The effect of substitutents on the strain energies of small ring compounds

The effect of substitutents on the strain energy (SE) of cyclic molecules is examined at the CBS, G2, and G2(MP2) levels of theory. Alkyl substitutents have a meaningful effect upon the SE of small ring compounds. gem-Dimethyl substitution lowers the strain energy of cyclopropanes, cyclobutanes, epoxides, and dimethyldioxirane (DMDO) by 6-10 kcal/mol relative to an unbranched acyclic reference molecule. The choice of the reference compound is especially important for geminal electronegative substitutents. The SE of 1,1-difluorocyclopropane is estimated to be 20.5 kcal/mol relative to acyclic reference molecule 1,3-difluoropropane but is 40.7 kcal/mol with respect to the thermodynamically more stable (DeltaE = -20.2 kcal/mol) isomeric reference compound 2,2-difluoropropane. The SE of dioxirane (DO) is estimated to be approximately 18 kcal/mol while the SE of DMDO is predicted to be approximately equal to 11 kcal/mol by using homodesmotic reactions that maintain a balanced group equivalency. The total energy (CBS-APNO) of DMDO is 2.6 kcal/mol lower than that of isomeric 1,2-dioxacyclopentane that has an estimated SE of 5 kcal/mol. The thermodynamic stability of DMDO is a consequence of its relatively strong C-H (BDE = 102.7 kcal/mol) and C-CH(3) (BDE = 98.9 kcal/mol) bonds. By comparison, the calculated sec-C-H and -C-CH(3) G2 bond dissociation energies in propane are 100.3 and 90.5 kcal/mol.     

Partial oxidation of propylene to propylene oxide over a neutral gold trimer in the gas phase: a density functional theory study

We report a B3LYP study of a novel mechanism for propylene epoxidation using H(2) and O(2) on a neutral Au(3) cluster, including full thermodynamics and pre-exponential factors. A side-on O(2) adsorption on Au(3) is followed by dissociative addition of H(2) across one of the Au-O bonds (DeltaE(act) = 2.2 kcal/mol), forming a hydroperoxy intermediate (OOH) and a lone H atom situated on the Au(3) cluster. The more electrophilic O atom (proximal to the Au) of the Au-OOH group attacks the C=C of an approaching propylene to form propylene oxide (PO) with an activation barrier of 19.6 kcal/mol. We predict the PO desorption energy from the Au(3) cluster with residual OH and H to be 11.5 kcal/mol. The catalytic cycle can be closed in two different ways. In the first subpathway, OH and H, hosted by the same terminal Au atom, combine to form water (DeltaE(act) = 26.5 kcal/mol). We attribute rather a high activation barrier of this step to the breaking of the partial bond between the H atom and the central Au atom in the transition state. Upon water desorption (DeltaE(des) = 9.9 kcal/mol), the Au(3) is regenerated (closure). In the second subpathway, H(2) is added across the Au-OH bond to form water and another Au-H bond (DeltaE(act) = 22.6 kcal/mol). Water spontaneously desorbs to form an obtuse angle Au(3) dihydride, with one H atom on the terminal Au atom and the other bridging the same terminal Au atom and the central Au atom. A slightly activated rearrangement to a symmetric triangular Au(3) intermediate with two equivalent Au-H bonds, addition of O(2) into the Au-H bond, and rotation reforms the hydroperoxy intermediate in the main cycle. On the basis of the DeltaG(act), which contains contribution from both pre-exponetial factor and activation energy, we identify the propylene epoxidation step as the actual rate-determining step (RDS) in both the pathways. The activation barrier of the RDS (epoxidation step: DeltaE(act) = 19.6 kcal/mol) is in the same range as that in the published computationally investigated olefin epoxidation mechanisms involving Ti sites (without Au involved) indicating that isolated Au clusters and possibly Au clusters on non-Ti supports can be active for gas-phase partial oxidation, even though cooperative mechanisms involving Au clusters/Ti-based-supports may be favored.     

