Showing papers on "Transition state published in 1995"

Theoretical study of the thermochemistry of molecules in the Si-O-H-C system

Abstract: A self-consistent set of thermochemical parameters for about 100 molecules in the Si−B−H−Cl system is obtained using a combination of ab initio electronic structure calculations and empirical corrections (the BAC-MP4 method). The species include stable and radical species as well as a few transition states. Trends in calculated heats of formation, bond dissociation enthalpies, and heats of reaction for various molecular decomposition channels are discussed. Silylboranes are most likely to decompose via the elimination of H2, HCl, or silylenes. The presence of the B atom reduces the energy required for the 1,1 elimination of H2 from the Si atom, relative to the analogous reaction in disilane.

Mechanism of Agrobacterium beta-glucosidase: kinetic analysis of the role of noncovalent enzyme/substrate interactions.

Abstract: The role of noncovalent interactions in the catalytic mechanism of the Agrobacterium faecalis beta-glucosidase was investigated by steady-state and pre-steady state kinetic analysis of the hydrolysis of a series of monosubstituted aryl glycosides, in which the hydroxyl groups on the glycone were substituted by hydrogen or fluorine. Contributions of each hydroxyl group to binding of these substrates at the ground state are relatively weak (interaction energies of 3.3 kJ/mol or smaller) but are much greater at the two transition states (glycosylation and deglycosylation). The strongest transition state interactions were at the 2 position (at least 18 and 22 kJ/mol for glycosylation and deglycosylation, respectively) with the interactions at the 3 and 6 positions contributing at least another 9 kJ/mol of binding energy at both transition states. The interaction at the 4 position is less crucial to transition state binding but important for stabilization of the glycosyl-enzyme intermediate. Comparison of observed rates with those for spontaneous hydrolysis of the same substrates provides evidence for oxocarbenium ion character at both transition states, that for deglycosylation apparently having the greater positive charge development at the anomeric center.

The nature of the transition state for enzyme-catalyzed phosphoryl transfer. Hydrolysis of O-aryl phosphorothioates by alkaline phosphatase.

Abstract: There has been much speculation that enzymes change the nature of the transition state for phosphoryl transfer from the dissociative transition state observed in solution reactions to an associative transition state at the enzyme's active site. This proposal can be tested by comparing linear free energy relationships (LFERs) for nonenzymatic and enzymatic reactions, provided that the specificity of the enzyme's binding site does not perturb the dependence of rate on the intrinsic reactivity of a series of substrates. The shallow binding groove of Escherichia coli alkaline phosphatase (AP) and its wide specificity suggest that this enzyme may be suited for such an approach. A second requirement of this approach is that the actual chemical step is rate-limiting. Comparisons of the reactions of aryl phosphorothioates and aryl phosphates support the previous conclusion that a nonchemical step limits kc,$& for reactions of aryl phosphates, but suggest that the chemical cleavage step is rate-limiting for the aryl phosphorothioates. We therefore determined the dependence of the rate of AP-catalyzed cleavage of a series of aryl phosphorothioates on the intrinsic reactivity of the substrates. The large negative values of group = -0.8 for the enzymatic reaction (kcat/&) and - 1.1 for the nonenzymatic hydrolysis reaction suggest that there is considerable dissociative character in both the enzymatic and nonenzymatic transition states. Despite the wide specificity of AP, certain substrates deviate from the LFER, underscoring that extreme care is required in applying LFERs to enzymatic reactions. The large negative value of Pleaving pup suggests that AP can achieve substantial catalysis via a transition state with dissociative character,

DFT study of the Diels–Alder reactions between ethylene with buta-1,3-diene and cyclopentadiene

Abstract: A set of DFT methods (Xa, HFB, S-VWN, B–LYP; B–HandH and Becke3-LYP) has been used for the calculation of the transition states and energy barriers of the Diels–Alder reaction of ethylene with buta-1,3-diene and cyclopentadiene. The pure DFT methods overestimate the bond lengths, while the hybrid methods give values much closer to those obtained by conventional ab initio methods. The ratio of the σ-forming bonds to the π-breaking bonds is in excellent correlation to the predicted electronic energy barriers for these reactions. The local spin density approximation (S–VWN) fails completely giving a negative value for the classical energy barrier height for the addition of ethylene to buta-1,3-diene. On the other hand, the vibrational adiabatic barrier heights predicted by B-LYP/6-31G** and the hybrid Becke3-LYP/6-31G** theoretical models are in excellent agreement with available experimental data for both reactions.

