Solid state13C NMR spectra of metal carbonyl clusters

The solid state¹³C NMR spectra of four¹³CO enriched carbonyl clusters having a tri-iron metallic core have been analyzed to provide structural and dynamic information. In Fe₃(CO)₁₂ (1), the high temperature spectra suggest the occurrence of large amplitude motions of the CO groups around their position at the vertexes of the coordination polyhedron in addition to the motion involving the Fe₃-triangle previously detected in the VT-¹³C MAS spectra.¹³C and³¹P NMR data of Fe₃(CO)₁₁PPh₃ (2) indicates the presence of one molecule in the asymmetric unit in apparent disagreement with the previously reported X-ray data. Furthermore, we show that structural information can be obtained from the chemical shift tensor components readily available from the analysis of the spinning sideband manifold.     

Solid State NMR of Drugs: Soluble Aspirin

Abstract

Natural abundance ¹³C NMR spectra of a soluble aspirin and model mixtures of acetylsalicylic acid with buffering components have been recorded in the solid state by using the combined techniques of cross polarization, high-power decoupling and magic-angle spinning. The solid-state spectrum of the soluble aspirin tablet showed more resonances than the solution spectrum. These multiplicities were originated in the buffer mixture containing citric and tartaric acid, as well as their salts. Solid-state ¹³C NMR was therefore found to provide information that is lost in the solution spectrum due to the fast proton exchange between the organic acids and their conjugated salts.     

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.     

Structure of red and orange fluorescein

The structures of colorless, red, and yellow-orange fluorescein have been investigated by a combination of solution and solid state¹³C NMR. It is demonstrated that the three forms have lactonic, zwitterionic, and quinoid structures, respectively. Conflicting X-ray, NMR, and IR structural evidence is discussed for samples of red fluorescein which cannot readily be obtained in definite crystalline form. It is concluded that solid-state¹³C NMR spectroscopy is superior by revealing primarily the molecular structure but being fairly insensitive toward lattice variations.The Editors regret the long delay in having this paper printed due to an unfortunate administrative error.     

13C NMR Spectra of 5- and 7-Bromo-6-ketosteroids and Related Compounds

¹³ C NMR spectra have been studied and signals of C atoms have been assigned for 5- and 7-bromo-6-ketosteroids, which are intermediates in the synthesis of ecdysteroids.     

Some Considerations about the Structure of 3(5)-Methylpyrazole

The behavior of neat 3(5)-methylpyrazole has been studied at low temperatures by DSC and by NMR (¹³C and ¹⁵N). The main conclusion is that the supercooled liquid is a mixture of four trimers formed by 3-methyl and 5-methyl tautomers.     

Solution structure of bis(acetoxy)iodoarenes as observed by O NMR spectroscopy

A group of para-substituted bis(acetoxy)iodoarenes has been studied by ¹⁷O and ¹³C NMR. Only one signal for all the oxygens of the acetoxy groups has been observed. Both ¹⁷O and ¹³C chemical shifts of this group show a strong invariance with para substitution. The absence of covalent I-O bonds and an ion pair structure is proposed for the title compounds.     

Synthesis and characterization of the [Ru(η5-C5Me4CF3)(CO)2]2 and Ru6(μ3-H)(η2-μ4-CO)2(μ-CO)(CO)12(η5-C5Me4CF3) complexes

The reaction of Ru₃(CO)₁₂ with tetramethyltrifluoromethylcyclopentadiene at various ratios of the reagents was studied. Refluxing of Ru₃(CO)₁₂ with a sixfold excess of tetramethyltrifluoromethylcyclopentadiene in octane in an inert atmosphere gave a complex, which is, according to X-ray diffraction data, a dimer,trans-[Ru(η⁵-C₅Me₄CF₃)(CO)₂]₂. The reaction under the same conditions but starting from Ru₃(CO)₁₂ and C₅Me₄CF₃H in 2∶1 molar ratio gave a hexaruthenium cluster [Ru₆(μ₃-H)(η₂-μ₄-CO)₂(μ-CO)(Co)₁₂(η⁵-C₅Me₄CF₂)], which was characterized by IR as well as¹H,¹³C, and¹⁹F NMR spectroscopy. According to X-ray diffraction data, an Ru₄ tetrahedron, in which two edges are bound by additional “briding” Ru atoms, constitutes the frame of this compound. This complex has one (η⁵-C₅Me₄CF₃) ligand, as well as one (μ₃-H) and two (η²-μ₄-CO) groups. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 507–512, March, 1998.     

