# Why is a negatively charged atom less likely to hydrogen bond than a neutral atom?

Can someone explain why a negatively charged atom let's say $$\ce{CH3SH}$$ is less likely to bond with an additional hydrogen than $$\ce{CH3S-}$$?

And can anyone relate this to nucleophilicity in a protic solvent?

• Wait, I didn't quite get it: what atom is negatively charged in $\ce{CH3SH}$ and how is that different from $\ce{CH3S-}$ (which is indeed negatively charged)? – Ivan Neretin Nov 12 '15 at 7:26
• @ATP You should edit question - right now it's nonsense – Mithoron Nov 12 '15 at 12:13

I'm going to assume you mean hydrogen bond with itself.

The reason that the $\ce{CH_3S^-}$ will not hydrogen bond is because there is no longer a hydrogen which has a significant dipole. When we had just $\ce{CH_3SH}$, the hydrogen attached to the sulfur is theoretically capable of hydrogen bonding$^1$ as thiol groups are known to hydrogen bond. I am not super confident that the hydrogen bond between one $\ce{CH_3SH}$ and another $\ce{CH_3SH}$ would be very strong if it is present at all. It is possible you could see the covalent nature of the hydrogen bond, but because the dipole-dipole interaction is so weak, you would barely find an equilibrium at all, or might even find a maximum in the vibrational potential energy surface. Perhaps o interest is that there are some hydrogen bonds which are known to blue-shift as opposed to the typical red-shift seen in hydrogen bound systems$^2$.

I would point out, however, that $\ce{CH_3S^-}$ will have a significant intermolecular attraction with any other molecule that has a positive dipole. This would be an ion-dipole force which, as far as I understand it, is a purely electrostatic interaction as opposed to hydrogen bonding which has covalent characteristics.

I hope that answer was a sufficient explanation.

$^1$YUAN, K., Liu, Y., Zhu, Y., Zhang, J., & Zhang, J. (2009). Structures and Properties of Halogen Bond and Hydrogen Bond Formed between CH3SH and HOCl. ACTA CHIMICA SINICA, 67(6), 499-506.

$^2$I highly recommend this paper if you are interested in understanding improper hydrogen bonds which would be those hydrogen bonds where you get a maximum in the vibrational potential energy surface which leads to a blue-shift at that equilibrium geometry. Also, it is written as a semi-exhaustive discussion of the topic for both biologists and chemists, so it quite accessible.

Joseph, J., & Jemmis, E. D. (2007). Red-, blue-, or no-shift in hydrogen bonds: a unified explanation. Journal of the American Chemical Society, 129(15), 4620-4632. Available for free here.