Showing papers on "Transition state published in 1996"

Lewis acid catalysis of a Diels-Alder reaction in water

Abstract: Here we report the first detailed study of a Diels-Alder (DA) reaction that is catalyzed by Lewis acids in water. The effect of Co2+, Ni2+, Cu2+ and Zn2+ ions as Lewis acid catalysts on the rate and endo-exo selectivity of the DA reaction between the bidentate dienophiles 3-phenyl-1-(2-pyridyl)-2-propen-1-ones (1a-e) and cyclo-pentadiene (2) in water has been studied. Relative to the uncatalyzed reaction in acetonitrile, catalysis by 0.010 M CU(NO3)(2) in water accelerates the reaction by a factor of 79 300. The kinetics of the catalyzed reaction were analyzed in terms of equilibrium constants for complexation of the Lewis acid with 1a-e and rate constants for the reaction of the resulting complexes with 2. The rate enhancement imposed upon the uncatalyzed DA reaction of substrates 1 with 2 by water is much more pronounced than that for the catalyzed reaction. The increase of the endo-exo selectivity induced by water in the uncatalyzed process is completely absent for the Lewis acid catalyzed reaction. The modest solvent and substituent effects observed for the catalyzed reaction indicate that the change in charge separation during the activation process is not larger than the corresponding change For the uncatalyzed reaction.

Activation of C-H and C-C Bonds by an Acidic Zeolite: A Density Functional Study

Abstract: Density functional theory is used to determine transition states and the corresponding energy barriers of the reactions related to C−H bond activation of hydrogen exchange and dehydrogenation of ethane catalyzed by a protonated zeolite as well as hydride transfer between methanol and a methoxide (CH3-zeolite) species. Additionally the C−C bond activation involved in the acid catalyzed cracking reaction of ethane was investigated. The computed activation barriers are 118 for hydrogen exchange, 202 for hydride transfer, 292 for cracking and finally 297 for dehydrogenation, all in kilojoules per mole. For the cracking reaction, two different transition states with the same activation barrier have been obtained, dependent on the approach of the ethane molecule to the zeolite cluster. A study of the relation between acidity and the structure of the zeolite shows that the transition state for the hydrogen exchange reaction is rather covalent and its geometry resembles the well-known carbonium ion, while the oth...

A perspective on biological catalysis.

Abstract: We have analysed enzyme catalysis through a re-examination of the reaction coordinate. The ground state of the enzyme-substrate complex is shown to be related to the transition state through the mean force acting along the reaction path; as such, catalytic strategies cannot be resolved into ground state destabilization versus transition state stabilization. We compare the role of active-site residues in the chemical step with the analogous role played by solvent molecules in the environment of the noncatalysed reaction. We conclude that enzyme catalysis is significantly enhanced by the ability of the enzyme to preorganize the reaction environment. This complementation of the enzyme to the substrate's transition state geometry acts to eliminate the slow components of solvent reorganization required for reactions in aqueous solution. Dramatically strong binding of the transition state geometry is not required.

Structure of the Transition States and Intermediates Formed in the Water-Exchange of Metal Hexaaqua Ions of the First Transition Series

Abstract: The structures of the transition states and intermediates formed in the water-exchange of hexaaqua complexes of the first row transition elements have been computed with ab initio methods at the Hartree−Fock or CAS-SCF level. As an approximation, water molecules in the second coordination sphere except one, bulk water, and anions have been neglected. For each of the three types of activation, namely associative, concerted, and dissociative mechanism, a representative transition metal complex has been studied, viz. Ti(OH2)63+, V(OH2)62+, and Ni(OH2)62+. Each type of mechanism proceeds via a characteristic transition state. For the A and D mechanisms, respectively, hepta- or pentacoordinated intermediates are formed, and their lifetimes were estimated based on the energy difference between that of the transition state and the corresponding intermediate. The computed activation energies are in agreement with the experimental values and are independent of the mechanism or the charge on the metal center. The b...

Quantumchemical study of the isobutane cracking on zeolites

Abstract: The ab initio quantumchemical investigation of the elementary steps of the catalytic isobutane cracking is presented. A reasonable agreement between experimental and theoretical activation energies is found. The obtained results demonstrate that the adsorbed carbenium and carbonium ions represent not the really existing reaction intermediates, but the high-energy transition states of the corresponding elementary reactions. This results in much higher activation energies than for the similar reactions in homogeneous super-acid solutions.

