MINDO/3 calculations of ESCA chemical shifts

Calculations of ESCA chemical shifts, using Jolly's equivalent core approximation and the MINDO/3 semi-empirical SCF MO method, have given results in reasonable agreement with experiment.     

Some simple basis sets for accurate 13c chemical shift calculations

Two relatively simple basis sets reproduce ¹³C chemical shifts in small molecules and simple first-row hydrides to an accuracy comparable to or better than previous more extensive bases. The bases are essentially triple-zeta valence sets with one set of d polarization functions on heavy atoms and double- or triple-zeta with no polarization on hydrogen. Hydrogen shifts are marginally acceptable on an absolute scale but surprisingly good on a relative scale.     

Use of13C chemical shift/charge density linear relationship for ranking chemical structures

A novel approach to molecular structure elucidation based on ranking chemical structures in agreement with the¹³C NMR chemical shift/charge density linear relationship is suggested. The structure having the lowest standard approximation error (SAE) is considered to be the correct one. Each ranked structure is additionally tested for the¹³C chemical shift equivalence corresponding to its constitutional symmetry (in terms of the charge densities).This paper is Part VI of the series Automatic assignment of¹³C NMR signals. For Part V see ref. [1].     

Dipole moment derivatives from MINDO/3 semi-empirical MO calculations

Dipole moment derivatives for CO, NO, CO₂, H₂O, HCN, BF₃, CH₄, C₂H₄, C₂H₆, CH₃F, F₂CO and H₂CO molecules have been evaluated using MINDO/3 MO calculations. The values are compared with those obtained by other semi-empirical MO methods.     

Radical hydroxylation of aromatic hydrocarbons examined by MINDO/3 methods

MINDO/3 calculations have been made on the potential-energy surfaces for the attachment of OH^. radicals to benzene (1) and naphthalene (2) in the vapor state. The activation energies of these reactions are calculated as 88 and 58 kJ/mole. while the enthalpies at 298K are calculated as –211 and –199 kJ/mol. The transition states in (1) and (2) lie closer to the reagents than the products on the reaction coordinate, while (1) has an earlier transition state than does (2). The transition states in these reactions have high dipole moments: 3.1 and 3.6 D, respectively, which are due to charge transfer from the hydrocarbons to the OH^.. Quantum-chemical calculations and kinetic data on the reactions of aromatic hydrocarbons with OH^. in aqueous solution indicate that the mechanism is probably not one involving electron transfer and a rate-limiting stage in the attachment. These processes are of high performance because the radicals are of high stability, while polar effects determine the selectivity.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 1, pp. 20–26, January–February, 1986.     

α-substituent effects in 13C NMR of hydrocarbons. Quantum chemical calculations

The ¹³C NMR α-substituent effects within the series methane to neopentane and ethene to isobutene have been calculated using Hartree—Fock perturbation theory with MINDO/3. The sign and order of magnitude of the calculated α-effects are in agreement with new experimental values, including effects of multiple substitution.     

13C chemical shift subsitituent parameters for alkyllithium compounds in hydrocarbon solvents

¹³C chemical shift substituent parameters are presented for carbons α, β, γ, and δ to the lithium atom based on the chemical shifts of 14 ⁶Li-enriched alkyllithium compounds. The chemical shift of the carbon α to lithium depends on the branching of alkyl group at the α-carbon and on the aggregation state of the alkyllithium compound. Increased branching results in increased upfield shifts. This is interpreted in terms of the varying electronic nature of the alkyllithium compounds. The chemical shift of the carbon β to lithium substitution is shifted downfield approximately 5 ppm from the corresponding carbon in the parent hydrocarbon, irregardless of the alkyl group or the aggregation state of the alkyllithium compound. The chemical shift of the γ-carbon depends on the steric requirements of the alkyl group. Carbons four or more bonds from lithium have the same chemical shift as those of the parent hydrocarbon. The derived chemical shift parameters are used to assign the α-carbons of two alkyllithium compounds formed from the reaction of t-butyllithium and trimethylvinylsilane.     

Locally dense basis sets for chemical shift calculations

Calculations of chemical shifts have been carried out using “locally dense” basis sets for the resonant atom of interest, and smaller, attenuated sets on other atoms in the molecule. For carbon, calculations involving a 6-311G(d) triply split valence set with polarization on the resonant atom and 3-21G atomic bases on other heavy atoms result in good agreement with experiment, and are virtually identical to those found employing the larger basis on all atoms. For species such as nitrogen, oxygen, and fluorine where standard balanced basis sets do not agree well with experiment, use of attenuated sets fail as well. The use of locally dense basis sets permits calculations previously impractical, and the successful application to carbon suggests that the chemical shift is most dependent on the local basis set, and less so on whether or not a balanced or unbalanced calculation is being carried out.     

萜类化合物13C NMR化学位移的定量构谱相关研究

利用原子电性作用矢量(Atomic electro-negativity interaction vector,AEIV)和原子杂化状态指数(Atomic hybridization state index,AHSI)对萜类化合物中的C原子进行结构表征并与其核磁共振碳谱(13C NMR)建立了优良的定量构谱相关(QSSR)模型.其中29个单萜类化合物中的290个C原子建模的计算值经留一法(Leave-one-out,LOO)交互校验(Cross-validation,CV)预测值的复相关系数(R)分别为0.9900和0.9867,进一步使用倍半萜、二萜、三萜化合物分子中65个C原子的13C NMR化学位移值来检测该模型的稳定性,模型预测值和观测值间复相关系数(R)为0.9777,取得了令人满意的结果.     

