On the correlation between the blue shift of hydrogen bonding and the proton donor-proton acceptor distance



It is demonstrated that in all types of hydrogen bonds (X[BOND]H···Y) there is a balance between the long-range attractive orbital interactions and short-range Pauli/nucleus repulsions. When the proton acceptor approaches the proton donor from distance, the hydrogen bonding energy becomes more negative at relatively large distance, goes through a minimum. and then starts to become less negative when the short-range repulsive forces come into effect Meanwhile, the X[BOND]H bond length increases at relatively large distances, goes through a maximum and starts to shorten when the short-range repulsive forces come into effect. Whether the hydrogen bond is red or blue shifted is dictated by the energy minimum position. If at the energy minimum position the X[BOND]H bond length is shorter than that for the free monomer, the hydrogen bond is blue shifted and vice versa. Further studies demonstrate that the recent report about the correlation of C[BOND]H bond lengths with proton donor-acceptor distance in F3C[BOND]H···OH2 and F3C[BOND]H···Cl is not fully correct because the authors conducted an inappropriate comparison. Furthermore, it is shown for the first time that the Pauli/nucleus repulsion theory is applicable to the blue-shifted hydrogen bonds in the X[BOND]H···π complexes and the blue-shifted lithium bonds in the X[BOND]Li···Y complexes.