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The pioneering spectroscopic observations of the methylzinc hydride [HZnCH3(X1A1)] molecule were reported previously by the Ziurys group [J. Am. Chem. Soc. 2010, 132, 17186-17192], and the possible formation mechanisms were suggested therein, including those with the participation of excited zinc atoms in reaction with methane. Herein, the ground singlet state and the lowest excited triplet state potential energy surfaces of the Zn + CH4 reaction have been explored using high-level electronic structure calculations with multireference second-order perturbation theory and coupled cluster singles and doubles with perturbative triples (CCSD(T)) methods in conjunction with all-electron basis sets (up to aug-cc-pV5Z) and scalar relativistic effects incorporated via the second-order Douglas-Kroll-Hess (DK) method. Based on the ab initio results, a plausible scenario for the formation of HZnCH3(X1A1) is proposed involving the activation of the C-H bond of methane by the lowest excited 3P state atomic zinc. Calculations also highlight the importance of an agostic-like Zn···H-C interactions in the pre-activation complex and good agreement between the structure of the HZnCH3(X1A1) molecule predicted at the DK-CCSD(T)/aug-cc-pVQZ-DK level of theory and that derived from rotational spectroscopy, as well as the discrepancies between the ab initio and density functional theory predictions.

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We have applied the CCSD(T)-F12a/cc-pVTZ-F12//CCSD(T)/cc-pVTZ level of theory to calculate energies for 22 reactions pertinent to the stability and reactivity of hardly isolable cyanoform (HC(CN)3). A number of exothermic processes has been indicated, especially the hydration. In the predicted mechanism for the gas-phase hydration of cyanoform, the H2O addition to the C≡N bond corresponds to a rate-limiting step, which is aided by an extra molecule of water. Also, for the cyanoform dihydrate (H2NC(OH)C(CN)CONH2) product, the experimentally identified compound, the more stable planar isomer exhibits intramolecular O-H···O═C (not N-H···O═C) H-bonding. Our calculated structures, binding energies, and NBO data for [HC(CN)3]n (n = 2,4) clusters suggest that the non-conventional C-H···N H-bonds contribute to their stability. Among the surveyed structures of the C≡N group incorporating products of reactions examined, the CCSD(T)/cc-pVTZ molecular parameters of cyanocarbons C2(CN)4, C2(CN)6, and C6(CN)6 can be regarded as the most accurate gas-phase values up-to-date.

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Although an isolation of elusive tricyanomethane HC(CN)3 was recently reported, the existence of other HC4N3 species has yet to be confirmed. In this work, the relative stabilities, spectroscopic features, and rearrangements of tricyanomethane and its four isomers are examined using single- (CCSD(T), CCSD(T)-F12) and multireference (MCSCF, MRPT2) methods. Tricyanomethane and dicyanoketenimine (NC)2CCNH, which are found to be the two most stable HC4N3 isomers lying within 9 kcal/mol, can be discriminated by their spectroscopic parameters. The predicted stepwise interconversion path relating HC(CN)3 and (NC)2CCNH features the HC4N3 species comprising the C-C-N ring moiety, with the largest barrier being associated with the initial H migration to one of the CN carbons. Adding a water molecule reduces the H migration barrier strongly and makes it possible to interconvert tricyanomethane to dicyanoketenimine in a "concerted" way.

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We present a comprehensive benchmark computational study which has explored a complete path of the anomerization reaction of bare d-erythrose involving a pair of the low-energy α- and ß-furanose anomers, the former of which was observed spectroscopically (Cabezas et al., Chem. Commun. 2013, 49, 10826). We find that the ring opening of the α-anomer yields the most stable open-chain tautomer which step is followed by the rotational interconversion of the open-chain rotamers and final ring closing to form the ß-anomer. Our results indicate the flatness of the reaction's potential energy surface (PES) corresponding to the rotational interconversion path and its sensitivity to the computational level. By using the explicitly correlated coupled cluster CCSD(T)-F12/cc-pVTZ-F12 energies, we determine the free energy barrier for the α-furanose ring-opening (rate-determining) step as 170.3 kJ/mol. The question of the number of water molecules (n) needed for optimal stabilization of the erythrose anomerization reaction rate-determining transition state is addressed by a systematic exploration of the PES of the ring opening in the α-anomer-(H2 O)n and various ß-anomer-(H2 O)n (n = 1-3) clusters using density functional and CCSD(T)-F12 computations. These computations suggest the lowest free energy barrier of the ring opening for doubly hydrated α-anomer, achieved by a mechanism that involves water-mediated multiple proton transfer coupled with the furanose CO bond breakage. Among the methods used, the G4 performed best against the CCSD(T)-F12 reference at estimating the ring-opening barrier heights for both the hydrated and bare erythrose conformers. Our results for the hydrated species are most relevant to an experimental study of the anomerization reaction of d-erythrose to be carried out in microsolvation environment. © 2016 Wiley Periodicals, Inc.

