Relative Populations Of Conformers Research Articles

Conformational analysis of jet cooled tryptophan analogs by molecular mechanics: Comparison with experiment

The possibility of predicting the geometries of jet cooled conformers of tryptophan analogs via molecular mechanics, employing MM2 (1987), was studied by carrying out computations on six different analog molecules. For three of the analogs, tryptamine in particular, the computationally predicted gas phase conformer geometries could be compared with experimentally determined geometries obtained from jet cooled samples. Some conformer geometries agreed qualitatively with experiment but the match did not extend to all cases. Most notably, the MM2 calculations failed to predict the eclipsed conformers of tryptamine that were deduced experimentally via analysis of rotational constants. They similarly failed to predict a planar geometry conformation found for 3-indole acetic acid. These two disagreements prompted a reconsideration of the possible geometries that yield the best match to the experimentally determined rotational constants for various jet cooled conformers. For tryptamine conformers D/E it was found that likely geometries might in fact be more than 30° from eclipsed structures. Some differences were also found for the Cα–Cβ and Cβ–Cγ dihedral angle values assigned to tryptamine conformers A/B and conformer F. Nonetheless, the overall agreement of the MM2 calculations with the refined tryptamine conformer geometries remains no more than qualitative. Reanalysis of the possible geometries of conformer A of 3-indole acetic acid that are consistent with the experimental rotational constants yielded agreement with the previous planar geometry assignment. It was also possible to use the molecular mechanics data to predict relative conformer populations and from this relative peak sizes of 0–0 transitions in the jet cooled excitation spectra. Considering all six cases, the match with experiment is again at best partial. The agreement is not good enough for reliable association of definite computed geometries with particular peaks in an experimental excitation spectrum.

Synthesis and characterization of model compounds for carbazole‐substituted N‐acylated linear polyethylenimine (PEI). Synthesis and 1H‐NMR spectra of N‐acylated diethylamine and N,N′‐dimethyl‐1,2‐ethanediamine containing 9‐carbazolylacetyl, (R,S)‐ or (R)‐2‐(9‐carbazolyl)propanoyl groups

AbstractThe synthesis of N,N‐diethyl‐9‐carbazolylacetamide (6), (R,S)‐ and (R)‐N,N‐diethyl‐2‐(9‐carbazolyl)propanamide (7), N,N′‐dimethyl‐N,N′‐di‐(9‐carbazolylacetyl)‐1,2‐ethanediamine (11), and (R)‐N,N′‐dimethyl‐N,N′‐di[2‐(9‐carbazolyl)propanoyl]‐1,2‐ethanediamine (13) is reported. The racemic compound, (R,S)‐2‐(9‐carbazolyl)propanoic acid (2), was resolved by partial crystallization of the diastereomeric salts formed between 2 and (+)‐α‐methylbenzylamine. The 1H‐NMR spectra of 6 and 7 showed magnetic nonequivalence of the chemically equivalent protons of the methyl and methylene groups in 6 and 7 due to partial double bond character of the amide bond. The upfield resonances corresponding to the two sets of methyl and methylene protons were assigned by the aromatic solvent‐induced shift (ASIS) method to the protons anti to the carbonyl oxygen in the conformation of amide bond in 6 and 7. The 1H‐NMR spectra of 11 and (R)‐13 were used to determine the population of anti‐anti, anti‐syn (syn‐anti) and syn‐syn conformers in the structures of these dimer model compounds; the relative conformer populations were 0.45:0.47:0.08 and 0.28±0.02:0.29±0.01:0.43±0.01 in 11 and (R)‐13.

Conformational analysis of organic carbonyl compounds. Part 4. A 1H and 13C nuclear magnetic resonance study of formyl and acetyl derivatives of benzo[b]furan

A conformational study of formyl and acetyl derivatives of benzo[b] furan provides evidence that in the case of 2- and 7-derivatives the E,Z-conformational mixture is solvent dependent, the Z-form prevailing in solvents with higher polarity. The presence of the same substituents in the 3- and 4-position gives compounds with predominantly Z-conformation and no change is caused by solvents. These results show an interestingly different behaviour by carbonyl derivatives of benzo[b] furan with respect to the same benzo[b] thiophene derivatives. The conformational analysis was carried out by the n.m.r. method by employing 1H and 13C chemical shifts, long-range proton–proton coupling constants, and lanthanide-induced shifts (LIS) simulation. For the derivatives substituted in position 7, the LIS method is not a suitable approach for the determination of conformer populations since, in solution, a chelate is formed where bonding of Eu to the oxygen atoms of the carbonyl and of the heterocyclic ring stabilizes the Z-form. A quantitative approach for determining the relative conformer populations of benzo[b] furan-7-carbaldehyde was set up by statistical manipulation of long-range proton–proton coupling constants, proton chemical shifts, and the classical theory of dielectrics.

Freezing of conformational isomer distribution in a supersonic jet detected by matrix infrared spectroscopy

Relative conformer populations in supersonic molecular beams have been studied by matrix IR spectroscopy. The supersonic expansion of 1,2-Difluoroethane was found to induce a conformer conversion (transgauche) , but the corresponding effect was not found to occur in the case of methyl nitrite. A reasonable explanation lies in the fact that the potential barrier hindering internal rotation is much higher in methyl nitrite than in 1,2-difluoroethane.

Phytochrome Models. IV. Conformational Heterogeneity and Photochemical Changes of Biliverdin Dimethyl Esters in Solution

AbstractSolutions of biliverdin dimethyl ester (I) and its monoprotonated form (IH+) exhibit a solvatochromic and thermochromic behaviour. Based on absorption, fluorescence and fluorescence excitation, and solvent‐induced circular dichroism (SICD) data, these properties are interpreted in terms of variations of the relative populations of conformers (including readily interconvertible configurational isomers). Thus, folded helical forms of I and IH+ are more abundant at room temperature in, e.g., ethanol than in ethyl lactate, and stretched conformations other than the already known stable (E, Z, Z) and (Z, Z, E) photoisomers are quite generally favoured with increasing temperature. Irradiation of I in ethanol at 173 K increases the population with stretched, thermally equilibrating conformers. IH+ in ethanol undergoes an irreversible photochemical change at room temperature or below. Its nature has not yet been identified.

Studies in nuclear magnetic resonance spectroscopy. Part VIII. Conformations and coupling constants in 1,2-disubstituted 3,3-dimethylbutanes

The 100 MHz spectra of a series of 1,2-disubstituted 3,3-dimethylbutanes have been analyzed exactly as ABC systems by use of iterative computer techniques. The chemical shifts are shown to agree reasonably well with those predicted from systems with similarly oriented substituents. The gauche(Jg) and trans(Jt) coupling constants were shown to be of the same sign and opposite in sign to Jgem. The values of the vicinal coupling constants and their relative insensitivity to concentration change suggest that these compounds exist predominantly as single conformers, viz. with gauche oriented 1,2-substituents. Neither Jg nor Jt correlates with the electronegativities or sizes of the substituents. Small differences observed in these parameters in this series of compounds may therefore reflect different relative populations of conformers.