Vibrational Overtone Spectroscopy of Saturated Hydrocarbons Dissolved in Liquefied Ar, Kr, Xe, and N₂

Abstract

This article presents a collection of vibrational overtone spectra of hydrocarbons in cryogenic solutions. Vibrational overtone spectra of ethane and propane dissolved in liquid argon and n-butane and isobutane dissolved in liquid krypton were recorded between 5000 and 14,000 cm(-1). Spectral regions for the first four overtones were measured using a Fourier transform spectrophotometer. The fifth overtone (Deltaupsilon = 6) spectra were recorded with a double beam (pump-probe) thermal lens technique using concentrations as low as 10-3 mole fraction. We obtained the C-H (Deltaupsilon = 6) spectra of (a) liquid ethane at 100 K and ethane in solutions in liquid Ar at 92 K and liquid N2 at 85 K, (b) liquid propane at 148 K and propane in liquid Ar at 93 K, (c) n-butane in liquid Kr at 129 K, (d) n-pentane in liquid Xe at 160 K, and (e) isobutane liquid at 135 K and isobutane in liquid Kr at 130 K. Local-mode parameters were calculated for primary, secondary, and tertiary C-H oscillators in solution and compared with gas-phase local-mode parameters. The peak frequency shift (Deltaomega) from gas phase to solution is explained by the change in harmonic frequency and anharmonicity in solution with respect to the gas-phase values. The bandwidth (Deltaomega1/2) of the (Deltaupsilon = 6) C-H absorption bands of ethane in solution can be explained in terms of collisions with the solvent molecules.

Vibrational overtone spectra and interactions of C2H4 and H2CCHCH3 in liquid Kr

“…Changes in harmonic frequency and anharmonicity in solution are responsible for the frequency shifts. A similar result was obtained for C 2 H 6 in liquid argon [2]. For larger molecules, in addition to changes in the Table 1 Observed frequencies, bandwidths (Dv 1/2 ), harmonic frequencies (v e ), and anharmonicities (v e x e ) in the C-H overtone spectrum of ethene dissolved in liquid krypton at 121 K and ethene in gas phase at room temperature.…”

“…This characteristic of vibrational overtones, could be used to facilitate the study of the influence of the solvent on frequency shifts and bandwidths. In a previous publication [2], the frequency shift of C-H absorptions of ethane in liquid argon was attributed to changes in harmonic frequency and anharmonicity of the vibration in the presence of liquid argon. The calculated frequency shift (Dv) in terms of the harmonic frequency and anharmonicity associated with the molecule in solution is calculated as: In these equations v(gas) and v(solution) are the experimental wavenumbers of the peak absorption for bands obtained for the vibrational overtones.…”

“…The average value of Dv = 60 cm À1 (for Dy = 5) is within the predicted value from Eq. (2). Changes in harmonic frequency and anharmonicity in solution are responsible for the frequency shifts.…”

“…The double beam (pump-probe) thermal lens technique for liquids has the sensitivity required to obtain absorption bands of high overtone levels of the solute in very dilute solutions [1]. In spectroscopic studies of hydrocarbons in cryogenic solutions [2][3][4], it has been found that the frequency shift (gas-solution) is due to changes in the potential energy parameters (harmonic frequency and anharmonicity) of the C-H bond of the solute due to interactions with the solvent. The bandwidths are also influenced by the solvent used.…”