Substituent Effects on 13 C Chemical Shifts in Aliphatic Molecular Systems. Dependence on Constitution and Stereochemistry. In: Eliel EL, Wilen SH, Allinger NL, editors. Topics in Stereochemistry. Hoboken: John Wiley & Sons, Inc.; 1986. pp. 219-324. [DOI: 10.1002/9780470147252.ch4]

NMR spectral properties of the tetramantanes - nanometer-sized diamondoids

Tetramantanes, and all diamondoid hydrocarbons, possess carbon frameworks that are superimposable upon the cubic diamond lattice. This characteristic is invaluable in assigning their (1)H and (13)C NMR spectra because it translates into repeating structural features, such as diamond-cage isobutyl moieties with distinctively complex methine to methylene signatures in COSY and HMBC data, connected to variable, but systematic linkages of methine and quaternary carbons. In all tetramantane C22H28 isomers, diamond-lattice structures result in long-range (4)JHH, W-coupling in COSY data, except where negated by symmetry; there are two highly symmetrical and one chiral tetramantane (showing seven (4)JHH). Isobutyl-cage methines of lower diamondoids and tetramantanes are the most shielded resonances in their (13)C spectra (<29.5 ppm). The isobutyl methylenes are bonded to additional methines and at least one quaternary carbon in the tetramantanes. W-couplings between these methines and methylenes clarify spin-network interconnections and detailed surface hydrogen stereochemistry. Vicinal couplings of the isobutyl methylenes reveal positions of the quaternary carbons: HMBC data then tie the more remote spin systems together. Diamondoid (13) C NMR chemical shifts are largely determined by α and β effects, however γ-shielding effects are important in [123]tetramantane. (1)H NMR chemical shifts generally correlate with numbers of 1,3-diaxial H-H interactions. Tight van der Waals contacts within [123]tetramantane's molecular groove, however, form improper hydrogen bonds, deshielding hydrogen nuclei inside the groove, while shielding those outside, indicated by Δδ of 1.47 ppm for geminal hydrogens bonded to C-3,21. These findings should be valuable in future NMR studies of diamondoids/nanodiamonds of increasing size.

Byrsonic Acid—the Clue to Floral Mimicry Involving Oil-Producing Flowers and Oil-Collecting Bees

Tetrapedia diversipes and other Apidae (Anthophoridae) may be deceived by floral similarities between Malpighiaceae and Orchidaceae of the Oncidiinae subtribe. The latter do not usually exudate floral oils. Thus, visitors may pollinate the flowers in a deceit/food/pollination syndrome. We studied the chemical compositions of Byrsonima intermedia (Malpighiaceae) floral oil and T. diversipes (Anthophoridae) cell provisions. From B. intermedia floral oil, we isolated a novel fatty acid (3R, 7R)-3,7-diacetoxy-docosanoic acid, here named byrsonic acid, and from T diversipes cell provisions we isolated two novel fatty acid derivatives 3,7-dihydroxy-eicosanoic acid and 3,7-dihydroxy-docosanoic acid, here named tetrapedic acids A and B, respectively. The three fatty acid derivatives have common features: possess long chains (20 or 22 carbon atoms) with no double bond and either hydroxy or acetoxy groups at carbons 3 and 7. This characteristic was also encountered in the fatty acid moiety of oncidinol (2S, 3'R, 7'R)-l-acetyl-2-[3', 7'-diacetoxyeicosanyl)-glycerol, a major floral oil constituent of several Oncidiinae species (Orchidaceae). Thus, both tetrapedic A (C20) and B (C22) could be the biotransformation products of oncidinol and byrsonic acid by T. diversipes hydrolases. These are the chemical clues for bee visitation and oil collecting from both plant species. The results indicate that the deceit/pollination syndrome should not be applied to all Oncidiinae flowers.

Substituent effects in the 13C NMR chemical shifts of alpha-mono-substituted acetonitriles

13C chemical shifts empirical calculations, through a very simple additivity relationship, for the alpha-methylene carbon of some alpha-mono-substituted acetonitriles, Y-CH(2)-CN (Y=H, F, Cl, Br, I, OMe, OEt, SMe, SEt, NMe(2), NEt(2), Me and Et), lead to similar, or even better, results in comparison to the reported values obtained through Quantum Mechanics methods. The observed deviations, for some substituents, are very similar for both approaches. This divergence between experimental and calculated, either empirically or theoretically, values are smaller than for the corresponding acetones, amides, acetic acids and methyl esters, which had been named non-additivity effects (or intramolecular interaction chemical shifts, ICS) and attributed to some orbital interactions. Here, these orbital interactions do not seem to be the main reason for the non-additivity effects in the empirical calculations, which must be due solely to the magnetic anisotropy of the heavy atom present in the substituent. These deviations, which were also observed in the theoretical calculations, were attributed in that case to the non-inclusion of relativistic effects and spin-orbit coupling in the Hamiltonian. Some divergence is also observed for the cyano carbon chemical shifts, probably due to the same reasons.

First direct discrimination of chiral phosphorus thionate (P=S) derivatives by multinuclear magnetic resonance spectroscopy in the presence of a chiral dirhodium complex

Enantiomeric ratios of compounds with P=S functionalities can be determined by 1H, 13C, and 31P NMR spectroscopic inspection of their diastereomeric complexes with (R)-Rh2(MTPA)4 (MTPA-H identical with methoxytrifluoromethylphenylacetic acid; Mosher's acid). This is the first facile and rapid spectroscopic method for chiral recognition in this class of compounds. Whereas complexation shifts Deltadelta are moderate or even negligible, significant signal dispersions Delta(nu) can be observed. Some rationalization about the complexation mode is presented. The NMR spectral characteristics of the free P=S compounds 1-9 are described in detail.

13 C NMR spectra of amino-alkylphosphonic acids

¹³ C chemical shifts and ³¹ P,¹³ C coupling constants are reported for 20 amino-alkylphosphonic acids and two phosphonic acids. The substituent-induced chemical shifts of the phosphonate group were calculated and their non-additivity in 1-aminoalkylphosphonic acids was observed. Two stable conformers of 4-aminobutylphosphonic acid were detected by ¹³ C NMR.