Effect of geminal substitution on the strain energy of dioxiranes. Origin of the low ring strain of dimethyldioxirane

The strain energies (SE) for dioxirane (DO) dimethyldioxirane (DMDO) and related dioxiranes have been examined by several methods using high-level computational schemes (G2, G2(MP2), CBS-Q). A series of calculated O-O, C-O, and O-H bond dissociation energies (G2) point to special problems associated with classical homodesmotic reactions involving peroxides. The relative SEs of DO, DMDO, methyl(trifluoromethyl)dioxirane (TFDO), and difluorodioxirane (DFDO) have been estimated by combination of the dioxirane with cyclopropane to form the corresponding 1,3-dioxacyclohexane. The relative SE predicted for DMDO (2) is 7 kcal/mol lower than that of DO, while the SE of 1,1-difluorodioxirane (4) is 8 kcal/mol higher. The most reactive dioxirane, methyl (trifluoromethyl)dioxirane (3), has an estimated SE just 1 kcal/mol greater than that of DO but 8 kcal/mol greater than that of DMDO. Six independent methods support the proposed SE for DO of 18 kcal/mol. The SE of the parent dioxirane (DO) has been estimated relative to six-membered ring reference compounds by dimerization of dioxirane and or its combination with cyclopropane. The relative SE of cyclic hydrocarbons, ethers and peroxides have been predicted by the insertion/extrusion of -CH(2)- and -O- fragments into their respective lower and next higher homologues. The moderated SE of DMDO (approximately equal to 11 kcal/mol) has also been estimated on the basis of group equivalent reactions. The unusual thermodynamic stability of DMDO is largely a consequence of combined geminal dimethyl and dioxa substitution effects and its associated strong C-H bonds and C-CH(3) bonds. The data clearly demonstrate that the reference compounds used to estimate the SE for highly substituted small ring cyclic compounds should reflect their molecular architecture having the same substitutents on carbon.     

Theoretical study on structures and stability of C4P isomers

The structures, energetics, spectroscopies, and stabilities of doublet C(4)P isomeric species are explored at the DFT/B3LYP, QCISD, and CCSD(T) (singlet-point) levels. A total of 12 minimum isomers and 27 interconversion transition states are located. At the CCSD(T)/6-311G(2df)//QCISD/6-311G(d)+ZPVE level, the lowest-lying isomer is a floppy CCCCP 1 (0.0 kcal/mol) mainly featuring a cumulenic structure |C=C=C=C=P*|, which differs much from the analogous C4N radical (|*C-C[triple bond]C-C[triple bond]N|). The quasi-linearity and the low bending mode of 1 are in contrast to the previous prediction. The second energetically followed isomer PC-cCCC 3 (14.9 kcal/mol) possesses a CCC ring-bonded to CP. The two low-lying isomers are separated by a high-energy ring-closure/open transition state (26.5 kcal/mol) and thus are very promising candidates for future laboratory and astrophysical detection. Furthermore, four high-energy isomers, that is, two bent isomers CCPCC 2 (68.4 kcal/mol) and CCPCC 2' (68.5 kcal/mol) and two cagelike species 10 (56.0 kcal/mol) and 11 (67.9 kcal/mol), are also stabilized by considerable barriers. The present work is the first detailed potential energy survey of CnP clusters and can provide useful information for the investigation of larger CnP radicals and for understanding the isomerism of P-doped C vaporization processes.     

