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Different chemical and spectroscopic techniques-diffuse reflectance infrared Fourier transform (DRIFT), surface-enhanced Raman spectroscopy (SERS), and 1H, 13C nuclear magnetic resonance (NMR) have been applied to investigate a peat humic acid (HA) separated by tangential ultrafiltration into different nominal molecular weight (NMW) fractions. Each fraction analyzed showed a characteristic DRIFT and NMR pattern. High nominal molecular weight fractions were mainly characterized by long chains of methyl and methylene groups and poorly substituted aromatic rings, while in low nominal molecular weight fractions (L-NMW), phenolic and oxygen-containing groups were predominant. A comparative study on fractions before and after treatment with 0.5 M HC1 was carried out. Purified fractions showed either an increase in the carboxylate and phenolic OH groups or an improvement in signal-to-noise ratio of their NMR spectra. The SERS study of NMW fractions allowed significative information on structure and conformation of these fractions. In particular, L-NMW fractions showed a great structural modification, when different alkaline extractants or treatment with HC1 were used. Humic-like substances obtained by catechol and gallic acid polymerization on metal surface were investigated using SERS. The SERS spectra of these polymers were compared and discussed with those of NMW HA fractions. © 2002 Elsevier Science B.V. All rights reserved.

We report the spectroscopic characterization of cGMP, which has an important biological role as a second messenger. NMR and Raman spectroscopies were used to characterize the cGMPH and cGMPNa structure in solution (water and dimethylsulfoxide). The spectroscopic results show that for cGMPH in dimethylsulfoxide the N7 position is free and the proton is localized on the phosphate group. By addition of 1 and 2 HCl equivalents to cGMPNa in the same solvent the first equivalent protonates the phosphate group while the second is localized on the N7 position of the guanine ring. On the contrary, for cGMPH in water the proton is mainly localized on the N7 guanine ring with an equilibrium depending on pH. Therefore, the solvent has a major effect in determining the protonation site in cGMPH. Solid state CP-MAS 13 C NMR spectrum of cGMPH suggests N7 protonation, phosphate group ionization and a sugar conformation different from that of the salt.

The structure of a 1,7,7-triaryl norbornane (compound 3) has been determined by X-ray diffraction and was found essentially equal to that predicted by Molecular Mechanics calculations. Restricted rotation of the aryl groups has been also observed by dynamic NMR spectroscopy in this compound and in a number of analogously substituted norbornanes. The aryl-norbornane bond rotation barriers were measured by line shape analysis of the 13C NMR spectra obtained at temperatures lower than -100 °C and were found to cover the range 6.0 to 7.9 kcal mol-1. An exception was the rotation involving the ortho-anisyl group in compound 5 which occurs near ambient temperature since the corresponding barrier is much higher (14.4 kcal mol-1). In one case (compound 4) configurational enantiomers could be separated by chiral HPLC and the corresponding CD spectra recorded.

The 13C NMR CP-MAS spectrum of 2-naphthylphenylsulfoxide in the solid state displays line broadening effects due to the restricted rotation about the Ph-S bond. Line shape simulation of the temperature dependent traces allowed the corresponding barrier to be determined in the solids (14.7 kcal mol-1). By making use of the information obtained from single crystal X-ray diffraction, this barrier could be satisfactorily reproduced by theoretical calculations (14.5 kcal mol-1) that take into account the correlated phenyl motion involving a large set of molecules in the crystalline state

The barriers for interconverting the conformational enantiomers (stereolabile atropisomers) of pyridine-substituted adamantane derivatives have been determined by dynamic 13C NMR spectroscopy. The trend of these values parallels that anticipated by MM calculations. In at least one case, the computed structure was found to agree with that obtained by single crystal X-ray diffraction. Also, it has been possible to achieve a physical separation of a pair of these stereolabile atropisomers at –60°C by means of the enantioselective cryogenic HPLC technique.

