Trimeric Model Compounds Research Articles

Theoretical study on the effect of the substituent groups on the homolysis of the ether bond in lignin trimer model compounds

The homolytic bond dissociation energies (BDEs) of Cα−O and Cβ−O bonds in 27 lignin trimer model compounds were calculated by employing density functional theory methods at M062X level with 6−31++G(d,p) basis set; the effects of various substituent groups (CH3, CH2OH and OCH3) at different positions on the BDEs of Cα−O and Cβ−O bonds were investigated. The results indicated that a single methoxyl group at R2 or R3 has a minor influence on the BDE of Cβ−O bond, whereas two methoxyl groups at R2 and R3 lead to an obvious decrease in the BDE of Cβ−O bond. The decrement in the BDE of Cβ−O bond from the methoxyl groups at R2 and R3 can be enhanced by the methoxyl groups at R4 and R5, but is hardly influenced by the substituent groups at R1. Meanwhile, the BDE of Cα−O bond is gradually reduced when the H atoms at R4 and R5 are successively substituted with methoxyl groups; the decrement in the BDE of Cα−O bond from the methoxyl groups at R4 and R5 can be strengthened by the methoxyl groups at R2 and R3. Furthermore, the methyl and hydroxymethyl groups at R1 can gradually increase the BDE of Cα−O bond and this effect is weakened when the H atoms at R2 and R3 are successively substituted with methoxyl groups. The methyl group at R1 has little influence on the BDE of Cβ−O bond, which is however dramatically increased by the hydroxymethyl group at R1.

Convenient preparation of a β-O-4-type lignin model trimer via KOH-catalyzed hydroxymethylation and a new protection method

Abstract Lignin, as the second most abundant biopolymer on earth, is one of the targets for plant biorefinery studies. Its complex chemical behavior is frequently studied by dimeric, trimeric, etc. model compounds, preferably with a β-O-4-type structure. In the present study, a convenient synthesis of a β-O-4-type trimeric model compound possessing a free syringylic hydroxyl has been investigated. Two key modifications were in focus: (1) Protection of the aliphatic hydroxyl groups of the starting phenolic dimer prior to the SN2 displacement reaction before introducing the syringylic moiety with 2,2-dimethoxypropane. (2) The hydroxymethylation step to introduce the full side chain moiety. When this reaction was performed in dioxane/water in the presence of a catalytic amount of KOH instead of K2CO3, the formation of a side product via dehydration was markedly reduced. In addition, a convenient method for introducing an α,β-epoxide structure in acetophenone is recommended.

ChemInform Abstract: Syntheses and X-Ray Structures for Model Compounds of a Pyrimidinediyl- Based Rigid-Rod Aromatic Polyamide.

A variety of model compounds for the pyrimidinediyl-based rigid-rod polyamide poly[imino-(pyrimidine-2,5-diyl)-imino-tetraphthaloyl] (PPYMT) was prepared, in order to compare their conformations to several model compounds of the related, fully aromatic polymer poly(p-phenyleneterephthalamide) (PPTA). In particular, the structures of N-(2-pyrimidyl)benzamide (PYMB) and its complexed form bis[(N-pyrimidin-2-yl)benzamide]nickel(II) dichloride (NiPYMB) were determined by X-ray diffraction. The molecular packing in these crystals provides us with a model for the possible ‘cross-linking’ of PPYMT fibers. The structures of the trimer model compounds N,N′ -bis(2-pyrimidyl)terephthalamide (PYTA) and N,N′ -bis(benzoyl)-2,5-diaminopyrimidine (BDAP) yield information about the conformation of PPYMT chains and are compared to analogous model compounds of PPTA.

Synthesis, Luminescence Properties, and Explosives Sensing with 1,1-Tetraphenylsilole- and 1,1-Silafluorene-vinylene Polymers

The syntheses, spectroscopic characterizations, and fluorescence quenching efficiencies of polymers and copolymers containing tetraphenylsilole- or silafluorene-vinylene repeat units are reported. These materials were prepared by catalytic hydrosilylation reactions between appropriate monomeric metallole alkynes and hydrides. Trimeric model compounds methyl(tetraphenyl)silole-vinylene trimer (1), methyl(tetraphenyl)silole-silafluorene-vinylene cotrimer (2), and methylsilafluorene-vinylene trimer (3) were synthesized to provide detailed structural and spectroscopic characteristics of the polymer backbone and to assess the extent of delocalization in the luminescent excited state. Poly(tetraphenylsilole-vinylene) (4), poly(tetraphenylsilole-silafluorene-vinylene) (5), and poly(silafluorene-vinylene) (6) maintain a regio-regular trans-vinylene Si−C backbone with possible ground state σ*−π and excited state σ*−π* conjugation through the vinylene bridge between metallole units. Fluorescence spectra of the polymers show an ∼13 nm bathochromic shift in λflu from their respective model compounds. Molecular weights (Mn) for these polymers and copolymers are in the range of 4000–4500. Detection of nitroaromatic explosives by solution-phase fluorescence quenching of polymers 4−6 was observed with Stern−Volmer constants in the range of 400−20 000 for TNT, DNT, and picric acid (PA). A surface detection method for the analysis of solid particulates of TNT, DNT, PA, RDX, HMX, Tetryl, TNG, and PETN is also described for silafluorene-containing polymers. Polymer 6 exhibited detection for all the preceding types of explosive residues with a 200 pg cm−2 detection limit for Tetryl. Polymers 4 and 5 exhibited only luminescence quenching with nitroaromatic explosives, revealing that the excited-state energy of the sensor plays a key role in the fluorescence detection of explosives.

