15N NMR Spectroscopy in Structural Analysis: An Update (2001 - 2005)

Abstract

Since our previous review article (Curr. Org. Chem. 2002, 6, 35), significant improvements and an array of 15 N NMR applications in structural analysis have been published. This report aims to update coverage of improvements in methodology and various types of applications published over the period 2001 -2005. Substantial progress in cryogenic probe technology and the commercial availability of cryoprobes have facilitated the measurement of 15 N NMR parameters.The number of solid-state applications has increased significantly during the past few years. In contrast to our previous review, this article covers 15 N solid-state studies. The 15 N NMR chemical shifts of organic molecules are routinely measured by using cross-polarization magic-angle spinning (CP/MAS) techniques. The principal values of the chemical shift tensors can also be determined. 1 H-15 N and 2 H-15 N distance measurements made by means of 1 H detection are currently used in NMR crystallography.User friendly quantum chemical programs allow for the routine calculation of 15 N chemical shielding and indirect spinspin coupling constants, especially using density functional theory (DFT).Applications of 15 N NMR spectroscopy in various fields of chemistry are summarized here. Major sections represent tautomerism, complexation, protonation, and hydrogen bonding. The other topics comprise N-alkylation, N-oxidation, regioisomerism, and changes in configuration or conformation.

Chemical Shift Tensors in Isomers of Adenine: Relation to Aromaticity of Purine Rings?

“…The experimental chemical shifts d st = 176.03 ppm for the carbonyl carbon and d st = 34.1 ppm for the nitrogen in glycine [49] were employed. Finally, the RMSDs between the theoretical and experimental data were calculated.…”

Electron Density Shift in Imidazolium Derivatives upon Complexation with Cucurbit[6]uril

“…A solution containing 1 equiv of [C 5 mim] + and 0.5 equiv of CB6 was used for the 15 N NMR measurements. [24] Two sets of signals corresponding to the free and bound forms of the guest were observed because of the slow exchange process.…”

Tautomerism and the Protonation/Deprotonation of Isocytosine in Liquid‐ and Solid‐States Studied by NMR Spectroscopy and Theoretical Calculations

“…The cluster approach and finite-charge models are commonly used. [47] In the cluster model, neighboring molecules or fragments are considered explicitly during the chemical shielding calculations. Their positions are taken, for example, from X-ray or neutrondiffraction studies.…”