Unimolecular Reactions of Ionized Alkanes: Theoretical Study of the Potential Energy Surface for CH3.bul. and CH4 Losses from Ionized Butane and Isobutane (original) (raw)
Related papers
Unimolecular Decompositions of Ionized Isopropyl Methyl Ether: An ab Initio and RRKM Study
Journal of the American Chemical Society, 1995
In connection with the unimolecular decompositions of ionized isopropyl methyl ether, namely, methane elimination and methyl radical loss, ab initio molecular orbital calculations at the UMP2, QCISD, and QCISD(T) levels of theory with the 6-31G(d) and 6-31G(d,p) basis sets have been used to investigate the relevant parts of the CdHl@ ground-state potential energy surface. The calculations demonstrate that at internal energies below the threshold for loss of a methyl radical, bond dissociation to an ion-neutral complex, [CH30+=CHCH3 CH3.1, and methane elimination therefrom both occur. Sets of reactant and transition state frequencies calculated at the UMP2/ 6-3 1G(d) level, as well as energetics based on QCISD(T)/6-3 lG(d,p)//UMP2/6-3 1G(d) + ZPVE calculations, were employed in RRKM calculations. By invoking quantum-mechanical barrier tunneling in the methane elimination channel, we were able to reproduce several experimental observations: (a) the methane elimination reaction dominates at internal energies near the threshold for dissociation; (b) the onsets for the methane and methyl losses differ by only a few tenths of a kilocalorie per mole; and (c) there is a very strong primary isotope effect of about 60 on the CH2D2 and CHD3 losses from CH30CH(CHD2)2'+. ' Universitat de Barcelona.
The Journal of Chemical Physics, 2006
A single trajectory ͑ST͒ direct dynamics approach is compared with quasiclassical trajectory ͑QCT͒ direct dynamics calculations for determining product energy partitioning in unimolecular dissociation. Three comparisons are made by simulating C 2 H 5 F → HF+ C 2 H 4 product energy partitioning for the MP2 / 6-31G * and MP2 / 6-311+ + G ** potential energy surfaces ͑PESs͒ and using the MP2 / 6-31G * PES for C 2 H 5 F dissociation as a model to simulate CHCl 2 CCl 3 → HCl +C 2 Cl 4 dissociation and its product energy partitioning. The trajectories are initiated at the transition state with fixed energy in reaction-coordinate translation E t ‡ . The QCT simulations have zero-point energy ͑ZPE͒ in the vibrational modes orthogonal to the reaction coordinate, while there is no ZPE for the STs. A semiquantitative agreement is obtained between the ST and QCT average percent product energy partitionings. The ST approach is used to study mass effects for product energy partitioning in HX͑X = F or Cl͒ elimination from halogenated alkanes by using the MP2 / 6-31G * PES for C 2 H 5 F dissociation and varying the masses of the C, H, and F atoms. There is, at most, only a small mass effect for partitioning of energy to HX vibration and rotation. In contrast, there are substantial mass effects for partitioning to relative translation and the polyatomic product's vibration and rotation. If the center of mass of the polyatomic product is located away from the C atom from which HX recoils, the polyatomic has substantial rotation energy. Polyatomic products, with heavy atoms such as Cl atoms replacing the H atoms, receive substantial vibration energy that is primarily transferred to the wag-bend motions. For E t ‡ of 1.0 kcal/ mol, the ST calculations give average percent partitionings to relative translation, polyatomic vibration, polyatomic rotation, HX vibration, and HX rotation of 74.9%, 6.8%, 1.5%, 14.4%, and 2.4% for C 2 H 5 F dissociation and 39.7%, 38.1%, 0.2%, 16.1%, and 5.9% for a model of CHCl 2 CCl 3 dissociation.
