Majerz Irena, Olovsson Ivar
Department of Materials Chemistry, Angström Laboratory, Box 538, SE-75121 Uppsala, Sweden.
Phys Chem Chem Phys. 2009 Mar 7;11(9):1297-302. doi: 10.1039/b814798a. Epub 2009 Feb 4.
The proton transfer in the intramolecular NHN(+) hydrogen bonds of selected proton sponges has been studied using theoretical calculations of the potential energy surfaces (PES). The proton-transfer trajectory follows very closely the lowest energy path, derived as the quantum-mechanical reaction coordinates (QMRC). The bond order is not conserved in the transfer process. Even in the most flexible proton sponges there are considerable constraints on the NN distance and the hydrogen bonds do not behave as intermolecular bonds. The curvature of QMRC is not a suitable criterion to distinguish between inter- and intramolecular NHN(+) bonds. It appears that the determining factor for linearity is the degree of constraint, which is most likely the strongest in the benzene proton sponges. In the naphthalene proton sponges with relatively short NN distances QMRC is more bent than in the benzene complexes with somewhat longer distances, opposite to what might be expected. It is important to note that in intramolecular complexes the PES is characterized by a single minimum, in contrast to a double minimum in intermolecular complexes. The experimentally determined NH bond lengths have been plotted on the potential energy surface and these points are all located on the QMRC curve, very close to the energy minimum of the PES. However, it is vital that the experimental X-ray hydrogen positions are then corrected to give the true internuclear NH distances.
利用势能面（PES）的理论计算研究了选定质子海绵分子内NHN(+)氢键中的质子转移。质子转移轨迹非常紧密地遵循作为量子力学反应坐标（QMRC）推导出来的最低能量路径。在转移过程中键级并不守恒。即使在最灵活的质子海绵中，NN距离也存在相当大的限制，并且氢键并不表现为分子间键。QMRC的曲率不是区分分子间和分子内NHN(+)键的合适标准。似乎线性的决定因素是限制程度，这在苯质子海绵中很可能是最强的。在NN距离相对较短的萘质子海绵中，QMRC比在距离稍长的苯配合物中更弯曲，这与预期相反。需要注意的是，与分子间配合物中的双最小值相反，分子内配合物中的PES具有单个最小值。已将实验测定的NH键长按势能面绘制，这些点都位于QMRC曲线上，非常接近PES的能量最小值。然而，至关重要的是，随后要校正实验X射线氢位置以给出真实的核间NH距离。
