Calculations , reaction pathways

In remembrance of the isolation of carbene 78 (Section m.C) it seems acceptable that 141, formed from 140 by a [l,2]-migration of the substituent, functions as a second intermediate. Carbene 141 can subsequently fragment into a nitrene 143 and carbon monoxide or open to an isocyanate 144, which alternatively may also be generated from an acid azide via acylnitrene 142. In comparison to calculated reaction pathways 75 78 and 82 81 (Schemes... 

An alternative means of calculating reaction pathways is employed in so-called global methods. These methods treat the entire path as a succession of points [71] which are found simultaneously. Methods of this type (for example the conjugate peak refinement algorithm [72], available in the TRAVEL module of CHARMM, which has the advantage of requiring only first derivatives of the energy) have been used to determine reaction paths in a number of proteins [4, 73]. 

Figure 8.5 Observed and calculated reaction pathways from aliphatic CgClg-species to hexachlorobenzene. Numbers are values (KJ/mol) calculated by B3LYP/6-31G(2d). (From H. Detert et al., to be published.)...

All the calculations were carried out with the Gaussian03 program,37 using the hybrid PBEO functional30,38 and the 6-31+G(d,p) basis set.39 Starting from the transition state, we have calculated reaction pathways for both static and dynamic approaches. On the one hand, we followed the minimum energy path between the transition state and... 

Because of the complexity of many reaction surfaces it may sometimes be necessary to calculate reaction pathways for several starting geometries and reaction coordinates a number of alternative transition state structures may thus be located. 

Figure 29. Modeling water assisted cleavage of a C-O-C ether bond calculated reaction pathways and energies of water hydrolysis of a dimethyl ether.

Figure 4. Calculated reaction pathways for the reaction of CH3NHNO2 + H products, based on BAC-MP4 heats of formation at 298 K for stable species and transition state structures. Energies are in kcal-mol ...

Fig. 9. Calculated reaction pathway for the reaction NH2(X) + NO( IT) N2(X) + H20 (X). flow reactor, flash photolysis, pulse radiolysis, IR MPD) are in good agreement thus, an...

Calculated reaction pathway for addition of acetaldehyde lithium enolate to formaldehyde. 

This is a question of reaction prediction. In fact, this is a deterministic system. If we knew the rules of chemistry completely, and understood chemical reactivity fully, we should be able to answer this question and to predict the outcome of a reaction. Thus, we might use quantum mechanical calculations for exploring the structure and energetics of various transition states in order to find out which reaction pathway is followed. This requires calculations of quite a high degree of sophistication. In addition, modeling the influence of solvents on... 

Example Researchers have used MX DO and. AMI setn i-etn pirical methods It) calculate possible reaction pathways for the interae-... 

The success of simple theoretical models in determining the properties of stable molecules may not carry over into reaction pathways. Therefore, ab initio calculations with larger basis sets may be more successful in locating transition structures than semi-empir-ical methods, or even methods using minimal or small basis sets. 

As the formation of betaines from amide-stabilized ylides is known to be reversible (in contrast with aryl- or semistabilized ylides, which can exhibit irreversible anti betaine formation see Section 1.2.1.3), the enantiodifferentiating step cannot be the C-C bond-forming step. B3LYP calculations of the individual steps along the reaction pathway have shown that in this instance ring-closure has the highest barrier and is most likely to be the enantiodifferentiating step of the reaction (Scheme 1.16) [25]. 

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