Topic
Conformational isomerism
About: Conformational isomerism is a research topic. Over the lifetime, 11563 publications have been published within this topic receiving 199312 citations.
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TL;DR: The results clearly support the conclusion drawn by Compagnon et al. that only one dominant isomer existed in the molecular beam at 85 K and add further evidence that the supersonic jet expansion or embedding helium droplets did not produce an equilibrium distribution.
Abstract: A systematic and extensive conformational search has been performed to characterize the gas-phase tryptophan structures. A total of 648 unique trial structures were generated by allowing for all combinations of internal single-bond rotamers and were optimized at the DFT/B3LYP/6-311G* level of theory. A total of 45 local minima conformers were found. Further optimization of the 45 conformers with B3LYP and MP2/6-311++G** did not produce meaningful structural change, and accurate geometries, dipole moments, rotational constants, harmonic frequencies, and relative energies were then determined. Combined with statistical mechanics principles, the conformational distributions of gas-phase tryptophan at different temperatures are shown. The results clearly support the conclusion drawn by Compagnon et al. that only one dominant isomer existed in the molecular beam at 85 K and add further evidence that the supersonic jet expansion or embedding helium droplets did not produce an equilibrium distribution.
69 citations
TL;DR: (F3C)F2SiONMe2 contains thus a nonclassical Si...N bond, and its properties can be regarded as a new model for the explanation of the old postulate of an alpha-effect in silicon chemistry, explaining the behavior of compounds with geminal Si and N atoms.
Abstract: (F3C)F2SiONMe2 was prepared from LiONMe2 and F3CSiF3. It was characterized by gas IR and multinuclear solution NMR spectroscopy and by mass spectrometry. Its structure was elucidated by single crystal X-ray crystallography and by gas electron diffraction. (It exists as a conformer mixture.) Important findings were extremely acute SiON angles [solid 74.1(1)°, gas anti 84.4(32)° and gauche 87.8(20)°] and short Si···N distances [solid 1.904(2) A]. The bending potential of the SiON unit was calculated at the MP2/6-311++G(3df,2dp) level of theory and appears very flat and highly asymmetric. The calculated atomic charges (NPA) are counterintuitive to the expected behavior for a classical Si−N dative bond, as upon formation of the Si···N bond electron density is transferred mainly from oxygen to nitrogen, while the silicon charge is almost unaffected. Despite the molecular topology of a three-membered ring, the topology of the electron density shows neither a bond critical point between Si and N atoms nor a ring...
69 citations
TL;DR: The single-conformation spectroscopy of two model γ-peptides has been studied under jet-cooled conditions in the gas phase, finding the structure is a natural extension of amide stacking, with the two terminal amides blocked from forming the amide tristack by formation of the C14 H-bond.
Abstract: The single-conformation spectroscopy of two model γ-peptides has been studied under jet-cooled conditions in the gas phase. The methyl-capped triamides, Ac-γ2-hPhe-γ2-hAla-NHMe and Ac-γ2-hAla-γ2-hPhe-NHMe, were probed by resonant two-photon ionization (R2PI) and resonant ion-dip infrared (RIDIR) spectroscopies. Four conformers of Ac-γ2-hPhe-γ2-hAla-NHMe and three of Ac-γ2-hAla-γ2-hPhe-NHMe were observed and spectroscopically interrogated. On the basis of comparison with the predictions of density functional theory calculations employing a dispersion-corrected functional (ωB97X-D/6-311++G(d,p)), all seven conformers have been assigned to particular conformational families. The preference for formation of nine-membered rings (C9) observed in a previous study [James, W. H., III et al., J. Am. Chem. Soc. 2009, 131, 14243] of the smaller analog, Ac-γ2-hPhe-NHMe, carries over to these triamides, with four of the seven conformers forming C9/C9 sequential double-ring structures, and one conformer a C9/C14 bifurca...
69 citations
TL;DR: In this paper, the conformation of metal derivatives of 2,3,7,8,12,13,17,18-octaethylporphyrin and 5-nitro were investigated using resonance Raman spectroscopy.
Abstract: The planar and nonplanar conformers of metal derivatives of 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) and 5-nitro-2,3,7,8,12,13,17,18-octaethylporphyrin (NO[sub 2]-OEP) are investigated using resonance Raman spectroscopy. The structural heterogeneity is assessed by analysis of the line shapes of the structure-sensitive Raman lines. First, heterogeneity in the conformation of the macrocycle has been detected in solutions of the nickel and cobalt derivatives of OEP, that is, both planar and nonplanar conformers are found to coexist at room temperature for these metal porphyrins but not for the Cu and Zn derivatives. The latter metals expand the porphyrin core, shifting the equilibrium entirely to the planar conformer. Second, we find that substitution with a single NO[sub 2] group at one of the methine-bridge carbons shifts this planar-nonplanar equilibrium substantially toward the nonplanar conformer. Thus, both crowding of the peripheral substituents and contracting of the porphyrin core (Ni(II) < Co(II) < Cu(II) < Zn(II)) displace the equilibrium toward the nonplanar conformer. Finally, the frequencies of several Raman lines correlate with structural parameters such as core size (obtained either from molecular mechanics calculations or from X-ray crystallographic studies). 30 refs., 6 figs., 3 tabs.
68 citations
TL;DR: It is shown that the right-handed single helix of the trimer occurs in two distinct forms: the major one corresponds to a B-DNA-like structure, S-S-S; the minor one has the sugar rings in a hitherto unsuspected S- S-N conformational sequence.
Abstract: Proton NMR studies at 360 MHz were carried out on the trinucleoside diphosphate dApdApdA, the dinucleoside monophosphate dApdA and the methylphosphate esters of the monomers pdA, pdAp and dAp. The compounds were also investigated by means of circular dichroism at varying temperatures. Complete NMR spectral assignments are given. The thermodynamics of stacking derived from the circular dichroic spectra was used to extrapolate the observed coupling constants, measured at a range of temperatures, to values appropriate to the pure stacked states of the dimer and the trimer. The data were interpreted in terms of the N and S deoxyribose pseudorotational ranges [Altona, C. and Sundaralingam, M. (1972) J. Am. Chem. Soc. 94, 8205--8212] and it is shown that the right-handed single helix of the trimer occurs in two distinct forms: the major one corresponds to a B-DNA-like structure, S-S-S; the minor one (30%) has the sugar rings in a hitherto unsuspected S-S-N conformational sequence. The dimer behaves quite similarly, a mixture of 70% S-S and 30% S-N stacks being indicated. The detailed geometry of teh S-type sugar rings is not invariable but appears to undergo a slight shift when another base stacks on top of a given nucleotide unit. The backbone angles of the fully stacked B-DNA-like single helix in solution are (starting the notation at P) beta = 187 degrees, gamma = 50 degrees, delta = 138 degrees, epsilon = 186 degrees.
68 citations