The Research Of Some HMBC Related Experiments

The heteronuclear multiple-bond correlation(HMBC) method is probably the single most important 2D NMR method for the structural elucidation of organic compounds.It provides correlation signals for C,H spin pairs indicating connectivities over two or more bonds and therefore can be used to build a molecular skeleton.This dissertation mainly focuses on the study and comparison of HMBC related experiments.In chapter one,the development of HMBC related experiments,which sorted by date are presented.In chapter two,a summary of the method for distinguishing between ²J_CH and ⁿJ_CH(n＞2) correlations was provided,which are ²J,³J-HMBC;1,1-ADEQUATE; H₂BC;²J/ⁿJ-HMBC.Chapter three and four concentrated on the study of spectrum comparisons among three HMBC experiments,ACCORD-HMBC,IMPEACH-MBC and CIGAR-HMBC. In order to find the contrast reason of the three experiments,a series of experiments have been done during this study,using a chain compounds-2-Ethylhexyl Acrylate and subsequently a ring compounds-sucrose as the sample.Firstly,we compared the three experiments,ACCORD(2-25Hz),IMPEACH-MBC(2-25Hz),CIGAR(2-25Hz) with HMBC(10Hz).Different results were obtained when HMBC is taken as the comparison experiment.In other words,there are differences among the three experiments,ACCORD-HMBC(2-25Hz),IMPEACH-MBC(2-25Hz) and CIGAR-HMBC (2-25Hz).The advantage of the three experiments is mainly attributed to the cross peak's long range coupling constants of the sample.Different from HMBC,in ACCORD-HMBC,IMPEACH-MBC and CIGAR-HMBC,¹³C GARP decoupling is applied during acquisition which improves the signal to noise ratio.This is the other reason for the differences.Then by varying the sampling ranges of long range couplings,comparison experiments under 6-10Hz,3-16Hz and 2-25Hz were done. Similar results were obtained from the two different samples.The differences among three experiments are mainly affected by the sampling ranges of long range couplings and the differences in pulse sequences.Analyzing the experiments,the IMPEACH-MBC,which suppresses F₁ modulation,showed more cross peaks compared with ACCORD-HMBC and CIGAR-HMBC.The comparison results of IMPEACH-MBC among three different sampling ranges futher indicated that there were more cross peaks in the spectrum when the sampling range of long range coupling was setted to 3-16 Hz.If the range is too small it can not cover all of the sample's long range coupling constants,if the range is too large it will brings the signal to noise ratio loss.