negative hyper-conjugation in mass spectrometry

https://dx.doi.org/10.1351/goldbook.NT07081
The effect of donation of electron density from filled π or nπ-orbitals to the symmetry adapted σ*-orbital(s) of σ-bond(s) of a molecular entity resulting in building π-character into bonds that nominally possess only σ-character. The consequences of the effect are, for example, in elongation of the C–F bond in the β-fluoroethyl anion and stabilization of the anti-periplanarconformation of fluoromethylamine.
NT07081
Source:
PAC, 1999, 71, 1919 (Glossary of terms used in theoretical organic chemistry) on page 1954