Laser Spectroscopy of Methanol Isotopologues in ⁴He Nanodroplets: Probing the Inertial Response around a Moderately...

High-resolution, mid-infrared spectra of methanol isotopologues (CH₃OH, CH₃OD, CD₃OH, and CD₃OD) embedded in superfluid helium nanodroplets have been obtained. For the normal isotopologue, we observed the CO stretching overtone band, the lines within which are 2× broader than in the fundamental for E species methanol and no different than the fundamental for A species methanol. For CH₃OD, we observed the CO stretching overtone band for the first time, which was characterized by narrow line widths for both nuclear spin species. Spectra in the CD₃ stretching bands were much broader, which is attributed to rapid relaxation to nearby anharmonically coupled vibrational state(s). Apparently the coupling is much stronger for CD₃OD, for which the rotational substructure is completely washed out. Inertial analyses of the rotationally resolved fundamental and overtone bands reveal that the moment of inertia of helium, Δ I_He, that couples to rotation decreases in going from the heavier (CD₃OH) to lighter (CH₃OH) isotopologues (i.e., with decreasing gas phase moment of inertia, I_G). The dependence of Δ I_He on I_G is larger than that found for other molecules in regions approaching the heavy and light rotor limits, which suggests a relatively large breakdown in the adiabatic following of helium density for this moderately light rotor.