Leaving group effects in gas-phase substitutions and eliminations

Using a methodology recently developed for studying the product distributions of gas-phase S(N)2 and E2 reactions, the effect of the leaving group on the reaction rate and branching ratio was investigated. Using a dianion as the nucleophile, reactions with a series of alkyl bromides, iodides, and trifluoroacetates were examined. The alkyl groups in the study are ethyl, n-propyl, n-butyl, isobutyl, isopropyl, sec-butyl, and tert-butyl. The data indicate that leaving group abilities are directly related to the exothermicities of the reaction processes in both the gas phase and the condensed phase. Gas-phase data give a reactivity order of iodide > trifluoroacetate > bromide for S(N)2 and E2 reactions. Previous condensed phase data indicate a reactivity order of iodide > bromide > trifluoroacetate for substitution reactions; however, the basicities of bromide and trifluoroacetate are reversed in the condensed phase so this reactivity pattern does reflect the relative reaction exothermicities. Aside from this variation, the gas-phase data parallel condensed phase data indicating that the substituent effects are rooted in the nature of the alkyl substrate rather than in differences in solvation. The experimental data are supported by calculations at the MP2/6-311+G(d,p)//MP2/6-31+(d) level.     

The kinetics and mechanisms of the gas-phase pyrolysis of ethyl 4-bromobutyrate. The ion-pair mechanism revisited

The gas-phase pyrolysis of ethyl 4-bromobutyrate has been investigated in a static system over the temperature range of 354.6–374.7°C and the pressure range of 51–126 torr. The elimination reaction in seasoned vessels and in the presence of at least twofold of a chain radical inhibitor is homogeneous, unimolecular, and obeys a first-order rate law. The rate coefficients are given by the Arrhenius equation log k₁(s^?1) = (13.31 ± 0.82) – (205.1 ± 8.6)kJ/mol/2.303RT. The partial rates for the parallel eliminations to normal dehydrobromination, lactone formation, and bromobutyric acid product have been estimated and reported. The carboethoxy substituent of the bromoester has been found to assist anchimerically the elimination process, where dihydrobromination and lactone formation arise from an intimate ion-pair mechanism.     

Kinetics and mechanisms of the homogeneous, unimolecular gas-phase elimination of trimethyl orthoacetate and trimethyl orthobutyrate

The gas-phase elimination kinetics of the title compounds have been examined over the temperature range of 310-369 degrees C and pressure range of 50-130 Torr. The reactions, in seasoned vessels, are homogeneous, unimolecular, and follow a first-order rate law. The products are methanol and the corresponding methyl ketene acetal. The rate coefficients are expressed by the Arrhenius equation: for trimethyl orthoacetate, log k1 (s(-1)) = [(13.58 +/- 0.10) - (194.7 +/- 1.2) (kJ mol(-1))](2.303RT)(-1)r = 0.9998; and for trimethyl orthobutyrate, log k1(s(-1)) = [(13.97 +/- 0.37) - (195.3 +/- 1.6) (kJ mol(-1))](2.303RT)(-1)r = 0.9997. These reactions are believed to proceed through a polar concerted four-membered cyclic transition state type of mechanism.     

Theoretic studies on the kinetics and mechanism of multi-channel gas-phase unimolecular reactions of 1-chloropropane and 2-chloropropane

The potential energy surfaces of the gas-phase unimolecular decomposition reactions of 1-chloropropane and 2-chloropropane are investigated by various quantum chemical methods including CCSD(T), CCSD, GBS-QB3, C3B3, MP4, MP2, and B3LYP. Modified strong collision/RRKM theory was used to calculate the unimolecular rate constants as a function of pressure and temperature. It is found that the major reaction pathway is the HCl elimination. It is predicted that bond dissociation reactions have negligible contribution to the overall rate constant. The computed rate constants are compared with the available experimental data.     

