Weak hydrogen bridges: a systematic theoretical study on the nature and strength of C--H...F--C interactions

We present a comparative study on the nature and strength of weak hydrogen bonding between the C(sp3)-H, C(sp2)-H, and C(sp)-H donor bonds and F-C(sp3) acceptors. The series of molecules CH3F.CH4 (2 a, 2 b), CH3F.C2H4 (3), CH3F.C2H2 (4), as well as model complexes of experimentally characterized 2-fluoro-2-phenylcyclopropane derivatives, C3H6.C3H5F (5 a, 5 b) and C3H5F.C3H5F (6) were investigated. Comparative studies were also performed for two conformers of the methane dimer (1 a, 1 b). The calculations were carried out in hierarchies of basis sets [SV(d,p), TZV(d,p), aug-TZV(d,p), TZV(2df,2pd), aug-TZV(2df,2pd), QZV(3d2fg,2pd), aug-QZV(3d2fg,2pdf)] by means of ab initio [HF, MP2, QCISD, QCISD(T)] methods and density functional theory (DFT/B3LYP, DFT/PBE). It is shown that well-balanced basis sets of at least TZV(2df,2pd) quality are needed for a proper description of the weakly bonded systems. In the case of 2, 3, 5, and 6, the dispersion interaction is the dominant term of the entire attraction, which is not accounted for at the B3LYP level. Significant electrostatic contributions are observed for 6 and 3. For 4, these forces have a dominating contribution to the hydrogen bonding. The C(sp)--H...F--C(sp(3)) interaction in 4, though weak, exhibits the same characteristics as conventional hydrogen bridges. Despite showing longer H.F/H contacts compared to 1 a, 2 a, and 5 a the bifurcated structures, 1 b, 2 b, 5 b, are characterized by larger dispersion interactions leading to stronger bonding. For the systems with only one H.F contact, the MP2/QZV(3d2fg,2pd) interaction energy increases in the order 2 a (-1.62 kJ mol(-1)), 3 (-2.79 kJ mol(-1)), 5 a (-5.97 kJ mol(-1)), 4 (-7.25 kJ mol(-1)), and 6 (-10.02 kJ mol(-1)). This contradicts the estimated proton donor ability of the C--H bonds (2 a<5 a<3<6<4).