= Hydrogen bond donor
{title2=$X-\mathrm H$}
A <hydrogen bond donor> is the $X-\mathrm H$ group supplying the attached <hydrogen> in a <hydrogen bond> $X-\mathrm H\cdots Y$. The donor atom $X$, or the whole <molecule> containing it, may also be called the donor. The terminology does not imply transfer of a free <proton>: in the ordinary <water> pair, the O–H <covalent bond> remains. Familiar donors include O–H groups in <water> and N–H groups in <ammonia>. See https://people.reed.edu/~glasfeld/Chem391/notes/ChIMF_2018.pdf[Andrew Glasfeld, Intermolecular Attraction, pp. 12–13: donor and acceptor roles].
\Image[/hydrogen-bond-donor-acceptor-site-map.png]
{title=<Hydrogen bond donor> and <hydrogen bond acceptor> roles depend on attached <hydrogen>, <protonation>, and <chemical resonance>}
{description=Toy site map for ordinary O–H and N–H associations, not measured geometry or an exhaustive list of <hydrogen bonds>. $R$ and $R'$ denote groups attached through <carbon>. Coloured labels identify possible sites, rather than count the number of simultaneous contacts.}
The attached <hydrogen> is essential to this role. At the <nitrogen> site, an unprotonated <amine> $R_3\mathrm N$ can be a <hydrogen bond acceptor>, but has no N–H <hydrogen bond donor>. <Protonation> gives $R_3\mathrm{NH}^+$: the new N–H group can donate, while that <nitrogen> no longer has the accepting <lone pair>. This is a change of chemical form, not a permanent label attached to a <chemical element>. Here $R$ denotes a group attached through <carbon>; the statement concerns the <nitrogen> site and does not rule out weaker C–H associations elsewhere. See https://openstax.org/books/organic-chemistry/pages/24-3-basicity-of-amines[McMurry, Organic Chemistry, 24.3: amine protonation] and https://people.reed.edu/~glasfeld/Chem391/notes/ChIMF_2018.pdf[Glasfeld: protonated amines as donors].
An <amide> with an N–H <covalent bond> can donate even though its <nitrogen> is ordinarily not an accepting site. For example, in $R-\mathrm C(=\mathrm O)-\mathrm{NH}-R'$, N–H is a <hydrogen bond donor> and the <carbonyl group>'s <oxygen> is a <hydrogen bond acceptor>. Replacing the remaining N–H by another group attached through <carbon> removes this N–H donor without removing the accepting <oxygen>. The analogous division of roles in a <peptide bond> helps explain <protein> structure. See https://people.reed.edu/~glasfeld/Chem391/notes/ChIMF_2018.pdf[Glasfeld: amide roles and biological interactions]. The useful question is therefore which site supplies the <hydrogen>, rather than whether the whole substance is simply “hydrogen bonding”.
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