1H NMR spectra part 31: 1H chemical shifts of amides in DMSO solvent

The ¹H chemical shifts of 48 amides in DMSO solvent are assigned and presented. The solvent shifts Δδ (DMSO‐CDCl₃) are large (1–2 ppm) for the NH protons but smaller and negative (?0.1 to ?0.2 ppm) for close range protons. A selection of the observed solvent shifts is compared with calculated shifts from the present model and from GIAO calculations. Those for the NH protons agree with both calculations, but other solvent shifts such as Δδ(CHO) are not well reproduced by the GIAO calculations. The ¹H chemical shifts of the amides in DMSO were analysed using a functional approach for near ( ≤ 3 bonds removed) protons and the electric field, magnetic anisotropy and steric effect of the amide group for more distant protons. The chemical shifts of the NH protons of acetanilide and benzamide vary linearly with the π density on the αN and βC atoms, respectively. The C=O anisotropy and steric effect are in general little changed from the values in CDCl₃. The effects of substituents F, Cl, Me on the NH proton shifts are reproduced. The electric field coefficient for the protons in DMSO is 90% of that in CDCl₃. There is no steric effect of the C=O oxygen on the NH proton in an NH…O=C hydrogen bond. The observed deshielding is due to the electric field effect. The calculated chemical shifts agree well with the observed shifts (RMS error of 0.106 ppm for the data set of 257 entries). Copyright © 2014 John Wiley & Sons, Ltd.     

1H NMR spectra. Part 29§: proton chemical shifts and couplings in esters—the conformational analysis of methyl γ‐butyrolactones

The ¹H NMR spectra of 35 cyclic and acyclic esters are analysed to give the ¹H chemical shifts and couplings. The substituent chemical shifts of the ester group were analysed using three‐bond (γ) effects for near protons and the electric field, magnetic anisotropy and steric effect of the ester group for more distant protons. The electric field is calculated from the partial atomic charges on the O?C = O atoms, and the asymmetric magnetic anisotropy of the carbonyl group acts at the midpoint of the C = O bond. The values of the anisotropies Δχ_parl and Δχ_perp were for the aliphatic esters 10.35 and ?18.84 and for the conjugated esters 7.33 and ?15.75 (×10^?6 ų/molecule). The oxygen steric coefficients found were 104.4 (aliphatic C = O), 45.5 (aromatic C = O) and 16.0 (C–O) (×10^?6 Å⁶/molecule). After parameterisation, the overall RMS error for the data set of 280 entries was 0.079 ppm. The strongly coupled ¹H NMR spectra of the 2‐methyl, 3‐methyl and 4‐methyl γ‐butyrolactones were analysed and the methyl conformational equilibrium obtained from the observed couplings. The observed versus calculated density functional theory (DFT) ΔG(ax‐eq) was 1.0 (1.01), 0.34 (0.54) and 0.65 (0.71) kcal/mol res. The shielding effect of a methyl cis to a proton in the five‐membered lactone rings is ?0.40 ±0.05 ppm and deshielding trans effect 0.12 ±0.05 ppm, which is common to both five and six membered rings. The cis/trans isomerism in the vinyl esters methyl acrylate, crotonate and methacrylate and methyl furoate was examined using the ¹H chemical shifts. The calculated shifts of both the cis and trans isomers were in good agreement with the observed shifts. Copyright © 2012 John Wiley & Sons, Ltd.     

Structural Studies of Self‐Folding Cavitands

Resorcinarene‐based cavitands 1a – c fold into a deep open‐ended cavity by means of intramolecular hydrogen bonds in both apolar solutions and the solid state. The X‐ray crystal‐structure analysis of cavitand 1a features a seam of secondary amide C=O⋅⋅⋅H−N interactions that bridge adjacent rings and are held in place by intra‐annular hydrogen bonds. This results in a cavity of 9.2×7.0 Å dimensions. The arrangement of the amides in 1a – 1c is cycloenantiomeric, with clock‐ and counterclockwise orientation of the head‐to‐tail amide sequence. Interconversion rates of the two enantiomers are controlled by solvent polarity: the rate is slow on the NMR time‐scale in aromatic solvents and CDCl₃, but fast in (D₆)acetone. The ¹H‐ and ¹³C‐NMR‐spectral analysis is in agreement with the crystallographic data. Chiral cavitand 1b with eight HN−C(O)−C*HMeEt ((+)‐(S)) groups on its upper rim exists as two cyclodiastereoisomers (in a ca. 3 : 1 ratio) in apolar solution. A `library' of 512 diastereoisomeric cavitands 1c is obtained as a mixture by using the corresponding racemic acid chloride.     

