A crossed molecular beam study on the formation of hexenediynyl radicals (H(2)CCCCCCH; C(6)H(3) (X(2)A')) via reactions of tricarbon molecules, C(3)(X(1)Sigma(g)(+)), with allene (H(2)CCCH(2); X(1)A(1)) and methylacetylene (CH(3)CCH; X(1)A(1))

Crossed molecular beams experiments have been utilized to investigate the reaction dynamics between two closed shell species, i.e. the reactions of tricarbon molecules, C(3)(X(1)Sigma(g)(+)), with allene (H(2)CCCH(2); X(1)A(1)), and with methylacetylene (CH(3)CCH; X(1)A(1)). Our investigations indicated that both these reactions featured characteristic threshold energies of 40-50 kJ mol(-1). The reaction dynamics are indirect and suggested the reactions proceeded via an initial addition of the tricarbon molecule to the unsaturated hydrocarbon molecules forming initially cyclic reaction intermediates of the generic formula C(6)H(4). The cyclic intermediates isomerize to yield eventually the acyclic isomers CH(3)CCCCCH (methylacetylene reaction) and H(2)CCCCCCH(2) (allene reaction). Both structures decompose via atomic hydrogen elimination to form the 1-hexene-3,4-diynyl-2 radical (C(6)H(3); H(2)CCCCCCH). Future flame studies utilizing the Advanced Light Source should therefore investigate the existence of 1-hexene-3,4-diynyl-2 radicals in high temperature methylacetylene and allene flames. Since the corresponding C(3)H(3), C(4)H(3), and C(5)H(3) radicals have been identified via their ionization potentials in combustion flames, the existence of the C(6)H(3) isomer 1-hexene-3,4-diynyl-2 can be predicted as well.     

Clar goblet and related non-Kekulé benzenoid LPAHs. A theoretical study

The results of an ab initio and semiempirical study of Clar Goblet (1), a C(38)H(18) non-Kekulé diradical LPAH, and its constitutional isomers 4 and 5 are reported. Planar D(2)(h)-1 was only 87.4 (triplet) and 83.8 (singlet) kJ/mol less stable than its planar Kekulé isomer C(2)(v)-6 (at (U)B3LYP/6-31G). Planar C(s)-4 was 63.6 (triplet) and 76.5 (singlet) kJ/mol less stable than 6. Overcrowded C(1)-5 was 80.1 (triplet) and 98.1 (singlet) kJ/mol less stable than 6. In concealed non-Kekulé 1, the singlet was more stable then the triplet by 3.6 kJ/mol, while in nonconcealed non-Kekulé 4 and 5, the triplets were more stable than the corresponding singlets by 12.9 and 18.1 kJ/mol, respectively, in accordance with theory. The spin density in 1, 4, and 5 is delocalized throughout the positions corresponding to active peri-peri coupling positions of the radical anion of naphthanthrone (2). The bond lengths in 1, 4, and 5 are in the range expected for aromatic compounds, except for the central carbon-carbon bonds, which are considerably elongated. A certain stabilization is evident in the homodesmotic reaction singlet-1 + 10 + 10 --> 11 + 3 + 3, indicating a "communication" between the two benzo[cd]pyrenyl radical (3) units of diradical 1. The HOMA indices indicate that in both singlet 1 and triplet 1 all of the rings except the central one have a significant aromatic character. The central ring is essentially antiaromatic, having negative HOMA index (-0.140 at UB3LYP/6-31+G). The stabilities of 1(2)(-) and 1(2+) are decreased relative to 3(-) and 3(+), respectively.     

Interconversions of Phenylcarbene, Cycloheptatetraene, Fulvenallene, and Benzocyclopropene. A Theoretical Study of the C(7)H(6) Energy Surface

