= Solution
For a positive <gyromagnetic ratio> the <magnetic moment> is parallel to the <spin angular momentum>; for a negative ratio it is antiparallel. The <Hamiltonian> $\hat H_Z=-\gamma B_0\hat I_z$ gives $E_m=-\gamma\hbar B_0m$. Thus \b[$m=+1/2$ is lower for the proton, whereas $m=-1/2$ is lower for nitrogen-15]. Their <spin> precession senses are opposite for a common positive field convention; measurable <absorption> <frequencies> and energy separations remain positive.
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