Conformational preferences for some 5-substituted 2-acetylthiophenes through infrared spectroscopy and theoretical calculations

The s-cis-trans isomerisms of some derivatives of thiophene (2-acetyl, AT; 2-acetyl-5-bromo, ABT and 2-acetyl-5-chloro, ACT) were analyzed, using data from deconvolution of their carbonyl absorption bands in two solvents (CCl4 and CHCl3). These infrared data showed that the O,S-cis conformer largely predominates in the studied solvents and that the same occurs in the gas phase, as observed from theoretical calculations. The latter results were obtained using B3LYP/6-311++G(3df,3p) and MP2/6-311++G(3df,3p) levels of theory, with zero-point energy correction. Moreover, the use of the IEFPCM (Integral Equation Formalism Polarizable Continuum Model) to take into account the solvent effects, using the same levels of theory, confirmed the results observed from infrared data. Low temperature 13C NMR spectra in CS2/CD2Cl2 (-90 °C) and in acetone-d6 (-80°C) did not show pairs of signals for each carbon, due to the known low energy barrier (～8 kcal mol(-1)) for the cis-trans interconversion. Data from NBO calculations show that the nO(2)→σS-C5* and nO(2)→σC2-C3* interactions occur only in the O,S-cis isomer and can explain its conformational preference.     

Theoretical and infrared studies on the conformational isomerism of trans-2-bromo-alkoxycyclohexanes

The infrared spectra of trans-2-bromo-alkoxycyclohexanes (alcoxy = OMe, OEt, O(i)Pr and O(t)Bu) were obtained for the neat liquid, and the C-Br stretching mode was quantitatively analyzed to give insight about the conformational isomerism of these compounds. Frequency calculations supported the band assignments, and the relative band intensities suggest that the diaxial conformer is prevalent for the methoxy and tert-butoxy derivatives (51 and 56%, respectively), while the diequatorial form is preponderant for the ethoxy and isopropoxy derivatives (76 and 77%, respectively). Therefore, the size of the alkoxy group plays a determinant role in determining the conformational preferences of the title compounds.     

Theoretical conformational study of the molecular structures of some bipyridine cardiotonics

Conformational features of amrinone and milrinone have been examined viaab initio (at STO-3G and 3–21G levels) MO theory. The calculated results suggest twisted structures for both the molecules. The higher barrier to rotation of the pyridine ring and restricted conformational flexibility of milrinone are observed to be two significant factors responsible for its greater cardiotonic activity as compared to that of amrinone.     

Determination of conformational preferences of dipeptides using vibrational spectroscopy

The NMR coupling constants ((3)J(H(N), H(alpha))) of dipeptides indicate that the backbone conformational preferences vary strikingly among dipeptides. These preferences are similar to those of residues in small peptides, denatured proteins, and the coil regions of native proteins. Detailed characterization of the conformational preferences of dipeptides is therefore of fundamental importance for understanding protein structure and folding. Here, we studied the conformational preferences of 13 dipeptides using infrared and Raman spectroscopy. The main advantage of vibrational spectroscopy over NMR spectroscopy is in its much shorter time scale, which enables the determination of the conformational preferences of short-lived states. Accuracy of structure determination using vibrational spectroscopy depends critically on identification of the vibrational parameters that are sensitive to changes in conformation. We show that the frequencies of the amide I band and the A12 ratio of the amide I components of dipeptides correlate with the (3)J(H(N), H(alpha)). These two infrared vibrational parameters are thus analogous to (3)J(H(N), H(alpha)), indicators for the preference for the dihedral angle phi. We also show that the intensities of the components of the amide III bands in infrared spectra and the intensities of the skeletal vibrations in Raman spectra are indicators of populations of the P(II), beta, and alpha(R) conformations. The results show that alanine dipeptide adopts predominantly a PII conformation. The population of the beta conformation increases in valine dipeptides. The populations of the alpha(R) conformation are generally small. These data are in accord with the electrostatic screening model of conformational preferences.     

Theoretical conformational analysis of some hydroxyazoic dyes

The optimum structures of p-hydroxy azobenzene, 4-phenyl azo 1-naphthalene and 4-phenyl azo 1-napthol are determined theoretically using the quantum mechanical MO method PCILO. The conformational minima are obtained by simultaneous optimization of the geometric parameters. The conformers equilibria and their temperature dependence, suggested by experimental dipole moments results, are discussed on the basis of our PCILO calculations.     

