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.     

A crossed beams study of the reaction of carbon atoms, C(3Pj), with vinyl cyanide, C2H3CN(X 1A')--investigating the formation of cyano propargyl radicals

The chemical dynamics of the reaction of ground state carbon atoms, C(3Pj), with vinyl cyanide, C2H3CN(X 1A'), were examined under single collision conditions at collision energies of 29.9 and 43.9 kJ mol(-1) using the crossed molecular beams approach. The experimental studies were combined with electronic structure calculations on the triplet C4H3N potential energy surface (H. F. Su, R. I. Kaiser, A. H. H. Chang, J. Chem. Phys., 2005, 122, 074320). Our investigations suggest that the reaction follows indirect scattering dynamics via addition of the carbon atom to the carbon-carbon double bond of the vinyl cyanide molecule yielding a cyano cyclopropylidene collision complex. The latter undergoes ring opening to form cis/trans triplet cyano allene which fragments predominantly to the 1-cyano propargyl radical via tight exit transition states; the 3-cyano propargyl isomer was inferred to be formed at least a factor of two less; also, no molecular hydrogen elimination channel was observed experimentally. These results are in agreement with the computational studies predicting solely the existence of a carbon versus hydrogen atom exchange pathway and the dominance of the 1-cyano propargyl radical product. The discovery of the cyano propargyl radical in the reaction of atomic carbon with vinyl cyanide under single collision conditions implies that this molecule can be an important reaction intermediate in combustion flames and also in extraterrestrial environments (cold molecular clouds, circumstellar envelopes of carbon stars) which could lead to the formation of cyano benzene (C6H5CN) upon reaction with a propargyl radical.     

On the directed gas phase synthesis of the imidoborane molecule (HNBH)--an isoelectronic molecule of acetylene (HCCH)

The elementary reaction of ground state boron atoms, (B((2)P(j))), with ammonia (NH(3)(X(1)A(1))) was conducted under single collision conditions at a collision energy of 20.5 ± 0.4 kJ mol(-1) in a crossed molecular beams machine. Combined with electronic structure calculations, our experimental results suggested that the reaction was initiated by a barrier-less addition of the boron atom to the nonbonding electron pair of the nitrogen atom forming a weakly bound BNH(3) collision complex. This intermediate underwent a hydrogen shift to a doublet HBNH(2) radical that decomposed via atomic hydrogen loss to at least the imidoborane (HBNH(X(1)Σ(+)) molecule, an isoelectronic species of acetylene (HCCH(X(1)Σ(g)(+))). Our studies are also discussed in light of the isoelectronic C(2)H(3) potential energy surface accessed via the isoelectronic carbon-methyl system.     

Crossed molecular beam studies of the reactions of allyl radicals, C3H5(X2A2), with methylacetylene (CH3CCH(X1A1)), allene (H2CCCH2(X1A1)), and their isotopomers

The chemical dynamics of the reaction of allyl radicals, C(3)H(5)(X(2)A(2)), with two C(3)H(4) isomers, methylacetylene (CH(3)CCH(X(1)A(1))) and allene (H(2)CCCH(2)(X(1)A(1))) together with their (partially) deuterated counterparts, were unraveled under single-collision conditions at collision energies of about 125 kJ mol(-1) utilizing a crossed molecular beam setup. The experiments indicate that the reactions are indirect via complex formation and proceed via an addition of the allyl radical with its terminal carbon atom to the terminal carbon atom of the allene and of methylacetylene (alpha-carbon atom) to form the intermediates H(2)CCHCH(2)CH(2)CCH(2) and H(2)CCHCH(2)CHCCH(3), respectively. The lifetimes of these intermediates are similar to their rotational periods but too short for a complete energy randomization to occur. Experiments with D4-allene and D4-methylacetylene verify explicitly that the allyl group stays intact: no hydrogen emission was observed but only the release of deuterium atoms from the perdeuterated reactants. Further isotopic substitution experiments with D3-methylacetylene combined with the nonstatistical nature of the reaction suggest that the intermediates decompose via hydrogen atom elimination to 1,3,5-hexatriene, H(2)CCHCH(2)CHCCH(2), and 1-hexen-4-yne, H(2)CCHCH(2)CCCH(3), respectively, via tight exit transition states located about 10-15 kJ mol(-1) above the separated products. The overall reactions were found to be endoergic by 98 +/- 4 kJ mol(-1) and have characteristic threshold energies to reaction between 105 and 110 kJ mol(-1). Implications of these findings to combustion and interstellar chemistry are discussed.     

