Gas-Phase reactions of NO 2− with nitrobenzenes and quinones. Electron transfer, clusters and formation of phenoxide and quinoxide negative ions. Use of NO 2 as a negative ion chemical ionization reagent

Abstract

Negative-ion/molecule reactions in systems containing methane as the major gas (∼ 4 torr), with NO 2 and compounds A at mtorr pressures were studied in a pulsed electron, high pressure mass spectrometer. The compounds A were substituted nitrobenzenes and quinones. All these A compounds have positive electron affinities. Three types of reactions were observed and examined. (1) Electron transfer: A − + NO 2 = A + NO 2 −. The exothermic electron transfer reactions proceeded with ADO collision rates for exothermicities from 30 to ∼ 10 kcal mol −1. Lower exothermicities led to low collision yields. (2) Adduct formation: NO 2 − + A = NO 2 −·A. The equilibria for adduct formation were determined. Stable adducts are formed when A has hydrogens with partial protic character. The stability of the adducts NO 2 −·A increased with increase in the electron affinity of A, when A was a substituted nitrobenzene. Substituents that increase the electron affinity of nitrobenzene are electron-withdrawing groups which also increase the protic character of the hydrogens involved in bonding in NO 2 −·A. (3) Some of the compounds A were converted to phenoxy negative ions on reaction with NO 2 −. For example, para-dinitrobenzene leads to formation of the para-nitrophenoxide negative ion. The oxy-negative-ion-forming reaction can be isomer specific. The utility of reaction types (1)–(3) is examined from the standpoint of negative ion chemical ionization where the reagent gas is NO 2 in methane and the reagent ion is NO 2 −.