Proximity and the heteroatom effects on the carbon-13 chemical shifts of methyl-substituted phenols,anilines and thiophenols

Upfield substituent-induced ¹³C chemical shifts for aryl carbons of polymethyl substituted benzenes, phenols, anilines and thiophenols were investigated as a function of the proximity between substituents X and CH₃ (X = CH₃, NH₂, OH and SH). The results indicate that the induced shifts of the substituted aryl carbons are, in general, independent of the polar substituent but depend on the number of adjacent substituted aryl carbons. A ?2.0 ppm upfield shift was found for a substituted aryl carbon adjacent to one substituted aryl carbon and a ?3.8 ppm upfield shift for a substituted aryl carbon bound by two substituted aryl carbons. It is suggested that the near additivity of the upfield shifts is the result of changes in the bond order between the aromatic ring carbons in the region of the substituted aryl carbons due to distortion of the ring. The ¹³C chemical shifts of the methyl substituents for methyl substituted phenols, anilines and thiophenols were determined, and it was found that the values could be predicted from the additivity parameters reported for the analogous methylbenzenes plus an additional pair-interaction term associated with the through-space electronic influence of the heteroatom.     

13C NMR spectroscopy of core heme carbons as a simple tool to elucidate the coordination state of ferric high-spin heme proteins

Evidence is presented demonstrating that the magnitudes of the 13C chemical shifts originating from heme meso carbons provide a straightforward diagnostic tool to elucidate the coordination state of high-spin heme proteins and enzymes. Pentacoordinate high-spin heme centers exhibit 13C meso shifts centered at approximately 250 ppm, whereas their hexacoordinate counterparts exhibit 13C shifts centered at approximately -80 ppm. The relatively small spectral window (400 to -100 ppm) covering the meso-13C shifts, the relatively narrow lines of these resonances, and the availability of biosynthetic methods to prepare 13C-labeled heme (Rivera, M.; Walker, F. A. Anal. Biochem. 1995, 230, 295-302) make this approach practical. The theoretical basis for the distinct chemical shifts observed for meso carbons from hexacoordinate high-spin hemes relative to their pentacoordinate counterparts are now well understood (Cheng, R.-J.; Chen, P. Y.; Lovell, T.; Liu, T.; Noodleman, L.; Case, D. A. J. Am. Chem. Soc. 2003, 125, 6774-6783), which indicates that the magnitude of the meso-carbon chemical shifts can be used as a simple and reliable diagnostic tool for determining the coordination state of the heme active sites, independent of the nature of the proximal ligand. Proof of the principle for the 13C NMR spectroscopic approach is demonstrated using hexa- and pentacoordinate myoglobin. Subsequently, 13C NMR spectroscopy has been used to unambiguously determine that a recently discovered heme protein from Shigella dysenteriae (ShuT) is pentacoordinate.     

Correlation of Electronic Effects in N-Alkylnicotinamides with NMR Chemical Shifts and Hydride Transfer Reactivity

The (13)C and (15)N NMR chemical shifts for ring atoms of a series of N-alkylnicotinamides are shown to be correlated with their reduction potentials and reactivities toward NaBH(3)CN. The nicotinamide compounds include N-ethyl-N-benzyl-N-[p-(trifluoromethyl)benzyl]-, N-(p-cyanobenzyl)-, N-(carbomethoxymethyl)-, and N-(cyanomethyl)nicotinamides. The values of delta()13(C) for all the ring carbons increase with increasing electron-withdrawing power of the N-alkyl substituent. The value for C-4 increases the most, a range of 2.4 ppm in this series, whereas those for other atoms increase on the order of 1 ppm. The value of delta()15(N) for N-1 decreases with increasing electron-withdrawing power over a range of 20 ppm. The NMR data indicate that inductive electron withdrawal by N-alkyl substituents polarizes the pi-electron system to decrease electron density on ring carbons and increase electron density on the ring nitrogen. The values of log k (second order) for reduction of these compounds by NaBH(3)CN are proportional to the values of delta()13(C) for C-4 and inversely proportional to delta()15(N) for N-1. The reduction potentials are proportional to delta()13(C). The substituent effects are qualitatively similar to the substrate-induced electrostatic effects on the nicotinamide ring of NAD(+) at the active site of UDP-galactose 4-epimerase (Burke, J. R.; Frey, P. A. Biochemistry 1993, 32, 13220-13230). However, they differ quantitatively in that the upfield perturbation at N-1 is smaller in the enzyme and the signal for C-6 is also shifted upfield. The substrate-induced enzymatic perturbation of electron density at C-4 of NAD(+) quantitatively accounts for its increase in reactivity at the active site, but the perturbation at N-1 is less closely correlated with reactivity.     

