Stepwise hydration of ionized aromatics. Energies, structures of the hydrated benzene cation, and the mechanism of deprotonation reactions

The stepwise binding energies (DeltaHdegree(n-1,n)) of 1-8 water molecules to benzene(.+) [Bz(.+)(H2O)n] were determined by equilibrium measurements using an ion mobility cell. The stepwise hydration energies, DeltaHdegree(n-1,n), are nearly constant at 8.5 +/- 1 kcal mol-1 from n = 1-6. Calculations show that in the n = 1-4 clusters, the benzene(.+) ion retains over 90% of the charge, and it is extremely solvated, that is, hydrogen bonded to an (H2O)n cluster. The binding energies and entropies are larger in the n = 7 and 8 clusters, suggesting cyclic or cage-like water structures. The concentration of the n = 3 cluster is always small, suggesting that deprotonation depletes this ion, consistent with the thermochemistry since associative deprotonation Bz(.+)(H2O)(n-1) + H2O-->C6H5. + (H2O)nH+ is thermoneutral or exothermic for n > or = 4. Associative intracluster proton transfer Bz(.+)(H2O)(n+1) + H2O-->C6H5.(H2O)nH+ would also be exothermic for n > or = 4, but lack of H/D exchange with D2O shows that the proton remains on C6H6(.+) in the observed Bz(.+)(H2O)n clusters. This suggests a barrier to intracluster proton transfer, and as a result, the [Bz(.+)(H2O)n]* activated complexes either undergo dissociative proton transfer, resulting in deprotonation and generation of (H2O)nH+, or become stabilized. The rate constant for the deprotonation reaction shows a uniquely large negative temperature coefficient of K = cT(-67+/-4) (or activation energy of -34+/- 1 kcal mol-1), caused by a multibody mechanism in which five or more components need to be assembled for the reaction.     

Formation and decomposition of chemically activated cyclopentoxy radicals from the c-C5H9 + O reaction

The formation and the decomposition of chemically activated cyclopentoxy radicals from the c-C5H9 + O reaction have been studied in the gas phase at room temperature. Two different experimental arrangements have been used. Arrangement A consisted of a laser-flash photolysis set up combined with quantitative Fourier transform infrared spectroscopy and allowed the determination of the stable products at 4 mbar. The c-C5H9 radicals were produced via the reaction c-C5H10 + Cl with chlorine atoms from the photolysis of CFCl3; the O atoms were generated by photolysis of SO2. Arrangement B, a conventional discharge flow-reactor with molecular beam sampling, was used to determine the rate coefficient. Here, the hydrocarbon radicals (c-C5H9, C2H5, CH2OCH3) were produced via the reaction of atomic fluorine with c-C5H10, C2H6, and CH3OCH3, respectively, and detected by mass spectrometry after laser photoionization. For the c-C5H9 + O reaction, the relative contributions of intermediate formation (c-C5H9O) and direct abstraction (c-C5H8 + OH) were found to be 68 +/- 5 and 32 +/- 4%, respectively. The decomposition products of the chemically activated intermediate could be identified, and the following relative branching fractions were obtained: c-C5H8O + H (31 +/- 2%), CH2CH(CH2)2CHO + H (40 +/- 5%), 2 C2H4 + H + CO (17 +/- 5%), and C3H4O + C2H4 + H (12 +/- 5%). Additionally, the product formation of the c-C5H8 + O reaction was studied, and the following relative yields were obtained (mol %): C2H4, 24%; C3H4O, 18%; c-C5H8O, 30%; c-C5H8O, 23%; 4-pentenal, 5%. The rate coefficient of the c-C5H9 + O reaction was determined relative to the reactions C2H5 + O and CH3OCH2 + O leading to k = (1.73 +/- 0.05) x 10(14) cm3 mol(-1) s(-1). The experimental branching fractions are analyzed in terms of statistical rate theory with molecular and transition-state data from quantum chemical calculations, and high-pressure limiting Arrhenius parameters for the unimolecular decomposition reactions of C5H9O species are derived.     

