Microwave spectrum and molecular structure calculations for η4-butadiene ruthenium tricarbonyl

The microwave spectrum of η⁴-butadiene ruthenium tricarbonyl was measured in the 5–15 GHz frequency range using a Flygare-Balle type pulsed beam Fourier transform microwave (FTMW) spectrometer. The rotational constants for the ¹⁰²Ru isotopologue were determined to have the following values: A = 932.20099(42), B = 858.03248(47) and C = 831.35161(37) MHz. The centrifugal distortion constant d_J is 0.0862(29)kHz. 22 a-dipole and 4c-dipole transitions were measured. Extensive high-level G16 calculations were made using DFT and MP2 methods with various basis sets, some including core-potentials (ECP). The best structure was calculated with Gaussian 16 using B3LYP/def2-QZVPP, which includes a core potential (ECP). Extensive all-electron calculations were made based on the best ECP structure to predict ¹⁰¹Ru and ⁹⁹Ru quadrupole coupling strengths. Quadrupole hyperfine structure splittings were measured for both ¹⁰¹Ru and ⁹⁹Ru. The hyperfine structure splittings for the ¹⁰¹Ru nuclear quadrupole were measured, yielding the values of 1.5χ_aa = 98.12(17) MHz and 0.25(χ_bb-χ_cc)  = 36.059(30). Measured hyperfine structure splittings for ⁹⁹Ru quadrupole coupling yielded the values of 1.5χ_aa = 16.99(77) MHz and 0.25(χ_bb-χ_cc)  = 6.23(32). These values are in reasonable agreement with some of the all-electron calculations.