A nitrogen-14 NMR study of some oximes and their isomeric structures

The isomeric structures of oximes, nitrones and nitroso compounds may be readily differentiated on the basis of their nitrogen chemical shifts. The tautomeric equilibria present in oximenitroso systems are easily observed by means of ¹⁴N NMR spectroscopy. An explanation of the range of chemical shifts found within the oxime and nitrone groups of compounds is presented, based upon the average excitation energy approximation to the paramagnetic term in the nuclear screening tensor. The increase in screening of the nitrogen found upon protonation of an oxime is analogous to that observed for pyridine-type structures and their N-oxides.     

29Si and 13C NMR spectra of permethylpolysilanes

²⁹Si, ¹³C and ¹H NMR spectra are reported for the series of linear permethylpolysilanes Me(SiMe₂)_nMe where n = 1 to 6, for the cyclic permethylpolysilanes (Me₂Si)_n where n = 5 to 8, and for a few related compounds. For linear polysilanes the ²⁹Si and ¹³C chemical shifts can be accurately calculated from simple additivity relationships based on the number of silicon atoms in α, β, γ and δ positions. Adjacent (α) silicon atoms lead to upfield shifts in the ²⁹Si and ¹³C resonances, whereas more remote silicon atoms lead to downfield shifts. The ²⁹Si chemical shifts of the polysilane chains are linearly related to the ¹³C shifts of the carbon atoms attached to the silicon. The ²⁹Si and ¹³C resonances of the cyclic silanes deviate from this relationship. Ring current effects arising from σ delocalization are suggested as an explanation for the deviations. Proton-coupled ²⁹Si NMR spectra are reported for Me₃SiSiMe₃ and for (Me₂Si)_n, n = 5 to 7.     

A 14N NMR study of the isomeric structures of urea and its analogues

In urea and its structural analogues differences in excess of 100 ppm are found in the nitrogen screening constants for the amide- and isoamide-type structures. The nitrogen chemical shifts are interpreted within the framework of Pople's gauge dependent atomic orbital approximation using INDO molecular orbitals. It is demonstrated that this method provides reasonable values for nitrogen absolute screening constants. The experimental data reported should be useful in examinations of tautomeric equilibria in amides and related structures.     

13C NMR, 1H NMR and IR spectra of a series of monochloroesters of aliphatic short chain carboxylic acids

¹³C chemical shifts for 14 isomeric monochloroesters of aliphatic carboxylic acids from propanoic acid to hexanoic acid have been determined. Comparisons are made with the literature values for methyl monochloro-octanoate isomers. Substituent effects for all positions are given. Characteristic IR absorption bands are presented and comparisons are made with regard to the isomeric structure. Connections are suggested between observed trends in IR absorption frequencies and some “abnormal” chlorine substituent effects on ¹³C chemical shifts. ¹H NMR spectra of these compounds are considered.     

Nitrogen NMR spectroscopy: Application to some substituted pyrroles

A series of ¹⁵N labeled 2-acylpyrroles was prepared and the nitrogen and proton n.m.r. spectra obtained. ¹⁵N chemical shifts for these compounds are reported for the first time. No correlation between the nitrogen chemical shift and any Hammett substituent constant could be found. No variation in J(¹⁵N? H) was observed for any compound with changes in solvent, temperature or concentration, ruling out any observable tautomeric equilibria for these systems. An increase in J(¹⁵N? H) with the addition of electron withdrawing groups indicates increasing polarization of the N? H bond and acidity of these molecules. Two and three bond ¹⁵N couplings are also reported.     

1H-,2H-,13C- and 14N-endor studies of coppinger's radicals in liquid solutions and in liquid crystals

Proton and non-proton ENDOR studies are reported on isotopically labelled Coppinger's radical (galvinoxyl) and on N-Coppinger's radical in isotropic solutions and in liquid-crystalline mesophases. The hyperfine coupling constant shifts have been measured over the temperature range of the nematic mesophase and an analysis of the shifts in terms of anisotropic hyperfine tensors and ordering parameters is given. A short phenomenological discussion of the ¹³C and¹⁴N ENDOR response is presented. Several methods for the sign determination of hyperfine coupling constants are discussed. Finally, a survey of the ENDOR spectroscopy on galvinoxyl type biradicals is given.     

