Origins of threefold rotational barriers of molecule containing two methyl groups: Ethyl propionate as paradigm

Origins of the rotational barriers of TG− form of ethyl propionate molecule have been investigated. The barrier heights, as determined from the Raman spectrum, are estimated to be 2.88 and 3.17 kcal/mol for the –CH3 (I) and –CH3 (II) methyl groups of the molecule respectively. The detail analyses suggest that the combined relaxations of the C2–C3, C2–C4 bond lengths and H10–C3–H11, C2–C4–H13 angles together play a significant role to control the barrier heights of methyl CH3 (I), CH3 (II) groups of the molecule.