A crossed beam and ab initio investigation on the formation of vinyl boron monoxide (C2H3BO; X1A') via reaction of boron monoxide (11BO; X2Σ+) with ethylene (C2H4; X1A(g))

The reaction dynamics of the boron monoxide radical ((11)BO; X(2)Σ(+)) with ethylene (C(2)H(4); X(1)A(g)) were investigated at a nominal collision energy of 12.2 kJ mol(-1) employing the crossed molecular beam technique and supported by ab initio and statistical (RRKM) calculations. The reaction is governed by indirect scattering dynamics with the boron monoxide radical attacking the carbon-carbon double bond of the ethylene molecule without entrance barrier with the boron atom. This addition leads to a doublet radical intermediate (O(11)BH(2)CCH(2)), which either undergoes unimolecular decomposition through hydrogen atom emission from the C1 atom via a tight transition state located about 13 kJ mol(-1) above the separated products or isomerizes via a hydrogen shift to the O(11)BHCCH(3) radical, which also can lose a hydrogen atom from the C1 atom. Both processes lead eventually to the formation of the vinyl boron monoxide molecule (C(2)H(3)BO; X(1)A'). The overall reaction was determined to be exoergic by about 40 kJ mol(-1). The reaction dynamics are also compared to the isoelectronic ethylene (C(2)H(4); X(1)A(g)) - cyano radical (CN; X(2)Σ(+)) system studied earlier.     

A crossed beam and ab initio investigation of the reaction of boron monoxide ((11)BO; X(2)Σ+) with acetylene (C2H2; X(1)Σ(g)+)

The reaction dynamics of boron monoxide (BO; X(2)Σ(+)) with acetylene (C(2)H(2); X(1)Σ(g)(+)) were investigated under single collision conditions at a collision energy of 13 kJ mol(-1) employing the crossed molecular beam technique; electronic structure RRKM calculations were conducted to complement the experimental data. The reaction was found to have no entrance barrier and proceeded via indirect scattering dynamics initiated by an addition of the boron monoxide radical with its boron atom to the carbon-carbon triple bond forming the O(11)BHCCH intermediate. The latter decomposed via hydrogen atom emission to form the linear O(11)BCCH product through a tight exit transition state. The experimentally observed sideways scattering suggests that the hydrogen atom leaves perpendicularly to the rotational plane of the decomposing complex and almost parallel to the total angular momentum vector. RRKM calculations indicate that a minor micro channel could involve a hydrogen migration in the initial collision to form an O(11)BCCH(2) intermediate, which in turn can also emit atomic hydrogen. The overall reaction to form O(11)BCCH plus atomic hydrogen from the separated reactants was determined to be exoergic by 62 ± 8 kJ mol(-1). The reaction dynamics were also compared with the isoelectronic reaction of the cyano radical (CN; X(2)Σ(+)) with acetylene (C(2)H(2); X(1)Σ(g)(+)) studied earlier.     

Reaction dynamics of phenyl radicals (C6H5, X2A') with methylacetylene (CH3CCH(X1A1)), allene (H2CCCH2(X1A1)), and their D4-isotopomers

Crossed molecular beam experiments were utilized to untangle the reaction dynamics to form 1-phenylmethylacetylene [CH(3)CCC(6)H(5)] and 1-phenylallene [C(6)H(5)HCCCH(2)] in the reactions of phenyl radicals with methylacetylene and allene, respectively, over a range of collision energies from 91.4 to 161.1 kJ mol(-1). Both reactions proceed via indirect scattering dynamics and are initiated by an addition of the phenyl radical to the terminal carbon atom of the methylacetylene and allene reactants to form short-lived doublet C(9)H(9) collision complexes CH(3)CCHC(6)H(5) and C(6)H(5)H(2)CCCH(2). Studies with isotopically labeled reactants and the information on the energetics of the reactions depict that the energy randomization in the decomposing intermediates is incomplete. The collision complexes undergo atomic hydrogen losses via tight exit transition states leading to 1-phenylmethylacetylene [CH(3)CCC(6)H(5)] and 1-phenylallene [C(6)H(5)HCCCH(2)]. The possible role of both C(9)H(8) isomers as precursors to PAHs in combustion flames and in the chemistry of circumstellar envelopes of dying carbon stars is discussed.     