The catalytic behavior in butadiene and n-butane oxidation of molybdovanadophosphoric acids with vanadium localized inside the primary (oxoanion) and/or the secondary structure is reported. The samples are characterized by infrared, [sup 31]P-NMR, [sup 51]V-NMR, and UV-visible diffuse reflectance spectroscopies in order to obtain information on the nature and localization of vanadium in the samples before reaction and the possible changes occurring during the course of the catalytic reaction. In particular, it is shown that vanadium localized initially in the secondary structure can exchange with the molybdenum atoms of the oxoanion during the catalytic reaction. Introduction of vanadium in the molybdophosphoric acid structure enhances the selective formation of maleic anhydride from the butadiene when vanadium is present both inside the oxoanion or localized in the secondary structure (before the catalytic tests), but the maximum in catalytic performance is found for different amounts of vanadium, depending on where the vanadium is localized initially. However, when present in the secondary structure, vanadium also has a negative influence on the activity of the heteropoly acid. On the contrary, in n-butane oxidation, the presence of vanadium enhances the rate of alkane activation due to the different rate-determining step. The presence of V ions also affectsmore » the maximum selectivity and yield to maleic anhydride from butane. V ions in the secondary structure are more selective at low conversion, while V ions inside the oxoanion are more selective at higher conversions and thus allow better maximum yields to maleic anhydride. 40 refs., 11 figs., 2 tabs.« less

A number of 1-naphthylimines (and some of the corresponding imonium chlorides), having the isopropyl group as a prochiral probe, have been synthesized and their configurations assigned by the combined use of NOE experiments and molecular mechanics calculations. This method also provides reliable information about the conformations adopted in solution. The existence of diastereotopic methyl groups in the isopropyl moiety indicates the presence of conformational enantiomers, due to the rotation about the naphthyl–imino bond in nonplanar conformers. The barriers for the stereomutation of such enantiomeric forms have been determined by line-shape analysis of the temperature-dependent 1H or 13C signals of the prochiral probe. Molecular mechanics calculations have also helped in suggesting which is the preferred stereomutation pathway and in predicting the existence of a derivative having configurationally stable enantiomers. Such a compound has been synthesized and is found to display the predicted property (atropisomerism), in that the two enantiomers can be separated.

The free energies of activation for the enantiomerization of the title compounds (Mes2C = X, Mes = 2,4,6-trimethylphenyl) were determined by dynamic NMR to be 4.6, 6.5, and 9.2 kcal mol-1 for X = O, S, and CH2, respectively. Single-crystal X-ray diffraction showed that the structure of dimesitylketone is that of a propeller (C2 symmetry) with the mesityl rings twisted by 50 degrees with respect to the plane of carbonyl. The same structure was predicted by molecular mechanics calculations, which also produced good agreement between computed and experimental barriers for a dynamic process where a disrotatory one-ring flip pathway reverses the helicity of the conformational enantiomers. Solid-state NMR spectra indicated that the enantiomerization barrier in the crystal must be much higher (at least 19 kcal mol-1) than that in solution. Contrary to the case of dimesitylketone, the calculated barrier of dimesitylethylene agrees better with the experimental value if the enantiomerization process is assumed to be a conrotatory two-ring flip pathway.

A number of 1-naphthylimines (and some of the corresponding imonium chlorides), having the isopropyl group as a prochiral probe, have been synthesized and their configurations assigned by the combined use of NOE experiments and molecular mechanics calculations. This method also provides reliable information about the conformations adopted in solution. The existence of diastereotopic methyl groups in the isopropyl moiety indicates the presence of conformational enantiomers, due to the rotation about the naphthyl–imino bond in nonplanar conformers. The barriers for the stereomutation of such enantiomeric forms have been determined by line-shape analysis of the temperature-dependent 1H or 13C signals of the prochiral probe. Molecular mechanics calculations have also helped in suggesting which is the preferred stereomutation pathway and in predicting the existence of a derivative having configurationally stable enantiomers. Such a compound has been synthesized and is found to display the predicted property (atropisomerism), in that the two enantiomers can be separated.

