Pike K J, Malde R P, Ashbrook S E, McManus J, Wimperis S
School of Chemistry, University of Exeter, UK.
Solid State Nucl Magn Reson. 2000 Jun;16(3):203-15. doi: 10.1016/s0926-2040(00)00081-3.
The question of whether or not higher-order (five-, seven- and nine-quantum) multiple-quantum magic angle spinning (MQMAS) experiments yield isotropic NMR spectra of half-integer quadrupolar nuclei with higher resolution than the basic three-quantum MAS experiment is examined. The frequency dispersion is shown theoretically to be greatly increased in higher-order MQMAS spectra, but it is argued that whether or not this translates into an increase in resolution depends upon the ratio of the homogeneous to inhomogeneous contributions to the isotropic linewidth. Experimentally, it is demonstrated using three-, five- and seven-quantum 45Sc MAS NMR and three- and five-quantum 27Al MAS NMR of crystalline samples that higher-order MQMAS experiments can yield a real and useful increase in resolution but that, owing to the presence of inhomogeneous broadening in the isotropic spectra, this increase is less than the theoretically predicted value. A number of practical issues relating to resolution in MQMAS NMR are also pointed out.
本文研究了高阶（五量子、七量子和九量子）多量子魔角旋转（MQMAS）实验是否能产生比基本三量子MAS实验具有更高分辨率的半整数四极核各向同性NMR谱。理论上表明，高阶MQMAS谱中的频率色散会大大增加，但有人认为，这是否转化为分辨率的提高取决于均匀和非均匀贡献对各向同性线宽的比例。通过对晶体样品进行三量子、五量子和七量子45Sc MAS NMR以及三量子和五量子27Al MAS NMR实验证明，高阶MQMAS实验确实能在分辨率上实现实际且有用的提高，但由于各向同性谱中存在非均匀展宽，这种提高小于理论预测值。还指出了一些与MQMAS NMR分辨率相关的实际问题。
