Nuclear Spin–Lattice Relaxation in Phenylacetylene

The temperature dependence of the proton and deuteron spin–lattice relaxation times $T_1$ in phenylacetylene and phenylacetylene‐d₁ has been measured by the adiabatic fast‐passage method. The intramolecular and intermolecular contributions to relaxation have been separated by the $T_1$ measurement of ring protons at different concentrations of phenylacetylene in CS₂ and C₆D₆. The deuteron electric field gradients in the R–C ≡ C–D compounds are relatively insensitive to the nature of the substituent R, and therefore the –C ≡ C–D grouping can be employed as a probe to study rotational motions in molecular liquids of the R–C ≡ C–D type. The reorientational correlation time, ${\tau }_q{\text{\hspace{0.17em}=\hspace{0.17em}5.5\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−12}}\sec$ in neat phenylacetylene‐d₁, and ${\tau }_q{\text{\hspace{0.17em}=\hspace{0.17em}3.3\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−12}}\sec$ at infinite dilution in CS₂ at 26°C, has been calculated from the deuteron $T_1$ and found in good agreement with those calculated from viscosity using both the microviscosity model and the Hill model. Analogous comparison of the experimental translational correlation time ${\tau }_t{\text{\hspace{0.17em}=\hspace{0.17em}1.7\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−10}}\sec$ for neat phenylacetylene at 26°C, and the calculated values has also been carried out.