Additives atomares punktdipol-(APUDI)-modell zur berechnung der 1h-NMR-werte von benzoiden kohlenwasserstoffen

Our empirical atomic point dipole (APUDI) model simply allows the calculation of ring-current produced chemical shifts of the ring protons of 17 planar benzenoid hydrocarbons with high accuracy. It simulates the effect of the anisotropy of diamagnetic susceptibility of the π-systems by assumption of two effective atomic magnetic point dipoles, located perpendicular above and below each carbon atom of the π-system at 70 pm, the distance of the maximum of π-electron density. Application of the McConnell equation on each of these point dipoles leads after summation for each ring proton j to a corresponding geometry factor GF_j which was correlated by linear least squares method with 160 literature values of experimental chemical shifts leading to a value of (- 12.5± 0.7) × 10^-36m³, the effective anisotropy of diamagnetic susceptibility per atomic point dipole with a correlation coefficient of 0.943. The chemical shift values may be obtained in the APUDI-model by the least squares equation δ^calc_j p me>= - 12.5 × 10^-36 GF_j - 5.16. This empirically determined slope may be interpreted as the average value of the z-component of the experimental diamagnetic susceptibility per atomic point dipole of these benzenoid aromatic compounds. Besides the simple applicability the APUDI-model yields superior results for the chemical shifts of sterically crowded protons in comparison to many known ring current models.     

Proton chemical shifts in NMR: Part 17. Chemical shifts in alkenes and anisotropic and steric effects of the double bond

The ¹H NMR spectra of a number of alkenes of known geometry were recorded in CDCl₃ solution and assigned, namely ethylene, propene, 4-methylcyclohexene, 1,4-dimethylcyclohexene, methylene cyclohexane (in CFCl₃–CD₂Cl₂ at 153 K), 5-methylene-2-norbornene, camphene, bicyclopentadiene, styrene and 9-vinylanthracene. These results together with literature data for other alkenes, i.e. 1,3- and 1,4-cyclohexadiene, norbornene, norbornadiene, bicyclo[2.2.2]oct-2-ene and α- and β-pinene, and other data allowed the determination of the olefinic shielding in these molecules. The shielding was analysed in terms of the magnetic anisotropy and steric effects of the double bond together with a model (CHARGE7) for the calculation of the two- and three-bond electronic effects. For the aromatic alkenes ring current and π-electron effects were included. This analysis showed that the double bond shielding arises from both anisotropic and steric effects. The anisotropy is due to the perpendicular term only with a value of Δχ(CC) of −12.1 × 10⁻⁶cm³mol⁻¹. There is also a steric deshielding term of 82.5/r⁶ (r in Å). The shielding along the π-axis changes sign from shielding at long range (>2.5 Å) to deshielding at short range (<2 Å). The model gives the first comprehensive calculation of the shielding of the alkene group. For the data set considered (172 proton chemical shifts) ranging from δ=0.48 to 8.39, the r.m.s. error of observed vs calculated shifts was 0.11 ppm. Copyright © 2001 John Wiley & Sons, Ltd.     

Correlation of substituted aromatic β‐diketones' characteristic protons chemical shifts with Hammett substituent constants

Influence of dibenzoylmethane's substituents in meta and para positions on chemical shift values of tautomers' characteristic protons was investigated in four solvents with ¹H NMR spectroscopy: acetone‐d₆, benzene‐d₆, CDCl₃ and deuterated dimethyl sulfoxide (DMSO‐d₆). It was proved that the influence of substituents on chemical shifts strongly depends on the kind of the solvent; the greatest changes were observed in benzene‐d₆ and the smallest in CDCl₃. In acetone‐d₆ and DMSO‐d₆, the influence of substituents on chemical shifts is similar and the most regular. It allowed a fair correlation of chemical shifts of para‐substituted dibenzoylmethane derivatives' characteristic protons with Hammett substituent constants in these solvents. In CDCl₃, characteristic protons' chemical shifts were near ¹H NMR spectroscopy measurement error limits, and, therefore, correlation with Hammett substituent constants in this solvent was unsatisfactory. In benzene, although the changes of chemical shifts are the most evident, the changes are also the most irregular, and, therefore, correlation in this solvent failed completely. Results of meta‐substituted derivatives were much more irregular, and their correlation with Hammett substituent constants was poor in all investigated solvents. Copyright © 2013 John Wiley & Sons, Ltd.     

