Melting point variation with molecule

As with alkanes, the boiling points and melting points of alkenes decrease with increasing molecular weight, but show some variations that depend on the shape of the molecule. Alkenes with the same molecular formula are isomers of one another if the position and the stereochemistry of the double bond differ. For example, there are four different acyclic structures that can be drawn for butene (C4H8). They have different b.p. and m.p. as follows. 

Complex schemes of polymorphism have been uncovered for TNDBN [47], DNFP [52], TNPAN [28], DPT [47] and OHMX [49]. The asymmetric unit of one of the polymorphs of TNDBN and also DNNC [98] contains two molecules having different conformations. This is a variation on the notion of conformational polymorphism which is well known with HMX. The polymorphism of TNPAN causes the compounds to have two different melting points [28]. Despite a careful look at many compounds, no clues were ever revealed about... 

---