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We present a detailed mechanistic study on the interaction and reaction of water monomer and water dimer with the smallest 3D aluminum particle (Al6) by employing density functional and explicitly correlated coupled cluster CCSD(T)-F12 theories. Water adsorption, dissociation, and dehydrogenation are considered. For the monomer reaction, where core-valence correlation and an extrapolation to the complete basis set limit is allowed for, our coupled cluster calculations predict the O-H dissociation barrier of about 2 kcal/mol. For the dimer reaction, two distinct reaction paths are identified, initiated by forming separate dimer complexes wherein (H2O)2 adsorbs mainly via the oxygen atom of the donor H2O molecule. The key O-H dissociation transition states of the dimer reaction involve a concerted migration of two H atoms resulting in the dissociation of the donor molecule and formation of the OH-water complex adsorbed on the metal cluster's surface. The most remarkable feature of both dimer reaction energy profiles is the lack of the overall energy barrier for the (rate-determining) O-H dissociation. The hydrogen bond acceptor molecule is suggested to have an extra catalytic effect on the O-H dissociation barrier of the hydrogen bond donor molecule by removing this barrier. A similar effect on the dehydrogenation step is indicated.

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D-Erythrose is a C4 monosaccharide with a biological and potential astrobiological relevance. We have investigated low-energy structures of d-erythrose and their interconversion in the gas phase with the highest-level calculations up-to-date. We have identified a number of structurally distinct furanose and open-chain isomers and predicted α ↔ α and ß â†” ß furanose interconversion pathways involving the O-H rotamers. We have estimated relative Gibbs free energies of the erythrose species based on the CCSD(T)/aug-cc-pVTZ electronic energies and MP2/aug-cc-pVTZ vibrational frequencies. By using natural bond orbital theory we have also quantified a stabilization of erythrose conformers and interconversion transition states by intramolecular H-bonds.

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The gas-phase reaction of the Al6(+) cation with a water molecule is investigated computationally by coupled cluster and density functional theories. Several low-energy paths of the mechanism for dihydrogen production from H2O by the positively charged aluminum cluster are identified. This reaction involves the initial formation of the association complex, exothermic by 25 kcal/mol, followed by the water dissociation and H2 elimination major steps, yielding the Al6O(+) product oxide with either the nonplanar or planar structure. The H2O dissociation on Al6(+) is the rate-determining step. Of the paths probed, the one kinetically most preferred leads from the O-H bond dissociation transition state lying below the separated reactants to the immediate HAl6OH(+) intermediate of the "open" type and involves further the more compact intermediate from which H2 is eliminated. The other reaction paths explored involve the activation enthalpy (at 0 K) for the rate-determining step of less than 2 kcal/mol relative to the Al6(+) + H2O. Natural population analysis based charges indicate that forming of H2 along the elimination coordinate is facilitated by the interaction of the hydridic and protic hydrogens. For the kinetically most favorable route detected, the coupled cluster singles and doubles with perturbative triples (CCSD(T)) relative energies calculated with the unrestricted and restricted HF references are in a good agreement. This investigation is relevant specifically to the recent mass spectrometric study of the reactivity of Aln(+) with water by Arakawa et al., and it provides a mechanistic insight into the formation of the observed AlnO(+) product oxide with n = 6.

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We performed large-scale second-order perturbation theory gas-phase calculations to study about five hundred structures of D-fructose. The two lowest energy fructose structures identified are ß-pyranoses possessing (2)C5 chair, with ΔG(298 K) of 6 kJ/mol, differing in orientation of the equatorially positioned hydroxymethyl group, gt and g'g, where the gt rotamer is the global minimum, consistent with the recent microwave spectroscopy study. We have found that interconversions from the fructose global minimum to the second and third most stable ß-pyranose rotamers involve the energy barriers of ca. 30 kJ/mol. Among numerous fructofuranose conformers discovered (about 250), a pair of the ((3)T2) α- and (E3) ß-anomers are energetically most preferred and lie at least 12 kJ/mol above the global minimum. We also found that the fructose open-chain structures lie significantly higher in energy than the most stable cyclic species. The commonly used M06-2X density functional performs well compared to MP2 and G4 theory at identifying the low-energy fructose minima, including the global one, and at reproducing their intramolecular H-bond geometric parameters. The lowest-energy gas-phase pyranose and furanose structures of fructose benefit from stabilization due to the cooperative or quasi-linear H-bonding and both endo and exo anomeric effects.