Study of molecular structures for arylidene derivatives of 3R‐methylcyclohexanone by 1H NMR and molecular simulation

Conformational states of the cyclohexanone ring were established for 3R‐methyl‐6‐(4‐phenylbenzylidene)cyclohexanone and several 2,6‐bis(4‐X‐benzylidene)‐3R‐methylcyclohexanones (X = Br, OCOCH₃ and C₆H₅) by ¹H NMR spectroscopy combined with molecular simulation using the semi‐empirical methods AM1 and PM3. The first compound studied contains only one arylidene group, and exists predominantly in a chair conformation of the cyclohexanone ring with an equatorial orientation of the methyl substituent in C₆D₆ and CDCl₃ solutions at room temperature (22–23 °C). In contrast, the 2,6‐bis(arylidene) derivatives of 3R‐methylcyclohexanone preferentially adopt conformations with an axially oriented methyl group. The extent of twisting of enone fragments was also characterized for the compounds studied based on simulation results and comparison of chemical shifts for the arylidene protons of appropriate model compounds. Copyright © 2003 John Wiley & Sons, Ltd.     

Counterion influence on chemical shifts in strychnine salts

The highly toxic plant alkaloid strychnine is often isolated in the form of the anion salt of its protonated tertiary amine. Here, we characterize the relative influence of different counterions on ¹H and ¹³C chemical shifts in several strychnine salts in D₂O, methanol‐d₄ (CD₃OD), and chloroform‐d (CDCl₃) solvents. In organic solvents but not in water, substantial variation in chemical shifts of protons near the tertiary amine was observed among different salts. These secondary shifts reveal differences in the way each anion influences electronic structure within the protonated amine. The distributions of secondary shifts allow salts to be easily distinguished from each other as well as from the free base form. Slight concentration dependence in chemical shifts of some protons near the amine was observed for two salts in CDCl₃, but this effect is small compared with the influence of the counterion. Distinct chemical shifts in different salt forms of the same compound may be useful as chemical forensic signatures for source attribution and sample matching of alkaloids such as strychnine and possibly other organic acid and base salts. Copyright © 2013 John Wiley & Sons, Ltd.     

Correlation between the reactivity and spectroscopic properties of N‐substituted secondary thioamides. New intramolecular N···H+···N binding approach and proton complexes based on thioamide ligation

On the basis of a comparison of chemical shifts and wavenumbers of several secondary thioamides and amides having monocationic substituents attached to thiocarbamoyl or carbamoyl groups by a polymethylene chain, new intramolecular unconventional N···H⁺···N hydrogen bonding effects were discovered. It is argued that the CH₂—N rotation is hindered and two ⁺H···NHCH₃ non‐equivalent protons occur in a proton spectrum of hydrochloride 1a (at 10.68 and 2.77 ppm, respectively) instead of two ⁺NH₂CH₃ protons. Presumably, the above steric factors inhibit the acidic hydrolysis of 1a (stabilized by strong intramolecular N···H⁺···N hydrogen bonds) to an amide and prevent intramolecular cyclization of 2a (stabilized by strong intramolecular neutral–neutral N···HN hydrogen bonds) to a cyclic amidine. Postulation of additional dihydrogen bond formation is helpful in understanding the spectroscopic differences of 4 and 5 . The above new bonding is also compared with intramolecular N···H—N⁺ hydrogen bonds in primary amine salts 7 and 8 . In contrast to 3 , a cooperative hydrogen bonded system is observed in 9 and 10 . The weak hydrogen bonds in 7 – 10 facilitate the hydrolysis and cyclization reactions of secondary thioamides. The spectroscopic data for secondary (thio)amides are especially useful for characterizing the electronic situation at the (thio)carbamoyl nitrogen atoms and they are perfectly correlated with the reactivity. Examples of chelation of protons by thioamides ( 11 and 12 ), which contain strongly electron‐donating pyrimidine groups, are presented to show the contribution of dihydrogen bonding in the protonation reaction similar to 1 and 4 . Copyright © 2003 John Wiley & Sons, Ltd.     

