Institute of Chemistry, Siberian Branch of the Russian Academy of Science, Favorski St. 1, 664033 Irkutsk, Russia.
Magn Reson Chem. 2010 Apr;48(4):309-17. doi: 10.1002/mrc.2579.
According to the density functional theory calculations, the X...H...N (X=N, O) intramolecular bifurcated (three-centered) hydrogen bond with one hydrogen donor and two hydrogen acceptors causes a significant decrease of the (1h)J(N,H) and (2h)J(N,N) coupling constants across the N-H...N hydrogen bond and an increase of the (1)J(N,H) coupling constant across the N-H covalent bond in the 2,5-disubstituted pyrroles. This occurs due to a weakening of the N-H...N hydrogen bridge resulting in a lengthening of the N...H distance and a decrease of the hydrogen bond angle at the bifurcated hydrogen bond formation. The gauge-independent atomic orbital calculations of the shielding constants suggest that a weakening of the N-H...N hydrogen bridge in case of the three-centered hydrogen bond yields a shielding of the bridge proton and deshielding of the acceptor nitrogen atom. The atoms-in-molecules analysis shows that an attenuation of the (1h)J(N,H) and (2h)J(N,N) couplings in the compounds with bifurcated hydrogen bond is connected with a decrease of the electron density rho(H...N) at the hydrogen bond critical point and Laplacian of this electron density nabla(2)rho(H...N). The natural bond orbital analysis suggests that the additional N-H...X interaction partly inhibits the charge transfer from the nitrogen lone pair to the sigma*(N-H) antibonding orbital across hydrogen bond weakening of the (1h)J(N,H) and (2h)J(N,N) trans-hydrogen bond couplings through Fermi-contact mechanism. An increase of the nitrogen s-character percentage of the N-H bond in consequence of the bifurcated hydrogen bonding leads to an increase of the (1)J(N,H) coupling constant across the N-H covalent bond and deshielding of the hydrogen donor nitrogen atom.
根据密度泛函理论计算，X…H…N（X=N，O）分子内分叉（三中心）氢键，一个氢供体和两个氢受体，导致（1h）J（N，H）和（2h）J（N，N）耦合常数在 N-H…N 氢键两侧显著降低，并且在 2,5-取代吡咯中 N-H 共价键的（1）J（N，H）耦合常数增加。这是由于 N-H…N 氢键的减弱导致 N…H 距离的延长和氢键形成时氢键角度的减小。屏蔽常数的无标度原子轨道计算表明，在三中心氢键的情况下，N-H…N 氢键的减弱导致桥质子的屏蔽和接受体氮原子的去屏蔽。分子中的原子分析表明，在具有分叉氢键的化合物中，（1h）J（N，H）和（2h）J（N，N）耦合的衰减与氢键临界点处的电子密度 rho（H…N）的减小以及该电子密度的拉普拉斯 nabla（2）rho（H…N）有关。自然键轨道分析表明，额外的 N-H…X 相互作用部分抑制了通过 Fermi 接触机制从氮孤对到 sigma*（N-H）反键轨道的电荷转移，从而削弱了（1h）J（N，H）和（2h）J（N，N）跨氢键耦合。由于分叉氢键，N-H 键的氮 s 特征百分比增加，导致 N-H 共价键的（1）J（N，H）耦合常数增加和氢供体氮原子的去屏蔽。
