Carbon, neutral ground state

It is easy to see that an immediate result of this theorem is that an odd-alternant system, which must have an odd number of MOs, must have a nonbonding E = a) MO that is not paired with another MO. It is also possible to show that the electron density is unity at every carbon for the neutral ground state of an alternant system. The proofs of the pairing theorem and some of its consequences are given in Appendix 5. 

The vertical IPs of CO deserve special attention because carbon monoxide is a reference compound for the application of photoelectron spectroscopy (PES) to the study of adsorption of gases on metallic surfaces. Hence, the IP of free CO is well-known and has been very accurately measured [62]. A number of very efficient theoretical methods specially devoted to the calculation of ionization energies can be found in the literature. Most of these are related to the so-called random phase approximation (RPA) [63]. The most common formulations result in the equation-of-motion coupled-cluster (EOM-CC) equations [59] and the one-particle Green s function equations [64,65] or similar formalisms [65,66]. These are powerful ways of dealing with IP calculations because the ionization energies are directly obtained as roots of the equations, and the repolarization or relaxation of the MOs upon ionization is implicitly taken into account [59]. In the present work we remain close to the Cl procedures so that a separate calculation is required for each state of the cation and of the ground state of the neutral to obtain the IP values. 

Spectroscopic studies with photo-CIDNP techniques revealed the existence of two distinct radical cations generated from hexamethyldewarben-zene, presumably rapidly interconverting. In one of these, the central carbon—carbon bond is significantly stretched and bears the unpaired spin density. In the second, the spin density is confined to one of the olefinic bonds. This example is the first to show conclusively that two different radical ion structures can correspond to a single minimum on the ground-state surface of the neutral (Roth et al., 1984). 

This is generally possible for neutral molecules in their ground states for an unsaturated carbon, one has nc = 1 for doubly-bonded nitrogen as in pyridine = 6, = 1, and for a simply-bonded one as in pyrrole... 

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