure, the hydrogen atom is weakly linked to both the carbon and oxygen atoms  

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When this initial guess is poor, you need a more sophisticated—albeit more expensive—means of generating the force constants. This is especially important for transition state optimizations. Gaussian provides a variety of alternate ways of generating them. Here are some of the most useful associated keywords consult the Gaussian User s Reference for a full description of their use ... 

Vinyl alcohol is a good system for discussing transition state optimizations. The (iptimizatior. to a transition state starting from the 9Sf form proceeds easily, making it a suitable introduction to the general topic. 

The transition state optimization (Opt=(TS,CakFC)) of the structure on the right converges in 12 steps. The UHF frequency calculation finds one imaginary frequency. Here is the associated normal mode ... 

An IRC calculation examines the reaction path leading down from a transition structure on a potential energy surface. Such a calculation starts at the saddle point and follows the path in both directions from the transition state, optimizing the geometry of the molecular system at each point along the path. In this way, an IRC calculation definitively connects two minima on the potential energy surface by a path which passes through the transition state between them. 

For our initial geometry for the transition structure, we ll detach one hydrogen from the carbon and increase the O-C-H bond angle. We specified the Opt=(TS,CalcFC) keyword in the route section, requesting an optimization to a transition state. The CalcFC option is used to compute the initial force constants, a technique which is generally helpful for transition state optimizations. We ve also included the Freq keyword so that a frequency calculation will automatically be run at the optimized geometry. 

A structural comparison of the calculated (B3LYP/6-311+G ) ts (transition state in the gas phase), ts-wc (transition state in the cluster of five extra water molecules), ts-CPCM (transition state within the CPCM-solvent model (B3LYP(CPCM)/6-311+G )) and ts-PCM (transition state optimized within the PCM-solvent model (B3LYP(PCM)/6-311+G )), shows no large differences (see Fig. 8), which is also valid for the precursor complexes (see Fig. 9). Modeling solvent effects shrinks in all cases the Be-0 bonds of the entering/leaving water molecules (159). 

Figure 5.21 Transition state optimized by ab initio DFT/B3LYP calculations for hydride-hydride exchange via the turnstile mechanism. (Reproduced with permission from ref. 32.)...

What is geometry optimization Why is this process for transition states (often called transition state optimization) more challenging than for minima ... 

Fig. 2 Geometries of the transition states optimized at the B3LYP/6-31+G(d) level for the first step of the hydrolysis of CH3COOCH3, (CH3)2CHCOOCH3, C6H5COOCH(CH3)2, the cocaine methyl-ester, and the cocaine benzoyl-ester [63], Internuclear distances are given in angstrom...

In the present study, the transition state structure for the ethylene insertion reaction 2a—>2b has been fully optimized by the standard algorithm of transition state optimization [13]. Most of the important geometric parameters are indicated in 2c. The Ni-C(ethyl) bond is elongated from 198A in the reactant 2a to 2.07A in 2c, and a partial C-C bond of 1.97A is... 

The geometry of the reactants, intermediates and products optimized at the B3LYP/6-311+G(3df, 2p) level is shown in Fig.l7, while those of the transition states optimized at the same level are displayed in Fig. 18. The singlet and triplet potential energy diagrams obtained at the G2M (CC2)//-... 

M.C. Milletti. Currently, he is a Graduate Student at Wayne State University under the direction of H.B. Schlegel. His research interests include the development of new methods for reaction path following, transition state optimization and the application of electronic structure theory to organometallic and inorganic chemistry. 

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