Computational study on the mechanisms and energetics of trimethylindium reactions with H2O and H2S

The reactions of trimethylindium (TMIn) with H2O and H2S are relevant to the chemical vapor deposition of indium oxide and indium sulfide thin films. The mechanisms and energetics of these reactions in the gas phase have been investigated by density functional theory and ab initio calculations using the CCSD(T)/[6-31G(d,p)+Lanl2dz]//B3LYP/[6-31G(d,p)+Lanl2dz] and CCSD(T)/[6-31G(d,p)+Lanl2dz] //MP2/[6-31G(d,p)+Lanl2dz] methods. The results of both methods are in good agreement for the optimized geometries and relative energies. When TMIn reacts with H2O and H2S, initial molecular complexes [(CH3)3In:OH2 (R1)] and [(CH3)3In:SH2 (R2)] are formed with 12.6 and 3.9 kcal/mol binding energies. Elimination of a CH4 molecule from each complex occurs with a similar energy barrier at TS1 (19.9 kcal/mol) and at TS3 (22.1 kcal/mol), respectively, giving stable intermediates (CH3)2InOH and (CH3)2InSH. The elimination of the second CH4 molecule from these intermediate products, however, has to overcome very high and much different barriers of 66.1 and 53.2 kcal/mol, respectively. In the case of DMIn with H2O and H2S reactions, formation of both InO and InS is exothermic by 3.1 and 30.8 kcal/mol respectively. On the basis of the predicted heats of formation of R1 and R2 at 0 K and -20.1 and 43.6 kcal/mol, the heats of formation of (CH3)2InOH, (CH3)2InSH, CH3InO, CH3InS, InO, and InS are estimated to be -20.6, 31.8, and 29.0 and 48.4, 35.5, and 58.5 kcal/mol, respectively. The values for InO and InS are in good agreement with available experimental data. A similar study on the reactions of (CH3)2In with H2O and H2S has been carried out; in these reactions CH3InOH and CH3InSH were found to be the key intermediate products.     

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.     

Mechanisms of the reactions of W AND W+ with H2O: computational studies

The mechanism of the reactions of W and W(+) with the water molecule have been studied for several lower-lying electronic states of tungsten centers at the CCSD(T)/6-311G(d,p)+SDD and B3LYP/6-31G(d,p)+SDD levels of theory. It is shown that these reactions are essentially multistate processes, during which lower-lying electronic states of the systems cross several times. They start with the formation of initial prereaction M(H(2)O) complexes with M-H(2)O bonding energies of 9.6 and 48.2 kcal/mol for M = W and W(+), followed by insertion of the metal center into an O-H bond with 20.0 and 53.3 kcal/mol barriers for neutral and cationic systems, respectively. The overall process of M + H(2)O --> t-HM(OH) is calculated to be highly exothermic, 48.4 and 48.8 kcal/mol for M = W and W(+). From the HM(OH) intermediate the reaction may proceed via several different channels, among which the stepwise HM(OH) --> HMO + H --> (H)(2)MO and concerted HM(OH) --> (H)(2)MO pathways are more favorable and can compete (energetically) with each other. For the neutral system (M = W), the concerted process is the most favorable, whereas for the charged system (M = W(+)), the stepwise pathway is slightly more favorable. From the energetically most favorable intermediate (H)(2)MO the reactions proceed via H(2)-molecule formation with a 53.1 kcal/mol activation barrier for the neutral system. For the cationic system, H-H formation and dissociation is an almost barrierless process. The overall reaction of W and W(+) with the water molecule leading to H(2) + MO formation is found to be exothermic by 48.2 and 39.8 kcal/mol, respectively. In the gas phase with the collision-less conditions the reactions W((7)S) + H(2)O --> H(2) + WO((3)Sigma(+)), and W(+)((6)D) + H(2)O --> H(2) + WO(+)((4)Sigma(+)) are expected to proceed via a 10.4 and 5.1 kcal/mol overall energy barrier corresponding to the first O-H dissociation at the TS1. On the basis of these PESs, we predict kinetic rate constants for the reactions of W and W(+) with H(2)O.     

