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.     

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 30: 1H chemical shifts in amides and the magnetic anisotropy,electric field and steric effects of the amide group

The ¹H spectra of 37 amides in CDCl₃ solvent were analysed and the chemical shifts obtained. The molecular geometries and conformational analysis of these amides were considered in detail. The NMR spectral assignments are of interest, e.g. the assignments of the formamide NH₂ protons reverse in going from CDCl₃ to more polar solvents. The substituent chemical shifts of the amide group in both aliphatic and aromatic amides were analysed using an approach based on neural network data for near (≤3 bonds removed) protons and the electric field, magnetic anisotropy, steric and for aromatic systems π effects of the amide group for more distant protons. The electric field is calculated from the partial atomic charges on the N.C═O atoms of the amide group. The magnetic anisotropy of the carbonyl group was reproduced with the asymmetric magnetic anisotropy acting at the midpoint of the carbonyl bond. The values of the anisotropies Δχ_parl and Δχ_perp were for the aliphatic amides 10.53 and ?23.67 (×10^?6 ų/molecule) and for the aromatic amides 2.12 and ?10.43 (×10^?6 ų/molecule). The nitrogen anisotropy was 7.62 (×10^?6 ų/molecule). These values are compared with previous literature values. The ¹H chemical shifts were calculated from the semi‐empirical approach and also by gauge‐independent atomic orbital calculations with the density functional theory method and B3LYP/6–31G⁺⁺ (d,p) basis set. The semi‐empirical approach gave good agreement with root mean square error of 0.081 ppm for the data set of 280 entries. The gauge‐independent atomic orbital approach was generally acceptable, but significant errors (ca. 1 ppm) were found for the NH and CHO protons and also for some other protons. Copyright © 2013 John Wiley & Sons, Ltd.     

13C and 1H NMR spectral studies of N-alkylmethylquinolinium salts

¹³C and ¹H chemical shifts of fourteen N-alkylmethylquinolinium salts in DMSO-d₆ are reported, and compared with those of the eleven corresponding methylquinoline bases. The influence of ring substitution by methyl groups in the salts and substitution at the nitrogen atom and the effect of the anion are discussed.     

Synthesis and fluorescence study of 3-aminoalkylamidonapthalimides

A new series of fluorescent 3-aminoalkylamidonapthalimides were synthesized starting form 1,8-naphthalic anhydride. The structure of these compounds was characterized by ¹H NMR, ¹³C NMR, IR and Mass spectral analysis. The solvent effect on ¹H and ¹³C NMR of these compounds was studied in CDCl₃, CDCl₃:DMSO-d₆ (7:3, v/v) and DMSO-d₆. NMR chemical shift of the ortho and para protons and meta carbons of naphthalene ring showed maximum variation on moving from CDCl₃ to DMSO-d₆. In CDCl₃ solvent naphthalene ring may exist in slightly puckered form while in DMSO-d₆ it attains maximum planar configuration. Fluorescent properties of the title compounds and their precursors were investigated in different solvents like chloroform, ethanol, acetonitrile, acetone, DMSO and water. 3-Aminoalkylamidonapthalimides exhibited improved fluorescence than their precursors. Cyclic amino derivatives yielded higher fluorescence quantum efficiency in protic solvents, ethanol and water. Acylic amino derivatives yielded high fluorescence quantum efficiency in chloroform solvent. The maximum fluorescence quantum yield up to 0.14 was found for butyl amine derivative in chloroform solvent. In general proton accepting nucleophilic solvents like acetone and DMSO quenched the fluorescence.     

NMR studies of interionic interactions in N-methylformamide

The NMR chemical shifts of alkali and thallium(I) salts with various monovalent anions have been measured in N-methylformamide solution. Lithium-7 chemical shifts are virtually concentration and counter-ion independent, presumably due to an absence of direct cation-anion interactions. The sodium-23, potassium-39 and cesium-133 chemical shifts of the salts studied depend on the anion and vary linearly with the concentration. The observed behavior can be accounted for by the formation of collisional ion pairs. On the other hand, the thallium-205 chemical shifts of thallium(I) nitrate and perchlorate were anion-dependent and varied non-linearly with the salt concentration. These results are indicative of contact ion pair formation; formation constants were calculated to be 2.6±0.4 M ^–1 for TlNO ₃ and 1.7±0.5 M ^–1 for TlClO ₄ . The cesium-133 NMR spectra of several mixed electrolyte systems also have been measured in N-methylformamide solution. The¹³³Cs chemical shifts also change linearly with the concentrations of the salts added to 0.10 M CsI/NMF solutions. The influence of the anions on the chemical shifts is the same as that observed for cesium salts alone.     

