Isomeric Dimers Research Articles

Jet spectroscopy of van der Waals dimers of 1-methoxynaphthalene: A laser induced fluorescence study

Excited state dynamics following S1←S0 transition of jet-cooled van der Waals complexes of 1-methoxynaphthalene has been investigated by measuring the laser-induced fluorescence excitation, dispersed fluorescence and two-color hole-burning spectra. The results give evidence of formation of two isomeric dimers, which show only sharp absorption and emit excimer fluorescence for excitations to the origin region of the S1 state. Excimer formation in one of the dimers occurs from the zero-point level of the locally excited state. The other dimer shows vibrational barrier and the yield of the excimer emission increases as the successive higher quanta of a low-frequency intermolecular mode are excited. The barrier in the second case has been attributed to the reorganization of the dimer geometry from the locally excited state to the excimer state. The characteristic enhancement of the yield of excimer emission with increase in excitation energy indicates the involvement of a tunneling mechanism for crossing the reorganization barrier.

Application of gas chromatography–tandem mass spectrometry to the analysis of inhibition of dimerisation of tributylphosphate under radiolysis: Identification of isomeric tributylphosphate-alkylbenzene inhibitor coupling products

Tributylphosphate (TBP), solvent used as extractant for reprocessing spent nuclear fuel, can dimerise under radiolysis. This occurs by radical radical recombination, leading to 10 isomeric dimers (TBP–TBP). These species are complexation agents and are responsible of fission product retention in the organic phase that increases the solvent degradation. In order to limit their formation two free radical inhibitors (In), isopropyl and 1,4-diisopropylbenzenes, were used. These additives reduce by about 50% the concentration of TBP–TBP dimers but this reduction is not strictly followed by TBP regeneration as mixed coupling products from TBP and inhibitor are detected. By using GC–MS–MS and selectively deuterated compounds, the identification of these different isomers (TBP–In) has been realised. From these identifications and from the analysis of the proportion of the different isomers, the major primary TBP radical generated under radiolysis was determined.

Excimer formation in the mixed dimers of naphthalene and 1-methoxynaphthalene in a supersonic jet

Electronic spectra for the S1←S0 transition of jet-cooled mixed clusters of naphthalene and 1-methoxynaphthalene have been measured by laser-induced fluorescence excitation and dispersed fluorescence spectroscopy. Under jet-cooled conditions, naphthalene forms two isomeric 1:1 mixed dimers with 1-methoxynaphthalene, which exhibit significant differences in excimer formation dynamics from the locally excited states. One of the isomeric dimers emits excimer fluorescence when excited to the electronic origin of the lowest excited state but the other shows a significant barrier to formation of the excimer. The energy gap between the S1 states of naphthalene and 1-methoxynaphthalene is ∽350 cm−1. The observation of excimer emission, particularly when excited to the S1 origin of one of the isomeric dimers, supports our earlier proposal that for naphthalene–methoxynaphthalene mixed dimers energy exchange or exciton resonance interactions are not significant for the stabilization of the mixed excimers. The charge-transfer (CT) interaction and overlap between π-molecular orbitals of two molecules at a suitable geometry are considered to be the primary factors for the stability of the excimers. At higher naphthalene vapour pressure we have observed a mixed trimer of 2:1 composition of naphthalene and 1-methoxynaphthalene. The trimer does not emit excimer fluorescence when excited with additional vibrational energy in S1 up to 650 cm−1. The observation indicates that the geometry of the trimer is very different from the stack configuration essential for formation of the excimer.

Unique Catalysis and Regioselectivity Observed in the Poly(C)-Directed RNA Dimer Formation from 2-MeImpG: Kinetic Analysis as a Function of Monomer and Polymer Concentration.