Ab initio calculations at the G2(MP2,SVP) and B-LYP/6-311+G(3df,2p)+ZPVE levels have been used to examine the potential energy surface of C(7)H(6). Fulvenallene (6) is the most stable C(7)H(6) isomer considered in this study. 1-Ethynylcyclopentadiene (9A), benzocyclopropene (10), and 1,2,4,6-cycloheptatetraene (4) lie 12, 29, and 49 kJ mol(-)(1), respectively, above 6. Phenylcarbene (1) is calculated is to have a triplet ((3)A") ground state, 16 kJ mol(-)(1) more stable than the singlet state ((1)A'). Interconversion of 1 and 4 is predicted to have a moderate activation barrier, with the involvement of a stable bicyclic intermediate (bicyclo[4.1.0]hepta-2,4,6-triene, 2). However, 2 is found to lie in a shallow potential energy well with a small barrier (8 kJ mol(-)(1)) to rearrangement to 4. At the G2(RMP2,SVP)//QCI level, the (3)A(2) and (3)B(1) triplet states of cycloheptatrienylidene ((3)3) are predicted to lie very close in energy. The singlet "aromatic" cycloheptatrienylidene ((1)3, (1)A(1)) is found to be a transition structure interconverting two chiral cyclic allenes (4) and it lies approximately 25 kJ mol(-)(1) below the triplet states. Bicyclo[3.2.0]hepta-1,3,6-triene (5) is predicted to be a stable equilibrium structure, lying in a significant energy well. Rearrangement of 4 to 5 constitutes the rate-determining step for the rearrangement of phenylcarbene to fulvenallene (6), the ethynylcyclopentadienes (9), and spiro[2.4]heptatriene (7). Rearrangement of 9A to 6, via a 1,4-H shift, requires a large barrier of 325 kJ mol(-)(1). Rearrangement of benzocyclopropene (10) to 6 involves a methylenecyclohexadienylidene intermediate (27) and is associated with an energy barrier of 223 kJ mol(-)(1). The calculated mechanisms and energetics for the interconversions of various C(7)H(6) isomers are in good accord with experimental results to date.     

Chemical dynamics of the formation of the 1,3-butadiynyl radical (C4H(X2Sigma+)) and its isotopomers

The reaction of dicarbon molecules in their electronic ground, C2(X1Sigma(g)+), and first excited state, C2(a3Pi(u)), with acetylene, C2H2(X1Sigma(g)+), to synthesize the 1,3-butadiynyl radical, C4H(X2Sigma+), plus a hydrogen atom was investigated at six different collision energies between 10.6 and 47.5 kJ mol(-1) under single collision conditions. These studies were contemplated by crossed molecular beam experiments of dicarbon with three acetylene isotopomers C2D2(X1Sigma(g)+), C2HD (X1Sigma+), and 13C2H2(X1Sigma(g)+) to elucidate the role of intersystem crossing (ISC) and of the symmetry of the reaction intermediate(s) on the center-of-mass functions. On the singlet surface, dicarbon was found to react with acetylene through an indirect reaction mechanism involving a diacetylene intermediate. The latter fragmented via a loose exit transition state via an emission of a hydrogen atom to form the 1,3-butadiynyl radical C4H(X2Sigma+). The D(infinity)(h) symmetry of the decomposing diacetylene intermediate results in collision-energy invariant, isotropic (flat) center-of-mass angular distributions of this microchannel. Isotopic substitution experiments suggested that at least at a collision energy of 29 kJ mol(-1), the diacetylene isotopomers are long-lived with respect to their rotational periods. On the triplet surface, the reaction involved three feasible addition complexes located in shallower potential energy wells as compared to singlet diacetylene. The involvement of the triplet surface accounted for the asymmetry of the center-of-mass angular distributions. The detection of the 1,3-butadiynyl radical, C4H(X2Sigma+), in the crossed beam reaction of dicarbon molecules with acetylene presents compelling evidence that the 1,3-butadiynyl radical can be formed via bimolecular reactions involving carbon clusters in extreme environments such as circumstellar envelopes of dying carbon stars and combustion flames.     

Unimolecular decomposition of chemically activated pentatetraene (H2CCCCCH2) intermediates: A crossed beams study of dicarbon molecule reactions with allene

