Synthesis and ring-opening copolymerization of cyclic aryl ester dimers

Two kinds of cyclic aryl ester dimers have been synthesized by reaction of phthaloyl dichloride with bisphenols via interfacial polycondensation. The cyclic dimers readily undergo anionic ring-opening polymerization or copolymerization in the melt by using sodium benzoate as the initiator, producing linear, high molecular weight polyesters. The contents of cyclic dimers in the homopolymers P1, P2, and copolymer P12 are 13.7%, 10.2%, 2.9%, respectively, which indicates that ring-opening copolymerization of cyclic dimers may impel the conversion of cyclic dimers and decrease the content of cyclic dimers in the resulting copolymer. Moreover, the isothermal chemorheology of the ring-opening copolymerization of cyclic dimers indicates that the reactivemoltenmixture has low shear viscosity and the viscosity increases slowly in the initial stage of ring-opening polymerization.     

Twisted perylene stereodimers reveal chiral molecular assembly codes

Unique perylene diastereomeric linear and cyclic dimers were synthesized from twisted perylene monomers, revealing that pi-stacking stereoisomerism imparted specific intermolecular self-assembly and intramolecular folding. Only the homochiral twisted tetrachloroperylene monomers cyclized via a cooperative reaction, forming the homochiral diastereomers. The heterochiral tetrachloroperylene monomers proceeded through a stepwise reaction and yielded a linear heterochiral dimer, which equilibrated with the linear homochiral dimers. The linear homochiral dimers cyclized to produce the same cyclic homochiral diastereomers. These results demonstrated that homochiral and heterochiral self-assemblies were two distinct molecular codes, directing two specific chemical pathways. The homochiral cyclic dimers remain isomerically pure at -20 degrees C but can be interconverted to the heterochiral cyclic dimer meso compound at room temperature. The diastereomers were readily separated by HPLC. While driven by solvophobic forces, foldable linear dimers synthesized from the same twisted monomers using phosphoramidite chemistry folded into homodimer and heterodimer, confirming the inherent molecular codes, which were dictated by the perylene chirality, ultimately gauged the weak pi-stack forces, and directed self-assembly and folding.     

Modelling the vibrational behaviour of the cyclic carboxylic acid dimer. SQM force field of the formic acid dimer

The vibrational behaviour of the cyclic carboxylic acid dimer is modelled through the scaled quantum mechanical (SQM) force field of the cyclic formic acid dimer. The results indicate that the SQM force field technique is very well applicable to hydrogen bonded molecules. The frequency shifts observed on hydrogen bonding can be related to the shifts observed on lowering the temperature. This study also confirms that a clear distinction between cyclic carboxylic acid dimers and catamers can be made through the difference between infrared and Raman frequencies, and it is proved here that these conditions are also valid for weaker hydrogen bonded cyclic carboxylic acid dimers.     

Molecular orbital treatments of hydrogen bonded systems. 2. Dimers of water and HCN

Calculations were performed to investigate the reliabilities of the CNDO/2, PRDDO, and MNDO approximate molecular orbital methods. Systems selected for study included the linear, cyclic, and bifurcated dimers of water as well as the linear and cyclic dimers of HCN. The PRDDO method was found to provide the most consistently accurate reproduction of ab initio and experimental data. CNDO/2 performed fairly well in a number of cases but yielded extremely poor results for the cyclic dimers of both H₂O and HCN. Hydrogen bond strengths were consistently underestimated by MNDO which also furnished erroneously large intermolecular separations. In addition, MNDO calculations indicate the bifurcated water dimer to be most stable in contrast to other quantum mechanical and experimental information.See Ref. [1] for Paper 1 of this series.     

A matrix infrared study of association of formamide

The association of formamide has been studied in argon. In the spectral region from 800 to 200 cm⁻¹ several characteristic bands due to association were found and assigned to the out-of-plane bending modes of the NH group. The widths of these low-frequency bands are of the same order of magnitude as those of the fundamental bands of the monomer. By comparing the spectra of self-associated species with spectra of heteroassociated species between formamide and N,N-dimethyl formamide, it was possible to assign association bands to the open dimers as well as to the cyclic dimers. A band appearing upon warming at about 230 cm⁻¹ has been assigned to a H-bond stretching mode. The results indicate that dimers trapped from the gas phase are predominantly open, and that both open and cyclic structures are formed upon diffusion-controlled association in the matrix.     

