Singlet, planar Subject

The ultraviolet spectroscopy of formaldehyde has been studied almost exhaustively, and there is an excellent review on this subject (171). A majority of the bands in the electric-dipole-forbidden vibronically allowed A 2 +X Aj transition have been assigned mostly due to the work of Brand (37), Robinson and DiGiorgio (196), Callomon and Innes (44), and Job, et al. (124). As briefly mentioned earlier, the ground electronic state (X) is planar and the first excited singlet state (A) is pyramidal. It is valid to use the C2V point group symmetry for both electronic states, rather than the C2 point group symmetry (see ref. 171), although the emission could certainly be treated as a -A" - 1A transition. 

These calculations show that, if vinyl alcohol and vinyl thiol indeed mainly exist in planar syn conformations, as previously stated, the situation can be very different when halogen substituents are present. In some cases, the anti conformation is more or less out-of-plane staggered and can become more stable than the syn one. The general trends of substituent effects can be discussed by molecular orbitals analysis and electrostatic interactions. The results are subject to experimental confirmation, at least in the case of enols since their conformers can be prepared by the action of singlet oxygen atom on substituted ethylen, and trapped in low temperature matrices. 

The singlet-triplet splitting of TMM has been the subject of numerous studies [19,55,58,59,67-73]. TMM has five relevant quasi-degenerate spectroscopic states, Aj, B2, A ( B2), Bi and Bi. The three lowest energy states are A ( B2), Ai and Bi. The experimentally measured singlet-triplet splitting for TMM is between the Aj singlet state and the Aj triplet state. The Ai state is C2V and planar, and is best described by two-determinant electronic... 

The predominant photoreaction of the [Pt(C2H4)Cl3] anion is loss of ethylene, which is accompanied by a little loss of m-chloride. This behaviour contrasts with the corresponding thermal reaction, where the predominant reaction is loss of the chloride trans to the ethylene. This contrast between thermal and photochemical behaviour is novel most photochemical reactions of platinum(ii) complexes follow the same course as the corresponding thermal reactions, but occur more quickly. The photoaquation of the ethylene ligand is subject to sensitization by acetone or by acetophenone, and therefore probably occurs via the first excited singlet state. A general model for photosubstitution reactions has been described, which is relevant to octahedral as well as to square-planar complexes. ... 

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