Carbon-13 NMR spectra of some 4-substituted phenacyl bromides

The ¹³C NMR signals for some 4- substituted phenacyl bromides were assigned. The experimental chemical shifts of the aromatic ring carbons are in close agreement with those calculated using substituent chemical shifts. Both the carbonyl and the α-methylene carbons exhibit upfield shifts compared with those of the corresponding 4-substituted acetophenones.     

Solution and solid state NMR studies of some selenium analogues of auranofin (an anti-arthritic gold drug)

Three mixed ligand complexes of gold(I) with phosphines and selenones, [Et₃PAuSe=C<]Br as analogues of auranofin (Et₃PAuSR) have been prepared and characterized by elemental analysis, IR and NMR methods. A decrease in the IR frequency of the C=Se mode of selenones upon complexation is indicative of selenone binding to gold(I) via a selenone group. An upfield shift in ¹³C NMR for the C=Se resonance of the selenones and downfield shifts in ³¹P NMR for the R₃P moiety are consistent with the selenium coordination to gold(I). ¹³C solid state NMR shows the chemical shift difference between free and bound selenone to gold(I) for ImSe and DiazSe to be ca 10 and 17?ppm respectively. Large ⁷⁷Se NMR chemical shifts (55?ppm) upon complexation in the solid state for [Et₃PAuDiazSe]Br compared to [Et₃PAuImSe]Br (10?ppm) indicates the former to be more stable and the Au–Se bond to be stronger than in the latter complex.     

Trends in chemical shift dispersion in fullerene derivatives. Local strain affects the magnetic environment of distant fullerene carbons

13C NMR chemical shift assignments for 1,2-C60H2 (1) and a series of 13C-labeled fullerene derivatives with three-, four-, and five-membered annulated rings (2-4) were assigned using 2D INADEQUATE spectroscopy and examined for trends that correspond to the changes in strain in the fullerene cage. Chemical shifts of equivalent carbons from 1-4 show that eight carbons trend downfield (carbons 5, 7, 8, 9, 11, 15, 16, 17) and the remaining six carbons (4, 6, 10, 12, 13, 14) trend upfield with increasing ring size. While the average chemical shift is nearly constant, the dispersion is greatest when the local strain is the least, in 1,2-C60H2 (1). 13C chemical shifts are not well correlated with trends in ring size, with strain as measured by the pyramidalization angle of nearby carbons, or with the geometry of the fullerene cage. We interpret the results as evidence that subtle geometrical changes lead to modulation of the strength of ring currents near the site of addition and, in turn, the magnetic field generated by these ring currents affects the chemical shift of carbons on the far side of the fullerene core. These results highlight ring currents as being critically important to the determination of 13C chemical shifts in fullerene derivatives.     

A density functional study of the 13C NMR chemical shifts in functionalized single-walled carbon nanotubes

The 13C NMR chemical shifts for functionalized (7,0), (8,0), (9,0), and (10,0) single-walled carbon nanotubes (SWNTs) have been studied computationally using gauge-including projector-augmented plane-wave (GIPAW) density functional theory (DFT). The functional groups NH, NCH3, NCH2OH, and CH2NHCH2 have been considered, and different sites where covalent addition or substitution may occur have been examined. The shifts of the carbons directly attached to the group are sensitive to the bond which has been functionalized and may, therefore, be used to identify whether the group has reacted with a parallel or a diagonal C-C bond. The addition of NH to a parallel bond renders the functionalized carbons formally sp3-hybridized, yielding shifts of around 44 ppm, independent of the SWNT radius. Reaction with a diagonal bond retains the formal sp2 hybridization of the substituted carbons, and their shifts are slightly lower or higher than those of the unsubstituted carbon atoms. The calculated 1H NMR shifts of protons in the functional groups are also dependent upon the SWNT-group interaction. Upon decreasing the degree of functionalization for the systems where the group is added to a parallel bond, the average chemical shift of the unfunctionalized carbons approaches that of the pristine tube. At the same time, the shifts of the functionalized carbons remain independent upon the degree of functionalization. For the SWNTs where N-R attaches to a parallel bond, the average shift of the sp2 carbons was found to be insensitive to the substituent R. Moreover, the shifts of the functionalized sp3 carbons, as well as of the carbons within the group itself, are independent of the SWNT radius. The results indicate that a wealth of knowledge may be obtained from the 13C NMR of functionalized SWNTs.     