Theory Does Not Support an Osmaoxetane Intermediate in the Osmium-Catalyzed Dihydroxylation of Olefins

Abstract: The origin of enantioselectivity in the dihydroxylation of H2C=CH(Ph) catalyzed by (DHQDJzPYDZOsO4 ((DHQD)iPYDZ = bis(dihydroquinidine)-3,6-pyridazine) is analyzed theoretically by means of hybrid QM/MM calculations with the IMOMM(Becke3LYP:MM3) method. Twelve different possible reaction paths are defined from the three possible regions of entry of the substrate and its four possible orientations and characterized through their respective transition states. The transition state with the lowest energy leads to the R product, in agreement with experimental results. The decomposition of the interaction energy between catalyst and substrate shows how the selectivity is essentially governed by stacking interactions between aromatic rings, with a leading role for the face-to-face interaction between the substrate and one of the quinoline rings of the catalyst.

Femtosecond real-time probing of reactions. XXI. Direct observation of transition-state dynamics and structure in charge-transfer reactions

Abstract: This paper in the series gives our full account of the preliminary results reported in a communication [Cheng, Zhong, and Zewail, J. Chem. Phys. 103, 5153 (1995)] on real‐time femtosecond (fs) studies of the transition state of charge‐transfer (CT)reactions, generally described as harpooning reactions. Here, in a series of experimental studies in a molecular beam, and with the help of molecular dynamics, we elucidate the microscopic elementary dynamics and the structure of the transition states for the isolated, bimolecular reaction of benzenes (electron donor) with iodine (electron acceptor). The transition state is directly reached by fs excitation into the CT state of the complex Bz⋅I_2, and the dynamics is followed by monitoring the product build up or the initial transition‐state decay.

Characterization of Transition States in Dichloro (1,4,7-Triazacyclononane) Copper (II)-Catalyzed Activated Phosphate Diester Hydrolysis

Abstract: The reaction mechanism for Cu[9]aneN3Cl2-catalyzed hydrolysis of ethyl 4-nitrophenyl phosphate was probed using kinetic isotope effects and isotope exchange experiments. The solvent deuterium isotope effect (Dk = 1.14), combined with the absence of 18O incorporation into 4-nitrophenol, suggests that hydrolysis proceeds through intramolecular attack of the metal-coordinated hydroxide at the phosphorus center. The secondary 15N isotope effect (15k = 1.0013 ± 0.0002) implies that loss of the leaving group occurs at the rate-limiting step with approximately 50% bond cleavage in the transition state. This study is one of the first applications of the secondary 15N isotope effect to simple metal-promoted hydrolysis reactions, and the result is consistent with concerted bond formation and cleavage. A mechanism consistent with the isotope studies is presented.

Structural consequences of the addition of lithium halides in enolization and aldol reactions

Abstract: Aggregates consisting of lithium halides with either lithium amides or lithium enolates have been characterized by X-ray crystallography. Two structural types, solvated heterodimers and heterotrimers, have been found for halide/amide combinations. Two compounds containing both lithium halide and lithium enolate have also been identified as heterodimers. Using this information, we propose a reaction sequence for enolization and subsequent aldol addition reaction involving halide-containing aggregates. An ab initio and PM3 theoretical study of model systems shows that solvated heterodimers between LiBr and either LiNH2 or LiOC(H)=CH2 are favored over the respective homodimers. Calculations reveal a stable eight-membered ring transition state for the enolization step between LiCl·LiNH2 and acetaldehyde. Two independent transition states, a 4,8 and a 4,4,6 ring system, were calculated for the model reaction between the heterotrimer [(LiNH2)2·LiCl] and acetaldehyde. Dissociation of donor solvent was computed t...

Hydrogen Transfer in 7-Azaindole

Abstract: Ab initio calculations are reported on 7-azaindole and the 1:1 complexes between 7-azaindole and water and methanol. Geometry optimizations using restricted Hartree−Fock wavefunctions and a double ζ plus polarization basis set were performed on the tautomeric minima and the transition states connecting these minima. Energetics were predicted using second-order perturbation theory. The ground state activation energy for tautomerization is predicted to decrease from nearly 60 kcal/mol in 7-azaindole to about 20 kcal/mol for the 1:1 complexes. Vertical excitation of the 1:1 complex with water, qualitatively estimated using singly excited configuration interaction, is predicted to reverse the order of stability of the two tautomers.

