The effect of an unsymmetric branched branch on the 13C NMR chemical shifts of main chain carbons in polyethylene

The ¹³C NMR spectra of copolymers of ethylene with 4‐methyl‐1‐hexene and 4‐methyl‐1‐pentene, respectively, were compared. The 4‐methyl‐1‐hexene/ethylene copolymer, which contains an unsymmetric 2‐methylbutyl branch, exhibits two distinct ¹³C NMR peaks for each of the pairwise methylenes spaced one, two, and three carbons from the backbone methine. The chemical shift differences for these pairwise methylenes are 57.4 Hz, 18.7 Hz, and 4.3 Hz, respectively, with chemical shift differences decreasing with increasing distance from the asymmetric carbon. The frequency differences for carbons farther from the branch were not distinguishable. The magnitude of the chemical shift difference also varies with temperature, with the first and second methylene carbon chemical shift differences decreasing with increasing temperature. The third carbon is almost unaffected by temperature variations. In contrast, the 4‐methyl‐1‐pentene/ethylene copolymer exhibits a single peak for each of the pairs of methylenes in the branch's vicinity. This is the first reported observation of a branched branch affecting the chemical shifts of main chain carbons in polyethylene containing short chain branches. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1210–1213, 2000     

Substituent effects on 33S NMR chemical shifts in sulphones

The ³³S NMR spectra of some selected sulphones demonstrate additive substituent-induced chemical shift (SCS) effects. In dimethyl sulphone (1), replacement of a methyl group by a vinyl or a phenyl group causes an SCS effect of ?7 to ?8.5 ppm or ?4 to ?5 ppm, respectively. The ³³S chemical shift in 1 is also sensitive to substitution of methyl protons. The ß-substituent effect for methyl and phenyl groups is in the range +7 to +8 ppm and +5.5 to +6 ppm, respectively.     

Influence of NMR Data Completeness on Structure Determinations of Homodimeric Proteins

The structure determination of homodimeric proteins by NMR using conventional NOESY experiments is still challenging due to the degeneracy of the chemical shifts in the identical monomers, which causes ambiguity in the NOE assignments. Residues involved in the interface between two monomers provide essential intermolecular NOEs for the structure determinations of homodimeric proteins. Hence NMR data, such as NOE peak lists and chemical shift assignments of these interface residues, play a crucial role for the successful structure determination of homodimeric proteins. This paper extends our previous report (Lin, Y.‐J.; Kirchner, D. K.; Güntert, P. J. Magn. Reson.­ 2012 , 222, 96) and investigates the influence of incomplete NOESY peak lists combined with incomplete ¹H chemical shift assignments of the interface residues on the structure determination of homodimeric proteins using the program CYANA. Data incompleteness was simulated by random omission of both NOESY cross peaks and interface ¹H chemical shifts. Our results for three proteins with different percentages of interface residues reveal that the algorithm can tolerate about 40–50% NOESY peak omission with complete interface chemical shift assignments, which indicates that partial NOESY peak omission does not cause severe problems when the interface chemical shifts are completely assigned. Combining NOESY peak omission with incomplete interface chemical shift assignments, the tolerance for interface chemical shift omission decreases with the extent of omitted NOESY peaks. The tolerance for unassigned interface side chain, methyl and aromatic chemical shifts is affected more strongly by NOESY peak omission than that for the omission of general interface ¹H chemical shifts including the backbone. In general about 10–30% peaks omission is tolerated in conjunction with missing chemical shift assignments. If more NOESY peaks are omitted calculations gradually become unstable and tend not to tolerate any missing interface chemical shifts. A large amount of omitted NOESY peaks, for instance 30% omission in our calculations, could decrease the tolerance for missing aromatic or methyl interface ¹H chemical shifts to as few as 2–4 missing chemical shifts, suggesting that complete aromatic and methyl ¹H chemical shift assignments are important when the NOESY peak data is significantly incomplete. Finally, for homodimeric proteins with a low percentage of interface residues, our results reveal that the omission of NOESY peaks, even at an extent of only 10%, can result in no tolerance against the omission of interface ¹H chemical shifts, suggesting that the completeness of both interface ¹H chemical shift assignments and NOESY peaks are important for the successful structure determination of proteins with a small homodimer interface.     

