Methyl-substituted Carbon Research Articles

Compositional evolution of particle-phase reaction products and water in the heterogeneous OH oxidation of model aqueous organic aerosols

Abstract. Organic compounds present at or near the surface of aqueous droplets can be efficiently oxidized by gas-phase OH radicals, which alter the molecular distribution of the reaction products within the droplet. A change in aerosol composition affects the hygroscopicity and leads to a concomitant response in the equilibrium amount of particle-phase water. The variation in the aerosol water content affects the aerosol size and physicochemical properties, which in turn governs the oxidation kinetics and chemistry. To attain better knowledge of the compositional evolution of aqueous organic droplets during oxidation, this work investigates the heterogeneous OH-radical-initiated oxidation of aqueous methylsuccinic acid (C5H8O4) droplets, a model compound for small branched dicarboxylic acids found in atmospheric aerosols, at a high relative humidity of 85 % through experimental and modeling approaches. Aerosol mass spectra measured by a soft atmospheric pressure ionization source (Direct Analysis in Real Time, DART) coupled with a high-resolution mass spectrometer reveal two major products: a five carbon atom (C5) hydroxyl functionalization product (C5H8O5) and a C4 fragmentation product (C4H6O3). These two products likely originate from the formation and subsequent reactions (intermolecular hydrogen abstraction and carbon–carbon bond scission) of tertiary alkoxy radicals resulting from the OH abstraction occurring at the methyl-substituted carbon site. Based on the identification of the reaction products, a kinetic model of oxidation (a two-product model) coupled with the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model is built to simulate the size and compositional changes of aqueous methylsuccinic acid droplets during oxidation. Model results show that at the maximum OH exposure, the droplets become slightly more hygroscopic after oxidation, as the mass fraction of water is predicted to increase from 0.362 to 0.424; however, the diameter of the droplets decreases by 6.1 %. This can be attributed to the formation of volatile fragmentation products that partition to the gas phase, leading to a net loss of organic species and associated particle-phase water, and thus a smaller droplet size. Overall, fragmentation and volatilization processes play a larger role than the functionalization process in determining the evolution of aerosol water content and droplet size at high-oxidation stages.

Absolute configuration of bioactive furanogermacrenones from Commiphora erythraea (Ehrenb) Engl. by computational analysis of their chiroptical properties

Absolute configurations (+)-(4S), (+)-(3S,4R), (−)-(2S,4S), and (−)-(1R,2R,4S) were assigned, for the first time, to bioactive furanogermacranes extracted from Commiphora erythraea resin by DFT computational analysis of their ORD curves and ECD spectra. This analysis established that all of these compounds share the same absolute configuration at the methyl-substituted carbon thus allowing us to hypothesize a biosynthetic relationship among these structurally related metabolites.

Theoretical Studies of Ring-Opening Reactions of Phenylcyclobutabenzenol and Its Reactions with Alkynes Catalyzed by Rhodium Complexes

DFT calculations have been carried out to study the Rh-catalyzed site selective ring-opening reactions of phenylcyclobutabenzenol (1a). Our calculations supported that the mechanism involves β-carbon elimination in a rhodium(I) phenylcyclobutabenzenolato complex followed by alcoholysis. The favorable β-carbon elimination involves the cleavage of the C(sp(2))-C(sp(3))(OH)(Ph) bond in 1a, which determines the site selectivity. The mechanism and regioselectivity of the related Rh-catalyzed insertion reaction of the asymmetric alkyne MeC≡CPh with phenylcyclobutabenzenol (1a) was also investigated. The key step involves the alkyne insertion into the Rh-C σ bond of a species resulting from the favorable β-carbon elimination in the rhodium(I) phenylcyclobutabenzenolato complex. The favorable insertion couples the metal-bonded carbon with the methyl-substituted carbon of MeC≡CPh.

