Topic
Transition state
About: Transition state is a research topic. Over the lifetime, 4978 publications have been published within this topic receiving 117965 citations. The topic is also known as: transition state of elementary reaction.
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TL;DR: The results show that a conventional Diels-Alder transition state conformation yields intermediates in all four investigated cases, but that these are too short-lived to be detected experimentally for the less activated reactants, and that introduction of a hydrogen bond donating catalyst favors a stepwise pathway even for less activated dienophiles.
Abstract: We have employed hybrid DFT and SCS-MP2 calculations at the SMD-PCM–6-311++G(2d,2p)//6-31+G(d) level to investigate the relationship between three possible channels for forming a Diels–Alder adduct from a highly nucleophilic diene and moderately to highly electrophilic dienophiles. We discuss geometries optimized using the B3LYP and M06-2X functionals with the 6-31+(d) basis set. The transition states and intermediates are characterized on the basis of geometric and electronic properties, and we also address the possibility of predicting detectability of a zwitterionic intermediate based on its relative stability. Our results show that a conventional Diels–Alder transition state conformation yields intermediates in all four investigated cases, but that these are too short-lived to be detected experimentally for the less activated reactants. The stepwise trans pathway, beginning with a conjugate addition-like transition state, becomes increasingly competitive with more activated reactants and is indeed fav...
53 citations
TL;DR: This investigation of the reaction in Escherichia coli and water involves QM/MM procedures to identify transition state structures in the water, enzyme, and gas phase followed by B3LYP/6-31+G* single-point computations which allow the determination of activation energies in water and in the E. coli enzyme.
Abstract: In previous research presentations, we have described the important features of the chorismate → prephenate reaction using molecular dynamics (MD) and thermodynamic integration studies. This investigation of the reaction in Escherichia coli and water involves QM/MM procedures (SCCDFTB/MM two-dimensional reaction coordinates to identify transition state structures in the water, enzyme, and gas phase followed by B3LYP/6-31+G* single-point computations which allow the determination of activation energies in water and in the E. coli enzyme). Computed activation energies of 11.3 kcal/mol in enzyme and 20.3 kcal/mol in water may be compared to the experimental values of 12.7 and 20.7 kcal/mol, respectively. The transition state structures in the gas phase, water, and enzyme are much the same. The transition states are characteristic of a concerted pericyclic rearrangement. The very small differences in the partial charges of O13 in NAC and TS support only a small preferential (10%) electrostatic stabilization o...
53 citations
TL;DR: A palladium catalyzed regioselective borylative ring-opening reaction of 2-arylaziridines to give β-amino-β-arylethylborates was developed.
Abstract: A palladium catalyzed regioselective borylative ring opening reaction of 2-arylaziridines to give β-amino-β-arylethylborates was developed. The reaction reported herein represents the first example of ring-opening borylation of non-vinylic aziridines and direct borylative C(sp3)-N bond cleavage of neutral organic substrates. NMR studies and density functional theory (DFT) calculations suggested that the active intermediate for the reaction is a PdL2 complex [L = P(t-Bu)2Me]. The multi-component artificial force-induced reaction method (MC-AFIR) located the transition states for the regioselectivity-determining aziridine ring opening that proceeds in an SN2 fashion, and explained the selectivity of the reaction. The full catalytic cycle consists of a selectivity-determining aziridine ring opening (oxidative addition), a proton transfer, phosphine ligand dissociation from the catalyst, boron-boron bond cleavage, and reductive elimination. Water is important to the drive the transmetalation step. The calculated overall mechanism and selectivity are consistent with the experimental results.
53 citations
TL;DR: In this article, the authors performed MO calculations for potential energy surfaces of unimolecular reactions of vinyl fluoride and discussed the mechanism of energy partitioning in the reaction product, and proposed an experiment that would determine which of the αα and αβ processes is dominant in the HF elimination reaction.
Abstract: Ab inito MO calculations have been performed for potential energy surfaces of unimolecular reactions of vinyl fluoride. The geometries and energies of the reactant, transition states, reaction intermediates, and product have been determined for ten elementary reactions both on the triplet and the singlet surfaces. The mechanism of the unimolecular dissociation reaction has been discussed in terms of the energy diagram obtained by the present calculation and compared with the interpretation derived from photosensitizer and IR‐multiphoton experiments. We have also calculated the intrinsic reaction coordinate (IRC) for the αα‐ and αβ‐HF direct elimination reactions and discussed the mechanism of energy partitioning in the reaction product. We propose an experiment that would determine which of the αα and αβ processes is dominant in the HF elimination reaction.
53 citations
TL;DR: The trend in reactivity of the thermal cycloisomerization reactions of 1,3-hexadien-5-ynes, A=B-C=D-E≡F, were explored and analyzed by using density functional theory at the M06-2X/def2-TZVPP level to show the aromaticity of the corresponding cyclic transition states and the computed activation barriers.
Abstract: The trend in reactivity of the thermal cycloisomerization reactions of 1,3-hexadien-5-ynes, A=B-C=D-E≡F, were explored and analyzed by using density functional theory at the M06-2X/def2-TZVPP level These reactions proceed through formally aromatic transition states to form a bent-allene intermediate with relatively high activation barriers Activation-strain analyses show that the major factor controlling this Hopf cyclization is the geometrical strain energy associated with the rotation of the terminal [A] group This rotation is necessary for achieving a favorable HOMO-LUMO overlap with the yne-moiety [F] associated with the formation of the new A-F single bond In addition, the relationship between the aromaticity of the corresponding cyclic transition states (all six-membered rings) and the computed activation barriers were analyzed The calculations also indicate that the aromatization of the bent-allene structures takes place through two consecutive 1,2-hydrogen shifts, the second one exhibiting negligible energy barriers Twisted! The barrier of Hopf cyclizations is primarily controlled by the activation strain (see figure, red) associated with twisting the terminal double bond, needed to achieve optimal HOMO-LUMO overlap and single-bond formation between ene and yne terminus (green) Substitution of a heteroatom, for example, NH, for the terminal CH
53 citations