Gas‐phase detection of the HBCC (X1Σ) molecule: a combined crossed beam and computational study of the B(2P)+C2H2(1Σg+) reaction

A novel supersonic beam of ground‐state boron atoms [B(²P)] was employed to investigate the reaction of B(²P) with acetylene [C₂H₂(¹Σ_g⁺)] at an average collision energy of 16.3±0.4 kJ mol^?1 at the most fundamental microscopic level. The crossed molecular beam technique was used to record time of flight spectra at mass to charge ratios of 36 (¹¹BC₂H⁺), 35 (¹⁰BC₂H⁺/¹¹BC₂⁺), and 34 (¹⁰BC₂⁺) at different laboratory angles. Forward‐convolution fitting of the laboratory data showed that only a product with the gross formula BC₂H was formed via a boron versus hydrogen exchange. By combining experimental results with electronic structure calculations, the conclusion was that the reaction proceeded via the initial addition of B(²P) to the two carbon atoms of acetylene, leading to the formation of a first intermediate, the borirene radical (c‐BC₂H₂). This intermediate underwent various isomerization processes on the BC₂H₂ potential energy surface before decomposing into the linear HBCC(X¹Σ) isomer via a hydrogen atom elimination. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1359–1365, 2001     

Crossed beam study of the atom-radical reaction of ground state carbon atoms (C(3P)) with the vinyl radical (C2H3(X2A'))

The atom-radical reaction of ground state carbon atoms (C((3)P)) with the vinyl radical (C(2)H(3)(X(2)A')) was conducted under single collision conditions at a collision energy of 32.3 ± 2.9 kJ mol(-1). The reaction dynamics were found to involve a complex forming reaction mechanism, which is initiated by the barrier-less addition of atomic carbon to the carbon-carbon-double bond of the vinyl radical forming a cyclic C(3)H(3) radical intermediate. The latter has a lifetime of at least 1.5 times its rotational period and decomposes via a tight exit transition state located about 45 kJ mol(-1) above the separated products through atomic hydrogen loss to the cyclopropenylidene isomer (c-C(3)H(2)) as detected toward cold molecular clouds and in star forming regions.     

Photofragment imaging study of the CH2CCH2OH radical intermediate of the OH+allene reaction

These velocity map imaging experiments characterize the photolytic generation of one of the two radical intermediates formed when OH reacts via an addition mechanism with allene. The CH2CCH2OH radical intermediate is generated photolytically from the photodissociation of 2-chloro-2-propen-1-ol at 193 nm. Detecting the Cl atoms using [2+1] resonance-enhanced multiphoton ionization evidences an isotropic angular distribution for the Cl+CH2CCH2OH photofragments, a spin-orbit branching ratio for Cl(2P1/2):Cl(2P3/2) of 0.28, and a bimodal recoil kinetic energy distribution. Conservation of momentum and energy allows us to determine from this data the internal energy distribution of the nascent CH2CCH2OH radical cofragment. To assess the possible subsequent decomposition pathways of this highly vibrationally excited radical intermediate, we include electronic structure calculations at the G3//B3LYP level of theory. They predict the isomerization and dissociation transition states en route from the initial CH2CCH2OH radical intermediate to the three most important product channels for the OH+allene reaction expected from this radical intermediate: formaldehyde+C2H3, H+acrolein, and ethene+CHO. We also calculate the intermediates and transition states en route from the other radical adduct, formed by addition of the OH to the center carbon of allene, to the ketene+CH3 product channel. We compare our results to a previous theoretical study of the O+allyl reaction conducted at the CBS-QB3 level of theory, as the two reactions include several common intermediates.     

Structure and bonding in cyclic isomers of BAl2Hnm (n=3-6, m=-2 to +1): preference for planar tetracoordination, pyramidal tricoordination, and divalency

The structure and energetics of cyclic BAl2Hnm (n=3-6, m=-2 to +1), calculated at the B3LYP/6-311+G** and QCISD(T)/6-311++G** levels, are compared with their corresponding homocyclic boron and aluminium analogues. Structures in which the boron and aluminium atoms have coordination numbers of up to six are found to be minima. There is a parallel between structure and bonding in isomers of BAl2H(3)2- and BSi2H3. The number of structures that contain hydrogens out of the BAl2 ring plane is found to increase from BAl2H3(2-) to BAl2H6+. Double bridging at one bond is common in BAl2H5 and BAl2H6+. Similarly, species with lone pairs on the divalent boron and aluminium atoms are found to be minima on the potential energy surface of BAl2H(3)2-. BAl2H4- (2 b) is the first example of a structure with planar tetracoordinate boron and aluminium atoms in the same structure. Bridging hydrogen atoms on the B--Al bond prefer not to be in the BAl2 plane so that the pi MO is stabilised by pi-sigma mixing. This stabilisation increases with increasing number of bridging hydrogen atoms. The order of stability of the individual structures is decided by optimising the preference for lower coordination at aluminium, a higher coordination at boron and more bridging hydrogen atoms between B--Al bonds. The relative stabilisation energy (RSE) for the minimum energy structures of BAl2Hnm that contain pi-delocalisation are compared with the corresponding homocyclic aluminium and boron analogues.     