The reactions dynamics of the dicarbon molecule C2 in the 1Sigma (g)+ singlet ground state and 3Pi(u) first excited triplet state with allene, H2CCCH2(X1A1), was investigated under single collision conditions using the crossed molecular beam approach at four collision energies between 13.6 and 49.4 kJ mol(-1). The experiments were combined with ab initio electronic structure calculations of the relevant stationary points on the singlet and triplet potential energy surfaces. Our investigations imply that the reactions are barrier-less and indirect on both the singlet and the triplet surfaces and proceed through bound C5H4 intermediates via addition of the dicarbon molecule to the carbon-carbon double bond (singlet surface) and to the terminal as well as central carbon atoms of the allene molecule (triplet surface). The initial collision complexes isomerize to form triplet and singlet pentatetraene intermediates (H2CCCCCH2) that decompose via atomic hydrogen loss to yield the 2,4-pentadiynyl-1 radical, HCCCCCH2(X2B1). These channels result in symmetric center-of-mass angular distributions. On the triplet surface, a second channel involves the existence of a nonsymmetric reaction intermediate (HCCCH2CCH) that fragments through atomic hydrogen emission to the 1,4-pentadiynyl-3 radical [C5H3(X2B1)HCCCHCCH]; this pathway was found to account for the backward scattered center-of-mass angular distributions at higher collision energies. The identification of two resonance-stabilized free C5H3 radicals (i.e., 2,4-pentadiynyl-1 and 1,4-pentadiynyl-3) suggests that these molecules can be important transient species in combustion flames and in the chemical evolution of the interstellar medium.     

Reactions of o-benzyne with propargyl and benzyl radicals: potential sources of polycyclic aromatic hydrocarbons in combustion

The kinetics and mechanisms of the reactions of o-benzyne with propargyl and benzyl radicals have been investigated computationally. The possible reaction pathways have been explored by quantum chemical calculations at the M06-2X/6-311+G(3df,2p)//B3LYP/6-311G(d,p) level and the mechanisms have been investigated by the Rice-Ramsperger-Kassel-Marcus theory/master-equation calculations. It was found that the o-benzyne associates with the propargyl and benzyl radicals without pronounced barriers and the activated adducts easily isomerize to five-membered ring species. Indenyl radical and fluorene + H were predicted to be dominantly produced by the reactions of o-benzyne with propargyl and benzyl radicals, respectively, with the rate constants close to the high-pressure limits at temperatures below 2000 K. The related reactions on the two potential energy surfaces, namely, the reaction between fulvenallenyl radical and acetylene and the decomposition reactions of indenyl and α-phenylbenzyl radicals were also investigated. The high reactivity of o-benzyne toward the resonance stabilized radicals suggested a potential role of o-benzyne as a precursor of polycyclic aromatic hydrocarbons in combustion.     

On the formation of resonantly stabilized C5H3 radicals--a crossed beam and ab initio study of the reaction of ground state carbon atoms with vinylacetylene

The formation of polycyclic aromatic hydrocarbons in combustion environments is linked to resonance stabilized free radicals. Here, we investigated the reaction dynamics of ground state carbon atoms, C((3)P(j)), with vinylacetylene at two collision energies of 18.8 kJ mol(-1) and 26.4 kJ mol(-1) employing the crossed molecular beam technique leading to two resonantly stabilized free radicals. The reaction was found to be governed by indirect scattering dynamics and to proceed without an entrance barrier through a long-lived collision complex to reach the products, n- and i-C(5)H(3) isomers via tight exit transition states. The reaction pathway taken is dependent on whether the carbon atom attacks the π electron density of the double or triple bond, both routes have been compared to the reactions of atomic carbon with ethylene and acetylene. Electronic structure/statistical theory calculations determined the product branching ratio to be 2:3 between the n- and i-C(5)H(3) isomers.     

Structure and Stability of Interstellar Molecule C_3S

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Dissociative addition of water to a main group 13 metal cluster: computational study of the reaction of gallium dimer with H2O

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.     

Increasing stability of the fullerenes with the number of carbon atoms: the experimental evidence

The values of the molar standard enthalpies of formation, Delta(f)H(o)(m)(C(76), cr) = (2705.6 +/- 37.7) kJ x mol(-1), Delta(f)H(o)(m)(C(78), cr) = (2766.5 +/- 36.7) kJ x mol(-1), and Delta(f)H(o)(m)(C(84), cr) = (2826.6 +/- 42.6) kJ x mol(-1), were determined from the energies of combustion, measured by microcombustion calorimetry on a high-purity sample of the D(2) isomer of fullerene C(76), as well as on a mixture of the two most abundant constitutional isomers of C(78) (C(2nu)-C(78) and D(3)-C(78)) and C(84) (D(2)-C(84), and D(2d)-C(84). These values, combined with the published data on the enthalpies of sublimation of each cluster, lead to the gas-phase enthalpies of formation, Delta(f)H(o)(m)(C(76), g) = (2911.6 +/- 37.9) kJ x mol(-1); Delta(f)H(o)(m)(C(78), g) = (2979.3 +/- 37.2) kJ x mol(-1), and Delta(f)H(o)(m)(C(84), (g)) = (3051.6 +/- 43.0) kJ x mol(-1), results that were found to compare well with those reported from density functional theory calculations. Values of enthalpies of atomization, strain energies, and the average C-C bond energy were also derived for each fullerene. A decreasing trend in the gas-phase enthalpy of formation and strain energy per carbon atom as the size of the cluster increases is found. This is the first experimental evidence that these fullerenes become more stable as they become larger. The derived experimental average C-C bond energy E(C-C) = 461.04 kJ x mol(-1) for fullerenes is close to the average bond energy E(C-C) = 462.8 kJ x mol(-1) for polycyclic aromatic hydrocarbons (PAHs).     

