Carbon-13 n.m.r. and CNDO/S study of 1,3-indandione and heterocyclic analogues

¹³C n.m.r. spectra are reported for 1,3-indandione, phthalic anhydride, thiophthalic anhydride, and phthalimide in chloroform-d solution. The ¹³C chemical shifts have also been calculated by means of Pople's perturbation theory using CNDO/S wave functions; the agreement between theoretical and experimental values is improved when an empirical charge-dependent term is added to the calculated diamagnetic and paramagneitc contributions.     

Carbon-13 NMR of 1,2-indandione and heterocyclic analogues. A reappraisal

¹³C NMR chemical shifts previously reported for 1,2-indandione and thionaphthenequinone have been found to be erroneous; the correct results have now been established.     

Carbon-13 n.m.r. studies of some saturated 1,2-oxazepine derivatives

The influence of incorporating an group, an oxygen atom and the fragment in a saturated 7-membered ring system on carbon-13 n.m.r. chemical shifts is examined, and also, the influence of the dioxolane ring moiety on the ¹³C chemical shifts in these ring systems. Substituent effects are generally additive except in cases where the ring is heavily substituted with methyl groups. A large upfield steric shift (γ effect) of 7–8 ppm is observed in two of the derivatives. An example of long range nonequivalence is also observed. Assignments of the ¹³C n.m.r. spectra have been made by comparison with model compounds, and from proton coupled ¹³C n.m.r. spectra. The synthesis of several new compounds is described.     

Carbon-13 n.m.r. study of naturally occurring xanthones

The ¹³C n.m.r. spectra of 36 naturally occurring xanthones are reported and all chemical shifts assigned. The shifts in substituted xanthones can be predicted from substituent effects evaluated for simple derivatives. The agreement between calculated and observed shifts decreases as the extent of substitution in the xanthone increases.     

Carbon-13 n.m.r. studies of methylquinolines and methylisoquinolines

Spectra are reported for all seven methylquinolines, nine dimethylquinolines, six methylisoquinolines and one dimethylisoquinoline. Substituent effects for the α, ortho, meta, para, vinylogous para and peri positions are reported for the monomethylcompounds. Additivity of these substituent effects is demonstrated for the dimethyl compounds which also exhibit a vicinal dimethyl steric effect.     

Carbon-13 n.m.r. spectroscopy of some Strychnos alkaloids

¹³C N.m.r. spectra have been determined for strychnine and a series of fourteen derivatives. The assignments are based on previously established data and on off-resonance decoupled spectra. The shifts resulting from the alterations in molecular structure are discussed and explained, in part, as a consequence of conformational changes. This detailed study indicates that changes are required in some previously published assignments.     

Carbon-13 n.m.r. characterization of indenyllithium in solution

¹³C N.m.r. chemical shifts obtained for indenyl- and cyclopentadienyllithium in several, mostly ethereal, solvents are presented. The chemical shift changes induced by varying the solvent are assumed to probe the actual π-electron density distribution, thus providing an insight into the structure of the ion-pair. The increase in the estimated π-electron densities obtained for the protonated aromatic carbons of indenyllithium by increasing the coordinating ability of the solvent gives evidence of a change of the ion-pair equilibrium towards solvent-separated species. The average chemical shifts for indenyl- and cyclopentadienyllithium were independent of the choice of solvent, a fact which supports the ion-pair model in all studied media.     

Carbon-13 n.m.r. analysis of selected strychnos alkaloids

The carbon-13 chemical shifts of strychnine and ten closely related analogues have been assigned and confirmed in several cases using selective proton decoupling techniques. Where additivity relationships are used the importance of having data on an array of structural analogues is emphasized. Quaternization at N-19 exerts long range polarization effects throughout the molecular framework. The effects of N-oxide formation on the carbon-13 shifts of 5 to 9-membered ring heterocycles and strychnine are discussed as aids to structural assignment. A Zwitterionic structure for strychnic acid is deduced.     

Carbon-13 n.m.r. studies of substituted bisalkylamino-s-triazines in acidic media

The carbon-13 n.m.r. spectra of six substituted bisalkylamino-s-triazines are reported. The site of protonation of 2-methoxy-s-triazines is shown to be at N-1 or N-3. The effect of trifluoroacetic acid on 2-chloro-s-triazines is also discussed.     

Carbon-13 n.m.r. spectra of branched carboxylic acids and their derivatives

¹³C n.m.r. spectra have been measured for 39 compounds with branched structures including carboxylic acids, their methyl and ethyl esters, nitriles and chlorinated esters. The results obtained indicate that the ¹³ C n.m.r. technique is applicable to structure assignment of acids and their derivatives containing various numbers of substituents on the chain. The dependence of the carboxylic carbon chemical shift on the number and structure of α-positioned substituents has been determined. Calculation of the chemical shifts for branched carboxylic acids, esters and nitriles from the corresponding increments using the additivity scheme is shown to be possible in principle.     

Carbon-13 n.m.r. of the 8-phosphabicyclo[3.2.1]octane system

¹³C Chemical shifts and ¹³C? ³¹P nuclear spin coupling constants have been determined for 26 8-phosphabicyclo[3.2.1]octane derivatives, namely phosphines, phosphine oxides, phosphine sulphides and one phosphonium salt. The influence of the phosphorus configuration on δ and ²J(PC) values was examined and other factors influencing the ²J(PC) coupling constant are discussed.     