Clusters of hydrated methane sulfonic acid CH3SO3H.(H2O)n (n = 1-5): a theoretical study

Ab initio and density functional methods have been used to examine the structures and energetics of the hydrated clusters of methane sulfonic acid (MSA), CH3SO3H.(H2O)n (n = 1-5). For small clusters with one or two water molecules, the most stable clusters have strong cyclic hydrogen bonds between the proton of OH group in MSA and the water molecules. With three or more water molecules, the proton transfer from MSA to water becomes possible, forming ion-pair structures between CH3SO3- and H3O+ moieties. For MSA.(H2O)3, the energy difference between the most stable ion pair and neutral structures are less than 1 kJ/mol, thus coexistence of neutral and ion-pair isomers are expected. For larger clusters with four and five water molecules, the ion-pair isomers are more stable (>10 kJ/mol) than the neutral ones; thus, proton transfer takes place. The ion-pair clusters can have direct hydrogen bond between CH3SO3- and H3O+ or indirect one through water molecule. For MSA.(H2O)5, the energy difference between ion pairs with direct and indirect hydrogen bonds are less than 1 kJ/mol; namely, the charge separation and acid ionization is energetically possible. The calculated IR spectra of stable isomers of MSA.(H2O)n clusters clearly demonstrate the significant red shift of OH stretching of MSA and hydrogen-bonded OH stretching of water molecules as the size of cluster increases.     

Ab initio and direct dynamics studies of the reaction of singlet methylene with acetylene and the lifetime of the cyclopropene complex

The energetics of the (1)CH(2) + C(2)H(2) --> H + C(3)H(3) reaction are accurately calculated using an extrapolated coupled-cluster/complete basis set (CBS) method based on the cc-pVDZ, cc-pVTZ, and cc-pVQZ basis sets. The reaction enthalpy (0 K) is predicted to be -20.33 kcal/mol. This reaction has no classical barrier in either the entrance or exit channel. However, there are several stable intermediates-cyclopropene (c-C(3)H(4)), allene (CH(2)CCH(2)), and propyne (CH(3)CCH)-along the minimum energy path. These intermediates with zero-point energy corrections lie below the reactants by 87.11 (c-C(3)H(4)), 109.69 (CH(2)CCH(2)), and 110.78 kcal/mol (CH(3)CCH). The vibrationally adiabatic ground-state (VAG) barrier height for c-C(3)H(4) isomerization to allene is obtained as 45.2 kcal/mol, and to propyne as 37.2 kcal/mol. In addition, the (1)CH(2) + C(2)H(2) reaction is investigated utilizing the dual-level "scaling all correlation" (SAC) ab initio method of Truhlar et al., i.e., the UCCSD(SAC)/cc-pVDZ theory. Results show that the reaction occurs via long-lived complexes. The lifetime of the cyclopropene intermediate is obtained as 3.2 +/- 0.4 ps. It is found that the intermediate propyne can be formed directly from reactants through the insertion of (1)CH(2) into a C-H bond of C(2)H(2). However, compared to the major mechanism in which the propyne is produced through a ring-opening of the cyclopropene complex, this reaction pathway is much less favorable. Finally, the theoretical thermal rate constant exhibits a negative temperature dependence, which is in excellent agreement with the previous results. The temperature dependence is consistent with the earlier RRKM results but weaker than the experimental observations at high temperatures.     

Mechanism of the hydration of carbon dioxide: direct participation of H2O versus microsolvation

Thermochemical parameters of carbonic acid and the stationary points on the neutral hydration pathways of carbon dioxide, CO 2 + nH 2O --> H 2CO 3 + ( n - 1)H 2O, with n = 1, 2, 3, and 4, were calculated using geometries optimized at the MP2/aug-cc-pVTZ level. Coupled-cluster theory (CCSD(T)) energies were extrapolated to the complete basis set limit in most cases and then used to evaluate heats of formation. A high energy barrier of approximately 50 kcal/mol was predicted for the addition of one water molecule to CO 2 ( n = 1). This barrier is lowered in cyclic H-bonded systems of CO 2 with water dimer and water trimer in which preassociation complexes are formed with binding energies of approximately 7 and 15 kcal/mol, respectively. For n = 2, a trimeric six-member cyclic transition state has an energy barrier of approximately 33 (gas phase) and a free energy barrier of approximately 31 (in a continuum solvent model of water at 298 K) kcal/mol, relative to the precomplex. For n = 3, two reactive pathways are possible with the first having all three water molecules involved in hydrogen transfer via an eight-member cycle, and in the second, the third water molecule is not directly involved in the hydrogen transfer but solvates the n = 2 transition state. In the gas phase, the two transition states have comparable energies of approximately 15 kcal/mol relative to separated reactants. The first path is favored over in aqueous solution by approximately 5 kcal/mol in free energy due to the formation of a structure resembling a (HCO 3 (-)/H 3OH 2O (+)) ion pair. Bulk solvation reduces the free energy barrier of the first path by approximately 10 kcal/mol for a free energy barrier of approximately 22 kcal/mol for the (CO 2 + 3H 2O) aq reaction. For n = 4, the transition state, in which a three-water chain takes part in the hydrogen transfer while the fourth water microsolvates the cluster, is energetically more favored than transition states incorporating two or four active water molecules. An energy barrier of approximately 20 (gas phase) and a free energy barrier of approximately 19 (in water) kcal/mol were derived for the CO 2 + 4H 2O reaction, and again formation of an ion pair is important. The calculated results confirm the crucial role of direct participation of three water molecules ( n = 3) in the eight-member cyclic TS for the CO 2 hydration reaction. Carbonic acid and its water complexes are consistently higher in energy (by approximately 6-7 kcal/mol) than the corresponding CO 2 complexes and can undergo more facile water-assisted dehydration processes.     