Quantum mechanical study of the ring-closing reaction of the hexatriene radical cation

The ring-closing reaction of hexatriene radical cation 1(*)(+) to 1,3-cyclohexadiene radical cation 2(*)(+) was studied computationally at the B3LYP/6-31G* and QCISD(T)/6-311G*//QCISD/6-31G* levels of theory. Both, concerted and stepwise mechanisms were initially considered for this reaction. Upon evaluation at the B3LYP level of theory, three of the possible pathways-a concerted C(2)-symmetric via transition structure 3(*)(+) and stepwise C(1)-symmetric pathways involving three-membered ring intermediate 5(*)(+) and four-membered ring intermediate 6(*)(+)-were rejected due to high-energy stationary points along the reaction pathway. The two remaining pathways were found to be of competing energy. The first proceeds through the asymmetric, concerted transition structure 4(*)(+) with an activation barrier E(a) = 16.2 kcal/mol and an overall exothermicity of -23.8 kcal/mol. The second pathway, beginning from the cis,cis,trans rotamer of 1(*)(+), proceeds by a stepwise pathway to the cyclohexadiene product with an overall exothermicity of -18.6 kcal/mol. The activation energy for the rate-determining step in this process, the formation of the intermediate bicyclo[3.1.0]hex-2-ene via transition structure 9(*)(+), was found to be 20.4 kcal/mol. More rigorous calculations of a smaller subsection of the potential energy hypersurface at the QCISD(T)//QCISD level confirmed these findings and emphasized the importance of conformational control of the reactant.     

Modern valence-bond description of chemical reaction mechanisms: the 1,3-dipolar addition of diazomethane to ethene

The electronic mechanism for the gas-phase concerted 1,3-dipolar cycloaddition of diazomethane (CH2N2) to ethene (C2H4) is described through spin-coupled (SC) calculations at a sequence of geometries along the intrinsic reaction coordinate obtained at the MP2/6-31G(d) level of theory. It is shown that the bonding rearrangements occurring during the course of this reaction follow a heterolytic pattern, characterized by the movement of three well-identifiable orbital pairs, which are initially responsible for the pi bond in ethene and the C-N pi bond and one of the N-N pi bonds in diazomethane and are retained throughout the entire reaction path from reactants to product. Taken together with our previous SC study of the electronic mechanism of the 1,3-dipolar cycloaddition of fulminic acid (HCNO) to ethyne (C2H2) (Theor. Chim. Acc. 1998, 100, 222), the results of the present work suggest strongly that most gas-phase concerted 1,3-dipolar cycloaddition reactions can be expected to follow a heterolytic mechanism of this type, which does not involve an aromatic transition state. The more conventional aspects of the gas-phase concerted 1,3-dipolar cycloaddition of diazomethane to ethene, including optimized transition structure geometry, electronic activation energy, activation barrier corrected for zero-point energies, standard enthalpy, entropy and Gibbs free energy of activation, have been calculated at the HF/6-31G(d), B3LYP/6-31G(d), MP2/6-31G(d), MP2/6-31G(d,p), QCISD/6-31G(d) and CCD/6-31G(d) levels of theory. We also report the CCD/6-311++G(2d, 2p)//CCD/6-31G(d), MP4(SDTQ)/6-311++G(2d,2p)//CCD/6-31G(d) and CCSD(T)/6-311++G(2d, 2p)//CCD/6-31G(d) electronic activation energies.     

