Red-Shifting versus Blue-Shifting Hydrogen Bonds: Perspective from Ab Initio Valence Bond Theory

Abstract

Both proper, red-shifting and improper, blue-shifting hydrogen bonds have been well-recognized with enormous experimental and computational studies. The current consensus is that there is no difference in nature between these two kinds of hydrogen bonds, where the electrostatic interaction dominates. Since most if not all the computational studies are based on molecular orbital theory, it would be interesting to gain insight into the hydrogen bonds with modern valence bond (VB) theory. In this work, we performed ab initio VBSCF computations on a series of hydrogen-bonding systems, where the sole hydrogen bond donor CF3H interacts with ten hydrogen bond acceptors Y (═NH2CH3, NH3, NH2Cl, OH(-), H2O, CH3OH, (CH3)2O, F(-), HF, or CH3F). This series includes four red-shifting and six blue-shifting hydrogen bonds. Consistent with existing findings in literature, VB-based energy decomposition analyses show that electrostatic interaction plays the dominating role and polarization plays the secondary role in all these hydrogen-bonding systems, and the charge transfer interaction, which denotes the hyperconjugation effect, contributes only slightly to the total interaction energy. As VB theory describes any real chemical bond in terms of pure covalent and ionic structures, our fragment interaction analysis reveals that with the approaching of a hydrogen bond acceptor Y, the covalent state of the F3C-H bond tends to blue-shift, due to the strong repulsion between the hydrogen atom and Y. In contrast, the ionic state F3C(-) H(+) leads to the red-shifting of the C-H vibrational frequency, owing to the attraction between the proton and Y. Thus, the relative weights of the covalent and ionic structures essentially determine the direction of frequency change. Indeed, we find the correlation between the structural weights and vibrational frequency changes.

Looking Inside the Intramolecular C−H∙∙∙O Hydrogen Bond in Lactams Derived from α-Methylbenzylamine

“…Since the blue-shifting of vibrational frequencies originates from the strong repulsion between the hydrogen atom and the acceptor, as well as the attraction between the donor and the acceptor atoms [ 37 ], it is expected to find these vibrational frequencies in the systems studied herein. Thereupon, the fundamental stretching frequencies of C−Hα bond for lactams 2 – 6 in both conformations (open and closed)—and consequently their corresponding relative infrared shift (RIS)—were calculated.…”

The definitive challenge of forming uncommon pseudo‐π···H–F and C···H–F hydrogen bonds on cyclic and cubic nonpolar hydrocarbons

“…The values of the frequency shifts and absorption intensity ratios are listed in Table 2. By everything that is known about the frequency shifts, [130,131] specially due to the formation of hydrogenbonded complexes, either red-shifts or blue-shifts observed in the stretch frequencies of the proton donors, incomparably, these are the more evident events to identify unambiguously the intermolecular systems. [132] By correlating this statement with the structural analysis, a direct relationship between the frequency shifts and the bond length variations is well defined, whose result ruled by Equation 3 yielded a linear coefficient (r 2 ) of −0.97 whose illustration is presented in Figure 2.…”

Insights into the cooperativity between multiple interactions of dimethyl sulfoxide with carbon dioxide and water

“…On the contrary, blue‐shifting HB which involves a contraction of A‐H distance and an increase of its stretching frequency remains a matter of debate. A number of hypotheses and models have been proposed to explain the origin of both HB types . Each model/hypothesis has its own advantages and limitations.…”