Analysis of the 13C-n.m.r. spectra of mono- and dimethylamphetamines

The ¹³C-n.m.r. spectra of the three monomethylainphetainines and the six dimethyl-amphetanunes and their hydrochlorides were determined. Spectra were distinctive and suitable for identification and authentication purposes. Signals were assigned by a self-consistent analysis of chemical shift differences between differently-substituted compounds in the series of both the bases and the salts. Determination of the effects of N-protonation on the chemical shifts of aromatic ¹³C signals was a valuable aid in resolving some ambiguities: C-1 shifted upfield by about 0.9 ppm, and methylated carbons ortho, meta, and para to the isopropylamine side-chain shifted downfield by about 3.3, 3.7, and 4.4 ppm, respectively. The data are useful in the authentication of reference materials employed in forensic analysis.     

The darcpluridata system: the 13C-n.m.r. data bank1

The capabilities of the DARC system are discussed and illustrated by the storage and retrieval functions of the ¹³C-n-m.r. data bank of the DARC PLURIDATA system. The data covered by the bank, as well as the input stream to the bank and validation of the spectra, are described. Particular stress is laid on the DARC structural retrieval system, which illustrates the interactive interrogation of a chemical data bank by means of the structural diagram of a molecule, i.e. the universal language in chemistry. The potential of the ¹³C-n.m.r. data bank in computer-aided structural elucidation is outlined.     

13C-n.m.r. spectra of acrylophenone (1-phenylprop-2-en-1-one) and ring-substituted acrylophenones

¹³C-n.m.r. chemical shifts of 10 acrylophenones (1-substituted phenylprop-2-en-1-ones) are reported. The additivity parameters for the substituent effect of the acryloyl group in the aromatic ring and of the benzoyl group in ethylene were calculated. Comparison of ethene chemical shifts in chalcones (1,3-diphenylpropenones) with calculated values yielded good results, showing that cross conjugation in these molecules is of little importance. Chemical shifts in acrylophenones and chalcones can therefore be calculated as linear combinations of the mutual effects of the constituent groups.     

Carbonyl carbon chemical shifts in the 13c spectra of cyclic ketones

The ketonic ¹³C NMR signals of the keto lactones 1 and haplophytine (4) occur at exceptionally high field. These upfield shifts are interpreted in the context of a general consideration of the structural factors affecting the carbonyl carbon chemical shifts of cyclic ketones. It is concluded that dipole-dipole interactions are the major sources of the upfield shifts in the cases of both 1 and 4.     

13C n.m.r. chemical shifts of carbodiimides

The ¹³C n.m.r. chemical shifts of the sp-hybridized carbons in dialkylcarbodiimides have values of δc ? 140. These shifts are compared with those of similarly hybridized carbons occuring in other classes of compounds.     

A minicomputer program based on additivity rules for the estimation of 13c-nmr chemical shifts

A minicomputer program for the estimation of ¹³C-n.m.r. chemical shifts based on simple additivity rules is described.     

Application of 13C-n.m.r. spectroscopy to study the mechanism of N-demethylation of [NMe13C] codeine by cell-free extracts of Cunninghamella bainieri

Codeine was synthesized with 90% enrichment of the N-methyl group with carbon-13. N-Demethylation of this substrate by cell-free extracts of Cunninghamella bainieri in an n.m.r, tube gave norcodeine and ¹³C-labelled formaldehyde. Fourier-transform ¹³C-n.m.r. spectroscopy was used to observe the N-demethylation process at selected temperatures. The labelled formaldehyde liberated was trapped with sodium sulphite, and the sulphite adduct, as well as intermediates, were located in the n.m.r, spectrum at each temperature. Intermediate resonances assignable to codeine-N-oxide were not detected during these enzyme-transformation studies. These data suggest that the observed ¹³C-n.m.r. signals correspond to the chemically labile carbinolamine intermediate formed during N-demethylation. A methine ¹³C signal was not observed. Thus, N-demethylation of codeine by Cunninghamella bainieri occurs by direct C-oxidation and not via an N-oxide intermediate.     

Relationship between reactivity and 13C chemical shifts of benzocyclobutenes

¹³C NMR spectra of benzocyclobutene derivatives have been investigated; substituent effects and the relationship between the chemical shifts and the reactivity of their cyclobutene ring are discussed.     

The influence of methylation on 13C chemical shifts of galactose derivatives

A comparison of the galactose and galactitol spectra with those of methylated derivatives provides the signal assignment of this hexose and its polyol. A formerly stated rule can be confirmed: Methylation of hexopyranose hydroxyls causes upfield shifts of about 4·5 ppm on β-carbons with axial OH groups. In the series of polyols O-methylation causes down-field shifts on the resonance of C_α of about 9–10 ppm, downfield shifts on that of C_β for about 0·6 to 0·9 ppm and upfield shifts of about 0·3 to 0·5 ppm on the C_γ-resonance.     

A microcomputer-base 1h-nuclear magnetic resonance search system for spectra

The development of integrated software for a microcomputer system is described for ¹H-NMR spectra search. The facilities include registration of newly generated reference data, addition of new data to the data bases and extraction of matched files for the query spectrum with good efficiency.     

A computer program for the prediction of 13-C-NMR chemical shifts of organic compounds

A computer program is described for the estimation of carbon-13 chemical shifts. It automatically selects and applies the additivity rules appropriate for the individual carbon atoms of the structure entered. Besides connectivity, the configuration and conformation can be entered and evaluated for some types of structure. All rule parameters (base values and increments) can be extended or modified by the user. New rules based on the corresponding linear relationships can also be added. A test against 168 807 known chemical shifts shows that the implemented rules cover roughly 97% of all cases. The standard deviation of the predicted values relative to the experimental values is 5.5 ppm.     

A substructure-oriented 13c-nmr chemical shift retrieval system

A computer program that uses on-line generated substructures of organic compounds as input and retrieves the corresponding distributions of ¹³C-n.m.r. chemical shifts is described and discussed. The procedure of creating the substructures and the main features of the retrieval philosophy are outlined. One search is worked out in detail to demonstrate the ability of the system.     

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.     

13C-NMR spectra of fluorinated molecules using 19F-13C polarization transfer

¹³C-NMR spectra of fluorinated molecules are difficult to observe under conventional broad-band proton decoupling. The large coupling constants involved make polarization transfer between fluorine and carbon, using INEPT or DEPT experiments very effective while broad band fluorine decoupling collapses the multiplet pattern.