= Hydrogen bond
{title2=$X-\mathrm H\cdots Y$}
{wiki}
A <hydrogen bond> is an attractive association involving a <hydrogen> attached to another <chemical atom>, and an accepting atom or group. In the IUPAC 2011 definition, the atom $X$ in $X-\mathrm H$ is more <electronegative> than <hydrogen>, and evidence of bond formation is required. The familiar strong examples use O–H or N–H groups, but restricting the concept to only nitrogen, oxygen and fluorine misses weaker cases. A short distance alone is not the entire definition.https://publications.iupac.org/pac/pdf/2011/pdf/8308x1637.pdf[Arunan et al., Definition of the hydrogen bond, IUPAC Recommendations 2011]{ref}
\Image[/water-hydrogen-bond-roles.png]
{title=Two water molecules play different hydrogen-bond roles while retaining their own covalent O–H bonds}
In the depicted <water> pair, one O–H group is the <hydrogen bond donor>; the other <oxygen> supplies the <hydrogen bond acceptor>. The solid O–H line denotes a <covalent bond> within a <water> <molecule>; the dotted H···O line denotes the <hydrogen bond> between them. One <water> <molecule> can play both roles in a larger arrangement because it has O–H groups and <lone pairs>.
This makes <hydrogen bonds> useful for understanding organization beyond a single <molecular formula>. They connect <water> <molecules> and contribute to <protein> folding and recognition between <molecules>. The same types of <chemical atoms> can support different associations depending on which donor and acceptor sites are exposed and how they are oriented. Alberts and colleagues discuss these noncovalent interactions in https://www.ncbi.nlm.nih.gov/books/NBK26883/[Molecular Biology of the Cell, the chemical components of a cell].
<Hydrogen bonds> are often directional, with an approximately straight donor–H–acceptor arrangement favourable in familiar cases; strength and preferred geometry depend on the system. The IUPAC definition uses converging experimental or theoretical evidence rather than one universal distance threshold. The author-hosted https://ipc.iisc.ac.in/~ea/iupac/iupac/IUPAC_finalreport_Nov_2009TG.pdf[2009 task-group draft, §§3.1–3.3] explains the definition and typical criteria leading to the recommendation.
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