<P> In organic chemistry, Carnelley's Rule, established in 1882 by Thomas Carnelley, states that high molecular symmetry is associated with high melting point . Carnelley based his rule on examination of 15,000 chemical compounds . For example, for three structural isomers with molecular formula C H the melting point increases in the series isopentane − 160 ° C (113 K) n - pentane − 129.8 ° C (143 K) and neopentane − 16.4 ° C (256.8 K). Likewise in xylenes and also dichlorobenzenes the melting point increases in the order meta, ortho and then para . Pyridine has a lower symmetry than benzene hence its lower melting point but the melting point again increases with diazine and triazines . Many cage - like compounds like adamantane and cubane with high symmetry have relatively high melting points . </P> <P> A high melting point results from a high heat of fusion, a low entropy of fusion, or a combination of both . In highly symmetrical molecules the crystal phase is densely packed with many efficient intermolecular interactions resulting in a higher enthalpy change on melting . </P> <P> An attempt to predict the bulk melting point of crystalline materials was first made in 1910 by Frederick Lindemann . The idea behind the theory was the observation that the average amplitude of thermal vibrations increases with increasing temperature . Melting initiates when the amplitude of vibration becomes large enough for adjacent atoms to partly occupy the same space . The Lindemann criterion states that melting is expected when the vibration root mean square amplitude exceeds a threshold value . </P> <P> Assuming that all atoms in a crystal vibrate with the same frequency ν, the average thermal energy can be estimated using the equipartition theorem as </P>

The melting point and freezing point of water