Ab initio and RRKM evidence that the McLafferty rearrangement of ionized n-butanal is stepwise

Hartree-Fock calculations with geometry optimization at a series of C(2)? C(3) distances predict a negligible reverse critical energy for the second step of the McLafferty rearrangement of ionized n-butanal, in contrast to a previous conclusion. The most favorable geometry for departure of the C₂H₄ has a dihedral angle of about 90° between the parting fragments. RRKM calculations were used to estimate rates of reactions associated with the McLafferty rearrangement These calculations indicate that the McLafferty rearrangement of the n-butanal ion is stepwise. The RRKM calculations predict competition of H exchange with decomposition up to much higher energies than actually occurs.     

The carbon-skeleton rearrangement in tropane alkaloid biosynthesis

High-level quantum chemistry calculations have been performed to examine the carbon-skeleton rearrangement of the tropane alkaloid littorine to hyoscyamine. Two pathways involving radical and carbocation intermediates have been investigated in this regard, namely, stepwise (or fragmentation-recombination) and concerted. The fragmentation products are calculated to be of high energy for both the radical- and carbocation-based mechanisms (136.3 and 170.9 kJ mol(-1), respectively). Similarly, the rearrangement barrier for the radical-based concerted pathway is calculated to be quite high (135.6 kJ mol(-1)). In contrast, the carbocation-based concerted pathway is found to be associated with a relatively low barrier (47.4 kJ mol(-1)). The ionization energy of the substrate-derived radical 3a is calculated to be 7.01 eV, suggesting that its oxidation to generate the substrate-derived carbocation 3b ought to be facile. In an attempt to investigate how an enzyme might modulate the rearrangement barriers, the separate and combined influences of partially protonating the migrating group and partially deprotonating the spectator OH group of the substrate were investigated. Such interactions can lead to significant reductions in the rearrangement barrier for both the radical- and carbocation-based concerted pathways, although the carbocation pathway continues to have significantly lower energy requirements. Also, the relatively high (gas-phase) acidity of the OH group of the product-related carbocation 4b indicates that the direct formation of hyoscyamine aldehyde (6) is a highly exothermic process. Although we would not wish to rule out alternative possibilities, our calculations suggest that a concerted rearrangement mechanism involving carbocations constitutes a viable low-energy pathway for the carbon-skeleton rearrangement in tropane alkaloid biosynthesis.     

Reactivity of 13,13-dibromo-2,4,9,11-tetraoxadispiro[5.0.5.1]tridecane toward organolithiums: remarkable resistance to the DMS rearrangement

Reactions of dibromocyclopropane 2a, containing two spiro-fused 1,3-dioxane rings, with MeLi gave only the methylation products 8 and 9 even at elevated temperatures. In contrast, the cyclohexane analogue 2b treated with MeLi underwent a smooth rearrangement to bicyclo[1.1.0]butane 11b at -78, -10, or +35 degrees C. Treatment of 2a with PhLi gave the alpha-Ph anion 13 as the only product, which underwent smooth methylation with MeI to give 14. Under the same conditions, 2b with PhLi gave bicyclo[1.1.0]butane 11b accompanied by bromophenyl derivative 8b. Treatment of either dibromide with t-BuLi gave a mixture of products including debrominated cyclopropanes 12. Experimental results were augmented with DFT calculations for salts 23 and MP2//DFT-level calculations for carbenes 22. They demonstrated a higher stability of the dioxane alpha-bromo anion with respect to alpha-elimination by 4.8 kcal/mol and also a lower tendency of the carbene 22a to undergo rearrangement by 4.0 kcal/mol than the cyclohexane analogues. These differences have been attributed to the inductive effect of the four oxygen atoms, which results in lower LUMO energy, the higher positive charge at the carbenic center, and the overall more electrophilic character of carbene 22a as compared to the cyclohexane derivative 22b. The rearrangement of carbenes 22 to the corresponding allenes 1, the thermodynamic products, requires a higher activation energy DeltaG(double dagger)(298) by 4.2 kcal/mol for dioxane and 6.4 kcal/mol for cyclohexane derivatives than for the formation of the bicyclo[1.1.0]butanes 11. The DeltaG(double dagger)(298) for intramolecular insertions to the CH bond is low and calculated as 6.0 kcal/mol for dioxane 22a and 2.0 kcal/mol for the formation of cyclohexane 22b.     

