Molecular motion in solid odd-numbered paraffin C19H40: Proton spin relaxation spectroscopy from 5.8 kHz to 86 MHz

Abstract

We describe the experimental setup of a continuously variable pulsed NMR spectrometer for solid‐state investigations, based on a fast magnetic field cycling technique (Zeeman‐field range H=0–1700 Oe, spin–lattice relaxation time range T₁?10 ms), and report on measurements of the proton relaxation time T₁ in solid odd‐numbered paraffin C₁₉H₄₀ (Larmor frequency range ω_L/2π=5.8 kHz–86 MHz, temperature range ϑ=−195 to +32°C). As a result of the extended frequency range one can identify three temperature regimes (I, II, III), where different relaxation mechanisms predominate. (I) In the low temperature regime the relaxation is determined by thermally activated rotational jumps of CH₃ end groups with uniform hindrance. (II) In the prerotator phase just below the rotational phase transition the relaxation is essentially caused by the additional mobility of CH₂ groups in thermally disordered molecules (kink defects); the T₁ dispersion can quantitatively be described by the one‐dimensional diffusion of kink defects in combination with competing rotational reorientations of the entire molecule. (III) In the rotator phase we observe T₁ contributions of two different CH₂ motions, supposed to be fast rotational reorientation of entire molecules and slow‐self‐diffusion or fluctuation of paraffin bundles, respectively. Our results agree for the prerotator phase with refined ideas of the Pechhold/Blasenbrey kink theory, but not for the rotator phase, where our analysis supports the Fischer/Strobl model of a premelting structure.