Spectra–structure relationships in carbon-13 nuclear magnetic resonance spectroscopy. Results from a large data base

A link between a substructure searching system and a ¹³C NMR data base has been established and permits the retrieval and examination of the chemical shifts associated with specific substructures. The means by which these searches are accomplished is described and the results from the searches are presented and discussed. The system is interactive, and can be used to locate in the data base the chemical shifts of carbon atoms in precisely defined environments. Alternatively, it may be used to learn the range of the chemical shifts possessed by particular types of carbon atoms, such as N-methyl or O-methyl carbons.     

NMR spectral studies. XIV. Carbon-13 nuclear magnetic resonance studies on β-lactams

Carbon-13 NMR spectra of a variety of mono- and bicyclic β-lactams have been measured and the chemical shifts of the ring carbon atoms have been assigned unambiguously. Effects of substituents on the chemical shifts of these carbons are also discussed.     

Determination of the 13C NMR chemical shifts in an a2u type iron(III) porphyrin cation radical

The 13C NMR chemical shifts of all the carbons in an a2u type iron(III) porphyrin radical cation, [Fe(TPP)Cl]+, have been determined for the first time by the titration method as well as by the chemical shift correlation; they are 2230, 1050, and -1910 ppm for the alpha-pyrrole, beta-pyrrole, and meso carbon atoms, respectively.     

Quantum chemical prediction of the 13C NMR shifts in alkyl and chlorocorannulenes: correction of chlorine effects

Prediction of the ¹³C NMR shifts of sym-pentachlorocorannulene and decachlorocorannulene provided impetus for the development of a correction scheme based on a regression of experimental and quantum chemical data. A training set of 15 compounds (18 carbon signals) comprising carbons atoms bearing 1–4 chlorine atoms leads to an estimated error per chlorine atom of about 10–12 ppm. Specifically, linear regression of the data obtained at B3LYP/cc-pVDZ leads to y = −3.77 + 13.11x, with R = 0.982. Ultimately, experiment and theory converge for sym-pentachlorocorannulene and decachlorocorannulene, the former by correction of the theory, the latter by collecting the proper experimental data. Contribution to the Mark S. Gordon 65th Birthday Festschrift Issue.     

2,3-二氰基-2,3-二(p-X苯基)丁二酸二乙酯的分子构型和取代基电子效应对13C NMR谱的影响

测定了meso-和dl-2,3-二氰基-2,3-二(p-X苯基)丁二酸二乙酯(X=OCH₃,CH₃,H,Cl,NO₂)的¹³CNMR谱。结果表明,中心碳-碳键两端相连基团的各碳原子的化学位移值相同,与dl-异构体相比,所有相应meso-异构体的乙氧羰基¹³CNMR吸收峰均处于高场。苯环对位取代基的Hammett基团常数σ与氰基碳原子和乙氧羰基中的羰基碳及次甲基碳的化学位移间线性相关,而且meso-异构体比dl-异构体有更好的线性关系。     

C NMR spectra of cyclomercurated ferrocenylimines: Substituent effects and conformations in the ferrocenes

Three series of cyclomercurated ferrocenylimines (2-chloromercurio-ferrocenylimines) have been studied using ¹³C NMR spectroscopy. Good to excellent linear relationships have been found to exist between the chemical shifts of the carbon atoms in the ferrocenyl moiety and normal Hammett substituent constants σ_m and σ_p. The δ values of the iminyl carbon atoms show excellent linear correlations with the σ values. In three series of the ferrocenes, the sensitivity of the carbon atoms to the substituent effect is different, which is discussed in terms of the twist angle between the N-phenyl ring and the C---C=N---C plane. The relative sensitivity of the chemical shifts on different positions in ferrocenyl moiety to the substituent effect has also been presented.     

Low field chemical shifts in 13carbon NMR spectra of 1,3-dehydroadamantane compounds possessing “inverted carbon atoms”

1,3-Dehydroadamantanes, which possess “inverted carbon atoms” in a cyclopropyl moiety, display low field ¹³C-NMR chemical shifts for carbon atoms situated across from the cyclopropyl group. This long range effect also shows in 2,4-dehydroadamantane and is therefore not due to the presence of especially strained inverted carbons.     