Determination of hydration equilibrium constants and pKa values of 4-piperidones in buffered water solutions

¹H and ¹³C chemical shift parameters are given for 4-piperidones and their derivatives in buffered aqueous solution. The ¹³C shift increments of methyl substituents on the nitrogen atom are discussed. The pH-shift dependence is studied in detail and pK_a values are given for ketone forms and hydration products. The hydration equilibria are measured as a function of pH and temperature.     

17O NMR spectroscopy: Intramolecular hydrogen bonding in hydroxypyridine carboxy esters

Natural abundance ¹⁷O nmr chemical shift data for 8 aryl esters and 10 pyridine carboxy esters, including 6 ortho-hydroxy esters, recorded in acetomitrile at 75° are reported. The carbonyl group ¹⁷O nmr chemical shift data for methyl 2-, 3- and 4-pyridinecarboxylate are correlated with σ⁺ constants. The hydrogen bonding component (Δδ_HB) to the ester carbonyl ¹⁷O nmr chemical shift for the intramolecular hydrogen bonded ortho-hydroxy systems are 9.8 ppm, 13.6 ppm and 4.3 ppm for benzoates, 2-pyridinecarboxylates and 4-pyridinecarboxylates, respectively. The relationships of the ester Δδ_HB values to other hydrogen bond acceptor Δδ_HB values are discussed.     

Carbon-13 NMR studies of thyroid hormones and model compounds. The conformational analysis of diphenyl ethers

The ¹³C spectra of a series of thyroid hormones and derivatives including thyroxine (T₄) sodium salt, T₄-N-acetyl methyl ester, triiodothyronine (T₃), T₃-sodium salt, T₃-methyl ester hydrochloride, sodium triiodothyroacetate and ethyl triiodothyropropionate were assigned using chemical shift trends from model compounds, coupling constants and relaxation times. The ¹³C spectra of a series of diphenyl ethers were assigned and the chemical shift trends observed were analyzed on the basis of conformational changes engendered by mesomeric or steric effects.     

Predicting 15N amide chemical shifts in proteins. I. An additive model for the backbone contribution

Because proteins adopt unique structures, chemically identical nuclei in proteins exhibit different chemical shifts. Amide ¹⁵N chemical shifts have been shown to vary over 20 ppm. The cause of these chemical shift inequivalencies is the different intra‐ and intermolecular interactions that individual nuclei experience at different locations in the protein structure. These chemical shift inequivalencies can be described as structural shifts, the difference between the actual chemical shift and the random coil chemical shift. As a first step toward the prediction of these amide ¹⁵N structural shifts, calculations have been carried out on acetyl‐glycine‐methyl amide to examine how a neighboring peptide group influences the amide ¹⁵N structural shifts. The ϕ,ψ dihedral angle space is completely surveyed, while all other geometrical variables are held fixed, to isolate the effect of the backbone conformation. Similar calculations for a limited number of conformations of acetyl‐glycine‐glycine‐methyl amide were carried out, where the effects of the two terminal peptide groups on the central amide ¹⁵N structural shift are examined. It is shown that the effect of the two adjacent groups can be accurately modeled by combining their individual effects additively. This provides a quite simple method to predict the backbone influence on amide ¹⁵N structural shifts in proteins. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 366–372, 2001     

13C NMR spectra and carbon-proton coupling constants of various methylangelicins

The ¹³C-nmr spectra of various methyl derivatives of angelicin are reported. The assignment of chemical shifts for all the C atoms has been achieved by using carbon-proton coupling constants, nuclear Overhauser effect consideration and shift effects caused by the introduction of methyl groups on various positions of the angelicin nucleus. Substituent effects on ¹³C chemical shifts and carbon-proton coupling constants are discussed.     

