Proton Magnetic Shielding Tensors from Spin-Rotation Measurements on H₂CO and NH₃

The spin-rotation tensor for H₂CO was obtained from very high resolution beam maser measurements of rotational transitions. Spin-rotation tensor elements are M_aa= −4.12 ± 0.36 kHz, M_bb= 2.45 ± 0.32 kHz, and M_cc= −2.10 ± 0.34 KHz in the principal axis system. These results are combined with previous molecular orbital calculations of diamagnetic shielding to obtain total absolute shielding tensor elements of σ_aa= 15.9, σ_bb= 40.7, and σ_cc= −1.7 ppm and an average shielding of 18.3 ppm with uncertainties of ±2 ppm or better. A similar analysis is carried out for NH₃. Spin-rotation tensors and shielding tensors are given for proton and nitrogen sites with estimated accuracy better than 1 ppm. For NH₃σ_zz(H) = 26.5 ppm and σ(H) = 31.2 ppm. The absolute shielding scale is discussed.