Loss Of CH Research Articles

Characterization of 4-Aryl-5-ethoxycarbonyl-6-methyl-3, 4-dihydropyrimidin-2 (1 H) -thiones by El-TOF Mass Spectrometry

The electron impact (EI) time-of-flight mass spectra (TOFMS) of the title compounds (thioxo-Biginelli compounds) were studied to establish their fragmentation pathways. With a TOF instrument of high resolution, the exact mass for each fragment was determined. These data were used to infer the molecular formulas and the ionic elemental compositions for all the compounds through interpretation by software. By further applying chemical intuition, majority of the fragmentation ions were fully assigned. All the compounds give strong signals (average 76%) for their molecular ions in the EI spectra. Two kinds of characteristic fragmentation pathways from the molecular ion were observed. One is related to the loss of the ester group, forming a resonance stabilized ion with a moderate abundance (average 40%). The other concerns the loss of the aromatic-ring radical, forming another resonance stabilized ion at m/z 199 with a high abundance (average 79%), from which further important fragmentations including the formation of an ion at m/z 171 (average 24%) by the loss of C2 H2 (via the McLafferty rearrangement), and an ion at m/z 153 (average 10%) by the elimination of a water molecule preceed. In addition, ion [M - Et˙]+ with a high abundance (average 62%) is a characteristic for the Biginelli compounds with an ethoxycarbonyl group. Differences and similarities among the fragmentations observed from the EI-TOFMS of oxo-Biginelli compounds are discussed.

Dissociative photoionization of mono-, di- and trimethylamine studied by a combined threshold photoelectron photoion coincidence spectroscopy and computational approach

Energy selected mono-, di- and trimethylamine ions were prepared by threshold photoelectron photoion coincidence spectroscopy (TPEPICO). Below 13 eV, the main dissociative photoionization path of these molecules is hydrogen atom loss. The ion time-of-flight (TOF) distributions and breakdown diagrams for H loss are analyzed in terms of the statistical RRKM theory, which includes tunneling. Experimental evidence, supported by quantum chemical calculations, indicates that the reverse barrier along the H loss potential energy curve for monomethylamine is 1.8 +/- 0.6 kJ mol(-1). Accurate dissociation onset energies are derived from the TOF simulation, and from this analysis we conclude that Delta(f)H degrees (298K)[CH(2)NH(2)(+)] = 750.4 +/- 1.3 kJ mol(-1) and Delta(f)H degrees (298K)[CH(2)NH(CH(3))(+)] = 710.9 +/- 2.8 kJ mol(-1). Quantum chemical calculations at the G3, G3B3, CBS-APNO and W1U levels are extensively used to support the experimental data. The comparison between experimental and ab initio isodesmic reaction heats also suggests that Delta(f)H degrees (298K)[N(CH(3))(3)] = -27.2 +/- 2 kJ mol(-1), and that the dimethylamine ionization energy is 8.32 +/- 0.03 eV, both of which are in slight disagreement with previous experimental values. Above 13 eV photon energy, additional dissociation channels appear besides the H atom loss, such as a sequential C(2)H(4) loss from trimethylamine for which a dissociation mechanism is proposed.

Laser-Induced Acoustic Desorption/Fourier Transform Ion Cyclotron Resonance Mass Spectrometry for Petroleum Distillate Analysis

Laser-induced acoustic desorption (LIAD) coupled with a 3-T Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR) allows the simultaneous analysis of both the nonpolar and polar components in petroleum distillates. The LIAD/FT-ICR method was validated by examining model compounds representative of the various classes of polar and nonpolar hydrocarbons commonly found in petroleum. LIAD successfully desorbs all the compounds as intact neutral molecules into the FT-ICR. Electron ionization (EI) at low energies (10 eV) and chemical ionization using cyclopentadienyl cobalt radical cation (CpCo*+) were employed to ionize the desorbed molecules. The EI experiments lead to extensive fragmentation of many of the hydrocarbon compounds studied. However, the CpCo*+ ion ionizes all the hydrocarbon compounds by producing only pseudomolecular ions without other fragmentation, with the exception of one compound (*CH3 loss occurs). Examination of two different petroleum distillate samples revealed hundreds of compounds. The most abundant components have an even molecular weight; i.e., they are likely to contain no (or possibly an even number of) nitrogen atoms.

