New data are presented for the proton spin relaxation time in gaseous ammonia (NH3) at 300 K. The characteristic nonlinear density dependence of the relaxation time at low densities is established. Old data for gaseous hydrogen chloride (HCl) are reanalyzed. The commonly used single relaxation time aproximation for the rotational levels is shown to be inadequate to fit either the NH3or the HCl data. Excellent fits of both sets of data are obtained using a two parameter, multilevel relaxation time approximation; the parameters deduced are the average rotational level linewidth and the dispersion. An effective intramolecular spinhyphen;rotation coupling constant is determined from the NH3data and it is shown to be consistent with the spinhyphen;rotation coupling constants deduced from microwave absorption experiments. Also an angular momentum reorientation cross section is computed from the NMR data and compared to other similar cross sections obtained from microwave, ultrasonic and viscomagnetic experiments. The reorientation process probed by the NMR experiment is shown to be dominated by elastic collisions.
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