Theoretical study of hydrogen abstraction and sulfur insertion in the reaction H2S + S

The reaction of H2S + S has been characterized at the multireference configuration interaction level with the geometries optimized using the aug-cc-pVTZ basis set and the single-point energy calculated using the aug-cc-pV(Q+d)Z basis set. As in the analogous reaction of H2 + S, the presence of an intersystem crossing enables products (SH + SH) to be formed on the singlet surface through S insertion, which bypasses the triplet barrier (19.1 kJ mol-1 relative to SH + SH) of the H abstraction route. This provides theoretical evidence for SH + SH formation without barrier beyond endothermicity at sufficiently low temperatures. The H abstraction route, however, is expected to be competitive at higher temperatures due to a much higher Arrhenius pre-exponential factor (6.9 x 10(14) cm3 mol-1 s-1 derived from TST calculation) than that of S insertion channel (3.7 x 10(13) cm3 mol-1 s-1, derived by a least-squares fit to the measurements). With a slightly higher transition-state barrier than that of the H abstraction channel, the production of S2 + H2 is less favored due to proceeding via intersystem crossing and insertion. While the formation of HSS + H is energetically unfavorable relative to SH + SH, recombination channels producing H2SS or the more stable HSSH are expected to occur under collisional stabilization conditions at high pressures.     

Mechanisms of the reactions of W and W+ with NOx (x=1, 2): a computational study

The mechanisms of the reactions of W and W+ with NOx (x=1, 2) were studied at the CCSD(T)/[SDD+6-311G(d)]//B3LYP/[SDD+6-31G(d)] level of theory. It was shown that the insertion pathway of the reaction W(7S)+NO2(2A1) is a multistate process, which involves several lower lying electronic states of numerous intermediates and transition states, and leads to oxidation, WO(3Sigma)+NO(2Pi), and/or nitration, WN(4Sigma)+O2(3Sigmag-), of the W-center. Oxidation products WO(3Sigma)+NO(2Pi) lie 87.6 kcal/mol below the reactants, while the nitration channel is only 31.0 kcal/mol exothermic. Furthermore, it was shown that nitration of W with NO2 is kinetically less favorable than its oxidation. The addition-dissociation pathway of the reaction W(7S)+NO2(2A1) proceeds via the octet (ground) state potential energy surface of the reaction, requires 3.3 kcal/mol barrier, and leads exclusively to oxidation products. Calculations show that oxidation of the W+ cation by NO2 is a barrierless process in the gas phase, proceeds exclusively via the insertion pathway, and is exothermic by 82.9 kcal/mol. The nitration of W+ by NO2 is only 14.1 kcal/mol exothermic and could be accessible only under high-temperature conditions. Reactions of M=W/W+ with NO are also barrierless processes in the gas phase and lead to the N-O insertion product NMO, which are 105.4 and 77.4 kcal/mol lower than the reactants for W and W+, respectively.     

The mechanisms of the reactions of W and W+ with COx (x=1, 2): a computational study

The mechanisms of the reactions of W and W+ with COx (x=1, 2) were studied at the CCSD(T)/[SDD+6-311G(d)]//B3LYP/[SDD+6-31G(d)] level of theory. It was shown that the gas-phase reaction of W with CO2 proceeds with a negligible barrier via an insertion pathway, W(7S)+CO2(1A1)-->W(eta2-OCO)(6A')-->OW(eta1-CO)(1A)-->WO (3Sigma+)+CO(1Sigma). This oxidation process is calculated to be exothermic by 32.4 kcal/mol. Possible intermediates of this reaction are the W(eta2-OCO) and OWCO complexes, among which the latter is 37.4 kcal/mol more stable and lies 39.7 and 7.3 kcal/mol lower than the reactants, W(7S)+CO2(1A1), and the products, WO (3Sigma+)+CO(1Sigma), respectively. The barrier separating W(eta2-OCO) from OWCO is 8.0 kcal/mol (relative to the W(eta2-OCO) complex), which may be characterized as a W+delta-(CO2)-delta charge-transfer complex. Ionization of W does not change the character of the reaction of W with CO2: the reaction of W+ with CO2, like its neutral analog, proceeds via an insertion pathway and leads to oxidation of the W-center. The overall reaction W+(6D) + CO2(1A1)-->W(eta1-OCO)+(6A)-->OW(eta1-CO)+(4A)-->WO+(4Sigma+)+CO(1Sigma) is calculated to be exothermic by 25.4 kcal/mol. The cationic reaction proceeds with a somewhat large (9.9 kcal/mol) barrier and produces two intermediates, W(eta1-OCO)+(6A) and OW(eta1-CO)+(4A). Intermediate W(eta1-OCO)+(6A) is 20.0 kcal/mol less stable than OW(eta1-CO)+(4A), and separated from the latter by a 35.2 kcal/mol barrier. Complex W(eta1-OCO)+(6A) is characterized as an ion-molecular complex type of W+-(CO2). Gas-phase reactions of M=W/W+ with CO lead to the formation of a W-carbonyl complex M(eta1-CO) for both M=W and W+. The C-O insertion product, OMC, lies by 5.2 and 69.3 kcal/mol higher than the corresponding M(eta1-CO) isomer, for M=W and W+, respectively, and is separated from the latter by a large energy barrier.     