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.     

Kinetics and product branching ratios of the reaction of (1)CH2 with H2 and D2

The reactions of singlet methylene (a(1)A1 (1)CH2) with hydrogen and deuterium have been studied by experimental and theoretical techniques. The rate coefficients for the removal of singlet methylene with H2 (k1) and D2 (k2) have been measured from 195 to 798 K and are essentially temperature-independent with values of k1 = (10.48 +/- 0.32) x 10(-11) cm(3) molecule(-1) s(-1) and k2 = (5.98 +/- 0.34) x 10(-11) cm(3) molecule(-1) s(-1), where the errors represent 2sigma, giving a ratio of k1/k2 = 1.75 +/- 0.11. In the reaction with H2, singlet methylene can be removed by reaction giving CH3 + H or deactivated to ground-state triplet methylene. Direct measurement of the H atom product showed that the fraction of relaxation decreased from 0.3 at 195 K to essentially zero at 398 K. For the reaction with deuterium, either H or D may be eliminated. Experimentally, the H:D ratio was determined to be 1.8 +/- 0.5 over the range 195-398 K. Theoretically, the reaction kinetics has been predicted with variable reaction coordinate transition state theory and with rigid-body trajectory simulations employing various high-level, ab initio-determined potential energy surfaces. The magnitudes of the calculated rate coefficients are in agreement with experiment, but the calculations show a significant negative temperature dependence that is not observed in the experimental results. The calculated and experimental H to D ratios from the reaction of singlet methylene with D2 are in good agreement, suggesting that the reaction proceeds entirely through the formation of a long-lived methane intermediate with a statistical distribution of energy.     

Unraveling the formation of HCPH(X2A') molecules in extraterrestrial environments: crossed molecular beam study of the reaction of carbon atoms, C(3Pj), with phosphine, PH3(X1A1)

The reaction between ground state carbon atoms, C(3P(j)), and phosphine, PH3(X(1)A1), was investigated at two collision energies of 21.1 and 42.5 kJ mol(-1) using the crossed molecular beam technique. The chemical dynamics extracted from the time-of-flight spectra and laboratory angular distributions combined with ab initio calculations propose that the reaction proceeds on the triplet surface via an addition of atomic carbon to the phosphorus atom. This leads to a triplet CPH3 complex. A successive hydrogen shift forms an HCPH2 intermediate. The latter was found to decompose through atomic hydrogen emission leading to the cis/trans-HCPH(X(2)A') reaction products. The identification of cis/trans-HCPH(X(2)A') molecules under single collision conditions presents a potential pathway to form the very first carbon-phosphorus bond in extraterrestrial environments like molecular clouds and circumstellar envelopes, and even in the postplume chemistry of the collision of comet Shoemaker-Levy 9 with Jupiter.     

A crossed beam and ab initio study of the reaction of atomic boron with ethylene

The reaction of atomic boron, B(2P), with the simplest alkene, C2H4, has been investigated under single collision conditions in crossed beam experiments with mass spectrometric detection. Our experimental data clearly showed that the atomic boron versus hydrogen exchange reaction led to molecule(s) of gross formula C2H3B via bound intermediate(s). According to the experimentally derived fraction of the available energy released as product translational energy, we propose that an important reaction pathways is the one leading to the borirene plus atomic hydrogen and/or the one leading to ethynylborane plus atomic hydrogen. The experimental results are accompanied by electronic structure calculations of the relevant potential energy surface and RRKM estimates of the product branching ratio. According to RRKM calculations, within the limit of complete energy randomization, the three isomers borirene, BH=C=CH2 and BH2-CCH, are all formed, with BH2-CCH being the dominant one. The discrepancies between the trend of the product translational energy distributions and the picture emerging from RRKM estimates are a symptom that a statistical treatment is not warranted for this system.     

A crossed molecular beam imaging study of the O(1D2)+HCl-->OH+Cl(2P J=3/2, 1/2) reaction.

