Composition as a method for data reduction: application to carbon-13 NMR chemical shifts

The concept of composition as the counterpart to partition is introduced and advocated for the discussion of molecular properties. In the partition approach an observable (experimental) quantity is fragmented into contributions which are non-observable but which hopefully maintain constancy for fragments (bonds) in similar environments and thus facilitate comparison of data. With the composition as an approach the role of fragments and whole are reversed: one starts with a collection of observable fragment properties (e.g., atomic chemical shifts of NMR spectra) and then constructs an abstract non-observable quantity representing the collection of fragments as a whole. If a so-derived quantity for different molecules shows some regularity, the initial loss of information in condensation of independent fragment data is compensated by insight into novel structural correlations. The approach is illustrated first by ordering isomers (e.g., nonanes C₉H₂₀) with respect to their content of special graph invariants p ₂ and p ₃ (numbers of paths of length two and length three, respectively) and then showing that the constructed global quantity derived from individual carbon-13 NMR chemical shifts shows a regular variation with p ₂ and p ₃, very similar to isomeric variations of numerous thermodynamic properties of nonanes. Subsequently it is outlined how the difference (p ₂ p ₃) leads to a correlation for mean carbon-13 chemical shifts in octanes and nonanes, taken as an illustration for the approach. It is expected that the outlined approach opens new avenues for data reduction and the search for structure-property correlations.Dedicated to Professor J. Koutecký on the occasion of his 65th birthdayOperated by Iowa State University for the US Department of Energy under Contract No. W-7405-ENG-82. This work was supported in part by the Office of the Director     

Nuclear magnetic resonance studies of aqueous solutions of alkylureas: Proton and carbon-13 chemical shifts

Aqueous solutions of methylated and ethylated derivatives of urea have been investigated by proton and carbon-13 magnetic resonance technique. The chemical shifts of water protons in solution of alkylureas, relative to that of pure water, are as a measure of the hydrogen-bonding reinforcing or weakening ability of these substances. The dependence of this effect on the concentration and on the temperature has also been studied. The results obtained indicate that different contrasting effects act in these solutions, among then the water structuring effect of the nonpolar surface of the alkylureas and the effect of the volume of the solute.¹³C chemical shifts of the carbonyl groups of the alkylureas give useful information about the interactions between water and the polar part of the ureas. Carbonyl-water interactions exist but are partially destroyed by increasing concentrations of the solute.     

13C NMR chemical shifts of carbonyl groups in substituted benzaldehydes and acetophenones: substituent chemical shift increments

13C NMR Substituent chemical shift (SCS) increments have been determined for the carbonyl carbon of a variety of substituted benzaldehydes and acetophenones. The 13C NMR chemical shift of the carbonyl carbon can be predicted for many di- and trisubstituted benzaldehydes and acetophenones through simple additivity of the SCS increments. The magnitude and sign of the SCS increments have been explored using Hartree-Fock 6-31G* calculations to determine the natural atomic charges of the carbonyl carbon. When a substituent capable of intermolecular hydrogen bonding is present, deviations from additivity on the order of 2 ppm are observed in dilution experiments; deviations of up to 6 ppm can result from intramolecular hydrogen bonding.     

Structure of irradiated polytetrafluoroethylene according to DFT calculations of NMR chemical shifts

Methods of density functional theory were used to calculate ¹H, ¹³C, and ¹⁹F magnetic shielding tensors for C₇F_nH_mO_l model molecules, which can arise as fragments from radiation exposure of polytetrafluoroethylene.     

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.     

致癌性胺离子反应过程中电荷和化学位移的计算

应用原子-键电负性均衡方法中的σπ模型(ABEEMσπ)计算了由致癌性胺离子所参与的反应过程中的电荷分布,所计算出的电荷分布可以和从头算很好的相关联,并且所需要的时间也大大的缩短;同时应用从头计算程序计算致癌性胺离子反应过程中的NMR化学位移.结果表明,在反应过程中电荷的变化和NMR化学位移的变化有很好的对应关系.     

QSAR study on N-containing corrosion inhibitors: Quantum chemical approach assisted by topological index

In this paper, QSAR study on N-containing corrosion inhibitors has been made using quantum chemical approach assisted by a novel topological index (). Two models were obtained and their efficiency was examined using statistics. The results indicate that inhibition efficiencies relate to some parameters of corrosion inhibitors, such as E_homo, polarizability, Dipole, frontier orbital charge density, the interaction mode between inhibitors and metal surface (feedback donor–acceptor coordination bonds), (steric hindrance of molecules). Predicted values are consistent with the experimental ones on the whole. The residuals range within the experimental error. So the models can be used to predict inhibition efficiencies of the same type molecules.     

