New Methods For Resolving J Coupling Constants In Nuclear Magnetic Resonance Spectroscopy

Nuclear magnetic resonance(NMR)has attracted much attention in the scientific community since it came into being.After decades of rapid development,it has been playing an important role in many fields such as physics,chemistry,biology and medicine.Nuclear magnetic resonance spectroscopy also gradually developed from the early one-dimensional spectrum to two-dimensional spectrum,and even multidimensional spectrum,providing more and more information.As two important parameters in NMR spectra,chemical shift and J coupling constants have always been hot topics in NMR research.In particular,homonuclear scalar coupling plays an indispensable role in elucidating molecular structure and molecular dynamics.However,there are complex multiplets due to 1H-1H scalar coupling spilttings.The identification and assignment of peaks in the crowded spectral region is complicated.Although many methods for resolving the J coupling constants have been proposed in recent years,there are still some shortcomings,such as dispersive components and strong axial peaks.Besides,selective homonuclear correlation experiments can also be used to analyze scalar coupling and measure scalar coupling values.However,anti-phase signals constitute to peaks in the spectrum,thus positive and negative components will cause self-cancelling and lineshape distortion,leading to the difficulty in spectral analysis.Here,we proposed two new methods to analyze J coupling constants.One is a postprocessing method for obtaining axial peaks free PSYCHEDELIC spectra.Based on the conventional Pell-Keeler method and combined with the interleaved acquisition mode,the PSYCHEDELIC spectra with pure absorptive lineshapes and no axial peaks can be obtained.The other is ISSS-COSY experiment,which is slice-selective in-phase selective homonuclear correlation spectra for soft-COSY,so as to avoid the problems of signal self-cancellation and lineshape distortion.This paper proposed a new post-processing method for PSYCHEDELIC spectra,which is a simple and improved method based on Pell-Keeler post-processing.Combined with the interleaved acquisition mode,the normal FID signals and the reversed FID signals can be obtained alternately,avoiding the influence of instrument instability on the spectrum.Thus,the absorptive 2D J spectrum can be obtained and axial peaks can be completely eliminated.This method can achieve high resolution and pure absorption spectra,and can clearly show the coupling network involving a given proton and accurately measure relevant scalar coupling constants.In this paper,we also propose a new method named ISSS-COSY to obtain in-phase soft-COSY spectra.It aims at the problem that conventional soft-COSY spectra have anti-phase signals and cannot be acquired in parallel.By combining the modules of perfect echo,selective pulses and gradient filtering,the active coupling signal modulated by sine is eliminated and the active coupling signal modulated by cosine is preserved.In addition,we also use ZS slice-selective method to realize parallel acquisition,which greatly reduces the experimental time.Therefore,ISSS-COSY experiment can obtain parallel acquisition of in-phase soft-COSY spectra,and avoid the problems of signal self-cancellation and lineshape distortion.