Del Bene Janet E, Alkorta Ibon, Elguero José
Department of Chemistry, Youngstown State University, Youngstown, Ohio 44555, USA.
Magn Reson Chem. 2008 May;46(5):457-63. doi: 10.1002/mrc.2199.
Ab initio calculations have been performed to obtain structures and coupling constants (1)J(N-H), (1h)J(H-N), and (2h)J(N-N) for models of proton sponges with symmetric and asymmetric N-H(+)-N intramolecular hydrogen bonds (IMHBs). For a given model, the asymmetric structure has a lower energy, a longer N-N distance, and a hydrogen bond which has a greater deviation from linearity. The computed values of (2h)J(N-N) for the models are significantly less than predicted values based on the distance dependence of (2h)J(N-N) for complexes with intermolecular N-H(+)-N hydrogen bonds. However, the reduced values of (2h)J(N-N) cannot be attributed solely to the distortion of the hydrogen bond in the models, but also reflect differences in s electron populations at the nitrogens in both the ground state and the excited states which couple to it through the Fermi-contact (FC) operator. Values of (2h)J(N-N) for IMHBs can be related quadratically to the N-N distances in the models, and demonstrate that there is no discrepancy between computed values of (2h)J(N-N) at the short N-N distances found in these systems and experimental data for proton sponges.
已进行从头算计算，以获得具有对称和不对称N-H(+)-N分子内氢键（IMHBs）的质子海绵模型的结构和耦合常数(1)J(N-H)、(1h)J(H-N)和(2h)J(N-N)。对于给定模型，不对称结构具有更低的能量、更长的N-N距离以及偏离线性程度更大的氢键。模型的(2h)J(N-N)计算值明显小于基于具有分子间N-H(+)-N氢键的配合物的(2h)J(N-N)距离依赖性所预测的值。然而，(2h)J(N-N)的降低值不能仅仅归因于模型中氢键的扭曲，还反映了基态和激发态氮原子上s电子布居的差异，这些差异通过费米接触（FC）算符与之耦合。IMHBs的(2h)J(N-N)值可以与模型中的N-N距离呈二次关系，并且表明在这些系统中发现的短N-N距离下(2h)J(N-N)的计算值与质子海绵的实验数据之间不存在差异。
