Thermodynamic cycle for the calculation of ab initio pKa values for hydroxamic acids
Thermodynamic cycle for the calculation of ab initio pKa values for hydroxamic acids
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DOI：
10.1016/j.theochem.2009.06.021
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发表时间：
2009-09-30
影响因子：
--
通讯作者：
Senthilnithy, Rajendram
中科院分区：
文献类型：
--
作者：
Dissanayake, Dhammike P.;Senthilnithy, Rajendram
本文提出了一种计算异羟肟酸pK（a）值（酸离解常数的负对数）的热力学循环。异羟肟酸主要作为酰胺异构体存在于水性介质中。异羟肟酸的酰胺形式具有两个去质子化位点，并且在去质子化后可以产生N-离子或O-离子。热力学循环包括异羟肟酸的气相N-H去质子化、N-离子向C-离子的溶剂相转化以及异羟肟酸分子和O-离子在水中的溶剂化。采用CBS-QB 3方法计算了12种异羟肟酸与相应阴离子的气相自由能差。在HF/6- 31 + G（d）/CPCM和HF/6-31+G（d）/CPCM水平上，采用HF 16 -31 + G（d）/CPCM几何构型，计算了水溶剂化吉布斯自由能变化.对于质子，使用气相形成自由能和溶剂化自由能变化的文献值。采用CBS-QB_3/CPCM计算了水介质中N ~-离子向O ~-离子转化的自由能变化。为此，根据羰基碳上的取代基将异羟肟酸分为两类。得到了脂肪族取代异羟肟酸和芳香族取代异羟肟酸的共同相变自由能差。pK（a）计算得出的均方根误差为0.32 pK（a）单位。(C)2009爱思唯尔有限公司版权所有。
A thermodynamic cycle to Calculate pK(a) values (Minus log of acid dissociation constants) of hydroxamic acids is presented. Hydroxamic acids exist mainly as amide isomers in the aqueous medium. The amide form of hydroxamic acids has two deprotonation sites and may yield either an N-ion or an O-ion upon deprotonation. The thermodynamic cycle proposed includes the gas-phase N-H deprotonation of the hydroxamic acid, the solvent phase transformation of the N-ion to the C-ion and the solvation of the hydroxamic acid molecule and the O-ion in water. The CBS-QB3 method was employed to obtain gas-phase free energy differences between 12 hydroxamic acids and their respective anions. The aqueous solvation Gibbs free energy changes were calculated at the HF/6-31G(d)/CPCM and HF/6-31+G(d)/CPCM levels of theory using HF16-31+G(d)/CPCM geometries. For the proton, literature values of the gas-phase free energy of formation and the solvation free energy change were used. The free energy change for the transformation of the N-ion to O-ion in the aqueous medium was calculated by employing CBS-QB3/CPCM in the aqueous medium. For this, the hydroxamic acids were divided in two classes according to the substituent at the carbonyl carbon. A common transformation free energy difference for aliphatic substituted hydroxamic acids and a separate common transformation free energy difference for aromatic substituted hydroxamic acids were obtained. The pK(a) calculation yielded a root mean square error of 0.32 pK(a) units. (C) 2009 Elsevier B.V. All rights reserved.