Toward an improved understanding of the glutamate mutase system

High-level quantum chemistry calculations have been used to examine the catalytic reactions of adenosylcobalamin-dependent glutamate mutase (GM) with the natural substrate (S)-glutamic acid. We have also examined the rearrangement of (S)-2-hydroxyglutaric acid, (S)-2-thiolglutaric acid, and 2-ketoglutaric acid, all of which have previously been shown to react as substrates or inhibitors of the enzyme. Our calculations support the notion that the 100-fold difference in kcat between glutamate and 2-hydroxyglutarate is associated with the relatively high energy of the glycolyl radical intermediate compared with the glycyl radical. More generally, calculations of radical stabilization energies for a variety of substituted glycyl radical analogues indicate that modifications at the radical center can profoundly affect the relative stability of the resulting radical, leading to important mechanistic consequences. We find that the formation of a thioglycolyl radical, derived from (S)-2-thiolglutaric acid, is highly dependent on the protonation state of sulfur. The neutral radical is found to be of stability similar to that of the glycolyl radical, whereas the S- form of the thioglycolyl radical is much more stable, thus providing a rationalization for the inhibition of the enzyme by the substrate analogue 2-thiolglutarate. Two possible rearrangement pathways have been examined for the reaction of GM with 2-ketoglutaric acid, for which previous experiments had suggested no rearrangement took place. The fragmentation-recombination pathway is associated with a fragmentation step that is very endothermic (by 102.2 kJ mol-1). In contrast, the addition-elimination pathway has significantly lower energy requirements. An alternative possibility, namely, that 2-ketoglutaric acid is bound in its hydrated form, 2,2-dihydroxyglutaric acid, also leads to a pathway with relatively low energy requirements, suggesting that some rearrangement might be expected under such circumstances.     

H-bonded network rearrangements in the S0, S1 and D0 states of neutral and cationic p-cresol(H2O)(NH3) complexes

The H-bonded network rearrangements in the S(0), S(1) and D(0) states of the neutral and cationic p-CreOH(H(2)O)(NH(3)) complexes were studied experimentally by means of (1 + 1)/(1 + 1') REMPI (Resonantly Enhanced MultiPhoton Ionization) and time resolved LIF (Laser Induced Fluorescence) spectroscopies combined with DFT (Density Functional Theory) calculations at the B3LYP/6-311G++(d,p) level. A comparison of the rearrangement process of the H-bonded network in the three states is given. Two cyclic H-bonded isomers were found on the S(0) potential energy surface and the results indicate that the rearrangement in this state is unlikely at the temperature of the supersonic expansion due to the presence of a high-energy barrier (7503 cm(-1)). On the other hand, the re-determination of the S(1) excited state lifetimes confirms that neither the H-bonded rearrangement nor the excited state hydrogen transfer (ESHT) reaction takes place in the S(1) state at the excitation energies of this work. Thus, it is concluded that the absorption of the second photon to reach the D(0) state takes place from the S(1) state of the cyclic-(OH-OH(2)-NH(3)) isomer. A preferential evaporation of H(2)O upon vertical ionization of the cyclic-(OH-OH(2)-NH(3)) isomer is observed which is consistent with a statistical redistribution of the internal energy. Nevertheless, our theoretical calculations suggest that initial excitation of the H-bonded network rearrangement modes may also play a role to leave the H(2)O molecule as a terminal moiety in a chain-(OH-NH(3)-OH(2))(+) isomer. The reaction pathway for the solvent rearrangement involves a double proton transfer process with a very low energy barrier (575 cm(-1)) that is overcome at the vertical ionization energy of the complex.     

Dissociation of the OCS+ ion in low-lying electronic states studied using multiconfiguration second-order perturbation theory