Hydroacridines: Part 33. An experimental and DFT study of the 13C NMR chemical shifts of the nitrosamines derived from the six stereoisomers of tetradecahydroacridine

The paper presents the experimental and DFT-calculated values of the ¹³C NMR chemical shifts of the six stereoisomers of tetradecahydroacridine and of the corresponding nitrosamines. Performing the DFT calculations using several combinations of functional and basis sets, it was found that the best experimental-calculated agreement was obtained for OPBE/6–311++G (dp) method. Considering the effect of N-nitrosation upon the ¹³C NMR chemical shifts of the C-α carbons of secondary amines, it was found that if following nitrosation both C-α carbons are shifted upfield or both are shifted downfield, then the resulted nitrosamine will have a sterically strained ─N═O group. If, however, one of the C-α is shifted upfield and the other is shifted downfield, then the ─N═O group will be strain-free or weakly strained. Our calculations predict strain energies of about 10–15 kcal mol⁻¹ in the first case and ≈0–6 kcal mol⁻¹ in the latter.     

Carbon-13 NMR studies of a series of benzylphenols

Carbon-13 NMR spectra of a series of benzylphenols and their O-alkylated derivatives were recorded to find the substituent effects of the benzyl, hydroxybenzyl and alkoxybenzyl groups on the ¹³C chemical shifts. It was found that the methylene bridge carbons show signal shifts mainly due to the mesomeric effects of the OH and OCH₃ substituents, and that in the case of ortho-substituted benzyl compounds, the methylene carbon signals exhibit upfield shifts due to both mesomeric and steric effects.     

1H and 13C NMR spectra of Strychnos alkaloids: Selected NMR updates

The PBE0/pcSseg-2//pcseg-2 calculations of ¹H and ¹³C NMR chemical shifts were performed for a classical series of 12 Strychnos alkaloids (except for the earlier studied parent strychnine), namely akuammicine, isostrychnine, rosibiline, tsilanine, spermostrychnine, diaboline, cyclostrychnine, henningsamide, strychnosilidine, strychnobrasiline, holstiine, and icajine. It was found that the calculated ¹H and ¹³C NMR chemical shifts show markedly good correlations with available experimental data, as characterized by a mean absolute error of 0.22 ppm for the range of 8 ppm for protons and 1.97 ppm for the range of 180 ppm for carbons. Complementarily, the present results provide essential NMR update and fill a gap in the NMR data of this distinguished group of vitally important natural products.     

Interaction of Cd and Zn with biologically important ligands characterized using solid-state NMR and ab initio calculations

For the first time, coordination geometry and structure of metal binding sites in biologically relevant systems are studied using chemical shift parameters obtained from solid-state NMR experiments and quantum chemical calculations. It is also the first extensive report looking at metal-imidazole interaction in the solid state. The principal values of the (113)Cd chemical shift anisotropy (CSA) tensor in crystalline cadmium histidinate and two different cadmium formates (hydrate and anhydrate) were experimentally measured to understand the effect of coordination number and geometry on (113)Cd CSA. Further, (13)C and (15)N chemical shifts have also been experimentally determined to examine the influence of cadmium on the chemical shifts of (15)N and (13)C nuclei present near the metal site in the cadmium-histidine complex. These values were then compared with the chemical shift values obtained from the isostructural bis(histidinato)zinc(II) complex as well as from the unbound histidine. The results show that the isotropic chemical shift values of the carboxyl carbons shift downfield and those of amino and imidazolic nitrogens shift upfield in the metal (Zn,Cd)-histidine complexes relative to the values of the unbound histidine sample. These shifts are in correspondence with the anticipated values based on the crystal structure. Ab initio calculations on the cadmium histidinate molecule show good agreement with the (113)Cd CSA tensors determined from solid-state NMR experiments on powder samples. (15)N chemical shifts for other model complexes, namely, zinc glycinate and zinc hexaimidazole chloride, are also considered to comprehend the effect of zinc binding on (15)N chemical shifts.     

Complete 1H, 13C and 15N NMR assignment of tirapazamine and related 1,2,4-benzotriazine N-oxides

1H, 13C and 15N NMR measurements (1D and 2D including 1H--15N gs-HMBC) have been carried out on 3-amino-1, 2,4-benzotriazine and a series of N-oxides and complete assignments established. N-Oxidation at any position resulted in large upfield shifts of the corresponding N-1 and N-2 resonances and downfield shifts for N-4 with the exception of the 3-amino-1,2,4-benzotriazine 1-oxide in which a small upfield shift of N-4 was observed. Density functional GIAO calculations of the 15N and 13C chemical shifts [B3LYP/6-31G(d)//B3LYP/6-311+G(2d,p)] gave good agreement with experimental values confirming the assignments. The combination of 13C and 15N NMR provides an unambiguous method for assigning the 1H and 13C resonances of N-oxides of 1,2,4-benzotriazines.     