Counterintuitive deshielding on the 13C NMR chemical shift for the trifluoromethyl anion

The trifluoromethyl anion (CF₃⁻) displays ¹³C NMR chemical shift (175.0 ppm) surprisingly larger than neutral (CHF₃, 122.2 ppm) and cation (CF₃⁺, 150.7 ppm) compounds. This unexpected deshielding effect for a carbanion is investigated by density functional theory calculations and decomposition analyses of the ¹³C shielding tensor into localized molecular orbital contributions. The present work determines the shielding mechanisms involved in the observed behaviour of the fluorinated anion species, shedding light on the experimental NMR data and demystify the classical correlation between electron density and NMR chemical shift. The presence of fluorine atoms induces the carbon lone pair to create a paramagnetic shielding on the carbon nucleus.     

Relations between the averaged 13C nuclear magnetic resonance chemical shift and the carcinogenic activity of polycyclic aromatic hydrocarbons.

13C Nuclear magnetic resonance (13C-NMR) spectra were measured for polycyclic aromatic hydrocarbons (PAH) to examine the correlation between their chemical shift and carcinogenicity. We confirmed our previous proposition that the carcinogenicity of PAH molecules can be predicted from the value of the averaged 13C-NMR chemical shift. It was also found, using the averaged chemical shift as a parameter, that several quantum chemical indices for the intermediate states of the metabolic transformation are correlated with the carcinogenic activity of PAH. This indicates that the averaged chemical shift can be applied to investigate the metabolic transformation of carcinogenicity.     

Use of an internal reference in 13C chemical shift measurements

The use of the nominal value of the chemical shift of the solvent to calibrate the spectrometer in ¹³C NMR spectroscopy was found to introduce errors due to the effect which the solute has on the solvent. In addition, hexamethyldisiloxane (HM) is proposed as an internal standard; owing to its high boiling point it is easier to manipulate than tetramethylsilane (TMS) and it is therefore possible to prepare solutions of known concentrations. In order to convert the data obtained with this standard to the TMS scale, the chemical shift of HM was determined in 16 solvents using cyclohexane as external standard in a spherical cell (5% w/w concentration of HM) as a function of the solvent factor g². Comparing these results with a similar plot obtained previously for TMS by other workers, it is possible to convert one type of data to the other by a simple linear expression.     

Reactivity of pyrrole pigments. Part 10 MINDO/3 calculations on bile pigments

The applicability of the MINDO/3 method is evaluated for tetrapyrrolic bile pigments: geometries from already calculated dipyrrolic partial models have been used to reduce the calculation time. Net atomic charges and reactivity parameters obtained from the frontier orbital model, i.e. HOMO and LUMO distribution on the molecule, are reported for several conformations.

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Reaktivität von Pyrrolpigmenten, 10. Mitt. MINDO/3-Rechnungen von Gallenpigmenten

Zusammenfassung Es wird der Einsatz von MINDO/3 für tetrapyrrolische Gallenpigmente beschrieben. Um die Rechenzeit herabzusetzen, wurden geometrische Daten verwendet, die vorher für dipyrrolische Partialstrukturen ermittelt worden waren. Für mehrere Konformationen werden Ladungsdichten und Reaktivitätsparameter nach dem Frontier-Orbital-Modell, nämlich HOMO- und LUMO-Verteilung, berichtet.

     

A general chemical shift decomposition method for hyperpolarized 13C metabolite magnetic resonance imaging

Metabolic imaging with hyperpolarized carbon‐13 allows sequential steps of metabolism to be detected in vivo. Potential applications in cancer, brain, muscular, myocardial, and hepatic metabolism suggest that clinical applications could be readily developed. A primary concern in imaging hyperpolarized nuclei is the irreversible decay of the enhanced magnetization back to thermal equilibrium. Multiple methods for rapid imaging of hyperpolarized substrates and their products have been proposed with a multi‐point Dixon method distinguishing itself as a robust protocol for imaging [1‐¹³C]pyruvate. We describe here a generalized chemical shift decomposition method that incorporates a single‐shot spiral imaging sequence plus a spectroscopic sequence to retain as much spin polarization as possible while allowing detection of metabolites that have a wide range of chemical shift values. The new method is demonstrated for hyperpolarized [1‐¹³C]pyruvate, [1‐¹³C]acetoacetate, and [2‐¹³C]dihydroxyacetone. Copyright © 2016 John Wiley & Sons, Ltd.     

A simplex optimized INDO calculation of 13C chemical shifts in hydrocarbons

The variable-size simplex optimization method is used to reparametrize the I + A and β parameters of an INDO approximation to the perturbed Hartree–Fock calculation of ¹³C chemical shifts in hydrocarbons. The absolute shifts for 39 nuclei in a set of molecules containing up to four carbons are reproduced within a standard error of 9.9 ppm for an unconstrained optimization and to a standard error of 10.0 ppm for an optimization constrained to yield gross atomic charges in agreement with double-zeta ab initio calculations.     

Automatic assignment of 13C NMR spectra based on the chemical shift/charge density relationship

An approach for the automatic assignment of ¹³C spectra, based on the chemical shift/charge density relationship, is suggested. All permutations of spectral signals are computer-generated, and for every permutation a least squares adjustment is carried out. The permutation presenting the highest correlation coefficient, or the lowest Hamilton's agreement factor, is considered to be correct. The application is exemplified by the ¹³C chemical shifts of a series of aromatic compounds. It is shown that more reliable assignment is achieved if the considered permutations are restricted by taking intoaccount the signal multiplicity.