INVESTIGATION OF IONIZATION AND FRAGMENTATION OF CHLOROANISOLE ISOMERS USING ELECTRON IMPACT MASS SPECTROMETRY AND SEMI-EMPIRICAL MOLECULAR ORBITAL CALCULATIONS

Abstract

An electron impact mass spectrometry is used in combination with deconvoluted first differential (DFD) technique in order to determine ionization energy values at threshold as well as higher energy states for the molecular ions [C7H70Cit" of chloroanisole isomers. Appearance energy values at threshold only are also measured for [C6H40Cl]+, [C sCl]\ [C7H10t, [Cs Clr and [C3H2Clt fragment ions produced from the three precursor molecular ions. The relative intensities of the prominent ions in the mass spectra of the three isomers are reported at 70 and 15 eV (nominal) for ionizing electron energy and are discussed in terms of the relative position of the two substituents (-Cl and OCH3) on the benzene ring. Also, the metastable peaks associated with formation and fragmentation of the prominent

fragment ions are recorded and the kinetic energy release (To.s ) values associated with these processes are determined. The values of ionization energy at threshold are measured at 8.46 ±0.06,

8.82±0.05 and 8.37+0.05 eV for o-, m- and p-chloroanisole isomers, respectively, and are explained as due to ionization of 1e 1 g electrons from 1t3 (B2 component) orbitals ofthe corresponding chloroanisole isomers. Higher energy levels have been detected in the ionization efficiency curves of the

molecular ions of the three isomers and some of these levels are tentatively explained by the author. Appearance energy values at threshold for [C6 0Clt fragment ions are measured at 11.85+0.09, 11.90+0.12 and 11.79±0.09 eV, for [C6H5Clt fragment ions at 11.37+0.06, 11.63±0.07 and

11.63+0.07 eV, for [C7H70t fragment ions at 10.44+0.05, 10.32±0.05 and 10.57±0.06 eV, for [C5IL;Cit fragment ions at 13.88±0.09, 13.33+0.08 and

13.67±0.11 and for [C3H2Cit fragment ions at 17.72±0.10, 17.53±0.08 and • 17.84+0.08 eV for ions produced from o-, m- and p-chloroanisole isomers, respectively. It is worthwhile to mention that appearance energy values for

nine out of fifteen studied fragment ions are reported for the first time.

The heat of formation, charge distribution and bond orders of the molecular chloroanisole isomers are calculated using the semiempirical molecular orbital calculation MNDO method. The MNDO method is used also to suggest the structural geometry of the molecular ions of chloro­ anisole as well as some of the studied fragment ions. The possible fragmentation processes for the formation of the

studied fragment ions have been suggested and thermodynamical threshold values ( Eth) for these processes (except for [C3H2Clt fragment ions) are calculated. The present results may indicate that [C6H40Clt ions are formed from the three precursors at threshold having a chlorophenoxide structure, [C6H5Clt ions are formed from the three precursors at threshold having a chlorobenzene structure, [C7H70t ions are formed from the three precursors at threshold having a phenoxymethylene structure while

[C5H4Clt ions are formed with one or mixture of linear structures. However the present results obtained for [C3H2Clt ions do not allow the suggestion of any structure(s) for the ions formed from the three precursors with reasonable certainty. Finally, the kinetic energy release (T0.5) values associated with the fragmentation of metastable [C7H70t suggest that the decomposing [C7H70t ions from o-, m- and p-chloroanisoles, have the same structure or mixture of structures at 70 eV, while the decomposing [C3H2Clt ions produced from o- and m-chloroanisoles have the same structure that is different from that obtained from p-chloroanisole at 70 eV.