Thermochemistry and Kinetics of the Thermal Decomposition of 1‐Chlorohexane

A theoretical kinetic study of the thermal decomposition of 1‐chlorohexane in gas phase between 600 and 1000 K was performed. Transition‐state theory and unimolecular reaction rate theory were combined with molecular information provided by quantum chemical calculations. Particularly, the B3LYP, BMK, M05–2X, and M06–2X formulations of the density functional theory (DFT) and the high‐level ab initio methods G3B3 and G4 were employed. The possible reaction channels for the thermal decomposition of 1‐chlorohexane were investigated, and the reaction takes place through the elimination of HCl with the formation of 1‐hexene. The derived high‐pressure limit rate coefficients are k _∞(600–1000 K) = (8 ± 5) × 10¹³ exp[‐((56.7 ± 0.4) kcal mol⁻¹/RT )] s⁻¹. The pressure effect over the reaction was analyzed from the calculation of the low‐pressure limit rate coefficients and the falloff curves. In addition, the standard enthalpies of formation at 298 K of −46.9 ± 1.5 kcal mol⁻¹ for 1‐chlorohexane and 5.8 ± 1.5 kcal mol⁻¹ for C₆H₁₃ radical were derived from isodesmic and isogiric reactions at high levels of theory.     

Arginine‐Facilitated α‐ and π‐Radical Migrations in Glycylarginyltryptophan Radical Cations

We have used model tripeptides GXW (with X being one of the amino acid residues glycine (G), alanine (A), leucine (L), phenylalanine (F), glutamic acid (E), histidine (H), lysine (K), or arginine (R)) to study the effects of the basicity of the amino acid residue on the radical migrations and dissociations of odd‐electron molecular peptide radical cations M^.+ in the gas phase. Low‐energy collision‐induced dissociation (CID) experiments revealed that the interconvertibility of the isomers [G^.XW]⁺ (radical centered on the N‐terminal α‐carbon atom) and [GXW]^.+ (radical centered on the π system of the indolyl ring) generally increased upon increasing the proton affinity of residue X. When X was arginine, the most basic amino acid, the two isomers were fully interconvertible and produced almost identical CID spectra despite the different locations of their initial radical sites. The presence of the very basic arginine residue allowed radical migrations to proceed readily among the [G^.RW]⁺ and [GRW]^.+ isomers prior to their dissociations. Density functional theory calculations revealed that the energy barriers for isomerizations among the α‐carbon‐centered radical [G^.RW]⁺, the π‐centered radical [GRW]^.+, and the β‐carbon‐centered radical [GRW_β^.]⁺ (ca. 32–36 kcal mol⁻¹) were comparable with those for their dissociations (ca. 32–34 kcal mol⁻¹). The arginine residue in these GRW radical cations tightly sequesters the proton, thereby resulting in minimal changes in the chemical environment during the radical migrations, in contrast to the situation for the analogous GGW system, in which the proton is inefficiently stabilized during the course of radical migration.     

Quantum-chemical calculation of a spillover model on a graphite support

The simplest quantum-chemical models of hydrogen spillover over a graphite-like surface as a proton or radical have been considered. The condensed planar C₂₄H₁₂ molecule was used as a model surface. Theab initio calculations of the interaction of hydrogen with the model surface were carried out by the restricted Hartree-Fock (HF) method in the STO-3G and 6-31 G^* basis sets. The radical hydrogen can not bind to such a surface, whereas the proton binds to it with an energy release of 186 kcal mol⁻¹. The activation energy of the transfer of the proton between two neighboring carbon atoms (10 kcal mol⁻¹) has been determined. The simplest model of the hydrogen migration as a proton over the model surface can be used for describing the spillover of hydrogen over the graphite surface. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 428–430, March, 1997.     

Computed Relative Populations of D2(22)‐C84 Endohedrals with Encapsulated Monomeric and Dimeric Water

Water monomer and dimer encapsulations into D₂(22)‐C₈₄ fullerene are evaluated. The encapsulation energy is computed at the M06‐2X/6‐31++G** level, and it is found that the monomer and dimer storage in C₈₄ yields an energy gain of 10.7 and 17.4 kcal mol^?1, respectively. Encapsulation equilibrium constants are computed by using partition functions based on the M06‐2X/6‐31G** and M06‐2X/6‐31++G** molecular data. Under high‐temperature/high‐pressure conditions, similar to that for the encapsulation of rare gases in fullerenes, the computed (H₂O)₂@C₈₄‐to‐H₂O@C₈₄ ratio is close to 1:2.     