Theoretical Prediction of EPR Coupling Constants for the Determination of the Selectivity in the OH Addition to Toluene

High-level theoretical methods based on density functional and Hartree−Fock−Moller−Plesset theories have been employed to study the selectivity of the OH radical attack on toluene and to predict expected hyperfine coupling constants of the isomeric addition adducts. In recent work, we have found that the adduct with OH• added to the methyl-substituted carbon atom (ipso addition) of the ring may be especially stable both in toluene and in the xylenes. However, since this kind of adduct does not form a phenol, it is not directly identified in the products. The calculations indicate that quite different values should be observed for the EPR signals of the methyl hydrogen atoms of the four isomers, suggesting that electron paramagnetic resonance measurements could provide decisive information on the selectivity of the OH• addition to toluene. The calculated values of the hyperfine coupling constants are compared with the experimental values reported by Jolibois et al. for thymine.

(1R,2s,3S,6R,9S)-5,5,10,10-tetrachloro-2-methyltricyclo

The title compound, C(12)H(10)Cl(4)O(2), has a pseudoasymmetric centre at the methyl-substituted carbon and, in the solid state, a boat-like conformation.

Ab initio studies on the Ziegler-Natta polymerization mechanisms of ethylene and propylene. Role of cocatalysis and stereoregulation

As a model of olefin polymerization for a heterogeneous Ziegler–Natta catalyst, the mechanisms of the insertion of ethylene and propylene into R-Ti-(Cl2AlH2)2+(R=CH3, C3H7) are studied by ab initio self-consistent field (SCF) methods and many-body perturbation theory. The structures of the reactant, the intermediate, the transition state, and the product are optimized by the Hartree–Fock molecular orbital calculation level. The reactions are classified into two steps: the first forms of olefin–Ti π complex, and the second step is the carbon–carbon bond formation through the pull–push mechanisms. The cocatalysis play a role in facilitating the pull–push mechanism with the Al–Cl bonds alternation. For the insertion reaction of propylene, the four-type conformations for attacking of propylene (two primary forms: methyl-substituted carbon attacks to the carbon side of Ti–C bond; two secondary forms: methyl-substituted carbon attack to Ti atom) are studied. For the complex formation, the secondary forms are more stable in energy than the primary forms. For the transition state, the primary forms are more stable in energy than the secondary forms. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 739–747, 1997

Ab initio studies on the Ziegler–Natta polymerization mechanisms of ethylene and propylene. Role of cocatalysis and stereoregulation

As a model of olefin polymerization for a heterogeneous Ziegler–Natta catalyst, the mechanisms of the insertion of ethylene and propylene into R-Ti-(Cl2AlH2)2+(R=CH3, C3H7) are studied by ab initio self-consistent field (SCF) methods and many-body perturbation theory. The structures of the reactant, the intermediate, the transition state, and the product are optimized by the Hartree–Fock molecular orbital calculation level. The reactions are classified into two steps: the first forms of olefin–Ti π complex, and the second step is the carbon–carbon bond formation through the pull–push mechanisms. The cocatalysis play a role in facilitating the pull–push mechanism with the Al–Cl bonds alternation. For the insertion reaction of propylene, the four-type conformations for attacking of propylene (two primary forms: methyl-substituted carbon attacks to the carbon side of Ti–C bond; two secondary forms: methyl-substituted carbon attack to Ti atom) are studied. For the complex formation, the secondary forms are more stable in energy than the primary forms. For the transition state, the primary forms are more stable in energy than the secondary forms. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 739–747, 1997

Clusters containing carbene ligands. 10. Transformation of an ynamine into an aminocarbene ligand by unsymmetric hydrogenation with a triosmium cluster complex

The reaction of ynamine MeC 2 NMe 2 with the lightly stabilized cluster complex Os 3 (CO) 9 (NCMe)(μ-OMe)(μ-H) (1) at 25°C yielded the product Os 3 (CO) 9 [μ 3 -η 3 -MeC(H)CNMe 2 ](μ-OMe)(μ-H) (2; 47%). Compound 2 was characterized crystallographically and was shown to consist of an open triosmium cluster with a triply bridging MeC(H)CNMe 2 ligand formed by the transfer of the hybride ligand to the methyl-substituted carbon atom of the ynamine

Clusters containing carbene ligands. 8. Formation of ligands containing (dialkylamino)carbene centers by a novel 1,1-hydrogenation of ynamine ligands