Computational study on the kinetics and mechanisms for the unimolecular decomposition of formic and oxalic acids

The kinetics and mechanisms for the unimolecular decomposition reactions of formic acid and oxalic acid have been studied computationally by the high-level G2M(CC1) method and microcanonical RRKM theory. There are two reaction pathways in the decomposition of formic acid: The dehydration process starting from the Z conformer is found to be the dominant, whereas the decarboxylation reaction starting from the E conformer is less competitive. The predicted rate constants for the dehydration and decarboxylation reactions are in good agreement with the experimental data. The calculated CO/CO2 ratio, 13.6-13.9 between 1300 and 2000 K, is in close agreement with the ratio of 10 measured experimentally by Hsu et al. (In The 19th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1983; p 89). For oxalic acid, its isomer with two intramolecular hydrogen bonds is the most stable structure, similar to earlier reports. Two primary decomposition channels of oxalic acid producing CO2+HOCOH with barriers of 33-36 kcal/mol and CO2+CO+H2O with a barrier of 39 kcal/mol were found. At high temperatures, the latter process becomes more competitive. The rate constant predicted for the formation of CO2 and HOCOH (the precursor of HCOOH) agrees well with available experimental data. The mechanism for the isomerization of HOCOH to HCOOH is also discussed.     

Effects of functional group interactions on the gas-phase methylation and dissociation of acids and esters

Functional group interactions have been observed to affect gas-phase ion-molecule chemistry in a quadrupole ion trap mass spectrometer. Gas-phase methylation and collisionactivated dissociation reactions of a series of related acids and esters allows an evaluation of the structural factors that influence reactivity and functional group interactions of these compounds. Examination of the [M+H]⁺ or [M+15]⁺ product ions by collision-activated dissociation has provided insight into the conformations from which diacids and diesters undergo electrophilic addition. Collision-activated dissociation has provided not only more detailed information on the structures of the ions, but also the data necessary for confident mechanistic interpretation. Labeling studies were done to probe fragmentation pathways. Upon activation of the [M+CD₃]⁺ products of dimethyl maleate and dimethyl succinate, formed from reaction of the neutrals with CD₃OCD ₂ ⁺ ions, a rapid interfunctional group methyl transfer causes scrambling of the methyls prior to elimination of dimethyl ether or methanol. The [M+15]⁺ ions of dimethyl maleate are believed to lose dimethyl ether through a rate-determining 1,6-methyl transfer, whereas the [M+15]⁺ ions of dimethyl succinate eliminate methanol through a rate-determining 1,5-proton transfer.     

Energy barriers to gas-phase unimolecular decomposition of mono- and dinitrotoluenes

Alternative paths of gas-phase unimolecular decomposition of three nitrotoluenes and six dinitrotoluenes, in particular homolytic dissociation of the C–N bond, nitro–nitrite rearrangement, intramolecular hydrogen transfer from the methyl to nitro group with formation of isomeric aci-nitrotoluenes, and various paths involving formation of bicyclic intermediates, have been studied at the DFT B3LYP/6-31+G(2df,p) level of theory using GAUSSIAN 09 software package. The most energetically favorable path for o-nitrotoluene and 2,3-, 2,4-, 2,5-, and 2,6-dinitrotoluenes is the formation of aci-nitrotoluenes. The effect of the substrate structure on the competition between different mechanisms of these reactions has been analyzed.     

Computational study on kinetics and mechanisms of unimolecular decomposition of succinic acid and its anhydride

The mechanisms and kinetics of unimolecular decomposition of succinic acid and its anhydride have been studied at the G2M(CC2) and microcanonical RRKM levels of theory. It was shown that the ZsgsZ conformer of succinic acid, with the Z-acid form and the gauche conformation around the central C-C bond, is its most stable conformer, whereas the lowest energy conformer with the E-acid form, ECGsZ, is only 3.1 kcal/mol higher in energy than the ZsgsZ. Three primary decomposition channels of succinic acid producing H2O + succinic anhydride with a barrier of 51.0 kcal/mol, H2O + OCC2H3COOH with a barrier of 75.7 kcal/mol and CO2 + C2H5COOH with a barrier of 71.9 kcal/mol were predicted. The dehydration process starting from the ECGCZ-conformer is found to be dominant, whereas the decarboxylation reaction starting from the ZsgsZ-conformer is only slightly less favorable. It was shown that the decomposition of succinic anhydride occurs via a concerted fragmentation mechanism (with a 69.6 kcal/mol barrier), leading to formation of CO + CO2 + C2H4 products. On the basis of the calculated potential energy surfaces of these reactions, the rate constants for unimolecular decomposition of succinic acid and its anhydride were predicted. In addition, the predicted rate constants for the unimolecular decomposition of C2H5COOH by decarboxylation (giving C2H6 + CO2) and dehydration (giving H3CCHCO + H2O) are in good agreement with available experimental data.     

