Water self-association dominates the formation of microsolvated molecular clusters which may give rise to complex structures resembling those of pure water clusters. We present a rotational study of the complex formamide-(HO) formed in a supersonic jet and several monosubstituted isotopologues. Formamide and water molecules form a four-body sequential cycle through N-H···O, O-H···O, and O-H···O═C hydrogen bonds, resulting in a chiral structure with a nonplanar skeleton that can be overlapped to that of water pentamer. The analysis of the N-nucleus quadrupole coupling effects shows the depletion of the electron density of the N atom lone pair with respect to the bare formamide that affects the amide group C-N and C═O distances. The study of the observed tunneling doublets shows that formamide-(HO) follows a path to invert its structure driven by the flipping of water subunits and passing through successive nonplanar configurations, a motion reminiscent of the pseudorotation of water pentamer.
Conformational flexibility is intrinsically related to the functionality of biomolecules. Elucidation of the potential energy surface is thus a necessary step towards understanding the mechanisms for molecular recognition such as docking of small organic molecules to larger macromolecular systems. In this work, we use broadband rotational spectroscopy in a molecular jet experiment to unravel the complex conformational space of citronellal. We observe fifteen conformations in the experimental conditions of the molecular jet, the highest number of conformers reported to date for a chiral molecule of this size using microwave spectroscopy. Studies of relative stability using different carrier gases in the supersonic expansion reveal conformational relaxation pathways that strongly favour ground-state structures with globular conformations. This study provides a blueprint of the complex conformational space of an important biosynthetic precursor and gives insights on the relation between its structure and biological functionality.
We
used jet-cooled broadband rotational spectroscopy to explore
the balance between π-stacking and hydrogen-bonding interactions
in the self-aggregation of thiophenol. Two different isomers were
detected for the thiophenol dimer, revealing dispersion-controlled
π-stacked structures anchored by a long S–H···S
sulfur hydrogen bond. The weak intermolecular forces allow for noticeable
internal dynamics in the dimers, as tunneling splittings are observed
for the global minimum. The large-amplitude motion is ascribed to
a concerted inversion motion between the two rings, exchanging the
roles of the proton donor and acceptor in the thiol groups. The determined
torsional barrier of
B
2
= 250.3 cm
–1
is consistent with theoretical predictions (290–502
cm
–1
) and the monomer barrier of 277.1(3) cm
–1
. For the thiophenol trimer, a symmetric top structure
was assigned in the spectrum. The results highlight the relevance
of substituent effects to modulate π-stacking geometries and
the role of the sulfur-centered hydrogen bonds.
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