Each compound consists of a sulfur-bridged triangular triosmium cluster. Compound 3 contains a bridging EtCN(Me)CH{sub 2} ligand. The ethyl-substituted carbon is a carbene center. Compound 4 contains a terminally coordinated carbene, EtC(NMe{sub 2}). Compound 5 contains a chelating EtCN(Me)CH{sub 2}. The ethyl-substituted carbon is a carbene center. Compound 6 contains a {eta}{sup 2}-H{sub 2}CC(H)C(H)NMe{sub 2} ligand. The carbene centers in compounds 3-5 were formed by 1,1-hydrogenations at the methyl-substituted carbon atom of the ynamine ligand in 1. In compounds 3 and 5 one of the N-methyl groups was metalated. The (dimethylamino)allyl ligand in 6 was formed by a 1,2-dihydrogenation of the ynamine ligand in 1 plus a CH activation on the methyl group on the carbon atom. Crystal data: for 3, space group Pbca, a = 12.884 (3) {angstrom}, b = 23.897 (5) {angstrom}, c = 12.522 (3) {angstrom}, 1,858 reflections, R = 0.034, R{sub w} = 0.033; for 4, space group P{anti 1}, a = 9.955 (1) {angstrom}, b = 13.971 (2) {angstrom}, c = 8.227 (1) {angstrom}, {alpha} = 102.91 (1){degree}, {beta} = 113.134 (9){degree}, {gamma} = 92.06 (1){degree}, 2,606 reflections, R = 0.027, R{sub w} = 0.031; for 5, space group P2{sub 1}/n, a = 9.260 (2)more » {angstrom}, b = 12.784 (3) {angstrom}, c = 16.734 (3) {angstrom}, {beta} = 90.60 (2){degree}, 2,163 reflections, R = 0.025, R{sub w} = 0.026; for 6, space group P2{sub 1}/n, a = 10.226 (3) {angstrom}, b = 12.589 (5) {angstrom}, c = 15.990 (7) {angstrom}, {beta} = 106.88 (3){degree}, 2,129 reflections, R = 0.036, R{sub w} = 0.039.« less

The crystal and molecular structure of 1,9-diamino-4-methyl-3,7-diazanonane-3,7-diacetato(2−)cobalt(III) nitrate monohydrate—a complex exhibiting large 13C NMR steric shifts

Past studies have shown major 13C NMR chemical shift differences between methylene carbon atoms proximal (In a γ position) and distal to a ligand methyl substituent in [Co(mddda)] + (mddda = 1,9-diamino-4-methyl-3,7-diazanonane-3,7-diacetate(2−), NH 2CH 2CH 2N(CH 2COO −)CH 2CH 2CH(CH 3)N(CH 2COO −)CH 2CH 2NH 2). Since the chemical shift differences have been attributed to γ steric shifts associated with the methyl group on the hexadentate ligand, X-ray crystal structure determination has been carried out on [Co(mddda)]NO 3·H 2O to examine the steric features. The compound crystallizes in the space group P2 1/c with a = 9.756(2) Å, b = 14.912(4) Å, c = 11.783(2)°, Å, β = 95.70(2)°, and Z = 4. Intensities were collected on an automated diffractometer and the structure was refined to a conventional R factor of 0.058. The crystal contains a racemic mixture of ΔΛΔ(R) and ΛΔΛ(S) enantiometric cationic complexes, where the R and S labels designate the absolute configurations of the chiral methyl-substituted carbon atom of the ligands. These diastereomers are those required to maintain an equatorial methyl group on the central six-membered chelate ring, whose conformations are λ skew-boat for the ΔΛΔ isomer and δ skew-boat for the ΛΔΛ isomer. The respective conformations of the two skewed five-membered chelate rings are δδ and λλ. Though most corresponding bond lengths and bond angles in the two portions of the molecule are nearly the same, those differences which are observed indicate nonbonded interactions between the methyl substituent and adjacent methylene groups. Consideration of these interactions and geometrical variations indicates that bond angle distortion may play an important role in the γ shifts observed.

The Reaction of Cycloalkanones with Copper(II) Halides. I. The Reaction of Cyclohexanones with Copper(II) Chloride

Abstract The reaction of cyclohexanones with copper(II) chloride gave polychlorinated cyclohexanediones directly in fairly good yields. With methylcyclohexanones, in general the type of product depends on the number of methylene groups between the carbonyl group and the methyl-substituted carbon in the starting material. A possible mechanism for the reaction is discussed.