The unimolecular elimination kinetics of benzaldoxime in the gas phase

The kinetics of the gas‐phase elimination of benzaldoxime was determined in a static reaction system over the temperature and pressure range 350°C–400°C and 56–140 Torr, respectively. The products obtained were benzonitrile and water. The reaction was found to be homogeneous, unimolecular, and tend to obey a first‐order rate law. The observed rate coefficient is represented by the following Arrhenius equation: According to kinetic and thermodynamic parameters, the reaction proceeds through a concerted, semi‐polar, four‐membered cyclic transition state type of mechanism. © 2007 Wiley Periodicals, Inc. 39: 145–147, 2007     

The mechanisms of the homogeneus,unimolecular elimination kinetics of ethyl 4-chlorobutyrate and 4-chlorobutyric acid in the gas phase

Ethyl 4-chlorobutyrate, which is reexamined, pyrolyzes at 350–410°C to ethylene, butyrolactone, and HCl. Under the reaction conditions, the primary product 4-chlorobutyric acid is responsible for the formation of γ-butyrolactone and HCl. In seasoned vessels, and in the presence of a free-radical inhibitor, the ester elimination is homogeneous, unimolecular, and follows a first-order rate law. For initial pressures from 69–147 Torr, the rate is given by the following Arrhenius expression: log k₁(s^?1) = (12.21 ± 0.26) ? (197.6 ± 3.3) kJ mol^?1 (2.303RT)^?1. The rates and product formation differ from the previous work on the chloroester pyrolysis. 4-Chlorobutyric acid, an intermediate product of the above substrate, was also pyrolyzed at 279–330°C with initial pressure within the range of 78–187 Torr. This reaction, which yields γ-butyrolactone and HCl, is also homogeneous, unimolecular, and obeys a first-order rate law. The rate coefficient, is given by the following Arrhenius equation: log k₁(s^?1) = (12.28 ± 0.41) ? (172.0 ± 4.6) kJ mol^?1 (2.303RT)^?1. The pyrolysis of ethyl chlorobutyrate proceeds by the normal mechanism of ester elimination. However, the intermediate 4-chlorobutyric acid was found to yield butyrolactone through anchimeric assistance of the COOH group and by an intimate ion pair-type of mechanism. Additional evidence of cyclic product and neighboring group participation is described and presented.     

The kinetics of the gas-phase reaction between ozone and alkenes

The rate law ? d[O₃]/ dt = k₁[A][O₃] + k₃[A][O₃]²/ (k₄ + k₅[O₂]) has been found to obtain for the reaction of ozone with allene and with 1,2-butadiene. We now find that this rate law also applies to the reaction of ozone with ethylene and presumably with all lower alkenes. This generalizes the inhibiting effect of oxygen and accounts for the simplifed rate law found in the presence of excess oxygen. Oxygen itself is a product of the ozone–ethylene reaction, and we find that as [O₃]₀ increases, the (O₂ formed)/(O₃ used) ratio approaches 1.5. Values of k₁, k₃/k₅ for ethylene are compared with those for allene, 1,3-butadiene, and propene. A generalized mechanism is postulated for the reaction of ozone with alkenes involving a chain sequence that produces oxygen and which accounts for the observed rate law. A specific mechanism is postulated for the reaction of O₃ with ethylene, and the thermochemistry of the chain sequence is examined in detail.     

