EFFECTS OF MOLECULAR CHARGE AND METHYL SUBSTITUTION ON PROTON-TRANSFER BETWEEN OXYGEN-ATOMS
EFFECTS OF MOLECULAR CHARGE AND METHYL SUBSTITUTION ON PROTON-TRANSFER BETWEEN OXYGEN-ATOMS
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DOI：
10.1021/ja00333a027
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发表时间：
1984-01-01
影响因子：
15
通讯作者：
SCHEINER, S
中科院分区：
文献类型：
--
作者：
HILLENBRAND, EA;SCHEINER, S
采用4-31G基组，通过从头算分子轨道计算，比较了(HO-H-OH)-、(H2O-H-OH2)+和(CH3OH-H-HOCH3)+中的质子转移。在所有3个体系中，氧间氢键长度的延长导致质子转移的能量势垒的增加。在研究的键长范围内，高达2.95 . ang。（H2O-H-OH2）+和(HO-H-OH)-的势垒高度相当，而(CH3OH-H-HOCH3)+的势垒略高。这些观测结果是根据质子化子系统的平衡氢氧键长度和电子密度云的空间范围来解释的。氢键的角度变形通常会导致转移势垒的增大。这两种阳离子体系的放大在性质上是相似的，而阴离子体系的放大则完全不同。静电相互作用的考虑可以直接解释两种系统之间的差异。检查伴随质子转移的电子密度重排，可以深入了解整体变化和甲基取代对过程的影响。甲基的极化性极大地促进了体系内密度的局部转移，而OH-的负电荷则允许密度从质子接受分子通过氢键向供体的净转移。
Proton transfers in (HO-H-OH)-, (H2O-H-OH2)+, and (CH3OH-H-HOCH3)+ are compared via ab initio molecular orbital calculations by using a 4-31G basis set. In all 3 systems, lengthening the interoxygen H bond length leads to increases in the energy barrier to proton transfer. In the range of bond lengths studied, up to 2.95 .ANG., the barrier heights for (H2O-H-OH2)+ and (HO-H-OH)- are quite similar while the barriers in (CH3OH-H-HOCH3)+ are somewhat higher. These observations are explained on the basis of equilibrium OH bond lengths in the protonated subsystems and spatial extent of electron-density clouds. Angular deformations of the H bonds generally lead to enlargements of the transfer barriers. These enlargements are qualitatively similar for the 2 cationic systems whereas the anion behaves quite differently. Considerations of electrostatic interactions may account for the disparities between the 2 types of systems in a straightforward manner. Examination of the electron-density rearrangements that accompany the proton transfer lead to insights into the effects of overall change and methyl substitution upon the process. The polarizability of the methyl group facilitates large contributions to the local shifts of density within the system while the negative charge of OH- permits a greater net transfer of density from the proton-accepting molecule across the H bond to the donor.