The singlet state

FIGURE 22. Optimized structures of disilene at MP2/6-31G (a) and CASSCF/6-31G (b). Bond lengths are in A, bond angles in deg. Data from References 195 and 202, respectively. 

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An individual radical from the RP may encounter a radical from a different RP to fomi what are known as random RPs or F pairs. F pairs which happen to be in the singlet state have a high probability of recombining, so the remaining F pairs will be in the triplet state. Consequently, the initial condition for F pairs is the triplet state in nearly all cases. 

A.rene-Olefin Photocydization Organic Photochemistry 1989, 10,357 - the photochemistry of benzene is dominated by the singlet state... 

To understand how the three triplet states have the same energy and why the singlet state has a different energy, and an energy different than the Ms= 0 triplet even though these two states are eomposed of the same two determinants, we proeeed as follows ... 

Notiee that all three triplet states involve atomie orbital oeeupaney in whieh one eleetron is on one atom while the other is on the seeond earbon atom. In eontrast, the singlet state plaees both eleetrons on one earbon (it eontains two terms one with the two eleetrons on the left earbon and the other with both eleetrons on the right earbon). 

The singlet state whieh must be eonstmeted orthogonal to the three singlet states (and in partieular to the 3S(S=l,Ms=0) state) ean be seen to be ... 

Ab initio molecular orbital calculations for the model systems RCN3S2 (R = H, NH2) show that these dithiatriazines are predicted to be ground state singlets with low-lying triplet excited states (Section 4.4). The singlet state is stabilized by a Jahn-Teller distortion from C2v to Cj symmetry. In this context the observed dimerization of these antiaromatic (eight r-electron) systems is readily understood. 

Run a stability calculation on the true (triplet) ground state of molecular oxygen. What is the energy difference between the ground state and the singlet state ... 

The blue colour of oxygen in the liquid and solid phases is due to electronic transitions by which molecules in the triplet ground state are excited to the singlet states. These transitions are normally forbidden in pure gaseous oxygen and, in any case, they occur in the infrared region of the spectrum at 7918 cm" ( Ag) and 13 195 cm" ( ]+). However, in the condensed phases a... 

The first two arrangements are singlet states (all electrons are paired), while the last is a triplet state (with two unpaired electrons). Experimentally, the ground state of methylene is a triplet, although much of methylene s chemistry (and that of substituted methylenes) is due to the singlet state. 

The photolysis of dimethyl sulphoxide (at 253.7 nm) in a wide range of solvents has been studied in detail176. Three primary reactions occur, namely (i) fragmentation into methyl radicals and methanesulphinyl radicals, equation (60), (ii) disproportionation into dimethyl sulphone and dimethyl sulphide, equation (61) and (iii) deactivation of the excited state to ground state dimethyl sulphoxide. All chemical processes occur through the singlet state. Further chemical reactions of the initial photochemical products produce species that have been oxidized relative to dimethyl sulphoxide. 

Postulate (i) follows from the fact that when two radicals, produced by whatever means, encounter each other, the interaction of the electron spin of one radical with that on the other radical can give rise to two mutually exclusive spin states, triplet and singlet. Random combination of the two possible electron spin states for the two electrons yields the three components of the triplet state, represented as T+i, To, and T i, and the singlet state, S. Throughout this article, S is assumed to be the singlet state of lowest energy. 

Since two electrons with symmetric space wavefunctions and antisymmetric space wavefunctions represent singlet and triplet states respectively, then obviously the triplet state (E ) is of lower energy than the singlet state E+) by an amount Had an attractive force... 

It should be noted that the above conclusions have been reached on strictly electrostatic grounds a spin property has not been invoked for the two electrons. From the variation of i/i along the box it can be shown that the singlet state is of higher energy than the triplet because the two electrons are more crowded together for (S-state) than for (T-state). Thus there is less interelectronic repulsion m the T-state. The quantity 2J j. is a measure of the effect of electron correlation which reduces the repulsive force between the two electron (Fermi correlation energy). 

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