Formation of a Criegee intermediate in the low-temperature oxidation of dimethyl sulfoxide

Dimethyl sulfoxide (DMSO) is the major sulfur-containing constituent of the Marine Boundary Layer. It is a significant source of H2SO4 aerosol/particles and methane sulfonic acid via atmospheric oxidation processes, where the mechanism is not established. In this study, several new, low-temperature pathways are revealed in the oxidation of DMSO using CBS-QB3 and G3MP2 multilevel and B3LYP hybrid density functional quantum chemical methods. Unlike analogous hydrocarbon peroxy radicals the chemically activated DMSO peroxy radical, [CH3S(=O)CH2OO*]*, predominantly undergoes simple dissociation to a methylsulfinyl radical CH3S*(=O) and a Criegee intermediate, CH2OO, with the barrier to dissociation 11.3 kcal mol(-1) below the energy of the CH3S(=O)CH2* + O2 reactants. The well depth for addition of O2 to the CH3S(=O)CH2 precursor radical is 29.6 kcal mol(-1) at the CBS-QB3 level of theory. We believe that this reaction may serve an important role in atmospheric photochemical and irradiated biological (oxygen-rich) media where formation of initial radicals is facilitated even at lower temperatures. The Criegee intermediate (carbonyl oxide, peroxymethylene) and sulfinyl radical can further decompose, resulting in additional chain branching. A second reaction channel important for oxidation processes includes formation (via intramolecular H atom transfer) and further decomposition of hydroperoxide methylsulfoxide radical, *CH2S(=O)CH2OOH over a low barrier of activation. The initial H-transfer reaction is similar and common in analogous hydrocarbon radical + O2 reactions; but the subsequent very low (3-6 kcal mol(-1)) barrier (14 kcal mol(-1) below the initial reagents) to beta-scission products is not common in HC systems. The low energy reaction of the hydroperoxide radical is a beta-scission elimination of *CH2S(=O)CH2OOH into the CH2=S=O + CH2O + *OH product set. This beta-scission barrier is low, because of the delocalization of the *CH2 radical center through the -S(=O) group, to the -CH2OOH fragment in the transition state structure. The hydroperoxide methylsulfoxide radical can also decompose via a second reaction channel of intramolecular OH migration, yielding formaldehyde and a sulfur-centered hydroxymethylsulfinyl radical HOCH2S*(=O). The barrier of activation relative to initial reagents is 4.2 kcal mol(-1). Heats of formation for DMSO, DMSO carbon-centered radical and Criegee intermediate are evaluated at 298 K as -35.97 +/- 0.05, 13.0 +/- 0.2 and 25.3 +/- 0.7 kcal mol(-1) respectively using isodesmic reaction analysis. The [CH3S*(=O) + CH2OO] product set is shown to form a van der Waals complex that results in O-atom transfer reaction and the formation of new products CH3SO2* radical and CH2O. Proper orientation of the Criegee intermediate and methylsulfinyl radical, as a pre-stabilized pre-reaction complex, assist the process. The DMSO radical reaction is also compared to that of acetonyl radical.     