Pre-steady-state transition-state analysis of the hydrolytic reaction catalyzed by purine nucleoside phosphorylase.

Abstract: The slow hydrolytic reaction catalyzed by calf spleen purine nucleoside phosphorylase [Kline, P. C., & Schramm, V. L. (1992) Biochemistry 31, 5964-5973] has been investigated using pre-steady-state kinetic isotope effects and solvolysis studies. The stoichiometric reaction between enzyme and inosine forms 1 mol of free ribose per trimer of purine nucleoside phosphorylase and a tightly bound complex of enzyme and hypoxanthine. The experimental kinetic isotope effects from [1'-3H]-, [2'-3H]-, [4'-3H]-, [5'-3H]-, [1'-14C]-, and [9-15N]inosine are 1.151 +/- 0.004, 1.145 +/- 0.003, 1.006 +/- 0.004, 1.028 +/- 0.005, 1.045 +/- 0.005, and 1.000 +/- 0.005, respectively, for the pre-steady-state conditions. Substrate trapping experiments demonstrated that there is no detectable forward commitment to catalysis for inosine hydrolysis. In contrast, bound inosine is 2.1 times more likely to form product than to dissociate when the enzyme-inosine complex is exposed to saturating PO4. The lack of an observed 9-15N isotope effect is consistent with an internal equilibrium between enzyme-inosine and the enzyme-hypoxanthine-ribose complex in which N9 of hypoxanthine is protonated. The equilibrium occurs as a consequence of slow product release and tightly bound hypoxanthine (Kd = 1.3 x 10(-12) M). This internal equilibrium has a minimal effect on the intrinsic kinetic isotope effects from ribose since equilibrium isotope effects for conversion of inosine to ribose are near unity. When the single-turnover hydrolytic reaction was accomplished in 20% methanol, approximately 85% of the product sugar was 1-methylribose. Under these conditions, the anion-binding pocket fills with solvent which competes for the oxocarbenium ion of inosine formed at the transition state. In the presence of arsenate, no methanolysis of inosine occurs [Kline, P. C., & Schramm, V. L. (1993) Biochemistry 32, 13212-13219]. The results define a transition state with oxocarbenium ion character and weak participation of the attacking solvent nucleophile. Electrostatic potential surfaces of the transition states indicate that arsenate anion is more effective in neutralizing the oxocarbenium ion than is H2O.

Density functional based studies of transition states and barriers for hydrogen exchange and abstraction reactions

Abstract: The overbinding that is inherent in existing local approximations to the density functional formalism has limited the usefulness of the local density approximation (LDA) for describing phenomena that are mediated by reaction barriers. Since the generalized gradient approximation (GGA) significantly decreases the overbinding, prospects for density functional based reaction dynamics are promising. Using both LDA and GGA functionals, we determined the transition state properties for four different reactions; H2+H→H+H2, CH4+H→CH3+H2,H+CH4→CH4 +H, and CH4+CH3 →CH3+CH4. Although we find that GGA still underestimates reaction barriers, our results show that this functional leads to significant improvements of the calculated reaction barriers and energetics.

