A Compatibility Study of a Secondary Amine Active Pharmaceutical Ingredient with Starch: Identification of a Novel Degradant Formed Between Desloratadine and a Starch Impurity Using LC–MSn and NMR Spectroscopy

On the utility of nonuniformly sampled two-dimensional NMR spectra in the pharmaceutical industry

In this work, we discuss representative examples of the application of nonuniform sampling (NUS) in small-molecule structure determination in a pharmaceutical research and development and quality control setting. We demonstrate the advantages of NUS over traditional sampling in various industrial applications of nuclear magnetic resonance (NMR). We propose an optimal trade-off between the quality and the time efficiency of 'routine' measurements, as demonstrated via a test sample of vinpocetine analyzed on a 'work horse' NMR spectrometer. In addition, we present case studies where the application of NUS contributed significantly to the successful completion of some challenging structure determination task at hand. To that end, in some cases, we combined NUS with a method of decoupling in the indirect dimension that gives 'constant time' nuclear Overhauser effect spectroscopy (NOESY) and 'constant time' heteronuclear multiple bond correlation (HMBC) spectra. NUS proved to be superior over traditional sampling even for the analysis of trace impurities, where the need to cope with a thousand-fold or even larger dynamic range and low signal-to-noise ratio for the impurity signals is quite a challenge.

Computational protocols for calculating 13C NMR chemical shifts

The most recent results dealing with the computation of ¹³C NMR chemical shifts in chemistry (small molecules, saturated, unsaturated and aromatic compounds, heterocycles, functional derivatives, coordination complexes, carbocations, and natural products) are reviewed, paying special attention to theoretical background and accuracy, the latter involving solvent effects, vibrational corrections, and relativistic effects.

High-resolution NMR spectroscopy in inhomogeneous fields

High-resolution NMR spectroscopy, providing information on chemical shifts, J coupling constants, multiplet patterns, and relative peak areas, is a mainstream tool for analysis of molecular structures, conformations, compositions, and dynamics. Generally, a homogeneous magnetic field is a prerequisite for obtaining high-resolution NMR information. Magnetic field inhomogeneity, whether from non-ideal experimental conditions or from intrinsic magnetic susceptibility discontinuities in samples, represents a hurdle for applications of high-resolution NMR. Numerous techniques have been proposed for measuring high-resolution NMR spectra free from the influence of inhomogeneous magnetic fields. Besides developments and improvements in NMR instrumentation, various types of experimental approaches have been established for recovering NMR information in inhomogeneous magnetic fields. Three main types are systematically described in this review. In addition, other high-resolution NMR approaches or data processing methods are also briefly described. All high-resolution NMR approaches covered in this review have individual advantages and disadvantages in practical applications, and no one technique is applicable to all practical circumstances. Hence, they are complementary for high-resolution NMR applications in inhomogeneous fields. The underlying mechanisms of these approaches are presented, together with analyses of their applicability and efficiency.