Probing Hydrogen Bond Energies by Mass Spectrometry

Mass spectrometry with desorption
electrospray ionization (DESI)
is demonstrated to be useful for probing the strength of hydrogen
bonding, exemplified by various complexes of benzothiazoles and carboxylic
acids in the solid state. Efficiencies for fragmentation of the complexes,
quantified by collision-induced dissociation (CID) technology, correspond
well with energies of the hydrogen bonds of O–H···N
and N–H···O bridging each pair of benzothiazole
and carboxylic acid. Linear correlations (with correlation factors
of 0.8953 and 0.9928) have been established for the calibration curves
of normalized collision energy at 100% fragmentation rate vs the length
between donor and acceptor (in the hydrogen bond of O–H···N)
as well as the slope of the fragmentation efficiency curve vs the
average length difference between O–H···N and
N–H···O in the complex. The mechanism responsible
for determination of the hydrogen bonds is proposed on the basis of
the experiments starting from the mixtures of the complexes as well
as labeling with deuterium. As a complement of previously available
methods (e.g., X-ray diffraction analysis), expectably, the proposed
mass spectrometric method seems to be versatile for probing hydrogen
bond energies.