Vibrational Excitation in Products of Nucleophilic Substitution: The Dissociation of Metastable X-(CH3Y) in the Gas Phase

Abstract

The relative kinetic energy distributions for the Y-+ CH3X nucleophilic substitution products from dissociation of metastable X-(CH3Y) (X = C1, Br; Y = Br, I) have been analyzed by means of ion kinetic energy spectroscopy, and the results modeled using statistical phase space theory. Comparison of the experimental distributions with the theoretical distributions predicted for statistical partitioning of the available energy reveals that the substitution products are translationally cold. The theoretically calculated distributions can be made to agree with experiment if most of the energy released in the dissociation is assumed to be unavailable for randomization, such that it cannot partition to relative translation. This unavailable energy must correspond to internal energy, most likely vibrational excitation, in the CH3X products. These results are consistent with recent theoretical predictions of non-RRKM dynamics in gas-phase s N 2 reactions.

Protonated nitro group as a gas-phase electrophile: Experimental and theoretical study of the cyclization of o-nitrodiphenyl ethers, amines, and sulfides

“…Reverse activation energy associated with loss of OH radical from P 12 or P 13 (ether 1) is minimal (≤ 3kJ/mol) and small for the other P 13 cases (≤ 15 kJ/mol, Tables 4a, 4b). These results are reasonable given that loss of OH from P 12 entails only N-OH bond cleavage, whereas, loss from P 13 involves ring relaxation to planarity with concomitant re-establishment of aromaticity. From P 12 , the OH radical loss would involve a simple homolytic bond cleavage, and the entropic factors in the transition states would be favorable to cleavage.…”

“…12 , from M 5 requires less energy than passing over the previous transition state, TS 4 ; hence there would be sufficient available energy to drive the first loss of OH radical (Tables 4a, 4b, Scheme 3). In addition, we performed CASSCF type calculations on M 5 and transition states for OH radical loss from even-electron M 5 : TS(M 5 -P 12 ) and TS(M 5 -P 13 ) yielding P 12 and P 13 , respectively (Table 4c). The results indicate, although direct comparison is not possible, that the reverse activation energy for the loss process is small for amine 9 and sulfide 5 and moderate for ether 1 cases.…”

“…Cycloaddition reactions involving ion-molecule reactions in the gas phase and comparison to reactions in solution are the subjects of a recent review [10]. These and other examples show the utility of a mass spectrometer as a "reaction vessel" for investigating gas-phase reactions of potential synthetic utility [11][12][13][14]. The gas-phase cyclizations of the radical cations of several disubstituted aromatic compounds led to the discovery of new methods [15,16] for the synthesis of heterocyclic compounds.…”

Thermochemistry of Hydrocarbon Radicals

“…25 Although not of thermochemical or combustion chemistry interest, this exothermic reaction has a classic double-well potential and the kinetic energy release had previously been studied by metastable ion dissociation, 26,27 providing an interesting test system for comparison with literature results. [26][27][28][29][30][31][32][33][34] At the lowest energies (near room temperature), the product velocity distributions are statistical as modeled by classical phase space theory (which disagrees with the metastable ion dissociation work but might be explained by the different experimental conditions). 27 However, at slightly higher kinetic energies, the product velocity distributions quickly become non-isotropic and show direct forward scattering of the CH 3 Cl product relative to the Cl !…”

“…[26][27][28][29][30][31][32][33][34] At the lowest energies (near room temperature), the product velocity distributions are statistical as modeled by classical phase space theory (which disagrees with the metastable ion dissociation work but might be explained by the different experimental conditions). 27 However, at slightly higher kinetic energies, the product velocity distributions quickly become non-isotropic and show direct forward scattering of the CH 3 Cl product relative to the Cl ! vector.…”