Activation Barriers for Addition of Methyl Radicals to Oxygen-Stabilized Carbocations

Activation barriers (DeltaHMe(double dagger)) for adding methyl radicals to ions of the general formula CH3CR=OCH3+ have been measured by looking at the threshold energies for the reverse reaction, dissociative photoionization of ethers of the general formula RC(CH3)2OCH3. Dissociation by loss of a methyl radical has more favorable thermochemistry than loss of R*, yet the onset of R* loss occurs at lower energies than loss of CH3*. In other words, the more endothermic dissociation exhibits a lower appearance energy. Contrathermodynamic ordering of appearance energies is observed for R = Et, nPr, iPr, tBu, and neopentyl. The sum of the appearance energy difference, DeltaAE, and the thermochemical difference (DeltaDeltaH, calculated using G3 theory) gives a lower bound for the barrier for adding methyl radical to CH3CR=OCH3+. More specifically, the difference between that activation barrier and the one for adding R* to (CH3)2C=OCH3+, DeltaHMe(double dagger)-DeltaHR(double dagger), equals DeltaAE + DeltaDeltaH and has values in the range 20-24 kJ mol(-1) for the homologous series investigated. There is no systematic trend with the steric bulk of R, and available evidence suggests that DeltaHR(double dagger) does not have a value >5 kJ mol(-1). The difference in barrier heights, DeltaHMe(double dagger)-DeltaHiPr(double dagger) for CH3* plus iPrC(CH3)=OX+ vs iPr* + (CH3)2C=OX+, has the same value, regardless of whether X = H or CH3. Mixing of higher energy electronic configurations provides a qualitative theoretical explanation for some (but not all) observed trends in barrier heights.

Fragmentation reactions of deprotonated peptides containing proline. The proline effect

The collision-induced dissociation (CID) fragmentation reactions of a variety of deprotonated peptides containing proline have been studied in detail using MS(2) and MS(3) experiments, deuterium labelling and accurate mass measurements when necessary. The [M--H--CO(2)](-) (a(2)) ion derived from H-Pro-Xxx-OH dipeptides shows an unusual fragmentation involving loss of C(2)H(4); this fragmentation reaction is not observed for larger peptides. The primary fragmentation reactions of deprotonated tripeptides with an N-terminal proline are formation of a(3) and y(1) ions. When proline is in the central position of tripeptides, a(3), y(2) and y(1) ions are the primary fragmentation products of [M--H](-), while when the proline is in the C-terminal position, a(3)and y(1) ions are the major primary products. In the latter case, the a(3) ion fragments primarily to the ''b(2) ion; further evidence is presented that the ''b(2) ions have a deprotonated oxazolone structure. Larger deprotonated peptides having at least two amino acid residues N-terminal to proline show a distinct preference for cleavage of the amide bond N-terminal to proline to form, mainly, the appropriate y ion. This proline effect is compared and contrasted with the similar proline effect observed in the fragmentation of protonated peptides containing proline.

Liquid chromatography/electrospray ionization mass spectrometry for the characterization of twenty‐three flavonoids in the extract of Dalbergia odorifera

A method incorporating high-performance liquid chromatography (HPLC) with electrospray ionization and tandem mass spectrometry, with parallel analysis by HPLC with UV detection using a diode-array detector, was developed for the qualitative characterization of flavonoids in D. odorifera. Twenty-three flavonoids, including six isoflavones, six neoflavones, four isoflavanones, three flavanones, two chalcones, one isoflavanonol and one pterocarpan, were unambiguously identified by comparing their retention times, UV and MS spectra with those of authentic compounds. Furthermore, the collision-induced dissociations of the [M-H]- ions were studied to clarify the MS behavior of the different types of flavonoids. In negative ion ESI-MS all the flavonoids yielded prominent [M-H]- ions in the first order mass spectra. Fragments involving losses of CH3*, H2O, CO, C2H2O, and CO2 were observed in the MS/MS spectra. Each of the seven types of flavonoid showed characteristic MS/MS fragmentation patterns. The isoflavanones, flavanones and chalcones were observed to undergo retro-Diels-Alder fragmentations. The spectra of almost all the neoflavonoids unexpectedly exhibited only [M-H-CH3]-* radical anions as base peaks without any further fragmentation. Substitution positions also remarkably influenced the fragmentation behavior, which could assist in distinction among the flavonoid isomers. The fragmentation rules deduced here could aid in the characterization of other flavonoids of these types.