Computational studies of the potential energy surface for O(3P)+H2S: Characterization of transition states and the enthalpy of formation of HSO and HOS

Abstract: Structures and vibrational frequencies for minima and 11 transition states on the O(3P)+H2S potential energy surface have been characterized at the MP2=FULL/6‐31G(d) level. GAUSSIAN‐2 theory was employed to calculate ΔHf,298 for HSO and HOS of −19.9 and −5.5 kJ mol−1, respectively. The kinetics of HSO=HOS isomerization are analyzed by Rice–Ramsperger–Kassel–Marcus theory. Transition state theory analysis for O+H2S suggests OH+HS is the dominant product channel, with a rate constant given by 1.24×10−16 (T/K)1.746 exp(−1457 K/T) cm3 molecule−1 s−1. Kinetic isotope effects and the branching ratio for H+HSO production are also analyzed. The other possible products H2+SO and H2O+S do not appear to be formed in single elementary steps, but low‐barrier pathways to these species via secondary reactions are identified. No bound adducts of O+H2S were found, but results for weakly bound triplet HOSH are presented. The likely kinetics for the reactions OH+SH→S(3P)+H2O, OH+SH→cis and trans 3HOSH, cis 3HOSH→HOS+H, and ...

An ab initio investigation on transition states and reactivity of chloroethane with OH radical

Abstract: The reaction C2H5Cl+⋅OH→C2H5Cl⋅+H2O (α and β abstraction) has been investigated by ab initio molecular orbital theory with several basis sets and levels of correlation. Optimized geometries and harmonic vibrational frequencies have been calculated for all reactants, transition states, and products at the (U)HF/6‐31G(d,p) and (U)MP2/6‐31G(d,p) levels of theory. The correlation energy is found to play an important role in determining the barrier heights and reaction enthalpies as well as the geometry and the vibrational frequencies of the transition states. A pseudocyclic transition state is found to be favorable to the β‐abstraction reaction since the participation of the chlorine substituent reduces the barrier height by 0.95 kcal/mol, through a relatively large inductive through‐space effect. The best results for the barrier heights and reaction enthalpies have been obtained using the second‐order Mo/ller–Plesset perturbation theory with spin projection employing the 6‐311+G(2d,p) basis set. A satisfacto...

Comparisons between statistics, dynamics, and experiment for the H+O2→OH+O reaction

Abstract: The accuracy of the variable reaction coordinate (VRC) implementation of transition state theory (TST) is investigated for the bimolecular reaction of H with O2 via direct comparisons with quantum scattering theory for J=0, classical trajectory simulations for a wide range of J, and experimental canonical rate constants. The DMBE IV potential energy surface of Varandas and co‐workers is employed in each of the theoretical calculations. The first two comparisons indicate that the VRC‐TST approach overestimates the cumulative reaction probability (CRP) for this reaction by a factor of 2.3, roughly independent of E and J for moderate energies. The trajectory simulations further indicate that this failure of TST is primarily the result of the rapid redissociation of a large fraction of the initially formed HO2. An estimate for the quantum CRP on the basis of the combined dynamical and statistical results is seen to provide a useful alternative to the more standard quasiclassical trajectory estimates. A therma...

Allosteric linkages between beta-site covalent transformations and alpha-site activation and deactivation in the tryptophan synthase bienzyme complex.

Abstract: This work examines two aspects of the catalytic mechanism and allosteric regulation of the tryptophan synthase bienzyme complex from Salmonella typhimurium: (a) the chemical mechanism by which indole and other nucleophiles react with the enzyme-bound alpha-aminoacrylate Schiff base intermediate, E(A-A), to form quinonoidal intermediates, E(Q), and (b) the effects of covalent transformations at the beta-site on the catalytic activity of the alpha-site. Transient kinetic studies in combination with alpha-secondary deuterium isotope effects are undertaken to determine the mechanism of nucleophile addition to E(A-A). These studies establish that nucleophilic attack is best described by a two-step reaction sequence consisting of a binding step that is followed by Michael addition to the conjugated double bond of E(A-A). Analysis of isotope effects suggests that the transition state for indole addition gives an E(A-A) beta-carbon that resembles an sp3 center, while the stronger nucleophiles, indoline and beta-mercaptoethanol, have transition states that appear to more closely resemble an sp2 beta-carbon. The effects of beta-site covalent transformations on alpha-site catalysis were studied using quasi-stable beta-site intermediates and the alpha-site substrate analogue 3-[6-nitroindole]-D-glycerol 3'-phosphate (6-nitro-IGP). It was found that the cleavage of 6-nitro-IGP is strongly activated by the formation of E(A-A) and various E(Q) species at the beta-site but not by external aldimine species. Therefore, we conclude that the conversion of the L-Ser external aldimine to E(A-A) is the beta-site process which activates the alpha-site, while conversion of E(Q) to the L-Trp external aldimine triggers deactivation of the alpha-site. These findings are discussed within the context of allosteric regulation of substrate channeling in tryptophan synthase catalysis.

