Molecular-orbitals singlet 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. 

From the orbital correlation diagram derived by Bryce-Smith [38], it was deduced that the ortho cycloaddition is forbidden from the lowest excited singlet state of benzene and the ground state of ethene. Van der Hart et al. [189] have constructed molecular orbital and state correlation diagrams for the ortho photocycloaddition of benzene to ethene. The molecular orbital correlation diagram differs from that given by Bryce-Smith, because natural correlations have been used. From a topological point of view, it seems less desirable to correlate the tt... 

HOMO and LLMO, also known as Frontier orbitals, are important in in tcrprcLitig results of a calculation (see Frontier Molecular Orbitals on page 141). You can use these m olecular orbiLals to comptiLe the lowest excited electronic singlet state of molecules and the ground states of radicals. 

If the mini her of electrons, N, is even, yon can haven dosed shell (as shown ) where the occupied orbitals each contain two electron s. For an odd n nrn her of electron s, at least on e orbital rn ust be singly occupied. In the example, three orbitals are occupied by-electron s and two orbitals arc nn occupied. Th e h ighest occupied nioleciilar orbital (HOMO is t[r), and the lowest unoccupied molecular orbital (LUMO) is The example above is a singlet, a state oh total spin S=0. Exciting one electron from the HOMO to the LUMO orbital would give one ol the I ollowing excited states ... 

The UHE wave function can also apply to singlet molecules. Usually, the results are the same as for the faster RHEmethod. That is, electrons prefer to pair, with an alpha electron sharing a molecular space orbital with a beta electron. Use the UHE method for singlet states only to avoid potential energy discontinuities when a covalent bond is broken and electrons can unpair (see Bond Breaking on page 46). 

Discuss, briefly, the valence molecular orbitals of AIH2 and the shape of the molecule in the ground and first excited singlet states. 

Using these molecular states, the weak absorption observed between 490 and 640 nm for Cbo in solution (Fig. 6) [67] is assigned to transitions between the singlet ground state So and the lowest excited singlet state 5i (associated with the tiu orbital and activated by vibronic coupling). 

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)... 

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... 

Fig. 13 Qualitative molecular orbital energy level diagram of the dimer d orbitals with the 14 electrons showing the electronic configuration 525 27i47i 4o2 (singlet) and the first excited state 828 2it4it 4a1CT 1 (triplet)...

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