On the Parallel Mechanism of the Dissociation of Energy-Selected P(CH3)3+ Ions

Energy selected trimethyl phosphine ions were prepared by threshold photoelectron photoion coincidence (TPEPICO) spectroscopy. This ion dissociates via H, CH(3), and CH(4) loss, the latter two involving hydrogen transfer steps. The ion time-of-flight distribution and the breakdown diagram are analyzed in terms of the statistical RRKM theory, which includes tunneling. Ab initio and DFT calculations provide the vibrational frequencies required for the RRKM modeling. CH(3) loss could produce both the P(CH(3))(2)(+) by a simple bond dissociation step, and the more stable HP(CH(2))CH(3)(+) ion by a hydrogen transfer step. Quantum chemical calculations are extensively used to uncover the reaction scheme, and they strongly suggest that the latter product is exclusively formed via an isomerization step in the energy range of the experiment. The data analysis, which includes modeling with the trimethyl phosphine thermal energy distribution, provides accurate onset energies for both H (E(0K) = 1024.1 +/- 3.5 kJ/mol) and CH(3) (E(0K) = 1024.8 +/- 3.5 kJ/mol) loss reactions. From this analysis, we conclude that the Delta(f)H(298K) degrees [HP(CH(2))(CH(3))(+)] = 783 +/- 8 kJ/mol and Delta(f)H(298K) degrees [P(CH(2))(CH(3))(2)(+)] = 711 +/- 8 kJ/mol.

Fragmentation of Trimethoprim and other Compounds Containing Alkoxy-Phenyl Groups in Electrospray Ionisation Tandem Mass Spectrometry

This work describes electrospray ionisation tandem mass spectrometry studies of trimethoprim and a series of structurally similar compounds containing alkoxy-phenyl groups; using accurate mass measurement to confirm the proposed fragmentations. Radical cations were observed in the spectra obtained for some of the compounds, as well as uncommon fragmentations showing losses of CH4 and C2H6, whereas other compounds showed the formation of even electron ions. Possible structures for the fragment ions have been proposed and explanations for the different types of fragmentations based on the structures of the compounds. In addition an alternate structure for a fragment ion previously reported for tandem mass spectrometry of trimethoprim has been proposed, based on accurate mass measurement.

Methyl Migration in the Metastably Decomposing Acetylphenyl Ions, CH3COC6H4+ (m/z 119), Followed by the Loss of CO

The metastable ion dissociations of o-, m-, and p-acetylphenyl ions, CH3COC6H4+ (m/z 119), generated upon electron ionization from o-, m-, and p-diacetylbenzenes (CH3COC6H4COCH3, Mw : 162, 1-3), have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometry in conjunction with thermochemistry. All these m/z 119 ions, which are generated by the consecutive losses of CH3 and CO from 1+·-3+·, dissociate to only the ion at m/z 91 by the loss of CO. These suggest that in the CH3COC6H4+ ions, the CH3 migration from the acetyl group to the benzene ring occurs prior to the loss of CO. The amounts of the kinetic energy release (KER) accompanying with the dissociation of the m/z 119 ions from compounds 1-3 to m/z 91 ion were larger than those of the corresponding, respective methylbenzoyl ions. In the m-, and p-CH3COC6H4+ ions, the charge migrates from the incipient charge site to the ortho position via the so-called hydrogen “ring-walk.” Then CH3 migrates from the acetyl group to the charge site at the ortho position in the benzene ring prior to the loss of CO. A part of the m/z 91 ions from o-, m-, and p-acetylphenyl ions may be generated via the migration of CH3 to the incipient charge site, followed by the loss of CO.

