An zbinitio spin-resrricred SCF and perturbative confintion interaction study of the silyi radical is presented-The vi-b_stionzl dependence of isotropic coupiin~ constsnts is investigtcd using doubIc-zct3 ztd double-zet3 pIus pokuizttion basis se"& The czdcuhtions predict P rxxrIy tetmhedml geometry for the mdic;lI with rm inversion bzurier of5.85 kuI/moI. The vibrdoid tratmcnt lesds to coupling eonStilnts (aSi = -199.30 G;~H = +.5_03 G) in excellent ngrccment xsith experiment. I_ Infroduction In recent years, increasing progress has been made in understanding the properties of free radicaIs in which the unpaired electron is located at an element other than carbon, especially silicon_ ESR spectra of SiHj trme been investigated in detail io the solid state [I -3J as we11 as in sohrtion [4> J _ All rest&s are consistent with a non-planar radical and the reported [6.7] high retention of optical activity in organosilyl radicals indicates a pyramidal conformation which may possess a noticeable inversion barrier in contrast to alkyi radicals_ However, there remain some problems in explaining satisfactorily the deviation from phnar structure of the radicaI [S-IO]. the value of the %i isotropic hyperfme couphng [2,3 J and the sign of the *H isotropic hyperfme coupling [ I--5,SJ _ Semi-empirical calculations give either a planar form ' Penmnent a,ddre Cattedra diChimicaTeorica. istituto Chimico dell thriversit3.80134 NzipIer. It3ly.