Modification of Activated Carbon Fibers and Their Characterizes of Adsorption for Xenon

Activated carbon fibers (ACFs) are characterizes by their highly developed internal surface area and porosity. Especially the development of micro- and mesopores is of great importance because it allows the carbons to adsorb large amounts and various types of chemicals from gases or liquids. Due to such a valuable feature, activated carbon fibers have been in use for many years. Porous carbon materials have now become extremely versatile adsorbents of major industrial significance. The high surface area and porosity of activated carbon fibers are the result of the activation process; physical or chemical activation. However, the complexity of the carbon structure, although voluminous research has been done and great effort has been made toward the control of pore size and its distribution^[1-3]. The recent development of industrial technology provides new application fields for porous carbons and,at the same time, requires the carbon to have a desired pore structure. To meet such a requirement, many novel approaches to control pore structure have been proposed.     

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.     

Investigation of substituent effects of chalcones by 13C n.m.r. spectroscopy

¹³C chemical shifts for 23 para- and meta-substituted chalcones of the types 1 and 2 have been determined. The aromatic shieldings are compared with previous results for other aromatic derivatives. Correlations of the ¹³C chemical shifts of vinyl carbons and carbonyl carbons as well as ring carbons with Hammett σ parameters, π electron densities and the reactivity parameters of Swain and Lupton provide a consistent picture of electronic effects transmitted through the carbon framework of the compounds studied.     

Tryptophan chemical shift in peptides and proteins: a solid state carbon-13 nuclear magnetic resonance spectroscopic and quantum chemical investigation

We have obtained the carbon-13 nuclear magnetic resonance spectra of a series of tryptophan-containing peptides and model systems, together with their X-ray crystallographic structures, and used quantum chemical methods to predict the (13)C NMR shifts (or shieldings) of all nonprotonated aromatic carbons (C(gamma), C(delta 2) and C(epsilon 2). Overall, there is generally good accord between theory and experiment. The chemical shifts of Trp C(gamma) in several proteins, hen egg white lysozyme, horse myoglobin, horse heart cytochrome c, and four carbonmonoxyhemoglobins, are also well predicted. The overall Trp C(gamma) shift range seen in the peptides and proteins is 11.4 ppm, and individual shifts (or shieldings) are predicted with an rms error of approximately 1.4 ppm (R value = 0.86). Unlike C(alpha) and N(H) chemical shifts, which are primarily a function of the backbone phi,psi torsion angles, the Trp C(gamma) shifts are shown to be correlated with the side-chain torsion angles chi(1) and chi(2) and appear to arise, at least in part, from gamma-gauche interactions with the backbone C' and N(H) atoms. This work helps solve the problem of the chemical shift nonequivalences of nonprotonated aromatic carbons in proteins first identified over 30 years ago and opens up the possibility of using aromatic carbon chemical shift information in structure determination.     

线形碳元素簇合物的成键性质

在ab initio 3-21G水平上, 用能量梯度法优化了线性碳元素簇合物C_n~e(n为成簇原子个数, e为电荷)的平衡几何结构. 所得的电离势随成簇原子个数的改变, 呈现出不同程度的奇偶交替变化趋势. 在ab initio计算基础上, 用Boys方法, 对其占据正则分子轨道进行定域化变换, 得到了它们的定域分子轨道. 对定域分子轨道性质的分析表明, 线性碳元素簇合物中, 主要键型有双中心σ和π健, 双中心弯键和三中心香蕉健, 以及多中心σ和π健. 这种键型的多样化可视为小元素簇的成健特征. 此外, 通过对其成键性质的分析, 讨论了线性碳元素簇的稳定性. 对于小碳元素簇, 化学键的共轭性对其稳定性具有十分显著的作用.     

Carbon-13 NMR of fluorocyclopropanes

Carbon-13 and hydrogen NMR data are presented for several cyclopropanes with one to four fluorine atoms attached to the ring. Substituents directly attached to a ring carbon are found to unshield that carbon to an extent increasing with their electronegativity, but remote substituents may have an opposite effect. Relative orientation of the substituent groups, especially if they are bulky, also has a strong influence on the ring-carbon shifts. Chemical shifts of carbons in methyl groups attached to the ring change to an extent which is additive in the effects of gem and cis substituents. One-bond coupling of ring carbons with fluorines is affected both by geminal and vicinal substituents, in a way generally related to their electronegativity. It is tentatively concluded that two-bond coupling between ring carbons and ring-attached fluorines depends in magnitude primarily on what groups are cis to the fluorine. Two-bond coupling between a carbon atom in a ring-attached methyl and the hydrogen gem to the methyl appears to be vanishingly small, but there is appreciable three-bond coupling of the methyl carbons to vicinal hydrogens.     