Substituent Effects on 13C NMR and 1H NMR Chemical Shifts of CH=N in Multi-substituted Benzylideneanilines

Fifty-three samples of multi-substituted benzylideneanilines XArCH=NArYs (abbreviated XBAYs) were synthesized and their NMR spectra were determined. An extensional study of substituent effects on the ¹H NMR chemical shifts (^δH(CH=N)) and ¹³C NMR chemical shifts (^δC(CH=N)) of the CH=N bridging group from di-substituted to multi-substituted XBAYs was made based on a total of 182 samples of XBAYs, together with the NMR data of other 129 samples of di-substituted XBAYs quoted from literatures. The results show that the substituent specific cross-interaction effect parameter \begin{document} $\left(\Delta\left(\displaystyle\sum\sigma\right)^2\right)$ \end{document} plays an important role in quantifying the ^δC(CH=N) values of XBAYs, but it is negligible for quantifying the ^δH(CH=N) values; the other substituent parameters also present different influences on the ^δC(CH=N) and ^δH(CH=N). On the whole, the contributions of X and Y to the ^δC(CH=N) of XBAYs are balanced, but the ^δH(CH=N) values of XBAYs mainly rely on the contributions of X.     

Synthesis and 1H, 13C and 15N NMR characterisation of substituted Pyrazolo[4,3-c]quinolines and related compounds

A wide range of novel and known alkyl substituted pyrazolo[4,3-c]quinolines and related compounds were synthesized. Some of these compounds can undergo tautomerism and differences have been noted in the structural assignments already published. For each of these compounds, a complete assignment was made using ¹H, ¹³C and ¹⁵N one- and two-dimensional NMR measurements, including NOESY, COSY, ¹H-¹³C HSQC and ¹H-¹³C HMBC. ¹H-¹⁵N HMBC NMR has proved crucial in determining the relevant tautomeric and protonated state of the compounds.     

15N and 17O NMR spectra of some nitrogen- and oxygen-containing polyfluoroaromatic compounds

The effect of substitution of hydrogen by fluorine on the ¹⁵N and ¹⁷O NMR chemical shifts in N-sulphinylarylamines, azoxy- benzenes, C,N-diarylnitrones and aromatic azomethines has been studied. Changes in nitrogen screening are controlled mainly by the radial factor <r^?3>_2p(N), which depends on the electron density on nitrogen, those in oxygen screening — by the two factors, the excitation energy ΔE^?1 and the radial factor <r^?3>_2p(0), which may operate in the same or opposite directions. The ¹⁵N and ¹⁷O NMR spectra of the above compounds did not show any specific effects of perfluorination.     

Untersuchungen zu einer IR-spektrophotometrischen Bestimmung von 15N und 18O

Zusammenfassung ¹⁵N bzw. ¹⁸O aus organischen Verbindungen muß zur IR-Analyse über NH₃ bzw. H₂O in geeignete Substanzen eingebaut werden. NH₃ kann quantitativ im mMol-Bereich durch Reaktion mit Sulfochloriden oder mit Halogen-nitrobenzolen gebunden werden; H₂O addiert sich quantitativ an Carbodiimide oder Inamine.Die Verschiebungen einiger IR-Absorptionsbanden der markierten Reaktionsprodukte werden angegeben. Die Abhängigkeit der Bandenhöhe der Carbonylabsorption des N, N-Di-p-tolyl-harnstoffs vom ¹⁸O-Gehalt wird demonstriert. Die Amplitude der NH-Absorption in Differenzspektren von ¹⁵N-2,4-Dinitro-anilin-Proben gegen ¹⁴N-2,4-Dinitroanilin ergibt eine Eichkurve, die zur Bestimmung des ¹⁵N-Gehaltes von Testsubstanzen benutzt wird. Der Anwendungsbereich des Verfahrens wird diskutiert.