Z and E rotamers of N‐formyl‐1‐bromo‐4‐hydroxy‐3‐methoxymorphinan‐6‐one and their interconversion as studied by 1H/13C NMR spectroscopy and quantum chemical calculations

N‐Formyl‐1‐bromo‐4‐hydroxy‐3‐methoxymorphinan‐6‐one (compound 2 ), an important intermediate in the NIH Opiate Total Synthesis, presumably exists as a mixture of two rotamers (Z and E) in both CHCl₃ and DMSO at room temperature due to the hindered rotation of its N‐C18 bond in the amide moiety. By comparing the experimental ¹H and ¹³C chemical shifts of a single rotamer and the mixture of compound 2 in CDCl₃ with the calculated chemical shifts of the geometry optimized Z and E rotamers utilizing density functional theory, the crystalline rotamer of compound 2 was characterized as having the E configuration. The energy barrier between the two rotamers was also determined with the temperature dependence of ¹H and ¹³C NMR coalescence experiments, and then compared with that from the reaction path for the interconversion of the two rotamers calculated at the level of B3LYP/6‐31G*. Detailed geometry of the ground state and the transition states of both rotamers are given and discussed. Copyright © 2012 This article is a US Government work and is in the public domain in the USA.     

Motional Anisotropy of 1-Phenyladamantane

¹³C-NMR longitudinal relaxation rates are analysed with the Woessner equations: in CDCl₃ solution at room temperature, 1-phenyladamantane has rotational diffusion coefficients R_‖ = 8.1 × 10¹⁰ rad · s^?1 and R_⊥ = 1.1 × 10¹⁰ rad · s^?1 corresponding to high motional anisotropy (σ = 7.4).     

1H NMR utilization of through-space effects in configurational assignments. I—application to nitriles; determination of the separate effects of the electric field and anisotropy of the CN group

The changes in chemical shift induced by isomerization for all the ring protons of the Z- and E-5,5-dimethyl-2-cyclohexenylidene acetonitriles depend only on the through-space effects of the cyano group. The configurational assignments were made taking into consideration the anisotropic and electric field effects, either separately or together. In the first case, the total effects are Δ_XCN^T=?14.7×10^?6cm³ mol^?1 and bμ_CN^T=14.7×10^?30 cm³, respectively. The second approach allows the estimation of the values Δ_XCN=?4.9 × 10^?6 cm³ mol^?1 and bμ_CN=9.8 × 10^?3 cm³, reflecting the combined contributions of magnetic anisotropy and electric field to the total effect.     

Encapsulation‐Induced Remarkable Stability of a Hydrogen‐Bonded Heterocapsule

Remarkably enhanced stability of the self‐assembled hydrogen‐bonded heterocapsule 1?2 by the encapsulation of 1,4‐bis(1‐propynyl)benzene 3 a was found with K_a=1.14×10⁹ M ^?1 in CDCl₃ and K_a2=1.59×10⁸ M ^?2 in CD₃OD/CDCl₃ (10 % v/v) at 298 K. The formation of 3 a @( 1?2 ) was enthalpically driven (ΔH°<0 and ΔS°<0) and there was a unique inflection point in the correlation between ΔH° versus ΔS° as a function of polar solvent content. The ab initio calculations revealed that favorable guest–capsule dispersion and electrostatic interactions between the acetylenic parts (triple bonds) of 3 a and the aromatic inner space of 1?2 , as well as less structural deformation of 1?2 upon encapsulation of 3 a , play important roles in the remarkable stability of 3 a @( 1?2 ).     

1H NMR spectra and electronic structure of 3-arylmethyl-idenephthalide and 3-arylthiomethylidenephthalide derivatives

The ¹H NMR spectra of trans-3-phenylmethylidenephthalide and trans-3-phenylthiomethylidenephthalide derivatives were investigated. After applying a correction for the anisotropy of substituents and/or for changes of ring current in the substituted aromatic ring, linear correlations were obtained between the chemical shifts of protons of the substituted phenyl group and the methine group and s? constants of substituents. The influence of the bridge heteroatom on the transfer of electronic effects of substituents through the molecules under study is discussed.     

