Absorption cross-sections for the 5th and 6th vibrational overtones in a series of short chained alcohols using incoherent broadband cavity enhanced-absorption spectroscopy (IBBCEAS)

Abstract

Absorption cross-sections for the 5th (6 $\leftarrow$ 0) and 6th (7 $\leftarrow$ 0) OH overtones for gas-phase methanol, ethanol, and isopropanol were measured using a slow flow cell and Incoherent Broadband Cavity-Enhanced Absorption Spectroscopy (IBBCEAS). Measurements were performed in two wavelength regions, 447-457 nm, and 508-518 nm, using two different instruments. The experimental results are consistent with previous computational predictions of the excitation energies for these transitions. Treating the OH stretch as a local mode allowed for calculation of the fundamental vibrational frequency ($ω_e$), anharmonicity constant ($ω_e x_e$), and the vertical dissociation energy (VDE) for each alcohol studied. The fundamental vibrational frequency is $3848 \pm 18 \, \text{cm}^{-1}$, $3807 \pm 55 \,\text{cm}^{-1}$, and $3813 \pm 63 \, \text{cm}^{-1}$ for methanol, ethanol, and isopropanol, respectively. The anharmonicity constant was measured to be $84.8 \pm 2.1 \, \text{cm}^{-1}$, $80.2 \pm 5.9 \, \text{cm}^{-1}$, and $84.4 \pm 6.8 \, \text{cm}^{-1}$ for methanol, ethanol, and isopropanol, respectively. The OH vertical dissociation energy was measured to be $499.4 \pm 18.4$ kJ/mol, $518.0 \pm 56.7$ kJ/mol, and $492.7 \pm 59.9$ kJ/mol. The spectroscopically measured values are compared to thermodynamically measured OH bond dissociation energies. The observed differences in previous measurements of the bond dissociation energies compared to the values reported herein can be explained due to the difference between vertical dissociation energies and bond dissociation energies. If the OH overtone stretching mode is excited in methanol to either the 5th or 6th overtone, the bimolecular reaction between methanol and O$_2$ becomes thermodynamically feasible and could contribute to formation of methoxy and HO$_2$ radicals under the proper combination of pressure and temperature.