  Electronic quantum coherence is a prerequisite for charge migration in
molecules and emerging molecular quantum technologies. It also underlies new
forms of spectroscopy and can enhance molecular function. Whether conical
intersections can create electronic coherence and for how long electronic
coherence can survive in aqueous environments has remained elusive and
controversial, respectively. Here, we use X-ray spectroscopy to realize a
breakthrough on both of these topics. We find that electronic relaxation from
the $^1$B$_{2\rm{u}}$($\pi\pi^*$) state of pyrazine through conical
intersections (CIs) induces previously unobserved electronic and vibrational
quantum coherences between the $^1$B$_{3\rm{u}}$(n$\pi^*$) and
$^1$A$_{\rm{u}}$(n$\pi^*$) states in the gas phase, which correspond to an
electronic ring current. These coherences are entirely suppressed when pyrazine
is dissolved in water. These observations, supported by the latest advances in
multiconfigurational electronic-structure calculations and non-adiabatic
dynamics simulations, confirm that CIs can create electronic coherences and
that aqueous solvation can decohere them in less than 40 fs. This study opens
the door to the investigation of the broad class of electronic coherences
created during light-induced molecular dynamics and to quantify their
susceptibility to aqueous solvation.
