Quantum chemistry calculations were performed to determine the effects of different lactone structures on abstraction reaction pathways. Previous studies are limited to 4-5-L ] and 4-4-L . Figure shows some of the results for five lactones, including three cases with different ring sizes (4-4-L, 5-5-L, 6-6-L; left column) and two of their methyl-substituted analogues (4-5-L, 5-6-L; middle column). In each case, the initial reaction considered is abstraction of a hydrogen atom from the lactone by a H radical produced elsewhere in the flame. The products are H2 and various lactone radicals (designated 44a, 44b, 55a, etc.), depending on which hydrogen atom is abstracted. The lactone radicals then undergo ring-opening unimolecular reactions to various linear intermediates (designated A, B, C, etc.). The red numbers in Fig. are the barriers to the ring-opening reactions. They vary significantly depending on the structure of the initial lactone. For the unsubstituted lactones (left column), the barriers systematically decrease as the ring becomes larger, from 30.1 and 30.5 kcal/mol for 4-4-L to 19.0 and 25.2 kcal/mol for 6-6-L. There is also a change in the order of the ring-opening channels. For 4-4-L the lowest barrier leads to the carbonyl intermediate B, but for 5-5-L and especially 6-6-L the lowest barriers lead to the carboxylic acid intermediates C and E. In the methyl-substituted lactones (middle column), the magnitude of the barrier decreases only very slightly as the ring becomes larger. However, the range of barriers narrows from 4.5 kcal/mol for 4-5-L to 1.1 kcal/mol for 5-6-L.