Predicted thermochemistry and unimolecular kinetics of nitrous sulfide

The geometry of N(2)S was obtained at the CCSD(T)/aug-cc-pV(T + d)Z level of theory and energies with coupled-cluster single double triple (CCSD(T)) and basis sets up to aug-cc-pV(6 + d)Z. After correction for anharmonic zero-point energy, core-valence correlation, correlation up to CCSDT(Q) and relativistic effects, D(0) for the N-S bond is estimated as 71.9 kJ mol(-1), and the corresponding thermochemistry for N(2)S is Δ(f)H(0)(°)=205.4 kJ mol(-1) and Δ(f)H(298)(°)=202.6 kJ mol(-1) with an uncertainty of ±2.5 kJ mol(-1). Using CCSD(T)/aug-cc-pV(T + d) theory the minimum energy crossing point between singlet and triplet potential energy curves is found at r(N-N) ≈ 1.105 ? and r(N-S) ≈ 2.232 ?, with an energy 72 kJ mol(-1) above N(2) + S((3)P). Application of Troe's unimolecular formalism yields the low-pressure-limiting rate constant for dissociation of N(2)S k(0) = 7.6 × 10(-10) exp(-126 kJ mol(-1)/RT) cm(3) molecule(-1) s(-1) over 700-2000 K. The estimated uncertainty is a factor of 4 arising from unknown parameters for energy transfer between N(2)S and Ar or N(2) bath gas. The thermochemistry and kinetics were included in a mechanism for CO/H(2)/H(2)S oxidation and the conclusion is that little NO is produced via subsequent chemistry of NNS.     

The role of nucleophile--electrophile interactions in the unimolecular and bimolecular gas-phase ion chemistry of peptides and related systems

This account describes the experimental tools (multi-stage mass spectrometric experiments, isotopic and structural labelling, kinetics and theoretical modelling) and physical organic concepts (influence of charge, the intermediacy of ion-molecule complexes, etc.) that can be used to unravel the mechanisms of gas-phase unimolecular and bimolecular ionic reactions of peptides. The role that nucleophile-electrophile interactions play in charge-directed reactions is highlighted for both unimolecular fragmentations (examples are illustrated for protonated sulfur-containing amino acids and peptides) and bimolecular ion-molecule reactions which cleave peptide bonds.     

Thermochemical kinetics of nitrogen compounds. V. The unimolecular thermal decomposition of diallylamine in the gas phase

The kinetics of the thermal decomposition of diallylamine to propylene and prop-2-enaldimine have been studied in the gas phase in presence of an excess of methylamine over the temperature range of 532.7 to 615.6°K, using a static reaction system. Methylamine reacted with the unstable primary product prop-2-enaldimine, forming the thermally stable N-methyl prop-2-enaldimine. First-order rate constants, based on the internal standard technique, fit the Arrhenius relationship log k(s^?1) = (11.04 ± 0.13) ? (37.11 ± 0.33 kcal/mole)/2.303 RT. They were independent on the initial total pressure (46–340 torr), the initial pressure of diallylamine (9.2–65 torr), or methylamine as well as the conversion attained. Despite an apparent surface sensitivity, the reaction is essentially homogeneous in nature as demonstrated by experiments carried out in a packed reaction vessel. The observed activation parameters for the title reaction together with those observed earlier for triallylamine and allylcyclohexylamine are consistent with the proposed concerted reaction mechanism involving a cyclic 6-center transition state. The observed substituent effects suggest a nonsynchronous mode of bond breaking and bond formation.     

On the unimolecular elimination of gas-phase iodine monobromide following excitation in the visible wavelength region

Dissociation dynamics of iodine monobromide has been theoretically investigated at numerous excitation wavelengths in the visible wavelength region (450–540 nm) by an accurate time-dependent dynamical simulation technique. The interatomic potential energy curves are derived from ab initio methods which are then employed in a Landau–Zener treatment to calculate the spin-orbit branching ratios in the two halogen products. The extinction coefficients, in arbitrary units, for the discrete electron transition are obtained through a numerical algorithm. Based on these elastic scattering calculations, the results of Br*: Br branching ratios are determined at several simulation wavelengths. Also the electronic properties are evaluated to elucidate the mechanism of chemical dynamics of the system. It is found that the branching dynamics can be clarified via the Landau–Zener model, which treats an avoided crossing between two states of a given nature and evaluates a nonadiabatic effect relevant to the dissociation of IBr molecule.     