Ab initio energies and product branching ratios for the O+C3H6 reaction

Intermediate and transition-state energies have been calculated for the O+C3H6 (propene) reaction using the compound ab initio CBS-QB3 and G3 methods in combination with density functional theory. The lowest-lying triplet and singlet potential energy surfaces of the O-C3H6 system were investigated. RRKM statistical theory was used to predict product branching fractions over the 300-3000 K temperature and 0.001-760 Torr pressure ranges. The oxygen atom adds to the C3H6 terminal olefinic carbon in the primary step to generate a nascent triplet biradical, CH3CHCH2O. On the triplet surface, unimolecular dissociation of CH3CHCH2O to yield H+CH3CHCHO is favored over the entire temperature range, although the competing H2CO+CH3CH product channel becomes significant at high temperature. Rearrangement of triplet CH3CHCH2O to CH3CH2CHO (propanal) via a 1,2 H-atom shift has a barrier of 122.3 kJ mol(-1), largely blocking this reaction channel and any subsequent dissociation products. Intersystem crossing of triplet CH3CHCH2O to the singlet surface, however, leads to facile rearrangement to singlet CH3CH2CHO, which dissociates via numerous product channels. Pressure was found to have little influence over the branching ratios under most conditions, suggesting that the vibrational self-relaxation rates for p相似文献   

Energetics of hydroxybenzoic acids and of the corresponding carboxyphenoxyl radicals. Intramolecular hydrogen bonding in 2-hydroxybenzoic acid

The energetics of the phenolic O-H bond in the three hydroxybenzoic acid isomers and of the intramolecular hydrogen O-H- - -O-C bond in 2-hydroxybenzoic acid, 2-OHBA, were investigated by using a combination of experimental and theoretical methods. The standard molar enthalpies of formation of monoclinic 3- and 4-hydroxybenzoic acids, at 298.15 K, were determined as Delta(f)(3-OHBA, cr) = -593.9 +/- 2.0 kJ x mol(-1) and Delta(f)(4-OHBA, cr) = -597.2 +/- 1.4 kJ x mol(-1), by combustion calorimetry. Calvet drop-sublimation calorimetric measurements on monoclinic samples of 2-, 3-, and 4-OHBA, led to the following enthalpy of sublimation values at 298.15 K: Delta(sub)(2-OHBA) = 94.4 +/- 0.4 kJ x mol(-1), Delta(sub)(3-OHBA) = 118.3 +/- 1.1 kJ x mol(-1), and Delta(sub)(4-OHBA) = 117.0 +/- 0.5 kJ x mol(-1). From the obtained Delta(f)(cr) and Delta(sub) values and the previously reported enthalpy of formation of monoclinic 2-OHBA (-591.7 +/- 1.3 kJ x mol(-1)), it was possible to derive Delta(f)(2-OHBA, g) = -497.3 +/- 1.4 kJ x mol(-1), Delta(f)(3-OHBA, g) = -475.6 +/- 2.3 kJ x mol(-1), and Delta(f)(4-OHBA, cr) = -480.2 +/- 1.5 kJ x mol(-1). These values, together with the enthalpies of isodesmic and isogyric gas-phase reactions predicted by density functional theory (B3PW91/aug-cc-pVDZ, MPW1PW91/aug-cc-pVDZ, and MPW1PW91/aug-cc-pVTZ) and the CBS-QMPW1 methods, were used to derive the enthalpies of formation of the gaseous 2-, 3-, and 4-carboxyphenoxyl radicals as (2-HOOCC(6)H(4)O(*), g) = -322.5 +/- 3.0 kJ.mol(-1) Delta(f)(3-HOOCC(6)H(4)O(*), g) = -310.0 +/- 3.0 kJ x mol(-1), and Delta(f)(4-HOOCC(6)H(4)O(*), g) = -318.2 +/- 3.0 kJ x mol(-1). The O-H bond dissociation enthalpies in 2-OHBA, 3-OHBA, and 4-OHBA were 392.8 +/- 3.3, 383.6 +/- 3.8, and 380.0 +/- 3.4 kJ x mol(-1), respectively. Finally, by using the ortho-para method, it was found that the H- - -O intramolecular hydrogen bond in the 2-carboxyphenoxyl radical is 25.7 kJ x mol(-1), which is ca. 6-9 kJ x mol(-1) above the one estimated in its parent (2-OHBA), viz. 20.2 kJ x mol(-1) (theoretical) or 17.1 +/- 2.1 kJ x mol(-1) (experimental).     

