A caution regarding the use of LAOCOON III for AA′BB′ spin systems

It has been found that LAOCOON III can fail to converge for some strongly coupled AA′BB′ spin systems unless trial coupling constants and chemical shifts within a few tenths of a Hz of the real parameters are used to initiate the iteration. This problem seemed linked to those spin systems possessing two or more pairs of energy levels which are nearly degenerate. In the cases where LAOCOON III failed, the program NMRENIT always yielded the correct, converged, iterative solution even with completely arbitrary starting parameters. The latter program also appears to accomplish a given number of iterations with an AA′BB′ system roughly three times as fast as the former program.     

Rotational isomerism—XVI: The AA′BB′X NMR spectrum and rotational isomerism of 2-fluoroethyl trichloroacetate

The complete AA′BB′X analysis of the ¹H and ¹⁹F NMR spectrum of CH₂F.CH₂OCO·CCl₃ ( 1 ) as a neat liquid and in various solvents is given and considered in detail. The differentiation of the two conjugate solutions is obtained both from the X spectrum and by analysis at two field strengths. The coupling constants obtained show that the compound exists almost entirely in the gauche rotamer in all solvents.     

Protonenresonanz-Spektroskopie ungesättigter Ringsysteme—XVI 7,7-Difluor-benzo-cyclopropen

The NMR-spectrum of 7·7-difluoro-benzo-cyclopropene ( 2 ) has been analysed to obtain chemical shifts and spin, spin-coupling constants: δ_AA′ = 7·6026, δ_BB′ = 7·4834 ppm; J_AB = 6·86, J_AA′ = 7·45, J_AB′ = 0·34 and J_BB′ = 1·89 Hz. Heteronuclear double resonance experiments have been used to establish a positive sign for ⁴J(H? F) (3.64 Hz) and a negative sign for ⁵J(H? F) (?0·33 Hz) in this molecule. The results are discussed with reference to the structure of 2 and the NMR data found for benzo-cyclopropene.     

Iterative analysis of the 1H NMR spectra of naphthalene (AA′ A″ A‴ BB′ B″ B‴ system) and two of its derivatives

The ¹H NMR spectra of naphthalene as an eight spin system, and two of its derivatives, 2,4-dichloro- and 2,4-dinitro-1-naphthol, have been completely analysed. The Indor technique has been very useful in the determination of the signs and magnitudes of very small coupling constants. An improved version of the Laocoon 3 program has greatly facilitated the spectral analysis through, first, the knowledge of the dependence of each transition upon the spectral parameters; second, some modifications in the iterative loop which avoid failure of convergence, and, third, the use of a slightly modified diagonalization subroutine which saves 20% CPU computer time. Our data suggest a rather small substituent effect on intra-ring couplings of the unsubstituted ring. The inter-ring coupling constants display a negative or positive sign according to an even or odd number of intervening bound, except the all-trans ⁶J which is positive. No substituent effect on the inter-ring couplings has been observed, except a small effect for the all-trans ⁶J.     

Carbon Magnetic Resonance Spectra of α, β, γ, δ- and α, β, α′, β′- Unsaturated Ketones

Carbon-13 spectra of a series of 26 unsaturated ketones (ortho- and para-cyclo-hexadienones and corresponding open-chain analogues) have been measured by Fourier-transform. Pulse spectroscopy. A complete analysis has been achieved by means of double resonance experiments using noise-modulated and coherent off-resonance proton irradiation and with the aid of non-decoupled spectra. Chemical shifts are interpreted in terms of charge distribution in the dienone system and of methyl substituent effects. Carbon chemical shifts were also obtained for O-protonated ortho- and para-cyclohexadienones. One-bond and long-range carbon-proton and carbon-fluorine spin coupling constants are reported for several compounds.     

Etude par Résonance Magnétique Nucléaire de dérivés phosphores. Etude de dithiaphospholanes-1,3,2

The proton NMR spectral analysis of eight different 1,3,2-dithiaphospholanes with various groups attached to the phosphorus atom has been performed. The AA′BB′X (X phosphorus atom) system shows that the two ³J(P? S? C? H) coupling constants have a small magnitude and opposite signs. Using the ³J(HH) values, the torsion about the C₄—C₅ bond has been evaluated. The conformational requirements in the two isomers of the 2 phenyl-4-methyl-1,3,2-dithiaphospholane are also discussed.     