Theoretical prediction of the heats of formation of C2H5O* radicals derived from ethanol and of the kinetics of beta-C-C scission in the ethoxy radical

Thermochemical parameters of three C(2)H(5)O* radicals derived from ethanol were reevaluated using coupled-cluster theory CCSD(T) calculations, with the aug-cc-pVnZ (n = D, T, Q) basis sets, that allow the CC energies to be extrapolated at the CBS limit. Theoretical results obtained for methanol and two CH(3)O* radicals were found to agree within +/-0.5 kcal/mol with the experiment values. A set of consistent values was determined for ethanol and its radicals: (a) heats of formation (298 K) DeltaHf(C(2)H(5)OH) = -56.4 +/- 0.8 kcal/mol (exptl: -56.21 +/- 0.12 kcal/mol), DeltaHf(CH(3)C*HOH) = -13.1 +/- 0.8 kcal/mol, DeltaHf(C*H(2)CH(2)OH) = -6.2 +/- 0.8 kcal/mol, and DeltaHf(CH(3)CH(2)O*) = -2.7 +/- 0.8 kcal/mol; (b) bond dissociation energies (BDEs) of ethanol (0 K) BDE(CH(3)CHOH-H) = 93.9 +/- 0.8 kcal/mol, BDE(CH(2)CH(2)OH-H) = 100.6 +/- 0.8 kcal/mol, and BDE(CH(3)CH(2)O-H) = 104.5 +/- 0.8 kcal/mol. The present results support the experimental ionization energies and electron affinities of the radicals, and appearance energy of (CH(3)CHOH+) cation. Beta-C-C bond scission in the ethoxy radical, CH(3)CH2O*, leading to the formation of C*H3 and CH(2)=O, is characterized by a C-C bond energy of 9.6 kcal/mol at 0 K, a zero-point-corrected energy barrier of E0++ = 17.2 kcal/mol, an activation energy of Ea = 18.0 kcal/mol and a high-pressure thermal rate coefficient of k(infinity)(298 K) = 3.9 s(-1), including a tunneling correction. The latter value is in excellent agreement with the value of 5.2 s(-1) from the most recent experimental kinetic data. Using RRKM theory, we obtain a general rate expression of k(T,p) = 1.26 x 10(9)p(0.793) exp(-15.5/RT) s(-1) in the temperature range (T) from 198 to 1998 K and pressure range (p) from 0.1 to 8360.1 Torr with N2 as the collision partners, where k(298 K, 760 Torr) = 2.7 s(-1), without tunneling and k = 3.2 s(-1) with the tunneling correction. Evidence is provided that heavy atom tunneling can play a role in the rate constant for beta-C-C bond scission in alkoxy radicals.     

Hydrogen atom transfer reactions of C2-, C4-, and C6-: bond dissociation energies of linear H-C2n- and H-C2n (n = 1, 2, 3)

The reactions of C2-, C4-, and C6- with D2O and ND3 and of C4- with CH3OH, CH4, and C2H6 have been investigated using guided ion beam tandem mass spectrometry. Hydrogen (or deuterium) atom transfer is the major product channel for each of the reactions. The reaction threshold energies for collisional activation are reported. Several of the reactions exhibit threshold energies in excess of the reaction endothermicity. Potential energy calculations using density functional theory show energy barriers for some of the reactions. Dynamic restrictions related to multiple wells along the reaction path may also contribute to elevated threshold energies. The results indicate that the reactions with D2O have the smallest excess threshold energies, which may therefore be used to derive lower limits on the C-H bond dissociation energies of the C2nH- and C2nH (n = 1-3) linear species. The experimental lower limits for the bond dissociation energies of the neutral radicals to linear products are D0(C2-H) >or= 460 +/- 15 kJ/mol, D0(C4-H) >or= 427 +/- 12 kJ/mol, and D0(C6-H) >or= 405 +/- 11 kJ/mol.     