The Journal of Physical Chemistry A, 1999
A theoretical study of the [C 3 ,H 5 ,N] •+ potential energy surface is presented. Ab initio molecular orbital calculations at the QCISD(T)/UMP2 level with the 6-31G(d,p) basis set show that acetonitrile N-methylide [CH 3 CNCH 2 ] •+ , a •+ , and N-methylketenimine [CH 3 NCCH 2 ] •+ , b •+ , are the most stable species among the 15 isomers considered, and a heat of formation of ∆ f H°2 98 ) 970 kJ/mol is proposed for both species. Detailed examination of the [C 3 ,H 5 ,N] •+ potential energy surface indicates that a •+ , b •+ , and related isomers are stable and distinct species in the gas phase, isolated by energy barriers as high as 300 kJ/mol. Their neutral equivalent, a and b, have also been studied, thus allowing their adiabatic ionization energies to be estimated: IE a (a) ) 7.1 eV and IE a (b) ) 8.0 eV. Finally, the theoretical study of a •+ and b •+ fragmentations provides an explanation for the similarity in their high-energy CID spectra by showing a possible isomerization of these species prior to dissociation.
International Journal of Mass Spectrometry and Ion Processes, 1997
The complete potential energy profile associated with the isomerization of ionized enol, [CH3CHCHOH] +, 1; distonic ion, [CH2CH2CHOH] +, 2; ionized propanal, [CH3CH2CHO] +, 3; ionized allyl alcohol, [CH2CHCH2OH] +, 4; and their dissociation into C2H5CO + + H, 5 has been constructed by means of molecular orbital calculations at the MP2/6-311 + G**//MP2/6-31 G* + ZPE level. Investigated reactions were: 1,2-, 1,3-and 1,4-hydrogen migrations, 1,2-HCOH migration 2 ~ 2' and the sigmatropie 1,3-OH shift, 4 ~ 4'. It is found that the energy-determining step of the overall process 1-. 2-, 3-* 5 is the 1,4-hydrogen atom migration 2-* 3. Another illustration of the key role played by the distonic species 2 is provided by the calculation of a very low critical energy for its degenerate isomerization 2 ~-2'. A rationalization of the experimental findings concerning the dissociation energetics and the metastable dissociations of 1-4 is given by RRKM calculations on the ab initio potential energy surface.
2009
The energetics of ethylenes and 1,3-butadienes may be interrelated by the reaction: RHC=CH 2 + H 2 C=CHR' - RHC=CH-CH=CHR' + H 2 . Shown earlier to be nearly enthalpically thermoneutral for a variety of hydrocarbon cases, we are now interested in the related energetics of halogenated alkenes and alkynes. Using quantum chemical calculations, we have studied this as recast as the isodesmic reactions: 2(H 2 C=CHX) + H2C=CH-CH=CH2→p,q-di-X-1,3-butadiene + 2H 2 C=CH 2 2(HC≡CX) + HC≡C-C≡CH → di-X-butadiyne + 2HC≡CH. Here p,q- = 1,3-; 1,4- and 2,3- with X = F, Cl, Br, and I. The halogen and location-dependent deviations from near enthalpic thermoneutrality are discussed.
Journal of the American …, 1992
Isomeric C 4H!' radical cations vinylacetylene (a), butatriene (b), methylene cyclopropene (c), and the nonaromatic cyclobutadiene (d), generated, respectively, from the neutral precursors 3-butyn-l-ol (1), 1,4-dichloro-2-butyne (2), benzene (3), and 7,8-benzotricyclo [4.2.2.0 2 • 5]deca-3,7,9-triene (4), undergo diagnostically different ion-molecule reactions with allene, isoprene, furan, and thiophene. It is speculated that adducts are generated by [2 + 2] cycloadditions with the first reagent and [4 + 2] Dials-Alder cycloadditions with
Chemical Physics, 2000
The reaction between ground state carbon atoms, C ~Pj ), and 1,3-butadiene, H 2CCHCHCH2, was studied at three averaged collision energies between 19.3 and 38.8 kJmol 21 using the crossed molecular beam technique. Our experimental data combined with electronic structure calculations show that the carbon atom adds barrierlessly to the p-orbital of the butadiene molecule via a loose, reactantlike transition state located at the centrifugal barrier. This process forms vinylcyclopropylidene which rotates in a plane almost perpendicular to the total angular momentum vector J around itsC-axis. The initial collision complex undergoes ring opening to a long-lived vinyl-substituted triplet allene molecule. This complex shows three reaction pathways. Two distinct H atom loss channels form 1and 3-vinylpropargyl radicals, HCCCHC 2H3(X A9) and H2CCCC2H3(X A9), through tight exit transition states located about 20 kJmol 21 above the products; the branching ratio of 1versus 3-vinylpropargyl radical...