CH4 Loss from (CH3)4N+ Revisited: How Does This High Energy Elimination Compete with •CH3 Loss?

The rate of CH4 elimination from the tetramethylammonium ion increases faster than simple •CH3 loss with increasing internal energy at both high and low energies. Improved understanding of this highly unusual competition is sought by ab initio theory and RRKM calculations. Geometries and energies of stationary points and pathways as traced by intrinsic reaction coordinate calculations are given. A transition state at an energy much higher than those of both the reactant (414 kJ mol−1 above) and the products (361 kJ mol−1 above) was found for methane elimination from (CH3)4N+. More importantly, this transition state was also 14 kJ mol−1 above one found for methyl loss. However, according to results obtained and presented by RRKM theory, methane elimination through this transition state would be too slow to compete with methyl loss. This transition state may be for a concerted or a complex-mediated process. It is unlikely that CH4 is actually lost by a concerted elimination because only a small fraction of the reverse activation energy becomes translational energy, whereas concerted eliminations usually convert substantial fractions of their reverse activation energies into translational energy. Also, complex-mediated CH4 loss is unlikely because such eliminations are usually very quickly overwhelmed by simple dissociation of the partners just above threshold with increasing energy, opposite to the behavior of the system studied. Thus it is concluded that CH4 elimination from (CH3)4N+ occurs by loss of •CH3 followed by loss of H• at all energies, even though that is a higher energy process than methane elimination. Kinetic shifts and a reverse activation energy for •CH3 loss appear to raise the energy in the ions dissociating in the field free regions high enough to dissociate (CH3)4N+• + to (CH3)2N=CH2+• + •CH3 + H•.

Electrospray tandem mass spectrometry of 2H‐chromenes

Several 2H-chromenes derived from carbazoles were analyzed by electrospray tandem mass spectrometry. The 2H-chromenes constitute an important class of compounds that exhibit photochromic activity. The fragmentation pathways of the protonated molecular species [M+H]+ were studied, and main fragmentation pathways of these compounds were identified. Fragmentation pathways of [M+D]+ ions were also studied in order to obtain information about the location of the ionizing proton or deuteron. It was found that the proton is not preferentially located on the nitrogen atom. The charge is preferentially located as a tertiary carbocation, resulting from the uptake of the proton (or deuteron) by the zwitterionic open structure of the chromenes. The major fragmentation occurred by cleavage of the gamma-bond relative to the carbocation center, leading to a fragment at m/z 191 (C5H11+ or C14H9N+), which are the most abundant fragment ions for almost all compounds. The presence of substituents in the chromene ring does not change this behavior. Other observed common fragmentation pathways included loss of CH3* (15 Da), loss of CO (28 Da), combined loss of CO and CH3 (43 Da), and loss of the phenyl ring via combined loss of C6H4 and CH3* (-91 Da) and combined loss of C6H6 and CO (-106 Da).

Low Energy Dissociation Processes of Ionized Cyclohexene: A Theoretical Insight,

The major dissociation reactions of the cyclohexene radical cation, 1, lead to the cyclopentenyl ion by methyl loss and to ionized 1,3-butadiene after elimination of C2H4. These two reactions are also observed during the Diels−Alder reaction between ionized butadiene and ethylene in the gas phase. The energetic and mechanistic aspects of the methyl loss process from the cyclohexene radical cation or the reaction between ionized butadiene and ethylene are discussed with the help of molecular orbital calculations at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) levels. Methyl loss is demonstrated to result from successive 1,2-hydrogen shifts and ring-contraction/ring-opening steps involving, as a crucial intermediate, ionized bicyclo[1,3,0]hexane rather than the distonic ion [CH2CH2CHCHCHCH2]•+ (one of the open forms of ionized cyclohexene). This latter one is however involved during the direct and retro Diels−Alder reactions. The CH3 and C2H4 loss rate curves of the cyclohexene ion are calculated using the Ric...