Negative-ion photoelectron spectroscopy, gas-phase acidity, and thermochemistry of the peroxyl radicals CH(3)OO and CH(3)CH(2)OO

Methyl, methyl-d(3), and ethyl hydroperoxide anions (CH(3)OO(-), CD(3)OO(-), and CH(3)CH(2)OO(-)) have been prepared by deprotonation of their respective hydroperoxides in a stream of helium buffer gas. Photodetachment with 364 nm (3.408 eV) radiation was used to measure the adiabatic electron affinities: EA[CH(3)OO, X(2)A' '] = 1.161 +/- 0.005 eV, EA[CD(3)OO, X(2)A' '] = 1.154 +/- 0.004 eV, and EA[CH(3)CH(2)OO, X(2)A' '] = 1.186 +/- 0.004 eV. The photoelectron spectra yield values for the term energies: Delta E(X(2)A' '-A (2)A')[CH(3)OO] = 0.914 +/- 0.005 eV, Delta E(X(2)A' '-A (2)A')[CD(3)OO] = 0.913 +/- 0.004 eV, and Delta E(X(2)A' '-A (2)A')[CH(3)CH(2)OO] = 0.938 +/- 0.004 eV. A localized RO-O stretching mode was observed near 1100 cm(-1) for the ground state of all three radicals, and low-frequency R-O-O bending modes are also reported. Proton-transfer kinetics of the hydroperoxides have been measured in a tandem flowing afterglow-selected ion flow tube (FA-SIFT) to determine the gas-phase acidity of the parent hydroperoxides: Delta(acid)G(298)(CH(3)OOH) = 367.6 +/- 0.7 kcal mol(-1), Delta(acid)G(298)(CD(3)OOH) = 367.9 +/- 0.9 kcal mol(-1), and Delta(acid)G(298)(CH(3)CH(2)OOH) = 363.9 +/- 2.0 kcal mol(-1). From these acidities we have derived the enthalpies of deprotonation: Delta(acid)H(298)(CH(3)OOH) = 374.6 +/- 1.0 kcal mol(-1), Delta(acid)H(298)(CD(3)OOH) = 374.9 +/- 1.1 kcal mol(-1), and Delta(acid)H(298)(CH(3)CH(2)OOH) = 371.0 +/- 2.2 kcal mol(-1). Use of the negative-ion acidity/EA cycle provides the ROO-H bond enthalpies: DH(298)(CH(3)OO-H) = 87.8 +/- 1.0 kcal mol(-1), DH(298)(CD(3)OO-H) = 87.9 +/- 1.1 kcal mol(-1), and DH(298)(CH(3)CH(2)OO-H) = 84.8 +/- 2.2 kcal mol(-1). We review the thermochemistry of the peroxyl radicals, CH(3)OO and CH(3)CH(2)OO. Using experimental bond enthalpies, DH(298)(ROO-H), and CBS/APNO ab initio electronic structure calculations for the energies of the corresponding hydroperoxides, we derive the heats of formation of the peroxyl radicals. The "electron affinity/acidity/CBS" cycle yields Delta(f)H(298)[CH(3)OO] = 4.8 +/- 1.2 kcal mol(-1) and Delta(f)H(298)[CH(3)CH(2)OO] = -6.8 +/- 2.3 kcal mol(-1).     

Imaging the dynamics of reactions between Cl atoms and the cyclic ethers oxirane and oxetane

Direct current slice velocity map ion images of the HCl(nu' = 0, J') products from the photoinitiated reactions of ground state Cl atoms with ethane, oxirane (c-C2H4O), and oxetane (c-C3H6O), at respective mean collision energies of 5.5, 6.5, and 7.3 kcal mol-1(-1), were analyzed using a Legendre moment fitting procedure. The experimental method and the fitting technique were tested by comparing the derived center-of-mass (CM) frame angular scattering distribution for the HCl(v' = 0, J' = 1) products from the reaction of Cl + C2H6 with those determined by Suits and co-workers from a crossed molecular beam experiment. For the Cl + c-C2H4O reaction, a broad, forward, and backward peaking CM frame angular distribution of HCl(nu' = 0, J' = 2) products was determined, with an average fraction of the available energy released as product translational energy of f t, equal to 0.52 +/- 0.18. The HCl consumes only 1% of the available energy, and conservation arguments dictate that the radical coproduct is significantly internally excited, corresponding to an average fraction of the available energy of f int(c-C2H3O), equal to 0.47 +/- 0.18. For the reaction of oxetane with Cl atoms, abstraction of H atoms is possible from carbon atoms from positions either alpha or beta to the O atom. The contributions to the reaction from these two H-atom abstraction channels were estimated to be 63 and 37%, consistent with an unbiased propensity for removal of alpha- and beta-H atoms that are present in 2:1 abundance. The angular scatter of products in the CM frame is also broad and forward-backward peaking and is reminiscent of the products of the Cl + CH3OH and CH3OCH3 reactions. The derived mean fraction of the available energy channelled into product translation is f t = 0.54 +/- 0.12 for each of the two abstraction pathways. With only a small amount of energy in the rotation of the HCl(nu' = 0), the remainder is accounted for by excitation of the radical coproduct internal modes, with f int(c-C3H5O) = 0.42 +/- 0.12 for both alpha- and beta-H abstraction. The broad product scattering in the CM frame observed for both reactions of Cl atoms with the cyclic ethers is consistent with reactive collisions over a wide range of impact parameters, as might be expected for barrierless reactions with loose transition states.     

