IR spectra and structural-dynamic anharmonic models of cyclohexanol

Background and Objectives: The construction of quantum mechanical structural-dynamic models of molecular systems has become a necessary part of studies of the structure, optical spectra and properties of matter. The results obtained in the harmonic approximation followed by a scaling procedure guarantee a reliable interpretation of the fundamental vibration bands of the measured vibrational spectra. However, if the spectrum has a complex structure determined by fundamental vibrations, overtones, composite frequencies, resonances, the interpretation is not complete. An approach based on taking into account mechanical anharmonicity makes it possible to significantly minimize the discrepancy between calculated and measured frequencies of fundamental vibrations, identify overtones and composite frequencies in the measured spectra, and establish the presence of resonant interactions. The advantage of this approach manifests itself to varying degrees in specific cases. In this article, using the density functional theory method, taking into account mechanical anharmonicity, structural-dynamic models of two conformers of the cyclohexanol molecule are constructed. The goal is to identify in this particular case the advantages of taking anharmonicity into account, which consists in minimizing the discrepancies between calculated and measured frequencies, identifying overtones, composite frequencies and resonances, refining and replenishing the interpretation of the measured spectra. Materials and Methods: Cyclohexanol (C6H11OH), used in many industries, has been the subject of extensive scientific research. The IR spectra of samples of crystalline phases II and III of cyclohexanol were used, measured on a Bruker IFS-88 Fourier spectrometer at a temperature of 12 K. Using the B3LYP/6-31G(d) method using the GAUSSIAN’03 software package, structural-dynamic models of conformers 1 and 2 isolated cyclohexanol molecules with an equatorial orientation of the hydroxyl group relative to the carbon backbone of the molecule. Results: The main parameters of adiabatic potentials have been calculated: minimum energies, optimal geometries, dipole moments of conformers 1 and 2 of an isolated cyclohexanol molecule. The frequencies of normal vibrations in the anharmonic approximation and the intensities of fundamental bands in the IR spectra have been calculated. A vibrational analysis of the IR spectra of cyclohexanol samples in crystalline phases II and III, measured in the range 400–3800 cm–1, has been carried out in order to identify overtones, vibrations of compounds and resonances. An assessment of the results obtained in comparison with those obtained in the harmonic approximation has been given. Their interpretation of the measured IR spectra has been refined. Conclusions: Based on the results of taking into account mechanical anharmonicity when constructing structural-dynamic models of isolated conformers 1 and 2 of cyclohexanol, it has been established that the agreement between the measured and calculated frequencies in the regions of 1080–1550 cm–1 and 2800–2940 cm–1 of the IR spectrum has significantly improved compared to harmonic scaled frequencies. The consequence of this improvement is a refinement of the interpretation of the frequencies ν₄₀ and ν₄₁ of the stretching vibrations of the C–H bonds closest to the core of the H-complex, and the band with a maximum at a frequency of 1517 cm–1 in the spectrum of crystalline phase II, which is a superposition of the composite vibrations ν₈ + ν₁₈, ν₅ + ν₂₃, ν₉ + ν₁₆. It has been established that resonance interactions in cyclohexanol conformers are small and resonances are unlikely. This conclusion is consistent with experimental data on the IR spectra of cyclohexanol.

cyclohexanol, IR spectrum, density functional method, molecular modeling, mechanical anharmonicity, normal fluctuation, composite frequency, overtone