Since theories on the behavior of dilute gases are in an advanced stage compared to condensed phases and complex materials, gas phase NMR measurements offer opportunities unique to the gas phase for fundamental understanding of the dependence of NMR quantities (shielding, J coupling) on the internal coordinates of the molecule (the property surfaces that mathematically describe how these molecular electronic properties change while the molecule carries out vibrational motions). The virial expansion is valid in the dilute gas, so that observations in the linear-density regime uniquely permit the determination of the linear-density dependence of NMR quantities, the second virial property coefficients that can be calculated in closed form by direct integration, if the intermolecular potential energy surface (PES) is known. Precise measurements afforded by NMR provide tested general theoretical treatments of intermolecular effects and intramolecular averaging that apply also to other molecular electronic properties. Spin–lattice relaxation times in the linear-density regime also provide a direct connection to the intermolecular PES; they can be calculated in terms of two well-defined cross-sections involving changes in molecular orientation and molecular rotational angular momentum sensitive to the anisotropy of the PES by well-established theory. Finally, observations of chemical exchange processes in gas phase NMR provide tests of kinetic theories for molecular rearrangements (RRKM) and transition state theory, and so provide tests of calculated transition states, reaction surfaces, and activation thermodynamic quantities such as ΔG⧧, ΔH⧧, ΔS⧧.