The utility of infrared spectroscopy for quantitative conformational analysis at a single temperature

The carbonyl stretching vibration of 2-bromocyclohexanone (1) has been measured in a variety of solvents. It is shown that its component intensities are not only dependent on the populations of the axial and equatorial conformers, but are also dependent on the molar absorptivities (epsilon ) which are specific for each conformer in each solvent. In CCl(4), the axial and equatorial conformers have epsilon values of 417 and 818 l mol(-1) x cm(-1), respectively, while in CH(3)CN solution, the values were 664 and 293 l mol(-1) x cm(-1). These results are supported by results of theoretical calculations of frequencies, which gave an intensity of 223.8 kM mol(-1) x(1782 cm(-1)) for the axial and 174.4 kM mol(-1) x (1802 cm(-1)) for the equatorial conformer, indicating that the axial conformer presents a larger molar absorptivity than the equatorial one in the vapor phase. Moreover, the results presented here clearly demonstrate that although infrared spectroscopy at a single temperature can be an important auxiliary technique for conformational analysis, it must not be used to quantify conformational preferences of a molecule if the absorption molar coefficients for each conformer are not known or not amenable to experimental determination.     

Theoretical investigation on conformational preferences and structural properties of 2-lithio-1,3-diphosphinane and 2-lithio-1,3-dimethyl-1,3-diphosphinane

A new family of dialphosphacyclohexane is introduced which has three nucleophilic centers. The conformational stabilities and structural properties of 2-lithio-1,3-diphosphinane and 2-lithio-1,3-dimethyl-1,3-diphosphinane were investigated computationally by DFT calculations and NBO analyses at B3LYP/6-31+G(d,p). Relative energy trend in 1,3-diphosphinane and 1,3-dimethyl-1,3-diphosphinane conformations explored from steric and hyperconjugative point of view. The stability trend of 2-lithio derivatives based on calculated relative energies in 1,3-diphosphinane is 1d > 1f > 1b > 1a > 1c > 1e and for 1,3-dimethyl-1,3-diphosphinane derivatives is 2f > 2d > 2b > 2a > 2c > 2e. Calculated NBO atomic charges indicate that high positive charge at lithium and small C–Li Wiberg bond indexes in these derivatives are demonstrators of ionic nature of the C–Li bonds. Stereoelectronic interactions, polarizability of phosphorus, and chelate formation between each of phosphorous and lithium are determining factors in stability trend observed in these derivatives. Ease of lithiation in bis(dimethylphosphino)methane, 1,3-diphosphinane, and 1,3-dimethyl-1,3-diphosphinane derivatives was estimated and compared by isodesmic reaction.     

Importance of entropy in the conformational equilibrium of phenylalanine: a matrix-isolation infrared spectroscopy and density functional theory study

The conformational behavior and infrared spectrum of l-phenylalanine were studied by matrix-isolation infrared spectroscopy and DFT [B3LYP/6-311++G(d,p)] calculations. The fourteen most stable structures were predicted to differ in energy by less than 10 kJ mol(-1), eight of them with abundances higher than 5% at the temperature of evaporation of the compound (423 K). Experimental results suggest that six conformers contribute to the spectrum of the isolated compound, whereas two conformers (IIb(3) and IIIb(3)) relax in matrix to a more stable form (IIb(2)) due to low energy barriers for conformational isomerization (conformational cooling). The two lowest-energy conformers (Ib(1), Ia) differ only in the arrangement of the amino acid group relative to the phenyl ring; they exhibit a relatively strong stabilizing intramolecular hydrogen bond of the O-H...N type and the carboxylic group in the trans configuration (O=C-O-H dihedral angle ca. 180 degrees ). Type II conformers have a weaker H-bond of the N-H...O=C type, but they bear the more favorable cis arrangement of the carboxylic group. Being considerably more flexible, type II conformers are stabilized by entropy and the relative abundances of two conformers of this type (IIb(2) and IIc(1)) are shown to significantly increase with temperature due to entropic stabilization. At 423 K, these conformers are found to be the first and third most abundant species present in the conformational equilibrium, with relative populations of ca. 15% each, whereas their populations could be expected to be only ca. 5% if entropy effects were not taken into consideration. Indeed, phenylalanine can be considered a notable example of a molecule where entropy plays an essential role in determining the relative abundance of the possible low-energy conformational states and then, the thermodynamics of the compound, even at moderate temperatures. Upon UV irradiation (lambda > 235 nm) of the matrix-isolated compound, unimolecular photodecomposition of phenylalanine is observed with production of CO(2) and phenethylamine.     

