For comparison, the S-Al 1 distance in Int-3 of 2-methylthiophene is much longer (3.96 Å) and the Mayer bond order is 0.0. Natural bond orbital (NBO) analysis. Second-order perturbation theory analysis of Fock matrix in the NBO basis show that the O-Al 1 interaction in Int-5 of 2-methylfuran is much stronger than the S-Al 1 interaction in Int-3 of 2-methylthiophene. These results hint that the ringexpansion of 2-methylfuran may be favoured, relative 2-methylthiophene, due to stabilisation of the transition state TS-3 (and preceding intermediate) by O-Al 1 coordination.