Geometry of the transition state

The geometry of the transition state is always unknown, so that r can only be guessed. This is probably the least serious of the problems. 

Dimethylborane+propene Cl depicts the transition state for addition of dimethylborane onto the terminal alkene carbon of propene. Examine and describe the vibration with the imaginary frequency. Which bonds stretch and compress the most What simultaneous changes in bonding are implied by these motions Simultaneously display the highest-occupied molecular orbital (HOMO) of propene and the lowest-unoccupied molecular orbital (LUMO) of dimethylborane. Is the overall geometry of the transition state consistent with constructive overlap between the two Explain. 

Obtain the energies of propene, dimethylborane, and 1-propyldimethyl borane, and calculate AH n for dimethylborane addition. Is this reaction exothermic or endothermic Use this result and the Hammond Postulate to predict whether the transition state will be more reactant like or more product like . Compare the geometry of the transition state to that of the reactants and products. Does the Hammond Postulate correctly anticipate the structure of the transition state Explain. 

Next, examine the highest-occupied and lowest-unoccupied molecular orbitals (HOMO and LUMO) of dichlorocarbene. Were the reaction a nucleophilic addition , how would you expect CCI2 to approach propene Were the reaction an electrophilic addition , how would you expect CCI2 to approach propene Which inteqDretation is more consistent with the geometry of the transition state ... 

Display the HOMO for cis-l,3,5-hexatriene. Which motion (conrotatory or disrotatory) insures bonding overlap Examine the geometry of the transition state for ring closure (hexatriene to cyclohexadiene). Is it consistent with the anticipated (conrotatory or disrotatory) motion of the terminal methylenes ... 

The stereochemical outcome of the reaction is determined by the geometry of the transition state for the Claisen rearrangement a chairlike conformation is preferred,and it proceeds strictly by an intramolecular pathway. It is therefore possible to predict the stereochemical course of the reaction, and thus the configuration of the stereogenic centers to be generated. This potential can be used for the planning of stereoselective syntheses e.g the synthesis of natural products. 

The frontier orbital interaction can be free from the symmetry restriction. A pair of the reaction sites is close to each other while the other pair of the sites is far from each other (Scheme 25b). This is the geometry of the transition state leading to diradical intermediates. 

Changes in AH are paralleled by changes of AS in such a direction that the resulting effect on reactivity is less than it would be if controlled by either AH or AS alone (compensation effect). Its cause is seen in steric or solvent effects (13, 116), affecting simultaneously the geometry of the transition state and the force constants (13, 37, 116). 

Geometry of the Transition State of Radical Abstraction Reaction... 

GEOMETRY OF THE TRANSITION STATE OF RADICAL ABSTRACTION REACTION... 

When the peroxyl radical reacts with the C—H bond of a polar molecule, for example alcohol, the polar interaction of the hydroxyl group influences the activation energy (see earlier) and the geometry of the transition state [34]. The parameters of the transition state of reaction EtOO + MeCH2OH calculated by the DFT method are the following [35] ... 

The polar interaction changes the geometry of the transition state of the reaction R02 + RH. Atoms C, H, O of the reaction center O H C of this reaction are in a straight line for the reaction of the peroxyl radical with a hydrocarbon. The reaction center O H C has an angular geometry in the reaction of the polar peroxyl radical with a polar molecule of the ketone. The interatomic distances rc H and i o n and angles peroxyl radical reactions with ketones calculated by the IPM method [79,80] are given in Table 8.15. 

Inspection of the optimized geometries of the transition states for cyclization suggested that steric hindrance is important in cases where N cyclizes toward F.87 This steric explanation of the rates and regiochemistry of cyclization was... 

In all these reactions only the exo approach of the dipole (614) has been observed. The high steric requirements of the tert-butoxy groups precluded the endo geometry of the transition state (Fig. 2.41). 

Given the definition of the geometry of the transition states in TST as the highest energy point in the minimum energy pathway from reactants to products, the formal definition of MEP is as follows. The MEP is, in one direction, the path of steepest descents from the transition state to reactants while, in the other direction, it is the path of steepest descents from transition state to products. For reasons which will not be discussed here, the formal definition of MEP includes the statement that the pathway is expressed in mass scaled Cartesian coordinates of the position of the atoms (introduced in Chapter 3, e.g. x is replaced by x = ). This simplifies... 

The sulfoxide-sulfimide interconversion is of considerable interest in connection with the possible geometries of the transition states or intermediates that might arise from nucleophilic substitution at... 

Apparently the geometry of the transition state for adsorption is approximately that of a ir-complexed olefin in that its structure seems to be only slightly distorted from that of the isolated alkene. However, this does not necessarily mean that the adsorbed state which is formed in the elementary reaction to which the stereochemistry refers is a tt complex, because the same geometry also represents a stage in the progression of olefin to the eclipsed 1,2-diadsorbed alkane. Hopefully other experi-... 

Similarly, from Fig. 15, the activation energy for hydrogen abstraction by H can be determined to be about 10-12 kcal/mol. This value again compares very well with the available experimental value of 12.5 kcal/mol (Kerr and Drew, 1987), As is also evident from Figs. 14 and 15, both reactions proceed via tight and early transition states, i.e., the interatomic distances remain within bonding distances and the geometry of the transition state exhibits reactantlike features. 

A molecule designed to mimic the properties, structure, and/or geometries of the transition state of a particular reaction. Such compounds are often potent inhibitors of enzymes. See Molecular Similarity... 

Eigure 10.17 Optimized geometry of the transition state TS1 calculated for proton transfer via dihydrogen-bonded complex W-H- -H-Cl. The distances are given in angstroms. (Reproduced with permission from ref. 9.)... 

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