∞ generated and posted on 2016.01.01 ∞
Attraction between stable partial charges as associated with polar substances.
That is, partial positives are attracted to partial negatives versus attraction between full charges as seen with ionic bonds (or with salt bridges). Hydrogen bonds particularly are between hydrogen atoms carrying partial positive charges and other atoms such as oxygen carrying partial negative chargers.
Note that "stable" is rather than "transient", with the latter seen instead with van der Waals interactions. Partial positive charges are associated particularly in hydrogen atoms that are bound to more electronegative atoms, again such as oxygen but also nitrogen. Partial negative charges are associated with oxygen, as noted, but also nitrogen atoms. Note that these are just one example of a variety of interactions between molecules/atoms that can be described as weak bonds (or weak chemical bonds).
Hydrogen bonds can be between molecules as well as within molecules. Between molecules includes in terms of the base pairing seen between the two strands making up the double helix of DNA. Within molecules is seen particularly within polypeptides making up proteins but also in terms of the complex secondary structure seen with RNA molecules such as tRNA and rRNA.
Hydrogen bonds and hydrogen bonding are responsible for the remarkable properties of water including that it is a liquid (rather than gas) at room temperature, that polar substances so readily dissolve in water, capillary action, hydrophobic exclusion, etc. Here the hydrogen bond forms between hydrogen atoms and oxygen atoms, i.e.,
H – O – H ⋯ O – H,
where the midline horizontal ellipsis is indicative of the hydrogen bond and the en dashes of polar covalent bonds.