Energetics of cresols and of methylphenoxyl radicals

Combustion calorimetry studies were used to determine the standard molar enthalpies of formation of o-, m-, and p-cresols, at 298.15 K, in the condensed state as Delta(f)H(m) degrees (o-CH(3)C(6)H(4)OH,cr) = -204.2 +/- 2.7 kJ.mol(-1), Delta(f)H(m) degrees (m-CH(3)C(6)H(4)OH,l) = -196.6 +/- 2.1 kJ.mol(-1), and Delta(f)H(m) degrees (p-CH(3)C(6)H(4)OH,cr) = -202.2 +/- 3.0 kJ.mol(-1). Calvet drop calorimetric measurements led to the following enthalpy of sublimation and vaporization values at 298.15 K: Delta(sub)H(m) degrees (o-CH(3)C(6)H(4)OH) = 73.74 +/- 0.46 kJ.mol(-1), Delta(vap)H(m) degrees (m-CH(3)C(6)H(4)OH) = 64.96 +/- 0.69 kJ.mol(-1), and Delta(sub)H(m) degrees (p-CH(3)C(6)H(4)OH) = 73.13 +/- 0.56 kJ.mol(-1). From the obtained Delta(f)H(m) degrees (l/cr) and Delta(vap)H(m) degrees /Delta(sub)H(m) degrees values, it was possible to derive Delta(f)H(m) degrees (o-CH(3)C(6)H(4)OH,g) = -130.5 +/- 2.7 kJ.mol(-1), Delta(f)H(m) degrees (m-CH(3)C(6)H(4)OH,g) = -131.6 +/- 2.2 kJ.mol(-1), and Delta(f)H(m) degrees (p-CH(3)C(6)H(4)OH,g) = -129.1 +/- 3.1 kJ.mol(-1). These values, together with the enthalpies of isodesmic and isogyric gas-phase reactions predicted by the B3LYP/cc-pVDZ, B3LYP/cc-pVTZ, B3P86/cc-pVDZ, B3P86/cc-pVTZ, MPW1PW91/cc-pVTZ, CBS-QB3, and CCSD/cc-pVDZ//B3LYP/cc-pVTZ methods, were used to obtain the differences between the enthalpy of formation of the phenoxyl radical and the enthalpies of formation of the three methylphenoxyl radicals: Delta(f)H(m) degrees (C(6)H(5)O*,g) - Delta(f)H(m) degrees (o-CH(3)C(6)H(4)O*,g) = 42.2 +/- 2.8 kJ.mol(-1), Delta(f)H(m) degrees (C(6)H(5)O*,g) - Delta(f)H(m) degrees (m-CH(3)C(6)H(4)O*,g) = 36.1 +/- 2.4 kJ.mol(-1), and Delta(f)H(m) degrees (C(6)H(5)O*,g) - Delta(f)H(m) degrees (p-CH(3)C(6)H(4)O*,g) = 38.6 +/- 3.2 kJ.mol(-1). The corresponding differences in O-H bond dissociation enthalpies were also derived as DH degrees (C(6)H(5)O-H) - DH degrees (o-CH(3)C(6)H(4)O-H) = 8.1 +/- 4.0 kJ.mol(-1), DH degrees (C(6)H(5)O-H) - DH degrees (m-CH(3)C(6)H(4)O-H) = 0.9 +/- 3.4 kJ.mol(-1), and DH degrees (C(6)H(5)O-H) - DH degrees (p-CH(3)C(6)H(4)O-H) = 5.9 +/- 4.5 kJ.mol(-1). Based on the differences in Gibbs energies of formation obtained from the enthalpic data mentioned above and from published or calculated entropy values, it is concluded that the relative stability of the cresols varies according to p-cresol < m-cresol < o-cresol, and that of the radicals follows the trend m-methylphenoxyl < p-methylphenoxyl < o-methylphenoxyl. It is also found that these tendencies are enthalpically controlled.     

