NMR Methods For Scalar Coupling Analysis And Spectral Resolution Enhancement

Nuclear magnetic resonance(NMR)is a powerful and versatile analytical tool widely applied in physics,chemistry and biology.It can provide molecule-level information and is capable for quantitative analysis.Scalar couplings provide valuable information on molecular structure and dynamics.The measurement of scalar coupling constants has long been an important issue in NMR spectroscopy.Scalar coupling provides fine structure of the spectrum,but it also increases spectral complexity.The analysis of scalar coupling is often hampered by limited spectral resolution and complicated multiplet patterns.This paper is aimed at designing new methods to enhance spectral resolution and improve experimental performance for the analysis of scalar coupling.We exploit the power of homonuclear decoupling technique and combine other NMR methods,and develop several methods for scalar coupling analysis and spectral resolution enhancement.The main content is as below:(1)A resolution-enhanced selective refocusing method is proposed.We use realtime refocusing and adjust the frequency of the spectrometer receiver to achieve decoupling,and employ doubly selective pulses to retain the scalar coupling between two specific spins.This method is able to resist the effect of field inhomogeneity,thus offering very narrow linewidths.Therefore,it can be used to measure very small coupling constants.It can offer spectra with high resolution even in inhomogeneous fields,ensuring the measurement of scalar coupling constants.Moreover,there are two versions for this method:real-time and pseudo 2D.These two versions can be selected for different situations,satisfying specific requirements.(2)Based on selective coupling allocation and disentanglement,a new type 2D Jedited spectroscopy dubbed CASCADE is developed for scalar coupling analysis.Through the control of scalar coupling by selective pulses,CASCADE can offer highresolution coupling information on both two dimensions,fully exploiting the power of 2D J spectroscopy.CASCADE method combines the advantages of regular 2D Jresolved spectroscopy and selective J measurement,but only cost the same time as a 2D J experiment.It can be used to measure scalar coupling constants involving the targeted proton and simplify multiplet structures of relevant protons,thus facilitating coupling analysis.(3)Because of limited chemical shift range and extensive signal splittings due to J couplings,there are often signal overlaps in normal 1D NMR spectra.Although pure shift spectroscopy offers very high resolution,it discards all scalar coupling information.A chemical shift upscaling method is proposed to enhance spectral resolution and resolve overlapped signals while retaining scalar coupling information.By using homonuclear decoupling module and adjusting evolution time,this method can offer upscaled chemical shifts and the upscaling factor can be controlled.In the obtained 1D spectrum,chemical shifts of resonances are upscaled,thus offering better signal dispersion.It is possible to separate signals that originally overlap with each other.Scalar couplings are not changed and accurate coupling constants can still be extracted.The chemical shift upscaling method provides an approach to achieve better signal dispersion and pushes the limit of magnetic field strength to some extent,facilitating signal assignment and quantitative analysis.