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Abstract: The pure rotational spectrum of perfluorobutyryl fluoride, C3F7COF, was measured using a pulsed nozzle, chirped pulse Fourier transform microwave spectrometer. A total of 280 transitions for the only observed conformer were recorded and analyzed. The rotational constants are A =1397.4330(1)MHz, B =619.17456(8)MHz, and C =592.06831(8)MHz. Quantum chemical calculations and the spectral analysis indicate that the observed conformer is the skew–anti form with calculated dihedral angles CCCO=120°, and CCCC≈170°. Conformational differences between perfluorobutyryl fluoride, perfluorobutyryl chloride, butyryl fluoride and butyryl chloride are discussed. [Copyright &y& Elsevier]