Excited-state double proton transfer dynamics of model DNA base pairs: 7-hydroxyquinoline dimers

The excited-state double proton transfer of model DNA base pairs, 7-hydroxyquinoline dimers, in benzene has been investigated using picosecond time-resolved fluorescence spectroscopy. Upon excitation, whereas singly hydrogen-bonded noncyclic dimers do not go through tautomerization within the relaxation time of 1400 ps, doubly hydrogen-bonded cyclic dimers undergo excited-state double proton transfer on the time scale of 25 ps to form tautomeric dimers, which subsequently undergo a conformational change in 180 ps to produce singly hydrogen-bonded tautomers. The rate constant of the double proton transfer reaction is temperature-independent, showing a large kinetic isotope effect of 5.2, suggesting that the rate is governed mostly by tunneling.     

Synthesis of polyaza macrocycles by palladium-catalyzed amination of 1,2-dibromobenzene and 2-bromo-1,3-dichlorobenzene

Palladium-catalyzed amination of 1,2-dibromobenzene with equimolar amounts of linear polyamines leads to the formation of polyaza macrocycles in low yields. The use of 2-bromo-1,3-dichlorobenzene as starting compound ensures considerably larger yields of the target macrocycles, and the reaction is accompanied by side formation of N-aryl- and N,N′-diaryl-substituted polyamines, as well as of cyclic oligomers. The yields strongly depend on the polyamine chain length. Reactions of excess 2-bromo-1,3-dichlorobenzene with a series of polyamines gave the corresponding N,N′-diarylpolyamines in high yields, and the latter reacted with the second polyamine molecule to form cyclic dimers. A relation between the yield of cyclic dimers and polyamine structure was revealed.     

Cyclic and linear liquid crystalline functionalized polyesters with main-chain ortho-linked units. synthesis and characterization of cyclic lc unimers and dimers with “u”-shaped rigid mesogenic units with alkyl side chains

Although ortho-diphenols had not been extensively used in the synthesis of LC esters, a great variety of molecular structures of low and high molecular weight LC esters containing high proportions of these units can be synthesized. In this paper we describe the synthesis and characterization of new series of low and high molecular weight cyclic and linear LC esters with mesogenic “U”-shaped rigid units with terminal groups which are alkyl chains. Cyclic oligoesters and linear polyesters were formed by the polycondensation of 4,4'-[1,10-decamethylenebis(oxy)]bis(cinnamic acid) with monosubstituted catechols which are the alkyl esters of 3,4-dihydroxybenzoic acid. Although the great importance that concomitant cyclization reactions have in polyesterifications involving high proportions of ortho-diphenols does not seem to have been considered until now, we have found mat these polyesterifications produced linear polyesters along with high proportions of cyclic oligoesters even when reaction conditions disfavored cyclization. Copolymerization with p-hydroxybenzoic acid decreased the amount of cyclic oligomers, however it was necessary to copolymerize with proportions of PB higher than 50 mol-% to get copolyesters with low proportions of cyclic oligomers. As far as we know we describe the first examples of cyclic LC oligoesters and cyclic LC unimers and dimers which display enantiotropic LC mesophases stable over broad ranges of temperature. Cyclic dimers display mesophases whose isotropization temperatures ( > 300°C) are much higher than that of their linear high molecular mass homologues. Cyclic LC unimers and dimers, linear LC polyesters and model compounds were characterized by FAB-MS, GPC, ¹H NMR, DSC and hot-stage polarized microscopy. All these compounds contain reactive C=C double bonds and can be crosslinked thermally and photochemically. Cyclic unimers and dimers can be polymerized termally to produce high molecular mass polymers.     

Chiral recognition in cyclic alpha-hydroxy carbonyl compounds: a theoretical study

A theoretical study (DFT and MP2) of the self-association of homochiral (RR or SS) and heterochiral (RS or SR) dimers of three series of cyclic alpha-hydroxy-carbonyl derivatives has been carried out. The solvation effect on the parent derivative dimers has been explored, showing nonsignificant changes in the configurations preferred but altering in some cases the homo/heterochiral preference of the dimers. The results in the gas phase of the systems with different substituents show a preference for the heterochiral dimers. The energetic results have been analyzed with the NBO and AIM methodologies. Optical rotatory power calculations of the monomers and homochiral dimers show large variations of this parameter depending on the substituents and the complexation.     

Ammonia dimer, linear or cyclic?

Ab initio calculations have been carried out for ammonia dimers with 4-31G and 6-31G* basis sets. The results are corrected for basis-set superposition error. For the ammonia dimer, the stability of the linear form is comparable to that of the cyclic form. The localized orbitals for both dimers are discussed.     

