All isomers of methylphenanthrene are formed when phenanthrene is reac
ted with methane and/or long chain n-alkanes in the presence of coals
or kaolinite in closed steel reactors in the range 230-360 degrees C.
In particular, the 3-methylphenanthrene isomer, for which a convincing
degradative precursor-controlled source has not been described, becom
es increasingly prominent at higher experimental temperature due to th
e 4-->3 methyl shift promoted by steric interaction at the 4- and 5-po
sitions. Therefore, a geosynthetic origin for the high concentrations
of 3-methylphenanthrene in mature carbonaceous sediments is indicated.
Furthermore, in a broader sense, the data demonstrate the need to con
sider synthetic, in addition to degradative, reactions in maturational
processes in sediments.