Complete active space self-consistent-field (CASSCF) and multiconfiguration second-order perturbation theory (CASPT2) calculations with atomic natural orbital basis sets were performed to investigate the S-loss direct dissociation of the 1 2Pi(X 2Pi), 2 2Pi(A 2Pi), 1 2Sigma+(B 2Sigma+), 1 4Sigma-, 1 2Sigma-, and 1 2Delta states of the OCS+ ion and the predissociations of the 1 2Pi, 2 2Pi, and 1 2Sigma+ states. Our calculations indicate that the S-loss dissociation products of the OCS(+) ion in the six states are the ground-state CO molecule plus the S+ ion in different electronic states. The CASPT2//CASSCF potential energy curves were calculated for the S-loss dissociation from the six states. The calculations indicate that the dissociation of the 1 4Sigma- state leads to the CO + S+ (4Su) products representing the first dissociation limit; the dissociations of the 1 2Pi, 1 2Sigma-, and 1 2Delta states lead to the CO + S+(2Du) products representing the second dissociation limit; and the dissociations of the 2 2Pi and 1 2Sigma+ states lead to the CO + S+(2Pu) products representing the third dissociation limit. Seams of the 1 2Pi-1 4Sigma-, 2 2Pi-1 4Sigma-, 2 2Pi-1 2Sigma-, 2 2Pi-1 2Delta, and 1 2Sigma(+)-1 4Sigma- potential energy surface intersections were calculated at the CASPT2 level, and the minima along the seams were located. The calculations indicate that within the experimental energy range (15.07-16.0 eV) the 2 2Pi(A 2Pi) state can be predissociated by 1 4Sigma- forming the S+(4Su) ion and can undergo internal conversion to 1 2Pi followed by the direct dissociation of 1 2Pi forming S+(2Du) and that within the experimental energy range (16.04-16.54 eV) the 1 2Sigma+(B 2Sigma+) state can be predissociated by 1 4Sigma- forming the S+(4Su) ion and can undergo internal conversion to 2 2Pi followed by the predissociation of 2 2Pi by 1 2Sigma- and 1 2Delta forming the S+(2Du) ion. These indications are in line with the experimental fact that both the 4Su and 2Du states of the S+ ion can be formed from the 2 2Pi and 1 2Sigma+ states of the OCS+ ion.     

Experimental and theoretical studies of charge transfer and deuterium ion transfer between D2O+ and C2H4

The charge transfer and deuterium ion transfer reactions between D(2)O(+) and C(2)H(4) have been studied using the crossed beam technique at relative collision energies below one electron volt and by density functional theory (DFT) calculations. Both direct and rearrangement charge transfer processes are observed, forming C(2)H(4) (+) and C(2)H(3)D(+), respectively. Independent of collision energy, deuterium ion transfer accounts for approximately 20% of the reactive collisions. Between 22 and 36 % of charge transfer collisions occur with rearrangement. In both charge transfer processes, comparison of the internal energy distributions of products with the photoelectron spectrum of C(2)H(4) shows that Franck-Condon factors determine energy disposal in these channels. DFT calculations provide evidence for transient intermediates that undergo H/D migration with rearrangement, but with minimal modification of the product energy distributions determined by long range electron transfer. The cross section for charge transfer with rearrangement is approximately 10(3) larger than predicted from the Rice-Ramsperger-Kassel-Marcus isomerization rate in transient complexes, suggesting a nonstatistical mechanism for H/D exchange. DFT calculations suggest that reactive trajectories for deuterium ion transfer follow a pathway in which a deuterium atom from D(2)O(+) approaches the pi-cloud of ethylene along the perpendicular bisector of the C-C bond. The product kinetic energy distributions exhibit structure consistent with vibrational motion of the D-atom in the bridged C(2)H(4)D(+) product perpendicular to the C-C bond. The reaction quantitatively transforms the reaction exothermicity into internal excitation of the products, consistent with mixed energy release in which the deuterium ion is transferred in a configuration in which both the breaking and the forming bonds are extended.     

Rate constants and branching ratios for the reaction of CH radicals with NH3: a combined experimental and theoretical study

The reaction between CH radicals and NH(3) molecules is known to be rapid down to at least 23 K {at which temperature k = (2.21 ± 0.17) × 10(-10) cm(3) molecule(-1) s(-1): Bocherel ; et al. J. Phys. Chem. 1996, 100, 3063}. However, there have been only limited theoretical investigations of this reaction and its products are not known. This paper reports (i) ab initio quantum chemical calculations on the energy paths that lead to various reaction products, (ii) calculations of the overall rate constant and branching ratios to different products using transition state and master equation methods, and (iii) an experimental determination of the H atom yield from the reaction. The ab initio calculations show that reaction occurs predominantly via the initial formation of a datively bound HC-NH(3) complex and reveal low energy pathways to three sets of reaction products: H(2)CNH + H, HCNH(2) + H, and CH(3) + NH. The transition state calculations indicate the roles of "outer" and "inner" transition states and yield rate constants between 20 and 320 K that are in moderate agreement with the experimental values. These calculations and those using the master equation approach show that the branching ratio for the most exothermic reaction, to H(2)CNH + H, is ca. 96% throughout the temperature range covered by the calculations, with those to HCNH(2) + H and CH(3) + NH being (4 ± 3)% and <0.3%, respectively. In the experiments, multiple photon dissociation of CHBr(3) was used to generate CH radicals and laser-induced fluorescence at 121.56 nm (VUV-LIF) was employed to observe H atoms. By comparing signals from CH + NH(3) with those from CH + CH(4), where the yield of H atoms is known to be unity, it is possible to estimate that the yield of H atoms from CH + NH(3) is equal to 0.89 ± 0.07 (2σ), in satisfactory agreement with the theoretical estimate.     

