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.

Theoretical Study on the Mechanism of the Dissociation of Benzene. The C5H3 + CH3 Product Channel

Abstract: Potential energy surfaces for the various mechanisms of the dissociation of C6H6 producing different isomers of the C5H3 radical have been investigated using the ab initio modified Gaussian-2 (G2M) method. The most stable structures of C5H3, H2CCCCCH (II1) and HCCC(H)CCH (II2), can be formed from benzene and fulvene through the pathways involving a 1,2-H shift in the carbon ring and the ring opening followed by the series of hydrogen shifts in the open chain structures of C6H6. The reaction is completed by the elimination of CH3 from C5H3−CH3. All the transition states along the reaction pathways are found to lie below ∼143 kcal/mol relative to benzene, while the heats of the reactions, benzene → H2CCCCCH + CH3 and benzene → HCCC(H)CCH + CH3, are calculated to be about 150 kcal/mol. On this basis, the C5H3 + CH3 product channel of the photofragmentation of benzene at 248 nm is expected to be a two-photon process. In total, 25 different isomers of C5H3 have been calculated. The most stable, II1 and II2, ar...

Metal-assisted coupling of oximes and nitriles: a synthetic, structural and theoretical study

Abstract: Chlorination of [Ph3PCH2Ph][PtCl3(EtCN)], obtained from the reaction of [PtCl2(EtCN)2] with [Ph3PCH2Ph]Cl, formed the platinum(IV) complex [Ph3PCH2Ph][PtCl5(EtCN)] which, at ambient temperature and both in solution and in the solid phase, hydrolyses to the ammonia compound [Ph3PCH2Ph][PtCl5(NH3)] and undergoes nucleophilic addition by ketoximes or amidoxime HONCR1R2 [R1R2 = Me2, C4H8, C5H10, C9H16, C9H18 or Ph(NH2)] to give the corresponding iminoacylated product [Ph3PCH2Ph][PtCl5{HNC(Et)ONCR1R2}]. All compounds were characterized by elemental analyses, FAB mass spectrometry, IR and 1H, 13C-{1H}, 31P-{1H} and 195Pt NMR spectroscopies. A crystal structure determination of [Ph3PCH2Ph][PtCl5{NHC(Et)ONC(C9H16)}] disclosed amidine one-end rather than the N,N-bidentate co-ordination mode of the N-donor ligand. The iminoacylation by oximes was investigated by ab initio methods (at RHF level using quasi-relativistic pseudopotentials for platinum) for [PtCl5(NCMe)]− which were also applied to the related neutral platinum(IV) [PtCl4(NCMe)2] and platinum(II) [PtCl2(NCMe)2] complexes. The calculations included geometry optimization of the starting and final complexes, location of possible transition states for the reaction discussed and intrinsic reaction coordinate calculations for one reaction. The results obtained provided an interpretation, on the basis of kinetic (activation energies) and thermodynamic (reaction energies) effects, for the order of reactivity observed [neutral PtIV > anionic PtIV > neutral PtII] and indicated that a mechanism based on nucleophilic addition of the protic nucleophile (undeprotonated oxime), to form a transition state with a four-membered NCOH ring, is energetically favoured relative to the alternative one involving prior deprotonation of the oxime, unless base-catalysed conditions are operating.

Rate constant of the HONO + HONO → H2O + NO + NO2 reaction from ab initio MO and TST calculations

Abstract: Kinetics and mechanism for the bimolecular decomposition of HONO have been studied by ab initio molecular orbital (G2M) and transition-state theory calculations. The reaction can take place by the interaction of a cis and a trans isomer or two cis or two trans isomers, via four-, five-, and six-member ring transition states, with decreasing reaction barriers as the size of the ring increases. The lowest energy path with a 13.7 kcal/mol barrier was found to occur by the six-member ring TS1 formed by the reaction of cis- and trans-HONO. A similar six-member ring TS (TS2) formed by two cis isomers has a barrier height of 15.1 kcal/mol, which is very close to the 5-ring TS formed by two trans isomers, 15.7 kcal/mol. The total rate constant computed with the ab initio MO results, including the three reaction channels mentioned above and an additional channel involving a five-member ring TS formed by a cis- and a trans isomer with a 17.7 kcal/mol barrier, can be represented by the three-parameter expression for...

Structure and Gas-Phase Thermochemistry of a Pd/Cu Complex: Studies on a Model for Transmetalation Transition States

Abstract: A heterobimetallic Pd(II)/Cu(I) complex was prepared and characterized by X-ray diffraction analysis. The crystal structure shows a remarkably short Pd–Cu bond and a trigonal ipso carbon atom. The Pd–Cu interaction, as determined by energy-resolved collision-induced dissociation cross-section experiments, models the net stabilizing energy of the Pd–Cu interaction in the transition state of the transmetalation step in Pd/Cu-catalyzed cross-coupling reactions. The bonding situation in the bimetallic dinuclear complex has been studied by atoms-in-molecules analysis.

Energetics and dynamics of proton transfer reactions along short water wires.

Abstract: Proton transfer (pT) reactions in biochemical processes are often mediated by chains of hydrogen-bonded water molecules. We use hybrid density functional calculations to study pT along quasi one-dimensional water arrays that connect an imidazolium–imidazole proton donor–acceptor pair. We characterize the structures of intermediates and transition states, the energetics, and the dynamics of the pT reactions, including vibrational contributions to kinetic isotope effects. In molecular dynamics simulations of pT transition paths, we find that for short water chains with four water molecules, the pT reactions are semi-concerted. The formation of a high-energy hydronium intermediate next to the proton-donating group is avoided by a simultaneous transfer of a proton from the donor to the first water molecule, and from the first water molecule into the water chain. Lowering the dielectric constant of the environment and increasing the water chain length both reduce the barrier for pT. We study the effect of the driving force on the energetics of the pT reaction by changing the proton affinity of the donor and acceptor groups through halogen and methyl substitutions. We find that the barrier of the pT reaction depends linearly on the proton affinity of the donor but is nearly independent of the proton affinity of the acceptor, corresponding to Bronsted slopes of one and zero, respectively.