As a part of our program on the mass spectral fragmentation of organic molecules, we examined the spectrum of ethyl p-aminobenzoate with the aid of high-resolution mass spectrometry. The conventional spectrum of this compound is shown in Fig. 1, in which six prominent peaks are observed. Correlation of these fragment ions is substantiated by the appearance of appropriate metastable ion peaks(Fig. 2). i)m/e 165 Ion(Molecular Ion, C₉H₁₁NO₂): The general discussion of the electron impact fragmentation process is briefly summarized. A schematic representation as to the localization of the positive charge in the molecular ion is put forward(Fig. 3). ii)m/e 137 Ion: This peak corresponds to M-28 and is accounted for by elimination of any one of three possible parts of the molecule(Fig. 4). Because methyl p-aminobenzoate does not give any significant amount of M-28peak(Fig. 5), the loss of C₂H₄from the ester alkyl group of the molecule is presumably responsible for this m/e 137 ion. The high-resolution spectrum of the peak shows only a single ion of the composition C₇H, NO₂, which is M-C₂H₄, as expected. The mechanism of the process is probably one of the McLafferty-type rearrangement involving migration of one hydrogen atom to the ester carbonyl group(Eq. 1). iii)m/e 120 Ion: The high-resolution spectrum(Fig. 6)indicates that this peak has two components, one of which being predominant. By using the molecular ion of n-propylbenzene as an internal standard (Fig. 7), the principal peak is shown to be 47m. m. u. lower than that of the hydrocarbon standard. This result establishes the composition of the ion as C₇H₆NO(Table I). The mechanism of the fragmentation is formulated as in Eqs. 2and3. iv)m/e 92 Ion: The peak has two components(Fig. 8), the mass difference of which is calculated to be23-24m. m. u. This suggests that the main fragment ion is C₆H₆N(Table II). The same result was obtained by the peak-matching technique with toluene as an internal standard in this case. It follows that the fragmentation pathway is expulsion of. CO from the m/e 120 ion (Eq. 4). v)m/e 65, 39, and 28 Ions: The peaks at m/e65and39are common in most aromatic compounds and their fragmentations have been well studied(Eq. 5). However, it is of interest to note that the m/e65ion is also formed by the simultaneous loss of CO and HCN from two different parts of the m/e 120 ion(metastable ion peak at35. 2). The CH₂N fragment is not liberated in any step of the above fragmentation pathways. This does not necessarily mean that no fragmentation takes place around the nitrogen moiety of the molecule. In fact, the high-resolution spectrum of m/e 28 peak shows relatively large fraction of the CH₂N ion(Fig. 9). This is explicable by assuming that, when the cleavage occurs at this part of the molecule, the positive charge preferentially remains on the nitrogen-containing fragment rather than on the aromatic ring. The fragmentations of ethyl p-aminobenzoate are thus established rigorously by the use of high-resolution spectra, and the results are summarized in Fig. 10.

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Mass spectrum is closely related with the structure of a compound. But it is not easy to determine the structure from its mass spectrum, because the behavior of molecule under electron bombardment is. rather complicated and in many cases there are several compounds which show the similar mass spectrum. It can be said that when the energy of the bombarding electrons changed, the dissociation process of a given molecule may be also change. We investigated the correlations between the structure of a compound and its mass spectrum at various energy bombardments, by using a CEC 21-103C mass spectrometer. The optimum operating condition and the reproducibility of mass spectrum at low energy bombardment are also described in this report. It was found that mass spectrum at low energy bombardment was tolerably reproducible and was closely related with the structure of the compound. The formation of the large majority of ions under low energy(2-4 eV higher than its ionization potential)bombardment is through the simple cleavage of a weak bond and the rearrangement. In some cases the rearrangement of parent ion gives the largest peak, such as the peak at m/e 33 in the spectrum of iso-butyl alcohol, at low energy bombardment. It is believed that very useful informations for correct interpretation of mass spectrum can be obtained by comparing the spectrum at low energy boambardment with that at ordinery measurement and the ionization efficiency curves are also useful.

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The mass spectral fragmentation patterns of hasubanan derivatives are summarized in terms of the most characteristic mass spectral peaks. Plausible mechabisms can be postulated in most instances for these ions and attention is called to the use of this information in the structure elucidation of alkaloid with hasubanan skeleton(those of not possesing ether bridge between C₄and C₅).

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