Predicting the energy of the water exchange reaction and free energy of solvation for the uranyl ion in aqueous solution

The structures and vibrational frequencies of UO2(H2O)4(2+) and UO2(H2O)5(2+) have been calculated using density functional theory and are in reasonable agreement with experiment. The energies of various reactions were calculated at the density functional theory (DFT) and MP2 levels; the latter provides the best results. Self-consistent reaction field calculations in the PCM and SCIPCM approximations predicted the free energy of the water exchange reaction, UO2(H2O)4(2+) + H2O <--> UO2(H2O)5(2+). The calculated free energies of reaction are very sensitive to the choice of radii (O and H) and isodensity values in the PCM and SCIPCM models, respectively. Results consistent with the experimental HEXS value of -1.19 +/- 0.42 kcal/mol (within 1-3 kcal/mol) are obtained with small cavities. The structures and vibrational frequencies of the clusters with second solvation shell waters: UO2(H2O)4(H2O)8(2+), UO2(H2O)4(H2O)10(2+), UO2(H2O)4(H2O)11(2+), UO2(H2O)5(H2O)7(2+), and UO2(H2O)5(H2O)10(2+), were calculated and are in better agreement with experiment as compared to reactions involving only UO2(H2O)4(2+) and UO2(H2O)5(2+). The MP2 reaction energies for water exchange gave gas-phase results that agreed with experiment in the range -5.5 to +3.3 kcal/mol. The results were improved by inclusion of a standard PCM model with differences of -1.2 to +2.7 kcal/mol. Rearrangement reactions based on an intramolecular isomerization leading to a redistribution of water in the two shells provide good values in comparison to experiment with values of Delta G(exchange) from -2.2 to -0.5 kcal/mol so the inclusion of a second hydration sphere accounts for most solvation effects. Calculation of the free energy of solvation of the uranyl cation yielded an upper bound to the solvation energy of -410 +/- 5 kcal/mol, consistent with the best experimental value of -421 +/- 15 kcal/mol.     

A photoelectron and TPEPICO investigation of the acetone radical cation

The valence shell photoelectron spectrum, threshold photoelectron spectrum, and threshold photoelectron photoion coincidence (TPEPICO) mass spectra of acetone have been measured using synchrotron radiation. New vibrational progressions have been observed and assigned in the X 2B2 state photoelectron bands of acetone-h6 and acetone-d6, and the influence of resonant autoionization on the threshold electron yield has been investigated. The dissociation thresholds for fragment ions up to 31 eV have been measured and compared to previous values. In addition, kinetic modeling of the threshold region for CH3* and CH4 loss leads to new values of 78 +/- 2 kJ mol(-1) and 75 +/- 2 kJ mol(-1), respectively, for the 0 K activation energies for these two processes. The result for the methyl loss channel is in reasonable agreement with, but slightly lower than, that of 83 +/- 1 kJ mol(-1) derived in a recent TPEPICO study by Fogleman et al. The modeling accounts for both low-energy dissociation channels at two different ion residence times in the mass spectrometer. Moreover, the effects of the ro-vibrational population distribution, the electron transmission efficiency, and the monochromator band-pass are included. The present activation energies yield a Delta(f)H298 for CH3CO+ of 655 +/- 3 kJ mol(-1), which is 4 kJ mol(-1) lower than that reported by Fogleman et al. The present Delta(f)H298 for CH3CO+ can be combined with the Delta(f)H298 for CH2CO (-47.5 +/- 1.6 kJ mol(-1)) and H+ (1530 kJ mol(-1)) to yield a 298 K proton affinity for ketene of 828 +/- 4 kJ mol(-1), in good agreement with the value (825 kJ mol(-1)) calculated at the G2 level of theory. The measured activation energy for CH4 loss leads to a Delta(f)H298 (CH2CO+*) of 873 +/- 3 kJ mol(-1).     

