| dc.description.abstract | The appearance of an intense yellow colour upon addition of crown ethers to TNB solutions indicates the formation of charge?transfer complexes. The association constants are very low, showing that the complexation is weak. An unusual feature is the formation of 1:1 complexes even by ethers containing more than one aromatic centre. Steric hindrance would not prevent the overlap of two acceptor molecules with the phenyl rings of di?substituted ethers to form 1:2 complexes, since the rings are well separated. However, if there is any contribution from n–? interaction, the acceptor molecule would have to project over the cavity, and simultaneous complexation of two molecules would not occur.
The increased stability observed with increasing ring size and number of aromatic centres may arise from two factors. One is the increased probability of association due to the larger number of available donor sites. Increasing ring size also favours molecular overlap, resulting in more stable complexes. It may not be appropriate to compare the K values of benzo?15?crown?5 with those of dibenzo?30?crown?10, since the spectral data suggest that the types of complexes formed by these two ethers are not the same. However, comparison of the K values of dibenzo?18?crown?6 and dibenzo?30?crown?10 shows that increased ring size favours better complexation. Since the donor capacities of these ethers are nearly equal, the increase in stability may arise from entropy factors.
The magnitudes of the ?H values are similar to those obtained for aromatic hydrocarbon donors. Comparison with other complexes indicates that these are predominantly ?–? type complexes.
The unusual positioning of the TNB molecule over benzo?15?crown?5 in the crystal structure suggests that, in this complex, charge transfer may be due to n–? interactions, with charge donation from the lone?pair orbitals of the oxygen atoms to the ??systems of the –NO? group and the benzene ring of TNB. Since the NMR spectral data also support these findings, it can be stated with confidence that benzo?15?crown?5 forms an n–? type complex with TNB. This is one of the few known cases where n–? contribution is significant. The absence of the 980?cm?¹ band in the IR spectrum of the benzo?15?crown?5–TNB complex can be explained by conformational changes in the ether upon complexation. Although the torsional angles do not differ from those of the free ether, the disappearance of this band may be attributed to a decrease in the angle between the plane of the oxygen atoms and the benzene ring.
The plain ring ethers form n–? type complexes, as does benzo?15?crown?5. The other ethers form ?–? type complexes with some contribution from n–? interactions. | |
