13C Hyperfine Splittings in the Electron Spin Resonance Spectra of HCO and FCO

The radicals H13CO and F13CO have been prepared in an argon matrix at 4°K by the addition of an H atom and an F atom, respectively, to CO enriched to 52% in 13C. The electron spin resonance spectra of these radicals contain large 13C hyperfine splittings in addition to the previously observed proton and fluorine hyperfine splittings. In HCO the principal components of the 13C hyperfine splitting tensor are: Ax(C)/h=365.7, Ay(C)/h=427.9, and Az(C)/h=338.9 Mc/sec. These results give an approximate value of 125 deg for the HCO bond angle and a value of 0.45 for the unpaired electron density on the carbon atom. The isotropic part of the 13C hyperfine splitting in FCO (some unusual complications made it impossible to determine the anisotropic terms) is A(C)/h=802.5 Mc/sec, where A(C) has the same sign as the fluorine hyperfine splitting constant. This result indicates that FCO is more sharply bent than HCO, the bond angle having an approximate value of 110 deg.