Complexation-induced chemical shifts--ab initio parameterization of transferable bond anisotropies

Complexation-induced changes in proton chemical shifts provide a potent tool for conformational analysis, being highly dependent on intermolecular orientation. An important contribution to these shifts arises from the molecular magnetisability anisotropy, or more specifically from the anisotropy of certain groups, such as aromatic rings and unsaturated bonds. While the influence of aromatic rings has been well characterised via the ring current effect, unsaturated bonds have received much less attention and prediction of complexation shifts is hampered by the lack of accurate anisotropy parameters for these bonds. We have therefore used ab initio calculations at the HF/aug-cc-pVDZ level to obtain bond anisotropies for C-H, N-H, C=O, C=C, C triple bond N, N=N, C triple bond C, and C triple bond N. Fitting the anisotropies to bond magnetic dipoles (the McConnell equation) gives non-transferable values for C-H and N-H bonds. We have therefore expanded in terms of bond magnetic dipoles, quadrupoles, and octopoles for double and triple bonds only, obtaining highly accurate shielding surfaces in all cases. The transferable nature of the anisotropies is confirmed by comparing with shifts obtained in larger molecules containing unsaturated bonds.     

The N.M.R. spectrum of coumarine partially oriented in a nematic mesophase

The proton magnetic resonance spectrum of coumarine oriented in a nematic mesophase has been recorded and analysed in terms of the proton-proton dipolar coupling constants and anisotropic chemical shifts. An analysis of the isotropic P.M.R. spectrum is also reported. The N.M.R. spectral parameters indicate that the molecule is planar and has a structure of the naphthalene type, with no apparent distortions in the aromatic ring. The internuclear proton distances derived from the N.M.R. experiment are reported, together with the orientation parameters.     

Hydrogen Bonding and the NMR Parameters in Dimethylformamide

The oxygen atom of dimethylformamide (DMF) is a good electron doaor site and in a proton donor solvent the solute-solvent interactions would include hydrogen bonding at this site. As this hydrogen bonding is expected to be relatively strong, it would predominate when both the lMF and the protonic acid are present at very low concentration in a solvent such as carbon tetrachloride, and the effect of this hydrogen bonding on the proton chemical shifts and coupling constants i n LMF can be observed. If the protonic acid is aromatic, such as phenol, and is in excess of the LMF, other form(s) of solut-olvent association would become significant and give rise to aromatic solvent induced shifts (ASIS), which would obscure the effect of hydrogen bonding at the oxygen site of the MF. The ASIS in DMF are well known, and the shifts have been attributed to the diamagnetic ring current effect of the aromatic solvent and specificsolut-olvent association.¹ More recently, the ASIS has been interpreted as the result of time-averaged general solvation around the polar site of the solute.²     

烯氢和烯碳化学位移的本质

对乙烯、共轭二烯、叠烯、羰基、烯胺、亚胺双键中的sp²杂化碳、氧、氮原子以及双键相连的质子或碳原子化学位移实验数据的分析表明, 目前化学位移理论中所采用的π-键在磁场中产生环流的假说与实验结果有严重矛盾. 核周电子云变形模型可较好地解决这些矛盾.     

Theoretical evaluation of NMR shifts in polycyclic aromatic hydrocarbons

ABSTRACT

Using computational chemistry methodology, we evaluate the proton magnetic shieldings and the corresponding chemical shifts of 12 polycyclic aromatic hydrocarbons that derive from chrysene, perylene and picene. Due to the large size of the studied compounds, we resort to density functional theory (DFT) and use it together with the B3LYP and the KT1 functionals. After a systematic method and basis set selection study carried out on methane, benzene and anthracene, the DFT(B3LYP) method and the 6-31G*, 6-31G** and 6-311++G** bases are selected to carry out the calculations, because of the efficiency in providing shifts close to the experimental data available. Additionally, we select the DFT(KT1) method together with the aug-pcS-1 basis set, and HF/6-31G* shifts are also calculated. In order to estimate the error in the theoretical results, we take as reference accurate experimental chemical shifts obtained for the molecules under investigation. Extra measurements are needed for this purpose and are included in the present work. The best combination of method and basis set is DFT(B3LYP)/6-31G**, proving to be very efficient in getting shifts close to experiment at a relatively low computational cost, and therefore we recommend it for the evaluation of proton shifts in larger polycyclic aromatic hydrocarbons.     