Vibrational spectral studies of solutions at elevated temperatures and pressures. IX. Acetic acid

Raman spectra of glacial acetic acid from 350 to 3700 cm^–1 have been measured at temperatures up to 275°C and at a pressure of 9 MPa. Raman spectra of aqueous solutions of acetic acid from 3.9 to 16 molar have been measured up to 200°C at a pressure of 7 MPa. The spectral region 800 to 1850 cm^–1 for both glacial acetic acid and its aqueous solutions have been studied in detail since this region is significantly affected by variations in temperature and concentration. An interpretation of the bands in this spectral region was made with the aid of factor analysis, difference spectroscopy, band resolution techniques and the existing extensive literature. The results suggest that the major equilibrium in glacial acetic acid is between cyclic and linear dimers; however, in aqueous solutions in the concentration range studied, mono- and di-hydrated dimers and cyclic dimers are the predominant species.     

Self-assembling cyclic peptides: molecular dynamics studies of dimers in polar and nonpolar solvents

The self-assembly of cyclic D,L-alpha-peptides into hollow nanotubes is a crucial mechanistic step in their application as antibacterial and drug-delivery agents. To understand this process, molecular dynamics (MD) simulations were performed on dimers of cyclic peptides formed from cyclo [(-L-Trp-D-N-MeLeu-)4-]2 and cyclo [(-L-Trp-D-Leu-)4-]2 subunits in nonpolar (nonane) and polar (water) solvent. The dimers were observed to be stable only in nonpolar solvent over the full 10 ns length of the MD trajectory. The behavior of the dimers in different solvents is rationalized in terms of the intersubunit hydrogen bonding, hydrogen bonding with the solvent, and planarity of the rings. It is shown that the phi and psi dihedral angles of a single uncapped ring in nonane lie in the beta-sheet region of the Ramachandran plot, and the ring stays in a flat conformation. Steered MD (SMD) simulations based on Jarzynski's equality were performed to obtain the potential of mean force as a function of the distance between the two rings of the capped dimer in nonane. It is also shown that a single peptide subunit prefers to reside close to the nonane/water interface rather than in bulk solvent because of the amphiphilic character of the peptide ring. The present MD results build the foundation for using MD simulations to study the mechanism of the formation of cyclic peptide nanotubes in lipid bilayers.     

Lipase-catalyzed ring-opening polymerization of molecularly pure cyclic oligomers for use in synthesis and chemical recycling of aliphatic polyesters

The lipase-catalyzed ROP of molecularly pure cyclic oligomers with a definite degree of oligomerization is analyzed with respect to the molecular weights of the resulting polymers and certain kinetic parameters of the enzymatic reaction. Cyclic BA dimers, trimers, and tetramers polymerize faster than the equivalent monomer; however, the latter produces PBA of significantly higher molecular weight. The reason is that the ring opening of the cyclic monomer is slow, leading to a lower initiator concentration than that produced by the cyclic BA dimer and trimer. Similarly, the cyclic BS dimer produces PBS of higher molecular weight than that obtained from the cyclic BS trimer.     

The associative behaviour of saturated alcohols with reference to the molecular structure in the OH proximity. An FT-IR temperature study.

The IR absorption pattern of the OH stretching vibration has been determined for six aliphatic alcohols in the temperature range 100–293 K. Differences found for the bandmaximum and shape have been related with the intramolecular structure in the OH proximity. It is shown that the tendency to self-associate upon temperature lowering decreases with increased shielding of the OH group.Beside the formation of linear dimers evidence is found for cyclic associates most likely dimers.     

Hydrogen-Bonded Cyclic Dimers at Large Compression: The Case of 1H-pyrrolo[3,2-h]quinoline and 2-(2′-pyridyl)pyrrole

1H-pyrrolo[3,2-h]qinoline (PQ) and 2-(2′-pyridyl)pyrrole (PP) are important systems in the study of proton-transfer reactions. These molecules possess hydrogen bond donor (pyrrole) and acceptor (pyridine) groups, which leads to the formation of cyclic dimers in their crystals. Herein, we present a joint experimental (Raman scattering) and computational (DFT modelling) study on the high-pressure behaviour of PQ and PP molecular crystals. Our results indicate that compression up to 10 GPa (100 kbar) leads to considerable strengthening of the intermolecular hydrogen bond within the cyclic dimers. However, the intramolecular N–H∙∙∙N interaction is either weakly affected by pressure, as witnessed in PQ, or weakened due to compression-induced distortions of the molecule, as was found for PP. Therefore, we propose that the compression of these systems should facilitate double proton transfer within the cyclic dimers of PQ and PP, while intramolecular transfer should either remain unaffected (for PQ) or weakened (for PP).     

