Molecular-orbitals triplet state

Statement (b) is partially correct - the ground state of O2 is a triplet state but it does not react as an electrophile in Diels-Alder reactions. The triplet ground state is with two unpaired electrons occupying different Jt (lHu) molecular orbitals. The electrophilic O2 molecule is one of the singlet states, the one that has two antiparallel electrons in one n molecular orbital, singlet state. The triplet state reacts as a radical. 

In case (ui) let us assume that Xyz (e.g. SO2 in reaction (3.25)) is formed in the singlet ground state the two electrons are thus placed in a single atomic or molecular orbital. Triplet species are excluded simply because their reactions are unlikely to be stereospecific. Consider, for example, the addition of a triplet carbene to an olefinic C=C bond, reaction (3.26). Because of the conservation of spin multiplicity an intermediate (triplet state) diradical is formed. 

The half-electron method can also apply to triplet states. For this calciilatioit. IlyperCheni populates selected molecular orbitals with pairs of half electron s. The final energy is computed by assigning the proper spins. 

The triplet excited state of H2 is obtained by promoting an electron to a higher-energy molecular orbital. This higher-energy (antibonding) orbital is written and can be considered to arise from two Is orbitals as follows ... 

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. 

Examine the highest-occupied molecular orbital (HOMO) of singlet methylene. Where is the pair of electrons, inplane or perpendicular to the plane Next, examine the electrostatic potential map. Where is the molecule most electron rich, in the o or the 7t system Where is the most electron poor Next, display the corresponding map for triplet methylene. Which molecule would you expect to be the better nucleophile The better electrophile Explain. Experimentally, one state of methylene shows both electrophilic and nucleophilic chemistry, while the other state exhibits chemistry typical of radicals. Which state does which Elaborate. 

The lowest excited states in molecular crystals are singlet and triplet excitons [3]. Since it costs coulombic energy to transfer an electron that has been excited optically from the HOMO (highest occupied molecular orbital) to the LUMC)... 

Hall, G. G., Proc. Roy. Soc. [London) A213, 113, The molecular orbital theory of chemical valency. XI. Bond energies, resonance energies and triplet state energies. ... 

Methoxy radical 210 Methyl-de-diazoniation 253 Methylene, energy of triplet state 178 Molecular orbital method, applications to ArNj structure and to dediazoniations - ab initio 84ff., 177ff., 270, 280... 

In this case the excited molecules produced on interaction with radiation undergo spin reversal to yield a triplet state with a much longer lifetime than that of the singlet excited state. One or more jt-bonds are broken in the triplet state since one of the n-electrons affected is in an antibonding n molecular orbital. This means that the o-bond is free to rotate and cis and trans isomers can be formed next to each other on recombination of the double bond. 

Fig. 1 A schematic illustration of the in-phase and out-of-phase combinations of the atomic orbitals into the bonding and antibonding molecular orbitals, respectively. The dissociation limit of a H molecule corresponds to a pure diradical with degenerate singlet and triplet states...

Molecular orbital calculations on ethylene indicate that the lowest energy excited singlet and triplet states have a twisted geometry.(2) This geometry helps minimize electron-electron repulsion. Figure 9.1 gives the calculated... 

Molecules with two or more unpaired electrons may be divided into two classes by far the most common examples are molecules where the unpaired electrons are contained in a set of degenerate atomic or molecular orbitals with qualitatively similar spatial distributions, e.g., an octahedral Cr(m) (4A2g) or Ni(n) (3A2g) complex, a ground state triplet molecule like 02, or the excited triplet states of naphthalene or benzophenone. 

The model clusters were chosen in accordance with the structure of PANI proposed in Refs. [1, 3], The present model of PANI also takes into account that under the influence of dopants (in this particular case, protons and anions (chlorine ions) which form bonds to PANI nitrogen), the spins of the highest occupied molecular orbital (HOMO) become unpaired, and PANI changes to triplet state. It should be noted that only in this state there is a considerable increase in PANI conductivity. 

---