Lattice Constants of 2-Methyldodecane, 2-Methylpentadecane, 2-Methylheptadecane and 2-Methyleicosane-Urea Adducts

It has been found that adductibilities of monomethylalkanes were different from those of normal alkanes because of a side methyl group of the guest molecules. In this study, precise lattice constants of 2-methyldodecane, 2-methylpentadecane, 2-methylheptadecane and 2-methyleicosane-urea adduct crystals were determined by powder X-ray diffraction method with the intention of comparing with those of normal alkane-urea adduct. The indexing of powder X-ray diffraction patterns of 2-methylalkane adducts showed that they belonged to hexagonal system as well as hexadecane adduct. The differences between the value of lattice constant co of hexadecane adducts and one of 2-methylalkane adducts were not significant but the values of ao of 2-methylalkane adduct were all larger than that of hexadecane adduct. Lattice constants as of 2-methylalkane adducts became smaller and tended to approach a constant value with the increasing carbon numbers of 2-methylalkane. From these results it was suggested that urea channels of 2-methylalkane adducts were expanded to a axis from that of hexadecane adduct owing to a side methyl group of guest 2-methylalkane molecule. The influence of a side methyl group on expansion of urea channel decreased with increasing of the chain, length of guest molecules. The each expansion was due to the stretching of one kind of N-H...0 bond of urea channel of normal alkane adducts. The latter estimation was supported by comparing the stretching energy of N-H...0 bond with the difference between heat of adduct formation of normal alkane and one of 2-methylalkane.