Peter Rudolf Seidl

Steric effects on carbon‐13 NMR shifts: carbon–hydrogen bond polarization contributions

The shielding observed in the chemical shifts of carbon atoms subjected to steric compression was originally attributed to carbon–hydrogen bond polarization. There is increasing evidence that this is not universally the case and theoretical studies reveal that changes in dihedral angles may be responsible for many steric effects. Hydrogen shifts, bond lengths and charge distributions were used as probes for these effects in methylnorbornanes and similar tetracyclic norbornyl hydrocarbons. Carbon–hydrogen bond polarization can make a significant contribution to shielding and can be distinguished from effects caused by changes in dihedral angles due to steric congestion.

Pharmaceuticals from natural products: current trends

The use of products extracted from plants for medicinal purposes can be traced to the beginnings of civilization and up until the end of the nineteenth century natural products were the principal source of medicines. Since then their relative importance has oscillated according to the strategies of large pharmaceutical companies. Now that these strategies are changing, there are new opportunities for countries like Brazil, in which a large proportion of the worlds biodiversity is located. There are, however, new circumstances that must be taken into consideration: material must be collected by groups which are formally authorized to do so and under the conditions of the Convention of Biological Diversity, the discovery process is being successively outsourced to smaller specialized firms and there is a growing integration with producers of cosmetics and phytomedicines.

Visualization and quantification of the anisotropic effect of C=C double bonds on 1H NMR spectra of highly congested hydrocarbons-indirect estimates of steric strain.

The anisotropic effect of the olefinic C=C double bond has been calculated by employing the NICS ( nucleus independent chemical shift) concept and visualized as an anisotropic cone by a through space NMR shielding grid. Sign and size of this spatial effect on (1)H chemical shifts of protons in norbornene, exo- and endo-2-methylnorbornenes, and in three highly congested tetracyclic norbornene analogs have been compared with the experimental (1)H NMR spectra as far as published. (1)H NMR spectra have also been calculated at the HF/6-31G* level of theory to get a full, comparable set of proton chemical shifts. Differences between delta( (1)H)/ppm and the calculated anisotropic effect of the CC double bond are discussed in terms of the steric compression that occurs in the compounds studied.

MOLECULAR MODELLING OF REAGENTS FOR FLOTATION PROCESSES

Abstract It has long been recognized that the selectivity of froth flotation processes is highly influenced by the specificity of interactions between minerals and reagents that are used to control the hydrophobic/hydrophilic character of the mineral/water interfaces. We have modelled mineral surfaces (hydroxy-apatite, calcite) and molecules of two polysaccharides (starch, ethyl-cellulose) which are candidates as depressing agents for calcite. A special parameter named “Total Fitting Number” (Ft) was developed to assess the steric compatibility between the structure of both minerals and polysaccharide species. The calculation of Ft was based on measurements of OH-OH distances that exist at reagent molecular structures versus CaCa distances that exist along the most common crystallographic orientations (hkl) exhibited by mineral particles. Higher values of Ft were found for the interaction calcite/starch than calcite/ethyl-cellulose. This finding suggests that starch is a more effective depressant for calcite than ethyl-cellulose. However, low values of Ft were found for the interaction apatite/starch and apatite/ethyl-cellulose. This fact means that both substances are unlikely to act as depressants for apatite. Microflotation response of apatite and calcite with sodium oleate (pH = 10.2) in the presence of both polysaccharides corroborates the model.

Conformational effects on NMR chemical shifts of half-cage alcohols calculated by GIAO-DFT

Abstract Half-cage compounds have played an important role in the investigation of the way steric compression affects physical and chemical properties of organic molecules. Recent theoretical studies of half-cage alcohols have also shown that rotation around the carbon–oxygen bond of the hydroxyl group leads to low-energy conformers in which hyperconjugation affects bond lengths, bond angles, and charge distribution on carbon and hydrogen atoms in its vicinity while charge distribution is also affected by electrostatic effects. Chemical shifts are also sensitive to such variations, but we found that in smaller model systems steric effects may strongly attenuate those due to hyperconjugation so we optimized geometries for low energy rotamers of ‘outside’ and ‘inside’ half-cage alcohols, where these effects can be separated, and calculated their respective hydrogen and carbon-13 chemical shifts by gauge-independent atomic orbital (GIAO) methods at the B3LYP/6-31G(d) level. Results are compared to those obtained for the corresponding norbornyl alcohols as well as for the half-cage hydrocarbon. Carbon-13 chemical shifts respond more strongly to effects owing to hyperconjugation while hydrogen chemical shifts are more sensitive to electrostatic effects due to the proximity of the hydroxyl group.