Ab initio calculations of the potential surface for rearrangement of 2,2,3,3-tetrafluoromethylenecyclopropane to 1-(difluoromethylene)-2,2-difluorocyclopropane

(4/4)CASSCF and CASPT2 calculations have been performed to understand the reason that addition of a second pair of geminal fluorines to methylenecyclopropane lowers the barrier to rearrangement by 6.7 kcal/mol more than addition of the first pair. Our calculations duplicate this experimental finding by Dolbier and co-workers. Our computational results confirm Dolbier's conjecture, that the non-additive lowering of Ea for the rearrangement of 2,2,3,3-tetrafluoromethylenecyclopropane (9) to 1-(difluoromethylene)-2,2-difluorocyclopropane (11) is due to destabilization of 9 by the presence of the vicinal CF2 groups in this fluorocarbon. In the course of exploring the potential energy surface for the rearrangement of 9, we have located a bond-stretch isomer (20) that differs from 9 by inversion of both CF2 groups. The enthalpy of 20 is computed to be 21.9 kcal/mol higher than that of 9, but direct interconversion of these two "bond-stretch invertomers" requires passage over a TS whose enthalpy is calculated to be 11.7 kcal/mol higher than that of 20.     

DFT study on the reaction mechanism and regioselectivity for the [1,2]-anionic rearrangement of 2-benzyloxypyridine derivatives

The reaction mechanism of the [1,2]-anionic rearrangement of 2-benzyloxypyridines has been investigated using DFT calculations. Calculated results indicate that: the deprotonation step is relatively fast and the rearrangement step is the rate-determining step; electron-donating group on the benzene ring decreases the activation energy of the rearrangement, which correlates with an increase in reaction yield, while electron-withdrawing groups show the opposite effect. The rearrangement is calculated to proceed by way of an oxirane-like transition state that had previously been postulated as a transient intermediate. Furthermore, the mechanism for the rearrangement of 2-(benzyloxy)nicotinonitrile was discussed. The quick formation of the five membered ring intermediate leads to the predominant formation of 2-phenylfuro[2,3-b]pyridin-3-amine. The calculation results indicate the possibilities of derivatizing the starting pyridyl ether as well as facilitating the rearrangement reaction by adding an appropriate electron-donating group on the benzene ring or electron-withdrawing group on the pyridine ring for future studies.     

The azulene-to-naphthalene rearrangement revisited: a DFT study of intramolecular and radical-promoted mechanisms

Intramolecular and radical-promoted mechanisms for the rearrangement of azulene to naphthalene are assessed with the aid of density functional calculations. All intramolecular mechanisms have very high activation energies (>/=350 kJ mol(-1) from azulene) and so can only be competitive at temperatures above 1000 degrees C. Two radical-promoted mechanisms, the methylene walk and spiran pathways, dominate the reaction below this temperature. The activation energy for an orbital symmetry-allowed mechanism via a bicyclobutane intermediate is 382 kJ mol(-1). The norcaradiene-vinylidene mechanism that has been proposed in order to explain the formation of small amounts of 1-phenyl-1-buten-3-ynes from flash thermolysis of azulene has an activation energy of 360 kJ mol(-1); subtle features of the B3LYP/6-31G(d) energy surface for this mechanism are discussed. All intermediates and transition states on the spiran and methylene walk radical-promoted pathways have been located at the B3LYP/6-31G(d) level. Interconversion of all n-H-azulyl radicals via hydrogen shifts was also examined, and hydrogen shifts around the five-membered ring are competitive with the mechanisms leading to rearrangement to naphthalene, but those around the seven-membered ring are not. Conversion of a tricyclic radical to the 9-H-naphthyl radical is the rate-limiting transition state on the spiran pathway, and lies 164.0 kJ mol(-1) above that of the 1-H-azulyl radical. The transition state for the degenerate hydrogen shift between the 9-H-azulyl and 10-H-azulyl radicals is 7.4 kJ mol(-1) lower. Partial equilibration of the intermediates in the spiran pathway via this shift may therefore occur, and this can account for the surprising formation of 1-methylnaphthalene from 2-methylazulene. The rate-limiting transition state for the methylene walk pathway involves the concerted transfer of a methylene group from one ring to the other and lies 182.3 kJ mol(-1) above that of the 1-H-azulyl radical. It is shown that rearrangement via a combination of 31% methylene walk and 69% spiran pathways can account semiquantitatively for all the products from 1-(13)C-azulene, 9-(13)C-azulene, and 4,7-(13)C(2)-azulene, in addition to accounting for the products from methylazulenes, and the formation of naphthalene-d(0) and -d(2) from azulene-4-d. It is also pointed out that a small extension to the spiran pathway could provide an alternative explanation for the formation of 1-phenyl-1-buten-3-ynes.     

