Quantum chemical calculations and analysis of FTIR, FT–Raman and UV–Vis spectra of temozolomide molecule

• FTIR, FT–Raman and UV–Vis spectra were recorded and analysed.
• Anharmonic frequencies were computed by VPT2, VSCF and CC–VSCF methods.
• Mode–Mode coupling strengths were computed using 2MR-QFF.
• The existence of intermolecular hydrogen bonding was investigated.

A combined experimental and theoretical study of the structure, vibrational and electronic spectra of temozolomide molecule, which is largely used in the treatment of brain tumours, is presented. FTIR (4000–400 cm−1) and FT–Raman spectra (4000‒50 cm−1) have been recorded and analysed using anharmonic frequency calculations using VPT2, VSCF and CC-VSCF levels of theory within B3LYP/6-311++G(d,p) framework. Anharmonic methods give accurate frequencies of fundamental modes, overtones as well as Fermi resonances and account for coupling of different modes. The anharmonic frequencies calculated using VPT2 and CC-VSCF methods show better agreement with the experimental data. Harmonic frequencies including solvent effects are also computed using IEF-PCM model. The magnitudes of coupling between pair of modes have been calculated using coupling integral based on 2MR-QFF approximation. Intermolecular interactions are discussed for three possible dimers of temozolomide. UV–Vis spectrum, examined in ethanol solvent, is compared with the calculated spectrum at TD-DFT/6-311++G(d,p) level of theory. The electronic properties, such as excitation energy, frontier molecular orbital energies and the assignments of the absorption bands are also discussed.

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