Geometry optimization in ab initio SCF calculations. VII. Proton affinities and ion structures calculated with floating orbital geometry optimization (FOGO)

The FOGO method is used to calculate absolute proton affinities of the molecules H₂, HF, NH₃, H₂O, CH₃OH, C₂H₅OH, H₂O₂, CH₂O, CO, and CH₂CO. Comparison with experimental values demonstrates that the geometrical and energetical data resulting from this type of ab initio calculation are of chemical accuracy. Predictive data for higher energy isomers, such as hydroxymethylene and ethynol are given as possible aid for the identification of these species.     

Correlation effects on energy curves for proton transfer. The cation [H5O2]+

Potential curves for proton transfer in [H₅O₂]⁺ and for the dissociation of one OH bond in [H₃O]⁺ were calculated by both ab initio and semi-empirical LCAO MO SCF CI methods. The energy barrier of the symmetric double minimum potential in [H₅O₂]⁺ is very sensitive to electron correlation. At an OO distance of 2.74 Å it decreases from the HF value of 9.5 kcal/mole to about 7.0 kcal/mole. The results of the semi-empirical calculations agree well with the ab initio data as long as only relative effects are regarded. The partitioning of correlation energy into contributions of individual electron pairs is very similar for proton transfer in [H₅O₂]⁺ and for the dissociation of one OH bond in [H₃O]⁺. In this example the proton transfer appears as a superposition of two “contracted ionic dissociation” processes. An interpretation of the behaviour of correlation during these processes is presented.     

<Emphasis Type="Italic">Ab initio</Emphasis> quantum-chemical study of the mechanism of methoxide ion formation in MOH/DMSO/CH<Subscript>3</Subscript>OH systems (M = Li,Na, K)

The profile of the reaction CH₃OH + MOH → CH₃OM + H₂O in the presence of an alkali (MOH, M = Li, Na, K) was investigated by the ab initio quantum-chemical method for the gas phase (with allowance for the solvent) within the continuum model. The proton transfer and the formation of the alkaline methoxide molecule in MOH/DMSO/CH₃OH systems (M = Li, Na, K) in the alkali-methanol pre-reaction complexes can take place without their preliminary dissociation and are barrier-free reactions.     

A Computational Study of the Kinetics and Mechanism for the C2H3 + CH3OH Reaction

The mechanism for the C₂H₃ + CH₃OH reaction has been investigated by the Gaussian‐4 (G4) method based on the geometric parameters of the stationary points optimized at the B3LYP/6–31G(2df, p) level of theory. Four transition states have been identified for the production of C₂H₄ + CH₃O (TSR/P1), C₂H₄ + CH₂OH (TSR/P2), C₂H₃OH + CH₃ (TSR/P3), and C₂H₃OCH₃ + H (TSR/P4) with the corresponding barriers 8.48, 9.25, 37.62, and 34.95 kcal/mol at the G4 level of theory, respectively. The rate constants and branching ratios for the two lower energy H‐abstraction reactions were calculated using canonical variational transition state theory with the Eckart tunneling correction at the temperature range 300–2500 K. The predicted rate constants have been compared with existing literature data, and the uncertainty has been discussed. The branching ratio calculation suggests that the channel producing CH₃O is dominant up to about 1070 K, above which the channel producing CH₂OH becomes very competitive.     

The structure and dissociation pathways of protonated methanol: An ab initio molecular orbital study

Ab initio molecular orbital calculations with moderately large polarization basis sets and including valence-electron correlation have been used to examine the structure and dissociation mechanisms of protonated methanol [CH₃OH₂]⁺. Stable isomers and transition structures have been characterized using gradient techniques. Protonated methanol is found to be the only stable isomer in the [CH₅O]⁺ potential surface. There is no evidence for a tightly-bound complex, [HOCH₂]⁺…?H₂, analogous to the preferred structure [CH₃]⁺…?H₂ of [CH₅]⁺. Protonated methanol is found to possess a pyramidal arrangement of bonds at the oxygen atom with a barrier to inversion of 8kJ mol^?1. The lowest energy fragmentation pathways are dissociation into methyl cation and water (predicted to require 284 kJ mol^?1 with zero reverse activation energy) and loss of molecular hydrogen (endothermic by 138 kJ mol^?1 but with a reverse activation barrier of 149 kJ mol^?1). The results offer a possible explanation as to why production of [CH₂OH]⁺ from the reaction of methyl cation with water is not observed. Other dissociation processes examined include loss of a hydrogen atom to yield the methylenoxonium radical cation or methanol radical cation (requiring 441 and 490 kJ mol^?1, respectively) and loss of a proton to yield neutral methanol (requiring 784 kJ mol^?1).     