Etude par RMN des 2,2′-Dihalogeno-Diethylethers—I: Effet de Température

NMR spectra of disubstituted ethers with the formula (X? CH₂? CH₂)₂O, X = Cl, Br, I, have been studied. Spectra of neat liquids at ambient temperature and spectra of these compounds in CS₂ at different temperatures have been analysed. It has been possible to assign the chemical shifts. In each case, the more populated rotamers have been determined. Additionally, for these three compounds dissolved in CS₂, it is shown that the gauche form becomes more stable with cooling. The authors propose a procedure, using spectral decomposition, which allows the analysis of AA′BB′spectra almost degenerated in A₂B₂ cases.     

Biocatalytic synthesis of polymers. II. Preparation of [AA–BB]x polyesters by porcine pancreatic lipase catalyzed transesterification in anhydrous,low polarity organic solvents

Enzyme-catalyzed preparation of polymers offers several potentially valuable advantages over the usual polymerization procedures. (1) Such polymerizations may allow the polymer to retain functionality that would be destroyed under normal polymerization conditions. (2) The selectivity provided by enzyme catalysts may permit polymers, including optically active polymers, to be prepared that are either not accessible or accessible only with difficulty by other methods. (3) The characteristics of the enzyme and the mild polymerization conditions may permit formation of polymers having highly regular sizes and backbone structures. This report describes the first successful use of an enzyme-catalyzed polycondensation to prepare a chiral (AA–BB)_x polyesters of more than a few repeat units. Polymerization of bis(2,2,2-trichloroethyl) alkanedioates (BB) with diols (AA) using the enzyme porcine pancreatic lipase (PPL) as a catalyst is detailed. The polycondensations were carried out at ambient temperature in anhydrous, low polarity organic solvents such as ether, THF, and methylene chloride. End group analysis by NMR provided M_n values of 1300–8200 daltons while GPC provided M_w values of 2800–14900 daltons for the polymers. Based on proton NMR spectra obtained during the polymerization, relatively rapid formation of an AA–BB “dimer” and an AA–BB–AA “trimer,” slower formation of a BB–AA–BB “trimer,” and subsequent condensation of these to give higher polymers are suggested to be components of the polymerization mechanism.     

Deceptive simplicity in NMR spectra of oriented molecules

The theory for deceptive simplicity in the NMR spectra of oriented molecules is discussed in terms of degeneracy of subspectra. The conditions for the degeneracy of AB, AB₂ and AA′BB′ spectra leading to deceptive simplicity in the spectra of the type ABX, AB₂X, AA′BB′X and AA′XX′ are reported. It is shown that the conditions for deceptive simplicity are easily fulfilled for ABX, AA′BB′X and AA′XX′ cases. It is further demonstrated that deceptive simplicity is not so common in AB₂X spectra. The proton spectra of partially oriented 2,4,5-trichloronitrobenzene (AB) and 2,6-dibromo-pyridine (AB₂) are use to illustrate degeneracy and those of p-chloro- and p-bromo-fluorobenzenes are reported to demonstrate deceptive simplicity.     

Proton magnetic resonance spectra of the atropisomers of ortho-hexaphenylene

The structures of the atropisomers of o-hexaphenylene are confirmed by their proton magnetic resonance spectra. The centrosymmetrical isomer 1a gives rise to an AA′BB′ spectrum, whereas the helical isomer 1b displays an AA′BB′ spectrum for two of its aromatic rings and an ABCD for the remaining four rings. Long-range coupling between the AA′BB′ and ABCD parts of 1b leads to a tentative assignment of the protons. An analysis of the spectrum of o-tetraphenylene ( 2 ) is also given.     