β-Diketiminato Scandium Chemistry: Synthesis, Characterization, and Thermal Behavior of Primary Amido Alkyl Derivatives

Treatment of the β-diketiminato-supported scandium dichlorides {[ArNC(R)CHC(R)NAr]ScCl2}n (Ar = 2,6-iPr2-C6H3; R = CH3, 1a, n = 2; R = tBu, 1b, n = 1) with 1 equiv of a lithium amide reagent LiN(H)R‘ (R‘ = tBu, 2,6-iPr2-C6H3) gave scandium amido derivatives. For 1a, use of LiN(H)tBu leads to the bis-amido derivative (6a) regardless of the equivalency of amide reagent employed, suggesting that facile ligand redistribution processes are operative when the ligand is the less bulky methyl-substituted example. For 1b, mono-amido chlorides 2b (R‘ = tBu) and 3b (R‘ = 2,6-iPr2-C6H3) are obtained in good yields, and these compounds can be alkylated with MeLi to provide mono-amido methyl compounds 4b (R‘ = tBu) and 5b (R‘ = 2,6-iPr2-C6H3). All four of these compounds were characterized crystallographically. The amido ligand occupies the exo coordination site exclusively, and there is no evidence in solution for a diastereomer with the amido group in the endo site. DFT calculations suggest that there is a strong steric preference and a slight electronic bias for the amido ligand to assume the exo position. Thermolysis of the amido methyl complex 4b leads to loss of CH4 and production of a scandacylic product, 7, formed via metalation with one of the N-aryl isopropyl methyl groups. This compound was characterized crystallographically. Deuterium labeling experiments suggest that 7 is produced via direct metalation and does not form via a scandium imido intermediate.

Unimolecular Reactions of Diethyl Malonate Cation in Gas-phase

Unimolecular reactions of diethyl malonate cation (CH3CH2OC(=O)CH2C(=O)OCH2CH3+· ; 1+·, Mw : 160) induced by electron ionization, have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling in conjunction with thermochemistry. In the metastable time window, the molecular ions 1+· lead not only to the formation of the major fragment ions m/z 133 ([M-C2H3]+) through a McLafferty + 1 rearrangement, but also to two minor fragment ions m/z 132 ([M-C2H4]+·) through a McLafferty rearrangement (or [M-CO]+·) and m/z 88 ([M-C3H4O2]+·). These dissociations are rationalized by the idea of ion-neutral complex. The m/z 88 ions are generated via at least two different routes. The m/z 133 ([M-C2H3]+) ions decompose into the ions at m/z 115, 106, and 105 by the losses of H2O, C2H3, and C2H4, respectively. A large part of the first fragment ions would be in the keto form and it decomposes into the m/z 71 and 43 ions by the losses of CO2 and C3H4O2 (or C2O3), and a small part would be in the enol form and decomposes into the m/z 87 ions by the loss of C2H4, not CO. The m/z 43 ions are isobaric, C2H3O+ and C3H7+.

Rate constant for the reaction CH3 + CH3 → C2H6 at T = 155 K and model calculation of the CH3 abundance in the atmospheres of Saturn and Neptune

The column abundances of CH3 observed by the Infrared Space Observatory (ISO) satellite on Saturn and Neptune were lower than predicted by atmospheric photochemical models, especially for Saturn. It has been suggested that the models underestimated the loss of CH3 due to poor knowledge of the rate constant k of the CH3 + CH3 self‐reaction at the low temperatures and pressures of these atmospheres. Motivated by this suggestion, we undertook a combined experimental and photochemical modeling study of the CH3 + CH3 reaction and its role in determining planetary CH3 abundances. In a discharge flow‐mass spectrometer system, k was measured at T = 155 K and three pressures of He. The results in units of cm3 molecule−1 s−1 are k(0.6 Torr) = 6.82 × 10−11, k(1.0 Torr) = 6.98 × 10−11, and k(1.5 Torr) = 6.91 × 10−11. Analytical expressions for k were derived that (1) are consistent with the present laboratory data at T = 155 K, our previous data at T = 202 K and 298 K, and those of other studies in He at T = 296–298 K and (2) have some theoretical basis to provide justification for extrapolation. The derived analytical expressions were then used in atmospheric photochemical models for both Saturn and Neptune. These model results reduced the disparity with observations of Saturn, but not with observations of Neptune. However, the disparity for Neptune is much smaller. The solution to the remaining excess CH3 prediction in the models relative to the ISO observations lies, to a large extent, elsewhere in the CH3 photochemistry or transport, not in the CH3 + CH3 rate.

