Wu W, Noble D L, Owers-Bradley J R, Horsewill A J
School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, UK.
J Magn Reson. 2005 Aug;175(2):210-21. doi: 10.1016/j.jmr.2005.04.003.
Concerted double proton transfer in the hydrogen bonds of a carboxylic acid dimer has been studied using 13C field-cycling NMR relaxometry. Heteronuclear 13C-1H dipolar interactions dominate the 13C spin-lattice relaxation which is significantly influenced by the polarisation state of the 1H Zeeman reservoir. The methodology of field-cycling experiments for such heteronuclear spin-coupled systems is studied experimentally and theoretically, including an investigation of various saturation-recovery and polarisation-recovery pulse sequence schemes. A theoretical model of the spin-lattice relaxation of this coupled system is presented which is corroborated by experiment. Spectral density components with frequencies omega(C), omega(C) + omega(H), and omega(C) - omega(H) are mapped out experimentally from the magnetic field dependence of the 13C and 1H spin-lattice relaxation and the proton transfer rate at low temperature is determined from their widths. Any dynamic isotope effect on the proton tunnelling in the hydrogen bond arising from 13C enrichment in the skeletal framework of the dimer is found to be smaller than experimental uncertainties (approximately 5%).
利用13C场循环核磁共振弛豫测量法研究了羧酸二聚体氢键中的协同双质子转移。异核13C-1H偶极相互作用主导了13C自旋晶格弛豫，而1H塞曼库的极化状态对其有显著影响。对这种异核自旋耦合系统的场循环实验方法进行了实验和理论研究，包括对各种饱和恢复和极化恢复脉冲序列方案的研究。提出了该耦合系统自旋晶格弛豫的理论模型，并得到了实验的证实。通过13C和1H自旋晶格弛豫对磁场的依赖性，实验绘制出了频率为ω(C)、ω(C)+ω(H)和ω(C)-ω(H)的谱密度分量，并根据其宽度确定了低温下的质子转移速率。发现二聚体骨架中13C富集引起的氢键中质子隧穿的任何动态同位素效应都小于实验不确定度（约5%）。
