aci-nitromethane -generated and characterized by neutralization reionization mass spectrometry

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.

Benzidine photodegradation: a mass spectrometry and UV spectroscopy combined study

The benzidine photodegradation process was studied using UV/Vis spectroscopy and electrospray ionization mass spectrometry (ESI-MS) combined with collision-induced dissociation (CID) and tandem mass spectrometry (MS/MS). Mass spectrometry was used to characterize benzidine and to identify and characterize possible degradation products and intermediates. The MS data showed that benzidine is quite persistent in aqueous medium. Moreover, the MS analysis enabled us to propose the following three degradation products/intermediates: 4'-nitro-4-biphenylamine, tetrahydroxybiphenyl and 4,4'-dinitrobiphenyl. For the benzidine molecular ion and protonated molecule and for the protonated molecules of the degradation products/intermediates detected, fragmentation patterns are proposed based on CID and MS/MS data. For the photodegradation process different catalysts were used, namely the commercial TiO2 Degussa P25, and the laboratory-prepared ZnO, TiO2 anatase and a titanium-zinc oxide with a perovskite type structure. Comparison of the different catalysts showed that degradation was favoured with the commercial TiO2. Nevertheless, the other catalysts appear to be promising and economic alternatives for potential future remediation studies.

Protonation sites in methyl nitrate and the formation of transient CH4NO3 radicals. A neutralization-reionization mass spectrometric and computational study

Protonation sites in methyl nitrate (1) were evaluated computationally at the Gaussian 2(MP2) level of ab initio theory. The methoxy oxygen was the most basic site that had a calculated proton affinity of PA = 728-738 kJ mol-1 depending on the optimization method used to calculate the equilibrium geometry of the CH3O(H)-NO2+ ion (2+). Protonation at the terminal oxygen atoms in methyl nitrate was less exothermic; the calculated proton affinities were 725, 722, and 712 kJ mol-1 for the formation of the syn-syn, anti-syn, and syn-anti ion rotamers 3a+, 3b+, and 3c+, respectively. Ion 2+ was prepared by an ion-molecule reaction of NO2+ with methanol and used to generate the transient CH3O(H)-NO2. radical (2) by femtosecond collisional electron transfer. Exothermic protonation of 1 produced a mixture of 3a(+)-3c+ with 2+ that was used to generate transient radicals 3a-3c. Radical 2 was found to be unbound and dissociated without barrier to methanol and NO2. Radicals 3a-3c were calculated to be weakly bound. When formed by vertical neutralization, 3a-3c dissociated completely on the 4.2 microseconds time scale of the experiment. The main dissociations of 3a-3c were formations of CH3O. + HONO and CH3ONO + OH.. The gas-phase chemistry of radicals 3a-3c and their dissociation products, as studied by neutralization-reionization mass spectrometry, was dominated by Franck-Condon effects on collisional neutralization and reionization. The adiabatic ionization energies of 3a-3c were calculated as 7.54, 7.57, and 7.66 eV, respectively.

Studies of unusual simple molecules by neutralization-reionization mass spectrometry

Reactive or unstable molecules are key intermediates in many important reactions, but can be difficult to prepare for experimental studies. Species with missing (:CH-OH) or extra (H3) substituents can often be formed conveniently in the gas phase by neutralizing a beam of a more stable ionic counterpart (CH = O+H, H3+). Reionization of the neutral after approximately 10(-6) seconds tests its stability, whereas its unimolecular chemistry can be probed by preparing it with different amounts of internal energy. The resulting neutral products are reionized and mass analyzed. Isomers are then characterized by ion dissociation and a third mass-analysis step. Many unusual molecules have been characterized with this technique, which can also be used to probe complex unimolecular chemistry, such as that of cyclobutadiene and ethylene oxide.