Knowledge Bank

When molecules are excited to regions of high state density (greater than 10 states/ $cm-1$), intramolecular vibrational redistribution (IVR) can occur. When a molecule contains a source of internal angular momentum, such as a methyl rotor, activation of the rotor via IVR flow can have profound effects on the rotational spectrum of the molecule. Because the total angular momentum of the molecule is a conserved quantity, internal angular momentum generated by activation of the internal rotor must be compensated for by counter-rotation of the frame. This counter-rotation of the frame produces a ``gyroscope effect'' within the molecule. In the presence of IVR, the inertial axes become time-dependent quantities where the rotation of the molecule is described somewhere between the asymmetric top and gyroscope limits. 3-butyn-2-one and 3-methyl but-1-yne are isoelectronic molecules with barrier heights to internal rotation that differ by almost an order of magnitude. Infrared excitation of the acetylenic CH stretches prepares the molecules with approximately $3330cm-1$ of internal energy which can be transferred to torsional motion via IVR. The resulting spectra were probed through a combination of infrared-microwave-Fourier transform microwave triple resonance techniques. The effects of internal angular momentum on the dynamic rotational spectra in the low barrier (3-butyn-2-one, gyroscope rotor) and high barrier (3-methyl but-1-yne, asymmetric top rotor) limits will be compared.