Substituent effects in the mass spectra of some γ- and β-substituted methyl butyrates

Abstract

Results are presented (relative ion abundances, I.P. and A.P. measurements) to illustrate substituent effects in the mass spectra of some y-and P-substituted methyl butyrates. It is argued that (i) ' sub-decomposition energy ' ions and (ii) competition in the source with fast fragmentation processes are important factors influencing parent/ daughter ion ratios, which are then rationalised in terms of the variation of these two factors with substituent. In certain cases a substituent effect on the energy of activation of the transition state is also invoked. The variation in parent/daughter ratios at different energies is explained in terms of relative A.P.'s and frequency factors. Mechanisms are discussed for the elimination of MeOH from M+ and for the McLafferty rearrangement. Albany, Albany, N.Y. 12203 BURSEY and McLafferty pioneered the study of substituent effects in mass spectrometry and their early work has been summarised.l Recent considerations 2-5of the basic factors involved in the formation, fragmentation, and collection of organic ions at low pressures in a mass spectrometer has led to recognition of complications in the interpretation of decomposition rates in terms of daughter/parent ion ratios. This has resulted in an enumeration of the variables which influence substituent effects, measured in terms of this parameter, in mass spectrometry. The situation is summarised in a recent review .'j

Absolute cross-sections for dissociative photoionization of some small esters

“…The m/z = 74 fragment ion is the dominant contributor to the total photoionization above 10.4 eV, augmented by the m/z = 71 C 4 H 7 O + and m/z = 87 C 4 H 7 O 2 + fragment ions with respective appearance energies of 10.9 ± 0.1 and 10.85 ± 0.1 eV. Howe et al [21] reported an appearance energy of 11.2 ± 0.2 eV for the m/z = 71 C 4 H 7 O + fragment ion, corresponding to direct loss of the CH 3 O radical. The CH 3 radical loss channel associated with formation of the m/z = 87 C 4 H 7 O 2 + fragment ion seen here has not been previously reported.…”

“…The m/z = 74 C 3 H 6 O 2 + and m/z = 71 C 4 H 7 O + fragment ions are prominent with respective appearance energies of 10.18 eV [52] and 11.2 ± 0.2 eV [21]. The structure of the m/z = 74 fragment ion was identified by Holmes and Lossing [52] to be the methyl acetate enol ion CH 2 COHOCH 3 + formed with the loss of C 2 H 4 by direct bond cleavage of the low-energy distonic CH 2 CH 2 CH 2 COHOCH 3 + ion, formed by prior isomerization of the methyl butanoate parent ion.…”

“…These studies have focused on the measurement of appearance energies for photofragment ions, the structural identities of these fragments and the dynamic mechanisms responsible for their formation. Dissociative ionization channels often arise from complex isomerization processes in competition with direct bond scission of the parent molecular ion [16][17][18][19][20][21][22][23][24][25]. Despite considerable interest in these intrinsic features of dissociative photoionization of esters, absolute photoionization cross-sections have been reported for only methyl and ethyl acetate [26].…”

“…Dissociative ionization of several small esters has been extensively studied by several groups, using a full range of experimental techniques [16][17][18][19][20][21][22][23][24][25]. These studies have focused on the measurement of appearance energies for photofragment ions, the structural identities of these fragments and the dynamic mechanisms responsible for their formation.…”

Photoionization of Two Potential Biofuel Additives: γ-Valerolactone and Methyl Butyrate

“…The production of esters as biofuel surrogates from vegetable oils and fatty acids through transesterification reactions has been studied extensively. − , MB has also shown promise of being generated from nonfood crops, − thus allowing it to qualify as a second-generation biofuel. The photoionization of MB has been studied by photoelectron spectroscopy and photoionization mass spectrometry. − Mazyar et al have analyzed the ethene loss dissociative photoionization dynamics after parent ion isomerization by PEPICO and provided a 0 K appearance energy, E 0 = 10.275 ± 0.010 eV, to the enolic form of the methyl acetate cation at m / z 74, along with the associated microcanonical rate curve. In a more recent photoionization mass spectrometry (PIMS) study to determine absolute photoionization cross sections, Wang et al have also proposed appearance energies of 10.90 ± 0.10 and 10.85 ± 0.10 eV for the m / z 71 and 87 daughter ions, respectively, along with an appearance energy of 10.15 ± 0.10 eV for the dominant m / z 74 fragment ion …”