Methanediol decomposition mechanisms: A study considering various ab initio approaches

The alternative decomposition reactions CH₂(OH)₂ → CH₂O + H₂O and CH₂(OH)₂ + H₂O → CH₂O + 2H₂O are investigated using the semiempirical PM 3 as well as the ab initio HF /3-21G , HF /6-31G , HF /6-31G **, and MP 2/6-31G ** calculations. Reactants, products, and appropriate transition states are located on corresponding potential energy surfaces and compared with those reported in earlier studies. © 1996 John Wiley & Sons, Inc.     

Reversible isomerization of a novel [FeFe]‑hydrogenase model complex and water-promoted electrocatalytic proton reduction

μ-(SCH(CH₂CH₃)CH₂S)-Fe₂(CO)₄(κ²-DPPE) (complex 1, DPPE is 1,2-bis (diphenylphosphor) ethane), which can be regarded as a model of the [FeFe]‑hydrogenase active site, was synthesized and characterized. The reversible isomerization of complex 1 under N₂ and CO atmosphere was demonstrated by cyclic voltammetry, IR spectroscopy and ³¹P NMR. Furthermore, we discovered that both the presence of a CO atmosphere and the addition of H₂O can independently trigger the same inversion of configuration of complex 1. The electrocatalytic proton reduction capacity of 1 was evaluated under varying conditions. It was found that addition of a little H₂O to CH₃CN can facilitate its efficiency of electrocatalytic proton reduction. The possible mechanism of transition between axial/basal and dibasal isomers and the function of H₂O in the electrocatalytic reaction are discussed.     

An ab initio quantum chemical study of reaction mechanisms in the C2H2/CH3OH/KOH/DMSO system

An ab initio quantum chemical study (MP2/6-311++G**//B3LYP/6-31+G*) of a number of possible interactions is performed for the gas phase system of acetylene—potassium hydroxide-dimethylsulfoxide(DMSO)—methanol and with regard to the solvent effect within the continuum model. Key structures in the vinylation reaction are shown to be methoxide ion complexes with the alkali metal hydroxide and acetylene molecules. The formation of these complexes results in the activation of the acetylene molecule and an increase in the nucleophilicity of the methoxide ion. In the C₂H₂/CH₃OH/KOH/DMSO reaction system, a proton exchange between the acetylene molecule and the anionic nucleophile ([OH]^- and [CH₃O]^-) is freely performed with the formation of systems with ethynideions, whereas the thermodynamically preferable formation of vinyl alcohol or methyl vinyl ether is determined by a barrier of 20 kcal/mol.     

Effect of Solvation on Chemical Reactions,I. Addition of a Single Water Molecule to the H− + H2O → OH− + H2 Reaction

The gas phase chemical reaction, H^? + H₂O → H₂ + OH, and the effect of an additional water molecule on the reaction, H^?(H₂O) + H₂O → H₂ + OH(H₂O), have been investigated. The optimal structures and energies of the reactants, products, two stable intermediates, and the transition state connecting the two intermediates have been determined. The additional water molecule does not affect the potential surface congruently: it destabilizes the H(H₂O) minimum, but stabilizes the H₂ ?OH minimum and the transition state connecting the two intermediates. However, it stabilizes the products more than the H₂ ?OH^? minimum. Finally, in line with the reduction in the barrier height, the transition state for the H(H₂0) to H₂ ?OH^? isomerization moves further along the reaction path.     

A Theoretical Investigation of the Decomposition Reactions of Ethyl Formate in the S0 State

This study revisits the stability of the possible conformations and the decomposition reactions of ethyl formate in the S₀ state using the (U)MP2, MP4SDTQ, CCSD(T), and (U)B3LYP methods with various basis sets. The transition states of the decomposition channels to HCOOH + C₂H₄, CO + CH₃CH₂OH, CH₂O + CH₃CHO, HCOH + CH₃CHO, C₂H₆ + CO₂, and H₂ + CH₂CHOCHO are determined. The microcanonical rate constants derived from the RRKM theory are calculated for each of the decomposition reactions. The high‐pressure limit rate constants are calculated for the decomposition channels to HCOOH + C₂H₄, CO + CH₃CH₂OH, and CH₂O + CH₃CHO.     