Ion‐Pairing of Phosphonium Salts in Solution: C?H⋅⋅⋅Halogen and C?H⋅⋅⋅π Hydrogen Bonds

The ¹H NMR chemical shifts of the C(α)? H protons of arylmethyl triphenylphosphonium ions in CD₂Cl₂ solution strongly depend on the counteranions X^?. The values for the benzhydryl derivatives Ph₂CH? PPh₃⁺ X^?, for example, range from δ_H=8.25 (X^?=Cl^?) over 6.23 (X^?=BF₄^?) to 5.72 ppm (X^?=BPh₄^?). Similar, albeit weaker, counterion‐induced shifts are observed for the ortho‐protons of all aryl groups. Concentration‐dependent NMR studies show that the large shifts result from the deshielding of the protons by the anions, which decreases in the order Cl^? > Br^? ? BF₄^? > SbF₆^?. For the less bulky derivatives PhCH₂? PPh₃⁺ X^?, we also find C? H???Ph interactions between C(α)? H and a phenyl group of the BPh₄^? anion, which result in upfield NMR chemical shifts of the C(α)? H protons. These interactions could also be observed in crystals of (p‐CF₃‐C₆H₄)CH₂? PPh₃⁺ BPh₄^?. However, the dominant effects causing the counterion‐induced shifts in the NMR spectra are the C? H???X^? hydrogen bonds between the phosphonium ion and anions, in particular Cl^? or Br^?. This observation contradicts earlier interpretations which assigned these shifts predominantly to the ring current of the BPh₄^? anions. The concentration dependence of the ¹H NMR chemical shifts allowed us to determine the dissociation constants of the phosphonium salts in CD₂Cl₂ solution. The cation–anion interactions increase with the acidity of the C(α)? H protons and the basicity of the anion. The existence of C? H???X^? hydrogen bonds between the cations and anions is confirmed by quantum chemical calculations of the ion pair structures, as well as by X‐ray analyses of the crystals. The IR spectra of the Cl^? and Br^? salts in CD₂Cl₂ solution show strong red‐shifts of the C? H stretch bands. The C? H stretch bands of the tetrafluoroborate salt PhCH₂? PPh₃⁺ BF₄^? in CD₂Cl₂, however, show a blue‐shift compared to the corresponding BPh₄^? salt.     

ANALYSIS OF THIOCYANATES BY NMR

Abstract

An unexpected nmr spectrum of β-phenyl ethyl thiocyanate in CDCl₃ solution has led to its analysis in a series of solvents of increasing polarity. Changes in polarity produce increments of ^δAB, transforming the signal from a singlet for 4 methylenic protons to a clear A₂B₂ pattern. Signals appear to split following increasing values of dielectric constants and dipole moments. Rotational isomerism is assumed to be the reason for it since the reaction field parameter depends directly on the dipole moments of rotamers, thus changes in their population affect the reaction field and ultimately the chemical shifts.     

Protonation of trimipramine salts of maleate, mesylate and hydrochloride observed by 1H, 13C and 15N NMR spectroscopy

Protonation of the tricyclic antidepressant drug trimipramine with maleic acid, methanesulfonic acid and hydrochloric acid was studied using 1H, 13C and 15N NMR spectroscopy at natural abundance. The effect of counter ions on the protonation was compared under identical conditions of solvent, concentration and temperature using homonuclear and heteronuclear one- and two-dimensional experiments. Differential protonation of the terminal tertiary amine nitrogen is determined from the indirect spin-spin couplings, chemical shifts, 13C relaxation data and variable-temperature experiments. In the maleate salt, only one of the acidic protons is involved in protonation, the other being associated with the anion moiety. 15N chemical shifts of the protonated nitrogens are nearly linearly related to the pK(a) of the constituent acid.     

Chemical ionization desorption mass spectrometry of some quaternary ammonium salts

The ammonia chemical ionization desorption spectra of N,N-dimethyl quaternary ammonium iodides in addition to high protonated molecular ion [M + H]⁺ intensity, show signals for an ion radical composed of N-methyl abstracted salt cation and ammonia [C + NH₃? CH₃]^+˙. These ions corresponding to the cation +2 show increased importance in the chemical ionization mode, using the same reagent gas. The technique of chemical ionization desorption appears suitable for the analysis of salts, and thus for the determination of the molecular weight of both anion and cation.     