A molecular mechanics and ab initio prediction of the 1H chemical shifts of pinanes

Molecular mechanics calculations plus the application of a refined Karplus equation gave the conformations of 19 pinanes. These range from a Y‐shaped geometry in the apopinene and α‐pinene series to a pseudo chair conformation in β‐pinene, nopinone and verbanone, a flattened chair in pinocarvone and the pinocarveols and a distorted Y shape for iso‐verbanone. These structures were then used as input to predict the ¹H chemical shifts of these compounds by semi‐empirical (¹H‐NMR spectra (HSPEC)) and ab initio gauge‐invariant atomic orbital (GIAO) calculations, the latter at the B3LYP hybrid density functional theory level using 6‐31++G** basis set. The two methods gave generally good agreement with the 184 observed shifts with root mean square (RMS) errors 0.07 ppm (HSPEC) and 0.10 ppm (GIAO), but the GIAO calculations gave several significant (>0.25 ppm) errors. One was for the H₃ proton in apopinenone and other α,β unsaturated ketones; the others occurred for protons in close proximity to hydroxyl groups. To provide more information, smaller analogues of known geometry and chemical shifts were subject to the same analysis. In cyclopentenone, the Gaussian geometry gave good agreement with the observed shifts, but the MMFF94, MMX and MM3 geometries all gave errors for different protons. These results show clearly that the molecular geometries of the α,β unsaturated ketones are responsible for the errors. The errors for the alcohols were examined using ethanol as model and were shown to be due to the different possible conformations of the OH group. Similar GIAO calculations on substituted methanes gave good agreement for the methyl compounds but poor agreement for di and tri halosubstituted methanes. The aforementioned method of molecular mechanics plus GIAO calculations is shown to be a very useful tool for the investigation of molecular geometries and conformations. However, multihalogen compounds may require different basis sets for accurate calculations. Copyright © 2017 John Wiley & Sons, Ltd.     

Conformational Analysis of the cis- and trans-Isomers of FK506 by NMR and Molecular Dynamics

The potent immunosuppressant drug FK506 ( 2 ) has been examined by ¹H- and ¹³C-NMR spectroscopy and NOE-restrained molecular dynamics to elucidate the conformation in solution. A combination of two- and three-dimensional NMR techniques was used to completely assign the ¹H- and ¹³C-NMR chemical shifts of the two configurational isomers resulting from the cis-trans isomerization about the single amide bond. Hetero- and homonuclear coupling constants were measured to assign the diastereotopic methylene protons at C(16), C(18), and C(23). Intramolecular H? H distances were defined from NOESY spectra recorded at ?30° in CDCl₃ and used as constraints in molecular-dynamics simulations. The conformational preferences of 2 in solution are discussed in light of the constitutional features recently proposed to be necessary for binding and activity.     

1H NMR studies of intramolecular hydrogen bonding in N-(pyridyl)amides of 6-methylpicolinic acid N-oxide

The ¹H NMR spectra of seven N-(pyridyl)amides of 6-methylpicolinic acid N-oxide in chloroform were obtained. The influence on the chemical shifts of the N? H protons of temperature, concentration and the CH₃ substituent in the pyridine ring was studied. The N? H protons were found to be shifted to low fields (～14 ppm) owing to the formation of strong intramolecular hydrogen bonding. The influence of the pyridine ring on the chemical shift of the N? H proton is comparable with the inductive effect of the p-nitrophenyl group. The hindered rotation around the N-pyridyl bond of N-(α-pyridyl)amides of 6-methylpicolinic acid in solution is discussed.     

Polypyridine‐Based Helical Amide Foldamer Channels: Rapid Transport of Water and Protons with High Ion Rejection

Synthetic strategies that enable rapid construction of covalent organic nanotubes with an angstrom‐scale tubular pore remain scarcely reported. Reported here is a remarkably simple and mild one‐pot polymerization protocol, employing POCl₃ as the polymerization agent. This protocol efficiently generates polypyridine amide foldamer‐based covalent organic nanotubes with a 2.8 nm length at a yield of 50 %. Trapping single‐file water chains in the 2.8 Å tubular cavity, rich in hydrogen‐bond donors and acceptors, these tubular polypyridine ensembles rapidly and selectively transport water at a rate of 1.6×10⁹ H₂O?S^?1?channel^?1 and protons at a speed as fast as gramicidin A, with a high rejection of ions.     

Substituent increments for the 1H-NMR. Chemical shifts of the 18- and 19-methyl protons of steroids. Part I: 9beta, 10alpha(retro)-steroids

This paper reports 261 substituent increments for the ¹H? NMR. chemical shifts (solvent: CDCl₃) of the 18- and 19-methyl protons of 9β, 10α(retro)-steroids relative to 5β,9β,10α,-androstane. The increments were calculated by a least-squares procedure from 1334 spectra of 759 different steroids.