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?
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2$\begingroup$ 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)? $\endgroup$– Ivan NeretinNov 12, 2015 at 7:26
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$\begingroup$ @ATP You should edit question - right now it's nonsense $\endgroup$– MithoronNov 12, 2015 at 12:13
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1 Answer
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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.