Gas phase generation of the neutrals H2CCCCO, HCCCCDO and CCCHCHO from anionic precursors. Rearrangements of HCCCCDO and CCCHCHO. A joint experimental and theoretical study

The reaction between O-. and MeO-CH2-C identical to C-CDO in the ion source of a VG ZAB 2HF mass spectrometer gives a number of product anions including [H2CCCCO]-. and [HCCCCDO]-. (in the ratio 1:5). Neutralisation-reionisation (NR+) of [H2CCCCO]-. results in the sequential two-electron vertical oxidation [H2CCCCO]-.-->H2CCCCO-->[H2CCCCO](+.). Singlet H2CCCCO lies 158 kJ mol-1 below the triplet [at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory]. The majority of neutrals H2CCCCO are stable for the microsecond duration of the NR experiment, but some are energized and decompose to give H2CCC and CO. A similar NR+ experiment with [HCCCCDO]-. yields neutrals HCCCCDO, some of which are excited and rearrange. Calculations show that it is the singlet form of HCCCCHO which rearranges (the singlet lies 36 kJ mol-1 above the ground state triplet): the rearrangement occurs by the sequential H transfer process, HCCCCHO-->HCC(CH)CO<--H2CCCCO. Neutral HCCCCHO needs an excess energy of only 43 kJ mol-1 to effect this reaction, which is exothermic by 230 kJ mol-1. Both HCC(CH)CO and H2CCCCO formed in this way should have sufficient excess energy to cause some loss of CO. The anions [CC(CH)CHO]-. and [CC(CD)CHO]-. are formed in the ion source of the mass spectrometer by the reactions of HO- with Me3SiC identical to C-CH = CHOMe and Me3SiC identical to C-CD = CHOMe respectively. NR+ of these anions indicate that energized forms of CC(CH)CHO and CC(CD)CHO may rearrange to isomer(s) which decompose by loss of CO. Singlet CC(CH)CHO rearranges to HCC(CH)CO and H2CCCCO, both of which are energized and fragment by loss of CO.     

Energetics of C-Cl, C-Br, and C-I bonds in haloacetic acids: enthalpies of formation of XCH2COOH (X=CI, Br, I) compounds and the carboxymethyl radical

The standard molar enthalpies of formation of chloro-, bromo-, and iodoacetic acids in the crystalline state, at 298.15 K, were determined as deltafH(o)m(C2H3O2Cl, cr alpha)=-(509.74+/- 0.49) kJ x mol(-1), deltafH(o)m(C2H3O2Br, cr I)-(466.98 +/- 1.08) kJ x mol(-1), and deltafH(o)m (C2H3O2I, cr)=-(415.44 +/- 1.53) kJ x mol(-1), respectively, by rotating-bomb combustion calorimetry. Vapor pressure versus temperature measurements by the Knudsen effusion method led to deltasubH(o)m(C2H3O2Cl)=(82.19 +/- 0.92) kJ x mol(-1), deltasubH(o)m(C2H3O2Br)=(83.50 +/- 2.95) kJ x mol(-1), and deltasubH(o)m-(C2H3O2I) = (86.47 +/- 1.02) kJ x mol(-1), at 298.15 K. From the obtained deltafH(o)m(cr) and deltasubH(o)m values it was possible to derive deltafH(o)m(C2H3O2Cl, g)=-(427.55 +/- 1.04) kJ x mol(-1), deltafH(o)m (C2H3O2Br, g)=-(383.48 +/- 3.14) kJ x mol(-1), and deltafH(o)m(C2H3O2I, g)=-(328.97 +/- 1.84) kJ x mol(-1). These data, taken with a published value of the enthalpy of formation of acetic acid, and the enthalpy of formation of the carboxymethyl radical, deltafH(o)m(CH2COOH, g)=-(238 +/- 2) kJ x mol(-1), obtained from density functional theory calculations, led to DHo(H-CH2COOH)=(412.8 +/- 3.2) kJ x mol(-1), DHo(Cl-CH2COOH)=(310.9 +/- 2.2) kJ x mol(-1), DHo(Br-CH2COOH)=(257.4 +/- 3.7) kJ x mol(-1), and DHo(I-CH2COOH)=(197.8 +/- 2.7) kJ x mol(-1). A discussion of the C-X bonding energetics in XCH2COOH, CH3X, C2H5X, C2H3X, and C6H5X (X=H, Cl, Br, I) compounds is presented.     

