The NMR Chemical Shift

Over the past decades the NMR chemical shift measurement in solutions has been applied to a vast range of problems in chemistry and biochemistry, and has revealed itself to be an invaluable microscopic probe. It has played an especially important role in the structural understanding of protein owing to its great sensitivity to the environment in which the probing atom is situated. Theories of the chemical shift in solution, on the other hand, have not been well developed owing to the lack of a theory for describing the electronic structure of a solvated molecule. 

In 2000 Yamazaki et al. proposed a successful theory [23] for computing the NMR chemical shift within the framework of the RISM-SCF 

Final expression of the nuclear magnetic shielding tensor can be obtained 

The proton chemical shifts in various solvents, obtained by the theory, are plotted as a function of temperature in Fig.2.12. The present method qualitatively reproduces the two observations in the recent experiment by Nakahara et al. with a modern NMR technique. [24] First, the proton chemical shifts of the solute water molecule are in the sequence over the temperature range studied, 

Secondly, the proton chemical shifts decrease with increasing temperature. These results clearly demonstrate the capability of the theory to predict experimental results. Analysis on the radial distribution function provides molecular level information on solvation structure and its relation to NMR chemical shifts. 

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Donor strengths, taken from ref. 207b, based upon the solvent effect on the symmetric stretching frequency of the soft Lewis acid HgBr2. Gutmann s donor number taken from ref 207b, based upon AHr for the process of coordination of an isolated solvent molecule to the moderately hard SbCL molecule in dichioroethane. ° Bulk donor number calculated as described in ref 209 from the solvent effect on the adsorption spectrum of VO(acac)2. Taken from ref 58, based on the NMR chemical shift of triethylphosphine oxide in the respective pure solvent. Taken from ref 61, based on the solvatochromic shift of a pyridinium-A-phenoxide betaine dye. 

The NMR chemical shifts of non-aromatic isothiazoles can be predicted with reasonable accuracy using standard substituent increments. A particular usefulness of NMR is its ability to distinguish between very similar compounds, and for this reason NMR finds application in pharmaceutical and other analyses. As an example CNMR allows ready distinction of the dlastereolsomers of dehydromethionine (14) and the possibility of detection of one dlastereolsomer in the presence of the other (79JOC2632). 

The stability of isothiazole derives from the fact that it has an aromatic delocalized ir-electron system. The NMR chemical shifts, which depend, inter alia, on ring currents, and the high stability of the molecular ions in mass spectrometry, are typical of aromatic compounds, and X-ray measurements confirm the partial double bond character of all the bonds of the ring. 

Analysis of the NMR chemical shifts of the triple bond carbon atoms of the substituted ynamines and comparison of these data with the corresponding values for compounds of the ethylene series suggest the presence of counterpolarization... 

In studies aimed at understanding the influence of structure on the reactivity of diazonium ions, Diener and Zollinger (1986) found that the NMR chemical shifts of the aromatic or heteroaromatic parent compounds provided a novel probe. This method can be applied both to substituted benzenediazonium ions and to various heteroaromatic diazonium ions, and it also provides semiquantitative information on the relative reactivities of the l,3,4-triazole-2-diazonium ion (12.5) and its deprotonated zwitterion (12.6). 

Thus, the enantiomeric contents in a pair of sulphoxides can be determined by the NMR chemical shifts in the methine or methylene protons in the two diastereomeric complexes which are stabilized by the hydrogen bond between the hydroxyl and the sulphinyl groups147-151 (Scheme 13). Similarly, the enantiomeric purity and absolute configurations of chiral sulphinate ester can be determined by measuring the H NMR shifts in the presence of the optically active alcohols152. 

Although 1- and/or 4-acylated hydroquinoxalines might not be considered as regular quinoxalone ketones, they are included here for pragmatic reasons. Examination of the NMR chemical shifts for H5 and H8 in l,4-diacyl-l,2,3,4-tetrahydroquinoxalines (197, R = Me, Et, Ph, etc.) indicated that the endo-exo conformation (shown) predominated in most such compounds at ambient temperatures. " However, this was disturbed in l,4-dibenzoyl-6,7-dimethyl-l,2,3,4-tetra-hydroquinoxaline (198), which showed a predominance of the exo-exo conformation (shown) below the coalescence temperature. ... 

