Rotational Spectroscopic Study of Quantum Solvation in Isotopologic (pH)-CO Clusters.

We report the Fourier transform microwave spectra of (pH)-CO, (pH)-CO, and (pH)-CO (N ≤ 8) clusters. We find that the frequencies of the a-type J = 1-0 transitions decrease to a minimum at N = 6 and then rapidly increase up to at least N = 8; this is similar to what was previously reported for (pH)-CO, for which the turnaround was found to correlate with an increase in the superfluid fraction of the pH component of the clusters [ Raston Phys. Rev. Lett. 2012 , 108 , 253402 ]. The data suggest that the turnaround in the transition frequency marks an abrupt decrease in the anisotropy of the potential (i.e., in going from N = 6 → 7 → 8), as evidenced from the isotopologic differences rapidly evolving from end-over-end to free-rotor character. Structurally, a more quantitative analysis of the anisotropy was hindered by the lack of accurate frequencies in the b-type series, and a simple Kraitchman analysis yielded unphysical results. In addition to comparing the transition frequencies of the different isotopologic clusters, we provide here more comprehensive details and further discussion of the initial measurements made on (pH)-CO.