Mass spectrometric study of negative ions from acetyl derivatives

A mass spectrometic technique capable of producing a low-energy and nearly monochromatic (fwhm ~ 0.2 eV) electron beam is developed and used to study the negative ions from five different acetyl derivatives CH3COX (X = Cl, C6H5, NH2, CH3, and H) at very low energies (0-10 eV). Negative ions are mainly produced by dissociative attachment and show resonance-like structures. To measure the excess energy imparted to the ions during ionization, a time-of-flight technique is applied. An ion with composition C2H30" is the common ion, and its appearance potential is different from different molecules. With use of the available data on bond energies and the excess energy equated to zero, electron affinity of the corresponding radical, C2H3CO, can be calculated; this varies between 2.3 and 2.7 eV for these five molecules. This value is rather high compared with the known electron affinity of CH3CO, 0.42 eV, measured by the technique of flowing afterglow, and is much closer to 1.817 eV, the reported electron affinity of its structural isomer, the enolate radical, CH2=CH—O. Therefore, isomerization during or after dissociation of the parent ion is suggested as a possibility