A meta effect in organic photochemistry? The case of SN1 reactions in methoxyphenyl derivatives

The photochemistry of isomeric methoxyphenyl chlorides and phosphates has been examined in different solvents (and in the presence of benzene) and found to involve the triplet state. With the chlorides, C-Cl bond homolysis occurs in cyclohexane and is superseded by heterolysis in polar media, while the phosphate group is detached (heterolytically) only in polar solvents. Under such conditions, the isomeric triplet methoxyphenyl cations are the first formed intermediates from both precursors, but intersystem crossing (isc) to the singlets can take place. Solvent addition (forming the acetanilide in MeCN, the ethers in alcohols, overall a SN1 solvolysis) is a diagnostic reaction for the singlet cation, as reduction and trapping by benzene are for the corresponding triplet. Solvolysis is most important with the meta isomer, for which the singlet is calculated (UB3LYP/6-31 g(d)) to be the ground state of the cation (DeltaE = 4 kcal/mol) and isc is efficient (kisc ca. 1 x 108 s-1), and occurs to some extent with the para isomer (isoenergetic spin states, kisc ca. 1.7 x 106 s-1). The triplet is the ground state with the ortho isomer, and in that case isc does not compete, although trapping by benzene is slow because of the hindering of C1 by the substituent. The position of the substituent thus determines the energetic order of the cation spin states, in particular through the selective stabilization of the singlet by the m-methoxy group, a novel case of "meta effect".     

Solvation properties of N-substituted cis and trans amides are not identical: significant enthalpy and entropy changes are revealed by the use of variable temperature 1H NMR in aqueous and chloroform solutions and ab initio calculations

The cis/trans conformational equilibrium of N-methyl formamide (NMF) and the sterically hindered tert-butylformamide (TBF) was investigated by the use of variable temperature gradient 1H NMR in aqueous solution and in the low dielectric constant and solvation ability solvent CDCl3 and various levels of first principles calculations. The trans isomer of NMF in aqueous solution is enthalpically favored relative to the cis (deltaH(o) = -5.79 +/- 0.18 kJ mol(-1)) with entropy differences at 298 K (298 x deltaS(o) = -0.23 +/- 0.17 kJ mol(-1)) playing a minor role. The experimental value of the enthalpy difference strongly decreases (deltaH(o) = -1.72 +/- 0.06 kJ mol(-1)), and the contribution of entropy at 298 K (298 x deltaS(o) = -1.87 +/- 0.06 kJ mol(-1)) increases in the case of the sterically hindered tert-butylformamide. The trans isomer of NMF in CDCl3 solution is enthalpically favored relative to the cis (deltaH(o) = -3.71 +/- 0.17 kJ mol(-1)) with entropy differences at 298 K (298 x deltaS(o) = 1.02 +/- 0.19 kJ mol(-1)) playing a minor role. In the sterically hindered tert-butylformamide, the trans isomer is enthalpically disfavored (deltaH(o) = 1.60 +/- 0.09 kJ mol(-1)) but is entropically favored (298 x deltaS(o) = 1.71 +/- 0.10 kJ mol(-1)). The results are compared with literature data of model peptides. It is concluded that, in amide bonds at 298 K and in the absence of strongly stabilizing sequence-specific inter-residue interactions involving side chains, the free energy difference of the cis/trans isomers and both the enthalpy and entropy contributions are strongly dependent on the N-alkyl substitution and the solvent. The significant decreasing enthalpic benefit of the trans isomer in CDCl3 compared to that in H2O, in the case of NMF and TBF, is partially offset by an adverse entropy contribution. This is in agreement with the general phenomenon of enthalpy versus entropy compensation. B3LY/6-311++G** and MP2/6-311++G** quantum chemical calculations confirm the stability orders of isomers and the deltaG decrease in going from water to CHCl3 as solvent. However, the absolute calculated values, especially for TBF, deviate significantly from the experimental values. Consideration of the solvent effects via the PCM approach on NMF x H2O and TBF x H2O supermolecules improves the agreement with the experimental results for TBF isomers, but not for NMF.     

