Heavy Analogs of Organic Free Radicals: Si‐, Ge‐, Sn‐ and Pb‐Centered Radicals

Abstract

Heavy Analogs of Organic Free Radicals: Si-, Ge-, Sn-and Pb-Centered Radicals 49 region of 205-250 nm (absorption maximum at ca. 215 nm), indicative of its pyramidal structure. 41 The pyramidality of the F 3 Si • radical, generated by the glow discharge of the disilane F 3 Si-SiF 3 , was deduced from the analysis of its rotational spectrum recorded during a recent microwave spectroscopy study. 42 EPR and CIDNP Spectroscopy EPR SpectroscopyThe first EPR spectra of the transient radicals H 3 E • (E = Si, Ge, Sn) were recorded in inert gas matrices at low temperatures. 43 Accordingly, the following EPR parameters were reported at 4.2 K for the above-mentioned radical species [g-factor, hfcc for the central nuclei ( 29 Si, 73 Ge, 119 Sn) in mT, matrix inert gas]: H 3 Si • (2.0030, 19, Xe); H 3 Ge • (2.0073, 7.5, Xe); H 3 Sn • (2.0170, 38, Kr). 43 The large values of the hfcc suggest the pronounced pyramidality of the H 3 E • radicals, whose structural parameters were computationally estimated as follows (s-character of the bond orbital, bond angles in degrees): H 3 Si • (0.285, 113.5); H 3 Ge • (0.295, 115); and H 3 Sn • (0.310, 117). The pyramidal geometry of the heavy analogs H 3 E • is in sharp contrast to the planar environment of the prototypical methyl radical H 3 C • , which revealed a resonance at g = 2.0020 in Xe matrix and for which the s-character of the bond orbital of 0.333 and bond angles of 120 • were calculated. 43 The subsequent EPR reinvestigation of the H 3 Si • radical, generated by either γ -irradiation of H 4 Si adsorbed on a silica gel surface at 77 K 44 or by the reaction of H 4 Si with H atoms in a Kr matrix, 45 disclosed very similar values for the 29 Si hfcc of 18.2 mT 44 and 19.0 mT. 45 The large hyperfine splitting, implying a large s-fraction of its SOMO, is consistent with the pyramidal configuration of the H 3 Si • radical, in accordance with Pauling's hypothesis that the AX 3 • -type radical is expected to be nonplanar if X is more electronegative than A. 46 The EPR spectrum of the H 3 Ge • radical, generated in a matrix of nonmagnetic Xe isotopes, has also been reported. 47 On further increasing the electronegativity of substituents, one should expect a progressive increase in the degree of pyramidalization. Thus, the EPR spectra of perhalogenated silyl radicals F 3 Si • 48 and Cl 3 Si • , 49 generated by either radiolysis of F 3 SiH in an SF 6 matrix or irradiation of SiCl 4 , revealed very large 29 Si hfcc values of 49.8 mT (F 3 Si • ), 48 41.6 mT (Cl 3 Si • in Cl 3 SiCH 3 matrix) 49a and 44.0 mT (Cl 3 Si • in Cl 4 Si matrix), 49b thus pointing to a strong pyramidalization at the silicon radical centers because of the great difference in electronegativities of the Si and F atoms. The pyramidality of silyl radicals successively increased along the series (CH 3 ) 3 Si • < (CH 3 ) 2 ClSi • < (CH 3 )Cl 2 Si • < Cl 3 Si • , as was manifested by the increasing values of their hfcc (129 < 215 < 308 < 440). 49b Simple trialkylsilyl radicals were proved to be pyramidal; however, the ex...