Photochemical Generation of Strained Cycloalkynes from Methylenecyclopropanes

The hydrocarbons 1‐cyclopentylidene‐1a,9b‐dihydro‐1H ‐cyclopropa[l ]phenanthrene and 1‐cyclobutylidene‐1a,9b‐dihydro‐1H ‐cyclopropa[l ]phenanthrene undergo photolysis in solution at ambient temperature to produce cyclohexyne and cyclopentyne, respectively. These strained cycloalkynes, formed via the putative cycloalkylidenecarbenes, were intercepted as Diels–Alder adducts. Calculations at the CCSD(T)/cc‐pVTZ//B3LYP/6‐31+G* level of theory show that singlet cyclopentylidenecarbene has to overcome a barrier of 9.1 kcal mol⁻¹ to rearrange into cyclohexyne (with ΔE for ring expansion=−15.1 kcal mol⁻¹). By contrast, cyclobutylidenecarbene only needs to surmount a barrier of 1.6 kcal mol⁻¹ to rearrange into cyclopentyne (with ΔE for ring expansion=−6.2 kcal mol⁻¹).     

Outer‐Sphere 2 e−/2 H+ Transfer Reactions of Ruthenium(II)‐Amine and Ruthenium(IV)‐Amido Complexes

A diverse set of 2 e⁻/2 H⁺ reactions are described that interconvert [Ru^II(bpy)(en*)₂]²⁺ and [Ru^IV(bpy)(en‐H*)₂]²⁺ (bpy=2,2′‐bipyridine, en*=H₂NCMe₂CMe₂NH₂, en*‐H=H₂NCMe₂CMe₂NH⁻), forming or cleaving different O−H, N−H, S−H, and C−H bonds. The reactions involve quinones, hydrazines, thiols, and 1,3‐cyclohexadiene. These proton‐coupled electron transfer reactions occur without substrate binding to the ruthenium center, but instead with precursor complex formation by hydrogen bonding. The free energies of the reactions vary over more than 90 kcal mol⁻¹, but the rates are more dependent on the type of X−H bond involved than the associated ΔG °. There is a kinetic preference for substrates that have the transferring hydrogen atoms in close proximity, such as ortho ‐tetrachlorobenzoquinone over its para ‐isomer and 1,3‐cyclohexadiene over its 1,4‐isomer, perhaps hinting at the potential for concerted 2 e⁻/2 H⁺ transfers.     

Vitamin C: an experimental and theoretical study on the gas‐phase structure and ion energetics of protonated ascorbic acid

In order to investigate the gas‐phase mechanisms of the acid catalyzed degradation of ascorbic acid (AA) to furan, we undertook a mass spectrometric (ESI/TQ/MS) and theoretical investigation at the B3LYP/6‐31 + G(d,p) level of theory. The gaseous reactant species, the protonated AA, [C₆H₈O₆]H⁺, were generated by electrospray ionization of a 10^?3 M H₂O/CH₃OH (1 : 1) AA solution. In order to structurally characterize the gaseous [C₆H₈O₆]H⁺ ionic reactants, we estimated the proton affinity and the gas‐phase basicity of AA by the extended Cooks's kinetic method and by computational methods at the B3LYP/6‐31 + G(d,p) level of theory. As expected, computational results identify the carbonyl oxygen atom (O2) of AA as the preferred protonation site. From the experimental proton affinity of 875.0 ± 12 kJ mol^?1 and protonation entropy ΔS_p 108.9 ± 2 J mol^?1 K^?1, a gas‐phase basicity value of AA of 842.5 ± 12 kJ mol^?1 at 298 K was obtained, which is in agreement with the value issuing from quantum mechanical computations. Copyright © 2016 John Wiley & Sons, Ltd.     

