13C-NMR. Spectra,Structure and Reactivity of Cyclic Oxocarbons

The structures of cyclic polyketones, their hydrates and reduction products were studied in different solvents by ¹³C-NMR. spectroscopy. In dimethylsulfoxide solution rhodizonic acid (1) , croconic acid (2) and squaric acid (3) exhibit signal averaging. In anhydrous tetrahydrofuran 1 and 2 could be observed as non-dynamic species. The spontaneous reactions of dodecahydroxycyclohexane (5) and decahydroxycyclopentane (‘leuconic acid’) (6) and the sequence of formation of ring-contracted products were investigated. Key intermediates could be clearly identified which support the mechanism proposed earlier involving a benzylic acid type rearrangement followed by decarboxylation and subsequent redox reactions.     

13C-NMR. Spectra of Pteridines

¹³C-NMR. spectra of pterin, xanthopterin, isoxanthopterin, leucopterin, lumazine and of the model compounds isocytosine and desamino-isocytosine have been measured as anions and cations in 1 M NaOD, CF₃COOH, H₂SO₄ and FSO₃H solutions. The spectra were analysed by means of heteronuclear double resonance, with the aid of non-decoupled spectra, and by spectral comparison. The results are interpreted in terms of the ionisation state of the pteridines in the four solvents and are compared with those obtained from ¹H-NMR. spectroscopy.     

13C-NMR. Spectra of Natural and Semi-synthetic Cannabinoids

The ¹³C-NMR. spectra of one natural and ten semi-synthetic cannabinoids were analyzed in detail. Assignments of the signals are based on their chemical shifts, splitting patterns in ¹H-off-resonance decoupling experiments and comparison with ¹³C-NMR. data of related cannabinoids. With some compounds final assignments were made by selective ¹H-decoupling experiments and incremental calculations.     

Low-Temperature 13C-NMR Spectra of 6Li- and 13C-Labelled Sulfur- and Selenium-Substituted Organolithium Derivatives

The ¹³C-NMR spectra of 19 different, singly, doubly, and triple ¹³C-labelled α-sulfur- and α-selenium-substituted ⁶Li-derivatives generated from methyl and phenyl thioethers, thioacetals, trithio-orthoesters and from their selenium analogues have been recorded in ethereal solutions (tetrahydrofuran (THF), 2-methyltetrahydrofuran (MTHF) at temperatures between ?30° and ?110°. The effects of H/Li-exchange upon chemical shifts and coupling constants, as well as the values and multiplicities of Li, ¹³C-coupling are interpreted in vie wof crystal structures of some of the same compounds. In two thirds of the cases studied, the H-decoupled ¹³C-NMR signals observed below ?80° were triplets, proving that the C-atoms are bonded to single ⁶Li-atoms. This is compatible either with monomeric or with dimeric, heteroatom-bridged structures. The direct ¹H, ¹³C- and ¹³C, ¹³C-coupling constants (¹J) decrease, the ¹³C, ⁷⁷Se-coupling constants increase upon lithiation. More striking is that the geminal coupling ¹³C-S-¹³C (²J) is too small to be observed in the non-metalated species, while it ranges from 3.7 to 7.5 Hz in the lithiated derivatives. These observations may be interpreted as resulting from delocalization of electron density from the carbonionic center towards the adjacent heteroatom.     

Microwave Spectra of o-Fluorotoluene and Its 13C Isotopic Species: Methyl Internal Rotation and Molecular Structure

The rotational spectra of o-fluorotoluene and its seven ¹³C isotopic species were recorded in the frequency range from 4 to 20 GHz with employment of pulsed molecular beam Fourier-transform microwave (MB-FTMW) spectrometers. The analysis of the spectra in the two lowest states of methyl internal rotation (torsional ground state, A and E species) was based on a asymmetric frame-rigid symmetric top Hamiltonian with inclusion of centrifugal distortion terms, yielding structural rotational constants, as well as the threefold barrier V ₃ to internal rotation and the angle(a,i) between the principal moment of inertia a axis and the internal rotor axis i. The rotational constants of all eight isotopomeres were used to derive the seven ¹³C r _s coordinates of the molecule.     

