  We present the first experimental characterization in molecular fragile
glassformers of a 'prepeak that appears significantly below the main peak of
the static structure factor. The temperature and density dependences of this
prepeak are studied via elastic neutron scattering experiments under high
pressure (up to 300 MPa) in m-toluidine and m-fluoroaniline. The prepeak
intensity increases with decreasing temperature, but it remains constant with
increasing pressure while its position and width stay roughly the same. These
features are opposite to those observed for the 'first sharp diffraction peak'
of network glasses. The origin of the phenomenon is analyzed with the help of
Monte Carlo simulations. We associate the prepeak to hydrogen-bond-induced
heterogeneities (or clusters) whose limited size results from exclusion effect
between benzene rings that prevents the extension of a hydrogen-bond network.
Implications for the dynamics of the liquid close to the glass transition are
finally considered.