Protonation of phenylboronic acid: Comparison of G3B3 and G2MP2 methods

G3B3 and G2MP2 calculations using Gaussian 03 have been carried out to investigate the protonation preferences for phenylboronic acid. All nine heavy atoms have been protonated in turn. With both methodologies, the two lowest protonation energies are obtained with the proton located either at the ipso carbon atom or at a hydroxyl oxygen atom. Within the G3B3 formalism, the lowest‐energy configuration by 4.3 kcal · mol^?1 is found when the proton is located at the ipso carbon, rather than at the electronegative oxygen atom. In the resulting structure, the phenyl ring has lost a significant amount of aromaticity. By contrast, calculations with G2MP2 show that protonation at the hydroxyl oxygen atom is favored by 7.7 kcal · mol^?1. Calculations using the polarizable continuum model (PCM) solvent method also give preference to protonation at the oxygen atom when water is used as the solvent. The preference for protonation at the ipso carbon found by the more accurate G3B3 method is unexpected and its implications in Suzuki coupling are discussed. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Short intermolecular N—Br...O=C contacts in 1,3‐dibromo‐5,5‐dimethylimidazolidine‐2,4‐dione

In the title compound, C₅H₆Br₂N₂O₂, all atoms except for the methyl group lie on a mirror plane in the space group Pnma (No. 62). All bond lengths are normal and the five‐membered ring is planar by symmetry. Two short intermolecular N—Br...O=C contacts [Br...O = 2.787 (2) and 2.8431 (19) Å] are present, originating primarily from the O‐atom lone pairs donating electron density to the antibonding orbitals of the N—Br bonds (delocalization energy transfers 3.27 and 2.11 kcal mol⁻¹). The total stabilization energies of the Br...O interactions are 3.4828 and 2.3504 kcal mol⁻¹.     

The proton‐coupled proton transfer mechanism,H2O catalysis,and hydrogen tunneling effects in the reaction of HNCH2 with HCOOH in the interstellar medium

The reaction HNCH₂ + HCOOH → H₂NCH₂COOH is supposed to be an important reaction related to the possible origin of amino acids on the early Earth. We find that it has an energy barrier of 87.37 kcal mol⁻¹ obtained with MP2/6‐311+G** in the gas phase, but it is likely enhanced to occur in the interstellar medium (ISM) through a proton‐coupled proton transfer reaction, initiated by HNCH₂ coupled with H₂⁺, H₃⁺, or H₃⁺O. H₂⁺, H₃⁺, and H₃⁺O serve as a donor of energy in the coupled reactions. H⁺, which is a key species to the coupled reactions, further, plays a catalytic role in reducing a barrier up to 14.14 kcal mol⁻¹. In the coupled reaction with H₃⁺O, H₂O, which can seize, transport, and deliver a proton from HCOOH to H₂NCH₂⁺, reduces a barrier up to 14.96 kcal mol⁻¹. A significant hydrogen‐tunneling pathway is predicted by the temperature dependences of k_H^CVT/SCT, calculated using the small curvature tunneling (SCT) approximation and canonical variational transition state theory (CVT). Hydrogen tunneling is another important mechanism to make the reaction happen in the ISM. The achieved results can be applied to discuss the origin of amino acids from the materials of the Earth itself. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Experimental and theoretical studies of the basicity and proton affinity of SiF4 and the structure of SiF4H+

A combined experimental and theoretical approach has been employed to establish the basicity and proton affinity of SiF₄ and the structure of SiF₄H⁺. The kinetics and energetics for the transfer of a proton between SiF₄, N₂, and Xe have been explored experimentally in helium at 0.35±0.02 torr and 297±3 K with a selected-ion flow tube apparatus. The results of equilibrium constant measurements are reported that provide a basicity and proton affinity for SiF₄ at 297±3 K of 111.4±1.0 and 117.7±1.2 kcal mol^?1, respectively. These values are more than 2.5 kcal mol^?1 lower than currently recommended values. The basicity order was determined to be GB(Xe)>GB(SiF₄)>GB(N₂), while the proton-affinity order was shown to be PA(Xe)>PA(N₂)>PA (SiF₄). Ab initio molecular orbital computations at MP4SDTQ(fc)/6-311++G(3df,3pd) using geometries from B3LYP/6-31+G(d,p) indicate a value for PA(SiF₄)=118.7 kcal mol^?1 that is in good agreement with experiment. Also, the most stable structure of SiF₄H⁺ is shown to correspond to a core SiF ₃ ⁺ cation solvated by HF with a binding energy of 43. 9 kcal mol^?1. Support for this structure is found in separate SIFT collision induced dissociation (CID) measurements that indicate exclusive loss of HF.     

