Majerz Irena, Olovsson Ivar
Department of Materials Chemistry, Angström Laboratory, Box 538, SE-75121, Uppsala, Sweden.
Phys Chem Chem Phys. 2008 Jun 7;10(21):3043-51. doi: 10.1039/b717815h. Epub 2008 Apr 8.
The potential energy curves for proton motion in NHN(+) hydrogen bonds have been calculated to investigate whether different methods of evaluation give different results: for linear H bonds most curves calculated along the NH direction are, as expected, identical with those along NN; for intramolecular H bonds it is very important to take into account the non-linearity and the potential energy curve calculated along the NH direction can be very far from the curve correctly describing the proton transfer. Other factors which influence the proton-transfer process are steric hindrance and presence of anions which modify the proton motion. In the analysis of the proton transfer process it is very important to take changes in the structure of the rest of the molecule into account, which is connected with exchange of energy with the surroundings. Comparison of adiabatic and non-adiabatic curves shows that they are significantly different for very bent hydrogen bonds and for hydrogen bonds with steric constraints for which the proton transfer process must be accompanied with relaxation of the whole molecule. Comparison of the potential-energy curves for compounds with very short H bonds emphasizes that the term 'strong H bond' needs to be qualified. For intermolecular H bonds shortening of the bond is connected with linearization. But for intramolecular H bonds the NN distance cannot be used as the only measure of H bond strength.
已计算出质子在NHN(+)氢键中运动的势能曲线，以研究不同的评估方法是否会给出不同的结果：对于线性氢键，如预期的那样，沿NH方向计算的大多数曲线与沿NN方向的曲线相同；对于分子内氢键，考虑其非线性非常重要，沿NH方向计算的势能曲线可能与正确描述质子转移的曲线相差甚远。影响质子转移过程的其他因素包括空间位阻和改变质子运动的阴离子的存在。在分析质子转移过程时，考虑分子其余部分结构的变化非常重要，这与与周围环境的能量交换有关。绝热曲线和非绝热曲线的比较表明，对于非常弯曲的氢键和具有空间限制的氢键，它们有显著差异，对于这些氢键，质子转移过程必须伴随着整个分子的弛豫。具有非常短氢键的化合物的势能曲线比较强调，“强氢键”这一术语需要加以限定。对于分子间氢键，键的缩短与线性化有关。但对于分子内氢键，NN距离不能作为氢键强度的唯一度量。