Neutralization-reionization mass spectrometry (NRMS). Structural information from vertical neutralization and reionization efficiencies

The neutralization-reionization mass spectra of alkane radical ions indicate significant differences between the structures and geometries of alkane molecules and their molecular ions, confirming recent ab initio predictions. Ionic isomers that are indistinguishable by collisionally-activated dissociation because of easy interconversion can be characterized by neutralization-reionization if the corresponding neutrals show different reactivities, as is demonstrated for the [C₂H₅]⁺/C₂H₅˙ system and for [C₂H₄O₂]⁺˙ isomers. For identification of mixtures of more than one neutral species, the relative efficiency for reionizing each neutral must be determined; e.g. the O₂ reionization efficiency of ˙CH₂OH radicals is ～4 times greater than that of CH₃O˙. This information and reference reionization spectra of CH₃O˙ and ˙CH₂OH show that metastable or collisionally activated methyl acetate cations lose CH₃O˙, not ˙CH₂OH as previously reported; the newly-formed CH₃O˙ undergoes partial (～20%) isomerization to ˙CH₂OH in the ～10^?6s before reionization. Similar results are obtained for [B(OCH₃)₃]⁺˙.     

Theoretical prediction regarding structural and thermodynamical characteristics of stable CH3PO2 isomers and unimolecular decomposition mechanisms of species CH3P(O)2, CH3O?PO,and CH2P(O)OH

The detailed isomerization and dissociation reaction potential energy profile of the CH₃PO₂ system was established at the UCCSD(T)/6‐311++G(3df,2p)//UB3LYP/6‐311++G(d,p) level of theory. Seventy minimum isomers were located and connected by 93 optimized interconversion transition states. Furthermore, 32 isomers with high kinetic stability were predicted to be possible candidates for further experimental detection. The bonding nature of the suggested stable isomers was analyzed while their molecular properties including heats of formation, adiabatic ionization potentials, and adiabatic electronic affinities were calculated at the G2, G2(MP2), G3, and CBS‐Q levels. Based on the isomerization and dissociation potential energy surface, possible unimolecular decomposition mechanisms and pathways of the low‐lying molecules CH₃P(?O)₂, CH₃O? P?O, and CH₂?P(?O)OH were discussed. Furthermore, the transition state theory rate constants of the primary unimolecular dissociation channels were also calculated. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009     

Spectroscopic Observation of the Hydroxy Position in Butanol Hydrates and Its Effect on Hydrate Stability

In this study, we investigate the crystal structures and phase equilibria of butanols+CH₄+H₂O systems to reveal the hydroxy group positioning and its effects on hydrate stability. Four clathrate hydrates formed by structural butanol isomers are identified with powder X‐ray diffraction (PXRD). In addition, Raman spectroscopy is used to analyze the guest distributions and inclusion behaviors of large alcohol molecules in these hydrate systems. The existence of a free OH indicates that guest molecules can be captured in the large cages of structure II hydrates without any hydrogen‐bonding interactions between the hydroxy group of the guests and the water‐host framework. However, Raman spectra of the binary (1‐butanol+CH₄) hydrate do not show the free OH signal, indicating that there could be possible hydrogen‐bonding interactions between the guests and hosts. We also measure the four‐phase equilibrium conditions of the butanols+CH₄+H₂O systems.     

The effect of water‐mediated catalysis on the intramolecular proton‐transfer reactions of the isomers of 5‐chlorouracil: a theoretical study

The geometrical structures and thermal energies (E), enthalpies (H) and Gibbs free energies (G) of 13 isomers of 5‐chlorouracil (5ClU) in the gas and water phases were investigated using the density functional theory (DFT) method at the M06‐2X/6‐311++g(3df,3pd) level. The isomers of 5ClU can be microhydrated at different molecular target sites. The mono‐ and dihydrated forms are the most stable in both the gas and water phases, and, because of the intermolecular interactions, the hydrations lead to a degree of change in the stability trend. Two types of isomerizations were considered: the internal H—O bond rotations in which the H atom rotates 180° around the C—O bond and the intramolecular proton‐transfer reactions in which an H atom is transferred between an O atom and a neighbouring N atom. The forward and backward energy barriers for isomerizations of nonhydrated 5ClU were calculated. In addition, 16 optimized transition‐state structures for water‐mediated catalysis on isomerizations of 5ClU were investigated. The forward and backward proton‐transfer energy barriers of water‐mediated catalysis on isomerizations of 5ClU were obtained. The results indicate that the catalytic effect of two H₂O molecules is much greater than that of one H₂O molecule in isomerizations of 5ClU.     

Proton and lithium cation affinities calculated with floating orbital geometry optimization (FOGO)

The FOGO method is used to calculate proton affinities and lithium cation affinities. The molecules of primary interest in this study are the methyl-substituted amines. In addition, the lithium cation affinity of HF, H₂O, CH₃OH, H₂CO, and HCN are calculated for comparison. Geometries of all species are fully optimized with a double-zeta (DZ) basis set, including polarization on hydrogen and the first-row elements by floating orbitals. Comparison with experimental values demonstrates that structural data and proton affinities resulting from this type of ab initio calculation are of chemical accuracy. The lithium cation affinities are also reasonably well reproduced, but the small experimental differences are not within the accuracy, which can be expected from this type of calculation.     