Cyclic dimers of tetrafluorobutatriene

1,1,4,4-Tetrafluorobutatriene polymerizes even at ?78?°C within a short time yielding a red insoluble polymer. Possible closed-shell cyclic dimers and oligomers resulting from several reaction paths were analyzed by computational methods??with CCSDT/cc-pVTZ as the highest order calculation and several other calculations of lower level. For a better understanding of fluorination effects, the perhydrogen triene dimers were included in this study. The destabilization of the central cumulenic double bond of tetrafluorobutatriene relative to ethylene and the further destabilizing fluorine substitution makes the formation of most of the dimers exothermic with only one exception for the perfluoro derivatives. Astonishingly, the geometry of some of the discussed molecules is highly affected by fluorine substitution, while for others there is nearly no effect on the geometry of the carbon backbone. ¹⁹F-NMR shifts of the potential dimers were calculated using the GIAO method.     

Polarized IR spectra of the hydrogen bond in two different oxindole polymorphs with cyclic dimers in their lattices

This article focuses on the problem of remarkably strong changes in the fine structure patterns of the ν(N-H) and ν(N-D) bands attributed to the hydrogen and deuterium bonds accompanying the phase transition, which occurs between two polymorphic forms of oxindole. The lattices of these two different crystals contain hydrogen-bonded cyclic dimers differ in their geometry parameters. The source of these differences in the polymorph spectral properties results from the geometry relations concerning the dimers constituting the lattice structural units. In the case of the "alpha" phase, the hydrogen bond lengths of the dimers differ by 0.18 ?. This leads to the "off-resonance exciton coupling" weakly involving the dimer hydrogen bonds. For the "beta" phase, with practically symmetric dimers in the lattice, the spectra become typical for centrosymmetric hydrogen bond systems due to the full resonance of the proton or deuteron vibrations.     

Synthesis of new porphyrins with peripheral conjugated chelates and their use for the preparation of porphyrin dimers linked by metal ions

This article describes the preparation of several new porphyrins bearing chelating peripheral groups fully conjugated with the macrocyclic pi-system. Treatment of a 2-nitro-meso-tetraarylporphyrin with phosphite gave a cyclic enamine, whose formylation gave an enaminoaldehyde. The thio analogue was obtained on treatment with Lawesson's reagent. The same reagent was also used to obtain the isomeric thioenaminoketone chelates. A enaminoketone ligand was prepared from a porphyrinic pyrrolone. All these ligands, as internal nickel complexes, could be metalated with palladium to yield porphyrinic dimers. The dimers obtained from enaminoketones and thioketones show a trans geometry, while in the enaminoaldehyde and -thioaldehyde series the cis isomer is thermodynamically favored. The bathochromic shifts of the electronic spectra of the aldehyde-derived dimers illustrate the strong electronic effect of peripheral metalation and dimerization. However, in the case of the pyrrolone-derived ligand, opposite effects were observed, due to partial reconstitution of the porphyrin chromophore on complexation. As with the dimers derived from enaminoketones, the dimers derived from the new ligands show typical splitting (up to 190 mV) of the electrochemical waves confirming large porphyrin-porphyrin interactions.     

Theoretical interpretation of the line shape of crystalline adipic acid

A general quantum theoretical approach of the upsilon(X-H) IR line shape of cyclic dimers of weakly H-bonded species in the crystal state is proposed. In this model, the adiabatic approximation (allowing to separate the high-frequency motion from the slow one of the H-bond bridge) is performed for each separate H-bond bridge of the dimer and a strong nonadiabatic correction is introduced into the model via the resonant exchange between the fast-mode excited states of the two moieties. Quantum indirect damping and Fermi resonances are taken into account. The present model reduces satisfactorily to many models in the literature dealing with more special situations. It has been applied to the cyclic dimers of adipic acid in the crystal phase. It correctly fits the experimental line shape of the hydrogenated compound and predicts satisfactorily the evolution in the line shapes with temperature and the change in the line shape with isotopic substitution.     

Adsorption studies of acetic acid dimers on activated charcoal from organic solvents

Acetic acid exists as dimers in organic solvents like benzene, toluene and xylene. Adsorption of dimeric acetic acid on activated charcoal (AC) at various temperatures from benzene, toluene and xylene solutions have been studied. The system obeys Langmuir isotherm, thus signifying a monolayer adsorption of dimers. Corrections on AC-solvent pore volume fillings, molecular cross sectional surface area of acetic acid dimers, the adsorption equilibrium constants, the free energy change and the enthalpy change values are computed at different temperatures for the three solvents. The adsorption process has been found to be physisorption type. The FTIR measurements show that the adsorbed acetic acid dimer seems to retain the cyclic structure against the open chain non-cyclic structure.