Secondary kinetic deuterium isotope effects. The CC cleavage of labeled tetramethylethylenediamine radical cations—Who gets to keep the electron?

Abstract

The simple cleavage of the CC bond in tetramethyl ethylenediamine molecular ions yields two fragments that are identical but for the positions of the charge and radical; nonetheless, the reactions of deuterium labeled analogs are accompanied by substantial secondary kinetic isotope effects. The underlying transition state zero-point vibrational energy differences depend particularly on the properties of the incipient radicals rather than on those of the charge-retaining products. The α-secondary effects arise primarily from vibrations related to deformation of the product-CH 2 N-and-CD 2 N-groups when the reactant is labeled at the central CC bond, whereas CH/CD stretching vibrations are the origin of the γ-secondary effects observed for reactants with labeled methyl groups. These zero-point energy differences are mainly due to hyperconjugative interactions (Bohlmann shifts); the secondary isotope effects on the dissociation of protonated amine dimers have a similar origin. # Dedicated to the memory of Nico M. M. Nibbering, for many years an important figure in European mass spectrometry research.

Secondary kinetic deuterium isotope effects on unimolecular cleavage reactions: Zero‐point vibrational energy and qualitative RRKM theory

“…CC-Cleavage gives rise to two identical fragments that only differ with regard to the position of charge and unpaired electron. The preferred location turns out to depend on deuterium substitution, resulting in remarkable isotope effects, in spite of the similarity of the products of reaction (Nielsen & Hammerum, 2017). Deuterium labeling at the central carbon atoms (Scheme 10a) gives rise to an α-secondary isotope effect as described above for simple amines, confirming that the zpve-consequences of the sp 3 → sp 2 conversion are more pronounced for the aminoalkyl radical than for the immonium ion.…”

A Critical Review of the Modelling Tools for the Reactive Transport of Organic Contaminants

“…While both processes can impact the isotopic fingerprint of hydrocarbons, kinetic fractionation tends to have a more pronounced effect on residues, resulting in an enrichment of substrates with heavier isotopes and products with lighter isotopes (known as kinetic isotope effect-KIE) [277]. The most significant isotope effects are associated with substitutions that influence reaction rates [278,279]. Determining KIE has become a standard measurement, with a wealth of KIE data available in the literature [280].…”

Energy‐dependent normal and unusually large inverse chlorine kinetic isotope effects of simple chlorohydrocarbons in collision‐induced dissociation by gas chromatography‐electron ionization‐tandem mass spectrometry

“…By means of MS, KIEs can be readily determined and applied to probing reaction mechanisms of molecules and ions in gaseous phase [8][9][10]. So far, most studies relevant to on-MS KIEs focused on hydrogen/deuterium (H/D) KIEs [5,[11][12][13], whereas a few concerned heavy-atom KIEs such as chlorine KIEs (Cl-KIEs) [14][15][16].…”