Synthesis, Characterization, and Optical and Electrochemical Properties of New 2,1,3-Benzoselenadiazole-Based CT-Type Copolymers

New alternating donor−acceptor charge-transfer (CT)-type copolymers consisting of didodecyloxy-p-phenylene (Ph), N-(4-dodecyloxyphenyl)carbazole (Cz), or N-hexyldiphenylamine (Da) unit (π-electron donor) and 2,1,3-benzoselenadiazole (BSe) unit (π-electron acceptor) were prepared by palladium-catalyzed Suzuki coupling reaction in 85−96% yields. The copolymers with the Ph and Cz units were soluble in common organic solvents and gave number-average molecular weights of 8300 and 6600, respectively, in GPC analysis; the copolymer with the Da unit was partly soluble in the solvents. The UV−vis absorption peak of the polymers appeared in the range of 420−530 nm in solutions and films, and the optical transition is considered to be accompanied by CT from the donor unit to the BSe unit. Quantum-chemical calculations of a trimeric model compound (Ph−BSe−Ph: 10) supported the notion that the optical activation of the copolymer involved the CT process. Cyclic voltammetry revealed that the copolymers were susceptible...

Polymerization of Methyl Phenyl Sulfoxide under Acidic Conditions: Synthesis and X-ray Structure Analysis of a Phenylene Sulfonium Polymer

Methyl phenyl sulfoxide was found to undergo self-polycondensation in triflic acid in the presence of diphenylamine to produce poly(methylsulfonio-1,4-phenylene triflate). Diphenylamine formed a charge-transfer (CT) complex with the hydroxy methyl phenyl sulfonium cation produced by the protonation of the sulfoxide, facilitating the electrophilic substitution reaction of the sulfonium cation onto the carbon atom of the terminal phenyl ring. The obtained polymer is a highly crystalline material whose structure was determined by X-ray analysis using the typical geometric parameters of the sulfoniophenylene dimer and trimer model compounds. The orthorhombic unit cell of the polymer (a = 10.875 Å, b = 10.449 Å, and c = 18.629 Å) contains eight monomeric units. The space group is Pbcn (#60). Two molecular chains along the c axis pass through the unit cell, one through the corner and the other through the center. The crystal structure of the polymer not only revealed a structural relevance to those of oxy- and thiophenylene polymers but also provided support for CT complex formation between the polymer and diphenylamine.

Phase Behaviors of Mainchain Liquid Crystals. 2H NMR and Pressure–Volume–Temperature Analysis of Dimer and Trimer Model Compounds

Our previous studies on the structure-property relation of dimer liquid crystals (LC), α,ω-bis(4-cyanobiphenyl-4′-yloxy)alkanes (CBA-n, n=9, 10), have been extended to include trimer compounds, 4,4′-bis[ω-(4-cyanobiphenyl-4′-yloxy)alkoxy]biphenyls (CBA-Tn, n=9,10), comprising three mesogenic units jointed by two intervening spacers. These nematic-LC-forming compounds were chosen as a model for the mainchain-type polymer LCs on the assumption that the spatial configuration and thermodynamic roles of the flexible spacer are nearly identical as long as the chemical structure of the repeating unit is similar. The 2H NMR method was extensively used to elucidate the orientational characteristics of these molecules in the mesophase. The analysis of binary mixtures with a low molar mass (monomer) LC gave an important information regarding the mesophase structure of CBA-Tn as well as CBA-n. The pressure–volume–temperature (PVT) relation was determined to estimate the constant–volume transition entropies at the Interphase such as crystal/nematic LC/isotropic melt. The results were found to be favorably compared with the conformational entropy changes derived on the basis of the rotational isomeric state analysis of the 2H NMR data. Finally, an important role of the flexible spacer was emphasized in determining the molecular ordering as well as thermodynamic properties of mainchain-type LCs.

