Spectroscopic, thermogravimetric studies and DFT calculations of pentoxyverine citrate ion-pairs with sulfonephthalein dyes

Pentoxyverine citrate (PEN-citrate) ion-pairs with six sulfonephthaleins acid dyes; bromocresol green (BCG), bromophenol blue (BPB), bromothymol blue (BTB), bromocresol purple (BCP), bromochlorophenol blue (BChPB) and bromoxylenol blue (BXB) have been prepared. A structural characterization and elucidation of these ion-pairs applying UV–visible spectroscopy, Fourier transform infrared absorption spectroscopy (FTIR), nuclear magnetic resonance (1H NMR) and mass spectrometry have been carried out. Moreover, the spectral characteristics; oscillator strengths, transition dipole moments, transition energy and resonance energy of the ion-pairs have been calculated. Further characterization of the prepared compounds was done with the aid of thermogravimetric analysis for the determination of their thermal stability. The energy of activation (Ea) for the degradation of the prepared ion-pairs and some of their thermodynamic properties such as activation enthalpy (
), activation entropy (
) and activation free energy (
) of each decomposition step using Horowitz-Metzger method have been calculated. The binding nature of a representative ion pair has been explored at a molecular level by performing quantum chemical calculations. Here, DFT based full geometry optimization has been carried out for PEN, neutral and charged (specifically anion) forms of bromophenol blue (BPB and BPB−), and the PEN–(BPB−) ion pair. The results indicated that this ion pair exhibits strong-to-medium hydrogen bond between the BPB sulfonate group and the PEN amino group with distance of 1.58 Å. Harmonic vibrations of BPB, PEN and PEN–(BPB−) ion pair have been calculated and the corresponding calculated spectra have been compared with the experimental Infrared spectra. Moreover, the UV–visible spectra and their corresponding excited states and electronic transitions for PEN–(BPB−) ion-pair have been investigated in DMSO.