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Investigations on an IR-spectrophotometric determination of ¹⁵N and ¹⁸O

¹⁵N or ¹⁸O from organic compounds must be converted into substances suitable for infra-red analysis via ammonia or water, respectively. NH₃ can be reacted quantitatively in the mmole scale with sulphochlorides or halonitrobenzenes; H₂O adds quantitatively to carbodiimides or inamines.The shifts of some IR-absorption bands of the labelled reaction products are given. The dependence of the carbonyl adsorbancy of N,N-di-p-tolyl-urea on the ¹⁸O-content is demonstrated. Prom the amplitude of the N-H-absorption in difference spectra of ¹⁵N-2,4-dinitro-aniline samples against ¹⁴N-2,4-dinitroaniline a calibration curve is obtained, which can be used for determining the ¹⁵N content of test substances. The scope of the method is discussed.

     

17O-enriched α-azohydroperoxides: 17O NMR spectroscopy, 17O-labeling reagents

¹⁷O-enriched α-azohydroperoxides, prepared by autoxidation, are efficient ¹⁷O-labeling reagents; ¹⁷O NMR (CD₃CN) of 2 showed broad signals at δ 254 and 204 PPM; the solvent dependence of the ¹⁷O chemical shifts and the kinetics of ionic oxidations are interrelated.     

NMR study (1H, 13C and 29Si) of the methylallylsilanes

¹H, ¹³C and ²⁹Si NMR data for the compounds (CH₃)_xSi(CH₂CHCH₂)_4-x are reported. The ¹H resonances from the π system are indicative of the electron-supplying inductive effect (+I) of the (CH₃SiCH₂, moiety but the corresponding ¹³C_π chemical shifts seem to be influenced by a sterically induced polarization of the C-H bonds. The ¹³C_All, ¹³C_Me and ²⁹Si chemical shift data reveal an important neighbour anisotropy contribution originating from the π system. Ultraviolet study of the compounds mentioned above gives indication of a σ—π conjugation in accordance with PES and ab initio results [1—5]. The trend observed in the various coupling constants is too small to be Interpreted.     

13C-Chemische verschiebungen und substituenteneffekte in polycyclisch konjugierten π-elektronensystemen mit fünf- und siebengliedrigen ringen1

In order to study the substituent influence on the ¹³C chemical shifts in polycyclic conjugated π-electron systems with five and seven membered rings we have studied the aceheptylene system with markedly alternating bond length— in contrast to azulene investigated earlier—as well as 5-azaazulene as a hetero-analogue of azulene. The substituent induced ¹³C chemical shifts Δδ_c observed in the monomethylazulenes, monomethylaceheptylenes and in 5-azaazulene are correlated with the atom-atom polarizibilities π_ij of the corresponding unsubstituted compound. For 5-aza-azulene it was further tested, if the ¹³C chemical shifts could be predicted using the ¹³C chemical shifts of 4-substituted derivatives (RCN, Cl, OEt) and the correlations given in the literature for the substituent induced ¹³C chemical shifts in the system benzene/butadiene. The results show that the influence of the π-electrons is markedly overshadowed in the methylazulenes and -aceheptylenes by other effects, but is clearly discernible in 5-azaazulene as a derivative of azulene; so the data can be used to predict the ¹³C chemical shifts of other azaazulenes.     

13c-chemische verschiebungen und substituenteneffekte in methylazulenen

The ¹³C chemical shifts of all possible monomethyl azulenes and of some polymethyl derivatives have been determined by the Pulse Fourier Transform technique and have been assigned on the basis of ¹³C,H coupling constants and/or splitting patterns taken from proton coupled ¹³C NMR spectra. The shift differences observed in the monomethyl derivatives as compared to azulene itself are additive as far as sterically unperturbed polymethyl derivatives are concerned and can be viewed on as substituent constants. As is shown by the results of HMO- and EHT-calculations the substituent induced shifts in monomethyl azulenes without sterical hindrance can be correlated with charge density changes, with the exception of the β -effect, which is proportional to the bond length resp. the π bond order.     

Calculation of the 14N and 2H quadrupole coupling tensor in trimeric imidazole

The ¹⁴N and ²H quadrupole coupling tensors at both nitrogen and at all hydrogen sites, have been calculated by ab initio SCF MO methods, in a trimeric imidazole molecule having the geometry found in the solid state. The ¹⁴N changes produced at N(1)H by hydrogen bonding are correctly reproduced to a noteworthy degree of precision, as is the ²H quadrupole interaction at the same site. The changes in the values of the three principal components of the tensor, which have been inferred from NQR measurements when imidazole passes from the gas to the solid state, are supported by the calculations, both for N(1)H and N(3).     

