The difference of the structures of the two isomers are shown in the picture. Generally, cyclic alkanes are much easier to break than straight-chained alkanes. When the molecules are cyclic, they are close to each other, thus lesser bond angles. Because they are closer, repulsion forces could be greater than attractive forces. That is why cyclohexane needs lesser energy of 936 kcal/mol compared to 941 kcal/mol because there is an additional straight-chained methyl substituent to break in methylcyclopentane.
