Discussion of the proton potential with proton-transfer equilibria: thermodynamic data and infrared continua as a function of temperature
作者:Rainer Krämer、Georg Zundel、Bogumił Brzezinski、Jerzy Olejnik
DOI:10.1039/ft9928801659
日期:——
The infrared continua caused with intramolecular hydrogen bonds of the type AH⋯B ⇌ A–⋯H+B have been studied as a function of the ΔpKa of the hydrogen-bond donor and acceptor as well as a function of the temperature. From the behaviour of these continua it is shown that the shape of the proton potential may change with decreasing temperature from a double minimum with the deeper well at the donor to proton potential with the deeper well at the acceptor. The In (KPT) over 1/T plots result, however, always in negative apparent ‘ΔH° values’. Hence with all such proton-transfer hydrogen bonds the deeper well of the double-minimum proton potentials should always be at the acceptor group.Using the reaction-field theory, it is shown that for proton-transfer equilibria the slope of these In KPT vs. 1/T plots remains linear since ∂P/∂T, i.e. the derivative of the Onsager parameter P against the temperature T is constant if it is considered with not too large a temperature range, and since ΔH° changes with temperature by a term which is linear in T. Therefore, from the In (KPT)vs. 1/T plots apparent ‘ΔH° values’ are obtained from which the true values can be calculated if ∂P/∂T is known.
我们研究了分子内氢键 AH⋯B ⇌ A-⋯H+B 类型引起的红外连续波,它是氢键供体和受体的 ΔpKa 的函数,也是温度的函数。这些连续曲线的行为表明,质子势的形状可能会随着温度的降低而改变,从供体处较深井的双最小值变为受体处较深井的质子势。然而,In (KPT) over 1/T 图的结果总是负的表观 "ΔH° 值"。因此,在所有这类质子转移氢键中,双最小质子势的深井应始终位于受体基团。利用反应场理论可以证明,对于质子转移平衡,由于 ∂P/∂T,即 Onsager 参数的导数,In KPT vs. 1/T 图的斜率保持线性。因此,从 In (KPT)vs. 1/T 图中可以得到明显的 "ΔH° 值",如果知道 ∂P/∂T,就可以从中计算出真实值。