Aromaticity and activation strain analysis of [3 + 2] cycloaddition reactions between group 14 heteroallenes and triple bonds

We have computationally explored the trend in reactivity of [3 + 2] cycloaddition reactions between H(2)E=C=PH and HC≡CH as the terminal position in the phosphaallene is varied along E = C, Si, Ge, Sn, Pb. The reaction barrier drops significantly from E = C (nearly 50 kcal/mol) to E = Si-Pb (ca. 20 kcal/mol). Activation strain analyses tie this trend to a reduction in activation strain in the heavier phosphaallene analogues which, in contrast to the parent compound H(2)C=C=PH, do already possess the bent geometry required in the TS.     

Ab initio study of the two competitive channels HO2 + H → H2 + O2 and HO2 + H → H2O + O

Saddle point geometries and barrier heights have been calculated for the H abstraction reaction HO₂(²A″)+H(²S) → H₂(¹Σ⁺_g)+O₂(³Σ⁻_g) and the concerted H approach-O removing reaction HO₂ (²A″)+H(²S) → H₂O(¹A₁)+O(³P) by using SDCI wavefunctions with a valence double-zeta plus polarization basis set. The saddle points are found to be of C_s symmetry and the barrier heights are respectively 5.3 and 19.8 kcal by including size consistent correction. Moreoever kinetic parameters have been evaluated within the framework of the TST theory. So activation energies and the rate constants are estimated to be respectively 2.3 kcal and 0.4×10⁹ ℓ mol⁻¹ s⁻¹ for the first reaction, 20.0 kcal and 5.4.10⁻⁵ ℓ mol⁻¹ s⁻¹ for the second. Comparison of these results with experimental determinations shows that hydrogen abstraction on HO₂ is an efficient mechanism for the formation of H₂ + O₂, while the concerted mechanism envisaged for the formation of H₂O + O is highly unlikely.     

Model identity SN2 reactions CH3X + X- (X = F, Cl, CN, OH, SH, NH2, PH2): Marcus theory analyzed

The structures of seven gas phase identity S(N)2 reactions of the form CH(3)X + X(-) have been characterized with seven distinct theoretical methods: RHF, B3LYP, BLYP, BP86, MP2, CCSD, and CCSD(T), in conjunction with basis sets of double and triple zeta quality. Additionally, the energetics of said reactions have been definitively computed using focal point analyses utilizing extrapolation to the one-particle limit for the Hartree-Fock and MP2 energies using basis sets of up to aug-cc-pV5Z quality, inclusion of higher order correlation effects [CCSD and CCSD(T)] with basis sets of aug-cc-pVTZ quality, and additional auxiliary terms for core correlation and scalar relativistic effects. Final net activation barriers for the reactions are E(b)(F,F)= -0.8, E(b)(Cl,Cl)= 1.6, E(b)(CN,CN)= 28.7, E(b)(OH,OH)= 14.3, E(b)(SH,SH)= 13.8, E(b)(NH2,NH2)= 28.6, and E(b)(PH2,PH2)= 25.7 kcal mol(-1). General trends in the energetics, specifically the performance of the density functionals, and the component energies of the focal point analyses are discussed. The utility of classic Marcus theory as a technique for barrier predictions has been carefully analyzed. The standard Marcus theory results show disparities of up to 9 kcal mol(-1) with respect to explicitly computed results. However, when alternative approaches to Marcus theory, independent of the well-depths, are considered, excellent performance is achieved, with the largest deviations being under 3 kcal mol(-1).     

Quantum chemical study of adsorption and dissociation of H2S on the gallium-rich GaAs (001)-4 x 2 surface

H(2)S adsorption and dissociation on the gallium-rich GaAs(001)-4 x 2 surface is investigated using hybrid density functional theory. Starting from chemisorbed H(2)S on the GaAs(001)-4 x 2 surface, two possible reaction routes have been proposed. We find that H(2)S adsorbs molecularly onto GaAs(001)-4 x 2 via the formation of a dative bond, and this process is exothermic with adsorption energy of 6.6 kcal/mol. For the first reaction route, one of the H atoms from the chemisorbed H(2)S is transferred to a second-layer As atom and the dissociated SH is inserted into the Ga-As bond with an activation barrier of 8.2 kcal/mol, which is found to be 29.3 kcal/mol more stable than the reactants. For the second case, the dissociated species may insert themselves into the Ga-Ga dimer resulting in the Ga-H-Ga and Ga-HS-Ga bridge-bonded states, which are found to be 29.8 and 22.2 kcal/mol more stable than the reactants, respectively. However, the calculations also show that the activation barrier (16.1 kcal/mol) for chemisorbed H(2)S dissociation through the second route is higher than the transfer of one H atom into a second-layer As atom. As a result, we conclude that sulfur insertion into the Ga-As bond is more kinetically favorable.     