Transition states of charge‐transfer reactions: Femtosecond dynamics and the concept of harpooning in the bimolecular reaction of benzene with iodine

Abstract: We describe an approach for real‐time studies of the transition‐state dynamics of charge‐transfer reactions. An application to the bimolecular reaction of benzene with iodine is reported. The measured 750±50 fs transient growth of the free I atom product elucidates the nature of the transition state and the mechanism for the dissociative charge‐transfer reaction. The mechanism is formulated in relation to the impact geometry and the dative bonding, which are crucial to condense‐phase and surface reactions, and is supported by molecular dynamics.

High-Valent Oxo, Methoxorhenium Complexes: Models for Intermediates and Transition States in Proton-Coupled Multi-Electron Transfer Reactions

Abstract: : Dioxorhenium(V) tetrapyridyl species are currently under active investigation as model systems for interfacial two-electron, two-proton transfer reaction sequences We now find that the corresponding oxo, methoxo complexes can be prepared from dioxo species and methyl trifluoromethanesulfonate. The new complexes behave nearly identically to the analogous oxo, hydroxo complexes-with one important exception: CH3(+), unlike H(+), does not dissociate from the oxo ligand. As a direct consequence, the usually elusive rhenium oxidation state, IV, is stabilized with respect to redox disproportionation and is observable for several complexes at high pH. The ability to detect this state, in turn leads to: (1) direct access to the formal reduction potentials for the isolated 1e(-) redox couples comprising the overall two electron transfer (key information for understanding multi-ET kinetics) (2) elucidation of the profound structural and energetic consequences of the initial protonation (methylation) step in the dioxorhenium(V) reduction kinetics.

Potential energy surface of the hno + no reaction. an ab initio molecular orbital study

Abstract: The potential energy surface of the HNO + NO reaction has been investigated by ab initio molecular orbital calculations at the QCISD(T)/6-311G(d,p)//UMP2/6-311G(d,p) + ZPE[UMP2/6-311G(d,p)] and Gaussian-2 (G2) levels of theory. The initial reaction step is NO association with the N atom of the HNO molecule to form the HN(O)NO intermediate, 2, overcoming the barrier 1[prime] of 9.5 kcal/mol. The reaction proceeds further by 1,3-hydrogen migration in HN(O)NO from nitrogen to oxygen via the transition state 3, which is much more favorable than 1,2-shift. This step is shown to be rate-determining, having a barrier of 21.6 kcal/mol. After the H shift, trans,cis-HONNO ([sup 2]A[double prime]) intermediate, 5a, is formed, which rearranges to trans,trans-HONNO ([sup 2]A[prime]), 7b. Finally, the latter dissociates to give the reaction products H[sub 2]O + OH. The energies of the transition states for internal rearrangements of HONNO as well as the transition state for HONNO ([sup 2]A[prime]) dissociation are calculated to be significantly lower than the rate-determining barrier for 1,3-hydrogen migration in HN(O)NO. 23 refs., 2 figs., 3 tabs.

An ab initio study of hydrogen abstraction by fluorine, chlorine and bromine atoms from ethane and propane

Abstract: The H-abstraction reaction by F, Cl and Br radicals from ethane and propane is studied using ab initio UHF, UHF/ MP2 and UHF/MP4 computational methods with the 6–31G ∗ basis set. It is found that in all the cases examined the reaction proceeds through a transition state where the halogen atom, the abstracted H atom and the C atom are almost collinear. The geometries of the various transition states are not significantly modified by the dynamic correlation corrections included with the MP2 treatment. However the correlation energy contributions on reactants and transition states are essential to obtain values of activation energy in reasonable agreement with experimental results. Also the trend of reactivity, which decreases in the direction F → Cl → Br, and the selectivity toward primary and secondary hydrogens are in good agreement with experiment. Another interesting point concerns the performance of the effective potential basis set LAN1DZ, which is found to yield reliable geometries for the transition states. So, more accurate activation energies can be obtained cheaply by single-point computations with the 6–31G ∗ basis set on LANL1DZ optimized geometries. A simple diabatic model is used to rationalize the trend of reactivity along the series F → Cl → Br and the greater reactivity of secondary versus primary hydrogens.

