The dissolution mechanism of cellulose in ionic liquids has been investigated by using cellobiose and 1-ethyl-3-methylimidazolium acetate (EmimAc) as a model system under various conditions with conventional and variable-temperature NMR spectroscopy. In DMSO-d6 solution, NMR data of the model system clearly suggest that hydrogen bonding is formed between hydroxyls of cellobiose and both anion and cation of EmimAc. The CH3COO− anion favors the formation of hydrogen bonds with hydrogen atoms of hydroxyls, and the aromatic protons in bulky cation [Emim]+, especially the most acidic H2, prefer to associate with the oxygen atoms of hydroxyls with less steric hindrance, while after acetylation of all hydroxyls in cellobiose the interactions between cellobiose octaacetate and EmimAc become very weak, implying that hydrogen bonding is the major reason of cellobiose solvation in EmimAc. Meanwhile the stoichiometric ratio of EmimAc/hydroxyl is estimated to be between 3:4 and 1:1 in the primary solvation shell, suggesting that there should be one anion or cation to form hydrogen bonds with two hydroxyl groups simultaneously. In situ and variable-temperature NMR spectra suggest the above mechanism also works in the real system.

NMR spectroscopic studies of cellobiose solvation in EmimAc aimed to understand the dissolution mechanism of cellulose in ionic liquids

Designing Defective Crystalline Carbon Nitride to Enable Selective CO2 Photoreduction in the Gas Phase

Publisher Summary Nitrogen nuclear magnetic resonance (NMR) is finding new applications. This chapter presents some of the applications of the nitrogen NMR. The element nitrogen in its natural occurrence has two isotopes; 14N and 15N. The nuclei of both possess magnetic moments; consequently, both give nuclear magnetic resonance (NMR) spectra and additionally may affect the spectra of other nuclei. The NMR spectra of nitrogen nuclei may provide an important contribution to the development of the general theory of chemical shifts, by allowing a rigorous check of current theories. This arises from the variety of nitrogen valence states occurring in the molecules, including more or less non-bonding electron pairs, many types of bonds of varying polarity and hydrogen bonding, as well as the common occurrence of nitrogen atoms in both neutral and ionic species. 14N nucleus has an electric quadrupole moment and this is responsible for producing broad-line NMR spectra. Broad lines often occur both in the spectrum of the 14N nucleus and in the spectrum of any nucleus spin coupled to the nitrogen nucleus that may preclude the detection of any spin-spin coupling between these nuclei. The electric quadrupole relaxation process of the 14N nucleus may influence the NMR spectra of nuclei that are spin-coupled to it. The most frequently encountered example is the N-H coupling. The chapter presents some examples of nitrogen chemical shifts in the organic and inorganic molecules. However, spin-spin coupling, involving nitrogen nuclei, is readily observed for the 15N isotope than for 14N. As 15N nuclei possess a spin of 1/2 and no quadrupole moment, these give NMR spectral patterns similar to those arising from lH and 19F interactions.

Nitrogen NMR Spectroscopy

Structural environments of carboxyl groups in natural organic molecules from terrestrial systems. Part 1: Infrared spectroscopy

Chemical shifts and protonation shifts in carbon-13 nuclear magnetic resonance studies of aqueous amines

The ^(13)C chemical shifts of each of the carbons of the simple aliphatic carboxylic acids and their corresponding anions from formic to valeric acid have been measured in aqueous solution. Perhaps surprisingly, ionization of an aliphatic acid results in a downfield shift of its carbon resonances. The results have been correlated with other substituent effects, with ^(13)C and H resonances in corresponding hydrocarbons, and with the distance between
the carboxylic(ate) group and the carbons undergoing resonance absorption.

Nuclear Magnetic Resonance Spectroscopy. ^(13)C Spectra of Aliphatic Carboxylic Acids and Carboxylate Anions

The ^(15)N and ^(13)C chemical shift changes have been determined for ethylenediaminetetraacetic acid (EDTA) as the result of changes in pH and complex formation with closed-shell metal ions. The carboxyl-carbon lac shifts in parts per million resulting from complex formation with metal ions are generally smaller than the concomitant
shifts of the ^(15)N resonances which themselves are not large. The comparatively small ^(15)N chemical shift
changes of EDTA upon metal complex formation are probably due to near-cancellation of variations in local paramagnetic
and diamagnetic shielding terms. This may be a general phenomenon for ligands lacking a large local paramagnetic contribution to the shielding of the nitrogen donor atoms.

Nuclear magnetic resonance spectroscopy. Nitrogen-15 and carbon-13 spectra of complexes of ethylenediaminetetraacetic acid (EDTA) with closed-shell metal ions

R. Hagen

Papers

Nuclear Magnetic Resonance Spectroscopy. ^(13)C Spectra of Aliphatic Carboxylic Acids and Carboxylate Anions

Nuclear magnetic resonance spectroscopy. Nitrogen-15 and carbon-13 spectra of complexes of ethylenediaminetetraacetic acid (EDTA) with closed-shell metal ions