A DFT computational study of the bis-silylation reaction of acetylene catalyzed by palladium complexes

In this paper we have investigated at the DFT(B3LYP) level the catalytic cycle for the bis-silylation reaction of alkynes promoted by palladium complexes. A model-system formed by an acetylene molecule, a disilane molecule, and the Pd(PH(3))(2) complex has been used. The most relevant features of this catalytic cycle can be summarized as follows: (i) The first step of the cycle is an oxidative addition involving H(3)Si-SiH(3) and Pd(PH(3))(2). It occurs easily and leads to the cis (SiH(3))(2)Pd(PH(3))(2) complex that is 5.39 kcal mol(-1) lower in energy than reactants. (ii) The transfer of the two silyl groups to the C-C triple bond does not occur in a concerted way, but involves many steps. (iii) The cis (SiH(3))(2)Pd(PH(3))(2) complex, obtained from the oxidative addition, is involved in the formation of the first C-Si bond (activation barrier of 18.34 kcal mol(-1)). The two intermediates that form in this step cannot lead directly to the formation of the final bis(silyl)ethene product. However, they can isomerize rather easily (the two possible isomerizations have a barrier of 16.79 and 7.17 kcal mol(-1)) to new more stable species. In both these new intermediates the second silyl group is adjacent to the acetylene moiety and the formation of the second C-Si bond can occur rapidly leading to the (Z)-bis(silyl)ethene, as experimentally observed. (iv) The whole catalytic process is exothermic by 41.54 kcal mol(-1), in quite good agreement with the experimental estimate of this quantity (about 40 kcal mol(-1)).     

Establishment of the C(2)H(5)+O(2) reaction mechanism: a combustion archetype

The celebrated C(2)H(5)+O(2) reaction is an archetype for hydrocarbon combustion, and the critical step in the process is the concerted elimination of HO(2) from the ethylperoxy intermediate (C(2)H(5)O(2)). Master equation kinetic models fitted to measured reaction rates place the concerted elimination barrier 3.0 kcal mol(-1) below the C(2)H(5)+O(2) reactants, whereas the best previous electronic structure computations yield a barrier more than 2.0 kcal mol(-1) higher. We resolve this discrepancy here by means of the most rigorous computations to date, using focal point methods to converge on the ab initio limit. Explicit computations were executed with basis sets as large as cc-pV5Z and correlation treatments as extensive as coupled cluster through full triples with a perturbative inclusion of quadruple excitations [CCSDT(Q)]. The final predicted barrier is -3.0 kcal mol(-1), bringing the concerted elimination mechanism into precise agreement with experiment. This work demonstrates that higher correlation treatments such as CCSDT(Q) are not only feasible on systems of chemical interest but are necessary to supply accuracy beyond 0.5 kcal mol(-1), which is not obtained with the "gold standard" CCSD(T) method. Finally, we compute the enthalpy of formation of C(2)H(5)O(2) to be Delta(f)H degrees (298 K)=-5.3+/-0.5 kcal mol(-1) and Delta(f)H degrees (0 K)=-1.5+/-0.5 kcal mol(-1).     

Hydrogen storage in LiAlH4: predictions of the crystal structures and reaction mechanisms of intermediate phases from quantum mechanics