Solid State NMR Studies of the Bromocyclohexane/Thiourea Inclusion Compound

Solid state ¹³C-MAS and ²H-NMR investigationshave been performed on the bromo-cyclohexane/thiourea inclusion compound.The main objective of this study was the evaluation of the molecularfeatures of the guest molecules and their changes over a large temperaturerange (100 K < T < 350 K). Particular emphasis was placed on themolecular behaviour in the vicinity of a solid–solid phase transitionat T = 237 K and in the low temperature phase, which was hitherto unknown.The ²H-NMR lineshape and relaxation studies reveal that, inthe low temperature phase, restricted but fast overall motions (MHz to GHzregion) of the guest molecules are dominant and reflect the distortedsymmetry of the thiourea channels. On heating above the solid–solidphase transition almost unrestricted overall motions appear, together with alarge degree of orientational disorder at higher temperature. Furthermore,the ring interconversion process presents a major relaxation process in theMHz region. The conformational order is unusual in the sense that the axialconformational state of the guest molecules is stabilized in the thioureachannels. It turns out that this unique property is also preserved in thelow temperature phase.     

1-Methyl-2(3,4)-(1-vinylpyrrol-2-yl)pyridinium iodides: some structural features revealed by 1H and 13C NMR spectroscopy

The reactions of 2-, 3-, and 4-(1-vinylpyrrol-2-yl)pyridines with methyl iodide afford the corresponding quaternary salts. Analysis of their ¹H and ¹³C NMR spectra showed that the quaternization of the nitrogen atom considerably enhances the -acceptor effect of the pyridine ring on the pyrrole ring and on the vinyl group. 1-Methyl-2-(1-vinylpyrrol-2-yl)pyridinium iodide contains no weak intramolecular C--H...N hydrogen bond present in the starting compound.     

Structure of Narcissus tazetta Glucomannan

Naturally acetylated glucomannan with MW 31,000 and []_D ²⁰ -29.1° was isolated from bulbs of Narcissus tazetta L. Chemical methods and IR and ¹³C NMR spectroscopies were used to show that the polysaccharide contains 12.9% O-Ac groups and consists of D-mannose and D-glucose in a 1:5.6 ratio that form a linear polymer through -1,4-glycoside bonds.     

Structure of 2-Amino-4-phenyl-9H-pyrimido[4,5-b]indole and Its Nitrate in Crystal and Solution

The crystal structures of 2-amino-4-phenyl-9H-pyrimido[4,5-b]indole and its nitrate were determined by XRD analysis. ¹³C NMR, XRD, and quantum-chemical data indicate that the acid proton is predominantly localized at the N(3) atom of the pyrimidine ring in both isolated nitrate molecules and in molecules in crystal and solution.     

Structural Assignment of Side Chain Liquid Crystalline Monomer and Polymer by 1-D and 2-D Solution NMR Studies

In the present study, the synthesis and characterization of a methacrylic-based side chain liquid crystalline monomer and its polymer were investigated with the aid of both 1-D and 2-D nuclear magnetic resonance (NMR) techniques. The mesophase characteristics of the monomer and polymer were determined by hot-stage optical polarized microscopy (HOPM). The nematic and smectic phases were observed for the monomer and polymer. Furthermore, ¹³C spin-lattice relaxation time measurements were performed on the above systems in order to monitor molecular dynamics. The present study provides an opportunity to carry out a systematic comparison of the evolution of structural as well as dynamical changes of the monomer and the polymer.     

Solid and solution study of tetranuclear zinc citrates with N-donor chelates

Tetranuclear zinc(II) citrates with N-heterocycle ligand, [Zn₄(Hcit)₂(phen)₄(H₂O)₄]·2NO₃·10H₂O (1) and [Zn₄(Hcit)₂(bpy)₄(H₂O)₆]·2NO₃·12H₂O (2) (H₄cit?=?citric acid, phen?=?1,10-phenanthroline, bpy?=?2,2′-bipyridine), were synthesized from aqueous solution and characterized by Infrared, Powder X-ray diffraction, fluorescence spectra, and X-ray structural analyses. Complexes 1 and 2 are centrosymmetric tetranuclear species with two crystallographically independent Zn(II) ions, which display different coordination geometries, while the citrate ligand shows similar coordination modes and bridges three different Zn ions. In 1, the Zn1 is five-coordinate by three oxygens from two citrates and two nitrogens from phen. There exist weak interactions between Zn1 and O4a atom of β-carboxy group of citrate (Zn1?O4a?=?2.895?Å). Zn2 is six-coordinate by two oxygens from one citrate, two nitrogens from phen, and two water molecules. Zn1 and Zn2 are bridged by one citrate to form a dinuclear unit, which is further extended to a tetranuclear unit. In 2, two citrates bridge two pairs of symmetry-related Zn(II) ions into a tetranuclear unit. The structural analyses of 1 and 2 indicate that different tetranuclear species are caused by arrangement of citrates with different steric hindrance of N-chelates. ¹H and ¹³C NMR spectra indicate that 2 remains coordinated in D₂O. Moreover, both 1 and 2 show strong fluorescent emission in dilute solution and solid state.     

