Singlet states identification

In the light of experimental difficulties associated with the identification of intermediates, a MINDO/3 quantum-mechanical study of the singlet state ( a2) of the cyclobutylidene to methylenecyclopropane rearrangement has been carried out. It has been proposed that the whole process is initiated by electrophilic attack from the C3 methylene group of cyclobutylidene at the empty p atomic orbital on the Cl carbene site, so that a shift of electron density towards Cl can take place to give the bicyclobutane-like nonclassical carbene intermediate 4. Finally, the bicyclobutane intermediate 4 undergoes a symmetry-allowed conrotatory bond-fission process to generate methylenecyclopropane. The activation enthalpy calculated for the two steps is 8 kcal mol-1.2... 

It is now necessary to describe in greater detail the energy states of the two singlet levels of molecular oxygen which are the subject of this review. Certain other levels of molecular oxygen are of importance in the discussions which follow, and it seems appropriate to mention them in this section. Optical transitions between O Aj) or 02(1S9+) and various other states of molecular oxygen afford one of the most reliable methods for the identification of the singlet states, and are considered in Section II-B. 

Rosenfeld et al. (144) have recorded the absorption spectrum of the fluorescent states of retinol, retinyl acetate, and retinyl-n-butylamine, using pulsed laser photolysis. Theoretical calculations (145) have closely reproudced the observed 435-nm band assuming that emission originates from the lowest - Ag state. Unfortunately, these results are not discriminative as far as identification of the lowest singlet state is concerned, since a strong absorption in the same region is also predicted for the 1B+ state (145). 

For open shells, the identification of the Fock-matrix elements is more difficult. For high-spin states, it is straightforward to identify the elements that correspond to the nonredundant rotations since these rotations mix orbitals of different occupations. The identification of the elements that correspond to redundant rotations is less straightforward, hi some cases, such as for the two-electron singlet state, an unambiguous identification cannot be made since rotations between singly occupied orbitals are not redundant. 

UV-vis spectra of matrix-isolated intermediates are not so informative as matrix IR spectra. As a rule, an assignment of the UV spectrum to any intermediate follows after the identification of the latter by IR or esr spectroscopy. However, UV-vis spectra may sometimes be especially useful. It is well known, for example, that the energy of electronic transitions in singlet ground-state carbenes differs from that of the triplet species. In this way UV spectroscopy allows one to identify the ground state of the intermediate stabilized in the matrix in particular cases. This will be exemplified below. 

Often, the observed species may undergo rearrangement either thermally or photochemically. The end products of such isomerizations tend to have singlet ground states, rendering IR spectroscopy the most powerful tool for elucidating these processes. If the end product is the result of a thermal reaction, then due to the low temperature, not many pathways are available. This facilitates identification and also allows for the possibility of carrying out kinetic measurements. 

The emitting species was found to be the singlet-excited state of 3-aminophthlate ion in both protic and aprotic solvents. This identification was made based on the equivalence of the chemiluminescence spectrum of luminol and the fluorescence spectrum of 3-AP ion . In different reaction media, slightly different maximum chemiluminescence wavelengths are observed (Table 2). The spectral shift observed when the system changes from aqueous media to DMSO or other aprotic solvents can be ascribed to a quinoidal form of 3-aminophthalate (26) formed in aprotic solvents (Scheme 15). ... 

All fluorocarbenes are ground state singlets. For laboratory use there are some precursors which thermally generate difluorocarbene.42 Its identification is usually made by a subsequent chemical insertion reaction. A few industrially important processes proceed via difluorocarbene. The thermal pyrolysis of chlorodifluoromethane (CHF2C1) for the production of tetrafluoroethene and hexafluoropropene gives the intermediate CF2 which dimerizes to the alkene. 

Photophysics in the condensed phase is much more involved than in solutions. In fact, there is much debate on the subject and the nature of the primary excitations are not easily assigned. Due to new mechanisms not yet elucidated or identified, singlet, triplet and charged states are all possible candidates for observation at short times, and there are experimental claims and evidence for each of them [97-103]. Defects and impurities may mask the identification of molecular states, which can also be substantially modified by intermolecular interactions [61,104,105]. 

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