The Effect of Microsolvation on E2 and SN2 Reactions: Theoretical Study of the Model System F− + C2H5F + nHF

Abstract: The model reaction system F− +C2H5F + nHF (n = 0–4) has been investigated by use of a density‐functional method, in order to achieve a qualitative understanding of the effect of solvation on the E2 and SN2 reactions. Two characteristic effects already occur upon monosolvation: a) the activation energies of the E2 and SN2 pathways increase significantly and even become positive, because reactants are more strongly solvated than transition states; b) the SN2 transition state is stabilized much more and becomes lower in energy than the anti‐E2 transition state. This agrees with general experience from gas‐ and condensed‐phase experiments. The solvation is analyzed from two complementary viewpoints: a) as an interaction between solvent molecules and the F−/C2H5F reaction system; b) as an interaction between the [F−, nHF] solvated base and the C2H5F substrate. The extent to which condensed‐phase characteristics can be modeled by this microsolvation approach is discussed.

Molecular Electrostatic Potential Analysis for Enzymatic Substrates, Competitive Inhibitors, and Transition-State Inhibitors

Abstract: Recent advances in the application of kinetic isotope effects to enzyme-catalyzed reactions have provided reliable information for enzymatic transition state structures. A method is presented for quantifying the similarity of substrates and inhibitors with their enzyme-stabilized transition states. On the basis of transition-state stabilization theory for enzymatic reactions, molecules most similar to the transition state structure bind with greatest affinity. Molecular similarity measures are applied to compare substrates, competitive inhibitors, and transition state inhibitors with the transition state structures stabilized by the enzymes AMP deaminase, adenosine deaminase, and AMP nucleosidase. (R)- and (S)-Coformycin 5‘-phosphate are inhibitors for AMP deaminase, with the R-species superior to its enantiomer. Formycin 5‘-phosphate 4-aminopyrazolo[3,4-d]pyrimidine-1-ribonucleotide, and tubercidin 5‘-phosphate inhibit AMP nucleosidase. The transition state for adenosine deaminase is analogous to that fo...

Bimolecular reactions observed by femtosecond detachment to aligned transition states: Inelastic and reactive dynamics

Abstract: With fs radical detachment and kinetic energy‐resolved time‐of‐flight (KETOF) mass spectrometry, we are able to study the transition state dynamics of the bimolecular reaction CH3I+I, inelastic and reactive channels; the collision complex is coherently formed (1.4 ps) and is long lived (1.7 ps). We also report studies of the dynamics of I2 formation. Direct clocking of the CH3I dissociation, hitherto unobserved, gives 150 fs for the C–I bond breakage time and 0.8 A for the repulsion length scale.

Reactivity of N-Phenyl-1-Aza-2-Cyano-1,3-Butadienes in the Diels−Alder Reaction

Abstract: It is found that N-phenyl-2-cyano-1-azadiene 4, prepared via a two-step, one-pot, sequence from acrylanilide, undergoes efficient [4 + 2] cycloaddition with a complete range of electron rich, electron poor, and neutral dienophiles under remarkably mild thermal conditions (90-120 degrees C for 20-48 h). Regiospecific formation of the alpha-cycloadduct wherein the dienophile substituent is alpha to nitrogen is observed for vinyl ethers and styrene, whereas the Diels-Alder reactions with methyl acrylate and methyl vinyl ketone (MVK) produce alpha/beta mixtures in which the alpha-cycloadduct is the major regioisomer (approximately 4-5:1). An essentially identical reaction pattern was observed in the Diels-Alder reaction of N-(p-methoxyphenyl)-2-cyano-1-azadiene 18 and the 4-methyl-substituted azadiene 27. For compound 19 derived from cycloaddition of 18 with ethyl vinyl ether, facile conversion to the dihydropyridine 21 through loss of EtOH on brief acid treatment was also noted. The 2,4-cis-disubstitution pattern confirmed by X-ray diffraction for the major cycloadduct 29 isolated from the reaction of 27 with styrene provides evidence for the endo mode of cycloaddition in the Diels-Alder reaction of N-phenyl(aryl)-2-cyano-1-azadienes. Calculation of the frontier orbital energies and coefficients, as well as the transition state geometries for the [4 + 2] cycloaddition of N-phenyl-2-cyano-1-azadiene 4 with methyl vinyl ether, styrene, and MVK were carried out at the RHF AM1 level (MOPAC, Version 5.0). The FMO treatment indicates that the reaction of 4 with methyl vinyl ether occurs under LUMO(diene) control, whereas in contrast, the corresponding cycloaddition with MVK occurs preferably under HOMO(diene) control. A high degree of asynchronicity is observed in the calculated transition states for reaction of 4 with the three representative dienophiles. In all cases the transition states leading to the alpha-cycloadducts are lower in energy than those giving the beta-products. However, at the AM1 level the exo cycloaddition mode is found to be the preferred, this result contrasting with experimental results for azadiene 27.