Left ventricular hypertrophy can be reversed by treatment of hypertension with captopril but the consequences of this regression are not yet fully described. We studied the maximal capacity of the hypertrophied and hypertrophy-reversed ventricle to generate pressure during transient total occlusion of the aorta, and also the left ventricular end-diastolic pressure required to meet this maximal effort. Two-kidney, one clip Goldblatt (renal hypertensive rats; RHR) hypertension was induced in 17 Wistar rats, eight of which were treated with captopril (RHR-C: 50 mg/kg given orally) from the fourth to the eighth week. Sham-operated controls (SC) remained untreated, or were treated with similar doses of captopril (SC-C). Significantly lower heart weights were found in RHR-C than in RHR (2.88 +/- 0.15 versus 2.38 +/- 0.04; P less than 0.001). During transient total occlusion of the aorta, the maximal intraventricular pressure developed in RHR-C was not significantly different from that in RHR, but left ventricular end-diastolic pressure was significantly less in RHR-C than in RHR (21.4 +/- 2.2 versus 34.3 +/- 3.8; P less than 0.01). The analysis of pressure-volume characteristics of the hypertrophied left ventricles and those in which hypertrophy was reversed revealed similar compliances between these two groups. Our data suggest that there was a mechanical improvement in the heart function after reversal of left ventricular hypertrophy.

Low-temperature NMR spectra show that the title compound exists as a pair of conformational enantiomers, generated by the restricted rotation about the C9-Pr (i) bond, the corresponding interconversion barrier being 6.9 kcal mol (-1). This interpretation is supported by theoretical (MM and DFT) calculations and by the experimental determination of the analogous barriers occurring in the related MeC=O and Bu(t)C=O derivatives.

By making use of low-temperature dynamic NMR spectroscopy, the rotation barriers about the sp3-sp2 bond have been determined in a number of hindered benzyl alcohols symmetrically substituted in the ortho positions, the substituents being F, Cl, Br, and Me. The free energies of activation covered the range 4.6-10.1 kcal mol-1. Ab initio computations matched satisfactorily the trend of these values and predicted the conformation adopted by these compounds. In one case, this result could be also confirmed by the X-ray diffraction structure. In the case of the corresponding methyl ethers two barriers could be measured, corresponding to the passage across two distinguishable transition states: the higher barriers covered the range 5.0-8.1 kcal mol-1 and the lower ones the range 4.7-6.2 kcal mol-1.

Carbinols of the Ar-C(OH)R(2) type, Ar being o-isopropylphenyl, exist as stereolabile syn-clinal (sc) and anti-periplanar (ap) atropisomers when R = Me, Et, i-Pr. In the latter compound, the major atropisomer also comprises two enantiomeric forms that interchange with a barrier of 6.4 kcal mol(-1). X-ray diffraction, NOE experiments, and ab initio calculations indicate that the sc-atropisomer is the more stable form when R = Me, i-Pr, t-Bu but is the less stable one when R = Et. NMR spectra at variable temperature allowed the determination of the barriers for the interconversion of the sc- into the ap-atropisomers (DeltaG++ = 7.6, 8.8, and 13.5 kcal mol(-1) for Me, Et, i-Pr, respectively). When R is a tert-butyl group, the two atropisomers are configurationally stable: the ap-atropisomer is obtained as the kinetic controlled compound, which can be transformed into the thermodynamically more stable sc-atropisomer with a free energy of activation of 29.3 kcal mol(-1). Both atropisomers exhibit restricted rotation of the tert-butyl moiety, the corresponding DeltaG++ values being 9.4 and 8.8 kcal mol(-1) for the sc- and ap-atropisomer, respectively.