A crossed molecular beam study is presented for the O((1)D(2))+HCl-->OH+Cl((2)P(J)) reaction at the collision energy of 6 kcal mol(-1). State-resolved doubly differential cross sections are obtained for the Cl((2)P(J=3/2) ) and Cl*((2)P(J=1/2) ) products by velocity-map ion imaging. Both products are slightly more forward scattered, which suggests a reaction mechanism without a long-lived intermediate in the ground electronic state. A small fraction (23 %) of the energy release into the translational degree of freedom indicates strong internal excitation of the counterpart OH radical. The contribution of the electronic excited states of O--HCl to the overall reaction is also examined from the doubly differential cross sections.     

Chemical dynamics of the CH(X2Π) + C2H4(X1A1g), CH(X2Π) + C2D4(X1A1g), and CD(X2Π) + C2H4(X1A1g) reactions studied under single collision conditions

The crossed beam reactions of the methylidyne radical with ethylene (CH(X(2)Π) + C(2)H(4)(X(1)A(1g))), methylidyne with D4-ethylene (CH(X(2)Π) + C(2)D(4)(X(1)A(1g))), and D1-methylidyne with ethylene (CD(X(2)Π) + C(2)H(4)(X(1)A(1g))) were conducted at nominal collision energies of 17-18 kJ mol(-1) to untangle the chemical dynamics involved in the formation of distinct C(3)H(4) isomers methylacetylene (CH(3)CCH), allene (H(2)CCCH(2)), and cyclopropene (c-C(3)H(4)) via C(3)H(5) intermediates. By tracing the atomic hydrogen and deuterium loss pathways, our experimental data suggest indirect scattering dynamics and an initial addition of the (D1)-methylidyne radical to the carbon-carbon double bond of the (D4)-ethylene reactant forming a cyclopropyl radical intermediate (c-C(3)H(5)/c-C(3)D(4)H/c-C(3)H(4)D). The latter was found to ring-open to the allyl radical (H(2)CCHCH(2)/D(2)CCHCD(2)/H(2)CCDCH(2)). This intermediate was found to be long lived with life times of at least five times its rotational period and decomposed via atomic hydrogen/deuterium loss from the central carbon atom (C2) to form allene via a rather loose exit transition state in an overall strongly exoergic reaction. Based on the experiments with partially deuterated reactants, no compelling evidence could be provided to support the formation of the cyclopropene and methylacetylene isomers under single collision conditions. Likewise, hydrogen/deuterium shifts in the allyl radical intermediates or an initial insertion of the (D1)-methylidyne radical into the carbon-hydrogen/deuterium bond of the (D4)-ethylene reactant were found to be-if at all-of minor importance. Our experiments propose that in hydrocarbon-rich atmospheres of planets and their moons such as Saturn's satellite Titan, the reaction of methylidyne radicals should lead predominantly to the hitherto elusive allene molecule in these reducing environments.     

Synthesis and characterization of a twin Cubane-type molybdenum-rhodium-sulfur cluster, [{Mo3RhCp*S4(H2O)7(O)}2]8+. X-ray structure of [{Mo3RhCpS4(H2O7O}2](CH3C6H4SO3)(8).14H2O

The reaction of [Mo(3)S(4)(H(2)O)(9)](4+) (1) with [(CpRhCl(2))(2)] afforded a novel rhodium-molybdenum cluster, [{Mo(3)RhCpS(4)(H(2)O)(7)(O)}(2)](8+) (2). X-ray structure analysis of [2](pts)(8).14H(2)O (pts(-) = CH(3)C(6)H(4)SO(3)(-)) has revealed the existence of a new oxo-bridged twin cubane-type core, (Mo(3)RhCpS(4))(2)(O)(2). The high affinity of the CpRh group for sulfur atoms in 1 seems to be the main driving force for this reaction. The strong Lewis acidity of the CpRh group in intermediate A, [Mo(3)RhCpS(4)(H(2)O)(9)](6+), caused a release of proton from one of the water molecules attached to the molybdenum atoms to give intermediate B, [Mo(3)RhCpS(4)(H(2)O)(8)(OH)](5+). The elimination of two water molecules from two intermediate B molecules, followed by the deprotonation reaction of hydroxo bridges, generated the twin cubane-type cluster 2. The formal oxidation states of rhodium and molybdenum atoms are the same before and after the reaction (i.e., Mo(IV)(3), Rh(III)). The Mo-O-Mo moieties in [2](pts)(8).14H(2)O are nearly linear with a bond angle of 164.3(3) degrees, and the basicity of the bridging oxygen atoms seems to be weak. For this reason, protonation at the bridging oxygen atoms does not occur even in a strongly acidic aqueous solution. The binding energy values of Mo 3d(5/2), Rh 3d(5/2), and C 1s obtained from X-ray photoelectron spectroscopy measurements for [2](pts)(8).14H(2)O are 229.8, 309.3, and 285 eV, respectively. The XPS measurements on the Rh 3d(5/2) binding energy indicate that the oxidation state of Rh is 3+. The binding energy of Mo 3d(5/2) (229.8 eV) compares with that observed for [1](pts)(4).7H(2)O (230.7 eV, Mo 3d(5/2)). A lower energy shift (0.9 eV) is observed in the binding energy of Mo 3d(5/2) for [2](pts)(8).14H(2)O. This energy shift may correspond to the coordination of an oxygen atom having a negative charge to the molybdenum atom.     

