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(sp 3 )H , C(sp 2 )H, and C(sp)H donor bonds and FC(sp 3 ) acceptors. The series of molecules CH 3 F⋅CH 4 ( 2 a , 2 b ), CH 3 F⋅C 2 H 4 ( 3 ), CH 3 F⋅C 2 H 2 ( 4 ), as well as model complexes of experimentally characterized 2‐fluoro‐2‐phenylcyclopropane derivatives, C 3 H 6 ⋅C 3 H 5 F ( 5 a , 5 b ) and C 3 H 5 F⋅C 3 H 5 F ( 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⋅⋅⋅FC(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 CH bonds ( 2 a < 5 a < 3 < 6 < 4 ).