Effect of Hydrogen Bonds on pKa Values: Importance of Networking

被引:81
作者
Shokri, Alireza [2 ]
Abedin, Azardokht [1 ]
Fattahi, Alireza [1 ]
Kass, Steven R. [2 ]
机构
[1] Sharif Univ Technol, Dept Chem, Tehran, Iran
[2] Univ Minnesota, Dept Chem, Minneapolis, MN 55455 USA
基金
美国国家科学基金会;
关键词
AMINO-ACID MUTAGENESIS; COMPLETE BASIS-SET; GAS-PHASE; DENSITY FUNCTIONALS; ORGANIC-ACIDS; ACTIVE-SITE; EQUILIBRIUM ACIDITIES; PROTEIN-STRUCTURE; CARBON ACIDS; AB-INITIO;
D O I
10.1021/ja3037349
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
The pK(a) of an acyclic aliphatic heptaol ((HOCH2CH2CH(OH)CH2)(3)COH) was measured in DMSO, and its gas-phase acidity is reported as well. This tertiary alcohol was found to be 10(21) times more acidic than tert-butyl alcohol in DMSO and an order of magnitude more acidic than acetic acid (i.e., pK(a) = 11.4 vs 12.3). This can be attributed to a 21.9 kcal mol(-1) stabilization of the charged oxygen center in the conjugate base by three hydrogen bonds and another 6.3 kcal mol(-1) stabilization resulting from an additional three hydrogen bonds between the uncharged primary and secondary hydroxyl groups. Charge delocalization by both the first and second solvation shells may be used to facilitate enzymatic reactions. Acidity constants of a series of polyols were also computed, and the combination of hydrogen-bonding and electron-withdrawing substituents was found to afford acids that are predicted to be extremely acidic in DMSO (i.e., pK(a) < 0). These hydrogen bond enhanced acids represent an attractive class of Bronsted acid catalysts.
引用
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页码:10646 / 10650
页数:5
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