Structure and bonding in cyclic isomers of B2AlH(n)(m) (n = 3-6, m = -2 to +1): a comparative study with B3H(n)(m), BAl2H(n)(m) and Al3H(n)(m)

The structure, bonding and energetics of B(2)AlH(n)(m) (n = 3-6, m = -2 to +1) are compared with corresponding homocyclic boron, aluminum analogues and BAl(2)H(n)(m) using density functional theory (DFT). Divalent to hexacoordinated boron and aluminum atoms are found in these species. The geometrical and bonding pattern in B(2)AlH(4)(-) is similar to that for B(2)SiH(4). Species with lone pairs on the divalent boron and aluminum atoms are found to be minima on the potential energy surface of B(2)AlH(3)(2-). A dramatic structural diversity is observed in going from B(3)H(n)(m) to B(2)AlH(n)(m), BAl(2)H(n)(m) and Al(3)H(n)(m) and this is attributable to the preference of lower coordination on aluminum, higher coordination on boron and the higher multicenter bonding capability of boron. The most stable structures of B(3)H(6)(+), B(2)AlH(5) and BAl(2)H(4)(-) and the trihydrogen bridged structure of Al(3)H(3)(2-) show an isostructural relationship, indicating the isolobal analogy between trivalent boron and divalent aluminum anion.     

Potential energy surface and product branching ratios for the reaction of dicarbon, C(2)(X(1)Sigma(g)(+)), with methylacetylene, CH(3)CCH(X(1)A(1)): an ab initio/RRKM study

Ab initio calculations of the potential energy surface for the C(2)(X(1)Sigma(g)(+)) + CH(3)CCH(X(1)A(1)) reaction have been carried at the G2M level of theory. The calculations show that the dicarbon molecule in the ground singlet electronic state can add to methylacetylene without a barrier producing a three-member or a four-member ring intermediate, which can rapidly rearrange to the most stable H(3)CCCCCH isomer on the C(5)H(4) singlet surface. This isomer can then lose a hydrogen atom (H) or molecular hydrogen (H(2)) from the CH(3) group with the formation of H(2)CCCCCH and HCCCCCH, respectively. Alternatively, H atom migrations and three-member-ring closure/opening rearrangements followed by H and H(2) losses can lead to other isomers of the C(5)H(3) and C(5)H(2) species. According to the calculated energetics, the C(2)(X(1)Sigma(g)(+)) + CH(3)CCH reaction is likely to be a major source of the C(5)H(3) radicals (in particular, the most stable H(2)CCCCCH and HCCCHCCH isomers, which are relevant to the formation of benzene through the reactions with CH(3)). Among heavy-fragment product channels, only C(3)H(3) + C(2)H and c-C(3)H(2) + C(2)H(2) might compete with C(5)H(3) + H and C(5)H(2) + H(2). RRKM calculations of reaction rate constants and product branching ratios depending on the reactive collision energy showed that the major reaction products are expected to be H(2)CCCCCH + H (64-66%) and HCCCHCCH + H (34-30%), with minor contributions from HCCCCCH + H(2) (1-2%), HCCCHCC + H(2) (up to 1%), C(3)H(3) + C(2)H (up to 1%), and c-C(3)H(2) + C(2)H(2) (up to 0.1%) if the energy randomization is complete. The calculations also indicate that the C(2)(X(1)Sigma(g)(+)) + CH(3)CCH(X(1)A(1)) reaction can proceed by direct H-abstraction of a methyl hydrogen to form C(3)H(3) + C(2)H almost without a barrier.