On the magnetic properties of conjugated molecules

Recent criticisms of the gauge factors usually employed in the ‘test-dipole’ method for the calculation of ‘ring current’ chemical shifts in conjugated molecules, are discussed. It is shown that, in a simple semi-empirical scheme of the London-McWeeny type, insertion of a dipole contribution into the vector potential appearing in the gauge factor, whilst having no effect on the calculated ‘ring current’ intensities, is algebraically analogous (and, at large distances from ring centres, numerically equivalent) to estimating the secondary field at the origin due to a set of classical line currents, as discussed originally by Salem; these ‘line currents’ are of the same magnitude as the quantum-mechanical ‘bond currents’ implicit in the ‘ring currents’ calculated using the simpler gauge factors originally due to London, but their contributions to the secondary magnetic field experienced by the peripheral protons, are estimated classically. Extensive numerical comparison is made between experimentally-observed proton chemical shifts in some conjugated hydrocarbons, and secondary fields estimated by this semi-classical formalism, and its predictions are found to correlate as well with experiment as do those of the original McWeeny approach. It is concluded that any further illegitimacy involved in the procedure of inserting a dipole contribution into the gauge factor, is evidently quite simply compensated for, numerically, by an appropriate empirical parametrization. Such empirical parametrizations are also thought to absorb errors due to all the other various approximations of the ‘ring current’ theories (with apparently unwarranted efficiency), and they should, therefore, be treated with more scepticism than has previously been thought necessary.     

Concentration‐Dependent Variation of 1H‐NMR Chemical Shifts of Aromatic Protons in Sampangine Derivatives

A concentration dependence of the ¹H‐NMR chemical shifts of the aromatic protons in sampangine derivatives with a fused imidazole ring is observed. This variation is probably ascribable to self‐association of the molecules through an intermolecular π‐stacking interaction of the aromatic rings. The quantitative variation is correlated with the calculated electrostatic potential for these derivatives. The concentration variation appears to be independent of the nature of the substitution in the imidazole ring.     

A proton magnetic resonance investigation of some weak interactions in solution

The proton chemical shifts of cyclohexane, methyl iodide and iodoform are measured in a number of solvents. A complete calculation of the contribution to the solute proton chemical shift arising from the magnetic anisotropy of cylindrically symmetric solvents is given. Although the formula predicts the direction of the observed shifts, the observed values for non-polar solutes are always much larger than the calculated values. Some possible reasons for this are given and discussed.

The variation of the proton chemical shifts of the polar solutes methyl iodide and iodoform in aliphatic solvents are shown to agree with present theories of these effects. However, in aromatic solvents considerable deviations from the theoretical line are found and these are postulated to arise from solute solvent complexes in which the dipole axis of the solute lies along the hexagonal axis of symmetry of the benzene ring with the protons towards the ring. From the variation of the proton chemical shifts of methyl iodide and iodoform in toluene solution with temperature the following parameters were obtained. For the methyl iodide toluene complex the energy and entropy of formation are 1·3 ± 0·5 kcals/mole and 4·9 ± 0·4 e.u. respectively. For the iodoform toluene complex the corresponding values are 1·6 ± 0·2 kcals/mole and 6·4 ± 0·2 e.u.     

High resolution hydrogen resonance spectra of some substituted ethylenes

The olefinic proton resonance spectra of a number of mono- and 1, 2 di-substituted ethylenes have been analysed as AB, ABC or ABX systems to obtain the coupling constants and chemical shifts. The values of J _trans (range observed 13·7 to 18·0 c/s in 14 compounds) were significantly larger than J _cis in similar molecules. The values of J _cis (range observed 6·5 to 12·3 c/s in 8 compounds) are abnormally large when the olefinic bond is conjugated to an aromatic ring. Coupling between adjacent protons in an olefinic methylene group (J _gem) is much smaller, and sometimes negative. The shielding of the lone vinyl proton in mono-substituted ethylenes is smaller than that of the methylene group and in two cases (the vinyl group directly bonded to an oxygen atom) the difference is so great as to give an approximation to the ABX condition, so that the methylene protons appear to be shielded to an unusually large extent.     

High-accuracy measurement of proton chemical shifts and coupling constants in substituted cyclic oligosaccharides using phase-sensitive two-dimensional soft experiments

The performances of soft COSY and RELAY experiments are demonstrated for substituted cyclic oligosaccharides. In association with soft NOESY, they allow fast sequencing of complex molecules and high-accuracy measurements of both chemical shifts and coupling constants, which are required for a complete determination of the three-dimensional structure of these compounds. These procedures are applied to a cyclodextrin derivative for which self-inclusion of the aromatic moiety in the hydrophobic cavity induces large ring current effects which can be used to derive the three-dimensional structure of this molecule.     

