Recently, nylon-11 and nylon-7 have been found to be ferroelectric and this ferroelectricity was related to hydrogen bonds between polymer chains. When the crystal structure of nylon-11 is complete and has little disordered structures, the ferroelectricity is difficult to be exhibited because of the fairly strong network of the hydrogen-bonding system. The ferroelectricity is obtained for nylon with the slightly disordered crystal structure. This was confirmed by measuring the vibrational frequency for the N-H bond stretching. For the nylon with the disordered crystal structure, the vibrational frequency was found to shift to the higher wavenumber, indicating the loosening of the hydrogen-bond in question. In the present paper, we calculate the vibrational frequency for the N-H bond stretching by using ab initio molecular orbital method in order to shed light on the relation between the degree of the disorder of the crystal and the magnitude of the shift of the vibrational frequency. In connection with this research, we had proposed a method, by which we can determine the structure of the molecule in the crystal. In this method, the geometry of the central molecule is optimized under the effect of the environmental molecules with the fixed geometry. After the optimization of the central molecule, the geometry of the environmental molecules is changed to that of the central molecule. This procedure is continued iteratively until the convergence in the geometry is obtained. We refer to this method as S-SCF method(structual SCF method). By using this S-SCF method, we obtained the intimate relation between the shift of the vibrational frequency and the degree of the disorder of the crystal and also the barrier for the polarization reversal in agreement with experimental data.