Existing problems in theoretical determination of red-shifted or blue-shifted hydrogen bond

Abstract

Both Hobza (Int. J. Quantum Chem., 90 (2002) 1071) and Li et al. (J. Am. Chem. Soc., 124 (2002) 9639) have predicted a blue-shifted or improper N–H⋯F hydrogen bond existing in the F 2N–H⋯FH complex with the method of Hartree–Fock calculation followed by Møller–Plesset correlation energy correction truncated at second order (MP2/6-31G** and MP2/6-311+G**, respectively). Unexpectedly, our density functional calculation (B3LYP) has predicted a red-shifted or normal N–H⋯F hydrogen bond in this complex whether the counterpoise-corrected gradient optimization and frequency calculation is applied or not. Changing the basis set from 6-311+G(d,p) to 6-311++G(3df,3pd) does not change the red-shifted hydrogen bond prediction. Experimental identification should be provided for evaluating relative performance of these two theoretical methods in predicting correct hydrogen bond stretching vibration frequency shift upon complex formation. X–H bond length change and its vibrational frequency change do not necessarily have monotonic corresponding relationship, i.e. blue-shift does not necessarily mean X–H bond length contraction, and vice versa. Much more attention or consideration should be paid in theoretical hydrogen bond type prediction such as red-shifted or blue-shifted.