We use the density functional theory and x-ray and neutron diffraction to investigate the crystal structures and reaction mechanisms of intermediate phases likely to be involved in decomposition of the potential hydrogen storage material LiAlH(4). First, we explore the decomposition mechanism of monoclinic LiAlH(4) into monoclinic Li(3)AlH(6) plus face-centered cubic (fcc) Al and hydrogen. We find that this reaction proceeds through a five-step mechanism with an overall activation barrier of 36.9 kcal/mol. The simulated x ray and neutron diffraction patterns from LiAlH(4) and Li(3)AlH(6) agree well with experimental data. On the other hand, the alternative decomposition of LiAlH(4) into LiAlH(2) plus H(2) is predicted to be unstable with respect to that through Li(3)AlH(6). Next, we investigate thermal decomposition of Li(3)AlH(6) into fcc LiH plus Al and hydrogen, occurring through a four-step mechanism with an activation barrier of 17.4 kcal/mol for the rate-limiting step. In the first and second steps, two Li atoms accept two H atoms from AlH(6) to form the stable Li-H-Li-H complex. Then, two sequential H(2) desorption steps are followed, which eventually result in fcc LiH plus fcc Al and hydrogen: Li(3)AlH(6)(monoclinic)-->3 LiH(fcc)+Al(fcc)+3/2 H(2) is endothermic by 15.8 kcal/mol. The dissociation energy of 15.8 kcal/mol per formula unit compares to experimental enthalpies in the range of 9.8-23.9 kcal/mol. Finally, we explore thermal decomposition of LiH, LiH(s)+Al(s)-->LiAl(s)+12H(2)(g) is endothermic by 4.6 kcal/mol. The B32 phase, which we predict as the lowest energy structure for LiAl, shows covalent bond characters in the Al-Al direction. Additionally, we determine that transformation of LiH plus Al into LiAlH is unstable with respect to transformation of LiH through LiAl.     

Radical-molecule reactions HCO/HOC + C2H2: mechanistic study

A detailed computational study is performed on the unknown radical-molecule reactions between HCO/HOC and acetylene (C2H2) at the CCSD(T)/6-311G(2d,p)//B3LYP/6-311G(d,p)+ZPVE, Gaussian-3//B3LYP/6-31G(d), and Gaussian-3//MP2(full)/6-31G(d) levels. For the HCO + C2H2 reaction, the most favorable pathway is direct C-addition forming the intermediate HC=CHCH=O followed by a 1,3-H-shift leading to H2C=CHC=O, which finally dissociates to the product C2H3 + CO. The overall reaction barrier is 13.8, 10.5, and 11.3 kcal/mol, respectively, at the three levels. The quasi-direct H-donation process to produce C2H3 + CO with barriers of 14.0, 14.1, and 14.1 kcal/mol is less competitive. Thus only at higher temperatures could the HCO + C2H2 reaction play a role. In contrast, the HOC + C2H2 reaction can barrierlessly generate C2H3 + CO via the quasi-direct H-donation mechanism proceeding via a prereactive complex with OH...C2 hydrogen bonding. This is suggestive of the potential importance of the HOC + C2H2 reaction in both combustion and interstellar processes. However, the direct C-addition channel is much less competitive. For both reactions, the possible formation of the intriguing interstellar molecules propadiene and propynal is also discussed. The present theoretical study represents the first attempt to probe the reaction mechanism between HOC and pi-systems. Future laboratory investigations on both reactions (particularly HOC + C2H2) are recommended.     

On the formation of radical dications of protonated amino acids in a "microsolution" of water or acetonitrile and their reactivity towards the solvent

In high-energy collisions (50 keV) between O2 and protonated amino acids AH+, radical dications AH2+* are formed for A = Phe, His, Met, Tyr, and Trp. When solvated by water or acetonitrile (S), AH2+*(S)1,2 are formed for A = Arg, His, Met, Tyr, and Trp. The stability of the hydrogen-deficient AH2+* in the "microsolution" depends on the energetics of the electron transfer reaction AH2+* +S --> AH++S+*, the hydrogen abstraction reaction AH2+*+S --> AH2(2+)+[S-H]*, and the proton transfer reaction AH2+* + S --> A+*+SH+. Using B3LYP/ 6-311+G(2d,p)//B3LYP/6-31+G(d) model chemistry, we describe these three reactions in detail for A=Tyr and find that the first two reactions are unfavorable whereas the third one is favorable. However, energy is required for the formation of Tyr+* and SH+ from TyrH2+*(S) to overcome the Coulomb barrier, which renders the complex observable with a life-time larger than 5 micros. The ionization energy, IE, of TyrH+ is calculated to be 11.1 eV in agreement with an experimental measurement of 10.1+/-2.1 eV ([IE(CH3CN)+IE(Tyr)]/ 2); hydration further lowers the IE by 0.3 eV [IE(TyrH+(H2O) = 10.8 eV, calculated]. We estimate the ionization energies of TrpH+, HisH+, and MetH+ to be 10.1+/-2.1 eV, 12.4+/-0.2 eV, and 12.4+/-0.2 eV, and that of PheH+ to be larger than 12.6 eV.     

