Computer-aided structure elucidation of organic compounds with the chemics system: Removal of Redundant Candidates by 13C-n.m.r. Prediction

A ¹³C-n.m.r. prediction module capable of removing inappropriate candidate structures given for an unknown compound based on the spectral data is introduced for the CHEMICS system. Given a set of candidate structures generated in the system, the routine may be used to prune off redundant candidates which have a predicted number of signals inconsistent with the observed number. It is shown that the addition of the examination module to the system makes structure elucidation by computer much more practical.     

N.m.r. characterisation of linkage isomers

Summary Halogenation of Ln^III, Ce^IV and ZrO^II -diketone/-ketoester derivatives and their mixed ligand complexes of the types Ln(AA)₂(Sal) and Ln(AA)(Sal)₂ by NCS, NBS and PyHBr₃ yield different isomeric products depending on the nature of the solvent medium, the reagent and the reaction time. The halogenation, if carried out in glacial acetic acid, irrespective of the reagent, yields the stable S-hall, product wherein the three chelate rings remain imperturbed in respect of metal coordination. When the reaction is carried out in 5% v/v DMF-CHCl₃ employingN-halosuccinimide and maintaining correct reaction times, it is possible to isolate individually three other isomeric products. The isomers prepared are Ln(OO)₃, Ln(OO)₂(OX), Ln(OO)(OX)₂ and Ln(OX)₃ where (OO) represents diketone oxygen linkage and (OX) represents diketone oxygen and substituted halogen linkage to the central metal ion. The four linkage isomers have been identified by a comparison of the number of observed³H n.m.r. or¹³C n.m.r. signals with those expected for a given isomer on the basis of symmetry considerations in the tris-chelated octahedral structures.     

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.     

Computer-aided n.m.r. spectra interpretation: Part 1. An artificial intelligence system

Some desirable hardware and software features of an artificial intelligence system tailored to the needs of a modern computerized n.m.r. laboratory are discussed. A system corresponding to this idea is described.     

13C n.m.r. spectra of cyclopropenes

¹³C n.m.r. chemical shifts and one bond ¹³C, ¹H coupling constants for cyclopropene and its 1- and 3-methyl derivatives as well as for methyl cyclopropane have been measured. The data for cyclopropene are 108·9 ppm and 228·2 Hz, 2·3 ppm and 167·0 Hz in the olefinic and allylic position, respectively. Substituent effects for the methyl group are discussed.     

Application of carbon-13 n.m.r. Spectroscopy to the structural investigation of ezomycins

Ezomycins A₁, A₂, B₁, B₂, C₁, C₂, D₁ and D₂, antifungal antibiotics, are anhydrooctose uronic acid nucleosides. Their novel structures were confirmed by studying their natural abundance ¹³C n.m.r. spectra in detail from gated proton-decoupled, long-range selective proton-decoupled and selective proton-decoupled spectra. The J(CH) value of the anomeric carbon signal (C-1″) supports the β-configuration.     

N.m.r. determination of wax esters in marine lipids

The proton n.m.r. spectra of lipids containing triglycerides and wax esters dissolved in CDCl₃ are characterized by seven sets of signals. The areas of the signals of terminal methyl group and of methylene protons of both wax ester and triglyceride were integrated. These were used to calculate the content of wax esters in lipids or oils. The rapid n.m.r. procedure is directly usable for natural lipids containing as low as 3 mg of wax esters in 50-mg samples with an error of about 7%. The method described compares favorably with t.l.c. determination.     

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.     

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.     

2,3,5,6-Tetramethylmorpholine: II—1H n.m.r. structural studies of the isomers of 2,3,5,6-tetramethylmorpholine

The structures of the six isomers of 2,3,5,6-tetramethyl-morpholine have been determined by means of proton nuclear magnetic resonance studies at 100 MHz. The spectra have been analysed in terms of spin–spin coupling constants and population distributions of the possible conformers at low temperatures. Also included in this paper are results from n.m.r. studies on the six 4-benzyl-2,3,5,6-tetramethylmorpholine derivatives, which give information about the stereochemistry of the methyl groups α to the amine group.     

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. 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.     

1H n.m.r. spectra of 6-acetoxy-2,4-cyclohexadienones

¹H n.m.r. spectra of 36 derivatives of 6-acetoxy-2,4-cyclohexadienones were analysed. All available evidence indicates that all members of the series have similar conformations which do not depart significantly from planarity. Substituent-induced chemical shifts and interproton coupling constants correlate well with those in the analogously substituted ethylene and benzene derivatives. In particular, it appears that a substituent on the central carbon atom exerts a similar influence on ⁴J(HH) across a ‘W’ path when the three intervening carbon atoms are a part of a benzene ring, an allylic system (i.e. sp³–sp²–sp²), a localized sp²–sp²–sp² system or a saturated (i.e. sp³–sp³–sp³) system.     

13C n.m.r. spectra of quercetin and rutin

The noise-decoupled and gated-decoupled ¹³C n.m.r. spectra of quercetin and rutin are studied and assignments are proposed. The previous assignments of carbons 2 and 4′, carbons 5 and 9, carbons 6 and 8 have been reversed.     

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.     

Electronic, e.p.r., cyclic voltammetric and biological activities of copper(II) complexes with macrocyclic ligands

Eight new macrocyclic ligands have been prepared by the reaction of the precursor diketone (benzil, glyoxal, diacetyl or 2,3-pentanedione) with a diamine (thiosemicarbazide or semicarbazide). Copper(II) complexes of these ligands have been synthesized and characterized by elemental analysis, molar conductance, magnetic susceptibility, i.r., u.v.–vis, ¹H-n.m.r., mass and e.p.r. spectral studies. Mass, n.m.r. and i.r. data indicate the condensation of the diamine and diketone and the whole molecular ion structure. g-Values are calculated for all of the complexes in polycrystalline form as well as in DMSO solution. Spin Hamiltonian values and bonding parameters have also been calculated which indicates that an unpaired electron is present in the orbital. The metal-ligand bonding parameters shows strong in-plane σ sigma and in-plane π bonding. The magnetic and spectral data indicate tetragonal geometry for all of the complexes except [CuH₂L⁴]Cl₂ and [CuH₂L⁴]Cl₂ which are square planar. From c.v. data reversible Cu^II/Cu^I couples are observed for these complexes. The macrocyclic complexes show more antibacterial and antiviral activity as compared to the ligands. The antibacterial activities of the compounds were tested against S. aureus, S. subtillis and E. coli.     

A remarkable accidental degeneracy in the1H-n.m.r. Spectrum of a (cyanoethene)platinum (II) complex

Summary In the¹H-n.m.r. spectra of (acetylacetonato)chloro-(²-cyanoetheiie)platinum(II), the three protons of the ligand appear to be equivalent.¹³C-n.m.r., i.r. and further¹H-n.m.r. studies indicate that the metal-ligand bonding is of orthodox -olefin type (as formulated in the name of the compound above). The two 2-hydrogens are made equivalent by fluxion, but there is an accidental degeneracy with 1-hydrogen atom.     

A general purpose n.m.r. microcell system suitable for volatile,air sensitive and toxic samples

A microcell system that is convenient to fill and which can be evacuated and/or filled with gas or vapour has been developed for handling tritiated n.m.r. samples and has subsequently proved to have a more general application. A 3 mm tube, after the removal of a 5 mm filling extension, is sleeved into a 5 mm n.m.r. tube or used directly in a reduced diameter probe.     

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.