Polycytidylate, poly(C), serves as a scaffold or template to direct and catalyze the synthesis of long oligoguanylates from guanosine 5'-phosphate 2-methylimidazolide, 2-MeImpG. In the absence of poly(C), small amounts of three isomeric dimers, i.e., the 2'-5'-, the 3'-5'-, and the pyrophosphate-linked, are formed slowly. In the presence of poly(C) oligomers that are primarily 3'-5'-linked are formed quickly and in high yield. Product analysis suggests that the oligomers are elongation products of the 3'-5'-linked dimer, abbreviated D. Assuming that D is formed slowly from two molecules of 2-MeImpG (Scheme 1) and elongates relatively fast, the initial rate of dimerization, d[D]/dt in M h(-1), was determined using two independent methods. The first method is based on the approximation that at the onset of the reaction the substrate is consumed only via hydrolysis and dimerization, and thus elongation can be neglected. The second, more accurate, method exploits the assertion that every oligomer was once a 3'-5'-linked dimer. Hence the concentration of D was obtained indirectly from the concentration of the oligomer products. These two methods gave comparable results. Experiments were run in aqueous solution in the presence of 1.0 M NaCl, 0.2 M MgCl(2) at pH 7.9 +/- 0.1 and 23 degrees C. Controls were run in the absence of poly(C) and in the presence of other polynucleotides. The kinetics were determined as a function of both monomer and polymer concentration the latter expressed in C equivalents. The kinetic data obtained in the presence of poly(C) confirmed an earlier conclusion regarding the remarkable effect of poly(C) on the formation of the 3'-5'-linked diguanylate. Initial dimerization rates were quantitatively correlated using a simple template-directed (TD) model that presumes cooperative binding (two association constants) of 2-MeImpG on poly(C) and reaction between adjacent template-bound molecules. The model allows for the estimation of the association constants and the intrinsic rate constant of dimerization, k(2). Insights into the detailed mechanism are also gained from this analysis. The fact that the proposed model can successfully correlate kinetic data that vary by more than 5000-fold between the slowest and the fastest reaction adds confidence and suggests the suitability of this model for describing TD reactions in general. It is anticipated that similar analysis of other known TD reactions may lead to clues that will facilitate the design of more efficient polynucleotide-synthesizing systems.

Vanadium (V) In Perchloric Acid : A Novel Use Of The Reagent For Dimerisation Of Some Naphthalene Derivatives

Oxidative dimerisation of 2-naphthol and its methyl ether into the respective 1, 1'-dimers was achieved in good yield and excellent purity through a one-step method using ammonium metavanadate and dilute perchloric acid. 1-Naphthol gave a mixture of isomeric dimers, although its methyl ether was converted almost quantitatively into the corresponding 4, 4'-dimer. This reagent could also dimerise 2-hydroxy-1, 4-naphthoquinone, a natural product.

ESR spectra of fullerence C70 in concentrated sulfuric acid

It was established by ESR spectroscopy that the reaction of C70 with concentrated H2SO4 resulted in the formation of isomeric dimers C140 +, and the action of H2SO4 on a toluene solution of C70 + gave the C70 radical cation. The structure of dimers was discussed.

Contrasting effects of platelet-derived growth factor (PDGF) isomers on mitogenesis, contraction and intracellular calcium concentration in human vascular smooth muscle.

The present study was aimed at characterizing the responses of human vascular smooth muscle to all three dimeric isomers of platelet-derived growth factor (PDGF-AA, -AB and -BB) in terms of mitogenesis, contraction and intracellular calcium concentration. The potential of interaction between PDGF and endothelin-1 (ET-1) was also investigated. All three PDGF isoforms (0.1-20 ng mL-1) stimulated DNA synthesis in cultured human coronary artery and saphenous vein vascular smooth muscle cells (VSMC), measured by [3H]thymidine incorporation. PDGF-AB and -BB elicited comparable large increases in DNA synthesis of maximum 595 +/- 149% (P = 0.001, n = 9) and 576 +/- 17% (P < 0.001, n = 5), respectively, whereas PDGF-AA was only weakly mitogenic (61 +/- 16% increase; P < 0.05, n = 3). At a threshold concentration, PDGF acted in synergy with ET-1 to enhance DNA synthesis (816 +/- 337% increase; P < 0.05, n = 7). In contrast to mitogenesis, none of the three PDGF isomers had any effect on contraction of human saphenous veins in vitro, nor did they affect the contractile response to ET-1, 5-HT or the thromboxane mimetic U46619. The effects of the three PDGF isomers on intracellular calcium ([Ca2+]i) rises in cultured human VSMC were heterogeneous, with PDGF-BB inducing the largest increase in [Ca2+]i (442 +/- 53 nmol L-1) vs. PDGF-AB (290 +/- 28 nmol L-1), whilst PDGF-AA had no effect. Both the responses to PDGF-AB and-BB relied upon intracellular calcium release, whilst only PDGF-AB showed additional dependence on influx of extracellular calcium. In summary, PDGF is strongly mitogenic and comitogenic with ET-1, despite not being a vasoconstrictor, for human VSMC. Also, human VSMC showed heterogeneous responses to the three PDGF isoforms. These results implicate PDGF, and in particular the PDGF receptor-beta, as important role players in the development of vascular smooth muscle-mediated intimal thickening in humans.

