Structural changes induced by water play a
pivotal role in chemistry and biology but remain
challenging to predict, measure, and control at molecular
level. Here we explore size-governed gas-phase
water aggregation in the flexible molecule 4-hydroxy-2-
butanone, modeling the conformational adaptability of
flexible substrates to host water scaffolds and the
preference for sequential droplet growth. The experiment
was conducted using broadband rotational spectroscopy,
rationalized with quantum chemical calculations.
Two different isomers were observed experimentally
from the di- to the pentahydrates (4-hydroxy-2-butanone-(
H2O)n=2–5), including the 18O isotopologues for
the di- and trihydrates. Interestingly, to accommodate
water molecules effectively, the heavy atom skeleton of
4-hydroxy-2-butanone reshapes in every observed isomer
and does not correspond to the stable conformer of
the free monomer. All solvates initiate from the alcohol
group (proton donor) but retain the carbonyl group as
secondary binding point. The water scaffolds closely
resemble those found in the pure water clusters,
balancing between the capability of 4-hydroxy-2-butanone
for steering the orientation and position of the
water molecules and the ability of water to modulate the
monomer’s conformation. The present work thus provides
an accurate molecular description on how torsionally
flexible molecules dynamically adapt to water along
progressing solvation.