Intramolecular proton transfer in malonaldehyde: accurate multilayer multi-configurational time-dependent Hartree calculations

Full-dimensional (multilayer) multi-configurational time-dependent Hartree calculations studying the intramolecular proton transfer in malonaldehyde based on a recent potential energy surface (PES) [Wang et al., J. Chem. Phys. 128, 224314 (2008)] are presented. The most accurate calculations yield a ground state tunneling splitting of 23.8 cm(-1) and a zero point energy of 14,678 cm(-1). Extensive convergence tests indicate an error margin of the quantum dynamics calculations for the tunneling splitting of about 0.2 cm(-1). These results are to be compared with the experimental value of the tunneling splitting of 21.58 cm(-1) and results of Monte Carlo calculations of Wang et al. on the same PES which yielded a zero point energy of 14,677.9 cm(-1) with statistical errors of 2-3 cm(-1) and a tunneling splitting of 21.6 cm(-1). The present data includes contributions resulting from the vibrational angular momenta to the tunneling splitting and the zero point energy of 0.2 cm(-1) and 2.4 cm(-1), respectively, which have been computed using a perturbative approach.     

Fragmentation mechanisms of protonated benzylamines. Electrospray ionisation-tandem mass spectrometry study and ab initio molecular orbital calculations

Our research into neurotransmitters in a biological fluid presented an opportunity to investigate the fragmentations under low collision energy characterising benzyl-amines protonated under electrospray ionisation (ESI) conditions in a triple quadrupole mass spectrometer. In this work we present the breakdown graphs of protonated 3,4-dihydroxybenzylamine, DHBAH(+), and 3-methoxy, 4-hydroxybenzylamine, HMBAH(+), at various source temperatures and various pressures in the collision cell, the collision energy varying from 0 to 46 eV in the laboratory frame. Both parent ions eliminate first NH(3) at very low collision energy. The fragmentations of [MH - NH(3)](+) occur at high collision energy and are quite different for DHBAH(+) and HMBAH(+): formation of [MH - NH(3) - H(2)O - CO](+) for the former; formation of the radical cation [MH - NH(3) - CH(3)](+.) for the latter. These fragmentations are interpreted by means of ab initio calculations up to the B3LYP/6-311+G(2d,2p) level of theory. The successive losses of H(2)O and CO involve first the rearrangement in two steps of benzylic ions formed by loss of NH(3) into tropylium ions. The transition states associated with this rearrangement are very high in energy (about 400 kJ mol(-1) above MH(+)) explaining (i). the absence of an ion corresponding to [DHBAH - NH(3) - H(2)O](+). The determining steps associated with the losses of H(2)O and with H(2)O + CO are located lower in energy than the transition states associated with the isomerisation of benzylic ions into tropylium ions; explaining (ii). the formation of the radical cation [MH - NH(3) - CH(3)](+.). The homolytic cleavage of CH(3)-O requires less energy than does the rearrangement.     