Ligand-Induced Selectivity in the Rhodium(II)-Catalyzed Reactions of α-Diazo Carbonyl Compounds†

Abstract: 3-Allyl-2,5-diazopentanedione and 3-butenyl-2,5-diazopentanedione were allowed to react with a trace amount of a rhodium(II) catalyst in methylene chloride at room temperature. The major products isolated correspond to the internal trapping of a carbonyl ylide as well as intramolecular cyclopropanation. Changing the catalyst from Rh2(OAc)4 to Rh2(cap)4 to Rh2(tfa)4 caused a significant alteration in product distribution. A rather unusual and unexpected regiochemical crossover in the cycloaddition occurred when Rh2(tfa)4 was used and is most likely due to complexation of the metal with the dipole. A computational approach to rationalize the observed product distribution was carried out. The thermodynamic stabilities of cycloaddition transition states were approximated from the computationally derived strain energies of ground state molecules using traditional force-field techniques. Globally minimized ground state energies were obtained for all possible cycloaddition products, and final strain energies wer...

Theoretical study on pyrolysis and sensitivity of energetic compounds. (2) Nitro derivatives of benzene

Abstract: The UHF-SCF-AM1 method has been employed to study the pyrolysis mechanism and impact sensitivity of nitro derivatives of benzene. Potential energy curves, transition states and activation energies of seven pyrolysis initiation reactions (homolysis of CNO2 bond into radicals) have been obtained here first. The molecular geometries of reactants, transition states and products of the seven reactions were fully optimized. It is found that there is a good linear relationship between the activation energies and bond orders of the weakest bond CNO2 in the molecule of each reactant. The linear correlation coefficient is 0.996. This result gives “the principle of the smallest bond order” powerful support and shows that the activation energy can also be used as a dynamic criterion of impact sensitivity.

Energetics and Dynamics for NO and CO Dissociation on Cu(100) and Cu(111)

Abstract: The dissociation of NO and CO has been studied on cluster models representing the copper(100) and -(111) single-crystal faces using density functional quantum calculations. For each surface, several possible reaction paths are proposed, for which we fully optimized the reactant, product, and transition states at the local density level (LDA). Nonlocal density functional calculations (NLDA) were then performed on these optimized geometries. The clusters we used, varying in size between 13 and 31 atoms, produced dissociation barriers and energies that were reasonably well converged with cluster size. Classical transition-state theory was used to calculate the rates of dissociation and recombination on the basis of computed frequencies of the predicted transition state and the reactant and product states. The transition states for NO and CO dissociation on all surfaces can be described as “tight” transition states corresponding to preexponentials for dissociation in the range 1010−1013 s-1. The dissociation ...

Hydride Exchange Processes in the Coordination Sphere of Transition Metal Complexes: The OsH3(BH4)(PR3)2 System

Abstract: The problem of intramolecular hydrogen atom exchange in the OsH3(BH4)(PR3)2 system is examined from both theoretical and experimental points of view, through ab initio MO calculations on the OsH3(BH4)(PH3)2 system at the MP2, MP4, and CCSD(T) computational levels and variable-temperature 1H NMR studies on the OsH3(BH4)(PiPr3)2 complex. Three different exchange processes are fully characterized from a theoretical point of view through location of intermediates and transition states. Experimental results supporting the existence of these three different exchange processes and providing definitely its intramolecular nature are also presented.

Modelling pka of carboxylic acids and chlorinated phenols

Abstract: Estimation of aqueous pKa through quantum chemical gas-phase and solution-phase calculations is investigated for 16 carboxylic acids and 15 phenols with experimental data taken from literature. Parameters based on the AM1 hamiltonian include enthalpy and free energy differences between the ground state of the ionizable compounds and their anion counterparts as well as respective transition states and intermediates along the reaction path of the aqueous proton transfer. With carboxylic acids, additional ab initio calculations are performed to evaluate the semiempirical level of theory. Aqueous solvation is modelled in three different ways: Application of continuum-solvation methods AM1-COSMO and AM1-SM2, microsolvation of the solutes through formation of clusters with three water molecules, and combination of both approaches to include both bulk water polarization and solute-solvent coupling effects. Regression equations with r values up to 0.93 for carboxylic acids and 0.96 for phenols suggest, that continuum-solvation models can be recommended to estimate aqueous pKa through electronic structure calculations.

