A laboratory and theoretical study of silicon hydroxide SiOH.

The rotational spectrum of the triatomic free radical SiOH in its X (2)A(') ground electronic state has been observed in a supersonic molecular beam by Fourier transform microwave spectroscopy. The fundamental (1(0,1)-->0(0,0)) transition has been detected for normal SiOH and for three rare isotopic species: (30)SiOH, Si(18)OH, and SiOD. The same transition has also been observed in two of three excited vibrational states, v(2) and v(3), for the most abundant species. Precise spectroscopic constants, including those that describe the effective spin doubling and hydrogen hyperfine structure, have been derived for each isotopic species or vibrational state. To complement the laboratory work, theoretical calculations of the structure, dipole moment, and energies of the X (2)A(') and low-lying 1 (2)A(") states have also been undertaken at the coupled cluster level of theory. In agreement with theoretical predictions, we conclude from the hyperfine constants that SiOH is a best described as a pi-type radical, with the unpaired electron localized on a p orbital on the silicon atom. Assuming a bond angle of 118.5 degrees , the Si-O bond length is 1.647(2) A and the O-H bond length is 0.969(4) A.