Department of Molecular Chemistry and Materials Science, Weizmann Institute of Science, 7610001, Rehovot, Israel.
Chemphyschem. 2022 Nov 18;23(22):e202200396. doi: 10.1002/cphc.202200396. Epub 2022 Aug 18.
Phosphinylidenes are an important class of organophosphorus compounds that can exhibit tautomerization between tricoordinated P(III) hydroxide (R R POH) and a pentacoordinated P(V) oxide (R R P(O)H) form. Herein we show, using the canonical variational transition state theory combined with multidimensional small-curvature tunneling approximation, the dominance of proton tunneling in the two-water-bridged tautomerizations of phosphinous acid and model phosphinylidenes comprising phosphosphinates, H-phosphonates, H-phosphinates and secondary phosphine oxides. Based on the studied system, the contribution of thermally-activated tunneling is predicted to speed up the semiclassical reaction rate by ca. threefold to as large as two orders of magnitude at 298.15 K in the gas phase. The large KIE and the concavity in the Arrhenius plots are further fingerprints of tunneling. The simulations also predicted that the rapid tunneling rate and short half-life span for the forward reaction, as opposed to the reverse reaction in fluorinated secondary phosphine oxides, would result in P(V) being elusive and only P(III) being isolable, which agrees with previous experiments where only P(III) was detected by IR and NMR spectroscopy. We also explored the role of solvent and predicted tunneling to be substantial.
膦烯是一类重要的有机磷化合物，可在三配位的 P(III)羟基（RR POH）和五配位的 P(V)氧化物（RR P(O)H）形式之间发生互变异构。本文采用正则变分过渡态理论结合多维小曲率隧道近似，证明了质子隧道在膦酸和包含膦酸酯、H-膦酸酯、H-次膦酸酯和二级氧化膦的模型磷烯的两种双水分子桥键互变异构中的主导作用。基于所研究的体系，预测热激活隧道的贡献可以在气相中 298.15 K 时将半经典反应速率提高约三倍，最高可达两个数量级。大的 KIE 和 Arrhenius 图的凹度进一步表明了隧道的存在。模拟还预测，快速的隧道速率和前向反应的短半衰期，与氟化二级氧化膦中的反向反应相反，将导致 P(V)难以捉摸，只有 P(III)可分离，这与以前的实验结果一致，IR 和 NMR 光谱仅检测到 P(III)。我们还探讨了溶剂的作用，并预测隧道效应非常显著。
