Direct Spectroscopic Evidence for a Hitherto Elusive 'Zwitterionic' Excited State

摘要

Direct experimental support for a theoretical concept in science is sometimes obtained in an area for which it was not originally proposed. The case we describe here is the simplest example of chemical bonding, the interaction of two electrons in weakly coupled orbitals on two atomic centers. As early as 1932 Mulliken analyzed the situation in the 7t-orbital domain of twisted ethylene and a few years later the (related) electronic structure of the a domain in "stretched" dihydrogen (i.e. an H, molecule in which the interatomic distance is increased well beyond its equilibrium value). In both cases four electronic states are specified (Fig. 1): two low-energy "diradical" states (singlet and triplet) with the electrons in separate orbitals. and two higher energy "zwitterionic" singlet states. The latter are derived from the symmetric and antisymmetric linear combination of wave functions representing the pairing of the two electrons in the two orbitals. It should be noted that these zwitterionic states do not have a net dipole moment in homosymmetric molecules. The fact that one of the states would represent the ground state in a real zwitterion is the reason for this potentially confusing terminology. However, in a polar environment (solution) these states may be subject to "sudden polarization" and display zwitterionic reactivity.