Nuclear Magnetic Relaxation of <i>n</i>-Alkanes in the Rotating Frame

Abstract

The rotating-frame nuclear magnetic relaxation time T1ρ has been measured for 10 normal alkanes ranging from C4H10 to C94H190. Data were obtained within the range −200° to +70°C for C94H190, C40H82, and C6H14. The high-temperature region is characterized by a process of high activation energy ascribed to chain rotation while the low-temperature region exhibits a T1ρ minimum arising from coupling of the entire spin system to the methyl-group rotation via spin diffusion. This information and the analogous T1 data yield an activation energy of 2.6 kcal/mole for the methyl-group rotation. The remaining compounds were examined in the vicinity of the T1ρ minimum at −190°C and relaxation times characterizing the intrinsic methyl relaxation and spin-diffusion process have been extracted from the data for three rf field strengths. Theoretical estimates of the spin-diffusion coefficients at low fields and methyl-proton relaxation times are in satisfactory agreement with the observed quantities.