13-C spin-rotation interaction and magnetic shielding at the carbon and oxygen nuclei in formaldehyde.

Abstract

A large-sized L-band microwave absorbtion cell was used to observe the 13C spin—rotation interaction in formaldehyde. The results yield | 32Maa+34(Mbb+Mcc)|=207.5±2.5 kc/sec. By making sign assumptions, the paramagnetic shielding at 13C is estimated to be σp=−215×10−6 compared to σp=−730×10−6 at the oxygen nucleus. Hybrid orbitals and the symmetric orthogonalization method are used to compute 10 one-electron properties in formaldehyde to compare with the LCAO SCF results. The hybrid orbitals chosen yield slightly better agreement with experiment than the SCF functions. However, the minimal basis set used here and in the SCF functions does not allow enough flexibility to fit all of the observables within the experimental error. Reasonable values of the carbon and oxygen diamagnetic shielding are chosen on the basis of the calculations and are combined with the 13C spin—rotation constants in this paper and the 17O spin—rotation constants in a previous work to yield the absolute shielding at the carbon and oxygen nuclei in formaldehyde. The values for the total magnetic shielding in formaldehyde are σ(carbon)=125×10−6 and σ(oxygen)=−268×10−6. The 17O–12C spectrum in the literature is reinterpreted to yield the 17O spin—rotation interaction in that molecule. Our interpretation for 17O–12C yields χaa=4.43±0.10 Mc/sec and Mbb=29±10 kc/sec. The 17O spin—rotation information is combined with a recent calculation of the oxygen diamagnetic shielding in CO to yield the absolute shielding at the oxygen nucleus which is σAv=−14×10−6.