HCN loss

Note The above mechanism of CO loss from phenols is perfectly analogous to HCN loss from aniline and other aminoarenes (Chap. 6.14.2). 

Fig. 6.53. El mass spectrum of pyridine. HCN loss represents the most important primary fragmentation of the molecular ion. Spectrum used by permission of NIST. NIST 2002.

In case of pyrrole molecular ion, HCN loss is somewhat less important than with indole, i.e., the [M-C2H2] ion at m/z 41 is more abundant than the [M-HCN] ion at m/z 40. This may in part be due to the twofold possibility to eliminate C2H2 (Fig. 6.55). The introduction of iV-substituents has similar effects as observed for the saturated heterocycles before, i.e., rearrangement fragmentations and a-cleavage of the substituent take control [230]... 

The mass spectrum of aniline has been known since the early days of mass spectrometry. [122] Initially, the observed [M-27] ion has been interpreted in terms of HCN loss (Fig. 6.56a). The mechanism for loss of the elements of [H, N, C] from aminoarenes is perfectly analogous to CO loss from phenols (Chap. 6.9.1). [231] More recently, it could be demonstrated that loss of hydrogen isocyanide, HNC, occurs rather than losing the more stable neutral species HCN, a behavior typical of ionized pyridine. [222]... 

Interestingly, the three isomeric aminopyridine molecular ions display ion chemistry similar to aniline molecular ions, i.e., metastable HNC loss, [222] instead of HCN loss which should also be possible due to the pyridine core of the molecule (Fig. 6.56b). 

Pyridinium dicyanomethides 372 react with benzyne (from anthranilic acid) to give, after HCN loss, cyanobenzoindolizines 373 in 10-35% yieldSimilarly, isoquinolinium dicyanomethide 375 gave 376 in 74% yield. The products (i.e. 373) are also 1,3-dipoles and can react with a second equivalent of benzyne (again, with loss of HCN) to give benzo[a]isoindolo[l,2,3-c /]indolizines 374, which are IStt annulenes. ... 

The HCN ice spectrum illustrates several aspects of HREELS. The strong dynamic dipole moment due to the libration mode causes multiple scattering from the molecules, so that there are several replica primary peaks almost as intense as the elastic peak. Consequently, each of the usual HCN loss peaks generates a spectrum with the same spectral intensity relationships as the no-loss peak. The multiple... 

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