Dielectric and molecular mechanics study of the γ relaxations of poly(chloroethyl methacrylate) and poly(chloropropyl methacrylate)

Cryogenic dielectric measurements corresponding to the γ-relaxation zone have been made for poly(chloroethyl methacrylate) and poly(chloropropyl methacrylate). Molecular mechanics calculations using the Allinger MM2 force field have been carried out on monomer and trimer model compounds in an attempt to gain some insight into the possible origin of the γ-relaxation process which occurs in these polymers at low temperatures. In both cases, we found that the molecular mechanism of oxylcarbonyl side group rotation combined with reasonable constraints on movements of the neighbouring side groups gave theoretical activation energy requirements which were in good agreement with those determined from the experiment.

Quantum Chemical Conformational Analysis on Isotactic Polyacrylonitrile Using Molecular Model Compounds

An attempt was made to decide the most stable structure for isotactic polyacrylonitrile using dimer and trimer model compounds with aids of semi-empirical quantum chemical methods (i.e., CNDO/2 and MINDO/3 methods) where possible conformations were considered by rotating main chain bond and bending nitrile triple bond from ordinary C-C-N straight bond at side chain. The results show that two adjacent nitrile groups have mutual attractive interactions and bend to get near to each other by assuming the prolonged CN bond length which reflects the tendency of double bond for CN groups in meso arrangement. This conclusion supports Kamide et al.’s proposal deduced from the systematic experimental results on chain stiffness and NMR peak assignment for stereoregular polyacrylonitrile. The bending nitrile bond structure of PAN was obtained for the first time from the view point of molecular orbital interactions in low molecular model compound.

Ion Binding Properties of Polycarboxylates Using Terbium(III) as a Fluorescent Probe: Viscosities and Coordinated Water Molecules in Polycarboxylate−Terbium(III) Complexes in Aqueous Solutions

The binding properties of trivalent ions to polyacrylate and its low molecular weight analogs (monomer, dimer, and trimer) were investigated in dilute aqueous solution (10 -2 -10 -3 M) using Tb 3+ ion as a fluorescent probe. The fluorescence intensity and lifetime of the Tb 3+ ion depend directly on the number of water molecules bound to their inner coordination sphere. The more efficiently ligands coordinate to Tb 3+ ion, the more water molecules are expelled and, consequently, the greater the fluorescence intensity and lifetime observed. Lifetime measurements in H 2 O and D 2 O showed that the number of water molecules coordinated to the Tb 3+ ion are 6.5, 6.1, 4.9, and 3.6 for monomer, dimer, trimer model compounds, and polyacrylate, respectively. The viscosities of Tb 3+ -polyacrylate complexes were measured in the presence of a large excess of sodium bromide. Viscosities (η sp /c) of the polyacrylate solutions follow Huggins' equation, and on addition of Tb 3+ ion, the intrinsic viscosities decrease abruptly from 700 to 40 mL/g. The mean end-to-end distances, 1/2 , for polyacrylate in Tb 3+ -polymer complexes were calculated using the Flory-Fox equation and were found to be reduced from 125 to 48 nm on addition of Tb 3+ . These results indicate that of the nine water molecules coordinated to Tb 3+ ion in aqueous solution, five to six are replaced with carboxylate groups attached to the polymer chain wrapped around the Tb 3+ .

Syntheses and X‐Ray Structures for Model Compounds of a Pyrimidinediyl‐Based Rigid‐Rod Aromatic Polyamide

AbstractA variety of model compounds for the pyrimidinediyl‐based rigid‐rod polyamide poly[imino‐(pyrimidine‐2,5‐diyl)‐imino‐tetraphthaloyl] (PPYMT) was prepared, in order to compare their conformations to several model compounds of the related, fully aromatic polymer poly(p‐phenyleneterephthalamide) (PPTA). In particular, the structures of N‐(2‐pyrimidyl)benzamide (PYMB) and its complexed form bis[(N‐pyrimidin‐2‐yl)benzamide]nickel(II) dichloride (NiPYMB) were determined by X‐ray diffraction. The molecular packing in these crystals provides us with a model for the possible ‘cross‐linking’ of PPYMT fibers. The structures of the trimer model compounds N,N′ ‐bis(2‐pyrimidyl)terephthalamide (PYTA) and N,N′ ‐bis(benzoyl)‐2,5‐diaminopyrimidine (BDAP) yield information about the conformation of PPYMT chains and are compared to analogous model compounds of PPTA.

Intramolecular Association of Viologen Dimer and Trimer Radical Cations. An Electrochemical and Spectroscopic Study

Abstract Viologen dimer and trimer model compounds were prepared and their reduced forms(i.e., radical cation) were investigated electrochemically as well as spectroscopically. Intramolecular association between two viologen radical cations takes place efficiently, which brings about changes in the electrode reduction processes of these compounds.