A close look at 13C CPMAS linewidths in solids for rigid, strongly coupled carbons under CW proton decoupling
Ambient-temperature 13C linewidth (LW) and transverse relaxation (TC2) data are presented for the natural-abundance crystalline carbons of linear polyethylene (LPE) under CW proton decoupling conditions and magic angle spinning (MAS). This linewidth behavior typifies that seen for rigid methylene carbons whose attached protons are also strongly coupled to other protons. These data are presented for two LPE samples (unoriented, melt-crystallized and uniaxially oriented, extruded) as a function of several parameters including static field (1.4 T < B0 < 9.4 T), proton decoupling field strength (38 kHz < nuH1 < 90 kHz), MAS frequency (0.5 kHz < nur < 8 kHz) and RF frequency offsets from resonance (-4 kHz < Deltanuoff < 4 kHz). It is the ubiquitous nature of off-resonance proton irradiation (ORPI) (arising from fixed or rotationally dependent deviations from the true proton resonance condition) which provides the focus for this work. Corresponding contributions, LW(ORPI), to the total LW are treated within the general framework of the effective-field picture of CW decoupling. Then, considering the presence of spin fluctuations characteristic of the strongly-dipolar-coupled protons of LPE, LW(ORPI) can be traced to orbit-dependent TC2 contributions to LW. Important dependences demonstrated and discussed include: (1) For "off-resonance" decoupling, there is a quadratic dependence of LW(ORPI) on (Deltanuoff/nuH1) and there is a strong dependence of the corresponding parabolic coefficient on nur. From the latter dependence, characteristic times for spin fluctuations are also estimated. (2) For "on-resonance" decoupling, LW(ORPI) is proportional to (nuH1)-2 and shows very little sensitivity to nur. These LW(ORPI) contributions become more important at higher B0 since the principle reason for ORPI is the chemical shift anisotropy (CSA) of the 13C-bonded protons. The difference in sensitivities of LW(ORPI) to nur for the off-resonance and the on-resonance cases is traced back, respectively, to the scalar property of Deltanuoff for RF frequency offsets and to the tensorial character of the proton CSA. Contributions from LW(ORPI), possibly much larger than those seen in LPE, can be expected when protons near 13C nuclei sense any non-scalar, rotor-position-dependent magnetic fields, e.g., (a) local dipolar fields associated with third, magnetic nuclei or (b) perturbing magnetic-susceptibility fields arising from paramagnetic or ferromagnetic inclusions in a sample. By understanding the contributions to LW in LPE, one can forecast much more precisely what the potential benefits will be from new decoupling schemes like the recently reported "two-pulse phase modulation" (TPPM) since TPPM is designed to reduce LW(ORPI). Aside from LW(ORPI) contributions, the experimental LW data cover parameter space where another broadening mechanism, namely, MAS-assisted dipolar fluctuations (MADF), is seen. This mechanism, also recognized by others, creates a TC2-type linewidth contribution which increases rapidly as nuH1 decreases and which additionally has some orbit dependence. If the current trend in 13CCPMAS is toward higher B0 and nur, the nur-dependent MADF contributions can easily dominate LW relative to the B0-dependent LW(ORPI) contributions. One can avoid serious MADF broadening; however, the minimum acceptable values of nuH1 for good decoupling rise rapidly with B0. Finally, a few LW measurements are made on methyl-alpha-d-glucopyranoside tetraacetate (MGT), a rigid, polycrystalline material containing carbons with 0, 1, 2, and 3 attached protons. The behaviors of the methylene, methine, and methyl carbons at 9.4 T are compared with the behavior of LPE. Copyright 1998 Academic Press.
VanderHart
DL
Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland, 20899, USA.
Campbell
GC