Solid state 15N NMR and theoretical studies of primary and secondary geometric H/D isotope effects on low-barrier NHN-hydrogen bonds
Solid state 15N NMR and theoretical studies of primary and secondary geometric H/D isotope effects on low-barrier NHN-hydrogen bonds
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DOI：
10.1021/ja9719790
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发表时间：
1998-04-01
影响因子：
15
通讯作者：
Janoschek, R
中科院分区：
文献类型：
--
作者：
Benedict, H;Limbach, HH;Janoschek, R
本文用高分辨和偶极固体N-15核磁共振相结合的方法，测定了强氢键双异氰盐[（CO）（5）Cr-C=N…我... N=C-Cr（CO）（5）]X-（+），其中X+ = AsPh 4+（2）和X+ = NPr 4+（3）。这些化合物在理论上已由线性系统[C=N.我... N=C]Li-（+）（1）。作用于阴离子的晶场是由多种固定碳原子产生的。Li距离。为了计算几何构型和NMR化学位移的动力学校正，采用了基于粗绝热近似的迭代方法，包括（i）在MP2/6-31+G（d，p）水平上从头计算能量超曲面，（ii）求解非谐共线氢分子运动的薛定谔方程，（iii）使用IGLO方法计算NMR化学位移。两个氢键距离r（1）= N…L和r（2）= L.发现当H被D取代时，N以相关的方式变化，作为X+的函数，即，电场在氢键处的作用。对于非常强的NHN-氢键，在实验上和理论上建立的相关性r（1）= f（r（2））显示出与先前建立的相关性良好的一致性（Steiner，Th.美国化学会杂志，化学通信1995，1331）基于许多弱氢键固体的中子衍射结构。对于q（1）= 0时的对称低势垒氢键，q（2）= r（1）+ r（2）-对应于线性氢键中重原子分离-作为质子从氢键中心的位错的函数的图q（1）= 1/2（r（1）-r（2））在约2.54埃处显示出最小值。这种情况下实现实验2。当q（1）不等于0时，得到非简谐单阱氢键，典型的为3。几何H/D同位素效应可以分为与氢原子位置q（1）= 1/2（r（1）-r（2））相关的初级效应和与重原子位置q（2）相关的次级效应。Ubbelohde先前已报告了继发性影响。这两种同位素效应被证明是在一个简单的经验的方式有关的氢键的几何形状和同位素分馏因素。最后指出氢桥中原子核的化学屏蔽是一次和二次几何同位素效应的定性探针。
Using a combination of high resolution and dipolar solid state N-15 NMR we have determined H/D isotope effects on the nitrogen-hydron (L = H, D) distances and N-15 chemical shielding tensors of strongly hydrogen bonded bisisocyanide salts of the type [(CO)(5)Cr-C=N ... L ... N=C-Cr(CO)(5)]X--(+), where X+ = AsPh4+ (2) and X+ = NPr4+ (3). These compounds have been modeled theoretically by the linear system [C=N ... L ... N=C]Li--(+) (1). The crystal field acting on the anion was generated by a variety of fixed C ... Li distances. For the calculation of dynamical corrections of geometries and NMR chemical shifts, an iterative procedure based on the crude adiabatic approximation was employed, consisting of (i) ab initio calculation of the energy hypersurface at the MP2/6-31+G(d,p) level, (ii) solution of the Schrodinger equation for the anharmonic collinear hydron motion, and (iii) NMR chemical shift calculations using the IGLO-method. The two hydrogen bond distances r(1) = N ... L and r(2) = L ... N are found to change in a correlated way when H is replaced by D, as a function of X+, i.e., of the electric field at the hydrogen bond site. The correlation r(1) = f(r(2)) established here experimentally and theoretically for very strong NHN-hydrogen bonds shows a good agreement with a correlation established previously (Steiner, Th. J. Chem. Soc., Chem. Commun. 1995, 1331) based on the neutron diffraction structures of a number of weakly hydrogen bonded solids. A plot of the sum q(2) = r(1) + r(2)-corresponding in a linear hydrogen bond to the heavy atom separation-as a function of the proton dislocation from the hydrogen bond center q(1) = 1/2(r(1)-r(2)) exhibits a minimum value at about 2.54 Angstrom for the symmetric low-barrier hydrogen bond at q(1) = 0. This situation is realized experimentally for 2. When q(1) not equal 0 anharmonic single well hydrogen bonds are obtained, typical for 3. The geometric H/D isotope effects can be split into a primary effect referring to the hydron position q(1) = 1/2(r(1)-r(2)) and a secondary effect referring to the heavy atom position q(2). Secondary effects have been reported previously by Ubbelohde. Both isotope effects are shown to be related in a simple empirical way to the hydrogen bond geometries and to the isotopic fractionation factors. Finally, it is shown that the chemical shielding of the nuclei in the hydrogen bridge is a qualitative probe for the primary and secondary geometric isotope effects.