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We present an extensive computational study of a complex conformational isomerism of two gas phase pentoses of biological and potential astrobiological importance, d-ribose and 2-deoxy-d-ribose. Both cyclic (α- and ß-pyranoses, α- and ß-furanoses) and open-chain isomers have been probed using second order Møller-Plesset perturbation theory (MP2), M06-2X density functional, and multi-level G4 methods. This study revealed a multitude of existing minima structures. Numerous furanose conformers found are described with the Altona and Sundaralingam pseudorotation parameters. In agreement with the recent gas-phase microwave (MW) investigation of Cocinero et al., the calculated free ribose isomers of lowest energy are the two ß-pyranoses with the (1)C4 and (4)C1 ring chair conformations. Both ß-pyranoses lie within 0.9kJ/mol in terms of ΔG(298K) (G4), thus challenge the computational methods used to predict the ribose global minimum. The calculated most favoured ribofuranose is the α-anomer having the twist (2)T1 ring conformation, put 10.4kJ/mol higher in ΔG than the global minimum. By contrast with d-ribose, the lowest energy 2-deoxy-d-ribose is the α-pyranose, with the most stable 2-deoxy-d-furanose (the α-anomer) being only 6.2kJ/mol higher in free energy. For both pentoses, the most favoured open-chain isomers are significantly higher in energy than the low-lying cyclic forms. A good overall agreement is observed between the M06-2X and MP2 results in terms of both the existing low-energy minima structures and intramolecular H-bonding geometrical parameters. The natural orbital analysis confirms the occuring of the endo- and exo-anomeric effects and maximization of intramolecular H-bonding in the lowest-lying pyranoses and furanoses of both sugars.

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We have investigated the lowest triplet and singlet potential energy surfaces (PESs) for the reaction of Ga(2) dimer with water. Under thermal conditions, we predict formation of the triplet ground state addition complex Ga(2)···OH(2)((3)B(1)) involving Ga···O···Ga bridge interaction. At the coupled cluster CCSD(T)/AE (CCSD(T)/ECP) computational levels, Ga(2)···OH(2)((3)B(1)) is bound by 5.5 (5.7) kcal/mol with respect to the ground state reactants Ga(2)((3)Π(u)) + H(2)O. Identification of the addition complex is in agreement with the experimental evidence from matrix isolation infrared (IR) spectroscopy reported recently by Macrae and Downs. The located minimum energy crossing points (MECPs) between the triplet and singlet energy surfaces on the entrance channel of Ga(2) + H(2)O are not expected to be energetically accessible under the matrix conditions, consistent with the lack of occurrence of Ga(2) insertion into the O-H bond under such conditions. The computed energies and harmonic and anharmonic vibrational frequencies for the triplet and singlet Ga(2)(H)(OH) insertion isomers indicate the singlet double-bridged Ga(µ-H)(µ-OH)Ga isomer to be the most stable and support the experimental IR identification of this species. The energy barrier for elimination of H(2) from the second most stable singlet HGa(µ-OH)Ga insertion isomer found to be 13.9 (12.9) kcal/mol is also consistent with the available experimental data.

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A comprehensive ab initio investigation using coupled cluster theory with the aug-cc-pVnZ, n = D,T basis sets is carried out to identify distinct structures of the Al(4)H(14)(-) cluster anion and to evaluate its fragmentation stability. Both thermodynamic and mechanistic aspects of the fragmentation reactions are studied. The observation of this so far the most hydrogenated aluminum tetramer was reported in the recent mass spectrometry study of Li et al. (2010) J Chem Phys 132:241103-241104. The four Al(4)H(14)(-) anion structures found are chain-like with the multiple-coordinate Al center and can be viewed approximately as comprising Al(2)H(7)(-) and Al(2)H(7) moieties. Locating computationally some of the Al(4)H(14)(-) minima on the correlated ab initio potential energy surfaces required the triple-zeta quality basis set to describe adequately the Al multi-coordinate bonding. For the two most stable Al(4)H(14)(-) isomers, the mechanism of their low-barrier interconversion is described. The dissociation of Al(4)H(14)(-) into the Al(2)H(7)(-) and Al(2)H(7) units is predicted to require 20-22 (10-13) kcal mol(-1) in terms of ΔH (ΔG) estimated at T = 298.15 K and p = 1 atm. However, Al(4)H(14)(-) is found to be a metastable species in the gas phase: the H(2) loss from the radical moiety of its most favorable isomer is exothermic by 18 kcal mol(-1) in terms of ΔH (298.15 K) and by 25 kcal mol(-1) in terms of ΔG(298.15 K), with the enthalpic/free energy barrier involved being less than 1 kcal mol(-1). By contrast with alane Al(4)H(14)(-), only a weakly bound complex between Ga(4)H(12)(-) and H(2) has been identified for the gallium analogue using the relativistic effective core potential.

