Ion-molecule reactions and proton affinities of methyl nitrite and nitromethane

Abstract At pressures of about 10 −3 Pa in an ion cyclotron resonance mass spectrometer equipped with a four-section cell, secondary ions of m/e = 45 and 76 are not formed in methyl nitrite, in contrast to their prominence in the isomer nitromethane. Secondary ions of m/e = 62 and 91 are formed in both isomers, but in methyl nitrite the parent ion of m/e = 61 reacts to give the secondary of m/e = 92 but not m/e = 62, whereas the converse holds in nitromethane. These observations may be explained by postulating that there is an exclusive structure for ions of m/e = 62 and 76 in each isomer: addition occurs on the central oxygen in methyl nitrite and on the terminal oxygens in nitromethane; in methyl nitrite, the parent ion is kinetically prohibited from attaining this structure in an ion-molecule collision. The primary ion of m/e = 60 reacts to give the secondary of m/e = 91 in both isomers: this is accepted as evidence that the structure of the ion of m/e = 60 is the same for both isomers. The proton affinity of methyl nitrite was found to be 787±9 kJ mol −1 , giving Δ H f (CH 3 ONOH + ) = 676 kJ mol −1 .