Zierkiewicz Wiktor, Michalska Danuta, Havlas Zdenek, Hobza Pavel
Institute of Inorganic Chemistry, Wrocław University of Technology, Wybrzeze Wyspiańskiego 27, 50-370 Wrocław, Poland.
Chemphyschem. 2002 Jun 17;3(6):511-8. doi: 10.1002/1439-7641(20020617)3:6<511::AID-CPHC511>3.0.CO;2-Y.
Weak hydrogen bonding was studied in the XH...OH2 and X3CH...OH2 complexes (X = F, Cl, Br, I) using the correlated MP2 ab initio method with relativistic Stuttgart/Dresden pseudopotentials and basis set (SDD). The accuracy of the method was tested for selected nonrelativistic complexes by performing MP2 calculations with all-electron basis sets (6-311G** and TZVPP). The characteristics of bonding in the hydrogen halide complexes correspond to the standard H-bonding (an elongation of the X-H bond and red shift of its stretch frequency), whereas those in the X3CH...OH2 complexes (X = F, Cl) are typical of improper blue-shifting H-bonding (a contraction of the CH bond and blue shift of the respective stretch frequency). A natural bond orbital analysis revealed some important differences between both classes of complexes: a) the electron density transfer (EDT) in the former complexes is considerably larger than that in the latter complexes: b) the EDT in the former complexes is almost completely directed to the sigma*-antibonding orbital of the XH bond, which causes a weakening of this bond, its elongation, and a concomitant decrease of the XH stretch frequency. In the latter complexes, only a small portion of the EDT goes to the sigma*-antibonding orbital of the CH bond of the proton donor and a larger part is transferred to the remote (nonparticipating) part of the proton donor. As a consequence, the structural reorganization of the proton donor occurred, leading to the contraction of the C-H bond. The fact that a small red shift of the C-H stretch frequency was found in bromoform-water and iodoform-water complexes was explained by the competition of both the above-mentioned mechanisms with dominating passage of electron density to the sigma*-antibonding orbital of the C-H bond. For an explanation of all the geometric features of both types of complexes, it is however necessary to consider both charge transfer and electrostatic effects. The electrostatic effects fail sometimes to interpret the geometry changes in the proton donor.
使用具有相对论性斯图加特/德累斯顿赝势和基组（SDD）的相关MP2从头算方法，对XH...OH₂和X₃CH...OH₂配合物（X = F、Cl、Br、I）中的弱氢键进行了研究。通过使用全电子基组（6 - 311G*和TZVPP）进行MP2计算，对选定的非相对论配合物测试了该方法的准确性。卤化氢配合物中的键合特征对应于标准氢键（X - H键伸长及其伸缩频率红移），而X₃CH...OH₂配合物（X = F、Cl）中的键合特征是典型的非标准蓝移氢键（C - H键收缩及其相应伸缩频率蓝移）。自然键轨道分析揭示了这两类配合物之间的一些重要差异：a）前一类配合物中的电子密度转移（EDT）比后一类配合物中的大得多；b）前一类配合物中的EDT几乎完全指向X - H键的σ反键轨道，这导致该键减弱、伸长，并伴随X - H伸缩频率降低。在后一类配合物中，只有一小部分EDT进入质子供体C - H键的σ反键轨道，而较大一部分转移到质子供体的远程（不参与）部分。结果，质子供体发生了结构重组，导致C - H键收缩。在溴仿 - 水和碘仿 - 水配合物中发现C - H伸缩频率有小的红移这一事实，是由上述两种机制的竞争来解释的，其中电子密度主要传递到C - H键的σ反键轨道。然而，为了解释这两类配合物的所有几何特征，有必要同时考虑电荷转移和静电效应。静电效应有时无法解释质子供体中的几何变化。
