Østergaard Lars F, Hammerum Steen
Department of Chemistry, University of Copenhagen, Universitetsparken 5, Copenhagen Ø, DK-2100, Denmark.
Mass Spectrom Rev. 2021 Nov;40(6):821-839. doi: 10.1002/mas.21660. Epub 2021 Jan 1.
Secondary kinetic isotope effects arise as the result of transition-state zero-point vibrational energy differences. Unimolecular simple cleavage reactions of gas-phase ions in mass spectrometers allow detailed studies of isotope effects on competing reactions, particularly when examined in intramolecular competition experiments where interpretation requires very few simplifying assumptions. The zero-point energy differences reflect changes of isotope sensitive vibrational properties, and both α- and β-secondary deuterium isotope effects are related to the sp → sp hybridization changes that accompany bond cleavage. Deuterium substitution three bonds or more removed from the bond broken also gives rise to isotope effects, but their origin is less easily interpreted. The magnitude and variation of the observed effects depend not only on zero-point energy differences; a number of additional factors play a role. The influence of the critical energy, the excess energy, the size of the reactant, and the presence of competing reactions can be rationalized within a simple, qualitative RRKM framework. The distinction between kinetic and thermodynamic isotope effects is not always obvious.
二级动力学同位素效应是过渡态零点振动能量差异的结果。质谱仪中气相离子的单分子简单裂解反应允许对同位素效应在竞争反应中的情况进行详细研究，特别是在分子内竞争实验中进行考察时，此时解释所需的简化假设极少。零点能量差异反映了同位素敏感振动性质的变化，并且α-和β-二级氘同位素效应都与键断裂时伴随的sp→sp杂化变化有关。在距断裂键三个或更多键处进行氘取代也会产生同位素效应，但其起源较难解释。观察到的效应的大小和变化不仅取决于零点能量差异；还有许多其他因素也起作用。临界能量、过剩能量、反应物大小以及竞争反应的存在等影响可以在一个简单的定性RRKM框架内得到合理说明。动力学同位素效应和热力学同位素效应之间的区别并不总是很明显。
