SOLID-STATE N-15 NMR AND THEORETICAL-STUDIES OF PRIMARY AND SECONDARYGEOMETRIC H D ISOTOPE EFFECTS ON LOW-BARRIER NHN-HYDROGEN BONDS/

Using a combination of high resolution and dipolar solid state N-15 NM R we have determined H/D isotope effects on the nitrogen-hydron (L = H , D) distances and N-15 chemical shielding tensors of strongly hydroge n 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 comp ounds have been modeled theoretically by the linear system [C=N ... L ... N=C]Li--(+) (1). The crystal field acting on the anion was generat ed by a variety of fixed C ... Li distances. For the calculation of dy namical corrections of geometries and NMR chemical shifts, an iterativ e procedure based on the crude adiabatic approximation was employed, c onsisting of (i) ab initio calculation of the energy hypersurface at t he MP2/6-31+G(d,p) level, (ii) solution of the Schrodinger equation fo r 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 w ay 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)) estab lished here experimentally and theoretically for very strong NHN-hydro gen bonds shows a good agreement with a correlation established previo usly (Steiner, Th. J. Chem. Soc., Chem. Commun. 1995, 1331) based on t he neutron diffraction structures of a number of weakly hydrogen bonde d solids. A plot of the sum q(2) = r(1) + r(2)-corresponding in a line ar hydrogen bond to the heavy atom separation-as a function of the pro ton dislocation from the hydrogen bond center q(1) = 1/2(r(1)-r(2)) ex hibits a minimum value at about 2.54 Angstrom for the symmetric low-ba rrier hydrogen bond at q(1) = 0. This situation is realized experiment ally for 2. When q(1) not equal 0 anharmonic single well hydrogen bond s are obtained, typical for 3. The geometric H/D isotope effects can b e 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 posi tion q(2). Secondary effects have been reported previously by Ubbelohd e. 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 nucle i in the hydrogen bridge is a qualitative probe for the primary and se condary geometric isotope effects.