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.

Roaming-mediated isomerization in the photodissociation of nitrobenzene

Abstract: Roaming reactions comprise a new class of reaction in which a molecule undergoes frustrated dissociation to radicals, followed by an intramolecular abstraction reaction. Nitro compounds have long been known to dissociate to give NO as a major product. However, rates based upon isomerization via calculated tight transition states are implausibly slow, so the key dissociation pathway for this important class of molecules remains obscure. Here, we present an imaging study of the photodissociation of nitrobenzene with state-specific detection of the resulting NO products. We observe a bimodal translational energy distribution in which the slow products are formed with low NO rotational excitation, and the fast component is associated with high rotational excitation. High-level ab initio calculations identified a 'roaming-type' saddle point on the ground state. Branching ratio calculations then show that thermal dissociation of nitrobenzene is dominated by 'roaming-mediated isomerization' to phenyl nitrite, which subsequently decomposes to give C(6)H(5)O + NO.

Theoretical Study of Two-State Reactivity of Transition Metal Cations: The ``Difficult'' Case of Iron Ion Interacting with Water, Ammonia, and Methane

Abstract: The potential energy surfaces corresponding to the dehydrogenation reaction of H 2 O, NH 3 , and CH 4 molecules by Fe + ( 6 D, 4 F) cation have been investigated in the framework of the density functional theory in its B3LYP formulation and employing a new optimized basis set for iron. In all cases, the low-spin ion-dipole complex, which is the most stable species on the respective potential energy hypersurfaces, is initially formed. In the second step, a hydrogen shift process leads to the formation of the insertion products, which are more stable in a low-spin state. From these intermediates, three dissociation channels have been considered. All of the results have been compared with existing experimental and theoretical data. Results show that the three insertion pathways are significantly different, although spin crossings between high- and low-spin surfaces are observed in all cases. The topological analysis of the electron localization function has been used to characterize the nature of the bonds for all of the minima and transition states along the paths.

Transition-state structures for N-glycoside hydrolysis of AMP by acid and by AMP nucleosidase in the presence and absence of allosteric activator

Abstract: The mechanism of acid and enzymatic hydrolysis of the N-glycosidic bond of AMP has been investigated by fitting experimentally observed kinetic isotope effects [Parkin, D. W., & Schramm, V. L. (1987) Biochemistry (preceding paper in this issue)] to calculated kinetic isotope effects for proposed transition-state structures. The sensitivity of the transition-state calculations was tested by "arying the transition-state structure and comparing changes in the calculated kinetic isotope effects with the experimental values of the isotope effect measurements. The kinetic isotope effects for the acid-catalyzed hydrolysis of AMP are best explained by a transition state with considerable oxycarbonium character in the ribose ring, significant bonding remaining to the departing adenine ring, participation of a water nucleophile, and protonation of the adenine ring. A transition-state structure without preassociation of the water nucleophile cannot be eliminated by the data. Enzymatic hydrolysis of the N-glycosidic bond of AMP by AMP nucleosidase from Azotobacter vinelandii was analyzed in the absence and presence of MgATP, the allosteric activator that increases Vmax approximately 200-fold. The transition states for enzyme-catalyzed hydrolysis that best explain the kinetic isotope effects involve early SN1 transition states with significant bond order in the glycosidic bond and protonation of the adenine base. The enzyme enforces participation of an enzyme-bound water molecule, which has weak bonding to C1' in the transition state. Activation of AMP nucleosidase by MgATP causes the bond order of the glycosidic bond in the transition state to increase significantly. Hyperconjugation in the ribosyl group is altered by enzymatic stabilization of the oxycarbonium ion. This change is consistent with the interaction of an amino acid on the enzyme. Together, these changes stabilize a carboxonium-like transition-state complex that occurs earlier in the reaction pathway than in the absence of allosteric activator. In addition to the allosteric changes that alter transition-state structure, the presence of other inductive effects that are unobserved by kinetic isotope measurements is also likely to increase the catalytic rate.

Implications of Transition State Confinement within Small Voids for Acid Catalysis

Abstract: The catalytic diversity of microporous aluminosilicates reflects their unique ability to confine transition states within intracrystalline voids of molecular dimensions and the number (but not the strength) of the protons that act as Bronsted acids. First-order rate constants for CH3OH conversion to dimethyl ether (DME) reflect the energy of transition states relative to those for gaseous and H-bonded CH3OH molecules; on zeolites, these constants depend exponentially on n-hexane physisorption energies for different void size and shape and proton location, indicating that van der Waals stabilization of transition states causes their different reactivity, without concomitant effects of void structure or proton location on acid strength. The dispersive contribution to adsorption enthalpies of DME, a proxy in shape and size for relevant transition states, was calculated using density functional theory and Lennard-Jones interactions on FAU, SFH, BEA, MOR, MTW, MFI, and MTT zeolites and averaged over all proton...