Mesomere Kationen. IV—13C-NMR-Spektren von Uronium-, Thiouronium- und Guanidiniumsalzen,sowie einigen Guanidinen

The ¹³C chemical shifts of uronium-, thiouronium- and guanidinum salts are reported, and compared with the shifts of the corresponding uncharged ureas, thioureas and guanidines. The shifts for the aromatic carbon atoms in these compounds compared with related phenols and thiophenols give insight into the charge distribution in the systems.     

Theoretical investigation on 1H and 13C NMR chemical shifts of small alkanes and chloroalkanes

Using density functional theory (DFT) with the B3LYP, PBE, and PBE0 exchange-correlation functionals as well as the Moller-Plesset second-order perturbation theory (MP2) combined with a series of rather extended basis sets, 1H and 13C chemical shifts of small alkanes and chloroalkanes (with different numbers of chlorine atoms on specific positions) have been simulated and compared to experimental data. For the 1H chemical shifts, theory tends to reproduce experiment within the limits of the experimental errors. In the case of 13C chemical shift, the differences between theory and experiment increase monotonically with the number of chlorine atoms and exhibit a deviation from additivity. This behavior is related to the saturation of the experimental 13C chemical shifts with the number of chlorine atoms, whereas the evolution is mostly linear at both DFT and MP2 levels of approximation. This difference has been traced back to the relativistic spin-orbit coupling effects, which are exalted as a result of the enhancement of the s character of the C atom when increasing the number of linked Cl atoms. Thus, it was demonstrated that not only electron correlation but also relativistic effects have to be considered for estimating the 13C chemical shifts when several Cl atoms are directly attached to the C atom. Linear (theory/experiment) regressions have then been performed for the different types of C atoms, i.e., bearing one, two, and three Cl atoms, with excellent correlation coefficients. The linear correlation relationships so obtained can then serve to predict and facilitate the interpretation of the nuclear magnetic resonance spectra of more complex compounds. Furthermore, by investigating the basis set effects, the correlation between the chemical shifts calculated using the 6-311 + G(2d,p) basis set and the more extended 6-311 + G(2df,p) and aug-cc-pvtz basis sets is excellent, demonstrating that the choice of the 6-311 + G(2d,p) basis set for calculating the 1H and 13C chemical shifts is relevant.     

Carbon-13 NMR and CNDO/2 studies of phenoxachalcogenins

The ¹³C chemical shifts of the carbon atoms in dibenzodioxin, phenoxathiin, phenoxaselenin and phenoxatellurin were determined in CDCl₃ solutions and assigned. The total (σ and π) charge densitites on the carbon atoms calculated by the CNDO/2 method without consideration of d-orbitals correlated well with the experimentally determined shifts. Rather good agreement was also found between experimental shifts and shifts calculated from ¹³C data for phenyl methyl chalcogenides on the assumption that a phenoxachalcogenin molecule can be assembled from C₆H₅O and C₆H₅X groups. Only the shifts of the carbon atoms bonded to the heavier chalcogen atoms show an upfield trend in the sequence O, S, Se, Te. All other shifts exhibit a downfield trend. These trends are rationalized in terms of the electronegativities, abilities to participate in π-interactions, and anisotropy effects of the chalcogen atoms.     

17O NMR spectra of some organotin(IV) compounds containing O,C,O-chelating ligands

The 17O chemical shifts of substituted benzyl ethers and a set of organotin(IV) derivatives containing O,C,O-chelating ligands were studied. Measured 17O chemical shifts were correlated with the additivity substituent increments for carbon atoms in the alkyl groups, and intramolecular Sn-O distance was obtained by X-ray diffraction techniques in the solid state.     

Application of 15N spectroscopy and dynamic NMR to the study of ureas,thioureas and their lewis acid adducts

Rotational barriers and ¹⁵N chemical shifts have been measured in a number of ureas and thioureas. As anticipated on the basis of the ¹⁵N shifts, several previously unobserved rotational barriers could be detected by using lanthanide reagents or a high field spectrometer. Nearly constant effects on both the rotational activation energy and the ¹⁵N shift are produced on going from ureas to the corresponding thioureas, and correlations are found between the ΔG? and δ¹⁵N values. The results are discussed in terms of lone pair delocalization, and anomalies with respect to the general behaviour are tentatively explained in the light of the effect of steric torsion in crowded structures on the ¹⁵N shifts and rotation barriers.     