A two-state computational investigation of methane C--H and ethane C--C oxidative addition to [CpM(PH3)]n+ (M = Co, Rh, Ir; n = 0, 1)

Reductive elimination of methane from methyl hydride half-sandwich phosphane complexes of the Group 9 metals has been investigated by DFT calculations on the model system [CpM(PH(3))(CH(3))(H)] (M = Co, Rh, Ir). For each metal, the unsaturated product has a triplet ground state; thus, spin crossover occurs during the reaction. All relevant stationary points on the two potential energy surfaces (PES) and the minimum energy crossing point (MECP) were optimized. Spin crossover occurs very near the sigma-CH(4) complex local minimum for the Co system, whereas the heavier Rh and Ir systems remain in the singlet state until the CH(4) molecule is almost completely expelled from the metal coordination sphere. No local sigma-CH(4) minimum was found for the Ir system. The energetic profiles agree with the nonexistence of the Co(III) methyl hydride complex and with the greater thermal stability of the Ir complex relative to the Rh complex. Reductive elimination of methane from the related oxidized complexes [CpM(PH(3))(CH(3))(H)](+) (M = Rh, Ir) proceeds entirely on the spin doublet PES, because the 15-electron [CpM(PH(3))](+) products have a doublet ground state. This process is thermodynamically favored by about 25 kcal mol(-1) relative to the corresponding neutral system. It is essentially barrierless for the Rh system and has a relatively small barrier (ca. 7.5 kcal mol(-1)) for the Ir system. In both cases, the reaction involves a sigma-CH(4) intermediate. Reductive elimination of ethane from [CpM(PH(3))(CH(3))(2)](+) (M = Rh, Ir) shows a similar thermodynamic profile, but is kinetically quite different from methane elimination from [CpM(PH(3))(CH(3))(H)](+): the reductive elimination barrier is much greater and does not involve a sigma-complex intermediate. The large difference in the calculated activation barriers (ca. 12.0 and ca. 30.5 kcal mol(-1) for the Rh and Ir systems, respectively) agrees with the experimental observation, for related systems, of oxidatively induced ethane elimination when M = Rh, whereas the related Ir systems prefer to decompose by alternative pathways.     

Comprehensive theoretical studies on the gas phase SN2 reactions of anionic nucleophiles toward chloroamine and N-chlorodimethylamine with inversion and retention mechanisms

The anionic S(N)2 reactions at neutral nitrogen, Nu(-) + NR(2)Cl → NR(2)Nu + Cl(-) (R = H, Me; Nu = F, Cl, Br, OH, SH, SeH, NH(2), PH(2), AsH(2)) have been systematically studied computationally at the modified G2(+) level. Two reaction mechanisms, inversion and retention of configuration, have been investigated. The main purposes of this work are to explore the reactivity trend of anions toward NR(2)Cl (R = H, Me), the steric effect on the potential energy surfaces, and the leaving ability of the anion in S(N)2@N reactions. Our calculations indicate that the complexation energies are determined by the gas basicity (GB) of the nucleophile and the electronegativity (EN) of the attacking atom, and the overall reaction barrier in the inversion pathway is basically controlled by the GB value of the nucleophile. The retention pathway in the reactions of NR(2)Cl with Nu(-) (Nu = F, Cl, Br, OH, SH, SeH) is energetically unfavorable due to the barriers being larger than those in the inversion pathway by more than 120 kJ mol(-1). Activation strain model analyses show that a higher deformation energy and a weaker interaction between deformed reactants lead to higher overall barriers in the reactions of NMe(2)Cl than those in the reactions of NH(2)Cl. Our studies on the reverse process of the title reactions suggest that the leaving ability of the anion in the gas phase anionic S(N)2@N reactions is mainly determined by the strength of the N-LG bond, which is related to the negative hyperconjugation inherent in NR(2)Nu (R = H, Me; Nu = HO, HS, HSe, NH(2), PH(2), AsH(2)).     