混合价硒代锡酸盐K2Sn2Se4的固相合成、晶体结构和反射光谱研究

采用反应性熔盐法在550℃下合成一种新型混合价硒代锡酸盐晶体(K2Sn2Se4).该晶体属于四方晶系,空间群为I4cm,晶胞参数a=0.81524(12)nm,b=0.81524(12)nm,c=0.67152(13)nm.K2Sn2Se4晶体具有一维链状结构,一维的1∞[Sn2Se4]2-链沿c轴方向无限扩展,K+插入链之间.K2Sn2Se4属于Zintl型化合物,具有半导体特性.漫反射光谱法研究结果表明,该化合物的光学能隙(Eg)为1.7eV.     

混合价多硒代锗酸盐K2Ge4Se8的固相合成、晶体结构和反射光谱研究

采用反应性熔盐法以n(K2Se3)∶n(Ge)∶n(Se)=2∶1∶6的摩尔比,在773 K下反应5 d,得到一种新的混合价硒代锗酸盐,K2Ge4Se8.该晶体属于单斜系,空间群为P21/c,晶胞参数a=0.73752(5) nm, b=1.2239(2) nm, c=1.7468(4) nm, β=96.02(3)°, V=1.5680(5) nm3, Z=4.偏差因子R1=0.0643, wR2=0.1547. K2Ge4Se8晶体具有层状结构,二维的 2∞[Ge4Se8]2-层负离子和K+正离子堆积而成,层与层按ABAB排列.K2Ge4Se8属于Zintl型化合物,属于半导体,反射光谱法测定的光学能隙(Eg)为1.8 eV.     

Crystal Structure Refinements of Cellulose Polymorphs using Solid State 13C Chemical Shifts

Force field methods in combination with chemical shift target functions are used to investigate the structures of cellulose I and II. Since diffraction investigations of biopolymers like cellulose II are only of poor resolution, different models for the structure are discussed in the literature. These models were used as the starting point for force field optimizations with ¹³C chemical shift target functions. In these optimizations additionally to the total energy a pseudo-energy is minimized that depends harmonically on the difference between calculated and observed chemical shifts. In the case of cellulose II all four criteria: (i) total energy, (ii) pseudo-energy, (iii) chemical shift rms (root mean square) difference, and (iv) deviation from the diffraction data favour the structure model of Kolpak and Blackwell with two antiparallel carbohydrate chains. The CH₂–OH group of one chain is in tg and that of the other chain in gt conformation. The chemical shift optimized fractional coordinates for cellulose II, I and I are presented together with the calculated and experimental ¹³C chemical shifts.     

13C-NMR. Spectra of 4-Substituted Quinuclidines. Polar effects,Part V

¹³C-NMR. sepctra of 37 4-substituted quinuclidinium perchlorates, 15 4-substituted quinuclidines and the corresponding 1-methylquinuclidinium iodides have been measured. The chemical shifts δ for all compounds lie in the expected range. No correlation is found between δ and the inductive substituent constant σ of the substituent. Abnormal shift differences between quinuclidines bearing a nucleofugal group and the corresponding protonated or N-methylated quinuclidinum salt are observed for the bridgehead carbon C(4). These differences are ascribed to incipient fragmentation, i.e. C, C-hyperconjugation in the ground state.     

Solid-State CP/MAS 13C-NMR Spectra of Oligolides derived from 3-hydroxybutanoic acid

The solid-state CP/MAS ¹³C-NMR spectra (cross-polarization/magic-angle spinning ¹³C-NMR) of eight lower cyclic and one linear oligomers and several polymers of (R)-3-hydroxybutanoic acid (3-HB) are reported. The polymeric samples of different origin and molecular weight give remarkably similar and well resolved spectra, indicating considerable similarity in the conformations of the molecules and homegeneity in the solid-state environment. The crystalline cyclic oligomers 1 – 8 containing 3–9 units of 3-HB give very well resolved spectra. The number of nonequivalent positions in the solid state can be identified and is in accord with structures from X-ray diffraction where these were determined. The spectra of the oligolides become increasingly similar to those of the polymer as the ring size increases. This spectral evidence supports the view of a homogeneous and well defined conformation for the polymeric material (as proposed previously, based on other experiments).     

13C-NMR spectra of cis-polymyrcene and cis-polyfarnesene

Homopolymers of myrcene and farnesene were prepared anionically in pure cyclohexane. The microstructure, determined from an analysis of the ¹³C-NMR spectrum and spin-lattice relaxation times, indicates the polymers are at least 85% cis-1,4, 10% cis-3,4, and under 3% trans-1,4.