Structural and energetics effects in the chemical ionization of halogen-substituted benzenes and toluenes

Abstract

The H2, D2, CH4, and CD4 chemical ionization (CI) mass spectra of XC6H5, C6H5CH2X, and XC6H4CH3 (X = Cl, Br, and I) have been determined. For the benzyl halides the dominant ion observed in all CI spectra is the m/q 91 (benzyl) ion formed by loss of HX(DX) from the protonated (deuteronated) molecule. The CI spectra of the isomeric halotoluenes differ substantially from the spectra of the benzyl halides indicating that isomerization to a common (tropylium-like) structure does not occur in CI. In the H2 CI of XC6H4Y (Y = H or CH3) the major fragment ions observed are YC6H5•H+, [Formula: see text], and YC6H4+, with the relative yields depending strongly on the halogen substituent. Neither the YC6H5•H+ ion, resulting from HX loss from the H3+•XC6H4Y complex, nor the YC6H4+ ion, resulting from HX loss from XC6H4Y•H+, are observed when X = I. On the other hand [Formula: see text], resulting from elimination of X• from XC6H4Y•H+, is observed only for X = Br and I. In the CH4 CI of XC6H4Y, in addition to XC6H4Y•H+ ions, CH3C6H4Y•H+ ions (HX loss from the CH5+•XC6H4Y complex) are observed for X = F and Cl, while [Formula: see text] ions (loss of X• from XC6H4Y•H+) are observed only when X = I. In addition, displacement of I• by C2H5+, and by C3H5+, is observed in the CH4 CI of the iodo-substituted compounds. The dependency of the products observed and their relative yields on the identity of the halogen substituent is discussed in relation to the reaction energetics.