Possible strategies toward the elusive tetraaminodisilene

In this paper we predict, using quantum mechanical calculations, which diaminosilylenes would dimerize to produce strongly bound tetraaminodisilenes, which so far have proven to be elusive. The central idea is that diaminosilylenes with a small singlet-triplet energy difference would dimerize to strongly bonded disilenes. Calculations at the B3LYP/6-311++G(3df,2p)//MP2/6-31G(d) level of theory showed that the energy difference between the singlet and the triplet states (DeltaE(ST)) of diaminosilylenes (R(2)N)(2)Si: (1) strongly depends on (i) the twist angle varphi between the SiN(2) and the R(2)N planes and (ii) the NSiN bond angle alpha at the divalent silicon. DeltaE(ST) decreases with increased twisting (larger varphi) and with widening of alpha. DeltaE(ST) is reduced from 70.7 kcal mol(-1) for planar (H(2)N)(2)Si: (1a) to DeltaE(ST) = 21.7 kcal mol(-1) when varphi is held at 90 degrees. Likewise, the bicyclic diaminosilylenes 1,4-diaza-7-silabicyclo[2.2.1]hepta-7-ylidene and 1,5-diaza-9-silabicyclo[3.3.1]nona-9-ylidene (4a,b), with the nitrogens in the bridgehead positions (varphi = 90 degrees), have DeltaE(ST) values of 45.1 and 38.3 kcal mol(-1), respectively. When dimerized, these silylenes form strongly bonded disilenes 5 (E(dim) = -32.2 kcal mol(-1) (4a) and -41.3 kcal mol(-1) (4b)) with Si=Si bond lengths of 2.239 A (4a) and 2.278 A (4b) (MP2/6-31G(d)//MP2/6-31G(d)). These theoretical predictions pave the way for the synthesis of the first strongly bonded tetraaminodisilene. Due to the steric requirements, also silyl substitution at nitrogen has a significant effect on DeltaE(ST) and [(H(3)Si)(2)N](2)Si: (1d) is predicted to form a stable Si=Si bonded dimer (E(dim)= -24.1 kcal mol(-1)). However, the larger size of the Me(3)Si substituent prevents the formation of a Si=Si bonded dimer of [(Me(3)Si)(2)N](2)Si: (1e).     

Gaseous reaction mechanism of C2F radical with water

The kinetic properties of the carbon-fluorine radicals are little understood except those of CFn (n =1-3). In this article, a detailed mechanistic study was reported on the gas-phase reaction between the simplest pi-bonded C2F radical and water as the first attempt to understand the chemical reactivity of the C2F radical. Various reaction channels are considered. The most kinetically competitive channel is the quasi-direct hydrogen-abstraction route forming P5 HCCF + OH. At the CCSD(T)/6-311+G(2d,2p)//B3LYP/6-311G(d,p)+ZPVE, CCSD(T)/6-311+G(3df,2p)//QCISD/6-311G(d,p)+ZPVE and Gaussian-3//B3LYP/6-31G(d) levels, the overall H-abstraction barriers (4.5, 4.7, and 4.2 kcal/mol) for the C2F + H2O reaction are comparable to the corresponding values (5.5, 3.7, and 5.7 kcal/mol) for the analogous C2H + H2O reaction. This suggests that C2F is a reactive radical like the extensively studied C2H, in contrast to the situation of the CF and CF2 radicals that have much lower reactivity than the corresponding hydrocarbon species. Thus, the C2F radical is expected to play an important role in the combustion processes of the carbon-fluorine chemistry. Furthermore, addition of a second H2O can catalyze the reaction with the H-abstraction barrier significantly reduced to a marginally zero value (0.5 kcal/mol). This is also indicative of the potential relevance of the title reactions in the low-temperature atmospheric chemistry.     

Reactivity of alkyl versus silyl peroxides. The consequences of 1, 2-silicon bridging on the epoxidation of alkenes with silyl hydroperoxides and bis(trialkylsilyl)peroxides

