FT-IR and FT-Raman spectroscopic studies of the mesogens with the general molecular formula 1-(p-n-alkyl phenyl) 3-(p-n-alkyloxyphenyl) propane 1,3-diones having alkyl chain lengths C-10, C-11 and C-12 are carried out. An analysis of the vibrational spectra of C-10, C-11 and C-12 at room temperature strongly suggests that while CI, probably belongs to a highly ordered monoclinic or orthorhombic type of lattice, C-10 and C-11 belong to a hexagonal or triclinic unit cell, having lesser degree of lattice order.