Gas-phase deprotonation of arylalkylamines. A collision-induced dissociation study

The collision-induced dissociation (CID) of deprotonated arylalkylamines of general formula R(1)C(6)H(4)CHR(2)CH(2)NR(3)(2) (where R(1) = H, OH, F or NO(2); R(2) = H or OH; R(3) = H or CH(3)) generated by negative chemical ionization with H(2)O and D(2)O as ionizing reagents, is discussed. The negative chemical ionization mass spectra show that, in the absence of a hydroxy group in the aromatic ring, deprotonation takes place at the benzylic position whereas the proton is lost from the OH group when present. The nitro compound forms only M(-.) ions. The CID spectra of the deprotonated molecules show that fragmentations are strongly dependent on the structural features of the molecules, namely the presence or absence of substituents in the aromatic ring or aliphatic chain. Copyright 1999 John Wiley & Sons, Ltd.     

The calculation of chemical shifts in conjugated molecules

Within the context of pi-electron theory an expression for the proton chemical shifts in conjugated molecules is derived via a current density approach. Using London integral approximations the method is applied to benzene and naphthalene. Provided a particular form of the London Approximation is used, theoretical values in good agreement with experiment are obtained.     

稠苯芳杂环及其烷基质子化学位移的计算

本文提出了予测稠苯芳杂环及其烷基链上质子化学位移的计算方法。 将稠苯芳杂环化合物用凯库勒式表示,计算式为为需考虑的苯环内的乙烯基效应。σ_mi,ci为各苯环的环流效应。σ_1,Hc为各芳杂环的屏蔽效应,对杂环上质子它就是该单独芳杂环上相应质子的δ值,对苯环上质子要将它分解为各结构因素的效应,即:σ_1,He=(1/2)^d-1δ_x=y(或σ_z)+σ_c-c·σ_m,H. σ_x-y与σ_z为杂原子或其基团的屏蔽效应,σ_c=c为存在于芳杂环中的乙烯基的效应,σ_m,Hc为芳杂环的环流效应,d为对不同质子所考虑的键数。有取代基时需考虑取代基的效应。计算环上烷基质子的公式为:δ=σ_p,CH3+ασ_c,CH3+βσ_t,CH3+σ_l,G σ_l,G为稠苯芳杂环基的某级效应。     

1H NMR study of the hetero-association of flavin-mononucleotide with mutagenic dyes: ethidium bromide and proflavine

The hetero-association of the vitamin B₂ derivative, flavin-mononucleotide (FMN), with a mutagenic dye, ethidium bromide (EB) or proflavine (PF), has been studied by 1D and 2D 500?MHz ¹H NMR spectroscopy. The variations of proton chemical shifts of both the vitamin and dye as a function of concentration and temperature were analysed in terms of the structural and thermodynamical properties of the FMN–EB and FMN–PF complexes in solution. The structures of the complexes were also investigated by observed intermolecular ROE contacts and molecular mechanics calculations. The results show that the 1?:?1 hetero-association complexes in solution are more stable than the self-association complexes, which is consistent with formation of an intermolecular hydrogen-bond in the hetero-complexes of FMN–EB and FMN–PF. Hence it is possible that the toxicity of aromatic molecules such as EB and PF may be reduced in vitro by the presence of FMN, partly because of the known antimutagenic action of FMN and partly because it has been shown in this work that there is an effective intermolecular association between the mutagens and the vitamin.     

Selective detection of the proton NMR spectra of molecules containing rare spins at natural abundance in liquid crystalline samples

It is shown that the proton NMR spectra of molecules containing rare spins at natural abundance dissolved in a liquid crystalline solvent can be obtained free from the strong lines from the spectrum of the abundant isotopomer by the 2D HSQC NMR experiment. The technique can also give the individual chemical shifts of the rare spins, and, for a molecule containing another abundant nucleus, such as fluorine, the rare spin--(19)F total anisotropic couplings are also obtained. The usefulness of the technique is demonstrated for molecules containing (13)C as the rare spins.     

