Carbon-13 nuclear magnetic resonance spectroscopy of polychlorinated diphenyl ethers

Carbon-13 n.m.r. chemical shifts are reported for diphenyl ether and several of its chlorinated analogs. Carbon shieldings were found to be dependent on the degree of steric interference to conjugative release by the oxygen atom. Substituent effects due to attached chlorine atoms seem to be more easily transmitted into the adjacent aryl moiety if the number of ortho chlorines is reduced. These results were clearly seen by a variable-by-variable plot display or by using a pattern recognition computer program. Spin-lattice relaxation times have been measured for several compounds mainly for assignment purposes, but they are also briefly discussed in view of possible interconversion mechanisms.     

Carbon-13 NMR studies of some organotin(IV) compounds

The ¹³C chemical shifts and ¹³C−¹¹⁹Sn, ¹¹⁷Sn coupling constants for several organotin(IV) compounds R_xSnCl_4−x (R = Me, Buⁿ, Ph; x = 1−4) have been measured in both inert (CDCl₃) and donor (DMSO-d₆) solvents, as have ¹³C data for the compounds R_xSnR′_4−x (R = Me, Ph; R′ = Buⁿ and R = Me; R′ = Ph; x = 1−3) and the compounds Me₃SnX (X = pseudo halide). The δ and ¹J(C-Sn) values appear to depend mainly on the type and number of substituents on tin and the donor ability of the solvent. There are linear relationships between the number of substituents (x) and both δ and ¹J(C-¹¹⁹Sn) for almost the R_xSnX_4−x series (R = Me, Buⁿ, Ph; X = Cl and R = Me, Buⁿ; X = Ph; x = 1−4), when measured in a single solvent, e.g. CDCl₃. There is an excellent linear relationship between ¹J(C-¹¹⁹Sn) and ²J(¹HC-¹¹⁹Sn) for the compounds Me_xSnCl_4−x. Determination of ¹³C data for Me₃SnCl and Ph₃SnCl in a range of solvents reveals that the value of ¹J(C-Sn) increases with the donor ability of the solvent.The marked increase in the values of ¹J(C-¹¹⁹Sn) in DMSO-d₆ for the compounds R_xSnCl_4−x(R = Me, Buⁿ,Ph) on going progressively from x = 4 to x = suggest tin coordination numbers of 4, 5, 6 and 6, respectively. Some additional physical data are presented for the isolated complexes from DMSO and the compounds Ph_xSnCl_4−x(x = 1−3) and Me₃SnX with X = N₃ or OCOMe.     

Carbon-13 NMR studies of benzoquinones

The assignment of all ring carbons of p-benzoquinones derived from perezone and from thymoquinone was completed using gated decoupled spectra. The long range proton-carbon couplings are discussed in terms of the degree of substitution of the quinone ring. The tautomeric interconversion of the two energetically equivalent forms of 2,5-dihydroxy-1,4-benzoquinones has been studied in various solvents and at several temperatures.     

Carbon-13 NMR spectra of some phosphorane-amino compounds

The ¹³C NMR spectra of a series of compounds of the type R¹P(NR₂²)_nPh_3?n X^? are reported. A net trend toward larger ¹J(PC) values with an increasing number of dialkylamino substituents is observed.     

Carbon-13 NMR studies of twenty-four methyl-substituted morpholines

Carbon-13 NMR spectra of all the isomers of monomethyl-, 2,3-, 2,5-, 2,6-, 3,5-dimethyl-, 2,3,5-, 2,3,6-trimethyl- and 2,3,5,6-tetramethylmorpholine have been obtained at both ambient (25 °C) and low temperature (～ ?100 to ?120 °C). The ring carbon shifts appear to be additive with respect to the position of the methyl groups. A good correlation between predicted and experimental shift values was obtained (r = 0.998₉). The values were used in an attempt to assign, conformationally, the ‘all cis’ isomer 2,3,5,6-tetramethylmorpholine, which from ¹HNMR spin–spin coupling studies has been unsuccessful. Methyl carbon shifts to high field were found for axially oriented carbons. The extracted ‘steric shift’ values for such carbons were compared to their corresponding proton shift data.     

Carbon-13 NMR studies of naturally occurring maytansinoids

The ¹³C NMR spectra of the antileukemic ansa macrolide maytansine and eight naturally occurring homologues were recorded. Signals were assigned to specific carbon atoms of the ansa macrolide ring and C-3 ester side chain using data from off-resonance decoupling, single frequency decoupling, and 180°–τ–907° double pulse experiments. The assignment of each carbon is discussed for maytansine and tabulated for the other eight maytansinoids.     

Carbon-13 NMR spectra of some tetra- and pentacyclic triterpene methyl ethers

The carbon-13 NMR spectra have been recorded for representatives of three previously unexamined classes of pentacyclic triterpenes—lupanes, fernanes and arboranes. In addition to the spectra of lupeol methyl ether, cylindrin and arundoin, the spectra of a number of other triterpene methyl ethers are also recorded. The effect of methyl ether formation on the shilding values for carbons in the 3β-hydroxy-4,4-dimethyl- ring A system is defined.     

