Solvent Effects on Nitrogen Chemical Shifts

Hanna Andersson; Anna-Carin C. Carlsson; Bijan Nekoueishahraki; Ulrika Brath; Máté Erdélyi    Department of Chemistry and Molecular Biology, University of Gothenburg, Gothenburg, Sweden

1 Introduction

Nitrogen atoms are ubiquitous in biologically significant secondary metabolites (alkaloids, cytokinins), biomacromolecules (proteins, peptides, DNA, RNA) as well as in synthetic organic substances, and commonly belong to atomic centers of importance for intra- and intermolecular interactions. Changes in the molecular environment, in interactions or in molecular topology influence nitrogen's electron density, and are thereby directly reflected in the magnitude of the nitrogen chemical shift. Due to its nonbonding electron pair, nitrogen often acts as Lewis base and hence may interact with protic solvents, for instance. Due to the central role of the lone electron pair of the nitrogen in interactions and reactions, the nitrogen NMR chemical shift is a sensitive parameter for monitoring complexation, protonation, N-alkylation and N-oxidation. The large, up to 1200 ppm, dispersion of the nitrogen chemical shift, and its high sensitivity to environmental changes makes nitrogen NMR into an exceptionally useful tool for structural and physicochemical studies.

The most abundant nitrogen isotope, 14N, has spin I = 1, resulting in tens to hundreds of Hz broad signals. Therefore the less abundant, 0.36%, yet spin I = ½ isotope 15N is the preferred nucleus for detection of nitrogen signals. As certain subfields published mostly 14N whereas others 15N NMR data, in this review both are reported. Due to major hardware improvements, such as the introduction of cryogenically cooled probe technologies, development of microprobes and high-field NMR magnets, along with the smooth availability of inverse detection pulse sequences (HSQC, HMBC), the sensitivity of 15N NMR has greatly improved, and thus 15N NMR data can be smoothly obtained even for low amounts of organic compounds, without the necessity of 15N labeling.

Nitrogen NMR spectroscopy has been repeatedly reviewed [1–10] over the past decades, typically with focus on its applicability in structural analysis. Although occasionally acknowledged, the influence of solvent on the magnitude of nitrogen chemical shifts has not yet been discussed in a comprehensive fashion. Herein, we review the available literature reports on the influence of the solvent on the nitrogen NMR chemical shift, collected from various fields of chemistry, and focusing on data of heterocycles and their metal coordination complexes, of nucleobases, nucleosides, nucleotides, and peptides. We do not attempt to review all the available literature, but discuss representative experimental examples to show the general trends. In order to reflect the common discussions of each subfield, the subchapters follow the nomenclature of the corresponding research area and focus on its most important questions. Computational prediction of chemical shifts is becoming a useful tool; however, it is not discussed in this review that has its emphasis on experimental data.

2 Referencing of Chemical Shifts

IUPAC recommends the use of a unified chemical shift scale, relative to the 1H resonance of tetramethylsilane set to 0 ppm for all nuclei [11,12]. In practice, this recommendation is not followed, but reference compounds are used to define the 0 ppm point of each nucleus. Most nuclei have one standard secondary reference, whereas 15N has two, external nitromethane (CH3NO2) for most cases, and ammonia (NH3) as an alternative for aqueous samples. Several additional nitrogen references were also applied in the literature, and therefore experimentally obtained conversion factors, Ξ, were introduced, which are the ratio of the secondary reference frequency to that of 1H in TMS in the same magnetic field. Consequently, following the measurement of ν(1HTMS) on any system, the 0 ppm reference frequency for any other nucleus may be calculated using the conversion factors. Wishart et al. reported conversion factors for NH3, and upon simultaneous measurements with CH3NO2 defined the conversion δ(CH3NO2) = δ(NH3) + 381.7 ppm, when the sample is measured in a superconducting magnet with the sample parallel to the magnetic field [13]. Previous measurements by Srinivasan et al. using a spectrometer in which the static magnetic field is perpendicular to the sample, defined δ(CH3NO2) = δ(NH3) + 380.2 ppm [14]. Because of the different bulk susceptibilities, the chemical shift conversion of the two techniques are truly different, and hence the conversion between δ(NH3) and δ(CH3NO2) is different.

The 15N NMR shifts (δ15N) are reported in ppm in this review, referenced to external nitromethane, δ(neat CH3NO2) = 0 ppm, which is the most common method of referencing δ15N in organic chemistry to date. Measurements are typically done by inserting a capillary of neat nitromethane into the NMR tube, and referencing its signal to 0 ppm. Correction for some of the most common yet not recommended 15N NMR standards are as follows: neat NH4Cl (− 339.5 ppm), 1 M aqueous NH4Br (− 352.9 ppm), 1 M aqueous 1,4-morpholine (− 347.9 ppm), 1 M N(CH3)4Br (− 336.5), DMF (− 275.3 ppm), 1 M urea in DMSO (− 303.2 ppm), pyridine (− 63.2 ppm), 1 M aqueous HNO3 (− 2.7 ppm), 1 M aqueous NaNO3 (− 3.7 ppm), when δ(neat CH3NO2) is defined as 0 ppm. Conversion factors for further nitrogen chemical shift references are given in [15].

3 Heterocycles

Nitrogen heterocyclic compounds are broadly distributed in nature, with many of them possessing useful biological activities, and are frequently utilized by various industries, making the understanding of their structural and electronic properties utmost important. Nitrogen NMR has become a common tool to assess the properties and interactions of heteroaromatic systems as the electron densities of π-excessive and π-deficient heteroaromatic compounds is well reflected by the chemical shift of their nitrogens. For example, the nitrogen NMR shift (δN) of pyridine (− 57.7 ppm) is much more deshielded than that of pyrrole (− 238.7 ppm) indicating their different electronic property and reactivity. The nitrogen of heterocyclic systems may interact with their solvent environment in various ways, including donation and acceptance of hydrogen bonds, modulation of the atomic electron densities, and shifting of tautomerization equilibria upon solvent polarity alterations, being the most important examples. For a quantitative experiment-based description of specific and nonspecific contributions to solvent-induced 15N NMR chemical shift variations, Webb and coworkers have developed an empirical equation, based on the Kamlet–Taft solvatochromic scale of solvent polarity [16–20]:

ij=σ0i+aiαj+biβj+siπ*j+diδj

where i and j represent the solute and solvent, respectively, α is the measure of the solvent hydrogen-bond donor (HBD) capacity,...