First-principles calculation of core-level binding energy shift in surface chemical processes

Combined with third generation synchrotron radiation light sources, X-ray photoelectron spectroscopy (XPS) with higher energy resolution, brilliance, enhanced surface sensitivity and photoemission cross section in real time found extensive applications in solid-gas interface chemistry. This paper reports the calculation of the core-level binding energy shifts (CLS) using the first-principles density functional theory. The interplay between the CLS calculations and XPS measurements to uncover the structures,...     

The application of empirical methods of 13C NMR chemical shift prediction as a filter for determining possible relative stereochemistry

The reliable determination of stereocenters contained within chemical structures usually requires utilization of NMR data, chemical derivatization, molecular modeling, quantum‐mechanical (QM) calculations and, if available, X‐ray analysis. In this article, we show that the number of stereoisomers which need to be thoroughly verified, can be significantly reduced by the application of NMR chemical shift calculation to the full stereoisomer set of possibilities using a fragmental approach based on HOSE codes. The applicability of this suggested method is illustrated using experimental data published for a series of complex chemical structures. Copyright © 2009 John Wiley & Sons, Ltd.     

Computation and analysis of 19F substituent chemical shifts of some bridgehead‐substituted polycyclic alkyl fluorides

The ¹⁹F NMR shieldings for several remotely substituted rigid polycyclic alkyl fluorides with common sets of substituents covering a wide range of electronic effects were calculated using the DFT‐GIAO theoretical model. The level of theory, B3LYP/6–311+G(2d,p), was chosen based on trial calculations which gave good agreement with experimental values where known. The optimized geometries were used to obtain various molecular parameters (fluorine natural charges, electron occupancies on fluorine of lone pairs and of the C? F bond, and hybridization states) by means of natural bond orbital (NBO) analysis which could help in understanding electronic transmission mechanisms underlying ¹⁹F substituent chemical shifts (SCS) in these systems. Linear regression analysis was employed to explore the relationship between the calculated ¹⁹F SCS and polar substituent constants and also the NBO derived molecular parameters. The ¹⁹F SCS are best described by an electronegativity parameter. The most pertinent molecular parameters appear to be the occupation number of the NBO p‐type fluorine lone pair and the occupation number of the C? F antibonding orbital. This trend suggests that in these types of rigid saturated systems hyperconjugative interactions play a key role in determining the ¹⁹F SCS. Electrostatic field effects appear to be relatively unimportant. Copyright © 2003 John Wiley & Sons, Ltd.     

Solid-state 13C NMR and quantum chemical investigation of metal diene complexes

This paper presents novel measurements and calculations of the olefinic (13)C chemical shift tensor principal values in several metal diene complexes. The experimental values and the calculations show shifts as large as 70 ppm with respect to the values in the parent olefinic compounds. These shifts are highly anisotropic, with the largest ones observed in the less shielded principal components and the smallest ones in the most shielded principal components of the tensor. The orientations of the principal components of the tensors remain, within 10 degrees , at their directions in ethylene and other olefinic compounds. The calculations, performed using the GIAO method and the LanDZ pseudopotential basis set, show good agreement with the experiments, and were used to establish definite evidence for the existence of a Cl-bridge structure in the bicyclo[2.2.1]hepta-2,5-diene (BCHD)dichlororuthenium(II) polymer.     

13 C NMR spectra of some indole derivatives

The effect of acyl and carboxyl groups in position 2 or 3 of the indole ring on the¹³C chemical shifts of the ring was studied, -, -, and -Increments of the indole and isatin rings for¹³ C chemical shifts of the substituants at the ring N-atom were determined.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 66–69, January, 1994.     

A computer program for the automatic estimation of H NMR chemical shifts

A computer program has been developed for predicting ¹H NMR chemical shifts. It automatically finds the various substructures of a given molecule for which additivity rules are available. Several strategies have been used to widen the range of applicability. with 200 test compounds, over 90% of the assigned chemical shifts of protons bonded to a carbon atom could be predicted. The mean deviation between observed and predicted values was 0.08 ppm with a standard deviation of 0.19ppm.     

