Proton hyperfine splitting in aromatic radicals and aromatic negative ions are treated by MO method, and the semi-empirical equation, Q=a
Nρ is examined theoretically to be true. Here a
N is the splitting constant refer to proton N, ρ is the unpaired electron density at the carbon atom adjacent to proton N, and Q is the semi-empirical constant to be same for all CH bond, the value of which is about -22.5 gauss. The essential approximations in this treatment are that the siglettriplet exitation energies ΔE
jo in σ-orbitals are replaced by the average value ΔE
av of them, and all σ-orbitals containing CH bond are transformed into δ-orbitals which are strongly localized to each CH fragment. That is to say, δ-orbital is approximated to be CH bond orbital between Is hydrogen orbital and Sp
2 hybrid carbon orbital, and ΔE
av is to be the siglet-triplet exitation energy in this bond orbital. These approximations are explained briefly to be adequate in aromatic systems. Therefore it is obvious that the proton hyperfine splitting calculations in aromatic systems take place the same results in the hypothetical CH fragment treatment and in the general treatment containing the entire set of aromatic system.
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