Isotope shifts and spin–spin coupling constants in the 13C and 17O NMR spectra of carbon monoxide and carbon dioxide

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Abstract
Oxygen isotope shifts on the 1 3C chemical shifts and carbonisotope shifts on the 1 7O chemical shifts in carbon monoxide and carbon dioxide are reported. Using models developed by Jameson, shielding derivatives with respect to bond lengths can be calculated using the measured isotope shifts. For carbon monoxide, the derivatives were calculated to be [∂σ (1 3C)/∂r] e =−456±15 ppm/Å and [∂σ (1 7O)/∂r] e =−1150±130 ppm/Å. Although earlier coupled Hartree–Fock calculations give a much lower value for [∂σ (1 7O)/∂r] e , recent a b i n i t i o calculations for carbon monoxide agree very well with our experimental results. Furthermore, the observed 1 8O/1 6O iostope shift is similar to values measured previously for a series of metal carbonyls. For carbon dioxide the iostope shift gives [∂σ (1 3C)/∂r] e =−214±17 ppm/Å which is in excellent agreement with the value obtained from a recent variable temperature gas phase NMR study. In addition, scalar spin–spin coupling constants, 1 J(1 3C,1 7O) were measured to be 16.4±0.1 Hz in carbon monoxide and 16.1±0.1 Hz in carbon dioxide. To our knowledge, these are the first directly measured carbon–oxygen coupling constants to be reported in the literature. From general trends in the periodic table, it seems likely that the sign of these coupling constants is positive.