The generic structural formula for a 1-alkyl-3-methylimid- azolium cation is where R is a -CH2(CH2)nCH3 alkyl group. The melting points of the salts that form between 1-alkyl-3-methylimidazolium cation and the PF6- anion are as follows: R = CH2CH3 (m.p. = 60 °C), R = CH2CH2CH3 (m.p. = 40 °C), r = CH2CH2CH2CH3 (m.p. = 10 °C), and R = CH2CH2CH2CH2CH2CH3 (m.p. = -61 °C). Why does the melting point decrease as the length of alkyl group increases?