Carbon, excited state

Figure 2.8 Electronic configuration excited-state carbon atom...

A second problem is more difficult to resolve If excited-state carbon uses two kinds of orbitals for bonding, 2s and 2p, how can it form four equivalent bonds Furthermore, if the three 2p orbitals in carbon are at angles of 90° to one another, and if the 2s orbital has no directionality, how can carbon form bonds with angles of 109.5° directed to the corners of a regular tetrahedron The answers to these questions were provided in 1931 by Linus Pauling, who introduced the idea of hybrid orbitals. 

Pauling showed that the quantum mechanical wave functions for s and p atomic orbitals derived from the Schrodinger wave equation (Section 5.7) can be mathematically combined to form a new set of equivalent wave functions called hybrid atomic orbitals. When one s orbital combines with three p orbitals, as occurs in an excited-state carbon atom, four equivalent hybrid orbitals, called sp3 hybrids, result. (The superscript 3 in the name sp3 tells how many p atomic orbitals are combined to construct the hybrid orbitals, not how many electrons occupy each orbital.)... 

The orffto-nitrobenzyl chromophore has also been employed for phosphate deprotection but, as in Eq. (12) for amides, this photochemistry again does not result from excited state carbon-oxygen bond cleavage [126,128]. Similarly, de-syl phosphates like 89 release phosphate photochemically but, as discussed in Sec. IV.D.3, by a pathway beginning with n,n excited state of the carbonyl group [13,126,129]. 

Similar to the fullerene ground state the singlet and triplet excited state properties of the carbon network are best discussed with respect to the tliree-dimensional symmetry. SurjDrisingly, the singlet excited state gives rise to a low emission fluorescence quantum yield of 1.0 x 10 [143]. Despite the highly constrained carbon network,... 

The meso carbon atom should present a carbenium structure with a low TT electron density in the ground state, in the excited state this carbon possesses the carbeniate structure (C ) with a high tt electron density (119). An electron-donating group in such a position should stabilize the ground state and rise the excited state to the highest level hypsochromic shift results as a whole. 

Subsequent studies (63,64) suggested that the nature of the chemical activation process was a one-electron oxidation of the fluorescer by (27) followed by decomposition of the dioxetanedione radical anion to a carbon dioxide radical anion. Back electron transfer to the radical cation of the fluorescer produced the excited state which emitted the luminescence characteristic of the fluorescent state of the emitter. The chemical activation mechanism was patterned after the CIEEL mechanism proposed for dioxetanones and dioxetanes discussed earher (65). Additional support for the CIEEL mechanism, was furnished by demonstration (66) that a linear correlation existed between the singlet excitation energy of the fluorescer and the chemiluminescence intensity which had been shown earher with dimethyl dioxetanone (67). 

In the lowest optieally excited state of the molecule, we have one eleetron (ti ) and one hole (/i ), each with spin 1/2 which couple through the Coulomb interaetion and can either form a singlet 5 state (5 = 0), or a triplet T state (S = 1). Since the electric dipole matrix element for optical transitions — ep A)/(me) does not depend on spin, there is a strong spin seleetion rule (AS = 0) for optical electric dipole transitions. This strong spin seleetion rule arises from the very weak spin-orbit interaction for carbon. Thus, to turn on electric dipole transitions, appropriate odd-parity vibrational modes must be admixed with the initial and (or) final electronic states, so that the w eak absorption below 2.5 eV involves optical transitions between appropriate vibronic levels. These vibronic levels are energetically favored by virtue... 

The isomerization of alkenes is believed to take place via an excited state in which the two sp carbons are twisted 90° with respect to one another. This state is referred to as the p (perpendicular) state. This geometry is believed to be the minimum-energy geometry for both the singlet and triplet excited states. 

An alternative description of the singlet excited state is a cyclopropylmethyl singlet diradical. Only one of the terminal carbons would be free to rotate in such a structure. 

These reactions are believed to proceed through a complex of the alkene with a singlet excited state of the aromatic compound (an exciplex). The alkene and aromatic ring are presumed to be oriented in such a manner that the alkene n system reacts with p orbitals on 1,3-carbons of the aromatic. The structure of the excited-state species has been probed in more detail using CAS-SCF ab initio calculations. ... 

This difference is due to the two lone pairs on the oxygen. Of the six valence electrons on the oxygen atom, two are involved in the double bond with the carbon, and the other four exist as two lone pairs. In Chapter 4, we ll examine the IR spectra for these two molecules. The orbitals suggest that we ll find very different frequencies for the two systems. In Chapter 9, we ll look at the transition to the first excited state in formaldehyde. ... 

The most significant treatment of excited states within the CNDO approach is that of Del Bene and Jaffe, who made three modifications to the original CNDO parameterization scheme. Two of the modifications were just minor tinkering with the integral evaluation, and need not concern us. The key point in their method was the treatment of the p parameters. Think of a pair of bonded carbon atoms in a large molecule. Some of the p-type basis functions on Ca will be aligned to those on Cb in a type interaction was reduced. They wrote... 

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