Israel Journal of Chemistry, 1993
The potential energy surface (PES) of [CI\NOj+ ions with an NCO connectivity has been explored by GAUSSIAN-l type calculations. Six isomers having an NCO backbone are found and characterized as minima on the PES; three of them are separated by isomerization barriers reasonably large enough to allow an experimental verification. The favored protonation site of neutral isocyanic acid (HNCO) is the nitrogen atom (proton affmity, calculated =171.4 kcal/mol; experimental =173 ± 1 kcal/ mol) leading to the most stable isomer H 2NCO+ (1). For the protonation of NCOH at the nitrogen atom we predict a proton affinity for the cyanic acid of 179.7 kcal/mol. The resulting ion HNCOH+(2) is 15.9 kcal/mol higher in energy than 1. The third stable isomer S having the NCO connectivity can be formally viewed as a complex between CN and the waterradical cation; Sis 72.9 kcal/mol less stable than~NCO+ (1). In addition, two highenergy isomers HN-C(H)O+(3) and HO-C(H)N+ (4) have been found. but according to the shape of the PES, they are predicted not to be accessible experimentally. The calculated thermochemical data are in excellect agreement with experimentally available values. The calculations are extended to the unimolecular fragmentation channels. which should be observed in mass-spectrometric experiments. The analysis of the results suggests that the hydrogen-atom elimination process is favored over all other dissociation channels, followed by loss of NI\. and NH. Helmut Schwarz, after four years as a technician in chemical industry, studied chemistry at Technische Universitiit (T.U.) Berlin and received his Ph.D. there with Ferdinand Bohlmann in 1972. Since 1978 he has been Professor of Chemistry atT.U. Berlin, although visiting appointments have taken him to Churchill College. Cambridge; ETH, Lausanne; Hebrew University, Jerusalem; and Technion. Haifa-among others. His awards include the LeibnizResearch Award of the Deutsche Forschungsgemeinschaft and an honorary degree from Hebrew University. and he is a member of several academies. Wolfram Koch obtained his doctorate in 1986 from T.U. Berlin under the tutelage of Prof. Schwarz and has been Professor of Theoretical Organic Chemistry there since 1992. In between, he was an IBM postdoctoral fellow at the IBM Almaden Research Center in San Jose. CA and a research staff member at the Institute for Supercomputing and Applied Mathematics at the mM Scientific Center in Heidelberg. He is the recipient of the 1987 Schering Award. Jan Hrusak, born in Novy Bor, Czechoslovakia, obtained his doctorate with Prof. G. Rasch at Technische Universitat Merseburg (Germany) in 1987. Since 1987 he has been at the Czech Academy of Sciences. Currently he is a visiting scientist at T.U. Berlin. His research interests are theoretical studies of ion-molecule reactions and calculations of small transition metal containing compounds. Max C. Holthausen, born in Hannover, Germany. studied at Universitat Gottingen, graduating in 1991. He is presently working with Prof. Koch in the field of organic species containing open shell transition metals. Norman Goldberg was born in Freiburg i. Br .• Germany, and grew up in Switzerland He studied at Universitiit Giessen with Prof. G. Maier. graduating in 1991. Presently heis working with Prof. Schwarz on small reactive species. Muhammad Iraqi. born in Tira, Israel. obtained his Ph.D. with Prof. C. Lifshitz at Hebrew University in 1990. Interested in the field of small cluster ions and systems of interstellar importance. he is currently a postdoctoral fellow with Prof. Schwarz.