Bifunctional even-electron ions. III. Fragmentation behaviour of aliphatic hydroxonium ions containing an additional carbomethoxy group

The primary and subsequent fragmentations of the bifunctional oxonium ions \documentclass{article}\pagestyle{empty}\begin{document}${\rm R} \!-\! \mathop {\rm C}\limits^ + ({\rm OH})\! -\! ({\rm CH}_2)_n \! - \! {\rm COOCH}_3 $\end{document} (n = 0?5), a, are dominated by functional group interactions. Loss of CH₃OH is the only appreciable primary fragmentation of the higher homoiogues, but for the lowest homologue (a₀) this reaction is missing. Instead, CO loss is observed. The next homologue (a₁) shov.s loss of CH₂CO besides loss of CH₃OH. The mode of the subsequent fragmentations is dependent on the chain length separating the functional group, and formation of cyclic ions is typical of the fragmentation behaviour of a₂ and a₃. Evidence for proton transfer from the carbonyl oxygen to the methoxy group of a protonated ester group is presented.     

Bifunctional even-electron ions. IV. Fragmentation behaviour of aliphatic methoxonium ions containing an additional carbomethoxy group

As for the corresponding hydroxonium ions, the methoxonium ions \documentclass{article}\pagestyle{empty}\begin{document}${\rm R} \!-\! \mathop {\rm C}\limits^ + ({\rm OCH}_2)\! -\! ({\rm CH}_2)_n \! - \! {\rm COOCH}_3 $\end{document} (n = 0?5), b, show as the main reactions those triggered by functional group interaction, but the overall decomposition rate is not much influenced. Loss of CH₃OH is the general primary fragmentation, originating preferentially from the COOCH₃ group. The lowest homologue (b₀) shows loss of C₃H₄O as the main primary reaction, while b₁ shows loss of CH₂CO besides CH₃OH loss. The subsequent fragmentations exhibit strong chain-length dependence. Methoxy anion migration is observed as a general reaction mode of fragment ions which still contain the COOCH₃ group. The fragmentation behaviour of a and b and of the corresponding methoxy-substituted bifunctional ions has been compared, and general conclusions have been drawn.     

The kinetics and mechanism of gas-phase N-oxidation of pyridine with hydrogen peroxide

The kinetics of N-monooxidation of 4-vinylpyridine as a π-deficient heteroaromatic compound under the conditions of gas-phase free-radical chain oxidation was studied. The experimental interference kinetic curves of synchronous hydrogen peroxide decomposition and 4-vinylpyridine N-oxidation reactions were obtained. The region of selective N-oxidation was determined and optimum conditions of N-oxide preparation found. The most probable mechanism was suggested. According to this mechanism, a key role in the free-radical N-oxidation of the substrate and its synchronization with the H₂O₂ decomposition reaction was played by HO₂ radicals.     

Use of gas-phase (1)H NMR to determine the kinetics of unimolecular rearrangement of 2,2-dichloro-1-methylenecyclopropane and the bimolecular dimerization of (dichloromethylene)cyclopropane

Gas-phase (1)H NMR analysis has been applied to investigate the kinetics of the unimolecular rearrangement of 2,2-dichloro-1-methylenecyclopropane (1) to (dichloromethylene)cyclopropane (2) [k(1) = 7.9 x 10(12) exp(-34.4 +/- 0.6 kcal mol(-1)/RT)], as well as for the subsequent second-order dimerization of 2 [k(2) = 2.4 x 10(6) exp(-18.5 +/- 1.1 kcal mol(-1)/RT)] to form 7,7,8,8-tetrachlorodispiro[2.0.2.2]octane (3)