Phosphorus-31 Nuclear Magnetic Relaxation Times in the PBr3–PBr2Cl–PBrCl2–PCl3 System

The NMR transverse and longitudinal relaxation times of ³¹P in a liquid mixture of PCl₃ and PBr₃, and in a liquid mixture of PBr₃, PBr₂Cl, PBrCl₂, and PCl₃ were measured at 40.5 MHz as a function of temperature from 80 to −100°C. Transverse relaxation times of ³⁵Cl in a liquid mixture of PCl₃ and PBr₃ and in neat liquid PCl₃ were measured over the temperature region 70–29°C. The ³⁵Cl relaxation time in PCl₃ is entirely due to a quadrupolar interaction mechanism, thus the reorientational correlation time of the PCl₃ molecule is determined from the 1/T₂ values and the known quadrupole coupling constant. The ³¹P transverse relaxation time is controlled by a scalar coupling interaction mechanism while the longitudinal relaxation time is controlled by a spin—rotation interaction mechanism at high temperatures and a combination of dipolar, scalar coupling, and anisotropic chemical shift interaction mechanisms at lower temperatures. The scalar coupling constants calculated for $\mathrm{P}{–}^{35}\mathrm{Cl}$ in PCl₃ and $\mathrm{P}{–}^{79}\mathrm{Br}$ in PBr₃ are 127 and 296 Hz, respectively. The value for PCl₃ is in excellent agreement with previous work, while that for PBr₃ is believed to be more accurate than the value reported from previous studies. The spin—rotation coupling constants for PBr₃, PBr₂Cl, PBrCl₂, and PCl₃ are 3.49, 4.41, 5.95, and 9.09 kHz, respectively. The values for PBr₃ and PCl₃ do not agree with those obtained from previous relaxation measurements, but are in excellent agreement with the theoretical values.