Quantum-Chemical Investigation of the Electronic and Spatial Structure of 1-Vinyltetrazoles

Calculations have been carried out of the total Mulliken charges on carbon and nitrogen atoms, the relative stability, and equilibrium populations of the s-cis(R) and s-trans(R) conformers of a series of 1-vinyl-5R-tetrazoles (R = H, Me, n-Bu, tert-Bu, Ph, NH₂, I, CF₃, NO₂) by the ab initio MP2/6-31G** method. Out-of-plane structures correspond to the stable s-cis(R) conformer, and the deviation from planarity grows regularly with an increase in the bulk of the substituent. The proportion of s-trans(R) conformation increases in the same series and reaches 100% for R = tert-C₄H₉. The data of quantum-chemical calculations are in agreement with the results of investigations of the spatial structure of 1-vinyl-5R-tetrazoles by ¹H and ¹³C NMR methods. Values of the total energies of the corresponding homodesmotic reactions were calculated to assess the effect of the nature of the substituent in position 5 of the tetrazole ring on the size of the conjugation energy in planar s-trans(R) conformations of the compounds investigated. The quantum-chemical calculations carried out show that the conjugation energy decreases regularly according to the increase in electron-withdrawing properties of the substituent in position 5 of the ring.__________Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 537–548, April, 2005.     

Highly regioselective synthesis of trifluoromethyl derivatives of pyrazolo[1,5-a]pyrimidines bearing fused cycloalkane rings using (2-ethoxycycloalkenyl)-2,2,2-trifluoroethanones

8-Trifluoromethyl-6,7-dihydro-5H-1,4,8a-triaza-s-indacene and 9-trifluoromethyl-5,6,7,8-tetrahydropyrazolo[5,1-b]quinazolines were efficiently generated by condensation of 5(3)-aminopyrazoles with (2-ethoxycycloalkenyl)-2,2,2-trifluoroethanones and isolated in excellent yields. The regiochemistry of the prepared compounds was established by ¹H, ¹³C and ¹⁹F NMR spectroscopy and X-ray diffraction analysis.     

13C NMR spectra of 17-substituted epimeric 17α and 17β-ethynylandrostenes and their hexacarbonyldicobalt complexes

¹³C NMR spectra of epimeric 17-R-substituted 17-ethynylandrostenes (R = H, OH, OAc, OMe, NCS) and 17-R-substituted 17-ethynylandrostenes (R = H, OH, OAc, OMe), and their hexacarbonyldicobalt complexes, as well as of ¹⁶-17-ethynylandrostenes and their hexacarbonyldicobalt complexes have been studied. Stereochemical dependence of C(12), C(14), C(18), C(20) and C(21) chemical shifts on the configuration of the substituents at C(17) has been demonstrated. The effect of Co-complexation on the spectral characteristics of Co-coordinated 17-ethynylandrostenes in both Stereochemical series has been investigated.Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 1, pp. 212–216, January, 1993.     

13C NMR Spectra of 6-Hydroximinosteroids of the Stigmastane Series

¹³ C NMR spectra were studied and signals of C atoms were assigned for 6-keto- and 6-hydroximinosteroids 1-10.     

Experimental and theoretical studies on group 1 metallacarboranes: Synthesis, structure and ab initio calculations of the NMR chemical shifts of the 1-(THF)-1-(TMEDA)-1-Na-2,4-(SiMe3)2-2,4-C2B4H5 and related carboranes

The reaction of gaseous HCl with either the disodium or dilithium compound of the [nido-2,4-(SiMe₃)₂-2,4-C₂B₄H₄]²⁻ dianion (I) in 1:1 stoichiometry in THF produced the monoprotonated species 1-Na(THF)₂-2,4-(SiMe₃)₂-2,4-C₂B₄H₅ (II) or 1-Li(THF)₂-2,4-(SiMe₃)₂-2,4-C₂B₄H₅ (III), in 81% and 80% yields, respectively. This method proved superior to that involving the direct reduction of the closo-C₂B₄ carborane by metal hydrides. II and III were characterized by elemental analysis, ¹H, ¹¹B and ¹³C NMR and IR spectra. Compound II was recrystallized from a mixture THF, hexane and TMEDA (1:2:1) to isolate colorless crystals of the mixed solvated species, 1-(THF)-1-(TMEDA)-1-Na-2,4-(SiMe₃)₂-2,4-C₂B₄H₅ (IV), which were subsequently used for X-ray diffraction studies. The structure of IV showed that the capping metal occupied the apical position above the open C₂B₃ face of the carborane and that a hydrogen atom was bridging the two adjacent boron atoms on that face. The ¹¹B and ¹³C NMR spectra calculated by GIAO (gauge independent atomic orbital) methods at the 6-311G^** level on the B3LYP/6-31G^* optimized geometries of I–III, and a number of related nido- and closo-carboranes, gave excellent agreement with experiment, even in compounds where electron correlation effects are known to be important.