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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