Conformational isomerism

About: Conformational isomerism is a research topic. Over the lifetime, 11563 publications have been published within this topic receiving 199312 citations.

Control of intramolecular orbital alignment in the photodissociation of thiophenol: conformational manipulation by chemical substitution.

Abstract: As scientists understand chemical reactions in atomic detail, many successful examples of reaction control have been reported in recent decades. These include active control using pulse shaping in terms of chirps or phases or passive control, such as vibration-mediated photodissociation. Apart from these studies, an interesting structure-based approach relying on conformational specificity has recently been introduced as an alternative tool for reaction control. In this approach, a particular conformational isomer is exclusively chosen, and its unique chemical reactivity is thoroughly examined. For instance, in the photodissociation of the cation of 1-iodopropane, a gauche or trans conformational isomer was selectively prepared by the mass-analyzed threshold ionization (MATI) technique, and it was found that different conformational isomers give rise to totally different reaction products. Another example of conformer-specific reaction control has been demonstrated by Suits and co-workers in the photodissociation of the gauche or cis conformational isomers of the cation of 1-propanal. In the latter case, one of two reaction channels proceeding in the ground state is preferentially controlled by conformational selection in the excited state. More recently, alanine and b-alanine, which are biological building blocks, were also found to be quite conformer-specific in their decarboxylation reactions upon ionization. The relationship between structure and chemical reactivity is essential for the understanding and control of chemical reactions, and therefore the study of the conformational specificity at the atomic level seems to be invaluable at the present time. Herein, we use the conformational change upon chemical substitution to control the branching ratio of two reaction channels in the photodissocation of thiophenol and its derivatives. It has been found that, as a result of S H(D) dissociation, the phenylthiyl radical is produced with its reactive singly occupied molecular orbital (SOMO) either perpendicular (X̃) or parallel (ffi) to the molecular plane. This so-called “intramolecular orbital alignment” is a novel concept in stereodynamics that could be developed into a useful tool in stereochemistry. Accordingly, control of the branching ratio naturally means that the intramolecular orbital alignment in the reactive phenylthiyl radical could be controlled by manipulation of the photodissociation dynamics. [D]Thiophenol (C6H5SD) is excited to the S2 state via the (np,s*) transition at a pump energy of 243 nm, with subsequent prompt S D dissociation, the time scale of which is less than 100 fs. Since the repulsive S2 state belongs to A’’ whereas the ground S0 state belongs to A’, the diabatic potential-energy surfaces of S2 and S0 cross along the S D elongation coordinate, as S2 and S0 diabatically correlate to the X̃ and ffi states of phenylthiyl (C6H5S), respectively. [18] This dissociation may be qualitatively described as the dynamics of a simple wavepacket on potential-energy surfaces (PES) of reduced dimensionality. In this picture, the bifurcation of the wavepacket into two different ffi and X̃ states of phenylthiyl takes place at the conical intersection (CI), which is formed at the planar geometry for which the plane of symmetry is maintained along the reaction coordinate (Figure 1).

Conformational Complexity of Succinic Acid and Its Monoanion in the Gas Phase and in Solution: Ab Initio Calculations and Monte Carlo Simulations

Abstract: Optimized structures and relative energies for conformers of succinic acid and its monoanion in the gas phase were obtained using ab initio molecular orbital calculations at the MP2/6-311+G^(**)//HF/6-31G^* and MP2/6-311+G^(**)//HF/6-31+G^* levels, respectively. The lowest energy conformer for succinic acid, designated ZsgsZ, has a gauche conformation about the central C2−C3 bond; the lowest energy conformer with an E-acid group and an internal hydrogen bond is ca. 3 kcal/mol higher in energy. The lowest energy structure for the monoanion, Ecgs, does have the expected internal hydrogen bond and is 15 kcal/mol more stable than any alternative. The ab initio results were used to determine corresponding torsional-energy parameters in the OPLS all-atom force field. This allowed application of statistical perturbation theory in Monte Carlo simulations to explore the effect of hydration on the conformational equilibria. The diacid and monoanion were both computed to be ca. 80% gauche in water at 25 °C. These results are in excellent agreement with NMR data. Though the conformational results are consistent with the gauche effect, their true origin requires a detailed understanding of the potential internal hydrogen bonding and solvation. Thus, in contrast to the monoanion's striking gas-phase preference, neither the diacid nor monoanion are computed to populate E conformers in aqueous solution.

13C NMR spectroscopy of the withanolides and other highly oxygenated C28 steroids

Abstract: The 13C NMR spectra of 29 highly oxygenated C, steroids with the withanolide skeleton, 24 of which are naturally occurring substances or acetates thereof, were recorded and the carbon signals fully assigned. The shifts of the heavily functionalized A/B ring system can serve as a ‘fingerprint’ for the different substitution patterns. A study of the effects of hydroxyl substitution in the ring D/side chain part of the molecules leads to conformational information, including preferred rotamers around the 17—20 and 20—22 bonds. Systems with both 17α- and 17β-oriented side chains are thus analysed.

Conformational analysis of canonical 2-deoxyribonucleotides. 2. Purine nucleotides.

Abstract: The molecular structure of different conformers of isolated canonical purine 2'-deoxyribonucleotides 2-deoxyadenosine-5'-phosphate (pdA) and 2'-deoxyguanosine-5'-phosphate (pdG) was optimized using the B3LYP/6-31G(d) method. The results of the calculations reveal that the geometrical parameters and relative stability of the conformers significantly depend on the nature of the nucleobase, its orientation, the conformation of the furanose ring, the charge of the phosphate group and the character of the intramolecular hydrogen bonds. Analysis of the electron density distribution in purine nucleotides reveals the existence of a number of intramolecular hydrogen bonds. In general, the south conformer has a lower energy at the anti orientation of the base, and both conformers occur as the most stable for the syn orientation of the nucleobases. The results of the calculations reveal that the geometry and relative energy of the conformers of purine DNTs may be easily tuned by the charge of the phosphate group.