Why does a molecule with a Thiol group form Hydrogen bonds? I'm doing the GRE Biology practice test, and question #1 asks:



The answer is E. I understand why II and IV are polar, but why is $\ce{R-S-H}$ polar? R is a side chain to the sulfur, sulfur's valence shell is full, and sulfur is highly electronegative. But without knowing what $\ce{R}$ is, how do we know the molecule's total polarity?
Actually it's all four.  See here for I.  The article describes a hydrogen bonding interaction between methane and water.  It's weaker than the "usual" hydrogen bonds but still contributes to forming methane-water clathrates.

The larger lesson here is that hydrogen bonding is not just the electrostatic interaction often described in textbooks.  It has a molecular orbital component that works even with primary bonds of low polarity.