Filamentous phage studied by magic-angle spinning NMR: resonance assignment and secondary structure of the coat protein in Pf1

Assignments are presented for resonances in the magic-angle spinning solid-state NMR spectra of the major coat protein subunit of the filamentous bacteriophage Pf1. NMR spectra were collected on uniformly 13C and 15N isotopically enriched, polyethylene glycol precipitated samples of fully infectious and hydrated phage. Site-specific assignments were achieved for 231 of the 251 labeled atoms (92%) of the 46-residue-long coat protein, including 136 of the 138 backbone atoms, by means of two- and three-dimensional 15N and 13C correlation experiments. A single chemical shift was observed for the vast majority of atoms, suggesting a single conformation for the 7300 subunits in the 36 MDa virion in its high-temperature form. On the other hand, multiple chemical shifts were observed for the Calpha, Cbeta, and Cgamma atoms of T5 in the helix terminus and the Calpha and Cbeta atoms of M42 in the DNA interaction domain. The chemical shifts of the backbone atoms indicate that the coat protein conformation involves a 40-residue continuous alpha-helix extending from residue 6 to the C-terminus.     

H and C NMR spectroscopy of 9-acridinones

The ¹H and ¹³C NMR spectra of 9-acridinone and its five derivatives dissolved in CDCl₃, CD₃CN and DMSO-d₆ were measured in order to reveal the influence of the constitution of the compounds and features of the solvents on chemical shifts and ¹H-¹H coupling constants. Experimental data were compared with theoretically predicted chemical shifts, on the GIAO/DFT level of theory, for DFT (B3LYP)/6-31G^∗∗ optimized geometries of molecules—also for four other 9-acridinones. This comparison helped to ascribe resonance signals in the spectra to relevant atoms and enabled revelation of relations between chemical shifts and physicochemical features of the compounds. It was found that experimentally or theoretically determined ¹H and ¹³C chemical shifts of selected atoms correlate with theoretically predicted values of dipole moments of the molecules, as well as bond lengths, atomic partial charges and energies of HOMO.     

13C NMR study on the methoxy carbon chemical shifts in chloro-substituted anisoles and guaiacols

The ¹³C NMR chemical shifts of methoxy carbons in chlorinated anisoles and guaiacols have been measured for acetone-d₆ solutions. Multiple linear regression analysis, and also ‘simple sum rule’ calculations, have been used to estimate the effects of the chlorine atoms (the position and degree of substitution) on the chemical shifts. The most important effects have shown to be due to the chlorine atoms adjacent to the methoxy and hydroxy substituents. For chlorinated guaiacols, the greatest effect is due to the chlorine atom adjacent to the methoxy group. For chlorinated anisoles, the substituents adjacent to the methoxy group (2,6-disubstitution) cause large effects. For both groups of compounds, the chemical shifts are also greatly influenced by the number of chlorine substituents. Using the three most important independent variables, the average differences between the observed and calculated chemical shifts are ca 0.2 ppm for anisoles and 0.1 ppm for guaiacols. For chloroguaiacols, the corresponding difference was only 0.1 ppm when calculations were performed using single substituent effects.     

15N NMR chemical shifts of ring substituted benzonitriles

15N chemical shifts in an extensive series of para (15) and meta (15) as well as ortho (8) substituted benzonitriles, X-C6H4-CN, were measured in deuteriochloroform solutions, using three different methods of referencing. The standard error of the average chemical shift was less than 0.03 ppm in most cases. The results are discussed for both empirical correlations with substituent parameters and quantum chemical calculations. The 15N chemical shifts calculated at the GIAO/B3LYP/6-31 + G*//B3LYP/6-31 + G* level reproduce the experimental values well, and include nitrogen atoms in the substituent groups (range of 300 ppm with slope 0.98 and R = 0.998, n = 43). The 15N shifts in hydroxybenzonitriles are affected by interaction with the OH group. Therefore, these derivatives are excluded from the correlation analysis. The resultant 15N chemical shift correlates well with substituent constants, both in the simple Hammett or DSP relationships and the 13C substituent-induced chemical shifts of the CN carbon.     