The bond dissociation energies for a series of silyl peroxides have been calculated at the G2 and CBS-Q levels of theory. A comparison is made with the O-O BDE of the corresponding dialkyl peroxides, and the effect of the O-O bond strength on the activation barrier for oxygen atom transfer is discussed. The O-O bond dissociation enthalpies (DeltaH(298)) for bis (trimethylsilyl) peroxide (1) and trimethylsilyl hydroperoxide (2) are 54.8 and 53.1 kcal/mol, respectively at the G2 (MP2) and CBS-Q levels of theory. The O-O bond dissociation energies computed at G2 and G2(MP2) levels for bis(tert-butyl) peroxide and tert-butyl hydroperoxide are 45.2 and 48.3 kcal/mol, respectively. The barrier height for 1,2-methyl migration from silicon to oxygen in trimethylsilyl hydroperoxide is 47.9 kcal/mol (MP4//MP2/6-31G). The activation energy for the oxidation of trimethylamine to its N-oxide by bis(trimethylsilyl) peroxide is 28.2 kcal/mol (B3LYP/6-311+G(3df,2p)// B3LYP/6-31G(d)). 1,2-Silicon bridging in the transition state for oxygen atom transfer to a nucleophilic amine results in a significant reduction in the barrier height. The barrier for the epoxidation of E-2-butene with bis(dimethyl(trifluoromethyl))silyl peroxide is 25.8 kcal/mol; a reduction of 7.5 kcal/mol relative to epoxidation with 1. The activation energy calculated for the epoxidation of E-2-butene with F(3)SiOOSiF(3) is reduced to only 2.2 kcal/mol reflecting the inductive effect of the electronegative fluorine atoms.     

Theoretical study and rate constant computation on the reaction HFCO + OH --> CFO + H2O

The potential energy surface, including the geometries and frequencies of the stationary points, of the reaction HFCO + OH is calculated using the MP2 method with 6-31+G(d,p) basis set, which shows that the direct hydrogen abstraction route is the most dominating channel with respect to addition and substitution channels. For the hydrogen abstraction reaction, the single-point energies are refined at the QCISD(T) method with 6-311++G(2df,2pd) basis set. The calculated standard reaction enthalpy and barrier height are -17.1 and 4.9 kcal mol(-1), respectively, at the QCISD(T)/6-311++G(2df,2pd)//MP2/6-31+G(d,p) level of theory. The reaction rate constants within 250-2500 K are calculated by the improved canonical variational transition state theory (ICVT) with small-curvature tunneling (SCT) correction at the QCISD(T)/6-311++G(2df,2pd)//MP2/6-31+G(d,p) level of theory. The fitted three-parameter formula is k = 2.875 x 10(-13) (T/1000)1.85 exp(-325.0/T) cm(3) molecule(-1) s(-1). The results indicate that the calculated ICVT/SCT rate constant is in agreement with the experimental data, and the tunneling effect in the lower temperature range plays an important role in computing the reaction rate constants.     

On the dimerization process of nitroso compounds

Summary Many organic C-nitroso compounds R-NO form stable dimers with a covalent NN bond. To gain insight into the dimerization reaction 2 R-NO (R-NO)₂ a theoretical study of the dimerization to atrans-form was performed using HNO as a model compound. Complete geometry optimizations were carried out at the HF, MP2 and QCISD levels using a 6–31G* basis. In the stationary points energies were calculated at the MP4(SDTQ) and QCISD(T) levels. For the equilibrium structure of the monomer and dimers stable RHF solutions were found, whereas for the TS UHF and UMPn calculations were applied. Extensive spin contamination was found in the UHF wavefunction, and projections up tos+4 were invoked. Relative energies were corrected for differences in ZPE. Calculations were made (a) for the least-motion path (C _2h symmetry) and (b) for a path with complete relaxation of all internal coordinates. Along the latter path a TS having virtuallyC _i symmetry was found. Along path (a) an activation energy of around 150 kcal/mol was predicted, in conformity with a symmetry forbidden reaction. On the relaxed path (b) the barrier to dimerization was estimated to be 10.7 kcal/mol at the MP4(SDTQ)//MP2 level, and 10.9 kcal/mol at the QCISD(T)//QCISD level. Unscaled ZPE corrections, calculated at the SCF level, changed these values to 12.7 and 12.9 kcal/mol, respectively. The reaction energy for the dimerization process is predicted to be – 17.2 kcal/mol at the MP4(SDTQ)//MP2 level corrected for ZPE. Calculations at the G1 level gave a corresponding value of – 16.4 kcal/mol. The equilibrium constant for the association to thetrans dimer is estimated to beK _p =259 atm, indicating that the dimer should be an observable species in the gas phase.     

