Nuclear Magnetic Relaxation of n-Alkanes in the Rotating Frame

Abstract

The rotating‐frame nuclear magnetic relaxation time T 1ρ 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 T 1ρ minimum arising from coupling of the entire spin system to the methyl‐group rotation via spin diffusion. This information and the analogous T 1 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 T 1ρ 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.