Coupling properties of imidazole dimer radical cation assisted by embedded water molecule: toward understanding of interaction character of hydrogen-bonded histidine residue side-chains

Geometry optimizations are performed at the DFTB3LYP6-311+G* level. Four intriguing coupling modes, totally eight stable structures are found in the potential energy surfaces of the water-assisted coupling of imidazole dimer radical cation. In these isomers, the water molecules are embedded between two imidazole moieties, and the oxygen atom is tridentate or quadridentate, respectively. The distinct redshifts of the vibrational frequencies of the O-H...N and N-H...O type H bonds indicate the strong interaction of two imidazole rings of respective isomer. Inspection of the highest occupied molecular orbital predicts the alterations of the geometry structures on oxidation and reduction. The low barrier of the fragment rotation demonstrates that the isomerization processes by experiencing the distinct transition states are easy to fulfill, especially for those with O-H...N and C-H...O H bonds. Both the energy difference of the 0 degrees-cis and 180 degrees-trans orientation and the barriers of the fragment rotation are lowered by the water assisting. The range of the zero point vibrational energy correction indicates that the influence on the complexes with N-H...O and O-H...N H bonds (0.13-0.17 kcal/mol) is more significant than those with O-H...N and C-H...O H bonds (+/-0.03 kcal/mol). The dissociation energies of these isomers indicate that the charges transfer easily through water in the dissociation process and then are distributed mainly over the imidazole ring connecting with water molecule. The isomer with proton transfer between imidazole fragments is the most stable one.     

The cysteine radical cation: structures and fragmentation pathways

A theoretical study on the structures, relative energies, isomerization reactions and fragmentation pathways of the cysteine radical cation, [NH(2)CH(CH(2)SH)COOH].+, is reported. Hybrid density functional theory (B3LYP) has been used in conjunction with the 6-311++G(d,p) basis set. The isomer at the global minimum, Captodative-1, has the structure NH(2)C.(CH(2)SH)C(OH)(2)+; the stability of this ion is attributed to the captodative effect in which the NH(2) functions as a powerful pi-electron donor and C(OH)(2)+ as a powerful pi-electron acceptor. Ion Distonic-S-1, H(3)N(+)CH(CH(2)S.)COOH, in which the radical is formally situated on the S atom, is higher in enthalpy (DeltaH degrees (0)) than Captodative-1 by 6.1 kcal mol(-1), but is lower in enthalpy than another isomer Distonic-C-1, H(3)N(+)C.(CH(2)SH)COOH, by 8.2 kcal mol(-1). Isomerization of the canonical radical cation of cysteine, [H(2)NCH(CH(2)SH)COOH].+, (Canonical-1), to Captodative-1 has an enthalpy of activation of 25.8 kcal mol(-1), while the barrier against isomerization of Canonical-1 to Distonic-S-1 is only 9.6 kcal mol(-1). Two additional transient tautomers, one with the radical located at C(alpha) and the charge on SH(2), and the other a carboxy radical with the charge on NH(3), are reported. Plausible fragmentation pathways (losses of small molecules, CO(2), CH(2)S, H(2)S and NH(3), and neutral radicals COOH. , HSCH(2). and NH(2).) from Canonical-1 are examined.     

Probing the barrier for CH2CHCO --> CH2CH + CO by the velocity map imaging method

This work determines the dissociation barrier height for CH2CHCO --> CH2CH + CO using two-dimensional product velocity map imaging. The CH2CHCO radical is prepared under collision-free conditions from C-Cl bond fission in the photodissociation of acryloyl chloride at 235 nm. The nascent CH2CHCO radicals that do not dissociate to CH2CH + CO, about 73% of all the radicals produced, are detected using 157-nm photoionization. The Cl(2P(3/2)) and Cl(2P(1/2)) atomic fragments, momentum matched to both the stable and unstable radicals, are detected state selectively by resonance-enhanced multiphoton ionization at 235 nm. By comparing the total translational energy release distribution P(E(T)) derived from the measured recoil velocities of the Cl atoms with that derived from the momentum-matched radical cophotofragments which do not dissociate, the energy threshold at which the CH2CHCO radicals begin to dissociate is determined. Based on this energy threshold and conservation of energy, and using calculated C-Cl bond energies for the precursor to produce CH2CHC*O or C*H2CHCO, respectively, we have determined the forward dissociation barriers for the radical to dissociate to vinyl + CO. The experimentally determined barrier for CH2CHC*O --> CH2CH + CO is 21+/-2 kcal mol(-1), and the computed energy difference between the CH2CHC*O and the C*H2CHCO forms of the radical gives the corresponding barrier for C*H2CHCO --> CH2CH + CO to be 23+/-2 kcal mol(-1). This experimental determination is compared with predictions from electronic structure methods, including coupled-cluster, density-functional, and composite Gaussian-3-based methods. The comparison shows that density-functional theory predicts too low an energy for the C*H2CHCO radical, and thus too high a barrier energy, whereas both the Gaussian-3 and the coupled-cluster methods yield predictions in good agreement with experiment. The experiment also shows that acryloyl chloride can be used as a photolytic precursor at 235 nm of thermodynamically stable CH2CHC*O radicals, most with an internal energy distribution ranging from approximately 3 to approximately 21 kcal mol(-1). We discuss the results with respect to the prior work on the O(3P) + propargyl reaction and the analogous O(3P) + allyl system.     