Toward direct determination of conformations of protein building units from multidimensional NMR experiments. V. NMR chemical shielding analysis of N-formyl-serinamide,a model for polar side-chain containing peptides

Knowledge of chemical shift-structure relationships could greatly facilitate the NMR chemical shift assignment and structure refinement processes that occur during peptide/protein structure determination via NMR spectroscopy. To determine whether such correlations exist for polar side chain containing amino acid residues the serine dipeptide model, For-L-Ser-NH(2), was studied. Using the GIAO-RHF/6-31+G(d) and GIAO-RHF/TZ2P levels of theory the NMR chemical shifts of all hydrogen ((1)H(N), (1)H(alpha), (1)H(beta1), (1)H(beta2)), carbon ((13)C(alpha), (13)C(beta), (13)C') and nitrogen ((15)N) atoms have been computed for all 44 stable conformers of For-L-Ser-NH(2). An attempt was made to establish correlation between chemical shift of each nucleus and the major conformational variables (omega(0), phi, psi, omega(1), chi,(1) and chi(2)). At both levels of theory a linear correlation can be observed between (1)H(alpha)/phi, (13)C(alpha)/phi, and (13)C(alpha)/psi. These results indicate that the backbone and side-chain structures of For-L-Ser-NH(2) have a strong influence on its chemical shifts.     

Analysis of the transition from normal modes to local modes in a system of two harmonically coupled Morse oscillators

Summary The system consisting of two Morse oscillators coupled via either a potential or a kinetic quadratic term is considered. The corresponding classical equations of motion have been numerically integrated and the initial conditions have been systematically analyzed in the regime of low total excitation energy of the system. Particular attention was paid to the full characterization of an intermediate type of motion, herein called transition mode, which appears at total energy values in between those typical of normal modes and those where local and normal modes coexist. A previously proposed perturbative approach (Jaffé C, Brumer P (1980) J Chem Phys 73:5646) is reanalyzed and compared with the results of numerical experiments, with the purpose of lending further support to the existence of transition modes.     

A study of the 17O carbonyl chemical shifts in substituted benzaldehydes

An analysis of 17O carbonyl chemical shifts of 50 substituted benzaldehydes leads to an empirical equation, delta(cal)(17O) = 564.0 + (sigma)(delta)o + (sigma)(delta)m + (sigma)(delta)p + C, for calculating 17O chemical shifts. This equation is based on a linear regression analysis using 11 substituent parameters and leads to good conformity with the expected data.     

Calculation of dissociation constants and chemical hardness of some biologically important molecules: A theoretical study

The gas‐phase geometries of neutral, protonated, and deprotonated forms of some biologically important molecules, alanine (Ala), glycine (Gly), phenylalanine (Phe), and tyrosine (Tyr), were optimized using density functional theory at B3LYP/6‐311++G(d) and the ab initio HF/6‐311++G(d) level of theories. The neutral and different stable ionic states of Ala, Gly, Phe, and Tyr have also been solvated in aqueous medium using polarizable continuum model for the determination of solvation free energies in the aqueous solution. The gas‐phase acidity constants of above four molecules have been also calculated at both levels of theories and found that the values calculated at HF/6‐311++G(d) method are in good agreement with experimental results. A thermodynamic cycle was used to determine the solvation free energies for the proton dissociation process in aqueous solution and the corresponding pK_a values of these molecules. The pK_a values calculated at B3LYP/6‐311++G(d) method are well supported by the experimental data with a mean absolute deviation 0.12 pK_a units. Additionally, the chemical hardness and the ionization potential (IP) for these molecules have been also explored at both the level of theories. The Tyr has less value of chemical hardness and IP at both levels of theories compared with other three molecules, Ala, Gly, and Phe. The calculated values of chemical hardness and IP are decreasing gradually with the substitution of the various functional groups in the side chain of the amino acids. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Structural study of 8-azole derivatives of protoberberine alkaloids: experimental and quantum chemical approach

New derivatives of protoberberine alkaloids were prepared by nucleophilic addition of some azoles (differing in bulkiness) to the iminium functionality of the quaternary protoberberine alkaloids. Compounds were structurally characterized mainly by ¹H and ¹³C NMR spectroscopy, and the structure of 8-carbazol-1-yl-7,8-dihydroberberine was determined using single-crystal X-ray diffraction. Additionally, conformational behaviors of five derivatives varying in bulkiness of the azole moiety have been investigated by low temperature NMR spectroscopy and quantum chemical calculations. Ring current effects of pyrrole and carbazole moieties on selected ¹H NMR resonances have been characterized, visualized, and discussed.