Theoretical study on germanium cyanide radical GeCN and its ions

A detailed theoretical study is performed on the hitherto unknown germanium cyanide radical and its ions. The (2)Pi state GeCN lies 5.0 kcal/mol lower than the (2)Pi state GeNC at the coupled-cluster theory including single and double excitations and perturbative inclusion of triple excitations [CCSD(T)]/6-311++G(3df)//quadratic configuration interaction with single and double excitations (QCISD)/6-311G(d)+zero-point vibrational energy (ZPVE) level. For interconversion between them, two electronic state pathways (2)A(') and (2)A(") are located, with the latter being 0.7 kcal/mol more favorable than the former. On the (2)A(") path, the GeCN-->GeNC and GeNC-->GeCN conversion barriers are 14.5 and 9.5 kcal/mol, respectively. The detailed singlet and triplet potential-energy surfaces of both the cationic and anionic GeCN species are also investigated. On the ground-state electronic hypersurface, singlet GeNC(+) is 4.6 kcal/mol more stable than singlet GeCN(+), whereas triplet GeNC(-) is 10.0 kcal/mol less stable than triplet GeCN(-). The relative energy difference between the GeCN(0,+/-) and GeNC(0,+/-) can be well correlated with the number of vacant orbitals on the Ge atom. The stability of the neutral and ionic CGeN and cyclic cGeCN is also discussed. The predicted structures, spectroscopies, ionization, and affinity energies as well as the Renner-Teller properties are expected to provide reliable estimates for future characterization of the potential GeCN and GeNC radicals as well as their ionic counterparts both in the laboratory and in the interstellar space.     

An ab initio correlated study of the potential energy surface for the HOBr.H2O complex

The potential energy surface (PES) for the HOBr.H(2)O complex has been investigated using second- and fourth-order M?ller-Plesset perturbation theory (MP2, MP4) and coupled cluster theory with single and doubles excitations (CCSD), and a perturbative approximation of triple excitations (CCSD-T), correlated ab initio levels of theory employing basis sets of triple zeta quality with polarization and diffuse functions up to the 6-311++G(3dp,3df ) standard Pople's basis set. Six stationary points being three minima, two first-order transition state (TS) structures and one second-order TS were located on the PES. The global minimum syn and the anti equilibrium structure are virtually degenerated [DeltaE(ele-nuc) approximately 0.3 kcal mol(-1), CCSD-T/6-311++G(3df,3pd) value], with the third minima being approximately 4 kcal mol(-1) away. IRC analysis was performed to confirm the correct connectivity of the two first-order TS structures. The CCSD-T/6-311++G(3df,3pd)//MP2/6-311G(d,p) barrier for the syn<-->anti interconversion is 0.3 kcal mol(-1), indicating that a mixture of the syn and anti forms of the HOBr.H(2)O complex is likely to exist.     

Thermolysis of alkyl sulfoxides and derivatives: a comparison of experiment and theory.

Gas-phase activation data were obtained for model sulfoxide elimination reactions. The activation enthalpy for methyl 3-phenylpropyl sulfoxide is 32.9 +/- 0.9 kcal/mol. Elimination by methyl vinyl sulfoxide to form acetylene has an enthalpic barrier of 41.6 +/- 0.8 kcal/mol and that of 3-phenylpropyl methanesulfinate to form hydrocinnamaldehyde is 34.6 +/- 0.6 kcal/mol. Calculations at the MP2/6-311+G(3df,2p)//MP2/6-31G(d,p) level for simplified models of these reactions provide barriers of 32.3, 40.3, and 32.7 kcal/mol, respectively. A series of other compounds are examined computationally, and it is shown that the substituent effects on the sulfoxide elimination reaction are much more straightforward to interpret if DeltaH data are available in addition to the usually determined DeltaH++. The activation enthalpy of the reverse addition reaction is also subject to structural variation and can usually be rationalized on the basis of nucleophilicity of the sulfur or polarity matching between the sulfenic acid and olefin derivative.     

