Ion—Molecule Reactions Involving Methyl Isocyanide and Methyl Cyanide

Abstract

Ion-molecule reactions involving methyl isocyanide and methyl cyanide have been performed in a new rf-only hexapole collision cell inserted in a large-scale tandem mass spectrometer. Beside protonation processes, N-methyl cyanogen ions (CH(3)N(+)CCN) and 1-methyleneiminium-1- ethylenium ions (CH(2)CN(+)CH(2)) have been produced in high yield and fully characterized by high-energy collisional activation. The unimolecular chemistry of the molecular ions of caffeine (1,3,7-trimethyl xanthine) has been revisited on the basis of these new results.

Internal Energy Effects on the Ion/Molecule Reactions of Ionized Methyl Isocyanide

“…It was therefore initially supposed that the m/z 81 ions result from an interaction between neutral MIc and the [M -H] + ions of MIc, a similar reaction being reported for methyl cyanide. 2 However, in an experiment using a beam of decelerated m/z 40 ions and neutral MIc in the hexapole reaction cell, we have not found any convincing evidence for such a process, the major reaction observed being a proton transfer to MIc (m/z 42).…”

“…1 the same phenomenon was observed for the interaction between both ionized and neutral MIc affording new cumulenic ions. 2 Isonitriles (rN=c:) and also carbon monoxide (o=c:) formally belong to the general family of vinylidenes (r 2 c=c:). However, these particular compounds are stabilized by delocalisation of non-bonding electrons of nitrogen and oxygen resulting in a drastic shortening of the Nc and oc bonds.…”

“…Electron ionization of MIc in a gas-tight ion source (self-chemical ionization conditions) has been studied recently in this laboratory. 2 the resulting cI mass spectrum shown in figure 1(a) indicates that the dominant products are detected at m/z 42 and 67. these ions, described respectively as C-protonated MIc (cH 3 N + ≡c-H) and N-methylated cyanogen (cH 3 N + ≡c-c≡N) are the result of largely exothermic ion/molecule reactions (229 mol -1 and 214 kJ mol -1 , respectively). It was proposed that the m/z 67 product ion was generated via a cumulenic dimer, cH 3 N=c=c=NcH 3…”

“…Due to the large dipole moment of methyl isocyanide, complexation with the radical cation is largely exothermic by 379 kJ mol -1 , a value based on a calculated heat of formation of 1051 kJ mol -1 for the m/z 82 ions in the E configuration. 2 A consecutive loss of a hydrogen atom appears to be allowed as the energy level for the fragment ion m/z 81 (plus H • ), 1314 kJ mol -1 , lies between the energy levels for the isolated species and the cumulenic radical cation (Scheme 2).…”

Aromatic Substitution Reactions between Ionized Benzene Derivatives and Neutral Methyl Isocyanide

“…Beside these thorough but limited studies, a need for the finding of other examples of nucleophilic aromatic substitution reactions is consequently evident in order to have a more general picture of this type of reaction and to provide new insight into its mechanism. In this context, we have recently shown that, in the gas phase, methyl isocyanide, CH 3 NC, is an efficient nucleophilic reagent toward cationic substrates. − This is the result of its high ionization energy (11.24 eV) and its relatively low proton affinity (839 kJ mol −1 ), which prevent charge and proton exchange processes, respectively. Moreover, the high dipole moment of CH 3 NC (3.85 D) favors its association with charged species and consequently increases its reaction efficiency.…”