Computational study on the 1,2-rearrangement in beta-(nitroxy)vinyl and beta-(acetoxy)vinyl radicals

The 1,2-nitroxyl and 1,2-acetoxyl rearrangement in beta-(nitroxy)vinyl and beta-(acetoxy)vinyl radicals 13a and 13b, respectively, has been studied for the gas phase with various ab initio and density functional methods. The energetically most favorable pathway for 13a is calculated to proceed via reversible fragmentation/radical addition through transition state I-19a. In the case of 13b, rearrangement through a five-membered ring transition state III-16b and the fragmentation/radical addition pathway via transition state I-19b are competing processes. Mulliken and natural population analysis reveal a certain degree of charge separation in III-16a/b that may indicate a potential solvent effect on the rearrangement rate. A stepwise group migration through a cyclic radical intermediate V-18a/b or rearrangement through a three-membered ring transition state II-15a/b can be ruled out for both vinyl radicals. A comparison of the results of the calculations with experimental findings provides important insights into the kinetics of "self-terminating radical oxygenations". A significant method dependence on the outcome of the calculations was observed, which revealed the unsuitability of the UHF, MP2, B3LYP, and mPW1PW91 methods for computing these radical rearrangement processes. The results from BHandHLYP/cc-pVDZ calculations showed the best agreement with single-point energy calculations performed at the QCISD and CCSD(T) levels of theory.     

Carbene and nitrene rearrangements: a theoretical study of cyclic allenes and carbenes,carbodiimides, and azirines

B3LYP/6-31G(d) calculations of structures, energies, and infrared spectra of several rearrangement products of (hetero)aromatic nitrenes and carbenes are reported. 3-Isoquinolylnitrene 36 ring closes to the azirine 37 prior to ring expansion to the potentially stable but unobserved seven-membered-ring carbodiimide 38 and diazacycloheptatrienylidene C(s)()-39S. A new, stable cycloheptatrienylidene, C(s)()-19S, is located on the naphthylcarbene energy surface. 4-Quinolylnitrene undergoes reaction via the azirine 50 in solution, but ring expansion to the stable seven-membered-ring ketenimine 47 under Ar matrix photolysis conditions. There is excellent agreement between calculated infrared spectra of 1,5-diazacyclohepta-1,2,4,6-tetraene 54 (obtained by photolysis of 4-pyridyl azide), 1-azacyclohepta-1,2,4,6-tetraene 5, 1-azacyclohepta-1,3,5,6-tetraene 55, and 1-azacyclohepta-1,3,4,6-tetraene 56 and the available experimental data.     

Experimental and theoretical analysis of the photochemistry and thermal reactivity of ethyl diazomalonate and its diazirino isomer. The role of molecular geometry in the decomposition of diazocarbonyl compounds

The photochemical or thermal decomposition of ethyl diazomalonate (1) or ethyl 3,3-diazirinedicarboxylate in methanol solutions yields the O-H insertion product 6, while products of the Wolff rearrangement were not detected in both cases. The analysis of temperature-dependent (13)C NMR spectra and the results of DFT B3LYP/6-311+G(3df,2p) and MP2/aug-cc-pVTZ//B3LYP/6-311+G(3df,2p) calculations allow us to conclude that diazodiester 1 predominantly exists in the Z,Z-conformation. In contrast, photolysis of the cyclic isopropylidene diazomalonate (3), which also has a Z,Z-configuration of the diazodicarbonyl moiety, results in a clean Wolff rearrangement. These observations allow us to conclude that the direction of the photodecomposition of diazomalonates is not controlled by the ground-state conformation. The quantum-mechanical analysis of the potential energy surfaces for the dediazotization of 1 and 3 suggests that the formation of a carbene as a discrete intermediate is controlled by the ability of the latter to adopt a conformation in which carbonyl groups are almost orthogonal to the carbene plane. The outcome of the photolysis of ethyl diazomalonate depends on the wavelength of irradiation. Irradiation with 254 nm light results in the loss of nitrogen and the formation of dicarboethoxycarbene (5, Phi(254) = 0.31), while at longer wavelengths, diazirine 2 becomes an important byproduct (Phi(350) = 0.09). This observation suggests that the formation of carbene 5 and isomerization to diazirine proceed from different electronically excited states of ethyl diazomalonate.     