含手性膦配体的钯（Ⅱ）配合物的二维核磁共振研究

利用一维和二维ＮＭＲ技术，对含有手性膦配体甲基－３脱氧－３（二苯膦基）－４，６－氧－苄叉基－α－Ｄ一吡喃阿卓糖苷（３－ＭＢＰＡ）和甲基－２－脱氧－２－（二苯膦基）－４，６－氧－苄叉基－α－Ｄ－吡喃阿卓糖苷（２－ＭＢＰＡ）的钯配合物ｔｒａｎｓ—［Ｐｄ（３－ＭＢＰＡＨ）２ＣＩ２」（１），ｔｒａｎｓ－［Ｐｄ（２－ＭＢＰＡＨ）２ＣＩ２］（２）和ｃｉｓ－［Ｐｄ（３－ＭＢＰＡ）２］（３），ｃｉｓ－［Ｐｄ（２－ＭＢＰＡ）２］（４）进行~１Ｈ和~（１３）Ｃ ＮＭＲ谱分析，归属了全部的~１Ｈ和~（１３）Ｃ ＮＭＲ谱线，并根据磷的化学位移及Ｒａｍａｎ谱确定化合物（３）和（４）是顺式构型，对实验中的一些现象也做了简单讨论。     

Synthesis and Spectroscopic Characterization of Silver(I) Complexes of Selenones

Silver(I) complexes of selenones, [LAgNO₃] and [AgL₂]NO₃ (where L is imidazolidine-2-selenone or diazinane-2-selenone and their derivatives) have been prepared and characterized by elemental analysis, IR and NMR (¹H, ¹³C and ¹⁰⁷Ag) spectroscopy. An upfield shift in the C=Se resonance of selenones in ¹³C NMR and a downfield shift in N-H resonance in ¹H NMR are consistent with selenium coordination to silver(I). In ¹⁰⁷Ag NMR, the AgNO₃signal is deshielded by 450-650 ppm on coordination to selenones. Greater upfield shifts in ¹³C NMR were observed for [LAgNO₃] compared to [AgL₂]NO₃complexes, whereas the opposite trend was observed for ¹H and¹⁰⁷Ag NMR chemical shifts.     

The 1H and 13C NMR chemical shifts of Strychnos alkaloids revisited at the DFT level

The density functional theory calculation of ¹H and ¹³C NMR chemical shifts in a series of ten 10 classically known Strychnos alkaloids with a strychnine skeleton was performed at the PBE0/pcSseg-2//pcseg-2 level. It was found that calculated ¹H and ¹³C NMR chemical shifts provided a markedly good correlation with experiment characterized by a mean absolute error of 0.08 ppm in the range of 7 ppm for protons and 1.67 ppm in the range of 150 ppm for carbons, so that a mean absolute percentage error was as small as ~1% in both cases.     

Computational studies of 13C NMR chemical shifts of saccharides

The (13)C NMR chemical shifts for alpha-D-lyxofuranose, alpha-D-lyxopyranose (1)C(4), alpha-D-lyxopyranose (4)C(1), alpha-D-glucopyranose (4)C(1), and alpha-D-glucofuranose have been studied at ab initio and density-functional theory levels using TZVP quality basis set. The methods were tested by calculating the nuclear magnetic shieldings for tetramethylsilane (TMS) at different levels of theory using large basis sets. Test calculations on the monosaccharides showed B3LYP(TZVP) and BP86(TZVP) to be cost-efficient levels of theory for calculation of NMR chemical shifts of carbohydrates. The accuracy of the molecular structures and chemical shifts calculated at the B3LYP(TZVP) level is comparable to those obtained at the MP2(TZVP) level. Solvent effects were considered by surrounding the saccharides by water molecules and also by employing a continuum solvent model. None of the applied methods to consider solvent effects was successful. The B3LYP(TZVP) and MP2(TZVP)(13)C NMR chemical shift calculations yielded without solvent and rovibrational corrections an average deviation of 5.4 ppm and 5.0 ppm between calculated and measured shifts. A closer agreement between calculated and measured chemical shifts can be obtained by using a reference compound that is structurally reminiscent of saccharides such as neat methanol. An accurate shielding reference for carbohydrates can be constructed by adding an empirical constant shift to the calculated chemical shifts, deduced from comparisons of B3LYP(TZVP) or BP86(TZVP) and measured chemical shifts of monosaccharides. The systematic deviation of about 3 ppm for O(1)H chemical shifts can be designed to hydrogen bonding, whereas solvent effects on the (1)H NMR chemical shifts of C(1)H were found to be small. At the B3LYP(TZVP) level, the barrier for the torsional motion of the hydroxyl group at C(6) in alpha-D-glucofuranose was calculated to 7.5 kcal mol(-1). The torsional displacement was found to introduce large changes of up to 10 ppm to the (13)C NMR chemical shifts yielding uncertainties of about +/-2 ppm in the chemical shifts.     