2+3] Cycloaddition of ethylene to transition metal oxo compounds. analysis of density functional results by marcus theory.

Density functional results on the [2+3] cycloaddition of ethylene to various transition metal complexes MO(3)(q) and LMO(3)(q) (q = -1, 0, 1) with M = Mo, W, Mn, Tc, Re, and Os and various ligands L = Cp, CH(3), Cl, and O show that the corresponding activation barriers DeltaE(double dagger) depend in quadratic fashion on the reaction energies DeltaE(0) as predicted by Marcus theory. A thermoneutral reaction is characterized by the intrinsic reaction barrier DeltaE(0) of 25.1 kcal/mol. Both ethylene [2+3] cycloaddition to an oxo complex and the corresponding homolytic M-O bond dissociation are controlled by the reducibility of the transition metal center. Indeed, from the easily calculated M-O bond dissociation energy of the oxo complex one can predict the reaction energy DeltaE(0) and hence, by Marcus theory, the corresponding activation barrier DeltaE. This allows a systematic representation of more than 25 barriers of [2+3] cycloaddition reactions that range from 5 to 70 kcal/mol.     

Predictions of substituent effects in thermal azide 1,3-dipolar cycloadditions: implications for dynamic combinatorial (reversible) and click (irreversible) chemistry

Substituent effects in 1,3-dipolar cycloadditions of azides with alkenes and alkynes were investigated with the high-accuracy CBS-QB3 method. The possibilities for noncatalytic activation and the reversibility or irreversibility of these reactions was explored; the possibilities for uses in dynamic combinatorial chemistry (DCC) or click chemistry were explored. The activation enthalpies for reactions of ethylene and acetylene with hydrazoic acid, formyl, phenyl-, methyl-, and methanesulfonylazides exhibit modest variation, with Delta H++ ranging from 17 to 20 kcal/mol. A detailed study of formylazide cycloadditions with various alkenes and alkynes reveals a narrow range of activation enthalpies (17-21 kcal/mol). The activation enthalpies for the reactions of azides with alkenes and alkynes are similar. FMO theory and distortion/interaction energy control have been used to rationalize the rates and regiochemistries of cycloadditions involving alkene dipolarophiles. Significantly, triazoles, formed from alkynes, are 30-40 kcal/mol more stable than tetrazolines formed from alkenes. On the basis of initial reactant concentrations, kinetic and thermodynamic values are suggested for the identification of reversible reactions that approach equilibrium over 24 h, as well as for fast irreversible reactions. Although azide cycloadditions are suitable for irreversible chemistry and are typically unsuitable for reversible applications, theoretical procedures established by these studies have provided guidelines for the prediction of useful reversible libraries.     

Computational design of S‐nitrosothiol “click” reactions

To address a long‐standing problem of finding efficient reactions for chemical labeling of protein‐based S‐nitrosothiols (RSNOs), we computationally explored hitherto unknown (3+2) cycloaddition RSNO reactions with alkynes and alkenes. Nonactivated RSNO cycloaddition reactions have high activation enthalpy (>20 kcal/mol at the CBS‐QB3 level) and compete with alternative S—N bond insertion pathway. However, the (3+2) cycloaddition reaction barriers can be dramatically lowered by coordination of a Lewis acid to the N atom of the —SNO group. To exploit this effect, we propose to use reagents with Lewis acid and a strain‐activated carbon–carbon multiple bond linked by a rigid scaffold, which can react with RSNOs with small activation enthalpies (～5 kcal/mol) and high reaction exothermicities (～40 kcal/mol). The proposed efficient RSNO cycloaddition reactions can be used for future development of practical RSNO labeling reactions. © 2013 Wiley Periodicals, Inc.     

