Assignment and analysis of the rotational spectrum of bromoform enabled by broadband FTMW spectroscopy

The rotational spectrum of the bromoform molecule is complicated by overlap of extensive hyperfine splitting structure of three bromine nuclei belonging to four isotopic species of comparable abundance. We have been able to achieve an unambiguous assignment of this spectrum on the basis of complete hyperfine patterns of the lowest-J rotational transitions recorded at conditions of supersonic expansion with chirped-pulse, broadband Fourier transform microwave spectroscopy. The mm-wave rotational spectrum was then also studied up to J = 131 and 318 GHz, and extensive measurements are reported for four isotopic species of HCBr₃ and four DCBr₃ species. Precise values of many spectroscopic constants have been determined from global fits of all measurements for a given isotopic species and have been substantiated by comparisons among the various isotopic species and with results of ab initio calculations. The experimental measurements allowed determination of the axial rotational constant C for the symmetric top species, evaluation of nuclear quadrupole hyperfine splitting constants in the principal quadrupole axes of the bromine nucleus, and extension of the range of known values of the h₃ splitting constant. The r_z geometry of HCBr₃ was also determined.