Theory of electron localization and its application to blue-shifting hydrogen bonds

Abstract

We propose a theory of electron localization or stabilization by electron localization through the interactions between occupied (i) and vacant (j*) orbitals under certain conditions, which have been believed so far to cause only electron delocalization. Electrons localize when the electrons redistributed by the interaction are more stable in the i-th occupied orbital than in the overlap region: h(ij*) > s(ij*)h(ii) for s(ij*) > 0. Electron delocalization occurs when h(ij*) < s(ij*)h(ii) for s(ij*) > 0. The h(ij*) and s(ij*)h(ii) terms represent the energy of the electrons in the overlap region and the energy of the redistributed electrons in the occupied orbital, respectively. The theory of electron localization is substantiated by the correlation of the C-H bond lengths of fluorinated methanes H(4-n)CF(n) (n = 1, 2, 3) to the electron population of the σ(CH) bonding orbital, and successfully applied to understanding blue-shifting hydrogen bonds in F(3)CH···X (X = CO, N(2), OC, Ne, OC(CH(3))(2)) and designing some proton donors, HCO(2)CH(3) and hypervalent molecules HPF(4) and HSF(5), for blue-shifting hydrogen bonds.

Structure, energy, vibrational spectrum, and Bader's analysis of π⋯H hydrogen bonds and H−δ⋯H+δdihydrogen bonds

“…[345][346][347][348][349] In the specific case of the formation of traditional hydrogen-bonded complexes, the variations in charge density provoke the appearance of red-and blue-shifts in the infrared spectrum often related to the proton donors. [350][351][352][353][354][355][356][357] Hitherto, the quantification of the charge transfer is fundamental in this regard [358][359][360][361][362][363][364] because these vibrational phenomena are immediate consequences of the drastic structural deformations in both proton donors and acceptors following complexation. [365][366][367] One interesting example of deformation on monoprotic acids is related to their bond lengths, [368][369] Fig.…”

C–H···O Interaction in Methanol–Water Solution Revealed from Raman Spectroscopy and Theoretical Calculations

“…The formation of the conventional hydrogen bond is a direct process where the weakening of the X–H bond is caused by electron density transfer (EDT) from the proton acceptor to the antibond orbital of the proton donor, whereas the formation of the blue-shifted hydrogen bond is a more complicated “two-step” process where the strengthening of the X–H bond results from structural reorganization induced by EDT from the donor to a remote part of the acceptor, which in turn leads to a shortening of the X–H bond. On the basis of a thorough theoretical calculation, Scheiner et al proposed that red-shifted and blue-shifted hydrogen bonds lead to a similar change in electron density of remote parts of the hydrogen bond donor and thus there are no fundamental distinctions between them. , The explanation on red-shifted and blue-shifted hydrogen bonds continues to be of interest nowadays. − …”

Hydrogen Bonds and Ionic Forms versus Polymerization of Imidazole at High Pressures

“…The cooperative C–H---π type interactions between the π electron clouds in the five-membered heterocyclic planar rings and the C–H bonds of the adjacent rings within the layer and between the layer are likely to strengthen when imidazole is compressed under pressure. ,,, Spectroscopic signatures of C–H---π interactions are diverse due to varied acid–base interactions and are quite different from that of the strong O–H---O and N–H---O interactions. While the latter ones are marked by red-shift of the O–H or N–H stretch vibrations, C–H stretching modes often show blue-shift upon hydrogen bond formation . The origin of blue-shift has been attributed to dispersion interaction or the negative sign of dipole moment derivative with respect to the stretching coordinate, charge transfer, polarization, etc., depending upon the chemical nature of the donor and acceptors. ,, …”

“…While the latter ones are marked by red-shift of the O−H or N−H stretch vibrations, 31 C−H stretching modes often show blue-shift upon hydrogen bond formation. 32 The origin of blue-shift has been attributed to dispersion interaction or the negative sign of dipole moment derivative with respect to the stretching coordinate, charge transfer, polarization, etc., depending upon the chemical nature of the donor and acceptors. 20,33,34 The geometry of the C−H---π hydrogen bond is flexible, and hence the C−H bond may point to the center of the aromatic ring or any atom of the ring for the interaction.…”