Lithium dialkylamides. 13C parameters and slow proton transfer

The changes in ¹³C chemical shifts for the structural change R₂NH → R₂NLi have been measured for a series of dialkylamines. These lithiation shifts are largest at the $\text{alpha}$ carbon (3.7 – 9.7 ppm) and decrease in the order α > β > γ > δ. The rates of lithium-hydrogen interchange between R₂NH and R′₂NLi have been determined. The activation energies are large (9 – 17 kcal/mole) and increase as the size of R or R′ increases. The slow exchange permits the direct measurement of acidity differences between pairs of amines using ¹³C nmr.     

195Pt NMR of organoplatinum compounds

The ¹⁹⁵Pt chemical shifts of several organoplatinum compounds in solution have been determined. The δ(¹⁹⁵Pt) values of the phosphine-Pt^II and -Pt⁰ compounds lie in separate ranges, and allow the metal-diene systems to be characterized either as metallacyclopentene or as η²-bonded diene. Although the two isomers of bis(η³-allyl)Pt (VIII) formally should be regarded as Pt^II compounds their ¹⁹⁵Pt shifts clearly lie in the region for Pt⁰ compounds. The large separation between the ¹⁹⁵Pt signals and the difference in ¹⁹⁵Pt-T₁ values for the two isomers of VIII are in accord with their having different geometries around the metal.     

Quantum-chemical investigation of structure and reactivity of pyrazol-5-ones and their thio- and seleno-analogs: X. Solvent effect on the chemical shifts of nuclei in the molecules of 3-methylpyrazol-5-ones and 1-phenyl-3-methylchalcogenepyrazol-5-ones and characteristics of tautomeric equilibrium in these compounds

By quantum-chemical DFT/GIAO method chemical shifts of all nuclei in the NMR spectra of 3-methylpyrazol-5-one and 1-phenyl-3-methylchalcogenopyrazol-5-ones in chloroform and dimethyl sulfoxide were calculated and analyzed using various solvation models. Low sensitivity to solvent of the chemical shfts of ¹³C and ¹H nuclei (except for “acidic” protons) calculated in the framework of various continuum models is revealed. Discrete and discrete-continuum models reflect well deshielding of the active centers of H-complexation and chemical shifts of “acidic” protons of the studied pyrazolones in solutions. Optimization of geometry of pyrazolones in solutions only slightly improves the agreement between the theoretically calculated and the experimental values. Shielding of nitrogen, oxygen, sulfur, and selenium atoms is more sensitive to the nature of solvent and to the nature of tautomeric forms. The methods of NMR spectroscopy allow to identify reliably the dominating tautomeric form but they are insufficient for the quantitative characterization of tautomeric equilibria.     

13C Pulse fourier transform NMR of menthol stereoisomers and related compounds

¹³C NMR spectra of menthol stereoisomers have been determined. The correlations of chemical shifts of these ring carbons with those of stereoisomeric 2-isopropylcyclohexanols are examined. Observed chemical shifts of 1-Me carbons are compared with those predicted from the chemical shifts of stereoisomeric 1-methyl-4-t-butylcyclohexanes. ¹³C NMR spectra of menthyl acetates, and cis and trans p -menthanes have also been examined.     

13C NMR investigation of tautomeric and acid-base equilibria of pyrazolone T and three analogs

Pyrazolone T and three derivatives have been characterized by ¹³C and, in part, ¹⁵N nmr at several pH values. The ¹³C chemical shifts have been assigned at, or near, the equivalence points and pK_a values of these four compounds. Closely situatued quaternary carbon signals were assigned by means of a heteronuclear chemical shift correlation (FLOCK) experiment which is sensitive to, and was optimized for, 3-bond C-H couplings. The ¹³C chemical shift data indicate the existence of both tautomeric and acid-base equilibria and demonstrate that the four congeners exist in surprisingly different forms at certain common pH values.