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

摘要

We describe the experimental setup of a continuously variable pulsed NMR spectrometer for solidhyphen;state investigations, based on a fast magnetic field cycling technique (Zeemanhyphen;field rangeH=0ndash;1700 Oe, spinndash;lattice relaxation time rangeT1quest;10 ms), and report on measurements of the proton relaxation timeT1in solid oddhyphen;numbered paraffin C19H40(Larmor frequency range ohgr;L/2pgr;=5.8 kHzndash;86 MHz, temperature range Vthgr;=minus;195 to +32deg;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 CH3end 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 CH2groups in thermally disordered molecules (kink defects); theT1dispersion can quantitatively be described by the onehyphen;dimensional diffusion of kink defects in combination with competing rotational reorientations of the entire molecule. (III) In the rotator phase we observeT1contributions of two different CH2motions, supposed to be fast rotational reorientation of entire molecules and slowhyphen;selfhyphen;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.