An Introduction to Spectroscopic Methods for the by F. Scheinmann (Eds.)

By F. Scheinmann (Eds.)

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In order to evaluate a given deduced structure it is customary to consider isomers, to postulate the spectra from these (see the example above, under fragmentations of com­ pounds of designated structure), and to suggest how the spectra might differ from the spectrum under consideration. It must be emphasized that some isomers cannot readily be distinguished by mass spectrometry ; other spectroscopic techniques are better suited for this. For example, it is difficult to decide the exact nature of a multibranched alkyl chain or to assign unequivocal orientation to some disubstituted benzenes.

To nitrogen to give m/e 102 and 104 (especially the latter which is also a scission a- to C — O); the ion of m/e 56 would be well stabilized by resonance. ·. Hk il , 1 m , |L ■|. 20 ■■■■■■■■■ ■ . . ■■ ■ . 40 60 ■■| 80 ■■ t "i nI 100 ■Ί■ , 120 y 140 ι , ■ , 160 ι■ ■ 1■ 180 m/e FIG. 23. Spectrum of the ethyl ester (LI) of methionine. (From K. Biemann, Massi Spectrometry : Or­ ganic Chemical Applications. Copyright 1962 by McGraw-Hill Book Company. ) m/e 150 would be probably of low abundance since other scissions would be more favour­ able.

OH becomes the morevolatile ether R—O—Si(CH3)3. The molecular ion peak of the derivative often is visible (silicon isotopes enable easy recognition), and instead of eliminations involving the —OH group the preferred fragmentations of the ether are scission of bonds β- to the ethereal oxygen to give oxonium ions, all of which are relatively of high abundance. Thus the trimethylsilyl ether (XLIII) of 3-hexanol gives a molecular ion, m/e 174, and three prominent fragments of m/e 159, 145, and 131 (each containing the silicon).

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