Solid State15N NMR and Theoretical Studies of Primary and Secondary Geometric H/D Isotope Effects on Low-Barrier NHN−Hydrogen Bonds

Abstract

Using a combination of high resolution and dipolar solid state 15N NMR we have determined H/D isotope effects on the nitrogen-hydron (L = H, D) distances and 15N chemical shielding tensors of strongly hydrogen bonded bisisocyanide salts of the type [(CO)5Cr−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 Schrödinger equation for the anharmonic collinear hydron motion, and (iii) NMR chemical shift calculations using the IGLO-method. The two hydrogen bond distances r1 ≡ N···L and r2 ≡ 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 r1 = f(r2) 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 q2 = r1 + r2corresponding in a linear hydrogen bond to the heavy atom separationas a function of the proton dislocation from the hydrogen bond center q1 = 1/2(r1−r2) exhibits a minimum value at about 2.54 Å for the symmetric low-barrier hydrogen bond at q1 = 0. This situation is realized experimentally for 2. When q1 ≠ 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 q1 = 1/2(r1−r2) and a secondary effect referring to the heavy atom position q2. 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.