Mechanistic studies of platinum(II)-catalyzed ethylene dimerization: determination of barriers to migratory insertion in diimine Pt(II) hydrido ethylene and ethyl ethylene intermediates

The diimine platinum(II) ethylene hydride complex [(N/\N)Pt(H)(ethylene)][BAr'4] (1, N/\N = [(2,6-Me2C6H3)N=C(An)-C(An)=N(2,6-Me2C6H3)], An = 1,8-naphthalenediyl, Ar' = 3,5-(CF3)2C6H3) was prepared by protonation of the diethyl complex (N/\N)PtEt2 with [H(OEt2)2][BAr'4]. The energy barrier to interchange of the platinum hydride with the olefinic hydrogens in 1 was determined to be 19.2 kcal/mol by spin saturation transfer experiments. Complex 1 initiates ethylene dimerization; the ethyl ethylene complex (N/\N)Pt(Et)(ethylene)+ (2) has been identified as the catalyst resting state. Trapping of 1 by ethylene to yield 2 is a second-order process; kinetic studies suggest this occurs via trapping of a reversibly formed beta-agostic ethyl complex. Complex 2 has been isolated and characterized by X-ray crystallography. The barrier to migratory insertion of 2, the turnover-limiting step in catalysis, was determined to be 29.8 kcal/mol. The 1-butene hydride complex, (N/\N)Pt(H)(1-butene)+ (3), is a key intermediate in the dimerization cycle and has also been isolated and characterized. Surprisingly rapid rates of degenerate associative exchange of free ethylene with bound ethylene in complexes 1 and 2 as well as the rate of degenerate exchange of free nitrile with bound nitrile in (N/\N)Pt(Et)(CH3CN)+ are reported.     

Theoretical study of rhodium(III)-catalyzed hydrogenation of carbon dioxide into formic acid. Significant differences in reactivity among rhodium(III), rhodium(I), and ruthenium(II) complexes

The title reaction was theoretically investigated, where cis-[RhH(2)(PH(3))(3)](+) and cis-[RhH(2)(PH(3))(2)(H(2)O)](+) were adopted as models of the catalyst. The first step of the catalytic cycle is the CO(2) insertion into the Rh(III)-H bond, of which the activation barrier (E(a)) is 47.2 and 28.4 kcal/mol in cis-[RhH(2)(PH(3))(3)](+) and cis-[RhH(2)(PH(3))(2)(H(2)O)](+), respectively, where DFT(B3LYP)-calculated E(a) values (kcal/mol unit) are given hereafter. These results indicate that an active species is not cis-[RhH(2)(PH(3))(3)](+) but cis-[RhH(2)(PH(3))(2)(H(2)O)](+). After the CO(2) insertion, two reaction courses are possible. In one course, the reaction proceeds through isomerization (E(a) = 2.8) of [RhH(eta(1)- OCOH)(PH(3))(2)(H(2)O)(2)](+), five-centered H-OCOH reductive elimination (E(a) = 2.7), and oxidative addition of H(2) to [Rh(PH(3))(2)(H(2)O)(2)](+) (E(a) = 5.8). In the other one, the reaction proceeds through isomerization of [RhH(eta(1)-OCOH)(PH(3))(2)(H(2)O)(H(2))](+) (E(a) = 5.9) and six-centered sigma-bond metathesis of [RhH(eta(1)-OCOH)(PH(3))(2)(H(2)O)](+) with H(2) (no barrier). RhH(PH(3))(2)-catalyzed hydrogenation of CO(2) proceeds through CO(2) insertion (E(a) = 1.6) and either the isomerization of Rh(eta(1)-OCOH)(PH(3))(2)(H(2)) (E(a) = 6.1) followed by the six-centered sigma-bond metathesis (E(a) = 0.3) or H(2) oxidative addition to Rh(eta(1)-OCOH)(PH(3))(2) (E(a) = 7.3) followed by isomerization of RhH(2)(eta(1)-OCOH)(PH(3))(2) (E(a) = 6.2) and the five-centered H-OCOH reductive elimination (E(a) = 1.9). From these results and our previous results of RuH(2)(PH(3))(4)-catalyzed hydrogenation of CO(2) (J. Am. Chem. Soc. 2000, 122, 3867), detailed discussion is presented concerning differences among Rh(III), Rh(I), and Ru(II) complexes.