The thermal reaction rate of muonium with methane (and ethane) in the gas phase

Abstract: Rates for the gas‐phase thermal reaction Mu+CH4→MuH+CH3 (Mu=μ+e−), have been measured using the μSR (muon spin rotation) technique, over the temperature range 625–820 K. A good fit is obtained to the usual Arrhenius expression, k=A exp(−Ea/RT), giving an activation energy Ea=24.6±0.9 kcal/mol, ∼12 kcal/mol higher than that of the H‐atom isotopic variant of this reaction, H+CH4→H2+CH3. This Ea difference is the largest yet seen at high temperatures between H and Mu in the gas phase, and seems much too high to be explained in terms of [zero‐point‐energy (ZPE)] differences in their respective transition states, indicating instead a dramatic difference in reaction dynamics. The possible sources of this difference include differing reactivities from vibrationally excited states and/or a more favorable tunneling path for the H+CH4 reaction due to its suspected much earlier (and thinner) reaction barrier. In contrast, the similar H‐atom abstraction reactions with H2 and C2H6 gave Ea differences which matched expectations based on ZPE shifts, suggesting a qualitative difference in dynamics between these otherwise homologous reactions. It is suggested that reaction from vibrationally excited states may be relatively more important in the case of the Mu+CH4 reaction.

Kinetics and Product Branching Ratios for the Reaction HCO + NO2

Abstract: The kinetics and product branching of the HCO + NO{sub 2} reaction have been studied at room temperature using laser flash kinetic spectroscopy and modeled with microcanonical variational RRKM theory. The rate constant for the disappearance of HCO radical at 296 K is (5.7 {+-} 0.9) x 10{sup -11} cm{sup 3} molecules{sup -1} s{sup -1}, and it is independent of the pressure of SF{sub 6} buffer gas up to 700 Torr. The CO{sub 2} yield is 52 {+-} 14%. Less than 10% of the reaction goes through the most exothermic product channel, HNO + CO{sub 2}. The least exothermic product channel. H + CO{sub 2} + NO, is responsible for the remaining CO{sub 2}. HONO has been observed, through not quantitatively, as a reaction product corresponding to the HONO + CO product channel. It must account for the 48 {+-} 14% of reaction which does not yield CO{sub 2}. The rates of formation of the collision complexes HCO(ONO) and HCO(NO{sub 2}) are calculated by variational RRKM theory to be comparable and only weakly temperature dependent. Branching ratios for the decomposition of these complexes are also calculated. They are found to match most experimental observations from 300 to 1600 K. 55more » refs., 6 figs., 1 tab.« less

Dynamics of persistent collision complexes in molecular beam reactive scattering

Abstract: New microcanonical theories for the angular distributions of reactive scattering arising from long-lived collision complexes and their application to a wide range of transition state geometries are reviewed. Transition states formed at the top of centrifugal barriers on potential energy surfaces without a potential energy barrier are well described by symmetric top transition states, provided that the loosening of bending mode vibrations into product rotation is included. A phase space description appropriate for collisions at small impact parameters can be incorporated into a consistent mathematical description. Tight transition states formed at the top of a potential energy barrier and involving H atom displacement are described by an asymmetrical top transition state theory which includes all the symmetric top configurations as limiting cases. However the effects of transition state flexibility rapidly obscure the observable consequences of preferred geometry for directions of dissociation awa...

