ESR of α-Copper Phthalocyanine. II. Powder Line Shape and MO Calculations

The ESR spectrum of polycrystalline α‐Cu phthalocyanine, reported by us previously, has been analyzed in detail by powder line shape and MO calculations. The superhyperfine (shf) tensors are shown to be essentially isotropic and hence the system can be described by an axial spin Hamiltonian. The low‐field spectrum consists of four well‐defined peaks from which the spin‐Hamiltonian parameters $g_{\Vert }$ and $A$ can be readily determined. The high‐field spectrum is a superposition of four sets of nine‐component lines over‐lapping to result in 15 almost equally spaced peaks. This fact suggests that the hyperfine (hf) constant $B$ is about twice the shf constant $a_N$ . This relation has been used to estimate the values of $\mathrm{g\perp ,\; B}$ , and $a_N$ . Only a slight adjustment of parameters was needed to synthesize a derivative line shape which fitted well to the experimental spectra. For one of the hf components $\mathrm{(M\hspace{0.17em}=\hspace{0.17em}}\frac{3}{2})$ , the resonance field is not a monotonic function of $\theta$ , the angle between molecular axis and the magnetic field. The effect of this anomaly on the spectrum, particularly the intensity ratio of the shf lines is discussed. The final set of parameters ${\mathrm{(g}}_{\Vert }\text{\hspace{0.17em}=\hspace{0.17em}2.158, g⊥\hspace{0.17em}=\hspace{0.17em}2.041, | A |\hspace{0.17em}=\hspace{0.17em}0.0212}{\mathrm{cm}}^{\text{−1}}\text{, | B |\hspace{0.17em}=\hspace{0.17em}0.0029}{\mathrm{cm}}^{\text{−1}}{\mathrm{,\; |\; a}}_N\text{|\hspace{0.17em}=\hspace{0.17em}0.0017}{\mathrm{cm}}^{\text{−1}})$ are in good agreement with those obtained by a semiempirical MO calculation which takes into account the out‐of‐plane $\pi$ bonding with the 16‐membered ring of the phthalocyanine molecule.