NMR Study of Blue-Shifting Hydrogen Bonds Formed by Fluoroform in Solution

Low-temperature (193 K) H-1, C-13 and N-13 NMR spectra of blue- and red-shifting H-bonded complexes formed by fluoroform with various proton acceptors were measured. Experimental NMR parameters were Plotted versus the ab initio calculated H-bond strength (MP2/6-31+G(d, p) interaction energy varies from similar to 5 LIP to 25 kJ . mol(-1) in (lie series). We show that experimental H-1 and N-15 shieldings its well its the H/D isotope effect on C-13 shielding change monotonously with the calculated H-bond strengthening. The C-13 chemical shift and the CH scalar coupling change non-monotonously and the extremum points are situated approximately in the region of transformation from blue to red-shifting H-bonds. The most informative NMR feature is the H/D isotope effect on N-15 shielding which changes its sign upon transformation from blue- to red-shifting H-bonds. To rationalize these observations, ab initio calculations of C-13 and N-15 shielding as functions of C...H and C...N distances were performed for complexes of CHF3 with acetonitrile (blue-shifting) and pyridine (red-shifting). The coupling Of the vibrations of the covalent Mid hydrogen bonds has been accounted for by direct computation of the distance C...N = integral(C...H) dependence. We demonstrate that the unusual sign of the H/D isotope effect on 15N chemical shift across it blue-shifting H-bond can be explained as a result of the inversion of the dynamic coupling of two vibrations.