Theoretical study of the conformational and electrostatic properties of C4-monosubstituted 2-azetidinones

The conformational preferences and electrostatic properties of 2-azetidinone, 4-(S)-methoxycarbonyl-2-azetidinone and 4-(R)-methyl-2-azetidinone have been studied in gas-phase, aqueous solution and CCl₄ solution using quantum mechanical methods. Gas-phase calculations were performed at the ab initio HF, MP2, and MP4 levels and solvent effects were investigated using a self-consistent reaction-field procedure adapted to the AM1 Hamiltonian. An almost planar arrangement was adopted by the-lactam ring in the three cases, whereas the alkoxycarbonyl side group was found to display a large conformational flexibility. The effects of the different solvents on the electrostatic properties of the three compounds were investigated by following the changes in both molecular electrostatic potentials and induced dipole moments. The resulting electrostatic parameters were used as static reactivity indices to predict the response of the systems to the attack of nucleophilic reagents. Theoretical results were compared with experimental data available on the structure and properties of-lactams. The validity of the method as a predicting tool was critically discussed.     

Theoretical conformational analysis of some agonists of the H1- and H2-receptors of histamine

The conformational equilibrium of -(aminoethyl)pyridines has been studied by molecular-mechanics using the geometry optimization. It has been determined that the equilibrium is characterized by the nearly statistical distribution of the ap- and sc-conformers about the bond. Substitution of the dimethylamino-group for the amino-group results in a slight shift of the equilibrium toward the ap-rotamers. The global minimum in -(ammoniumethyl)pyridines corresponds to the conformation with the +sc-orientation about the central C-C bond and the -sc-orientation (in the case of the dimethylammonium derivatives) about the bond. In the latter conformation, the bond of the ammonium group is oriented toward the pyridine cycle. For charged compounds, the regular orthogonal conformation of the aromatic-ring plate is distorted (the vicinal C-C bond becomes eclipsed).Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1833–1840, August, 1991.The authors are grateful to P. P. Shagidullin and A. Kh. Plyamovatyi for their interest in this work and for fruitful discussions.     

The role of internal H-bonding in the conformational preferences of some molecules of the formula CH3CHYCH2X

The conformational energies of 1-amino-2-propanol, 2-amino-1-propanol and 1,2-diaminopropane are studied using ab initio molecular orbital theory employing minimal (STO-3G) and extended (4-31G) basis sets. Calculations at both levels of theory generally favor conformations stabilized by internal H-bonding for all molecules considered. Results are first presented for conformations employing assumed geometries. Since the conformational energy differences as found by the initial set of calculations are in some cases rather small it then becomes necessary to introduce geometry optimizations into the study at the minimal STO-3G level. In addition, to get a better estimate of the energy differences of the various conformations 4-31G calculations are performed on the STO-3G optimized structures. These latter results indicate the following, (a) For 1-amino-2-propanol only one conformation that is stabilized by intramolecular H-bonding is low in energy; this has the methyl and amino groups anti. The other H-bonded conformer, where the methyl and amino groups are gauche, is predicted to be ca. 1.2 kcal mol^?1 less stable. Similar findings for this molecule have recently been provided by micro-wave spectroscopy. (b) For 2-amino-1-propanol the two H-bonded conformers are only separated by about 0.5 kcal mol^?1, with the anti conformer being more stable. Micro-wave spectroscopy again supports these calculations. (c) For 1,2-diaminopropane the gauche conformer is predicted to be of rather high energy (ca. 2.5 kcal mol^?1) compared to the corresponding anti H-bonded conformer. The value of 2.5 kcal mol^?1should be taken as an upper limit, since the geometry optimization of the gauche conformer of 1,2-diaminopropane is incomplete compared to the optimization carried out for the anti conformer.     

Computational study of conformational preferences of thioamide-containing azaglycine peptides

The effect of thioamide substitution on the conformational stability of an azaglycine-containing peptide, For-AzaGly-NH2 (1), was investigated for the sake of finding possible applications by using ab initio and DFT methods. As model compounds, For-[psiCSNH]-AzaGly-NH2 (2), For-AzaGly-[psiCSNH]-NH2 (3), and For-[psiCSNH]-AzaGly-[psiCSNH]-NH2 (4) were used. Two-dimensional phi-psi potential energy surfaces (PESs) for 2-4 were calculated at the B3LYP/6-31G*//HF/6-31G* level in gas (epsilon = 1.0) and in water (epsilon = 78.4) by applying the isodensity polarizable continuum model (IPCM) method. On the basis of these PESs, the minimum energy conformations for 2-4 were characterized at the B3LYP level with 6-31G*, 6-311G**, and 6-31+G** basis sets. The remarkable structural effect of thioamide substitution for 2-4 is that beta-strand structure is observed as a global or local minimum. The minima of 2-4 are also compared with those for glycine and thioamide-containing glycine peptides. Our theoretical results demonstrate that compounds 2-4 would be used to design controllable secondary structures.     

