The Effects of 1H Decoupling Strength on 13C MAS Spectral Resolution for Solid Polymer Samples

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NM130015E

High resolution 1H decoupled 13C MAS spectra (CPMAS and DDMAS spectra), exhibiting shoulders and splittings, may yield the information on tacticity distribution and sample heterogeneity. Figure shown below is 13C CPMAS spectra for syndiotactic polypropylene, observed with (a) 1H decoupling RF field strength ν1 of 116kHz and (b) ν1 of 80.6kHz.

13C CPMAS spectra for syndiotactic polypropylene

Obviously, strong 1H decoupling irradiation improves spectral resolution:In (a), one can recognize distinct shoulders for outer CH2 and CH3, as well as a conspicuous splitting for CH.

13C-1H dipolar interaction νCH, ca 20kHz, can be averaged out by 1H-13C heteronuclear decoupling ν1, whether 80kHz or 120kHz. However, for most organic solid samples, “the third nucleus” of 1H in addition to 13C and 1H lowers 13C spectral resolution. CH2 as a three-spin model system involves not only two 13C-1H dipolar interactions of νCH=20kHz but also 1H-1H dipolar interaction of νHH=20kHz. 1H-1H dipolar interaction cannot perfectly be eliminated using multiple pulse techniques and lowers 13C spectral resolution through high-order perturbation cross terms with 1H chemical shift anisotropy. On the contrary, 1H-1H dipolar interaction left without manipulation would be removed via “self-decoupling” phenomena, but MAS prevents such self-decoupling. TPPM decoupling may recouple MAS-reduced 1H-1H dipolar interaction, thereby improving 13C spectral resolution via self-decoupling.

Reference

P. Hodgkinson, Prog. NMR Spect., 46, 197-222 (2005).

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