The ESR lineshape of one electron spin coupled by anisotropic hyperfine interaction to two nuclei is calculated when two different configurations of the molecule interchange by a simple reaction scheme. The spectrum calculation includes the anisotropic electron Zeeman interaction and the nonsecular terms of the electron spin. The lineshape, which simulates both a nonsaturated CW or a pulse experiment, is calculated numerically by using a density matrix theory within the Liouville formalism. The theory is tested against the X-band spectra of-CH2 in ZnAc single crystal for three different orientations of the magnetic field. The present theoretical lineshape reproduces all the experimental “forbidden” transitions and predicts that they are important for certain crystal orientations. The calculated reorientation barrier of the methylene in the present work is 7.17 kcal/mol which is closer to an estimated minimum value of 9 kcal/mol, than the previous value of 5–6 kcal/mol obtained by an analysis with the modified Bloch equations.