Evolution of Nucleic‐Acid‐Based Constitutional Dynamic Networks Revealing Adaptive and Emergent Functions

The evolution of networks is a fundamental unresolved issue in developing the area of systems chemistry. We introduce a versatile rewiring mechanism that leads to the emergence of nucleic‐acid‐based constitutional dynamic networks (CDNs). A two‐component constituent AA′ functionalized with a Mg²⁺‐ion‐dependent DNAzyme activator unit forms a complex with an intact hairpin H_BB′ composed of B and B′ sequences. Cleavage of H_BB′ leads to the two‐component constituent BB′, and its rewiring with AA′ yields CDN X composed of the equilibrated constituents AA′, AB′, BA′, and BB′. In analogy, subjecting AA′ to an intact hairpin H_CC′ leads to the formation of CDN Y consisting of AA′, AC′, CA′, and CC′. Subjecting AA′ to the mixture of H_BB′ and H_CC′ evolves the [3×3] CDN Z, composed of nine constituents, thus demonstrating hierarchical adaptive properties. Furthermore, the DNAzyme units associated with the constituents are applied to tailor emerging catalytic functions from the different CDNs.     

Michael addition polymerizations of difunctional amines (AA′) and triacrylamides (B3)

Novel hyperbranched poly(amido amine)s containing tertiary amines on the backbones and acryl or secondary amines as the surface groups were successfully synthesized via the Michael addition polymerizations of a triacrylamide [1,3,5‐triacryloylhexahydro‐1,3,5‐triazine (TT)] and a difunctional amine [n‐butylamine (BA)] NMR techniques were used to clarify the structures of hyperbranched polymers and polymerization mechanism. The reactivity of the secondary amine formed in situ was much lower than that of the primary amines in BA. When the feed molar ratio was 1:1 TT/BA, the secondary amine formed in situ was almost kept out of the reaction before the BA (AA′) and TT (B₃) monomers were consumed, and this led to the formation of A′B₂ intermediates containing one secondary amine group and two acryl groups. The self‐polymerization of the A′B₂ intermediates produced hyperbranched polymers bearing acryl as surface groups. For the polymerization with the feed molar ratio of 1:2 TT/BA, A′₂B intermediates containing one acryl group and two secondary amine groups were accumulated until self‐polymerization started; the self‐polymerization of the intermediates formed hyperbranched polymers with secondary amines as their surface groups. Modifications of surface functional groups were studied to form new hyperbranched polymers. The hyperbranched poly(amido amine)s were amorphous. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6226–6242, 2006     

Long range H,H coupling constants in cycloheptatrienes

The ¹H NMR spectra of 1,6-dicarbomethoxycyclohepta-1,3,5-triene ( 2 ) and 3,4-benzocyclohepta-1,3,5-triene ( 3 ) have been analysed in terms of chemical shifts and coupling constants. A method for the assignment of δ(AA′) and δ(BB′) in AA′BB′ systems, based on the observation of ¹³C satellites in the ¹H NMR spectrum, is described. For J(1,5) in 3 , a negative sign was established. Spin tickling experiments were used to determine the positive sign for J(2,5) in 2 and the negative sign for J(2,7) in 3. The conformation of 3 is discussed.     

Synthesis and total 1H-NMR assignment of naphtho[1′,2′:4,5]thieno[2,3-c][1,10]phenanthroline and naphtho[2′,1′:4,5]-thieno[2,3-c] [1,10]phenanthroline

Naphtho[1′,2′:4,5]thieno[2,3-c][1,10]phenanthroline and naphtho[2′,1′:4,5]thieno[2,3-c][1,10]phenanthroline, two novel polycyclic heterocyclic ring systems, have been synthesized in four steps from known starting materials. The total ¹H nmr spectral assignments were made using a COSY experiment to identify the spin systems.     

The synthesis of some 4,4′-disubstituted-2,2′-biquinolines

2,2′-Biquinoline dioxide and 4,4′-dichloro-2,2′-biquinoline have been used for the preparation of the following 4,4′-disubstituted-2,2′-biquinolines: dimethoxy, diethoxy, dihydroxy, dipiperidino, dipyrrolidino, dibromo, diphenoxy, dithiophenoxy, di(p-chloro-phenoxy), di(p-bromophenoxy), di(p-fluorophenoxy), di(β-naphthoxy) and the dinitro dioxide. Molar extinction coefficients have been determined for several of the copper (I) complexes of these compounds.     

Separating Para and Ortho Water

Water exists as two nuclear‐spin isomers, para and ortho, determined by the overall spin of its two hydrogen nuclei. For isolated water molecules, the conversion between these isomers is forbidden and they act as different molecular species. Yet, these species are not readily separated, and no pure para sample has been produced. Accordingly, little is known about their specific physical and chemical properties, conversion mechanisms, or interactions. The production of isolated samples of both spin isomers is demonstrated in pure beams of para and ortho water in their respective absolute ground state. These single‐quantum‐state samples are ideal targets for unraveling spin‐conversion mechanisms, for precision spectroscopy and fundamental symmetry‐breaking studies, and for spin‐enhanced applications, for example laboratory astrophysics and astrochemistry or hypersensitized NMR experiments.     