The effects of water vapor on the CH3O2 self-reaction and reaction with HO2

The gas phase reactions of CH3O2 + CH3O2, HO2 + HO2, and CH3O2 + HO2 in the presence of water vapor have been studied at temperatures between 263 and 303 K using laser flash photolysis coupled with UV time-resolved absorption detection at 220 and 260 nm. Water vapor concentrations were quantified using tunable diode laser spectroscopy operating in the mid-IR. The HO2 self-reaction rate constant is significantly enhanced by water vapor, consistent with what others have reported, whereas the CH3O2 self-reaction and the cross-reaction (CH3O2 + HO2) rate constants are nearly unaffected. The enhancement in the HO2 self-reaction rate coefficient occurs because of the formation of a strongly bound (6.9 kcal mol(-1)) HO2 x H2O complex during the reaction mechanism where the H2O acts as an energy chaperone. The nominal impact of water vapor on the CH3O2 self-reaction rate coefficient is consistent with recent high level ab initio calculations that predict a weakly bound CH3O2 x H2O complex (2.3 kcal mol(-1)). The smaller binding energy of the CH3O2 x H2O complex does not favor its formation and consequent participation in the methyl peroxy self-reaction mechanism.     

Theoretical studies on cycloaddition reactions between the 2-aza-1,3-butadiene cation and olefins

Density functional (B3LYP) calculations, using the 6-31G basis set, have been employed to study the title reactions. For the model reaction (H(2)C=C-NH(+)=CH(2) + H(2)C=CH(2)), a complex has been formed with 6.2 kcal/mol of stabilization energy and the transition state is 4.0 kcal/mol above this complex, but 2.1 kcal/mol below the reactants. However, the substituent effects are quite remarkable. When ethene is substituted by electron-withdrawing group CN, the reaction could also yield six-membered-ring products, but the energy barriers are all more than 7 kcal/mol, which shows that CN group unfavors the reaction. The other substituents, such as CH(3)O and CH(3) groups, have also been considered in the present work, and the results show that they are favorable for the formation of six-membered-ring adducts. The calculated results have been rationalized with frontier orbital interaction and topological analysis.     

Density functional investigation of reaction of borohydride cation BH2+ with propylene

The reactions of BH2+ with propylene (CH2=CHCH3) to form both the adducts BC3H8+ and the H2-elimination products BC3H6+ + H2 have been investigated at the density functional B3LYP/6-311G(d,p) level of theory. It is shown that the electrophilic attacks of BH2+ towards two olefinic carbons of H2C=CHCH3 and two subsequent 1,3-H-shifts may form four low-lying BC3H8+ isomers (with the relative energies in parentheses in kcal/mol): 1 BH2+.CH2CHCH3 (0.0), 1' BH2+.CH3CHCH2 (6.3), 3 BHCH2CH2CH3+ (4.3), and 4 BHCH(CH3)2+ (5.0), respectively. On the other hand, further H2-eliminations may also occur easily between B-C bonds of isomers 1 and 1' and between C-C bonds of isomers 3 and 4 to form two dissociation products (P1) HBCHCHCH3+ + H2 and (P2) HBC(CH3)CH2+ + H2, with H2-elimination from isomer 1 to be energetically most favorable. According to our calculated mechanism, the collisional stabilization processes of low-lying isomers 1, 1', 3, and 4 may compete extensively with their H2-eliminations processes for the title reaction, leading mainly to some linear carborane cations. This study may be helpful for understanding the stereochemical aspects of borohydride cations towards alkylenes.     

The structures and electronic states of zinc-water clusters Zn(n)(H2O)(m) (n = 1-32 and m = 1-3)

Ab initio and Density Functional Theory (DFT) calculations have been carried out for zinc-water clusters Zn(n)-(H2O)(m) (n = 1-32 and m = 1-3, where n and m are the numbers of zinc atoms and water molecules, respectively) to elucidate the structure and electronic states of the clusters and the interaction of zinc cluster with water molecules. The binding energies of H2O to zinc clusters were small at n = 2-3 (2.3-4.2 kcal mol(-1)), whereas the energy increased significantly in n = 4 (9.0 kcal mol(-1)). Also, the binding nature of H2O was changed at n = 4. The cluster size dependency of the binding energy of H2O accorded well with that of the natural population of electrons in the 4p orbital of the zinc atom. In the larger clusters (n > 20), it was found that the zinc atoms in surface regions of the zinc cluster have a positive charge, whereas those in the interior region have a negative charge with the large electron population in the 4p orbital. The interaction of H2O with the zinc clusters were discussed on the basis of the theoretical results.     

