1H and 19F spin-lattice relaxation and CH3 or CF3 reorientation in molecular solids containing both H and F atoms

Abstract

The dynamics of methyl (CH3) and fluoromethyl (CF3) groups in organic molecular (van der Waals) solids can be exploited to survey their local environments. We report solid state 1H and 19F spin-lattice relaxation experiments in polycrystalline 3-trifluoromethoxycinnamic acid, along with an X-ray diffraction determination of the molecular and crystal structure, to investigate the intramolecular and intermolecular interactions that determine the properties that characterize the CF3 reorientation. The molecule is of no particular interest; it simply provides a motionless backbone (on the nuclear magnetic resonance (NMR) time scale) to investigate CF3 reorientation occurring on the NMR time scale. The effects of 19F–19F and 19F–1H spin-spin dipolar interactions on the complicated nonexponential NMR relaxation provide independent inputs into determining a model for CF3 reorientation. As such, these experiments provide much more information than when only one spin species (usually 1H) is present. In Sec. IV, which can be read immediately after the Introduction without reading the rest of the paper, we compare the barrier to CH3 and CF3 reorientation in seven organic solids and separate this barrier into intramolecular and intermolecular components.

Monitoring a simple hydrolysis process in an organic solid by observing methyl group rotation

“…However, as a rough guide, we can consider the electronic structure barrier calculations by values in related compounds [11].…”

“…The wait time t w in the pulse sequence was sufficiently long to allow the magnetization to return to its equilibrium value within 0.1% [11]. This is particularly important if the relaxation is nonexponential.…”

“…The 1 H spin-lattice relaxation results from the modulation of 1 H-1 H spin-spin (dipolar) interactions by methyl group (CH 3 ) rotation [11,21]. When methyl group rotation is responsible for the relaxation, the relaxation is nonexponential near the maximum in the relaxation rate and at higher temperatures [2,[8][9][10][22][23][24][25][26][27][28].…”

“…where R* is the characteristic relaxation rate and β is the stretching parameter [11,22,[29][30][31][32][33][34][35]. If the relaxation is exponential to within experimental uncertainty, β = 1 and R* is labeled R. This occurs in pure compounds like 1 and 2 at temperatures below the maximum in the relaxation rate [2,[8][9][10][22][23][24][25][26][27][28].…”

“…The fitted NMR activation energies in the pure samples are in reasonable agreement with the barrier heights for methyl group rotation determined by electronic structure calculations in clusters of molecules based on the Xray structures of the pure crystals, both of which are reported here. The calculations in both isolated molecules and in the clusters allow us, independently of all the experimental techniques, to determine, approximately, the intramolecular and intermolecular contributions to the methyl group rotational barrier [11].…”