The only physical property which has been studied for substituted vinylidene sets is the nmr chemical shift of the vinylidene proton in substituted ethylenes and in tra s-l,2-disubstituted ethylenes. The first attempt at correlating chemical shift data for substituted ethylenes with the Hammett equation appears to be the work of Banwell and Sheppard (53), who reported a correlation of A2 values with the or constants, the A2 values being defined by the equation... 

Six-coordinate organoiron porphyrin nitrosyl complexes, Fe(Por)(R)(NO), were prepared from Fe(Por)R (Por = OEP or TPP R = Me, n-Bu, aryl) with NO gas. The NMR chemical shifts were typical of diamagnetic complexes, and the oxidation state of iron was assigned as iron(ll). ... 

Oxygen-17 NMR spectroscopy has an immense potential for structural analysis of cyclic sulfoxides and sulfones as well as for providing insight into the nature of bonding within these two functional groups . Indeed, in addition to data concerning the NMR chemical shifts for several cyclic sulfoxides and sulfones, NMR chemical shift differences between several diastereotopic sulfonyl oxygens in both cyclic and acyclic systems have been reported . 

Similarly, the determined by the NMR chemical shift of CHCI3 in the presence of sulphoxides can be correlated well with the association constants obtained both on the basis of the IR stretching shift (Avqh) of phenol in the presence of sulphoxide and also with the F NMR chemical shifts of p-fluorophenol ((5, ). 

The H NMR spectra of the epimeric cyclohexanols in DMSO reveal that the hydroxyl proton in the axial alcohol shows a resonance absorption at a higher field than in the equatorial one, indicating that the conformational effect of the hydrogen bond influences the NMR chemical shifts . 

FIG. 9 Schematic representation of the delivery sites of PrBe (left) and BzOH (right) in the EPC SUV determined by the NMR chemical shift difference. (From Ref. 46. Copyright 1999 American Chemical Society.)... 

The NMR chemical shifts of the polymers 10 and 11 are only slightly dependent on the molecular weight. The NMR spectrum of the polymer 10 shows three shrap resonances at 6 0.34, 0.94, and 7.28 ppm, due to MeSi, EtSi, and phenylene ring protons, while the polymer 11 reveals shrap resonances at 6 0.62, 7.28, and 7.35 ppm, attributed to MeSi and phenyl ring protons. The polymers 10 and 11 show characteristic strong absorption bands at 262 and 254 nm, respectively, significantly red-shifted relative to pentamethylphenyl-disilane which exhibits an absorption band at 231 nm. 

The basicity of cyclopropenones was determined by examining the disappearance of typical IR-absorptions (see p. 38) on protonation or the change in the -NMR chemical shifts of three-ring substituents as a function of H0 in solutions of various acidity. From these measurements half-protonation was found to occur as follows ... 

The relationship between the structure of 1,2,4-thiadiazolidines and their H NMR spectral solvent effects has been studied by measurement of the NMR chemical shift differences (Av) of 39 derivatives in various solvents (CgDg, CCU) for methyl or methylene groups attached to an sp2-hybridized nitrogen, Av correlates linearly with Hammett a constants and for those attached to an sp3 4-hybridized nitrogen, with Taft a° constants <1982AHC285>. 

Quantum chemical calculations predict the primary ethylcation, C H/" to have /(-hydrido-bridgcd structure 5 which is 6-8 kcal/mol more stable than the Kekule line-bond structure for the primary cation 4. The ethylcation is not stable enough to be observable directly in superacid media. The NMR chemical shifts were calculated for both isomers 4 and 5 using the GIAO-MP2 method for CCSD optimized structures.23... 

These highly fluorinated homopolymers (7,8) derived from hydroxy fluor-ovinyl ethers (5,6), in contrast to typtical highly fluorinated compounds and polymers, exhibit unusually good solubility in common organic solvents. The NMR chemical shifts of residual protons in these polymers are highly sensitive to the polarity of the solvent as shown in Table 4.3. 

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