Esca. results versus other physical and chemical data

X-ray photoemission spectra yield quantities of very direct interest in physics and chemistry. In this paper the relations of these spectra to other data and concepts are discussed. Both initial-state and final-state properties may be studied: the former are treated first. Charge distributions in molecules alter the effective (Coulomb plus exchange) potential experienced by core electrons in molecular ground states, there by shifting their binding energies. The shifts can be calculated by $\text{ab}$$\text{initio}$ methods or more directly by using potential models based on intermediate-level molecular-orbital theories such as INDO. One version, the ground-state potential model (GPM) yields good predictions of core-level shifts among atoms in similar environments. Alternatively, the measured shifts may be used to derive charges on individual atoms in molecules. It is more difficult to derive charges in solids in this way, but a characteristic splitting in the more tightly-bound valence bands yields a direct measure of ionicity in simple binary compounds of the zinc-blende and rocksalt structures. Atomic orbital composition of molecular orbitals can be deduced from photoemission spectra. In solids such as diamond and graphite comparison of photoemission spectra with x-ray emission spectra yields the atomic-orbital composition of the valence bands. Turning to final-state properties, the spectra are dominated by relaxation effects. Again a simple approach—the relaxation potential model (RPM)—predicts core-level shifts well for cases in which the atomic environments are varied substantially. Among ammonia and the methylamines, for example, the N(ls) shifts are predicted correctly by RPM, while GPM reverses the order. For paramagnetic molecules RPM predicts electron charge transfer toward the positive hole but usually spin transfer away, in agreement with experiment. Extra-atomic relaxation in metals, a many-body effect, is manifest both as a contribution to the binding energy and as line-shape asymmetry. Delocalized valence electrons also show relaxation shifts that can be understood as polari zation of the electron gas toward the “Coulomb hole”. Auger lines show larger relaxation shifts. Comparison of core-level or Auger shifts in nonmetallic solids separately is questionable because there is no reference level, but intercomparison of the two is meaningful. Finally, core-level binding-energy trends in series of simple alcohols, etc., agree quantitatively with proton affinities and core-level shifts in other functional groups. This suggests extending the concept of Lewis basicity to include lone pairs of core electrons. Thus, core-level shifts measure the chemical reactivity—a quantity of great chemical importance that depends on both initial- and final-state properties—rather directly. Rela xation energies are shown to be the dominant cause of trends in the lowest ionization potentials of simple alcohols and amines.     

N-芳基取代甲基丙烯酰胺的1H-NMR分析

本文采用酰氯路线由甲基丙烯酰氯同苯胺、对甲苯胺、邻甲苯胺、对溴苯胺、对硝基苯胺、邻硝基苯胺、α-萘胺等反应制得了一系列N-芳基取代甲基丙烯酰胺(N-ArMA).给出了这些新单体的¹H-NMR谱图.研究了这些单体中苯环上不同取代基及同一取代基在不同取代位置上对质子的化学位移的影响.研究结果表明,不同的取代基由于不同的电子效应使苯环上质子峰发生位移,按向低场位移顺序是: -NO₂ >-Br >-H >-CH₃ 同样的取代基在邻位取代使苯环质子移向低场更显著.化学位移的实测值与经验公式计算值比较一致.     

NMR chemical shift study of the methanol—benzene-d6 system

The OH proton chemical shifts of methanol referred to the CH₃ proton in the methanol — benzene-d₆ system have been measured from room temperature up to the supercritical region. MD simulations have also been performed at the same composition as experiment at 575 K. The chemical shift results in the supercritical region are in good agreement with those of pure methanol at the same density within experimental errors. The results of NMR and MD simulation suggest that the hydrogen-bonded structure of methanol is practically not affected by addition of benzene molecules.     

Molecular Spectral Analysis and Ab Initio Calculations of bis(3-aminopyridinium) Tetrachlorocuprate (II), 3-Ammoniumpyridinium Tetrachlorocuprate (II), and bis(3-aminopyridinium) Hexachlorodicuprate (II)

The aromatic character, distortion, and stabilization as a result of single and double protonation of 3-aminopyridine like three different complex salts were studied by infrared-, ultraviolet spectral analysis, proton nuclear magnetic resonance, and quantum chemical ab initio calculations. Linear-dichroic infrared spectroscopy was applied for identification of the infrared bands. The correlation structure-spectroscopic properties of the model systems are determined: bis(3-aminopyridinium) tetrachlorocuprate (II) salt, where the ring nitrogen atom participates in protonation; 3-ammoniumpyridinium tetrachlorocuprate (II) salt, where both nitrogen atoms are protonated; and a complex with copper (II) bis(3-aminopyridinium) hexachlorodicuprate (II), where the metal ion is coordinated through amino group.