Theoretical characterization of structures and vibrational frequencies for intermediates and transition states in the reaction of NH2 with NO

Abstract: Features of the potential energy surface for the reaction of NH 2 with NO have been characterized at the complete active space self-consistent field level of theory with a correlation-consistent polarized valence double-zeta basis set. The same large active space, sufficient to describe all of the key reaction channels, was used throughout. In this way a consistent set of structural parameters and a consistent set of harmonic vibrational frequencies for use in statistical RRKM treatment were obtained. The overall energetics were investigated further by performing single-point large-scale configuration interaction calculations using polarized valence triple-zeta and augmented polarized double-zeta basis sets.

Molecular modelling of xylose isomerase catalysis: the role of electrostatics and charge transfer to metals.

Abstract: The two main steps of the mechanism of xylose-xylulose conversion catalysed by D-xylose isomerase, the ring opening of xylose and the isomerization of the opened product by hydride transfer, were investigated by molecular mechanical and molecular orbital techniques. The activation energies calculated for these reactions clearly showed that hydrogen transfer is the rate-determining step of the enzymatic isomerization and that Mg2+ ions activate whereas Zn2+ ions inhibit the reaction, in agreement with the experiments. The remarkable differences between the net charges of these ions found by molecular orbital calculations and the inspection of the protein electrostatic potential around the reaction intermediates indicate that the main role of bivalent metal ions should be the electrostatic stabilization of the substrate transition states. In order to propose a more detailed mechanism, an attempt was made to clarify the effects of nearby residues (e.g. His54, Asp57, Lys183, Asp257) in the reaction. Different isomerization mechanisms, such as through an enediol intermediate, were examined and could be excluded, in addition to the charge-relay mechanism during the ring opening.

Molecular mechanics study of zirconocenium catalyzed isospecific polymerization of propylene

Abstract: Stereocontrol energy (ΔE0) is investigated as a measure of enantioselectivity of ansa-zircoocenium catalyst in propylene polymerization; it was calculated with MM2 (molecular mechanics) force field using π complex (°C) and transition state (TS) geometries obtained by ab initio molecular orbital methods. Both rac-ethylenebis (1-η5-indenyl) - (1) and rac-ethylenebis (1-η5-4,5,7,8-tetrahydroindenyl) (2) zirconocenium species are isospecific in either the π-complexes or the transition states. The stereoselectivity is greater if there is α-agostic interaction; it is lowered in the cases of β and γ agostic interactions. The 13C-NMR steric pentad distribution indicates the poly(propylene) to be much less stereoregular than that predicted by ΔE0. Following the occurrence of a regiochemical insertion error, the addition of another monomer via any mode is prohibitively unfavorable. The catalyst suffers loss of stereospecificity as temperature of polymerization increases. Insertion via transition states involving different agostic interactions could be one explanation for the observed loss. © 1995 John Wiley & Sons, Inc.

Theoretical studies on the mechanisms of proton transfer in Schiff bases

Abstract: Proton transfer reactions of salicylideneaniline (SA), N -tetrachlorosalicylideneaniline (Cl 4 SA) and N -tetrachlorosalicylideneaniline-1-pyrenylamine (Cl 4 SPY) have been investigated by using a semiempirical SCF MO method with an energy gradient technique. Thermochromic reactions involve the proton transfer from the hydroxyl oxygen to the imine nitrogen via six-membered ring transition states. From the calculated potential barriers, 81.27 and 87.27 kJ mol −1 for Cl 4 SA and Cl 4 SPY, respectively, it can be seen that the thermochromic reactions proceed easily at room temperature. The analyses of molecular orbitals and electronic distributions show that the thermochromism is also due to a change in the π-electron configuration induced by the proton transfer. Intermolecular interaction makes the thermochromic reaction of Cl 4 SPY easier to proceed. Photochromic reaction of SA is initiated by excitation of the enol form to its first excited singlet state S 1 . The population in the S 1 state is transferred either to the second excited singlet state of the keto form (S′ 2 ) along an adiabatic pathway or to the lowest excited triplet state of the keto form (T′ 1 ) via intersystem crossing, and finally relaxing to the trans-keto form (S′' 0 ) or cis-keto form (S′ 0 ).