Halide ion effect on the 1H NMR chemical shifts of the residual protons in commonly employed deuterated solvents with tetra-n-butylammonium chloride – Part 2

Many ¹H NMR spectroscopic studies involving supramolecular binding of tetra-n-butylammonium halides (TBAX) with a variety of molecular receptors have been reported to date. Previously we demonstrated that the reference residual proton signal of the deuterochloroform solvent itself in TBAX solutions shifted downfield in a linear TBAX concentration-dependent relationship. We now report that a similar downfield chemical shift behaviour of the residual protons of other commonly employed deuterated solvents with TBACl can be seen for dichloromethane-d₂ and acetonitrile-d₃, but in acetone-d₆, methanol-d₄ and DMSO-d₆, upfield shifts are observed. A hypothesis based on Density Functional Theory (DFT) modelling is presented to account for this behaviour.     

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.     

Isolation and Structural Characterization of Di‐ and Tetra‐protonated Forms of the Macrocyclic Hexaamine trans‐6,13‐Dimethyl‐1,4,8‐11‐tetraazacyclodecane‐6,13‐diamine

Cations derived by protonation of the ligand title compound (L¹) have been structurally characterized in their di‐ and tetra‐ protonated forms in the salts [H₂L¹][ClO₄]₂·2H₂O and [H₄L¹][ZnCl₄]₂·4H₂O. In both structures, one half of the formula unit comprises the asymmetric unit of the structure, the macrocycle being centrosymmetric, with the two macrocycles adopting similar conformations. In both salts, a pair of diagonally opposed macrocyclic secondary amine groups are protonated; in the [H₄L¹]⁴⁺ salt, the additional pair of protons are accommodated on the exocyclic pendant amine groups. The dispositions of the pendent amines differ between the two structures, being ‘equatorial’ with respect to the macrocyclic ring in the [H₂L¹]²⁺ salt, and ‘axial’ in the [H₄L¹]⁴⁺ salt. In other structurally characterized compounds containing [H₄L¹]⁴⁺ the equatorial disposition was found in the ferricyanide adduct, while in the tetraperchlorate salt the axial disposition was identified. The differences in disposition of the exocyclic groups are ascribed to the extensive H‐bonding in the lattices.     

H and C NMR spectroscopy of 9-acridinones

The ¹H and ¹³C NMR spectra of 9-acridinone and its five derivatives dissolved in CDCl₃, CD₃CN and DMSO-d₆ were measured in order to reveal the influence of the constitution of the compounds and features of the solvents on chemical shifts and ¹H-¹H coupling constants. Experimental data were compared with theoretically predicted chemical shifts, on the GIAO/DFT level of theory, for DFT (B3LYP)/6-31G^∗∗ optimized geometries of molecules—also for four other 9-acridinones. This comparison helped to ascribe resonance signals in the spectra to relevant atoms and enabled revelation of relations between chemical shifts and physicochemical features of the compounds. It was found that experimentally or theoretically determined ¹H and ¹³C chemical shifts of selected atoms correlate with theoretically predicted values of dipole moments of the molecules, as well as bond lengths, atomic partial charges and energies of HOMO.     

Halide ion effect on the chloroform chemical shift in supramolecular complexation studies with tetra-n-butylammonium salts: a 1H NMR and X-ray study

A ¹H NMR spectroscopic study of tetra-n-butylammonium halides (TBAX: X = Cl^? , Br^?  or I^? ) in CDCl₃ solutions was conducted. Complexation studies of TBAX salts with different host molecules using ¹H NMR in CDCl₃ have previously revealed that the reference residual CHCl₃ proton signal had been shifted downfield. The aim of the study was to quantify the extent of these chemical shift changes with TBAX salts. Linear concentration–chemical shift relationships in each case were obtained from the resulting titration plots obtained from the addition of the TBAX salts alone to CDCl₃. Interactions in the solid state as determined by X-ray crystallography support the solution-state investigations indicating halide ion–chloroform proton interactions.     

