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关于分叉氢键对模型吡咯中的 1J(N,H)、1hJ(N,H)、2hJ(N,N)耦合和 1H、15N 屏蔽的理论研究。

Theoretical study of bifurcated hydrogen bonding effects on the 1J(N,H), 1hJ(N,H), 2hJ(N,N) couplings and 1H, 15N shieldings in model pyrroles.

机构信息

Institute of Chemistry, Siberian Branch of the Russian Academy of Science, Favorski St. 1, 664033 Irkutsk, Russia.

出版信息

Magn Reson Chem. 2010 Apr;48(4):309-17. doi: 10.1002/mrc.2579.

Abstract

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.

摘要

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。

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