Variational transition state theory: Application to a symmetric exchange reaction in water

Abstract: Variational transition state theory (VTST) is applied for the first time to a chemical reaction in a liquid. The theory provides accurate estimates of reaction rates and leads to well defined microscopic friction functions. The structure of the optimized planar dividing surface provides insight into the range of solute–solvent interactions for which there is an appreciable effect on the reaction dynamics. The VTST method also allows for separation of the frictional effects of solvent translation, rotation, and stretch modes. The numerical cost is less than an analogous molecular dynamics reactive flux computation and the insight gained is greater.

Extended gorin model for radical-radical recombination reactions

Abstract: Radical-radical recombination reactions (e.g., CH3 + CH3 reversible arrow C2H6) proceed with no barrier through simple-fission transition states. The application of transition state theory (TST) to these reactions is discussed, achieving a new understanding of the dividing surface and dynamical assumption implicit in all TST treatments of these reactions. A reinterpretation of the modified Gorin model for such transition states is discussed which removes several inconsistencies from this model and greatly improves data prediction and interpretation for radical-radical recombination reactions (and the reverse unimolecular dissociations) in the gas phase. The suggested model is an extension of the basic Gorin approach, which treats the transition state as consisting of two moieties which have the same vibrational and rotational properties as the fully separated fragments. The method discussed here proposes an improvement of the modified Gorin model Hamiltonian that better describes simple-fission reaction dynamics by completely excluding trajectories occurring with unfavorable orientations of the combining moieties from the transition state theory rate coefficient. This new approach is sufficiently simple that the description is applicable to any system and thus can be routinely implemented with modest computational resources. Comparison with experiment and with more precise theoretical descriptions for ethane and neopentane decomposition reactions shows that this treatment provides quantitative agreement for ethane. It is also concluded that more sophisticated treatments of transitional modes than afforded by hindered rotor models are needed for the description of transition states with bulky moieties at elevated temperatures, such as the neopentane decomposition system described here.

Time-Resolved Photodissociation of the Molecular Ions of Propyl Phenyl Ethers

Abstract: The fragmentation rate of n-propyl phenyl ether molecular ions has been measured by time-resolved photodissociation (TRPD) in the region of 2 eV internal energy, giving first-order rate constants on the order of 104 s-l. RRKM fitting suggests a dissociation energy barrier of 1.47 eV. Decomposition of the isopropyl isomer is at least 30 times faster in this energy region. Dissociative photoionization of isopropyl phenyl ether in a supersonic jet by laser resonance-enhanced multiphoton ionization (REMPI) shows that hydrogen scrambling is insignificant for this isomer in the 4.5 eV energy range, in contrast to the earlier finding of extensive hydrogen randomization under those conditions for the n-propyl isomer. PhOD'+ is the only fragment detected in the TRPD or the REMPI of (CD&CHOPh. The REMPI results (guided by SCF calculations) also indicate that the two methyl groups are not equivalent in the most stable conformation of neutral isopropyl phenyl ether. A quantitative reaction coordinate is proposed for the n-propyl isomer dissociation, which postulates separate transition states for hydride migration and for proton transfer and an intermediate (iPr+ Pho') ion-neutral complex. The faster dissociation of the isopropyl isomer is not consistent with the energetics predicted by UHF calculations, and it seems likely that that dissociation proceeds via a different mechanism. Positive ions derived from a variety of alkyl aryl ethers decompose in the gas phase via the intermediacy of ion-neutral complexes, as represented in eq 1 for a radical cation.' Evidence for this mechanism derives largely from studies on rearrange- ments of the alkyl moiety, R', prior to the formation of observable fragments. These rearrangements, as revealed by analysis of neutral products of the decomposition2 and by isotopic labeling experiments in the mass spectrometer,'^^-^ are those that would be expected of a free alkyl cation with a short (~1O-l~ s) lifetime. ~~

Gas-phase nucleophilic displacement reactions

Abstract: Displacement reactions of each of a variety of anionic nucleophiles reacting with each of a variety of neutrals have been studied by pulsed ion cyclotron resonance (ICR) spectroscopy. Rate constants for these reactions are interpreted in terms of a three-step reaction sequence. RRKM calculations are used to obtain information about the energy of transition states. The origin of the barrier to reaction in solution is discussed.