Level Anti‐Crossings are a Key Factor for Understanding para‐Hydrogen‐Induced Hyperpolarization in SABRE Experiments

Various hyperpolarization methods are able to enhance the sensitivity of nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) by several orders of magnitude. Among these methods are para‐hydrogen‐induced polarization (PHIP) and signal amplification by reversible exchange (SABRE), which exploit the strong nuclear alignment of para‐hydrogen. Several SABRE experiments have been reported but, so far, it has not been possible to account for the experimentally observed sign and magnetic‐field dependence of substrate polarization. Herein, we present an analysis based on level anti‐crossings (LACs), which provides a complete understanding of the SABRE effect. The field‐dependence of both net and anti‐phase polarization is measured for several ligands, which can be reproduced by the theory. The similar SABRE field‐dependence for different ligands is also explained. In general, the LAC concept allows complex spin dynamics to be unraveled, and is crucial for optimizing the performance of novel hyperpolarization methods in NMR and MRI techniques.     

Tethering for Selective Synthesis of 2,2′‐Biphenols: The Acetal Method

2,2'‐Biphenols are a large and diverse group of compounds with exceptional properties both as ligands and bioactive agents. Traditional methods for their synthesis by oxidative dimerisation are often problematic and lead to mixtures of ortho‐ and para‐connected regioisomers. To compound these issues, an intermolecular dimerisation strategy is often inappropriate for the synthesis of heterodimers. The ‘acetal method’ provides a solution for these problems: stepwise tethering of two monomeric phenols enables heterodimer synthesis, enforces ortho regioselectivity and allows relatively facile and selective intramolecular reactions to take place. The resulting dibenzo[1,3]dioxepines have been analysed by quantum chemical calculations to obtain information about the activation barrier for ring flip between the enantiomers. Hydrolytic removal of the dioxepine acetal unit revealed the 2,2′‐biphenol target.     

Conformational dynamics of bis(BF2)‐2,2′‐bidipyrrins revealed by through‐space 13C?19F and 19F?19F couplings

The conformation of [bis‐(N,N′‐difluoroboryl)]‐3,3′‐diethyl‐4,4′,8,8′,9,9′,10,10′‐octamethyl‐2,2′‐bidipyrrin (1) in solution was studied by analyzing the ¹³C? ¹⁹F and ¹⁹F? ¹⁹F through‐space spin–spin couplings. The ¹H and ¹³C NMR spectra were assigned on the basis of nuclear Overhauser effect spectroscopy (NOESY), heteronuclear single‐quantum correlation (HSQC), and heteronuclear multiple‐bond correlation (HMBC) experiments. The ¹⁹F spectrum of 1 was compared with that of 2‐ethyl‐1,3,5,6,7‐pentamethyl‐4,4‐difluoro‐4‐bor‐3a,4a‐diaza‐s‐indacen (2). The ¹⁹F? ¹⁹F through‐space spin? spin coupling in 1 was thus assigned and the coupling constant was obtained by simulating the coupling patterns. The obtained conformation of 1 was compared with those of the known complexes [bis‐(N,N′‐difluoroboryl)]‐3,3′,8,8′,9,9′‐hexaethyl‐4,4′,10,10′‐tetramethyl‐6,6′‐(4‐methylphenyl)‐2,2′‐bidipyrrin (3)and [bis‐(N,N′‐difluoroboryl)]‐9,9′‐diethyl‐4,4′,8,8′,10,10′‐hexamethyl‐3,3′‐bis(methoxycarbonylethyl)‐2,2′‐bidipyrrin (4). The conformational dynamics of 1, 3, and 4 was surveyed by observing the temperature dependence of the through‐space coupling constants between 253 and 333 K. The ¹³C? ¹⁹F and ¹⁹F? ¹⁹F through‐space spin–spin couplings thus confirm similar conformations of different BisBODIPYs in solution in contrast to earlier findings in the solid state. Copyright © 2009 John Wiley & Sons, Ltd.