A theoretical study of C4H5N isomers: Comparison of cyano and isocyano as substituents

Ab initio molecular orbital calculations using a 3-21G basis set have been used to optimize geometries for pyrrole, CH₃(X)CCH₂, CH₃(H)CCHX (both cis and trans), c-C₃H₅X, and CH₂CHCH₂X, where X is CN and NC. In all the alkenyl derivatives methyl groups are found to adopt the conformation in which the methyl hydrogen eclipses the double bond. 6-31G*∥3-21G level calculations show the alkenyl cyanides to be of similar energy to pyrrole, but the isocyanides are ～20 kcal mol^?1 higher in energy. For both substituents the cyclopropyl derivatives are higher in energy by ～10 kcal mol^?1. At the 6-31G* level ring strain is 27.7 kcal mol^?1 for the cyanide and 30.6 kcal mol^?1 for the isocyanide. Data on the relative energies of RCN and RNC are compared when R is (i) a saturated hydrocarbon, (ii) an unsaturated hydrocarbon, (iii) an α-carbenium ion, (iv) an allyl cation, and (v) an α-carbanion.     

Stable Oxindolyl‐Based Analogues of Chichibabin's and Müller's Hydrocarbons

Chichibabin's and Müller's hydrocarbons are classical open‐shell singlet diradicaloids but they are highly reactive. Herein we report the successful synthesis of their respective stable analogues, OxR‐2 and OxR‐3 , based on the newly developed oxindolyl radical. X‐ray crystallographic analysis on OxR‐2 reveals a planar quinoidal backbone similar to Chichibabin's hydrocarbon, in accordance with its small diradical character (y₀=11.1 %) and large singlet–triplet gap (ΔE_S‐T=−10.8 kcal mol⁻¹). Variable‐temperature NMR studies on OxR‐2 disclose a slow cis/trans isomerization process in solution through a diradical transition state, with a moderate energy barrier (ΔG^≠_298K=15–16 kcal mol⁻¹). OxR‐3 exhibits a much larger diradical character (y₀=80.6 %) and a smaller singlet–triplet gap (ΔE_S‐T=−3.5 kcal mol⁻¹), and thus can be easily populated to paramagnetic triplet diradical. Our studies provide a new type of stable carbon‐centered monoradical and diradicaloid.     

Bonding Energetics of Palladium Amido/Aryloxide Complexes in DMSO: Implications for Palladium-Mediated Aniline Activation

Thermodynamic knowledge of the metal–ligand (M−L) σ-bond strength is crucial to understanding metal-mediated transformations. Here, we developed a method for determining the Pd−X (X=OR and NHAr) bond heterolysis energies (ΔG_het(Pd−X)) in DMSO taking [(tmeda)PdArX] (tmeda=N,N,N′,N′-tetramethylethylenediamine) as the model complexes. The ΔG_het(Pd−X) scales span a range of 2.6–9.0 kcal mol⁻¹ for ΔG_het(Pd−O) values and of 14.5–19.5 kcal mol⁻¹ for ΔG_het(Pd−N) values, respectively, implying a facile heterolytic detachment of the Pd ligands. Structure-reactivity analyses of a modeling Pd-mediated X−H bond activation reveal that the M−X bond metathesis is dominated by differences of the X−H and Pd−X bond strengths, the former being more influential. The ΔG_het(Pd−X) and pK_a(X−H) parameters enable regulation of reaction thermodynamics and chemoselectivity and diagnosing the probability of aniline activation with Pd−X complexes.     

Influence of metal ion on chelate–aryl stacking interactions

CCSD(T)/CBS and DFT methods are employed to study the stacking interactions of acetylacetonate‐type (acac‐type) chelates of nickel, palladium, and platinum with benzene. The strongest chelate–aryl stacking interactions are formed by nickel and palladium chelate, with interaction energies of −5.75 kcal mol⁻¹ and −5.73 kcal mol⁻¹, while the interaction of platinum chelate is weaker, with interaction energy of −5.36 kcal mol⁻¹. These interaction energies are significantly stronger than stacking of two benzenes, −2.73 kcal mol⁻¹. The strongest nickel and palladium chelate–aryl interactions are with benzene center above the metal area, while the strongest platinum chelate–aryl interaction is with the benzene center above the C2 atom of the acac‐type chelate ring. These preferences arise from very different electrostatic potentials above the metal ions, ranging from very positive above nickel to slightly negative above platinum. While the differences in electrostatic potentials above metal atoms cause different geometries with the most stable interaction among the three metals, the dispersion (correlation energy) component is the largest contribution to the total interaction energy for all three metals.     

