Units are mdyn Å-1 for stretching and mdyn Å rad-2 for angle deformations
aThis work, bFrom Ref [77] for promethazine.
On the other hand, the f(νC-N)R force constants also present lower values in solution due to the fact that the involved N4-C19 and N4-C17 bonds belong to N4 atoms of pyridine rings and, hence, these are also acceptors of H bonds. The other scaled force constants in both species practically present the same values. If now the constants are compared with the reported for both forms of promethazine, we observed important differences in the scaled f(νN-H) andf(νN-CH3) force constants. The low values observed in both forms of hydrochloride promethazine can be attributed to the ionic N-H··· Cl bonds because the electronegativity of Cl atoms produces a strong enlargement of N-H bonds diminishing the values while the low value in the f(νN-CH3) is related to the different groups linked to tertiary N atoms acceptors of H bonds. In CQ, the tertiary N2 atoms are linked to two CH2-CH3 groups while in promethazine the tertiary N atoms are linked to two CH3 groups.