Saturated amine oxides: part 10. hydroacridines: part 32. linear 17O/13C and 13C/13C chemical shift correlations between saturated azaheterocyclic N‐methylamine N‐oxides,and the methiodides of their parent amines

Two kinds of good linear correlations were found between the chemical shifts of saturated six‐membered azaheterocyclic N‐methylamine N‐oxides and the chemical shifts of the methiodides of their parent amines. One of the correlations occurs between the ¹⁷O chemical shift of the N⁺―O^– oxygen in the N‐oxides and the ¹³C chemical shift of the N⁺―CH₃ methyl group analogously situated in the appropriate methiodide (r = 0.9778). This correlation enables unambiguous configuration assignment of the N⁺―O^– bond, even if the experimentally observed ¹⁷O chemical shift of only one N‐epimer is available, provided the ¹³C chemical shifts of both N⁺―CH₃ groups in the methiodide are known and assigned; furthermore, it can be used also for the estimation of ¹⁷O chemical shifts of the N⁺―O^– oxygens in N‐epimeric pairs of N‐oxides, for which observed ¹⁷O data hardly become available. The second correlation is observed between the ¹³C chemical shift of the N⁺―CH₃ methyl group in the N‐oxides and the ¹³C chemical shift of the N⁺―CH₃ methyl group analogously situated in the appropriate methiodide (r = 0.9785). It can be used for safe configuration assignment of the N⁺―CH₃ group and, indirectly, also of the N⁺―O^– bond in an amine N‐oxide, even if no ¹⁷O NMR data, and the ¹³C chemical shift of only one N‐epimer is available. Copyright © 2015 John Wiley & Sons, Ltd.     

Chemical shift non-equivalence of the isopropyl methyl groups of two 10-isopropoxyergoline-8β-methanol derivatives

The chemical shift non-equivalence values (about 1 ppm) of the isopropyl methyl groups in the ¹H and ¹³C NMR spectra of the compounds having structure 1 is interpreted as due to the existence of a highly preferred conformation.     

Trimethylsilylation - Aid in NMR Analysis of Oligosaccharides. Assigment of 29Si and 13C NMR Spectra of Trimethylsilylated Methyl β-D-Xylobiosides by 2D NMR

The ¹H, ¹³C and ²⁹Si NMR spectra of methyl β-D-xylopyranoside and three methyl β-D-xylopyranosyl-β-D-xylopyranosides have been measured and assigned by two-dimensional NMR spectroscopy. According to the determined proton-proton coupling constants, the ring conformer ratio is essential ly the same in the studied compounds. The assigned chemical shifts provide correct substituent chemical shifts for assignments in the spectra of higher trimethylsilylated xylooligosaccharides. Heteronuclear chemical shift correlated 2D NMR spectroscopy is proven to be a usable experimental method for ²⁹Si NMR line assignment in carbohydrates. The assigned silicon shifts identify the site of glycosidation.     

A two-dimensional NMR study of angelic and tiglic acid

The controversy about the ¹³C NMR assignment of the methyl groups in angelates and tiglates was settled by two-dimensional ¹H? ¹³C chemical shift correlations for angelic and tiglic acid.     

Melamine–formaldehyde resins: Constitutional characterization by fourier transform 13C-NMR spectroscopy

The random chemical structures of melamine–formaldehyde resins, including methylated melamine–formaldehyde resins and urea–melamine formaldehyde resins, were investigated by ¹³C-NMR spectroscopy (Fourier transform). All the combined formaldehydes, methylol and methyl ether groups, methylene structures, and dimethylene ether structures were assigned. A ¹³C chemical shift of methylene carbon occurred by substitution of other constituents of the methylene group for a proton of the adjacent monosubstituted nitrogen atom, as shown in a ¹³C-NMR spectrum of urea–formaldehyde resins. It was found that the chemical shift of each corresponding carbon of both melamine resins and urea–melamine resins was almost superimposed with that of urea resins.     

Frequency dependence of nuclear magnetic resonance parameters in poly(methyl methacrylate)

Using 26 NMR spectrometers, the Research Group on NMR, the Society of Polymer Science, Japan observed the ¹H NMR chemical shift, resolution, and signal intensity; ¹³C NMR chemical shift, resolution, and signal intensity; the effect from initiator fragment signal; ¹H spin-lattice relaxation times; ¹³C spin-lattice relaxation times; and ¹³C nuclear Overhauser enhancement of radically polymerized poly(methyl methacrylate). Excellent reliability was found after comparison between the data from different spectrometers. Molecular motion of this polymer was analyzed with a term of 3τ model.     

