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

Abstract

The radicals W 3 CO and p13CO 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 lac. The electron spin resonance spectra. of these radicals contain large lac hyperfine splittings in addition to the previously observed proton and fluorme hyperfine splittings. In HCO the principal components of the 13C hyperfine splitting tensor are: Ax(C)/h= 365.7, A.(C)/h=427.9, and A.(C)/h=338.9 Me/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 ma~e it impossible. to determine the anisotropic terms) is A(C)/h=802.5 Me/sec, where A(C) has the same sign as the fluonne hyperfine splitting constant. This result indicates that FCO is more sharply bent than HCO, the bond angle having an approximate value of 110 deg.

13C Hyperfine interactions in CD3C60 and the distribution of unpaired spin on the C60 cage

“…With the recent observation of BC hyperfine interactions (inclusive of sign) for MuC 60 [8], it seemed appropriate to remeasure the BC hyperfine interactions in, for example, CH 3C60 and to assign as many as possible to specific carbons on the C 60 surface. (Muonium (Mu) is a radioactive "isotope" of hydrogen with a mass 0.113 times that of H. The spin of Mu+ is 1/2 [9]). …”

“…This means that unpaired spin is permitted by symmetry in all C(2s) and radially directed C(2p) atomic orbitals. However, the isotropic BC hyperfine interactions of these radicals are very small by comparison with, for example, that of the formyl radical (HCO, a c = 134.8 Gauss [9]), which also belongs to the A' representation of the C, point group. This is not surprising, since the unpaired spin in RC 60 radicals is predominantly in radially-directed C(2p) atomic orbitals, and direct C(2s) contributions arising from the non-planarity of the C 60 framework are negligible [7].…”

An ab initio study of the electronic structure and isotropic hyperfine coupling constants of HCO, FCO, and HBF using different gaussian basis sets

“…However, the spin density on oxygen in both cases is low (-0.03 in HCO; 0.06 in FCO), and though the absolute magnitudes are uncertain in view of the limitations of the population analysis, it is clear that these values are very much less than the value suggested by Cochran and coworkers of 0.28 [19], and the carbon spin density (0.72) is also higher than their value (0.53). Calculations with basis D on HBF give a('H) = 380 G, a("B) = 193 G, and a(19F) = 61 G. Our previous work on BF, [24] with a similar basis set gave calculated values of a(l0B) = 280 G and a(19F) = 190 G ; again rather similar relative magnitudes.…”

“…The calculated hyperfine coupling constants with the largest basis set are a('H) = 81.8G, a(I3C) = 143.56, and a("O) = -6.4G which compare with the experimental values aE('H) = 137G and aE(13C) = 134.76 [19]. Table VI compares these values with those calculated by other workers, using both semiempirical and ab initio wave functions.…”

Theoretical study of isotropic hyperfine coupling constants in small radicals by MINDO/3 method

“…In columns 5 and 6 we list annihilated spin densities and calculated hyperfine couplings, respectively, for nuclei given in column 4. The last column of this table contains experimental data (37)(38)(39)(40)(41)(42)(43)(44)(45)(46)(47)(48)(49)(50)(51)(52)(53)(54)(55)(56).…”