For three rare isotopologues of carbon dioxide, 18O12C18O (828, according to a well-established shorthand notation), 17O12C18O (728), and 18O13C18O (838), 3923, 4318, and 1058 empirical rovibrational energy levels, respectively, are determined, using the MARVEL (Measured Active Rotational–Vibrational Energy Levels) protocol and code. For the isotopologues 828/728/838, the analysis of their spectroscopic network is based on 11 353(7665)/11 313(7700)/2155(1595) measured(unique) transitions, belonging to 165/113/50 vibrational bands, respectively. The measured transitions collected from the literature span the regions 953–12 570 cm−1 (for 828), 628–8197 cm−1 (for 728), and 600–7918 cm−1 (for 838). The number of critically evaluated and recommended energy levels of this study are 3923, 4318, and 1058 for the 828, 728, and 838 isotopologues of CO2, respectively. Comparison of the empirical rovibrational energy levels determined in this study with their counterparts in two published databases, CDSD-2019, Ames-2021 and HITRAN shows very good overall agreement.
Chirped-pulse Fourier-transform microwave (CP-FTMW) spectroscopy has been used to measure the spectra of ten previously unobserved fluoroethylene (FE)/CO2 clusters, (FE)x(CO2)y, with x from 1 to 4 and y from 0 to 4. Multiple spectra were recorded with varying concentrations of CO2 in the sample, and at least 400,000 free induction decays averaged per data set. This allowed implementation of data-centered approaches, using intensity variation, to identify subsets of transitions belonging to the same cluster species or those of similar composition, simplifying the assignment process for the complex mixture of clusters present. All spectra were fitted to Watson A-reduction Hamiltonians, including quartic distortion constants, with very few species requiring higher order distortion constants for satisfactory fits to be obtained. Computational data at MP2/6-311++G(2d,2p) and ωB97X-D/6-31+G(d,p) levels suggested FE:CO2 ratios and likely structural arrangements of each cluster based on comparisons with experimental rotational constants.
The ground state rotational spectrum of 1,1-dichloro-1-silacyclohex-2-ene has been recorded using a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer. Several isotopologues in their natural abundances have been observed in the free-jet expansion, and their spectra assigned, making it possible to present a partial heavy-atom substitution structure. Furthermore, the high resolution of this technique allows the complicated hyperfine splitting pattern to be largely deconvoluted. As a result, the on-diagonal nuclear quadrupole coupling constants for the two chlorine atoms have been determined for all observed isotopologues. Additionally, χbc is determined for both chlorine atoms of the parent species. The quadrupole coupling tensors for the parent species have been diagonalised, noting some assumptions have been made pertaining to the off-diagonal nuclear quadrupole coupling constants in the principal axis system, to yield reasonable values of χzz and η which are then compared.