First-principles calculation of 17O and 25Mg NMR shieldings in MgO at finite temperature: rovibrational effect in solids

The temperature dependence of (17)O and (25)Mg NMR chemical shifts in solid MgO have been calculated using a first-principles approach. Density functional theory, pseudopotentials, a plane-wave basis set, and periodic boundary conditions were used both to describe the motion of the nuclei and to compute the NMR chemical shifts. The chemical shifts were obtained using the gauge-including projector augmented wave method. In a crystalline solid, the temperature dependence is due to both (i) the variation of the averaged equilibrium structure and (ii) the fluctuation of the atoms around this structure. In MgO, the equilibrium structure at each temperature is uniquely defined by the cubic lattice parameters, which we take from experiment. We evaluate the effect of the fluctuations within a quasiharmonic approximation. In particular, the dynamical matrix, defining the harmonic Hamiltonian, has been computed for each equilibrium volume. This harmonic Hamiltonian was used to generate nuclear configurations that obey quantum statistical mechanics. The chemical shifts were averaged over these nuclear configurations. The results reproduce the previously published experimental NMR data measured on MgO between room temperature and 1000 degrees C. It is shown that the chemical shift behavior with temperature cannot be explained by thermal expansion alone. Vibrational corrections due to the fluctuations of atoms around their equilibrium position are crucial to reproduce the experimental results.     

13C NMR studies of hydrocarbon guests in synthetic structure H gas hydrates: experiment and computation

(13)C NMR chemical shifts were measured for pure (neat) liquids and synthetic binary hydrate samples (with methane help gas) for 2-methylbutane, 2,2-dimethylbutane, 2,3-dimethylbutane, 2-methylpentane, 3-methylpentane, methylcyclopentane, and methylcyclohexane and ternary structure H (sH) clathrate hydrates of n-pentane and n-hexane with methane and 2,2-dimethylbutane, all of which form sH hydrates. The (13)C chemical shifts of the guest atoms in the hydrate are different from those in the free form, with some carbon atoms shifting specifically upfield. Such changes can be attributed to conformational changes upon fitting the large guest molecules in hydrate cages and/or interactions between the guests and the water molecules of the hydrate cages. In addition, powder X-ray diffraction measurements revealed that for the hexagonal unit cell, the lattice parameter along the a-axis changes with guest hydrate former molecule size and shape (in the range of 0.1 ?) but a much smaller change in the c-axis (in the range of 0.01 ?) is observed. The (13)C NMR chemical shifts for the pure hydrocarbons and all conformers were calculated using the gauge invariant atomic orbital method at the MP2/6-311+G(2d,p) level of theory to quantify the variation of the chemical shifts with the dihedral angles of the guest molecules. Calculated and measured chemical shifts are compared to determine the relative contribution of changes in the conformation and guest-water interactions to the change in chemical shift of the guest upon clathrate hydrate formation. Understanding factors that affect experimental chemical shifts for the enclathrated hydrocarbons will help in assigning spectra for complex hydrates recovered from natural sites.     

(s)-多沙唑嗪的核磁共振理论和实验研究

报道了(s)-多沙唑嗪的1H、13C、DEPT、1H-1H COSY等的NMR波谱数据, 并对1H、13C NMR信号进行了指认. (s)-多沙唑嗪分子中含有9个季碳原子, 其中绝大部分通过常规实验的方法难以指认. 应用量子化学规范不变原子轨道(GIAO)的Becke-3-Lee-Yang-Parr(B3LYP)和Hartree-Fock(HF)方法, 分别在6-21G基组下计算了标题化合物的13C NMR化学位移值. 计算结果表明, 理论计算数据与实验结果吻合较好, 可以帮助对(s)-多沙唑嗪季碳原子NMR位移信号进行指认.     

Estimation and prediction of 13C NMR chemical shifts of carbon atoms in both alcohols and thiols

Quantitative structure-spectrum relationship calculations of ¹³C NMR chemical shifts of both 302 carbon atoms in 56 alcohols and 62 carbon atoms in 15 thiols are described using several parameters: the atomic ionicity index (INI), the polarizability effect index (PEI), and stereoscopic effect parameters (?) of the compounds. The ¹³C NMR chemical shifts for these compounds of both alcohols and thiols can be estimated through the multiple linear regression (MLR). A MLR model was built with variable screening by the stepwise multiple regression and examined by validation on its stability. The correlation coefficient of the established model as well as the leave-one-out cross-validation was 0.9724 and 0.9716 respectively. The results obviously indicate that INI and ? are linearly related with ¹³C NMR chemical shifts, which provides a new method for calculating ¹³C NMR chemical shifts in the compounds of both alcohols and thiols.     