Natural abundance 17O NMR spectroscopy of heterocyclic N-oxides and Di N-oxides. Structural effects

The ¹⁷O chemical shift data for a series of azine N-oxides, diazine N-oxides and di-N-oxides at natural abundance are reported. Isomeric methyl substituted quinoline N-oxides exhibited chemical shifts which are interpreted in terms of electronic and compressional effects. The ¹⁷O chemical shift for 8-methylquinoline N-oxide (370 ppm) is deshielded by 25 ppm more than predicted, based upon electronic considerations. The ¹⁷O chemical shift for the N-oxide of 8-hydroxyquinoline (289 ppm) is substantially shielded as a result of intramolecular hydrogen bonding. The relative ¹⁷O chemical shifts for diazine N-oxides of pyrazine, pyridazine and pyrimidine follow predictions based on back donation considerations. Because of solubility limitations, spectra of only two N,N′-dioxides were obtained. The chemical shift of benzopyrazine di N-oxide in acetonitrile was shielded by 18 ppm compared to that of its mono N-oxide.     

13C-NMR spectra and carbon-proton coupling constants of variously annulated furocoumarins

The ¹³C-nmr spectra of variously annulated methylfurocoumarins are reported. The assignments of chemical shifts for all the C resonances has been achieved by using carbon-proton coupling constants, relaxation efficiency considerations and shift effects caused by the introduction of methyl groups at various positions of the furocoumarin nucleus. Substituent effects on ¹³C chemical shifts and carbon-proton coupling constants are discussed.     

Stereochemical aspects of proton chemical shifts. VII. The γ effects of hydroxyl,methoxyl and methyl substituents

The ¹H NMR chemical shifts of some hydroxy, methoxy or methyl substituted trans-decalins, trans-1, 3-dioxadecalins and cyclohexanes are reported. It is concluded that the replacement in a g⁺g⁺ H? C? C? C? H fragment of one hydrogen by hydroxy, methoxy or methyl results in a modest (0.1 ppm) upfield shift of the other hydrogen atom. Experimental limitations to the transferability of shift increments from one molecular environment to another are demonstrated. The syntheses of 1α,5β-dimethoxy- and 1β,5α-dimethoxy-trans-decalin are given.     

NMR study of some derivatives of dithiocarbazic acids

The NMR spectra of a series of methyl and phenyl derivatives of dithiocarbazic acid have been examined. The value of the chemical shift of the S-Me group is found to be 2·3–2·6 ppm, while for N²-Me and N³-Meδ is in the range 3·4–3·6 ppm, and 2·4–2·7 ppm, respectively.Present data suggest that the rotation about the SCN and SCS bonds is sterically hindered.     

13C NMR study of N-unsubstituted aziridines: I—steric effects,pseudoconjugation

¹³C chemical shifts for twenty-nine alkyl and phenyl substituted N-unsubstituted aziridines have been measured. Additivity parameters for methyl, phenyl and aziridyl carbons have been derived with the aim of testing the consistency of the assignments made on the basis of chemical shift considerations and off-resonance decoupling information. The observed chemical shifts are discussed in terms of steric and pseudoconjugation effects.     

Assignments for the 1H-NMR of alkoxy groups in syndiotactic methacrylate copolymers—I: Methyl methacrylate-methacrylic acid copolymers and methyl methacrylate-diphenylmethyl methacrylate copolymers

Unusual assignments have been observed for the ¹H-NMR of alkoxy groups in syndiotactic methyl methacrylate-methacrylic acid (MMA-MAA) copolymers and methyl methacrylate-diphenylmethyl methacrylate (MMA-DPMMA) copolymers. Thereby, the alkoxy groups OCH₃ and OCH show a degenerate pentad assignment, in as much as the two monomer units nearest to the central monomer unit exert no differentiating influence on chemical shift, in contrast to the two next-to-nearest monomer units. By the use of copolymers possessing a tendency toward alternation with respect to compositional statistics, it is possible to distinguish between a degenerate pentad and a normal triad assignment. The reason for the degenerate pentad assignment is seen in specific conformations of the pentads, leading to the elimination of the differentiating influence on chemical shift for the two monomer units nearest to the central unit.