NMR investigation of the electrostatic effect in binding of a neuropeptide, achatin-I, to phosphatidylcholine bilayers

Achatin-I (Gly1-d-Phe2-Ala3-Asp4), known as a neuropeptide containing a d-amino acid, binds to the surface of a zwitterionic phosphatidylcholine (PC) membrane only when the peptide N-terminal amino group is in the ionized state, NH3+ (Kimura, T.; Okamura, E.; Matubayasi, N.; Asami, K.; Nakahara, M. Biophys. J. 2004, 87, 375-385). To gain mechanistic insights into how the binding equilibrium is delicately controlled by the ionization state of the N-terminal amino group, peptide-lipid binding interactions are investigated by selectively enriched 15N (at the N-terminus) and natural-abundance 13C NMR spectroscopy. Upon binding to the PC membrane, the 15N NMR of the N-terminal NH3(+) shifts upfield. This observation supports a mechanism that the role of the N-terminal NH3(+) in stabilizing the binding state is through electrostatic attraction with a headgroup negative charge, i.e., PO4(-). Interestingly, when the side chain beta-carboxyl group in Asp4 is deionized at acidic pH, the 15N signal of the N-terminal NH3(+) exhibits no significant chemical-shift change upon membrane binding of achatin-I. The Asp4 side chain thus regulates efficiency of the electrostatic binding between the peptide N-terminal NH3(+) and the lipid headgroup PO4(-). 13C chemical shifts in the hydrophobic D-Phe2 residue are largely perturbed upon membrane binding, in the case where the side chain beta-CO2(-) in Asp4 is deionized; the deionization of Asp4 beta-CO2(-) increases the net hydrophobicity of achatin-I with a reduction of both the electrostatic hydration and the electrostatic attraction with the headgroup N(CH3)3(+) in the most superficial region of the PC membrane, resulting in deeper anchoring of the phenyl ring. Hence, the electrostatic effect of the side chain beta-CO2(-) in Asp4 floats achatin-I on the PC membrane surface, and the binding equilibrium is sensitively controlled by the ionization state of the N-terminal NH3(+).     

The phenylnitrene rearrangement in the inner phase of a hemicarcerand

The photochemistry of phenyl azide 1 and 13C-labeled phenyl azide 13C-1 incarcerated inside a hemicarcerand 4 was investigated. Low-temperature photolysis of hemicarceplex 41 and 413C-1 yields incarcerated 1-azacyclohepta-1,2,4,6-tetraene 42 and 413C-2 (18-50%), respectively, which were characterized by low-temperature FT-IR and 1H NMR and 13C NMR spectroscopy. After correction for the hemicarcerand-induced upfield shift, the 13C chemical shifts of incarcerated 13C-2 compare very well (Deltadelta 相似文献   

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.     

How reliable are GIAO calculations of 1H and 13C NMR chemical shifts? A statistical analysis and empirical corrections at DFT (PBE/3z) level

Reliability of calculated (1)H and (13)C NMR chemical shifts for various classes of organic compounds obtained with gauge-invariant atomic orbital (GIAO) approach has been studied at the PBE/3ζ level (as implemented in PRIRODA code) using linear regression analysis with experimental data. Empirical corrections for the calculated chemical shifts δ(H,calc) = δ(PBE/3ζ) - 0.08 ppm (RMS 0.18 ppm, MAD 0.66 ppm) and δ(C,calc) = δ(PBE/) (3) (ζ) - 6.35 ppm (RMS 3.09 ppm, MAD 9.42 ppm) have been developed using the sets of 263 and 308 experimental values for (1)H and (13)C chemical shifts, respectively. The confidence intervals of NMR chemical shifts at 95% confidence probability are δ(H,calc) ± 0.35 ppm for (1)H and δC,calc) ± 6.05 ppm for (13)C.