Statistical Learning of NMR tensors from 2D Isotropic/Anisotropic Correlation Nuclear Magnetic Resonance Spectra

Abstract

Many linear inversion problems involving Fredholm integrals of the first kind are frequently encountered in the field of magnetic resonance. One important application is the direct inversion of a solid-state NMR spectrum containing multiple overlapping anisotropic subspectra to obtain a distribution of the tensor parameters. Because of the ill-conditioned nature of this inverse problem, we investigate the use of the TSVD-S-LASSO based regularization method, which (a) stabilizes the solution and (b) promotes sparsity and smoothness in the solution. We also propose a unambiguous representation for the anisotropy parameters using a piecewise polar coordinate system to minimize rank deficiency in the inversion kernel. To obtain the optimum tensor parameter distribution, we implement the k-fold cross-validation, a statistical learning method, to determine the hyperparameters of the regularized inverse problem. In this article, we provide the details of the linear- inversion method along with numerous illustrative applications on purely anisotropic NMR spectra, both synthetic as well as experimental two-dimensional spectra correlating the isotropic and anisotropic frequencies.

Publication
J. Chem. Phys.