Theoretical study of the effects of substitution,solvation, and structure on the interaction between nitriles and methanol

We report an investigation on intermolecular interactions in R? CN ··· H? OCH₃ (R = H, CH₃, F, Cl, NO₂, OH, SH, SCH₃, CHO, COCH₃, CH₂Cl, CH₂F, CH₂OH, CH₂COOH, CF₃, SCOCH₃, SCF₃, OCHF₂, CH₂CF₃, CH₂OCH₃, and CH₂CH₂OH) complexes using density functional theory. The calculations were conducted on B3LYP/6‐311++G** level of theory for optimization of geometries of complexes and monomers. An improper hydrogen bonding (HB) in the H₃CO? H ··· NC? R complexes was observed in that N atom of the nitriles functions acts as a proton acceptor. Furthermore, quantum theory of “Atoms in Molecules” (AIM) and natural bond orbital (NBO) method were applied to analyze H‐bond interactions in respective complexes. The electron density (ρ) and Laplacian (?²ρ) properties, estimated by atoms in molecules calculations, indicate that H ··· N bond possesses low ρ and positive ?²ρ values, which are in agreement with partially covalent character of the HBs, whereas O? H bonds have negative ?²ρ values. In addition, the weak intermolecular force due to dipole–dipole interaction (U) is also considered for analysis. The examination of HB in these complexes by quantum theory of NBO method fairly supports the ab initio results. Natural population analysis data, the electron density, and Laplacian properties, as well as, the ν(O? H) and γ(O? H) frequencies of complexes, calculated at the B3LYP/6‐311++G** level of theory, are used to evaluate the HB interactions. The calculated geometrical parameters and conformational analysis in water phase solution show that the H₃CO? H ··· NC? R complexes in water are more stable than that in gas phase. The obtained results demonstrated a strong influence of the R substituent on the properties of complexes. Numerous correlations between topological, geometrical, thermodynamic properties, and energetic parameters were also found. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Structure and formation of gaseous [C4H5O]+ Ions. II–Ions of the type HCC?\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}(OH)(CH3)

Loss of an alkyl group X? from acetylenic alcohols HC?C? CX(OH)(CH₃) and gas phase protonation of HC?C? CO? CH₃ are both shown to yield stable HC?C? \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}(OH)(CH₃) ions. Ions of this structure are unique among all other [C₄H₅O]⁺ isomers by having m/z 43 [C₂H₃O]⁺ as base peak in both the metastable ion and collisional activation spectra. It is concluded that the composite metastable peak for formation of m/z 43 corresponds to two distinct reaction profiles which lead to the same product ion, CH₃\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}?O, and neutral, HC?CH. It is further shown that the [C₄H₅O]⁺ ions from related alcohols (like HC?C? CH(OH)(CH₃)) which have an α-H atom available for isomerization into energy rich allenyl type molecular ions, consist of a second stable structure, H₂C?\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm C}\limits^{\rm + } $\end{document}? C(OH)?CH₂.     

Evaluated and estimated kinetic data for phase reactions of the hydroperoxyl radical

Methods are discussed for the production and detection of the hydroperoxyl radical for use in gas phase kinetic studies. Rate constants for gas phase reactions of the hydroperoxyl radical with itself, H₂, H₂O, CO, NO, SO₂, O₃, C₂H₆, C₃H₈, i-and n-C₄H₁₀, C₂H₄, i-C₄H₈, HCHO, C₂H₅CHO, n-C₃H₇CHO, Br, O, OH, and H are critically evaluated. Recommended or estimated rate constant expressions with associated error limits are given applicable over specified temperature ranges (normally 300–1000°K). The reactivity of HO₂ compared with OH, O, H, F, Cl, Br, CH₃, and CH₃O is presented in tabular form and the implications for atmospheric chemistry are discussed.     

An SCF and CI Study of the 1,3 Shift in the HX?CHY⇌XCH?YH Isoelectronic Series: X,YCH2, NH,and O

Ab initio molecular orbital calculations at SCF level with the 3-21G, 6-31G, and 6-31G** basis sets and CI level with the 6-31G basis set have been carried out for an isoelectronic series HX? CH?Y and X?CH? YH, where X, Y can be CH₂, NH, and O. Optimized structures (3-21G and 6-31G**) for both tautomers and the 1,3 hydrogen shift transition states are reported. The relative stabilities of the isomers and the barriers of the 1,3 shift are discussed in terms of proton affinities and bond orders. It is shown that both the relative stabilities of the tautomers and the relative barrier heights can be explained qualitatively using simple proton affinity arguments and that the barrier heights are quantitatively related to bond orders.