Singlet electronic states

Much of the interest in carbene chemistry derives from the complexity induced by the presence of two electronic states, singlet and triplet. In this respect, it is crucial... 

Reaction 8.6 produces atomic oxygen in an excited (lD) electronic state, singlet oxygen. This is important in the troposphere too, as 0( D) reacts with water vapor to produce chemically reactive hydroxyl radicals, -OH ... 

Either type of transition (n,7r and 7t,Jt ) and electronic state (singlet, triplet) may participate in the first stage of this reaction, which is rationalized by invoking diradical intermediates ... 

Afree radical (with just one unpaired electron) is described as an electronic doublet because, in an external magnetic field, the electron can only exist in one of two possible spin states ( up or down ). By contrast, a pair or radicals, or a biradical (a species with two unpaired electrons in the same molecule) can exist in either of two electronic states singlet or triplet. In the singlet state the electrons are paired (opposite spin) a singlet radical pair (or biradical) is thus diamagnetic (W5 = I -1 = 0) and not observable by EPR. The radical pair above is shown in the singlet state. 

The luminescence properties of rare-earth enolate complexes are largely dependent on the position of the excited electronic states (singlet and triplet) of the ligands. The experimental S states are usually obtained from the electronic absorption spectra of the metal complex in solution. On the other hand, the phosphorescence spectra of solid state Gd + complexes, obtained from the broad emission bands assigned to T Sq transitions, give information about the triplet state position of the enolate ligands (Figure 2). 

Free carbenes exist in two different electronic states, singlet and triplet,4 which are represented in structures 4 and 5. The singlet state has one lone electron pair, whereas the triplet state has two unpaired electrons. 

The situation in singlet A electronic states of triatomic molecules with linear equilibrium geometry is presented in Figme 2. This vibronic structure can be interpreted in a completely analogous way as above for n species. Note that in A electronic states there is a single unique level for K =, but for each other K 0 series there are two levels with a unique character. 

This dialog box also contains the option for specifying that the molecular system ism the first excited singlet stale (Next lowest or the Lowest electronic state. 

An example of an El forbidden but "vibronically allowed" transition is provided by the singlet n ==> ti transition of H2CO that was discussed earlier in this section. As detailed there, the ground electronic state has Ai symmetry, and the n ==> 71 state is of 1A2 symmetry, so the El transition integral... 

Quantum mechanics calculations use either of two forms of the wave function Restricted Hartree-Fock (RHF) or Unrestricted Hartree-Fock (UHF). Use the RHF wave function for singlet electronic states, such as the ground states of stable organic molecules. 

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