Coulomb Explosion Dynamics of Multiply Charged para-Nitrotoluene Cations.

This work explores Coulomb explosion (CE) dissociation pathways in multiply charged cations of para-nitrotoluene (PNT), a model compound for nitroaromatic energetic molecules. Experiments using strong-field ionization and mass spectrometry indicate that metastable cations PNT2+ and PNT3+ undergo CE to produce NO2+ and NO+. The experimentally measured kinetic energy release from CE upon formation of NO2+ and NO+ agrees qualitatively with the kinetic energy release predicted by computations of the reaction pathways in PNT2+ and PNT3+ using density functional theory (DFT). Both DFT computations and mass spectrometry identified additional products from CE of highly charged PNTq+ cations with q > 3. The dynamical timescales required for direct CE of PNT2+ and PNT3+ to produce NO2+ were estimated to be 200 and 90 fs, respectively, using ultrafast disruptive probing measurements.

Dissociation of Singly and Multiply Charged Nitromethane Cations: Femtosecond Laser Mass Spectrometry and Theoretical Modeling.

Dissociation pathways of singly- and multiply charged gas-phase nitromethane cations were investigated with strong-field laser photoionization mass spectrometry and density functional theory computations. There are multiple isomers of the singly charged nitromethane radical cation, several of which can be accessed by rearrangement of the parent CH3-NO2 structure with low energy barriers. While direct cleavage of the C-N bond from the parent nitromethane cation produces NO2+ and CH3+, rearrangement prior to dissociation accounts for fragmentation products including NO+, CH2OH+, and CH2NO+. Extensive Coulomb explosion in fragment ions observed at high laser intensity indicates that rapid dissociation of multiply charged nitromethane cations produces additional species such as CH2+, H+, and NO22+.  On the basis of analysis of Coulomb explosion in the mass spectral signals and pathway calculations, sufficiently intense laser fields can remove four or more electrons from nitromethane.

Hydrogen-Deuterium Exchange in Basic Near-Critical and Supercritical Media: An Experimental and Theoretical Study.

Treatment of homo- and heterocyclic aromatic substrates with basic deuterium oxide under near- or supercritical conditions results in rapid base-catalyzed hydrogen-deuterium exchange (HDE) in aromatic and benzylic positions. It has been postulated that HDE follows a simple deprotonation-reprotonation mechanism, but little evidence has been provided to date. This study correlates experimentally observed proton exchanges in n-butylbenzene with ab initio calculations of the acidities and potential energy (PE) profiles. In addition to providing further support for carbanion intermediacy in HDE, these results offer new insights into substrate acidities in near- and supercritical aqueous media and the optimal conditions required for their isotope exchange.

Rearrangement reactions of lithiated oxiranes.

The first computational study of the rearrangement reactions of oxiranes initiated by lithium dialkylamides is presented. Aside from the well-known carbenoid insertion pathways, both ß-elimination and α-lithiation have been suggested as the exclusive mechanism by which oxiranes react in the presence of organolithium bases. The products of the former are allyl alcohols (and, in some cases, dienes) and are ketones in the case of the latter. The computational studies reported in this work indicate that both mechanisms could be simultaneously operational. In particular, our work shows that the allyl alcohols from ß-elimination are unlikely to undergo 1,3-hydrogen transfer to the vinyl alcohols and thus to the ketones, suggesting that ketones are formed through the opening of the oxirane ring after α-substitution. Elimination of LiOH from the lithiated allyl alcohol is found to result in the diene product. Low activation barriers for ß-elimination are offered as the explanation for the few special cases where the allyl alcohol is the dominant or exclusive product. These findings are consistent with the product distributions observed in several experiments.

Carbenoid alkene insertion reactions of oxiranyllithiums.

The first computational investigations of the carbenoid reactions of α-lithiated dimethyl ether (methoxymethyllithium) and the intramolecular and intermolecular reactions of lithiated epoxides with the alkene double bond to yield cyclopropane rings are presented. These reactions represent the full spectrum of known carbenoid pathways to cyclopropanation. The reaction of Li-CH(2)-O-CH(3) with ethylene proceeds exclusively through a two-step carbolithiation pathway, the intramolecular reaction of 1,2-epoxy-5-hexene follows either the carbometalation or a concerted methylene transfer pathway (the former is energetically more favorable), and the reaction of lithiated ethylene oxide (oxiranyllithium) with ethylene, the main focus of this paper, appears to proceed exclusively by the methylene transfer mechanism. In the case of these latter reactions, the free energy of activation for cyclopropanation tends to decrease with the higher aggregation states. Formation of tetramers or higher aggregates is favorable in nonpolar solvents, but in strongly coordinating solvents such as tetrahydrofuran (THF), steric factors appear to limit aggregate sizes to the dimer. In the case of 1,2-epoxy-5-hexene, consideration of competing reaction pathways provide an explanation for the observed product distribution.