The Journal of Chemical Physics, 2006
The ionization energies for methylene ͑CH 2 ͒, methyl ͑CH 3 ͒, ethynyl ͑C 2 H͒, vinyl ͑C 2 H 3 ͒, ethyl ͑C 2 H 5 ͒, propargyl ͑C 3 H 3 ͒, and allyl ͑C 3 H 5 ͒ radicals have been calculated by the wave-function-based ab initio CCSD͑T͒/CBS approach, which involves the approximation to the complete basis set ͑CBS͒ limit at the coupled-cluster level with single and double excitations plus a quasiperturbative triple excitation ͓CCSD͑T͔͒. When it is appropriate, the zero-point vibrational energy correction, the core-valence electronic correction, the scalar relativistic effect correction, the diagonal Born-Oppenheimer correction, and the high-order correlation correction have also been made in these calculations. The comparison between the computed ionization energy ͑IE͒ values and the highly precise experimental IE values determined in previous pulsed field ionization-photoelectron ͑PFI-PE͒ studies indicates that the CCSD͑T͒/CBS method is capable of providing accurate IE predictions for these hydrocarbon radicals achieving error limits well within ±10 meV. The benchmarking of the CCSD͑T͒/CBS IE predictions by the PFI-PE experimental results also lends strong support for the conclusion that the CCSD͑T͒/CBS approach with high-level energy corrections can serve as a valuable alternative for reliable IE determination of radicals, particularly for those radicals with very unfavorable Franck-Condon factors for photoionization transitions near their ionization thresholds.
Phys. Chem. Chem. Phys., 2011
This study uses computational chemistry and statistical reaction rate theory to investigate the chemically activated reaction of diacetylene (butadiyne, C 4 H 2) with the propargyl radical (C H 2 CCH) and the reaction of acetylene (C 2 H 2) with the i-C 5 H 3 (CH 2 CCCC H) and n-C 5 H 3 (CHCC HCCH) radicals. A detailed G3SX-level C 7 H 5 energy surface demonstrates that the C 3 H 3 + C 4 H 2 and C 5 H 3 + C 2 H 2 addition reactions proceed with moderate barriers, on the order of 10 to 15 kcal mol À1 , and form activated open-chain C 7 H 5 species that can isomerize to the fulvenallenyl radical with the highest barrier still significantly below the entrance channel energy. Higher-energy pathways are available leading to other C 7 H 5 isomers and to a number of C 7 H 4 species + H. Rate constants in the large multiple-well (15) multiple-channel (30) chemically activated system are obtained from a stochastic solution of the one-dimensional master equation, with RRKM theory for microcanonical rate constants. The dominant products of the C 4 H 2 + C 3 H 3 reaction at combustion-relevant temperatures and pressures are i-C 5 H 3 + C 2 H 2 and CH 2 CCHCCCCH + H, along with several quenched C 7 H 5 intermediate species below 1500 K. The major products in the n-C 5 H 3 + C 2 H 2 reaction are i-C 5 H 3 + C 2 H 2 and a number of C 7 H 4 species + H, with C 7 H 5 radical stabilization at lower temperatures. The i-C 5 H 3 + C 2 H 2 reaction predominantly leads to C 7 H 4 + H and to stabilized C 7 H 5 products. The title reactions may play an important role in polycyclic aromatic hydrocarbon (PAH) formation in combustion systems. The C 7 H 5 potential energy surface developed here also provides insight into several other important reacting gas-phase systems relevant to combustion and astrochemistry, including C 2 H + the C 3 H 4 isomers propyne and allene, benzyne + CH, benzene + C(3 P), and C 7 H 5 radical decomposition, for which some preliminary analysis is presented.