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 ³ → sp ² 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.

中文翻译：

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二次动力学氘同位素对单分子裂解反应的影响：零点振动能和定性 RRKM 理论

过渡态零点振动能量差异会导致二次动力学同位素效应。质谱仪中气相离子的单分子简单裂解反应允许详细研究同位素对竞争反应的影响，特别是在分子内竞争实验中检查时，解释需要很少的简化假设。零点能量差异反映了同位素敏感振动特性的变化，α-和β-二次氘同位素效应均与sp ³  →  sp ^{2 有关}伴随键断裂的杂交变化。从断裂的键中去除三个或更多键的氘取代也会产生同位素效应，但它们的起源不太容易解释。观察到的效应的大小和变化不仅取决于零点能量差异；许多其他因素也起作用。临界能量、过剩能量、反应物的大小和竞争反应的存在的影响可以在一个简单的定性 RRKM 框架内合理化。动力学和热力学同位素效应之间的区别并不总是很明显。