Abstract
Oxygen-17 screening constants are obtained within the CNDO/S and INDO frameworks by using gauge-dependent atomic orbitals in the molecular orbital description. The calculations include the excited singlet states which are mixed with the ground state by an external magnetic field. Oxygen-17 chemical shifts, relative to water, are found to be satisfactorily accounted for by the CNDO/S calculations but less reasonably by the INDO results. Calculations of the various contributions to the local paramagnetic term show that the lowest-energy n → π ∗ transition makes the largest contribution for carbonyl oxygen atoms while the lowest-energy σ → π ∗ transition is dominant for CO, CO 2, and NO 2 +. In addition to the average value of the 17O screening tensor, the diagonal components of the paramagnetic term are also obtained by CNDO/S calculations. These data are compared with experimental results and ab initio data where available.
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