Quantum scattering calculations on the NH3+OH→NH2+H2O reaction

Abstract: Quantum scattering calculations on the NH3+OH→NH2+H2O reaction have been performed at energies up to 0.8 eV. The rotating bond approximation is used, treating NH2 as a pseudoatom. The OH rotation and a reactive N–H stretch of NH3 are treated explicitly as well as the bending motion and one OH local stretch vibration of H2O. A reduced dimensionality potential energy surface is developed. It has accurate reactant and product rovibrational energy levels for the modes explicitly treated in the scattering calculations and incorporates the zero point energy of the other modes. Quantized transition states gating the flux are found and mode selectivity is observed. Reactants in their ground rovibrational states produce mainly ground state H2O and vibrationally excited NH3 produces mainly vibrationally excited H2O. Rate constants are obtained using an adiabatic approach to account for all degrees of freedom not explicitly treated in the scattering calculations. Tunneling makes a dominant contribution to the rate constants, which are in reasonable agreement with previous theoretical and experimental work.

Theoretical Study on the Mechanism of the Superacid-Catalyzed Unimolecular Isomerization of n-Butane and 1-Butene

Abstract: Owing to the practical interest of the acid-catalyzed isomerization of n-butane and 1-butene, the mechanism of the isomerization and scrambling reactions of the n-butyl cation has been studied theoretically using ab initio methods which include electron correlation and extended basis sets. It has been found that the protonated cyclopropane ring does not appear as a common intermediate for carbon scrambling and branching isomerization of the n-butyl cation, since it is a transition state and not a minimum on the potential energy surface. The transition states for both reactions have been determined and the activation energies calculated. These values are in very good agreement with those obtained experimentally.

Ab Initio Study on the Substituent Effect in the Transition State of Keto−Enol Tautomerism of Acetyl Derivatives

Abstract: Ab initio molecular orbital calculations have been performed on the intramolecular tautomerism of some acetyl derivatives, CH3COX (X = H, BH2, CH3, NH2, OH, F, Cl, CN, NC). All stationary points and the keto, enol, and transition structures were optimized at the HF/6-31G* and MP2(full)/6-31G* levels of theory and confirmed by frequency calculations. Single-point calculations at MP4(FC)/6-311++G**//MP2(Full)/6-31G* were also carried out for all stationary points. The intrinsic reaction coordinate (IRC) for the tautomeric processes were traced to connect the transition structures and the corresponding substituted tautomeric pairs. The natural bond orbital (NBO) analyses on the transition states show that the interactions of the lone pair electrons on the oxygen atom and the σ* C−H bonds have a significant effect on their stabilities, which consequently affects the activation energies of the tautomeric processes. The energy barriers calculated at various levels of theory are reported for each tautomeric inte...

Theoretical Study of the Deprotonation of Nitriles, RCH2CN: Ab Initio and PM3 Calculations of Intermediate Aggregates and Transition States

Abstract: The intermediates and transition state for the deprotonation of acetonitrile, 7, with dimeric lithiumamide [LiNH2]2, 8, which leads to the dianion complex [CH2CN/NH2]2- 2 Li+, 14, have been calculated by ab initio (MP2(full)6-31+G*//MP2(full)6-31+G*), DFT (Becke3LYP/6-311+G*, DGAUSS DZP/A1), and semiempirical methods. This reaction is initiated by the formation of a 4-ring-dimer/nitrile complex, 9, which rearranges to give an “open dimer” complex 10. This step is (energetically) the most expensive, requiring 11.4 kcal/mol. The complex 10 is a suitable precursor for easy C to N proton transfer via transition state 11, which lies just 1.2 kcal/mol above 10. The reaction proceeds via 12, a second “open dimer” and finally, after extrusion of NH3, yields 14. The PM3 and ab initio results for the relative energies of these structures are in acceptable agreement; the only exception is the overestimation of the stability of the complex 9 by PM3 (Figure 1). Therefore, PM3 allows comparison of the inter- and intram...

