Molecular Constants of Carbon Monoxide at v = 0, 1, 2, and 3: A Vibrational Spectroscopy Experiment in Physical Chemistry

Abstract

The vibrational-rotational spectrum of a diatomic molecule is treated in most laboratory-experiment textbooks. The analysis usually refers to the fundamental absorption of the HCl molecule; the spectrum is analyzed to obtain the equilibrium rotational constant, the moment of inertia, and the internuclear distance. Parameters such as harmonic frequency, anharmonicity of the vibration, and centrifugal distortion are not obtained experimentally, but the students are usually encouraged to find the values in the literature.The experiment described in this article deals with the vibrational-rotational transitions in carbon monoxide. The advantage of using CO is that the fundamental and first overtone are around 2143 and 4259 cm^-1, and the two regions can be covered with a standard FT IR. The experiment studies the vibrational-rotational transitions of the fundamental and the first and second overtones of CO. The experiment illustrates the decrease in absorption coefficient as a function of the excited vibrational quantum number and the magnitude of the changes in structural parameters, such as rotational constant, moment of inertia, and internuclear distance. The experiment also allows the students to obtain spectroscopic parameters, such as harmonic frequency, anharmonicity, rotation-vibration interaction constant, and centrifugal distortion.