Ab initio charge distributions in half-cage compounds

Abstract Charge distributions in half-cage inside and outside isomer alcohols were obtained by STO-3G ab initio calculations. Input optimized geometries were calculated by MM2 and MNDO methods. The MM2 geometry accompanied neutron-diffraction data on relative bond lengths more closely, but this fact had little influence on charge distributions. In C-H bonds under strong non-bonded pressure the electron density maximum of the hydrogen atom is shifted toward the bound carbon resulting in a significant increase in population and relative p character. In such cases, the influence of steric compression may be comparable to that of bond angles in determining hybridization.

Critical Variables for the Characterization of Asphaltenes Extracted from Vacuum Residues

Abstract Asphaltenes represent a solubility class of petroleum constituents that has been associated with a number of problems that arise from the use of heavier crude oils. As they are defined by their solubility and not their chemical structure, there are often certain difficulties in distinguishing them from other solubility fractions. In a widely adopted separation procedure, saturates generally consist of naphthenes and paraffins while aromatics, resins, and asphaltenes are often quite similar polynuclear aromatic species which may vary in the number of heteroatoms and sizes of ring systems and side-chains that are present. In our attempts to characterize asphaltenes extracted from vacuum residues, we found that the procedures employed for separating asphaltenes in a certain sample have a significant effect on the analytical data that is generated from it. We have thus investigated the influence of different variables on vacuum residues obtained from two Brazilian and one Venezuelan crude and suggest that the most important factors are: the proportion of heptane to sample, treatment by ultra-sound and recovery of asphaltenes in residues by toluene. Combinations of these variables can be employed to hinder aggregation and afford a better separation of asphaltenes and resins. The analytical techniques that were used to confirm this are: NMR (1H and 13C), gel permeation chromatography (GPC), infrared spectroscopy (IR), and elemental analysis. Information on the types of aromatic ring substitution patterns and terminal methyl groups may be obtained from spectroscopic techniques particularly from NMR.

NMR analysis of asphaltenes separated from vacuum residues by selected solvents

Abstract The characterization of petroleum asphaltenes from different sources has been the object of much current research. However, even employing standard separation methods to obtain representative samples, certain variations in properties are observed. In the present work, we report the results of combining solvent extraction with isolation procedures. We fractionated three asphaltenes obtained from vacuum residues by the IP‐143 method by ethyl acetate, acetone, methyl isobutyl ketone, acetyl acetone, and carbon tetrachloride as single solvents or in a given sucession. The extracts were isolated, weighed and analyzed by IR and 1H NMR. Our results indicate that the solubility of asphaltene constituents in these solvents is affected by the presence of certain aromatic ring systems and saturated chains or rings. The solubility of certain constituents does not depend exclusively on the polarity of the solvent.

Structures of half-cage pentacyclododecane derivatives: skeletal deformations due to steric compression-decompression

Abstract Hexachlorinated half-cage isomer acetates may be obtained from isodrin in acetic acid under the proper conditions and separated in highly pure form. The X-ray crystal structure of the “inside” acetate shows that it is still more sterically congested than the previously reported “outside” one. Bond lengths and angles for both isomers reveal large deviations from those normally found in saturated systems providing interesting models for the investigation of steric effects on chemical bonding.

Steric and electronic contributions to conformational effects on chemical shifts of acyclic alcohols

Abstract Our calculations on bi- and polycyclic alcohols reveal that the Mulliken charge distribution and chemical shift patterns due to hyperconjugation of lone pairs on oxygen with neighboring groups break down or are attenuated for certain spatial relationships of the hydroxyl group. Since in strained ring systems other effects on these parameters may be present, we applied a similar analysis to acyclic alcohols. Calculations at the B3LYP/6-31G ∗ level on conformers of methanol, ethanol, 1- and 2-propanol, 2-methyl-1-propanol, 2-methyl-2-propanol, 2-butanol, 2-methyl-2-butanol, 1- 2- and 3-pentanol and 2-methyl-3-pentanol, where hyperconjugation may be present, reveal steric effects as modifiers of hyperconjugative patterns affecting carbon-13 chemical shifts in such alcohols. Contrary to what is observed in bi- and policyclic systems, where electrostatic effects interfere with effects due to hyperconjugation, these steric effects may be the main cause for the attenuation of deshielding of nuclei that are subject to hyperconjugation. Electrostatic effects are also present but they do not interfere with hyperconjugation by lone pairs. Conformational effects fall off sharply after the third carbon in the chain.