Switching of global minima of novel germylenic reactive intermediates via halogens (X): C2GeH2 vs. C2GeHX at ab initio and DFT levels

For 30 C₂GeHX germylenic isomers, one cyclic structure, X-germacyclopropenylidene, and three acyclics are considered, which include: ethynyl-X-germylene, X-vinylidenegermylene, and (X-ethynyl)germylene (X = H, F, Cl, and Br). The global minimum among six isomeric C₂GeH₂ (where X = H), is found to be cyclic, aromatic, singlet germacyclopropenylidene. In contrast, among the 24 corresponding halogermylenes, C₂GeHX (where X = F, Cl, and Br), the global minima switch to acyclic, singlet ethynylhalogermylenes, at eight reasonably high ab initio and DFT levels. The direct resonance interaction between X and the divalent center Ge in the singlet acyclic ethynylhalogermylene structures, is claimed to justify switching of the calculated global minima in the halo derivatives. GIAO-NICS calculations indicate that the X-germacyclopropenylidene isomer is more aromatic for X = H than X = F, Cl, or Br. The angle ∠XGeC bending potential energy curves show the singlet and triplet ethynylgermylene crossing at ≈146°, for X = H.     

Theoretical study on the reaction mechanism of the gas-phase H2/CO2/Ni(3D) system

The ground-state potential energy surface (PES) in the gas-phase H2/CO2/Ni(3D) system is investigated at the CCSD(T)//B3LYP/6-311+G(2d,2p) levels in order to explore the possible reaction mechanism of the reverse water gas shift reaction catalyzed by Ni(3D). The calculations predict that the C-O bond cleavage of CO2 assisted by co-interacted H2 is prior to the dissociation of the H2, and the most feasible reaction path for Ni(3D) + H2 + CO2 --> Ni(3D) + H2O + CO is endothermic by 12.5 kJ mol(-1) with an energy barrier of 103.9 kJ mol(-1). The rate-determining step for the overall reaction is predicted to be the hydrogen migration with water formation. The promotion effect of H2 on the cleavage of C-O bond in CO2 is also discussed and compared with the analogous reaction of Ni(3D) + CO2 --> NiO + CO, and the difference between triplet and singlet H2/CO2/Ni systems is also discussed.     

Energy-resolved photoionization of alkylperoxy radicals and the stability of their cations

The photoionization of alkylperoxy radicals has been investigated using a newly developed experimental apparatus that combines the tunability of the vacuum ultraviolet radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory with time-resolved mass spectrometry. Methylperoxy (CH(3)OO) and ethylperoxy (C(2)H(5)OO) radicals are produced by the reaction of pulsed, photolytically produced alkyl radicals with molecular oxygen, and the mass spectrum of the reacting mixture is monitored in time by using synchrotron-photoionization with a double-focusing mass spectrometer. The kinetics of product formation is used to confirm the origins and assignments of ionized species. The photoionization efficiency curve for CH(3)OO has been measured, and an adiabatic ionization energy of (10.33 +/- 0.05) eV was determined with the aid of Franck-Condon spectral simulations, including ionization to the lowest triplet and singlet cation states. Using the appearance energy of CH(3)(+) from CH(3)OO, an enthalpy of formation for CH(3)OO of Delta(f) (CH(3)OO) = (22.4 +/- 5) kJ mol(-1) is derived. The enthalpy of formation of CH(3)OO(+) is derived as Delta(f) = (1019 +/- 7) kJ mol(-1) and the CH(3)(+)-OO bond energy as (CH(3)(+) - O(2)) = (80 +/- 7) kJ mol(-1). The C(2)H(5)OO(+) signal is not detectable; however, the time profile of the ethyl cation signal suggests its formation from dissociative ionization of C(2)H(5)OO. Electronic structure calculations suggest that hyperconjugation reduces the stability of the ethylperoxy cation, making the C(2)H(5)OO(+) ground state only slightly bound with respect to the ground-state products, C(2)H(5)(+) and O(2). The value of the measured appearance energy of C(2)H(5)(+) is consistent with dissociative ionization of C(2)H(5)OO via the Franck-Condon favored ionization to the ? (1)A' state of C(2)H(5)OO(+).