Search results for "Intramolecular force"
showing 10 items of 599 documents
N‐[tert‐Butoxycarbonylglycyl‐(Z)‐α,β‐dehydrophenylalanylglycyl‐(E)‐α,β‐dehydrophenylalanyl]glycine methyl ester dihydrate
2006
The title pentapeptide, Boc0—Gly1–ΔZPhe2—Gly3–ΔEPhe4—Gly5—OMe, C30H35N5O8·2H2O, adopts the type I β-turn conformation for the ΔZPhe2—Gly3 residues. It is stabilized by a 4\rightarrow1 intramolecular hydrogen bond between the ΔEPhe4 NH and Gly1 CO groups. All the amino acid residues in the pentapeptide sequence are linked trans to each other. The crystal structure is stabilized by intra- and intermolecular hydrogen bonds.
The acidity of calix[5]arenes and their linear analogues
1999
Abstract Five new calix[5]arenes containing a single p-nitrophenol unit as the most acidic phenolic unit have been synthesised by (3+2) fragment condensation of a trimer with a p-nitrophenol in the middle with various bishydroxymethylated alkanediyl diphenols. Their first acid constant (pKal) has been determined in 2-methoxyethanol/water (9:1) at 22°C by optical titration. The pKal values are distinctly lower (ΔpKa > 2) than for the trimer, while no difference has been found in comparison to structurally analogous calix[4]arenes. Rigidification of the calix[5]arene skeleton by introducing a single alkanediyl bridge opposite to the p-nitrophenol unit has no effect on pKal. All values are in …
Timolol derivatives. I. X-ray, NMR and theoretical studies of the crystallization of (S)-timolol O,O-diacetyl-l-tartaric acid monoester
1993
Abstract The absolute configurations of (S)-timolol hemihydrate and (S)-timolol O,O-diacetyl-(R,R)-tartaric acid monoester were determined by single crystal X-ray diffraction. An NMR analysis based on the temperature dependence of vicinal coupling constants was carried out to characterize the conformational behaviour of the S,R,R- and R,R,R-forms in solution. The same conformation as in crystalline state was also found in solution, although with a rather low preference over some other conformations. Results of theoretical calculations using MNDO and AMBER force field methods are reported. An infinite chain of hydrogen bonds, along with other favourable inter- and intramolecular forces that …
The Nature of Transannular Interactions in E4N4 and E82+ (E = S, Se)
2015
The electronic structures of tetrachalcogen tetranitrides, E4N4, and octachalcogen dications, E8(2+), and the nature of their intramolecular E···E interactions (E = S, Se) was studied with high-level theoretical methods. The results reveal that the singlet ground states of both systems have a surprisingly large correlation contribution which functions to weaken and therefore lengthen the cross-ring E-E bond. The observed correlation effects are primarily static in E4N4, whereas in E8(2+) the dynamic part largely governs the total correlation contribution. The presented description of bonding is the first that gives an all-inclusive picture of the origin of cross-ring interactions in E4N4 an…
Structural, magnetic and calorimetric studies of a crystalline phase of the spin crossover compound [Fe(tzpy)2(NCSe)2]
2013
The compound [Fe(tzpy)2(NCSe)2] (tzpy = 3-(2-pyridyl)-[1,2,3]triazolo[1,5-a]pyridine)) has been synthesized and its crystal structure, magnetic behavior and calorimetric properties investigated. Samples constituted of single crystals of [Fe(tzpy)2(NCSe) 2] display a relatively cooperative spin-state change centered at T1/2 ¿ 251.7 K with a hysteresis loop 3.5 K wide. The average enthalpy (¿H) and entropy (¿S) changes upon the spin crossover behavior (SCO) obtained from DSC measurements are 11.1 ± 0.4 kJ mol -1 and 44.5 ± 3 J K-1 mol-1, respectively. The magnetic and calorimetric data have been satisfactorily simulated using the mean-field regular solution model (Slichter-Drickamer) and the …
Theoretical Study of the Twisted Intramolecular Charge Transfer in 1-Phenylpyrrole
2000
Ab initio results for the electronic spectra of 1-phenylpyrrole are presented. Vertical, emission, and nonvertical excitation energies have been computed using multiconfigurational second-order per...
ChemInform Abstract: Microwave-Assisted Organocatalytic Enantioselective Intramolecular Aza-Michael Reaction with α,β-Unsaturated Ketones.
2012
The combination of an epi-hydroquinine catalyst and pentafluoropropionic acid accelerates the process.
Cyclizations via Frustrated Lewis Pairs: Lewis Acid Induced Intramolecular Additions of Amines to Olefins and Alkynes
2010
ChemInform Abstract: Intramolecular Michael Reaction of tert-Butylsulfinyl Ketimines: Asymmetric Synthesis of 3-Substituted Indanones.
2012
The reaction proceeds in the presence of the Ruppert—Prakash reagent (Tms-CF3) with either TBAT or TBAF as a base.
Do the Intramolecular π Interactions Improve the Stability of Ionic, Pyridine-Carbene-Based Iridium(III) Complexes?
2013
Throughout the last years one of the most intensive research topics in light-emitting electrochemical cells (LECs) focused on the design of blue-emitting, ionic iridium(III) complexes. To this end, the most recent strategy is the use of carbene-based ancillary ligands. Although blue LECs have been successfully fabricated, the stability has been noted as the main drawback. To overcome this problem, Zhang et al. have recently explored the use of π interactions to enhance the strength of pyridine-carbene-based complexes. The authors suggested that the use of intramolecular π–π stacking interactions by means of pendant phenyl rings to improve the stability of LECs is not as effective as in devi…