Nuclear Magnetic Antishielding of Nuclei in Molecules. Magnetic Moments of F19, N14, and N15

Abstract

The combination of molecular beam data on spin-rotational interactions in molecules with chemical shift data has been used to calculate the paramagnetic part of the nuclear magnetic shielding constant for F in HF and ${\mathrm{F}}_2$. With the assumption of the sign of the spin-rotational constant in $\mathrm{N}_2^{}{}_{}{}^{15}$ as positive (i.e., a net negative rotational magnetic field at the nitrogen nucleus), the paramagnetic part of the nuclear magnetic shielding constant in ${\mathrm{N}}_2$ has been calculated. The results, when combined with reliable calculations of the diamagnetic part of the shielding constant, yield the total shielding constants. These are found to be: F in $\mathrm{HF}:\sigma =(414.9\mathrm{}\pm 1.4)\times {10}^{-6\mathrm{}}$, F in ${\mathrm{F}}_2:\sigma =(-210\mathrm{}\pm 8.0)\times {10}^{-6\mathrm{}}$, N in ${\mathrm{N}}_2:\sigma =(-101\mathrm{}\pm 25.0)\times {10}^{-6\mathrm{}}$, and demonstrate the phenomenon of nuclear magnetic antishielding in ${\mathrm{F}}_2$ and ${\mathrm{N}}_2$, as well as in other compounds. Use of these shielding constants permits considerable improvement in the estimates of the bare nuclear magnetic moments of fluorine and nitrogen. The results are ${\mu }_{\mathrm{N}}\left(\mathrm{F}\right)=2.628353\mathrm{}\pm 0.000005$ nm, ${\mu }_{\mathrm{N}}\left({\mathrm{N}}^{14}\right)=0.403562\mathrm{}\pm 0.000010$ nm, ${\mu }_{\mathrm{N}}\left({\mathrm{N}}^{15}\right)=-0.283049\mathrm{}\pm 0.000007$ nm.