Carbon-13 n.m.r. investigation on the nitrogen methylation of the mono- and diazanaphthalenes

The ¹³C n.m.r. spectra of the N-methylated mono- and diazanaphthalenes have been recorded and analysed. It has been shown that N-methylation as well as N-protonation in cinnoline occur predominantly at the β-nitrogen atom. N-methylation and N-protonation show a similar effect on the ¹³C chemical shifts.     

Carbon-13 NMR and CNDO/2 studies of phenoxachalcogenins

The ¹³C chemical shifts of the carbon atoms in dibenzodioxin, phenoxathiin, phenoxaselenin and phenoxatellurin were determined in CDCl₃ solutions and assigned. The total (σ and π) charge densitites on the carbon atoms calculated by the CNDO/2 method without consideration of d-orbitals correlated well with the experimentally determined shifts. Rather good agreement was also found between experimental shifts and shifts calculated from ¹³C data for phenyl methyl chalcogenides on the assumption that a phenoxachalcogenin molecule can be assembled from C₆H₅O and C₆H₅X groups. Only the shifts of the carbon atoms bonded to the heavier chalcogen atoms show an upfield trend in the sequence O, S, Se, Te. All other shifts exhibit a downfield trend. These trends are rationalized in terms of the electronegativities, abilities to participate in π-interactions, and anisotropy effects of the chalcogen atoms.     

Carbon-13 n.m.r. spectra of saturated heterocycles. Part VII—Thianium Salts

The ¹³C n.m.r. spectra of 53 thianium, S-methylthianium, S-alkylthianium and S-methyl-1-thiadecalinium salts, most of them substituted with methyl groups in the ring, have been recorded. The chemical shifts of the ring carbons in these thianium and S-methylthianium salts and the S-methyls in the S-methylthianium salts have been analyzed in terms of additive parameters of the methyl substituents which are compared to those previously determined for the parent thianes. Comparison is also made with other charged species.     

Carbon-13 n.m.r. investigation of 2-arylaminothiazolines and analogous thiazines,thiazepines and their amides

The ¹³C n.m.r. spectra of 2-arylaminothiazoline, -thiazine and -thiazepine derivatives, and also those of some N-substituted isomeric pairs were recorded and assigned. The structure of the individual isomers, which as a result of contradictory spectroscopic data had earlier been ambiguous, was established. In addition, the tautomeric structure of some N-unsubstituted compounds was determined.     

The structure of 1,2-dithioles studied by 13C n.m.r. spectroscopy

The charge separation between the dithiole ring and oxygen in 3-(1′,2′-dithiole-3′-ylidene)-6-methyl-2,3-dihydropyran-2,4-diones and 3-(1′,2′-dithiole-3′-ylidene)-2,3-dihydrobenzo[b] pyran-2,4-diones has been determined by ¹³C and ¹⁹F n.m.r. spectroscopy. By both methods it is found that these compounds are relatively polar. A correlation is established between the ¹³C chemical shift of some carbon atoms and the S2p binding energies measured by e.s.c.a. in the 1,2-dithiole derivatives. This correlation shows that these ¹³C shifts depend on the positive charge taken by the 1,2-dithiole ring and can then constitute a convenient evaluation of this charge.     

Carbon-13 isotope effect on proton decoupled 13C and 31P n.m.r. spectra of bis(diphenylphosphino)acetylene

High resolution proton decoupled ¹³C and ³¹P n.m.r. spectra of bis(diphenylphosphino)acetylene in (CD₃)₂SO and CDCl₃ are analysed as ABX spectra to give the relative chemical shifts of the ¹³C and ³¹P nuclei as well as the spin–spin coupling constants ³J(PP) and ⁿJ(PC). The differences in ³¹P shieldings are due to secondary ¹³C isotope effects which have been observed to be negligible over more than two bonds.     

Carbon-13 n.m.r. studies: 68—diastereomeric nonequivalence and the characterization of some bisnorbornylpinacols

The sodium-ammonia reduction of racemic norcamphor, exo-3-methylnorcamphor and 3,3-dimethyl-norcamphor (camphenilone) leads to substantial yields (60–90%) of the corresponding 2,2′-bisnorbornyl-2,2′-diols as diastereomeric mixtures. A combination of their ¹H, ¹³C and infrared spectra together with the results of periodic acid and lead tetraacetate oxidations permits stereochemical assignments for these six pinacols.     

Carbon-13 n.m.r. studies. 65—13C spectra of some tricyclo[3.2.1.02,4]octane derivatives

The ¹³C n.m.r. spectra of 18 derivatives of the tricyclo[3.2.1.0^2,4]octanes have been determined. This series includes methyl, hydroxyl and oxo substituted examples to compare the effects of these substituents on the skeletal carbon shieldings with those observed for the corresponding norbornanes and bicyclo[3.2.1]octanes. In general, the trends are similar and the perturbations associated with closely neighboring groups follow a consistent pattern. The shielding data for the exo–exo and exo–endo isomers of tetracyclo[3.3.1.0^2,40^6,8]nonane are also reported.     

CNDO/BW study of prototropy in 2-acetyl-1,3-indandione

Conclusions CNDO/BW methods have been used to determine optimal conformations of potentially prototropic forms of 2-acetyl-1,3-indandione and optimal paths for intramolecular proton transfer within the molecule. Possible mechanisms for external prototropy are discussed.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1747–1751, August, 1981.