Proton and Fluorine Spin–Lattice Relaxation and Spin–Rotational Interaction in Liquid 1,3,5- and 1,2,4-Trifluorobenzenes

The proton and fluorine spin–lattice relaxation times, $T_1$ , have been measured in degassed liquid 1,3,5‐ and 1,2,4‐trifluorobenzenes (135TFB and 124TFB) using pulsed NMR techniques at a frequency of 30 MHz. Within experimental error, the relaxation of each species could always be described with a single exponential. The data are analyzed in terms of dipolar and spin–rotational (SR) contributions to $T_1$ . The proton–fluorine dipolar interaction is theoretically approximated so that a single $T_1$ may be used to analyze the relaxation of each nucleus. The proton relaxation was measured from the mp to 100°C and is dominated by dipolar interactions. The fluorine relaxation times, measured from − 20°C (supercooled liquid) to approximately 220°C, are dominated by dipolar interactions at the lower temperatures and by SR interactions at the higher temperatures. The inter‐ and intramolecular contributions to $T_1$ were separated for F at 0°C and for H at 0 and 25°C by measuring relaxation as a function of dilution in C₆D₆. The intramolecular $T_1$ values in seconds are: 135TFB:F = 33 ± 2, H = 63 ± 4 (0°) and 87 ± 3 (25°); 124TFB:F = 35.5 ± 2, H = 48 ± 3 (0°) and 55 ± 3 (25°). The dipolar correlation times obtained from H relaxation are (4.8 ± 0.2) × 10⁻¹² sec for 135TFB and (3.1 ± 0.2) × 10⁻¹² sec for 124TFB at 0°C. Rather inaccurate estimates of the SR contribution to fluorine $T_1$ and the fluorine SR interaction constant ${\text{(2C⊥}}^2{\mathrm{\hspace{0.17em}+\hspace{0.17em}C}}_{\Vert }^2{)}^{\text{1/2}}$ are made and yield at 0°C the approximate values 70 sec and 2.3 kHz, respectively. The data are compared with the theories and experimental results of other investigators.