Microwave spectroscopic study of the SiF3 radical: Spin-rotation interaction and molecular structure

The trifluorosilyl radical has been produced by glow discharge in hexafluorodisilane in a free space cell. The rotational spectrum of the radical has been measured from the 330 (N=22−21) GHz region down to the 90 (N=6−5) and 75 (N=5−4) GHz regions. In the lower N transitions the K=1 hyperfine components showed a distinct splitting. From the splitting of the K=1 lines it was concluded that the spin-rotation interaction constant of trifluorosilyl radical has a positive sign, which is different from that of trifluoromethyl. This difference is discussed in terms of the electronic states. The simplified discussion based on the Mulliken–Walsh diagram of the energy level is used to rationalize the difference in the sign of the interaction constants in the two radicals. The Si–F bond length is determined from the rotational constant B0 to be 1.565 Å when the bond angle F–Si–F is assumed to be 109.9° deduced in the matrix infrared spectroscopy. The spin density on the fluorine atoms is derived from the hyperfine coupling constants. The obtained molecular structure is compared with those of related molecules.