N‐Methylacridinium Salts: Carbon Lewis Acids in Frustrated Lewis Pairs for σ‐Bond Activation and Catalytic Reductions

N‐methylacridinium salts are Lewis acids with high hydride ion affinity but low oxophilicity. The cation forms a Lewis adduct with 4‐(N,N‐dimethylamino)pyridine but a frustrated Lewis pair (FLP) with the weaker base 2,6‐lutidine which activates H₂, even in the presence of H₂O. Anion effects dominate reactivity, with both solubility and rate of H₂ cleavage showing marked anion dependency. With the optimal anion, a N‐methylacridinium salt catalyzes the reductive transfer hydrogenation and hydrosilylation of aldimines through amine–boranes and silanes, respectively. Furthermore, the same salt is active for the catalytic dehydrosilylation of alcohols (primary, secondary, tertiary, and ArOH) by silanes with no observable over‐reduction to the alkanes.     

Structural studies by 1H and 13C dynamic n.m.r.: Part III alternative protonation sites in carbonyl conjugated enamines of the type R1?C(O)?CH?CH?NR2R3

¹³C magnetic resonance spectra of several enamino ketones with secondary and tertiary amino groups were obtained for trifluoroacetic acid solutions. In both series O-protonation is predominant and the chemical shifts are related to the electron density changes with respect to the parent base. The spectra of the tertiary compounds are interpreted in terms of slow rotation around the C–1? C–2 and C–3? N bonds discernible at room temperature. O-protonated forms of the secondary enamino ketones undergo further reaction on C–2 yielding pyridinium salts. The mechanism of formation of the quaternary salts is interpreted and the additivity parameters of the ¹³C n.m.r. chemical shifts in the pyridinium ions is briefly discussed.     

Interactions and Encapsulation of Vitamins C,B3, and B6 with Dendrimers in Water

Titrations of commercial diaminobutane (DAB) and polyamidoamine (PAMAM) dendrimers by vitamins C (ascorbic acid, AA), B₃ (nicotinic acid), and B₆ (pyridoxine) were monitored by ¹H NMR spectroscopy using the chemical shifts of both dendrimer and vitamin protons and analyzed by comparison with the titration of propylamine. Quaternarizations of the terminal primary amino groups and intradendritic tertiary amino groups, which are nearly quantitative with vitamin C, were characterized by more or less sharp variations (Δδ) of the ¹H chemical shift (δ) at the equivalence points. The peripheral primary amino groups of the DAB dendrimers were quaternarized first, but not selectively, whereas a sharp chemical‐shift variation was recorded for the inner methylene protons near the tertiary amines, thereby indicating encapsulation, when all the dendritic amines were quaternarized. With DAB‐G5‐64‐NH₂, some excess acid is required to protonate the inner amino groups, presumably because of basicity decrease due to excess charge repulsion. On the other hand, this selectivity was not observed with PAMAM dendrimers. The special case of the titration of the dendrimers by vitamin B₆ indicates only dominant supramolecular hydrogen‐bonding interactions and no quaternarization, with core amino groups being privileged, which indicates the strong tendency to encapsulate vitamins. With vitamin B₃, a carboxylic acid, titration of DAB‐G3‐16‐NH₂ shows that only six peripheral amino groups are protonated on average, even with excess vitamin B₃, because protonation is all the more difficult due to increased charge repulsion, as positive charges accumulate around the dendrimer. Inner amino groups interact with this vitamin, however, thus indicating encapsulation presumably with supramolecular hydrogen bonding without much charge transfer.     

Complexation Between (O‐Methyl)6‐2,6‐Helic[6]arene and Tertiary Ammonium Salts: Acid/Base‐ or Chloride‐Ion‐Responsive Host–Guest Systems and Synthesis of [2]Rotaxane

Complexation between (O ‐methyl)₆‐2,6‐helic[6]arene and a series of tertiary ammonium salts was described. It was found that the macrocycle could form stable complexes with the tested aromatic and aliphatic tertiary ammonium salts, which were evidenced by ¹H NMR spectra, ESI mass spectra, and DFT calculations. In particular, the binding and release process of the guests in the complexes could be efficiently controlled by acid/base or chloride ions, which represents the first acid/base‐ and chloride‐ion‐responsive host–guest systems based on macrocyclic arenes and protonated tertiary ammonium salts. Moreover, the first 2,6‐helic[6]arene‐based [2]rotaxane was also synthesized from the condensation between the host–guest complex and isocyanate.