On the Nature of Blueshifting Hydrogen Bonds

Abstract

The block-localized wave function (BLW) method can derive the energetic, geometrical, and spectral changes with the deactivation of electron delocalization, and thus provide a unique way to elucidate the origin of improper, blueshifting hydrogen bonds versus proper, redshifting hydrogen bonds. A detailed analysis of the interactions of F(3)CH with NH(3) and OH(2) shows that blueshifting is a long-range phenomenon. Since among the various energy components contributing to hydrogen bonds, only the electrostatic interaction has long-range characteristics, we conclude that the contraction and blueshifting of a hydrogen bond is largely caused by electrostatic interactions. On the other hand, lengthening and redshifting is primarily due to the short-range n(Y)→σ*(X-H) hyperconjugation. The competition between these two opposing factors determines the final frequency change direction, for example, redshifting in F(3)CH⋅⋅⋅NH(3) and blueshifting in F(3)CH⋅⋅⋅OH(2). This mechanism works well in the series F(n)Cl(3)-n CH⋅⋅⋅Y (n=0-3, Y=NH(3), OH(2), SH(2)) and other systems. One exception is the complex of water and benzene. We observe the lengthening and redshifting of the O-H bond of water even with the electron transfer between benzene and water completely quenched. A distance-dependent analysis for this system reveals that the long-range electrostatic interaction is again responsible for the initial lengthening and redshifting.

The origins of the directionality of noncovalent intermolecular interactions#

“…The optimal electron‐localized state actually corresponds to a van der Waals complex where the CT effect is strictly absent. Thus, the geometrical and energetic changes or even spectral changes due to the electron transfer effect can be examined …”

“…Though the nature of hydrogen bonds is still somewhat controversial, the current consensus is that hydrogen bonds are predominantly electrostatic with minor covalent characters, and it is the latter that is responsible for the bond directionality. The covalency is conferred by the charge transfer (CT) (or hyperconjugation) effect, which results from the overlap between an occupied orbital on one end and a virtual orbital on another end, and is directional …”

Continuum in the X‐Z‐‐‐Y weak bonds: Z= main group elements

“…Although most of the recent studies on the nature of blue‐shifting Z‐bonds involve H‐bond and halogen‐bond, a few investigations on the blue‐shifting nature of chalcogen bonded complexes are also known. The available explanation for red‐shifting is the Charge Transfer (CT) from acceptor group (Y) to the σ* Anti‐Bonding Molecular Orbital (ABMO) of the donor (X‐Z) molecule.…”

Interpretation of Spectroscopic Markers of Hydrogen Bonds

“…An umber of different rationales have been proposed, [14,72,[84][85][86][87][88][89][90][91][92][93][94] but all frameworks are organized around as et of competing forces, some pullingt he mode to lower frequencya nd others in the reverse direction. An umber of different rationales have been proposed, [14,72,[84][85][86][87][88][89][90][91][92][93][94] but all frameworks are organized around as et of competing forces, some pullingt he mode to lower frequencya nd others in the reverse direction.…”

“…The changes in the A-H stretching frequencies induced in the proton donor molecule by HB formation have an entirely different source. An umber of different rationales have been proposed, [14,72,[84][85][86][87][88][89][90][91][92][93][94] but all frameworks are organized around as et of competing forces, some pullingt he mode to lower frequencya nd others in the reverse direction. In the majority of HBs, the former forces are stronger,b ut blue shifts can occur when the latter overwhelm the former,w ith CH···O HBs the most commone xample.…”