Structural studies of 4-aryloctahydro-pyrido[1,2-c]pyrimidine derivatives

¹³C cross-polarisation magic angle spinning NMR data have been reported for four derivatives of 4-aryl-octahydro-pyrido[1,2-c]pyrimidine-1,3-dione and the X-ray diffraction data for two (with 2′-Me and 2′-OMe). The crystal structures show the presence of centrosymmetric cyclic dimers with intermolecular C1O?H-N or C3O?H-N hydrogen bonds, the configuration at the chiral centres (C4 and C4_a) was determined as RR (SS). The twisting of aromatic ring at C4 with respect to the pyrido[1,2-c]pyrimidine skeleton is about 68-109°.     

Analysis of curing reaction of liquid-crystalline epoxy compositions by using temperature-modulated DSC TOPEM®

Journal of Thermal Analysis and Calorimetry - The curing behaviour of the composition of a liquid-crystalline diepoxy monomer (LCEM) with the central triaromatic mesogenic group was studied using...     

Impact of the ΔPhe configuration on the Boc-Gly-ΔPhe-NHMe conformation: experiment and theory

Conformational propensities of N-t-butoxycarbonyl-glycine-(E/Z)-dehydrophenylalanine N′-methylamides (Boc-Gly-(E/Z)-ΔPhe-NHMe) in chloroform were investigated by NMR and IR techniques. The low-temperature crystal structure of the E isomer was determined by single crystal X-ray diffraction and the experimental data were elaborated by theoretical calculations using DFT (B3LYP, M06-2X) and MP2 approaches. The β-turn tendencies for both isomers were determined in the gas phase and in the presence of solvent. The obtained results reveal that the configuration of ΔPhe residue significantly affects the conformations of the studied dehydropeptides. The tendency to adopt β-turn conformations is significantly lower for the E isomer (Boc-Gly-(E)-ΔPhe-NHMe), both in gas phase and in chloroform solution.

     

100 Picosecond Diffraction Catches Structural Transients of Laser‐Pulse Triggered Switching in a Spin‐Crossover Crystal

We study by 100 picosecond X‐ray diffraction the photo‐switching dynamics of single crystal of the orthorhombic polymorph of the spin‐crossover complex [(TPA)Fe(TCC)]PF₆, in which TPA=tris(2‐pyridyl methyl)amine, TCC^2?=3,4,5,6‐Cl₄‐Catecholate^2?. In the frame of the emerging field of dynamical structural science, this is made possible by using optical pump/X‐ray probe techniques, which allow following in real time structural reorganization at intra‐ and intermolecular levels associated with the change of spin state in the crystal. We use here the time structure of the synchrotron radiation generating 100 picosecond X‐ray pulses, coupled to 100 fs laser excitation. This study has revealed a rich variety of structural reorganizations, associated with the different steps of the dynamical process. Three consecutive regimes are evidenced in the time domain: 1) local molecular photo‐switching with structural reorganization at constant volume, 2) volume relaxation with inhomogeneous distribution of local temperatures, 3) homogenization of the crystal in the transient state 100 µs after laser excitation. These findings are fundamentally different from those of conventional diffraction studies of long‐lived photoinduced high spin states. The time‐resolution used here with picosecond X‐ray diffraction probes different physical quantities on their intrinsic time‐scale, shedding new light on the successive processes driving macroscopic switching in a functionalized material. These results pave the way for structural studies away from equilibrium and represent a first step toward femtosecond crystallography.     

Deuterium isotope effects on hydrophobic interactions: the importance of dispersion interactions in the hydrophobic phase

Hydrogen/deuterium isotope effects on hydrophobic binding were examined by means of reversed-phase chromatographic separation of protiated and deuterated isotopologue pairs for a set of 10 nonpolar and low-polarity compounds with 10 stationary phases having alkyl and aryl groups bonded to the silica surface. It was found that protiated compounds bind to nonpolar moieties attached to silica more strongly than deuterated ones, demonstrating that the CH/CD bonds of the solutes are weakened or have less restricted motions when bound in the stationary phase compared with the aqueous solvent (mobile phase). The interactions responsible for binding have been further characterized by studies of the effects of changes in mobile phase composition, temperature dependence of binding, and QSRR (quantitative structure-chromatographic retention relationship) analysis, demonstrating the importance of enthalpic effects in binding and differentiation between the isotopologues. To explain our results showing the active role of the hydrophobic (stationary) phase we propose a plausible model that includes specific contributions from aromatic edge-to-face attractive interactions and attractive interactions of aliphatic groups with the pi clouds of aromatic groups present as the solute or in the stationary phase.     

Alkoxy/siloxy group exchange in the system vinyltrialkoxysilane-iridium(I) siloxide complex

A study of reactions of dimeric siloxide iridium complex, [[(cod)Ir(mu-OSiMe3)]2] (1) with vinyltriethoxysilane and vinyltrimethoxysilane has revealed a new type of the reation--alkoxy group transfer from silicon to iridium with a simultaneous transfer of a siloxy group from iridium to silicon--as a result of which vinyldialkoxytrimethyldisiloxane and dimeric alkoxide iridium complex [[(cod)Ir(mu-OR)]2] (3) are formed. The structure of [[(cod)Ir(mu-OEt)]2] (3a) has been solved by X-ray diffraction.     

Microwave assisted sulfopropylation of N-heterocycles using 1,3-propane sultone

Dramatic rate acceleration was observed for N-sulfopropylation of heterocyclic compounds (1a-h) using 1,3-propane sultone(2) under microwave irradiation affording the N-sulfopropylated compounds (3a-h) in 68–95% yield.