Carbon-13 and proton NMR studies of 1,4-benzodiazepines

One bond and long range ¹H-¹H, ¹H-¹⁹F, ¹³C-¹H and ¹³C-¹⁹F coupling constants for six 1,4-benzodiazepines are described. An unambiguous assignment of their carbon-13 resonances is carried out based on chemical shift theory and on the analysis of the fine splittings caused by one bond and long range couplings. Chemical shift and coupling constant values are reported for diazepam, chlordiazepoxide, oxazepam, nitrazepam, chlonazepam and flunitrazepam.     

Carbon-13 NMR spectra of Piper alkamides and related compounds

¹³C NMR spectra of Piper alkamides and related compounds have been completely assigned on the basis of splitting patterns in the coupled spectra, application of additivity relationships and internal comparison of chemical shifts.     

Carbon-13 NMR studies of a series of benzylphenols

Carbon-13 NMR spectra of a series of benzylphenols and their O-alkylated derivatives were recorded to find the substituent effects of the benzyl, hydroxybenzyl and alkoxybenzyl groups on the ¹³C chemical shifts. It was found that the methylene bridge carbons show signal shifts mainly due to the mesomeric effects of the OH and OCH₃ substituents, and that in the case of ortho-substituted benzyl compounds, the methylene carbon signals exhibit upfield shifts due to both mesomeric and steric effects.     

Carbon-13 NMR spectra of some polychloro hydrocarbons and related compounds

Data derived from the carbon-13 NMR spectra of 37 organic polychloro compounds allow one to identify readily the ? CHCl₂, ? CCl₂? and ? CH₂Cl groups, the ¹³C signals of which are registered in the shift ranges of 67 to 78 (80), 85 to 96 and 38 to 55 (59) ppm (from TMS), respectively, and have the distinctive one bond spin-spin coupling constants ¹J(C? H) 170 to 184 Hz (for the ? CHCl₂ groups) and 147 to 158 Hz (for the ? CH₂Cl groups). The ? CCl₂CH₂CH₂Cl fragment features characteristic diamagnetic shieldings of the ? CCl₂? and ? CH₂Cl that may be related to increased electron density on both of these groups.     

Carbon-13 NMR spectra of retinal1 and its related compounds

Carbon-13 NMR spectra of all-trans retinal₁ and vitamin A₁ were measured by the pulse Fourier transform method in CCl₄. All peaks in these spectra were assigned from considerations of chemical shifts, half proton-decoupled spectra, spin-lattice relaxation times and induced chemical shift by the addition of shift reagent. The carbon-13 NMR spectrum was also measured for all-trans retinal₁ which had been exposed to the sunlight for three hours, and the induced isomer was proposed to be 11-s-cis retinal₁.     

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 NMR studies on 3-phenyl-as-triazine 4-oxides

The substituent chemical shifts of a series of para substituted-3-phenyl-as-triazine 4-oxides were studied using the Swain-Lupton F and R values. This data was compared to that obtained for parasubstituted biphenyls, phenyl pyrroles and phenyl furans. It was found that the substituent chemical shifts in the triazines behaved in a similar manner to that in the biphenyl system.     

Carbon-13 NMR studies on some 5-substituted quinoxalines

Eleven 5-substituted quinoxalines (NO₂, NH₂, COOH, OCH₃, CH₃, OH, F, Cl, Br, I, CN, the latter five not reported previously) have been synthesized by standard methods. Their ¹³C NMR spectra have been measured in DMSO-d₆ and assigned on the basis of substituent parameters, by line widths and by intensities. The chemical shifts compare favorably with those calculated using benzene substituent parameters, and are very close to those of corresponding carbons in 1-substituted phenazines. The correlation with the chemical shifts of the corresponding positions in 1-substituted naphthalenes is also close except for those of carbons 4a and 8a in the quinoxalines which, due to their proximity to nitrogen, are downfield (in some cases 12 ppm) of the signals of the corresponding carbons in naphthalene. 5-Fluoroquinoxaline was also measured in CDCl₃, CD₃COCD₃, CD₃CN, CD₃OD, C₆D₆ and CD₃COOD. In all solvents an abnormally low ²J(CF) (～ 12 Hz) was found for C-4a and no C? F spin-spin splitting could be detected for the three-bond coupling of C-8a. Similar abnormalities were found in 2-fluoroaniline and 2-fluoroacetanilide. There are linear relationships between the Q parameter of the substituent and the chemical shift of carbons 4a, 5 and 6. A linear relationship also exists between the chemical shift of C-8 (‘para’ position) and the Hammett σ_p parameter of the substituent.     

Carbon-13 NMR spectral studies on some labdane diterpenoids

Carbon-13 NMR signal assignments of the labdane diterpenoids andrographolide and 14-deoxyandrographolide, along with their acetates, and isodeoxyandrographolide have been made. This study indicates that the C-6 and C-11 resonance assignments of isovirescenol-B, made earlier, should be interchanged.     

Carbon-13 NMR of polybutadiene

By using the carbon-13 NMR technique, it is shown that there are no cis-1,4–trans-1,4 linkages in a n-BuLi-catalyzed polybutadiene. The polymer consists of “blocks” of cis-1,4 units and trans-1,4 units separated by isolated vinyl units. Preliminary evidence suggests this might also be true for other types of 1,3-diene polymerization. Some of the implications of this finding on the mechanism of polymerization are discussed. Tacticity triad distributions are readily determined in polybutadienes with high 1,2 addition contents.