Mechanisms and kinetics for the thermal decomposition of 2-azido-N,N-dimethylethanamine (DMAZ)

To gain insight into the mechanisms and kinetics of 2-azido-N,N-dimethylethanamine's (DMAZ's) thermal decomposition postulated reaction paths were simulated with ab initio and density functional theory quantum chemistry models. Four reaction types were modeled: (i) spin-allowed and spin-forbidden paths involving N-N(2) bond fission and nitrene formation, (ii) HN(3) elimination with the formation of (dimethylamino)ethylene, (iii) N-N(2) bond fission with the formation of molecules with three- or four-membered heterocyclic rings, and (iv) simple scission of C-H, C-N, and C-C bonds. The geometries of stationary points of the reactions were obtained with a MPWB1K/6-31+G(d,p) model. To locate and model the geometries of minimum energy intersystem crossing points for triplet nitrene formation and isomerization, unrestricted broken spin symmetry calculations were performed. Employed to model an analogous path for methyl azide's decomposition, this approach was found to yield results similar to those obtained with a CASSCF(10,8)/aug-cc-pVDZ model. Of the four reaction types studied, N-N(2) bond fissions with singlet or triplet nitrene formation were found to have the lowest barriers. Barriers for paths to cyclic products were found to be 2-4 kcal/mol higher. Kinetic rate expressions for individual paths were derived from the quantum chemistry results, and spin-allowed nitrene formation was found to be dominant at all temperatures and pressures examined. The expression 2.69 × 10(9) (s(-1))T(1.405) exp(-39.0 (kcal/mol)/RT), which was derived from QCISD(T)/6-31++G(3df,2p)//MPWB1K/6-31+G(d,p) results, was found to be representative of this reaction's gas-phase rate. Adjusted on the basis of results from self-consistent reaction field models to account for solvation by n-dodecane, the expression became 1.11 × 10(9) (s(-1))T(1.480) exp(-37.6 (kcal/mol)/RT). Utilizing this result and others derived in the study, a model of the decomposition of n-dodecane-solvated DMAZ was constructed, and it generated simulations that well-reproduce previously published measured data for the process.     

Structural Distortion of Cycloalkynes Influences Cycloaddition Rates both by Strain and Interaction Energies

The reactivities of 2-butyne, cycloheptyne, cyclooctyne, and cyclononyne in the 1,3-dipolar cycloaddition reaction with methyl azide were evaluated through DFT calculations at the M06-2X/6-311++G(d)//M06-2X/6-31+G(d) level of theory. Computed activation free energies for the cycloadditions of cycloalkynes are 16.5–22.0 kcal mol⁻¹ lower in energy than that of the acyclic 2-butyne. The strained or predistorted nature of cycloalkynes is often solely used to rationalize this significant rate enhancement. Our distortion/interaction–activation strain analysis has been revealed that the degree of geometrical predistortion of the cycloalkyne ground-state geometries acts to enhance reactivity compared with that of acyclic alkynes through three distinct mechanisms, not only due to (i) a reduced strain or distortion energy, but also to (ii) a smaller HOMO–LUMO gap, and (iii) an enhanced orbital overlap, which both contribute to more stabilizing orbital interactions.     

Infrared spectroscopy of ozone-water complex in a neon matrix

Matrix isolation infrared spectroscopy has been applied to study an ozone-water complex of atmospheric interest. The complex was identified in the spectral region of three normal modes of ozone and water. Ab initio calculation at MP4(SDQ), QCISD, and CCSD(T) levels indicates the existence of only one stable conformer, which accords with the present experimental result. This conformer belongs to the Cs symmetry group where two molecular planes of ozone and water are perpendicular to the Cs symmetry plane. The binding energy was calculated to be 1.89 kcal/mol at the CCSD(T)/6-311++G(3df,3pd)//CCSD(T)/6-311++G(d,p) level of theory. The formation constant and atmospheric abundance of the ozone-water complex are estimated using the thermodynamic and spectroscopic data obtained.