Intramolecular hydrogen bonding (HB) is a complex phenomenon
that
extends beyond a simple valence event, affecting the core electrons
of a molecule. Salicylic acid (SA) and its conformers provide an excellent
model compound for studying intramolecular HB as the proton donor
(H) and acceptor (O) can be toggled by rotating the C–O and
C–C bonds to form up to seven potential conformers through
various HB. In this study, we computationally investigated intramolecular
interactions in SA conformers with and without such HB, by examining
their calculated O 1s core electron-binding energy (CEBE) and 1H NMR chemical shifts validated using recent measurements.
The quantum mechanically stable SA conformers are fully defined by
three rotatable bonds in the compound, which are abstracted as SA(η1η2η3) digital structures,
where ηi = 0 if the ηi angles match
the most stable SA conformer (000) and ηi = 1 otherwise. Our findings suggest that the stability is
dominated by the appearance of the intergroup intramolecular HB of
Hp···O (where O is in the carboxylic acid
functional group and Hp is the phenolic proton in −OHp), and η3 serves as a switch of such HB.
As a result, the (η1η20) SA conformers
containing such Hp···O HB are more stable
than other SA conformers (η1η21)
without such the Hp···O HB. The present
density functional theory calculations reveal that this Hp···O HB results in splitting of the O 1s CEBEs of
two hydroxyl groups (−OH) by up to 1 eV and deshielding the
Hp proton 1H NMR (δHp) up to
11.68 ppm for the (η1η20) conformers.
Without such Hp···O HB, the O 1s XPS binding
energies of two −OH groups will be closely located in the same
band, and the 1H NMR chemical shift of the Hp atom will be as small as an 4.09 ppm SA conformer [SA-G(101)]. The
present study indicates that the O 1s CEBE splitting between two −OH
groups serves as an indicator of the presence of the Hp···O HB in SA conformers, which is also supported
by the 1H NMR results