We describe herein the investigation of the stereodynamic processes occurring in a series of 1-aryl-2,2-dimethylindanols, by dynamic NMR. When the aryl moiety is a mesityl or a 2-methyl-1-naphthyl, the rotational barrier exceeds the 25 kcal/mol, so that stable atropisomers are observed. In two cases, all the chiral-atropisomeric species have been separated by enantioselective HPLC, and the comparison between theoretical and experimental electronic circular dichroism spectra allowed the absolute configuration assignment of all the isolated species to be obtained. Chirality, 2011. © 2011 Wiley-Liss, Inc.

Dynamic NMR spectroscopy can determine energy barriers due to internal motion over the range of about 4.5-23 kcalmol -1 . Conformational analysis of the frozen conformations can be simulated and interpreted by reliable theoretical calculations based mainly on density functional theory (DFT). The same calculations can identify transition states and predict the values of energy barriers involved in stereodynamic processes. This review describes recent advances in the experimental and theoretical approaches used in the field of the stereodynamic analysis, reporting a number of examples taken from the recent literature.

The stereodynamic processes and conformational preferences of two classes of aryl fluorenyl ketones have been investigated by means of dynamic NMR spectroscopy, DFT calculations and X-ray diffraction. When the aryl substituent has two hydrogens in the ortho positions, its rotation is independent of that of the fluorene ring. On the contrary, if the two ortho hydrogens are replaced by the bulkier methyl groups (e.g. mesityl fluorenyl ketones), the motion of the aryl ring interacts with the fluorene, and the two rings rotate in a correlated manner.

Fluorene derivatives, having substituted benzyl groups bonded to position 9, have been investigated by variable temperature NMR spectroscopy. Stereodynamic processes involving restricted rotation about the fluorenyl-CH2 and aryl-CH2 bonds have been observed, leading to conformers and enantiomeric forms, and the corresponding barriers were determined by line shape simulation. These dynamic processes are interpreted as being due to correlated rotation pathways, on the basis of DFT computations that satisfactorily reproduce the experimental barriers. The structures of two such compounds were also determined by single-crystal X-ray diffraction.

NMR spectra of biphenyl derivatives bearing a single CR2OH substituent in the ortho position indicate that they exist as sp (more stable) and ap (less stable) conformers, due to the restricted rotation about the Ar-CR2OH bond. When R = Et (compound 2) the corresponding rotation barrier was determined (7.5 kcal mol-1) by line shape simulation of the low-temperature NMR spectra. Introduction of the prochiral i-Pr group in the position 3' of a biphenyl with the CMe2OH substituent in the position 2 (4) allowed the determination of the enantiomerization barrier (due to the Ar-Ar bond rotation) for the stereolabile axially chiral atropisomers (13.95 kcal mol(-1)). DFT computations of these barriers were all in agreement with the experiments. Biphenyls bearing two CR2OH groups in the 2,2' positions were found to exist as configurationally stable atropisomers: when R = Me (7) they were separated by enantioselective HPLC and the absolute configuration assigned on the basis of the corresponding CD spectra. In solution, compounds 6 (R = H) and 7 (R = Me) were found to originate a dimer, due to H-bond interactions between two enantiomers. In the case of 7, the free energy of activation (9.5 kcal mol-1) for the exchange of the monomer with the dimer could be measured, for the first time, by dynamic NMR. The conformational preferences, predicted by computations for the biphenyls with two CR2OH substituents in the 2,2' positions, were confirmed by X-ray diffraction in the case of R = H (6), R = Me (7), and R = i-Pr (9).

The ring inversion barrier for thianthrene tetraoxide was determined by making use of the variable temperature 13C NMR spectra of the 2,7-diisopropyl derivative (DeltaG(double dagger) = 6.5 kcal mol(-1)). The barrier is lower than that measured for a trans thianthrene dioxide derivative (DeltaG(double dagger) = 9.35 kcal mol(-1)). These results agree well with ab initio theoretical predictions.