1

Why does hydrogen bonding require a lone pair of electrons on the partially negative atom? They would still be partially negative even without the lone pair right? Or does a bond exist, but is much more negligible compared to when a lone pair is there?

Hayden Soares
  • 151
  • 1
  • 9
  • 5
    In a H-bond electron density is donated to the 'naked' proton, so this electron density has to come from somewhere. There needs to be some orbital overlap, so there must be a lone pair. – H.Linkhorn Dec 21 '20 at 14:01

2 Answers2

1

You don't need a lone pair. Water can hydrogen bond to methane, attacking the back side of one of the carbon-hydrogen bonds in the methane. In a molecular orbital the sigma bonding orbitals in the methane are sufficiently delocalized to serve in place of lone pairs, overlapping the antibonding orbitals in the water [1]:

Abstract (from [1])

Quantum chemical calculations were performed at different levels of theory (SCF, DFT, MP2, and CCSD(T)) to determine the geometry and electronic structure of the HOH···CH4 complex formed by water and methane molecules, in which water is a proton donor and methane carbon (sp 3) is an acceptor. The charge distribution on the atoms of the complex was analyzed by the CHelpG method and Hirshfeld population analysis; both methods revealed the transfer of electron charge from methane to water. According to the natural bond orbital (NBO) analysis data, the charge transfer upon complexation is caused by the interaction between the σ orbital of the axial С–H bond of methane directed along the line of the O–H···C hydrogen bridge and the antibonding σ* orbital of the О–H bond of the water molecule. Topological analysis of electron density in the HOH···CH4 complex by the AIM method showed that the parameters of the critical point of the bond between hydrogen and acceptor (carbon atom) for the O–H···C interaction are typical for Н-bonded systems (the magnitude of electron density at the critical point of the bond, the sign and value of the Laplacian). It was concluded that the intermolecular interaction in the complex can be defined as an Н bond of O–H···σ(С–H) type, whose energy was found to be 0.9 kcal/mol in MP2/aug-cc-pVQZ calculations including the basis set superposition error (BSSE).

Reference

1. A. N. Isaev,"O–H···C hydrogen bond in the methane–water complex", Russian Journal of Physical Chemistry A 90(2016), 1978–1985.

Mithoron
  • 4,546
  • 14
  • 40
  • 61
Oscar Lanzi
  • 56,895
  • 4
  • 89
  • 175
  • 1
    That's sure interesting (not being a hard proof, though), but I have a feeling OP simply doesn't get how h-bond works... – Mithoron Dec 21 '20 at 19:33
  • 1
    The hydrogen bond in $\ce{CH5^+}$ is so weak if compared to $\ce{H3O+}$ that it looks more like van der Wals forces. – Maurice Dec 21 '20 at 19:56
-2

Hydrogen bonds occurs when a H atom is bound to a highly electronegative atom like oxygen. In such a bond O-H, the 1s electron of hydrogen is mainly localized between the H and O nuclei. So, at the outer part of the bond, the H nucleus is nearly not surrounded any more by an electron layer. The H nucleus is nearly "naked". Nothing prevents it from touching and attracting electrons from the vicinity, and specially these electrons which are not engaged in a bond, like those in outer layer of the Oxygen atoms. This electrostatic attraction is called a Hydrogen bond.

Maurice
  • 28,241
  • 3
  • 29
  • 61
  • 1
    There's so many errors in this post that, the list could be longer then your answer. See for example https://chemistry.stackexchange.com/a/5506/9961 – Mithoron Dec 21 '20 at 19:26
  • I'm actually surprised. With nearly 10k rep I assume your answers can be better than this. – Oscar Lanzi Dec 21 '20 at 20:27
  • What do you have against my explanation of the Hydrogen bond ? It is made for high schools students. – Maurice Dec 21 '20 at 21:44
  • We should be teaching high school students the truth, not an oversimplified stereotype. – Oscar Lanzi Dec 22 '20 at 00:41
  • High school students cannot understand Schrödinger equation. – Maurice Dec 22 '20 at 11:14
  • If they understand molecular orbitals, they can understand a proper renderinf of hydrogen bonds. – Oscar Lanzi Dec 22 '20 at 11:17
  • @Oscar Lanzi. Even so, the explanation would not differ much from what I have written 20 hours ago. Of course the words "naked" and "touching electrons" are nearly a non-sense. But they are well understood by the average student. It is not so easy to be on a level with the average high school student. At least, that is nearly what H. Linkhorn has written in the very beginning of this discussion. And he got points for this answer ! – Maurice Dec 22 '20 at 12:40