  We report $^{59}$Co, $^{93}$Nb, and $^{121}$Sb nuclear magnetic resonance
(NMR) measurements combined with density functional theory (DFT) calculations
on a series of half-Heusler semiconductors, including NbCoSn, ZrCoSb, TaFeSb
and NbFeSb, to better understand their electronic properties and general
composition-dependent trends. These materials are of interest as potentially
high efficiency thermoelectric materials. Compared to the other materials, we
find that ZrCoSb tends to have a relatively large amount of local disorder,
apparently antisite defects. This contributes to a small excitation gap
corresponding to an impurity band near the band edge. In NbCoSn and TaFeSb,
Curie-Weiss-type behavior is revealed, which indicates a small density of
interacting paramagnetic defects. Very large paramagnetic chemical shifts are
observed associated with a Van Vleck mechanism due to closely spaced $d$ bands
splitting between the conduction and valence bands. Meanwhile, DFT methods were
generally successful in reproducing the chemical shift trend for these
half-Heusler materials, and we identify an enhancement of the larger-magnitude
shifts, which we connect to electron interaction effects. The general trend is
connected to changes in $d$-electron hybridization across the series.