Atomic layer deposition of Al2O3 on H-passivated Si. I. Initial surface reaction pathways with H/Si(100)-2×1

Aluminum oxide (Al2O3) grown by atomic layer deposition (ALD) is currently under investigation for use as a high-κ gate dielectric alternative to SiO2. Cluster calculations employing hybrid density functional theory have been carried out to examine the chemical reaction pathways between the ALD precursors, trimethylaluminum (TMA) and H2O, with the H/Si(100)-2×1 surface. Results obtained using Si9H14 and Si15H20, dimer and double dimer clusters to represent the surface active site are in good agreement, providing a consistent view of reaction energetics on the H/Si(100)-2×1 surface. The adsorption energies for TMA and H2O on the surface are calculated to be 0.02 and 0.15 eV, respectively. For the reaction between H2O and the H/Si(100)-2×1 surface, hydroxylation of the surface accompanied by loss of H2 was found to be the preferred pathway having an activation energy and overall reaction enthalpy of 1.60 eV and −0.75 eV, both of which are ⩾0.70 eV lower than the corresponding values for the possible H/D exchange reaction. TMA exposure of the H/Si(100)-2×1 surface favors the deposition of –Al(CH3)2 with loss of CH4, having a barrier height of 1.30 eV and reaction enthalpy of −0.31 eV, which are 0.10 and 0.40 eV lower than the surface methylation pathway (H/CH3 exchange) and 2.64 and 0.45 eV lower in energy than the H2 loss reaction, that results in the deposition of –CH2–Al(CH3)2 to the surface. Therefore, the dominant reactions identified in this work are those with direct implication in the Al2O3 ALD growth mechanism, leading to the formation of Si–O and Si–Al species on the H/Si(100)-2×1 surface.

Unimolecular Gas-Phase Reactions of Diethyl Phthalate, Isophthalate, and Terephthalate upon Electron Ionization

Unimolecular gas-phase reactions of diethyl phthalate (1), isophthalate (2), and terephthalate (3), upon electron ionization, have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometry and deuterium labelling. The metastable molecular ions (1)+ decompose to give exclusively the ions m/z 176 ([M – CH3CH2OH]+) and not the ions by the loss of CH3CH2O as proposed earlier in the literature. The metastable molecular ions (2)+ and (3)+ fragment differently from (1)+ and lead not only to the formation of the major fragment ions m/z 194 ([M − CH2CH2]+) via a McLafferty rearrangement but also to minor fragment ions m/z 193 ([M – CH2CH3]+).Yet, molecular ions decomposing in the ion source all show as primary fragmentation channel the loss of CH3CH2O to give the ions at m/z 177, which further dissociate to give the ions at m/z 149 through the loss of C2H4 or CO, indicating the resulting ions are +COC6H4COOH and +C6H4COOCH2CH3. The +COC6H4COOH ions decompose into the m/z 121, 93, and 65 ions by the consecutive losses of three carbon monoxide molecules, respectively. Prior to the second CO loss, a migration of the OH group to the benzene ring occurs. During the metastable fragmentation of the +C6H4COOCH2CH3 ions no ethoxy migration occurs, in contrast to the methoxy migration taking place in the metastable decomposition of the lower homologue +C6H4COOCH3 ions.

