3.7. Force constants
The harmonic force fields for the S(-) and R(+) forms of CQ in both media, calculated with the SQMFF method and the Molvib program by using the B3LYP/6-311++G** level of theory, were used to obtain the corresponding scaled force constants [38-40]. Hence, the scaled force constants for some groups of both forms of CQ are summarized inTable 6 compared with those reported in gas phase for the two hydrochloride forms of antihistaminic promethazine agent by using the B3LYP/6-31G* method [77]. First, regarding the scaled force constants for the S(-) and R(+) forms of CQ in both media we observed that the f(νN-H) ,f(νN-(CH2-CH3)2 and f(νC-N)R force constants present slight differences in both media, being their values lower in solution. These lower values for both forms are related to the enlargement of involved N3-H30, N2-C10, N2-C11, N4-C19 and N4-C17 bonds. The N3-H30 bonds are donors of H bonds and, as a consequence they are hydrated in solution while the N2 atoms are acceptors of H bonds increasing the N2-C10 and N2-C11 distances in solution, as observed in Table 3.
Table 6 . Scaled internal force constants for S(-) and R(+) forms of chloroquine in gas phase and aqueous solution by using the B3LYP/6-311++G** method.