Adsorption and dissociation of carbon trioxide on Ag(100)

Using density functional theory methods, we have studied carbon trioxide, its adsorption and dissociation on Ag(100). In the gas phase, two isomers are found, D_3h and C_2v, with the latter of 2.0 kcal mol^?1 lower in energy at the PW91PW91/6?31G(d) level. For CO₃ on Ag(100), the calculated adsorption energy is 91.2 and 89.1 kcal mol^?1 for the bi‐coord perpendicular and tri‐coord parallel structures, respectively. Upon the adsorption, 0.50 ～ 0.56 electron is transferred from silver to CO₃, indicative of significant ionic characters of the adsorbate‐surface bonding. In addition, the geometry of CO₃ is largely changed by its strong interaction with silver. For CO_3(ad) → O_(ad) + CO_2(gas), the energy barrier is calculated to be 19.8 kcal mol^?1 through the bi‐coord path. The process is endothermic with an enthalpy change of +17.3 ～ +26.7 kcal mol^?1 and the weakly chemisorbed CO₂ is identified as an intermediate on the potential energy surface. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Ethyl 3‐[1‐(5,5‐dimethyl‐2‐oxo‐1,3,2‐dioxaphosphorin‐2‐yl)propan‐2‐ylidene]carbazate: a combined X‐ray and density functional theory (DFT) study

In the title compound, C₁₁H₂₁N₂O₅P, one of the two carbazate N atoms is involved in the C=N double bond and the H atom of the second N atom is engaged in an intramolecular hydrogen bond with an O atom from the dimethylphosphorin‐2‐yl group, which is in an uncommon cis position with respect to the carbamate group. The cohesion of the crystal structure is also reinforced by weak intermolecular hydrogen bonds. Density functional theory (DFT) calculations at the B3LYP/6‐311++g(2d,2p) level revealed the lowest energy structure to have a Z configuration at the C=N bond, which is consistent with the configuration found in the X‐ray crystal structure, as well as a less stable E counterpart which lies 2.0 kcal mol⁻¹ higher in potential energy. Correlations between the experimental and computational studies are discussed.     

Thermochemical properties for n‐propyl,iso‐propyl,and tert‐butyl nitroalkanes,alkyl nitrites,and their carbon‐centered radicals

Density functional theory (DFT) based calculations are performed on a series of alkyl nitrites and nitroalkanes representing large‐scale primary, secondary, and tertiary nitro compounds and their radicals resulting from the loss of their skeletal hydrogen atoms. Geometries, vibration frequencies, and thermochemical properties [S°(T) and C°_p(T) (10 K ? T ? 5000 K)] are calculated at the B3LYP/6‐31G(d,p) DFT level. Δ_fH°₂₉₈ values are from B3LYP/6‐31G(d,p), B3LYP/6‐31+G(2d,2p), and the composite CBS‐QB3 levels. Potential energy barriers for the internal rotations have been computed at the B3LYP/6‐31G(d,p) level of theory, and the lower barrier contributions are incorporated into entropy and heat capacity data. The standard enthalpies of formation at 298 K are evaluated using isodesmic reaction schemes with several work reactions for each species. Recommended values derived from the most stable conformers of respective nitro‐ and nitrite isomers include ?30.57 and ?28.44 kcal mol^?1 for n‐propane‐, ?33.89 and ?32.32 kcal mol^?1 for iso‐propane‐, ?42.78 and ?41.36 kcal mol^?1 for tert‐butane‐nitro compounds and nitrites, respectively. Entropy and heat capacity values are also reported for the lower homologues: nitromethane, nitroethane, and corresponding nitrites. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 181–199, 2010     

A structural and theoretical study of intermolecular interactions in nicotinohydrazide dihydrochloride

In the title compound [systematic name: 3‐(azaniumylcarbamoyl)pyridinium dichloride], C₆H₉N₃O²⁺·2Cl⁻, the ions are connected by N—H...Cl hydrogen bonds to form layers and C—H...Cl interactions expand the layers into a three‐dimensional net. The energies of the N—H...Cl interactions range from typical for very weak interactions (0.17 kcal mol⁻¹) to those observed for relatively strong interactions (29.1 kcal mol⁻¹). C—H...Cl interactions can be classified as weak and mildly strong (energies ranging from 2.2 to 8.2 kcal mol⁻¹). Despite the short contacts existing between the parallel aromatic rings of the cations, π–π interactions do not occur.