Evidence from the gas-phase electronic absorption spectra of catechol borane, phthalan, 1,3-benzodioxole, and coumaran regarding the geometry of the five-membered ring

Abstract

Abstract The A˜-X˜ electronic absorption systems of catechol borane (CB), phthalan (PHTH), 1,3-benzodioxole (BDO), and coumaran (COU) have been investigated with a view to obtaining information regarding the coplanarity, or otherwise, of the skeletal atom in position 2 in the five-membered ring with the other skeletal atoms in the molecule. The evidence which has been considered concerns the regularity, or irregularity, of the sequence in the puckering vibration of the group in position 2 and the possibility of an intensity alternation in any series of rotational fine structure which can be assigned successive values of the prolate symmetric-rotor quantum number Ka. The spectrum of CB shows regular, intense sequences and an intensity alternation in the rotational fine structure which is consistent with 10:6 for Ka even:odd. The evidence is therefore very strong that CB is planar not only in the ground, X˜, but also in the excited, A˜, electronic state. Planarity is consistent with the fact that the five-membered ring is aromatic. The spectrum of PHTH shows an intensity alternation consistent with 13:11 for Ka even:odd. Intense sequences are regular, but two unexpected high-wavenumber sequences cast slight doubt on the conclusion that the skeleton is planar in both electronic states. BDO shows an intensity alternation consistent with 7:9 for Ka even:odd, but the sequence structure is very complex and it is concluded that the skeleton of the molecule is either planar, or nonplanar with a low barrier, in either or both electronic states. The intense sequence structure of COU shows no regularity, and there can be no intensity alternation because of the low symmetry. No conclusions can be drawn regarding planarity. The results for all these molecules contrast with information obtained previously (J. Mol. Spectrosc., 66, 452–464 (1977); 68, 499 (1977)) for indane that it is nonplanar in both electronic states with a barrier above the v = 7 level of the puckering vibration. The nonplanarity of indane is rationalized in terms of resistance to eclipsing of the CH2 groups. In none of the other molecules does the possibility of eclipsing arise.