Configurational and conformational aspects of some 5-methylenedisubstituted-3-pyrrolin-2-ones

The influence of the disubstitution at the exocyclic carbon atom of 5-methylene-3-pyrrolin-2-ones upon the configurational and conformational equilibria is studied. The results obtained confirm and extend the observations reported in the literature about the factors determining the configurational and conformational equilibria in monosubstituted systems, i.e.: 5-arylmethylene-3—pyrrolin-2-ones and 5(1H)-pyrromethenones.

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Zur Konfiguration und Konformation von einigen 5-Methylen-disubstituierten-3-pyrrolin-2-onen

Zusammenfassung Es wird der Einfluß der Disubstitution des exocyclischen C-Atoms von 5-Methylen-3-pyrrolin-2-onen auf das Konfigurations- und Konformationsgleich-gewicht untersucht. Die erhaltenen Resultate stehen im Einklang mit der Literatur, die sich auf monosubstituierte Verbindungen [5-Arylmethylen-3-pyrrolin-2-one, 5 (1H)-Pyrromethenone und strukturverwandte Gallenpigmente] beziehen, und vertiefen das Verständnis der Faktoren, die für diese Gleichgewichte maßgebend sind.

     

One-pot synthesis of 1-monosubstituted-1, 2, 3-triazoles from 2-methyl-3-butyn-2-ol

An efficient method for the synthesis of 1-monosubstituted-1, 2, 3-triazoles from 2-methyl-3-butyn-2-ol under copper catalyst conditions has been developed through a one-step one-pot sequence. This method provides a concise and efficient pathway to synthesize 1-monosubstituted-1, 2, 3-triazole derivatives in good to excellent yields.     

Theoretical study of the conformational features of triosmium clusters

The article summarizes the results of the study of conformational states of triosmium clusters with organic ligands. Special attention is paid to the structural features of molecular carbonyl cluster complexes with M _n (CO) _m L organic ligands. The effect of the structure surface charge, solvent, cluster crystalline state, and intermolecular and intramolecular interactions on the cluster conformational state is also determined.     

Theoretical study of the conformational energy hypersurface of cyclotrisarcosyl

The multidimensional Conformational Potential Energy Hypersurface (PEHS) of cyclotrisarcosyl was comprehensively investigated at the DFT (B3LYP/6-31G(d), B3LYP/6-31G(d,p) and B3LYP/6-311++G(d,p)), levels of theory. The equilibrium structures, their relative stability, and the Transition State (TS) structures involved in the conformational interconversion pathways were analyzed. Aug-cc-pVTZ//B3LYP/6-311++G(d,p) and MP2/6-31G(d)//B3LYP/6-311++G(d,p) single point calculations predict a symmetric cis-cis-cis crown conformation as the energetically preferred form for this compound, which is in agreement with the experimental data. The conformational interconversion between the global minimum and the twist form requires 20.88 kcal mol-1 at the MP2/6-31G(d)//B3LYP/6-311++G(d,p) level of theory. Our results allow us to form a concise idea about the internal intricacies of the PEHSs of this cyclic tripeptide, describing the conformations as well as the conformational interconversion processes in this hypersurface. In addition, a comparative analysis between the conformational behaviors of cyclotrisarcosyl with that previously reported for cyclotriglycine was carried out     

Theoretical and experimental vibrational spectroscopy study of K3Ln(PO4)2 phosphates

The monoclinic phosphates with K3Ln(PO4)2 (Ln=rare earth) formula were synthesized. Their infrared and Raman spectra have been reported and analysed. The results of a force field calculation for K3Nd(PO4)2 are presented.     

Time-resolved infrared spectroscopy of K3Ta3B2O12 photocatalysts for water splitting

Electrons photoexcited in K(3)Ta(3)B(2)O(12), an efficient photocatalyst for the water-splitting reaction driven by ultraviolet light, were observed using time-resolved IR absorption spectroscopy with microsecond resolution. When the catalyst was irradiated with 266 nm light pulses, a structureless absorption appeared at 3000-1500 cm(-1). The absorption was assigned to the optical transition of electrons that were band gap-excited and then trapped in mid-gap states. The absorbance decayed with a time delay because of the electron-hole recombination. The rate of recombination in an argon atmosphere was sensitive to the composition of the starting material used in the catalyst preparation. The electron decay was accelerated by exposing the catalyst to water vapor. The degree of acceleration was qualitatively correlated with the H(2) production rate observed during steady-state light irradiation.