Formation and reactions of cyanocyclohexadienyl complexes of manganese(I)

Reactions of the cationic arene complexes [(C6H6 –nMen)Mn(CO)3]+ with cyanide ion under mild conditions yield exo-cyanocyclohexadienyl complexes (C6H6 –nMenCN)Mn(CO)3 in which the cyano-group is attached to an unsubstituted carbon atom: the hexamethylbenzene complex reacts differently to form (C6Me6)Mn(CO)2CN. The preference for addition to an unsubstituted carbon atom is (at least in part) kinetic rather than thermodynamic in origin, since the compound formed by cyanide addition to the mesitylene complex rearranges in chloroform solution to give some of the isomer in which the cyano-group is attached to a methyl-substituted carbon atom.Decomposition of the cyanocyclohexadienyl complexes in the solid state or in solution yields either (C6H6 –nMen)Mn(CO)2CN or free arene and a polymeric manganese complex according to the reaction conditions used. Conventional hydride-abstracting reagents remove the cyano-group from the complexes, regenerating [(C6H6 –nMen)Mn(CO)3]+, but oxidation with cerie ion liberates the cyano-arene C6H5 –nMenCN uncontaminated by C6H6 –nMen.

Thermal rearrangements of aryldimethylvinylidenecyclopropanes and aryldimethylenecyclopropanes

1-Dimethylvinylidene-2-methyl-2-phenylcyclopropane (1a) undergoes thermal rearrangement in solution (130°) or in the vapour phase (350°) to give 2-isopropylidene-1-methylene-3-methyl-3-phenylcyclopropane (6a) exclusively. A kinetic study in solution over the temperature range 120–155° yields values for the activation parameters of 30·4 kcal mol–1 for Ea, and –2·5 e.u. for ΔS‡, which are shown to be consistent with the intermediacy of an orthogonal diradical. Similar rearrangements are undergone by a series of six related dimethylvinylidene cyclopropanes. Rearrangement of cis- and trans-1-dimethylvinylidene-2-methyl-3-phenylcyclopropanes (9) and (10) is kinetically controlled at 130° yielding mainly syn-1-ethylidene-2-isopropylidine-3-phenylcyclopropane (11); equilibration of the latter compound with the anti-isomer (12) became significant at 180°. The relative rates of rearrangement of compounds (9) and (10) to products (11) and (12) are explained in terms of the interactions of the substituents bonded to the phenyl-substituted carbon atom and the substituents and the incipient half-filled p-orbital on the methyl-substituted carbon atom. At higher temperatures (530°) the cyclopropanes (1a) and (6a) and 3,3-diphenyl-2-isopropylidene-1-methylenecyclopropane (6e) undergo further rearrangement to 1,2,3-trisubstituted indene derivatives.

The crystal and molecular structure of 2-bromolumisantonin

The molecular structure of lumisantonin, C15H18O3, a photoirradiation product of α-santonin, has been determined from a three-dimensional X-ray diffraction crystal structure analysis of its 2-bromo-derivative. The crystals are orthorhombic, space group P212121, with four molecules in a unit cell, a 11·60 ± 0·02, b 16·043 ± 0·008, c, 7·876 ± 0·005 Å. After the bromine atomic co-ordinates had been postulated from Patterson projection syntheses, the structure was solved semi-automatically by the heavy-atom method using an electron density map scanning computer programme. The final atomic parameters were obtained from a full-matrix least-squares refinement with 911 photographically recorded and visually measured data. The final R was 0·136. The structure consists of four fused substituted rings; a cyclopentenone ring and a distorted cyclohexane ring are both cis-fused to a doubly methyl-substituted cyclopropane ring, and a methyl-substituted γ-lactone ring is trans-fused to the cyclohexane ring. The configurations at the four asymmetric carbon atoms confirm the now accepted stereochemistry of α-santonin, including the recently revised configuration at the methyl-substituted carbon atom of the γ-lactone ring. The packing of the molecules is completely determined by van der Waals interactions.