Theoretical force-field model for blue-shifted hydrogen bonds with fluoromethanes

It is demonstrated that the contraction of the C-H bonds of fluoromethanes in some subclass of blue-shifting complexes and the concomitant blue shifts of their stretching vibrational modes are a direct consequence of the intramolecular mode coupling between C-H and C-F bonds. This subclass of the so-called blue-shifted complexes includes those blue-shifting complexes, where the C-F bond of fluoromethanes participates in the pre-formation of the dominating C-F⋯H-Y halogen-hydrogen bond with the conventional proton-donor molecule Y-H. The formation of the latter elongates this 'spectator' C-F bond that in turn causes the C-H bond(s) of fluoromethanes to contract. It is therefore an intrinsic, intramolecular feature of the fluoromethanes that underlies the phenomenon of an 'intramolecular negative response'. This phenomenon is a key mechanism of a blue shift of this subclass of blue-shifted complexes. To understand the origin of the intrinsic intramolecular negative response of the fluoromethanes, a physical and analytically solvable model of the intramolecular mode coupling that is mainly based on the harmonic force field ansatz is proposed. It is explicitly shown that this model fairly agrees with the calculated contractions of the C-H bonds in the fluoromethanes-hydrogen fluoride complexes CHnF4−n⋯(HF)1⩽m⩽4 (1 ⩽ n ⩽ 3). The model concludes that it is the intramolecular mode coupling patterns of the fluoromethanes that entail their intrinsic negative response. The related enigmatic nature of the formation of the complexes CHF3⋯OH2 and CHF3⋯NH3 is revisited invoking particularly the traditional approach of embedding the studied system into an external homogeneous electric field.