Search results for "Intramolecular force"
showing 10 items of 599 documents
From CO2 to dimethyl carbonate with dialkyldimethoxystannanes: the key role of monomeric species.
2011
International audience; The formation of dimethyl carbonate (DMC) from CO(2) and methanol with the dimer [n-Bu(2)Sn(OCH(3))(2)](2) was investigated by experimental kinetics in support of DFT calculations. Under the reaction conditions (357-423 K, 10-20 MPa), identical initial rates are observed with three different reacting mixtures, CO(2)/toluene, supercritical CO(2), and CO(2)/methanol, and are consistent with the formation of monomeric di-n-butyltin(iv) species. An intramolecular mechanism is, therefore, proposed with an Arrhenius activation energy amounting to 104 ± 10 kJ mol(-1) for DMC synthesis. DFT calculations on the [(CH(3))(2)Sn(OCH(3))(2)](2)/CO(2) system show that the exothermi…
Experimental and DFT Studies on Competitive Heterocyclic Rearrangements. Part 2:1 A One-Atom Side-Chain versus the Classic Three-Atom Side-Chain (Bou…
2007
The experimental investigation of the base-catalyzed rearrangements of 3-acylamino-1,2,4-oxadiazoles evidenced a new reaction pathway which competes with the well-known ring-degenerate Boulton-Katritzky rearrangement (BKR). The new reaction consists of a one-atom side-chain rearrangement that is base-activated, occurs at a higher temperature than the BKR, and irreversibly leads to the corresponding 2-acylamino-1,3,4-oxadiazoles. An extensive DFT study is reported to elucidate the proposed reaction mechanism and to compare the three possible inherent routes: (i) the reversible three-atom side-chain ring-degenerate BKR, (ii) the ring contraction-ring expansion route (RCRE), and (iii) the one-…
The Reaction Mechanism of Spirocylization and Stereoselectivity Studies for the Calyculin C16 -C25 Fragment
2005
The mechanism of the double intramolecular hetero-Michael addition, a key reaction in the planned synthesis of the natural product calyculin C, has been studied by NMR. The cyclization follows Baldwin’s rules and proceeds first through a six-membered ring closure (6-endo-dig), followed by a five-membered ring cyclization (5-exo-trig). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Kinetik der Bromierung von Phenolen und phenolischen Mehrkernverbindungen, 5. Mitt.
1984
The kinetics of the bromination of six differently substituted 2,6-bis(hydroxybenzyl)phenols having only one reactivepara position at the phenolic unit in the middle of the molecule were studied in acetic acid at 22°C. The reaction rate decreases if intramolecular hydrogen bonds between one or two hydroxy groups of the adjacent phenolic units and the hydroxy group of the reacting unit become possible, and it is especially low, if these hydrogen bonds are directed to the middle by bulky substituents inortho position. This must be explained by a smaller +M-effect of the hydroxy group of the reacting unit. A kinetic isotope effect is observed in deutero acetic acid, where the reaction rate is …
Dynamics of hyaluronan aqueous solutions as assessed by fast field cycling NMR relaxometry.