Density functional study of the mechanism of the Beckmann rearrangement catalyzed by H-ZSM-5: a cluster and embedded cluster study

The mechanism of the Beckmann rearrangement (BR) catalyzed by the ZSM-5 zeolite has been investigated by both the quantum cluster and embedded cluster approaches at the B3LYP level of theory using the 6-31G(d,p) basis set. Single-point calculations were carried out at the MP2/6-311G(d,p) level of theory to improve energetic properties. The embedded cluster model suggests that the initial step of the Beckmann rearrangement is not the O-protonated oxime but the N-protonated oxime. The energy barriers derived from the proton shuttle of the N-bound to the O-bound isomer are determined to be approximately 99 and approximately 40 kJ/mol for the embedded cluster and quantum cluster approaches, respectively. The difference in the activation energy is due mainly to the effect of the Madelung potential from the zeolite framework. The next step is the rearrangement step, which is the transformation of the O-protonated oxime to be an enol-formed amide compound, formimidic acid. The activation energy, at the rearrangement step, is calculated to be approximately 125 and approximately 270 kJ/mol for the embedded cluster and quantum cluster approaches, respectively. The final step is the tautomerization step which transforms the enol-form to the keto-form, formamide compound. The energy barrier for tautomerization is calculated to be 123 and 151 kJ/mol for the embedded cluster and quantum cluster approaches, respectively. These calculated results suggest that the rate-determining step of the vapor phase of the Beckmann rearrangement on H-ZSM-5 is the rearrangement or tautomerization step.     

An experimental and theoretical study of the reactions OIO+NO and OIO+OH

The kinetics of the reaction OIO+NO were studied by pulsed laser photolysis/time-resolved cavity ring-down spectroscopy, yielding k(235-320 K)=7.6(+4.0)(-3.1) x 10(-13) exp[(607+/-128)/T] cm3 molecule-1 s-1. Quantum calculations on the OIO+NO potential-energy surface show that the reactants form a weakly bound OIONO intermediate, which then dissociates to the products IO+NO2. Rice-Ramsberger-Kassel-Markus (RRKM) calculations on this surface are in good accord with the experimental result. The most stable potential product, IONO2, cannot form because of the significant rearrangement of OIONO that would be required. The reaction OIO+OH was then investigated by quantum calculations of the relevant stationary points on its potential-energy surface. The very stable HOIO2 molecule can form by direct recombination, but the bimolecular reaction channels to HO2+IO and HOI+O2 are closed because of significant energy barriers. RRKM calculations of the HOIO2 recombination rate coefficient yield krec,0=1.5x10(-27) (T/300 K)(-3.93) cm6 molecule-2 s-1, krec,infinity=5.5x10(-10) exp(46/T) cm3 molecule-1 s-1, and Fc=0.30. The rate coefficients of both reactions are fast enough around 290 K and 1 atm pressure for these reactions to play a potentially important role in the gas phase and aerosol chemistry in the marine boundary layer of the atmosphere.     

New insights on reaction dynamics from formaldehyde photodissociation

We review the photodissociation dynamics of formaldehyde with an emphasis on recent calculations that make use of a global ab initio-based potential energy surface for the S(0) state. These calculations together with recent experiments reveal striking departures from conventional transition state theory for the formation of the molecular products H(2) + CO. The evidence for this departure is reviewed in detail by examining properties of the new potential surface and results of quasiclassical trajectory dynamics calculations using this surface. We also review very recent work on the dynamics governing the formation of radical products, H + HCO. These products can be formed on the T(1) surface as well as the S(0) one, and we present some results contrasting the dynamics on these two surfaces. This work makes use of a new semi-global ab initio-based T(1) potential energy surface.     

Energy selective excision of CN- following electron attachment to hexafluoroacetone azine ((CF3)2C=N-N=C(CF3)2)

Low energy electron attachment (DEA) to hexafluoroacetone azine (HFAA) leads to a remarkable energy selective excision of CN(-) within a pronounced resonance located at 1.35 eV. The underlying dissociative electron attachment (DEA) reaction involves multiple bond cleavages and rearrangement within the neutral products. A series of further fragment ions (F(-), CF(3)(-), (CF(3))(2)C(-) and (CF(3))(2)CN(-)) are observed from resonant features above 2 eV and only (CF(3))(2)CN(-) is additionally formed within a narrow resonance below 1 eV. In contrast to CN(-) all the remaining fragment ions can be formed by simple bond cleavages with (CF(3))(2)CN(-) being the result of a symmetric decomposition of the target molecule by cleavage of the (N-N) bond with the excess charge localised on either of the identical fragments. Our ab initio calculations predict an adiabatic electron affinity of HFAA close to 2 eV with the geometry of the relaxed anion considerably distorted with respect to that of the neutral molecule.