Unimolecular Gas-Phase Reactions of Ionized Organo-Silicon Compounds. Part XVI. (CH3)3SiOCH2CH3 and (CH3)3SiOCH2CF3

The unimolecular metastable decompositions of ethoxytrimethylsilane ((CH3)3SiOCH2CH3; 1, MW: 118) and its fluorine analogue, 2,2,2-trifluoroethoxytrimethylsilane ((CH3)3SiOCH2CF3, 2; MW: 172) induced by electron ionization, have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling. Both molecular ions are formed in low abundance. The fragmentation of 1+ · is different from that of 2+ · , except for the loss of CH3 from the corresponding molecular ions. The MIKE spectra of the [M - CH3]+ ions from 1+ · and 2+ · gave five and three fragment ions, respectively. Two of the five reactions for 1 leading to the formation of the ion at m/z 101, (CH3)2Si+OCH = CH2, and m/z 61, CH3Si+HOH, concern new mechanistic pathways, which were not described in previous reports. Skeletal rearrangement by among others F and CF3 migrations, occurs during the fragmentation of the [M - CH3]+ ions from 2+ · . The m/z 61 ions from 2+ · are both CH2OCF+ (or CH2FCO+ etc.) and CH3Si+HOH.

Ab Initio Studies on the Thermal Dissociation Channels of cis- and trans-Azomethane

The unimolecular dissociation of azomethane is studied by first sampling the reaction channels with a density functional theory based ab initio molecular dynamics method and then by mapping out the reaction barrier and transition structure for each channel with Gaussian based ab initio method at G3 and B3LYP/6-31G(d) levels. A number of new dissociation channels are identified, and all of them involve the migration of H atoms or the breaking of a C−H bond. There is also notable difference among these channels for trans- and cis-azomethane. In the gas phase, a CH4 loss channel could be present in the dissociation of cis-azomethane. For N−N bond cleavage observed on metal surfaces, the hydrogen migration from C to N atoms, by either 1,2- or 1,3-hydrogen shifts, is the first step. Further H migration could result in N−N cleavage and produce HCN and CH3NH2, both observed in previous surface experiments. The concerted C−H bond cleavages in cis-azomethane could also produce H2.

Rate Constant for the Recombination Reaction CH3 + CH3 → C2H6 at T = 298 and 202 K.

The recombination of methyl radicals is the major loss process for methyl in the atmospheres of Saturn and Neptune. The serious disagreement between observed and calculated levels of CH3 has led to suggestions that the atmospheric models greatly underestimated the loss of CH3 due to poor knowledge of the rate of the reaction CH3 + CH3 + M → C2H6 + M at the low temperatures and pressures of these atmospheric systems. In an attempt to resolve this problem, the absolute rate constant for the self-reaction of CH3 has been measured using the discharge-flow kinetic technique coupled to mass spectrometric detection at T = 202 and 298 K and P = 0.6-2.0 Torr nominal pressure (He). CH3 was produced by the reaction of F with CH4, with [CH4] in large excess over [F], and detected by low energy (11 eV) electron impact ionization at m/ z = 15. The results were obtained by graphical analysis of plots of the reciprocal of the CH3 signal vs reaction time. At T = 298 K, k 1(0.6 Torr) = (2.15 ± 0.42) × 10-11 cm3 molecule-1 s-1 and k 1(1 Torr) = (2.44 ± 0.52) × 10-11 cm3 molecule-1 s-1. At T = 202 K, the rate constant increased from k 1(0.6 Torr) = (5.04 ± 1.15) × 10-11 cm3 molecule-1 s-1 to k 1(1.0 Torr) = (5.25 ± 1.43) × 10-11 cm3 molecule-1 s-1 to k 1(2.0 Torr) = (6.52 ± 1.54) × 10-11 cm3 molecule-1 s-1, indicating that the reaction is in the falloff region. Klippenstein and Harding had previously calculated rate constant falloff curves for this self-reaction in Ar buffer gas. Transforming these results for a He buffer gas suggest little change in the energy removal per collision, -〈Δ E〉d, with decreasing temperature and also indicate that -〈Δ E〉d for He buffer gas is approximately half of that for Argon. Since the experimental results seem to at least partially affirm the validity of the Klippenstein and Harding calculations, we suggest that, in atmospheric models of the outer planets, use of the theoretical results for k 1 is preferable to extrapolation of laboratory data to pressures and temperatures well beyond the range of the experiments.