Blue Shift Of The Hydrogen Bond Causes Of Theoretical Studies

Hydrogen bond has been one of the most interesting areas mainly due to its vital role in many chemical and biological processes. The improper blue-shifted hydrogen bond has attracted great interest to many scientist since it was detected. In the last 10-odd years, numerous theoretical studies on the nature of improper blue-shifted hydrogen bond have been reported.Improper blue-shifted hydrogen bond is characterized by contraction of the X-H bond in the electron donor and a concomitant increase (blue shift) of the X-H stretch frequency. It's IR spectrum character differs from that of the red-shifted hydrogen bond. Now, two prevailing theories are applied in rationalizing the origin of blue-shifted H-bond. One theory, suggested by Hobza and his co-workers, is based on reorganization of electronic density in electron acceptor. The other, developed by Weinhold et al., is based on rehybridization. It must be emphasized that previous studies on the origin of blue shifts completely focused on the C-H bond directly participating in H-bond formation. However, some experiments indicated that blue shifts also exist in some bonds not directly participating in H-bond formation. In a very recent paper, Parreira et al. have studied the blue shift of C-H in formic acid―HF and formate anion―HF/H2O/NH3/CH4 complexes. Their results showed that pronounced electron-density transfer (EDT) from the electron donor to the electron acceptor led to reorganization of the electron density (ED) in electron donor, and resulted in formation of a new electronic structure and the blue shift of C-H bond not directly participating in H-bond formation. To further explore the origin of the blue shift for C-H bond not directly participating in H-bond formation, we present a theoretical investigation on the complexes formed by formamide and small molecules such as HF et al.The most important results are as follows:i. Whether C-H and C-N bonds not directly participating in H-bond formation in NH2CHO···HF/H2O/H2S/NH3 complexes is red-shifted or blue-shifted depends on the electron density redistribution and rehybridization.ii. For formamide-methanol complexes, the result again proved that the origin of the red and blue shifts of some bonds not directly participating in H-bond formation was due to the result of the electron density redistribution and rehybridization effect .iii. The red and blue shifts of C-H bond in formic acid—HF/H2O/NH3 and formate anion—HF/H2O/NH3/CH4 complexes can be also explained by redistribution and rehybridization. The red and blue shifts of some bonds not directly participating in H-bond formation in some complexes can't be explained only by electron density redistribution.Electron-density transfer (EDT) from the proton acceptor to the proton donor in hydrogen bond complexes leads to formation of a new electronic structure and changes of geometry of monomer. Hobza's electron density redistribution theory and Weinhold's rehybridization theory not only can explain the origin of blue-shifted H-bond for some hydrogen bond complexes, but also can explain the origin of the red and blue shifts of some bonds not directly participating in H-bond formation.