# Hydrogen bonds - why not in HCl? [duplicate]

In a textbook1, I found the following clear definition of hydrogen bonds:

The strongest secondary bonding type, the hydrogen bond, is a special case of polar molecule bonding. It occurs between molecules in which hydrogen is covalently bonded to fluorine (as in $\ce{HF}$), oxygen (as in $\ce{H_2O}$), and nitrogen (as in $\ce{NH_3}$). For each $\ce{H-F}$, $\ce{H-O}$, or $\ce{H-N}$ bond, the single hydrogen electron is shared with the other atom. Thus, the hydrogen end of the bond is essentially a positively charged bare proton that is unscreened by any electrons.

This is not an "etc." definition. It is made clear that we only call it a hydrogen bond when we have either of the three bonds: $\ce{H-F}$, $\ce{H-O}$ or $\ce{H-N}$.

My question is why this definition excludes compounds such as $\ce{HCl}$? $\ce{HCl}$ and $\ce{HF}$ are very similar with one covalent bond that pulls the hydrogen electron towards their "shared" region, exposing the hydrogen atom as a positive end of this permanent dipole. Both can create secondary dipole bonds to adjacent molecules of the same type.

Why is $\ce{HF}$ called a hydrogen bond while $\ce{HCl}$ is not?

1 Materials Science and Engineering by W. D. Callister & D. G. Rethwisch, Wiley, 8th edition

## marked as duplicate by Mithoron, A.K., a-cyclohexane-molecule, Jon Custer, Todd MinehardtAug 29 '18 at 23:24

• HCl and HF are less similar than you think. – Ivan Neretin Aug 29 '18 at 12:38
• @IvanNeretin Please enlighten me. – Steeven Aug 29 '18 at 13:04
• As you surely know, HF is more polar. Now consider this: it is really, really much more polar. – Ivan Neretin Aug 29 '18 at 13:06

Very very related.

Hydrogen bonds to chlorine atoms are known. In the gas phase HCl dimers are hydrogen bonded but the bond has only about half the dissociation energy of $\ce{(HF)2}$ which has a larger electrostatic attraction energy. The adduct $\ce{HCl\cdots OH2}$ is also known and has a similar hydrogen bond energy to that of $\ce{HOH\cdots OH2}$.
Using neutron diffraction hydrogen bonds have been studied in many crystals. From a study of many types of amino acid crystals (and derivatives thereof) the normal $\ce{HOH\cdots O}$ hydrogen bond is found to have a range of lengths, from about 0.175 to 0.2 nm with most lengths at around 0.185 nm. The $\ce{OH\cdots Cl}$ hydrogen bond peaks about 0.2 nm with an range of $\approx \pm$ 0.05 nm. The $\ce{NH^+\cdots Cl}$ bond is longer peaking at 0.21 nm.