An ab Initio Quantum Mechanical Model for the Catalytic Mechanism of HIV-1 Protease

Abstract: The catalytic mechanism of the HIV-1 protease (HIV-PR) is studied through ab initio theoretical model calculations. This model consists of a formate/formic acid pair, a structurally important water molecule, and a formamide molecule. The proposed catalytic mechanism is composed of five steps, two of which are transition states separated by a third step (an intermediate state). The remaining two steps are related to product release. The overall forward hydrolysis reaction barrier is approximately 22 kcal/mol, with a reverse hydrolysis barrier of approximately 34 kcal/mol at the RHF/6-31G* level. The second transition state is related to a nucleophilic attack of the water molecule on the carbon atom of the substrate scissile bond, and is essential for the collapse of the substrate. That the transition state structures of HIV-PR have not been identified makes a theoretical study of this kind particularly valuable for understanding the HIV-PR mechanism.

Decomposition Pathways for a Model TiN Chemical Vapor Deposition Precursor

Abstract: A computational study of small-molecule elimination, modeling chemical vapor deposition (CVD) pathways, from a model single-source precursor of titanium nitride is reported. A comparison of possible multiply bonded intermediates is made. Calculated geometries for all species agree well with experiment. Transition states for energetically favorable 1,2-elimination to form multiply bonded intermediates are four-centered geometries with a “kite-shape” (i.e., one obtuse and three acute angles). Processes involving the transfer of H from an amido (NHR) ligand to the leaving group (X) are predicted to be the most favorable. In cases where a group other than H (in this case, methyl) is transferred from the amido (NMeR) to X, the TS has a square shape and a much higher barrier to 1,2-elimination. Pathways to cyclic intermediates and β-H elimination pathways are also compared with 1,2-elimination. The β-H elimination process to form an organic imine and a Ti-amido is found to be kinetically and thermodynamically d...

Analysis of the changes on the potential energy surface of Menshutkin reactions induced by external perturbations

Abstract: A quantitative analysis of the changes induced by solvation and static uniform electric fields on the potential energy surface of the SN2 Menshutkin-type reaction between ammonia and methyl chloride has been performed with the help of different indexes. These indexes have been defined to account for the structural and electronic degree of advance of the transition state with respect to the reactant complex and ion pair product through the use of geometrical parameters, dipole moments and electron density distributions. Indexes reveal that external perturbations yield transition states which are both electronically and structurally advanced as compared to the transition state in the gas phase. The overall study is complemented with HOMO-LUMO orbital considerations, quantification of the global charge density redistributions by means of quantum molecular self-similarity measures and an analysis of the topological features of electron density distributions.

Quantum chemical investigations of the thermal and photoinduced proton-transfer reactions of 2-(2',4'-dinitrobenzyl)pyridine

Abstract: Quantum chemical calculations of the proton-transfer system 2-(2‘,4‘-dinitrobenzyl)pyridine (DNBP) and of derivatives with larger aromatic bases are presented. These systems undergo photochemical isomerizations from a stable form to two different photoisomers. Ab initio methods are employed to investigate the isomerization of methylimine to vinylamine as a model system for the proton-transfer reaction of DNBP. The ground and excited states of both isomers and of the transition state for the isomerization are studied with Hartree−Fock and MRD-CI (multireference singles and doubles CI) methods. The isomerizations of the larger system DNBP and its derivatives are investigated with semiempirical PM3-SCF and PM3-MRD-CI methods. The intrinsic reaction coordinate formalism is used for calculating the reaction pathways from the optimized transition states. The relative stabilities of the various isomers and the mechanism of the intramolecular proton-transfer reaction are discussed. The ortho-nitro group assists t...

Reactivity of Strained Compounds: Is Ground State Destabilization the Major Cause for Rate Enhancement?1

Abstract: Reaction and activation energies were computationally determined for the nucleophilic attacks of OH- on 1-cyanobicyclobutane, 2-cyanobicyclobutane, and propionitrile using ab initio methods at the RHF/6-31+G* level. In the first reaction the central bond of the bicyclobutane moiety is cleaved. In the second reaction a side bond is fissioned, and in the third reaction an unstrained reference C−C bond is cleaved. The reaction energies are −38.3, −34, and −0.1 kcal, and the activation energies are 4.4, 30.6, and 41.6 kcal, respectively. Based on these data, traditional analysis suggests that the percent of strain relieved at the transition states of the first two reactions which have nearly the same thermodynamic driving force is 97% and 32%, respectively. These values, according to the linear free energy relationship approach point to an early transition state for the first reaction and a late transition state for the side bond cleavage. Examination of the computed geometrical parameters shows the opposite ...