A fast and easy computational method to calculate the (13)C NMR chemical shift of organic species adsorbed on the zeolite surface

Calculations of the 13C NMR chemical shifts for the methoxy and ethoxy groups adsorbed on Y and ZSM-5 zeolites were computed at GIAO/B3LYP/6-31+G*//MM+ level of theory, using a cluster representing a real part of the zeolites. The Y zeolite was represented by a cluster with 168 atoms, while ZSM-5 was represented by a cluster with 144 atoms. The calculated chemical shifts agreed well with reported experimental values, showing that the difference in chemical shifts is associated with differences in the geometry of the alkoxides on the two zeolites.     

13C and proton NMR spectra of 2(1H)pyrazinones

¹³C and proton NMR spectra data are given for eleven 2(1H)pyraziones. Assignments of chemical shifts were made by methods which included: deuterium exchange with certain protons of 3-alkyl substituents; change of chemical shifts of certain carbon atoms with change in pH; the use of long-range coupling constants for ¹³C to protons; and various correlations among assigned spectra.     

A solid state 13C NMR, crystallographic, and quantum chemical investigation of chemical shifts and hydrogen bonding in histidine dipeptides

We report the first solid-state NMR, crystallographic, and quantum chemical investigation of the origins of the 13C NMR chemical shifts of the imidazole group in histidine-containing dipeptides. The chemical shift ranges for Cgamma and Cdelta2 seen in eight crystalline dipeptides were very large (12.7-13.8 ppm); the shifts were highly correlated (R2= 0.90) and were dominated by ring tautomer effects and intermolecular interactions. A similar correlation was found in proteins, but only for buried residues. The imidazole 13C NMR chemical shifts were predicted with an overall rms error of 1.6-1.9 ppm over a 26 ppm range, by using quantum chemical methods. Incorporation of hydrogen bond partner molecules was found to be essential in order to reproduce the chemical shifts seen experimentally. Using AIM (atoms in molecules) theory we found that essentially all interactions were of a closed shell nature and the hydrogen bond critical point properties were highly correlated with the N...H...O (average R2= 0.93) and Nepsilon2...H...N (average R2= 0.98) hydrogen bond lengths. For Cepsilon1, the 13C chemical shifts were also highly correlated with each of these properties (at the Nepsilon2 site), indicating the dominance of intermolecular interactions for Cepsilon1. These results open up the way to analyzing 13C NMR chemical shifts, tautomer states (from Cdelta2, Cepsilon1 shifts), and hydrogen bond properties (from Cepsilon1 shifts) of histidine residue in proteins and should be applicable to imidazole-containing drug molecules bound to proteins, as well.     

15N NMR chemical shifts for the identification of dipyrrolic structures

A variety of dipyrromethanes and dipyrromethenes have been prepared, and their 15N NMR chemical shifts have been measured by two-dimensional correlation to 1H NMR signals. The nitrogen atoms in five examples of dipyrromethanes consistently exhibit chemical shifts around -231 ppm, relative to nitromethane. Seven examples of hydrobromide salts of meso-unsubstituted dipyrromethenes consistently display 15N chemical shifts around -210 ppm, while their corresponding zinc(II) complexes exhibit chemical shifts around -170 ppm. The presence of electron-withdrawing substituents on one of the pyrrolic rings of dipyrromethenes affects the chemical shifts of both of the nitrogen nuclei in the molecule. Boron difluoride complexes of meso-unsubstituted dipyrromethenes display 15N chemical shifts around -190 ppm. Two examples of free-base dipyrromethenes bearing substituents at the meso-position exhibit 15N chemical shifts at approximately -156 ppm, and for the zinc complexes of these compounds at -162 ppm. One-bond nitrogen-hydrogen coupling constants, when measurable, were consistently in the range of -96 Hz. Since the measured 15N chemical shifts have such a high regularity correlated to structure, they can be used as diagnostic indications for identifying the structure of dipyrrolic compounds.     

