Variation in the Quadrupole Coupling Constant with Vibrational State in the Methyl Halides

The J=0→1 microwave rotational transitions in the states v=1 of the vibrations v3(a1) and v6(e) in methyl chloride and the state v=1 of the vibration v3(a1) in methyl bromide have been studied. The quadrupole coupling constants eqQ of chlorine and bromine in these states were determined. These were compared with the values determined for the ground state. No variation of the quadrupole coupling constants with vibrational state was observed. The accuracy of the measurements requires the variation of eqQ with vibrational state to be less than 0.07 percent in methyl bromide. The rotational constants and vibration-rotation constants for these states were also determined.