Unimolecular rate constants for HX or DX elimination (X = F, Cl) from chemically activated CF3CH2CH2Cl, C2H5CH2Cl, and C2D5CH2Cl: threshold energies for HF and HCl elimination

Vibrationally activated CF(3)CH(2)CH(2)Cl molecules were prepared with 94 kcal mol(-1) of vibrational energy by the combination of CF(3)CH(2) and CH(2)Cl radicals and with 101 kcal mol(-1) of energy by the combination of CF(3) and CH(2)CH(2)Cl radicals at room temperature. The unimolecular rate constants for elimination of HCl from CF(3)CH(2)CH(2)Cl were 1.2 x 10(7) and 0.24 x 10(7) s(-1) with 101 and 94 kcal mol(-1), respectively. The product branching ratio, k(HCl)/k(HF), was 80 +/- 25. Activated CH(3)CH(2)CH(2)Cl and CD(3)CD(2)CH(2)Cl molecules with 90 kcal mol(-1) of energy were prepared by recombination of C(2)H(5) (or C(2)D(5)) radicals with CH(2)Cl radicals. The unimolecular rate constant for HCl elimination was 8.7 x 10(7) s(-1), and the kinetic isotope effect was 4.0. Unified transition-state models obtained from density-functional theory calculations, with treatment of torsions as hindered internal rotors for the molecules and the transition states, were employed in the calculation of the RRKM rate constants for CF(3)CH(2)CH(2)Cl and CH(3)CH(2)CH(2)Cl. Fitting the calculated rate constants from RRKM theory to the experimental values provided threshold energies, E(0), of 58 and 71 kcal mol(-1) for the elimination of HCl or HF, respectively, from CF(3)CH(2)CH(2)Cl and 54 kcal mol(-1) for HCl elimination from CH(3)CH(2)CH(2)Cl. Using the hindered-rotor model, threshold energies for HF elimination also were reassigned from previously published chemical activation data for CF(3)CH(2)CH(3,) CF(3)CH(2)CF(3), CH(3)CH(2)CH(2)F, CH(3)CHFCH(3), and CH(3)CF(2)CH(3). In an appendix, the method used to assign threshold energies was tested and verified using the combined thermal and chemical activation data for C(2)H(5)Cl, C(2)H(5)F, and CH(3)CF(3).     

Cooperative interaction of H3O+ with 1,3-alternate tetrapropoxycalix[4]arene: NMR and theoretical study

Interaction of H3O+ or H5O2+ with 1,3-alternate tetrapropoxycalix[4]arene (1) was studied in nitrobenzene and dichloromethane using 1H and 13C NMR including transverse and rotating-frame relaxations and density functional level of theory (DFT) quantum calculations. According to NMR, the ion forms an equimolecular complex with 1 with the equilibrium constant K being 3.97 x 10(3) L.mol(-1) at 296 K. The ions are bound by strong hydrogen bonds to the phenoxy-oxygen atoms of one half of 1 and by a medium-strong hydrogen bond to the pi system of the aromatic rings of the other half. The complex appears to have C(4h) symmetry in NMR even when cooling its solution down to 213 K, which could be due either to a genuine symmetry of the complex (if the ion is H5O2+) or to fast structure averaging by ion exchange processes (if the ion is H3O+). Therefore, the dynamics of the system was studied. Using two independent NMR methods (transverse and rotating-frame relaxation), two different exchange processes were discerned with correlation times 25 x 10(-6) and 5 x 10(-6) s, the first being clearly intermolecular and the other being apparently intramolecular. The energetic aspects of the possible exchange processes were examined by DFT quantum calculations. Rotation of H3O+ ion within one binding site with the energy barrier 8.13 kcal/mol is easily possible. Intermolecular exchange by freeing the ion from the complex has too high a barrier but cooperative interaction of the ion with additional water molecules makes it viable. The intramolecular exchange (or hopping) of the H3O+ ion between the two sites of the molecule is not viable in the classical manner, the barrier being 25.6 kcal/mol. Quantum tunneling of the ion is highly improbable, too. Alternative mechanisms including concerted two-ion intermolecular exchange and cooperative interaction with another bound water molecule including complexation with proton dihydrate H5O2+ are discussed.