1H and 13C NMR spectra, structure and physicochemical features of phenyl acridine-9-carboxylates and 10-methyl-9-(phenoxycarbonyl)acridinium trifluoromethanesulphonates--alkyl substituted in the phenyl fragment

The 1H and 13C NMR spectra of twelve phenyl acridine-9-carboxylates--alkyl-substituted in the phenyl fragment--and their 10-methyl-9-(phenoxycarbonyl)acridinium salts dissolved in CD3CN, CD3OD, CDCl3 and DMSO-d6 were recorded in order to examine the influence of the structure of these compounds and the properties of the solvents on chemical shifts and 1H-(1)H coupling constants. Experimental data were compared with 1H and 13C chemical shifts predicted at the GIAO/DFT level of theory for DFT(B3LYP)/6-31G** optimised geometries of molecules, as well as with values of 1H chemical shifts and 1H-(1)H coupling constants, estimated using ACD/HNMR database software to ensure that the assignment was correct. To investigate the relations between chemical shifts and selected structural or physicochemical characteristics of the target compounds, the values of several of these parameters were determined at the DFT or HF levels of theory. The HOMO and LUMO energies obtained at the HF level yielded the ionisation potentials and electron affinities of molecules. The DFT method provided atomic partial charges, dipole moments, LCAO coefficients of pz LUMO of selected C atoms, and angles reflecting characteristic structural features of the compounds. It was found that the experimentally determined 1H and 13C chemical shifts of certain atoms relate to the predicted dipole moments, the angles between the acridine and phenyl moieties, and the LCAO coefficients of the pz LUMO of the C atoms believed to participate in the initial step of the oxidation of the target compounds. The spectral and physicochemical characteristics of the target compounds were investigated in the context of their chemiluminogenic ability.     

Carbon-13 NMR spectra of chromium pentacarbonyl carbenoid complexes

The carbon-13 NMR spectra of six carbenoid complexes of the type (OC)₅Cr-(CXX′) have been recorded. The carbenoid carbon atoms are all markedly deshielded (chemical shifts vs. TMS in the range ?271 to ?360). With only one minor inversion of uncertain significance, the chemical shifts correlate well with the expected ability of the X and X′ groups to engage in dative π bonding to the carbenoid carbon atom. The longitudinal relaxation times for both carbenoid and carbonyl carbon atoms in (OC)₅Cr[C(CH₃)(OC₂H₅)] are 1 – 2 sec. The chemical shift difference for carbonyl carbon atoms cis and trans to the carbenoid ligand is essentially invariant in the six compounds.     

An NMR, IR and theoretical investigation of (1)H chemical shifts and hydrogen bonding in phenols

The change in (1)H NMR chemical shifts upon hydrogen bonding was investigated using both experimental and theoretical methods. The (1)H NMR spectra of a number of phenols were recorded in CDCl(3) and DMSO solvents. For phenol, 2- and 4-cyanophenol and 2-nitrophenol the OH chemical shifts were measured as a function of concentration in CDCl(3). The plots were all linear with concentration, the gradients varying from 0.940 (phenol) to 7.85 (4-cyanophenol) ppm/M because of competing inter- and intramolecular hydrogen bonding. Ab initio calculations of a model acetone/phenol system showed that the OH shielding was linear with the H...O=C distance (R) for R < 2.1 A with a shielding coefficient of - 7.8 ppm/A and proportional to cos(2)phi where phi is the H...O=C--C dihedral angle. Other geometrical parameters had little effect. It was also found that the nuclear shielding profile is unrelated to the hydrogen bonding energy profile. The dependence of the OH chemical shift on the pi density on the oxygen atom was determined as ca 40 ppm/pi electron. This factor is similar to that for NH but four times the value for sp(2) hybridized carbon atoms. The introduction of these effects into the CHARGE programme allowed the calculation of the (1)H chemical shifts of the compounds studied. The CHARGE calculations were compared with those from the ACD database and from GIAO calculations. The CHARGE calculations were more accurate than other calculations both when all the shifts were considered and also when the OH shifts were excluded. The calculations from the ACD and GIAO approaches were reasonable when the OH shifts were excluded but not as good when all the shifts were considered. The poor treatment of the OH shifts in the GIAO calculations is very likely due to the lack of explicit solvent effects in these calculations.