Unique chemical reactivity of a graphene nanoribbon's zigzag edge

The zigzag edge of a graphene nanoribbon possesses a unique electronic state that is near the Fermi level and localized at the edge carbon atoms. The authors investigate the chemical reactivity of these zigzag edge sites by examining their reaction energetics with common radicals from first principles. A "partial radical" concept for the edge carbon atoms is introduced to characterize their chemical reactivity, and the validity of this concept is verified by comparing the dissociation energies of edge-radical bonds with similar bonds in molecules. In addition, the uniqueness of the zigzag-edged graphene nanoribbon is further demonstrated by comparing it with other forms of sp2 carbons, including a graphene sheet, nanotubes, and an armchair-edged graphene nanoribbon.     

Importance of structural and chemical heterogeneity of activated carbon surfaces for adsorption of dibenzothiophene

The performance of various activated carbons obtained from different carbon precursors (i.e., plastic waste, coal, and wood) as adsorbents for the desulfurization of liquid hydrocarbon fuels was evaluated. To increase surface heterogeneity, the carbon surface was modified by oxidation with ammonium persulfate. The results showed the importance of activated carbon pore sizes and surface chemistry for the adsorption of dibenzothiophene (DBT) from liquid phase. Adsorption of DBT on activated carbons is governed by two types of contributions: physical and chemical interactions. The former include dispersive interactions in the microporous network of the carbons. While the volume of micropores governs the amount physisorbed, mesopores control the kinetics of the process. On the other hand, introduction of surface functional groups enhances the performance of the activated carbons as a result of specific interactions between the acidic centers of the carbon and the basic structure of DBT molecule as well as sulfur-sulfur interactions.     

邻、间、对-二甲氧基亚甲基苯及衍生物双自由基体系的自旋耦 合规律

采用量子化学abinitio法对具有甲氧基的碳、氧双自由基邻、间、对二甲氧基亚甲基苯及衍生物体系基态自旋耦合规律进行研究,得到非平面共轭体系中自由基之间磁性耦合的拓朴规则:共轭体系中,两个自由基之间以偶数个碳原子耦合,则有效交换积分J~i~j<0,体系具有低自旋基态;两个自由基之间以奇数个碳原子耦合,则J~i~j>0,体系具有高自旋基态。自由基性质对自旋耦合的影响较大,正离子自由基间磁性耦合能力较强,这些结论为有机磁性材料的分子设计与实验合成提供了理论依据。     

Small binary iron-carbon clusters with persistent high magnetic moments. A theoretical characterization

Characterization of the structural and electronic properties of binary iron-carbon clusters composed by six iron atoms and with up to nine carbon atoms was carried out with density functional theory calculations. Neutral, cations (q = +1), and anions (q = −1), some of them experimentally detected, were studied. The formation of dimers and trimers of carbon atoms over the iron surface were preferred. Moreover, some large carbon chains, with up to five atoms, were determined. High spin states emerged for the ground states, with multiplicities above 16, for all clusters independently of the number of carbon atoms attached to the iron core. All neutral clusters were stable because fragmentation (into carbon chains), dissociation (of a single carbon atom), and detachment of all carbons need high amounts of energy. Reactive species were defined by small HOMO-LUMO gaps. Charge transfer, to the carbon atoms, increased as the carbon content increased, producing, for some cases, an even-odd behavior for the magnetic moment of the Fe₆C_n particles.     

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.     

Infrared spectroscopy of copper-resveratrol complexes: a joint experimental and theoretical study

Infrared multiple-photon dissociation spectroscopy has been used to record vibrational spectra of charged copper-resveratrol complexes in the 3500-3700 cm(-1) and 1100-1900 cm(-1) regions. Minimum energy structures have been determined by density functional theory calculations using plane waves and pseudopotentials. In particular, the copper(I)-resveratrol complex presents a tetra-coordinated metal bound with two carbon atoms of the alkenyl moiety and two closest carbons of the adjoining resorcinol ring. For these geometries vibrational spectra have been calculated by using linear response theory. The good agreement between experimental and calculated IR spectra for the selected species confirms the overall reliability of the proposed geometries.     

Photoelectron spectroscopy (PES) study of gas-phase 2,5-(2,2-dithienyl)diethynyl-thiophene (TRIM): an experimental and theoretical study

The X-ray photoelectron spectra of gaseous 2,5-(2,2^′-dithienyl)diethynyl-thiophene (TRIM) in the C 1s and S 2p core level regions have been recorded by means of synchrotron radiation (SR) at ELETTRA. For all the non-equivalent carbon atoms of TRIM, ΔSCF calculations of the C 1s photoelectron spectra, have also been performed. All the carbons of the aromatic rings are perturbed by an electron charge withdrawing. The ethyne carbon atoms are negatively shifted in energy. The relative broadening of the S 2p accounts for the presence of two non-equivalent sulphur atoms. The S2p_3/2 ionization potential value is lowered with respect to thiophene by −0.63 eV.