Knowledge Bank

The molecular Zeeman effect of nitrogen-15 formamide has been observed under high resolution. The data yield the molecular g-values of $gaa=-0.2843\pm 0.001,gbb=0.0649\pm 0.0004$, and $gcc=-0.0117\pm 0.0004$, and the magnetic susceptibility anisotropies $2\chi aa-\chi bb-\chi cc=2.2\pm 0.7$ and $2\chi bb-chicc-\chi aa=8.0\pm 0.5$. These values give molecular quadrupole moments of $Qaa=-0.33\pm 0.52,Qbb=3.42\pm 0.44$, and $Qcc=-3.09\pm 0.84$. The measured values of the susceptibility anisotropy were then used with McConnell’s theory of shielding to predict the change in chemical shift during the helix to coil transition in poly-L-alanine. The results are compared to experimental and theoretical values.