2010
Fast field cycling (FFC) NMR relaxometry has been used to study the conformational properties of aqueous solutions of hyaluronan (HYA) at three concentrations in the range 10 to 25 mg mL(-1). Results revealed that, irrespective of the solution concentration, three different hydration layers surround hyaluronan. The inner layer consists of water molecules strongly retained in the proximity of the HYA surface. Because of their strong interactions with HYA, water molecules in this inner hydration layer are subject to very slow dynamics and have the largest correlation times. The other two hydration layers are made of water molecules which are located progressively further from the HYA surface.…
Mononuclear Fe(III) and tetranuclear [Fe(III)Gd(III)]2 complexes with a Schiff-base ligand derived from the o-vanillin: Synthesis, crystal structures…
2011
The mononuclear high-spin iron(III) complexes [Fe(3-MeOsalpn)Cl(H 2 O)] ( 1 ) and [Fe(3-MeOsalpn)(NCS)(H 2 O)]·0.5CH 3 CN ( 2 ) and the tetranuclear oxo-bridged compound [{Fe(3-MeOsalpn)Gd(NO 3 ) 3 } 2 (μ-O)]·CH 3 CN ( 3 ) [3-MeOsalpn 2− = N , N ′-propylenebis(3-methoxysalicylideneiminate)] have been prepared and magneto-structurally characterised. The iron(III) ion in 1 and 2 is six-coordinated in a somewhat distorted octahedral surrounding with the two phenolate-oxygens and two imine-nitrogens from the Schiff-base building the equatorial plane and a water ( 1 and 2 ) and a chloro ( 1 )/thiocyanate-nitrogen ( 2 ) in the axial positions. The neutral mononuclear units of 1 and 2 are assemb…
C3-symmetric trinuclear copper(ii) species as tectons in crystal engineering
2013
Three new complexes have been obtained using C3-symmetric trinuclear complexes as tectons; [Cu3(felden)(NCS)3(dmf)3] (1), [Cu3(felden)(mand)3]·(C2H5)2O (2), and [Cu3(felden)(dca)3(C2H5OH)]·2H2O (3) (H3felden is the Schiff base resulting from the condensation of 2,4,6-triformylphloroglucinol with N,N-dimethylethylenediamine, mand− is the anion of the R-mandelic acid and dca− is the dicyanamide anion). Compounds 1 and 2 are discrete trinuclear species, while compound 3 is a 2D coordination polymer, constructed from trinuclear nodes and dicyanamido spacers. The variable-temperature magnetic properties of 1–3 have been investigated and they reveal the occurrence of weak antiferromagnetic intera…
The influence of solvent on conformational properties of peptides with Aib residue—a DFT study
2017
The conformational propensities of the Aib residue on the example of two model peptides Ac-Aib-NHMe (1) and Ac-Aib-NMe2 (2), were studied by B3LYP and M06-2X functionals, in the gas phase and in the polar solvents. To verify the reliability of selected functionals, we also performed MP2 calculations for the tested molecules in vacuum. Polarizable continuum models (PCM and SMD) were used to estimate the solvent effect. Ramachandran maps were calculated to find all energy minima. Noncovalent intramolecular interactions due to hydrogen-bonds and dipole attractions between carbonyl groups are responsible for the relative stabilities of the conformers. In order to verify the theoretical results,…
Structural study of oxalamide compounds: 1H, 13C, and DFT calculations
2011
Abstract The conformational properties of some N-alkyl, N,N′-dialky, and tetraalkyloxalamides have been investigated, in vacuo and in solvent using DFT methods at the B3LYP/6-31G∗∗ computational level. Special emphasis has been given on oxalamides with substituents of the type –CH2CH2OH. In oxalamides with the N–H group (N-alkyl and N,N′-dialky), the most stable conformations are those in which the oxalamide moiety adopts a planar s-trans arrangement and the amide bonds are trans. A different situation appears in the case of tetraalkyloxalamides, in which the oxalamide moiety always adopts a skewed arrangement and there are conformations with similar energy. A careful study of 13C and 1H NM…
Intramolecular Dipole Reorientation of Amino Compounds and their Interaction with Solvent Molecules. I
1972
Abstract The dielectric absorption of dilute solutions of aniline, methyl substituted anilines, chlorine substituted anilines, N,N,N',N'-tetramethyl-p-phenylene diamine and 4-aminobiphenyl in various sol-vents has been measured between 0.3 and 135 GHz. The measurements have been carried out at 20 °C and for some mesitylene solutions also at -30 °C and 60 °C. The absorption curves have been resolved into multiple absorption regions. A far infrared (FIR) term had to be included in each analysis. Its contribution to the total dipole reorientation depends on the solvent and on the mobility of the substituted groups in the phenyl ring. The obtained group relaxation times are longer than those re…