On the change of the NMR isotropic shielding in blue- and red-shifted complexes of F3CH

The change in the 1H and 13C isotropic magnetic shielding constant of F3CH on formation of the H-bonded F3CH⋯Y complexes, Y = F−, Cl−, NH3, N(CD3)3, FH, ClH, OH2, SH2, N2, CO, OCO, Ar, NCH and O(CH3)2, were determined by GIAO MP2 ab initio and DFT computations. A good correlation between the sign of frequency shift and the change of the proton magnetic shielding was obtained, i.e., red-shifted complexes show the characteristic downfield shift of the isotropic proton magnetic resonance while in blue-shifted complexes the resonance is shifted upfield or shifted downfield but to a much smaller extent than in red-shifted complexes. Similar results were obtained for the 13C shifts. Counterpoise correction and basis set variation of the magnetic shielding were also considered.