Four alkyl diamondoids,
1-pentyladamantane (PA), 2-butyladamantane
(BA), 2-propyladamantane (2-PrA), and 2-ethyladamantane (EA), were
synthesized on a preparative scale, and key fuel properties, including
density, net heat of combustion, low-temperature viscosity, and derived
cetane number (DCN), were measured. The fuel molecules had densities
up to 17% higher than conventional jet fuel and volumetric net heats
of combustion comparable to or exceeding that of the synthetic missile
fuel JP-10. Remarkably, the alkyl diamondoid fuels had DCNs in the
range of 42–49, which should allow for their efficient combustion
in diesel engines. PA, BA, and 2-PrA all had essentially the same
DCN (∼49), while the short alkyl chain in EA lowered the value
to 42.7. Bond dissociation energies for the formation of diradicals
from 2-PrA and PA were calculated with density functional theory methods
to gain a qualitative understanding of potential combustion mechanisms.
Ring opening of the adamantane core during combustion is proposed
to explain the high cetane number of these fuels. The alkyl diamondoids
described in this report are the first examples of multicyclic hydrocarbons
that combine extraordinary densities (>0.9 g/mL) with DCNs comparable
to or exceeding that of conventional diesel fuel.