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We have investigated the mechanism of the reaction of H abstraction from n-butanol by the hydroxyl radical (HO*) using high level ab initio methods in conjunction with the correlation consistent basis sets up to quadruple-zeta quality (cc-pVQZ). This reaction is of significance in the atmosphere and combustion. The focus of the study has been on the relative importance of the abstractions from the specific n-butanol sites and on the role of reaction intermediates involved. Our results show that abstractions from the C(alpha) and C(gamma) positions are kinetically most favored and nearly barrierless, with barrier height estimates of 0.10 and 0.47 kcal/mol, respectively, at the CCSD(T)/cc-pVQZ level. We have determined that the indicated barrier height order, C(alpha) < C(gamma) < C(beta) < C(delta) < OH, parallels that for the n-butanol bond dissociation energies established recently. The kinetically and thermodynamically most favored C(alpha) abstraction occurs via a mechanism including the formation of the n-butanol...HO* prereaction complex. The weakly bound postreaction complexes between the product radicals and H(2)O have been identified for all the specific site abstraction reactions, with their calculated CCSD(T) binding energies of up to about 3 kcal/mol after correcting for the basis set superposition error. G3 method has been found to yield consistent results with those obtained from the CCSD(T) calculations for the predicted orders of both the H abstraction barrier heights and their exothermicities.

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Effects of zeolite support on reactivity of Pd 4 cluster toward dihydrogen molecules were studied at the DFT level using T6 (six-ring) and T24 (sodalite cage) clusters as models of zeolite FAU. It has been found that Pd 4 cluster binds to O-centers of T6 cluster via eta (3) and eta (2) coordination modes, leading to three different T6/Pd 4 clusters. For the energetically most stable triplet state T6/Pd 4 structures, the energy of interaction between Pd 4 and the constrained T6 ring is calculated to be ca. -5 kcal/mol. Encapsulating Pd 4 in a sodalite cage (T24) with the full relaxation of cluster geometry resulted in the Pd 4-zeolite interaction energy of -7.4 kcal/mol after correcting for basis set superposition error. The H-H bond activation barrier associated with the first H 2 addition to the triplet state T6/Pd 4 clusters (Delta E 0/Delta H, kcal/mol) varies from (2.2/0.7) to (3.2/2.0) to (4.8/3.5), depending on the path. Comparison of the calculated H 2 addition barriers for the T6-supported and gas-phase Pd 4 indicates that embedding of Pd 4 on zeolite reduces this barrier slightly (by 1.8/2.1 kcal/mol). Interestingly, the characteristic gas phase Pd 4-H 2 active site structural motif has been preserved in the T6-supported transition state structures. The heat of the reaction of the addition of first H 2 to the triplet state T6/Pd 4 ranges from (-17.6/-18.9) to (-21.8/-23.5) for the paths considered. The addition of the second, third and fourth H 2 molecules to the respective first H 2 addition products leads to the dissociative addition product only for the continuation of the single first H 2 addition path.

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Based on second-order perturbation theory (MP2) predictions with large 6-311 + + G(3df, 3pd) basis set we have reviewed the possible structures and stabilities of a series of neutral MH(n)(M = Al, Ga; n = 4, 5, 6) species. For AlH4 and AlH5, our results confirm the previous theoretical findings, which indicate the dihydrogen C(s) complexes (2A') AlH2(H2) and (1A') AlH3(H2), respectively, as the lowest energy isomers. We found, similarly, C(s) (2A') GaH2(H2) and (1A') GaH3(H2) van der Waals complexes as the most stable species of the gallium analogues GaH4 and GaH5. The calculated H2 dissociation energies (D(e)) for AlH2(H2) and AlH3(H2) are of the order 1.8-2.5 kcalmol(-1), whereas this range of values for GaH2(H2) and GaH3(H2) is 1.4-1.8 kcalmol(-1) . Symmetry-adapted perturbation theory (SAPT) was used to analyze the interaction energies of these dihydrogen complexes (for n = 5) to determine why the Ga species show a smaller binding energy than the Al species. The SAPT partitioning of the interaction energy showed significant differences between AlH3(H2) and GaH3(H2), resulting from the much stronger "hydride" character of the aluminum species. The experimental observation of AlH2(H2) and AlH3(H2), and likely GaH3(H2), via low-temperature matrix isolation has been reported recently by Pullumbi et al. and Andrews et al., supporting the theoretical predictions. For n = 6, we found the degenerate C2(2A) and C(s)(2A') MH2(H2)2 "double H2" type van der Waals complexes as the lowest energy species for both M = Al and Ga.

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