Preparation of volatile molecular lithium-niobium alkoxides. Crystal structures of [Nb(μ-OCH 2SiMe 3)(OCH 2SiMe 3) 4] 2 and [LiNb( μ3-OCH 2SiMe 3)-( μ2-OCH 2SiMe 3) 2(OCH 2SiMe 3] 2

New heterometallic alkoxides [LiNb(OCH 2CMe 3) 6] 2 ( 4), LiNb(OCH 2CMe 3) 6(Me 3CCH 2OH) 2 ( 5), and [LiNb(OCH 2SiMe 3) 6] 2 ( 6) have been prepared from the corresponding homometallic alkoxides. All are moderately soluble in hydrocarbons and sublime in moderate to good recoveries. The structure of 6 determined crystallographically contains an Nb 2Li 2( μ-O) 4( μ 3-O) 2 central-core geometry. Compound 4 appears to be isostructural on the basis of 1H NMR data. Bulk pyrolysis of 4 gives crystalline LiNbO 3 under N 2 at 600°C. However, LiNbO 3 films are not obtained from 4 by spray MOCVD under conditions that give LiNbO 3 films from [LiNb(OEt) 6] ∞. Crystallographic structure determination for the precursor compound [Nb(μ-OCH 2SiMe 3)(OCH 2SiMe 3) 4] 2 ( 3) reveals an edge-shared biotahedral geometry in which the μ-O atoms are distinctly pyramidal. This structure is maintained in solution; the pyramidal bridge configurations allow coexistence of syn and anti dimeric isomers. Precursor 3 also exhibits a dimer-monomer equilibrium for which ΔH = 63(3) kJ/mol and ΔS = 180(10) J/(K mol).

Identification of isomeric tributylphosphate dimers formed by radiolysis using tandem mass spectrometry and stable isotopic labelling

Tributylphosphate (TBP) dimers, formed during radiolysis of the nuclear fuel reprocessing solvent, were studied using tandem mass spectrometry (MS/MS), stable isotopic labelling ( 2 H) and synthetic model compounds. The structures of isomeric dimers and their lower homologues were established, using GC/MS as well as electron impact and chemical ionisation GC/tandem mass spectrometry, with an extensive use of ion chromatograms of characteristic fragment ions from collision-induced decomposition in both nondeuterated and selectively deuterated series. An explanation of isomer ratios and of the restricted number of lower homologues as well as a radiolytic mechanism are advanced. The TBP-dimer fraction represented less than ~ 0.03% of the total mixture obtained from radiolysis of solvent.

Relative cyanide cation (+CN) affinities of pyridines determined by the kinetic method using multiple‐stage (MS3) mass spectrometry

AbstractIon–molecule reactions occurring in a pentaquadrupole mass spectrometer are used to generate and characterize ions in which one or two pyridine molecules are bound by a +CN cation. Cyanide cation binds strongly to the nitrogen atom of pyridine to generate a mono‐adduct, which undergoes pyridine exchange reactions and from which one can generate the dipyridine adduct in low abundance. The dimeric ions have two structures, loosely bound and covalently bound, and both fragment to yield the constituent cyanide‐bound monomers. In the case of dimers comprised of meta‐substituted alkylpyridines, there is a quantitative correlation between relative cyanide cation affinity, as measured using the kinetic method, and literature values of relative proton affinities. These dimers fragment analogously to the corresponding H+‐ and Cl+‐ bound dimers, and on this basis are assigned analogous structures, viz. the loosely bound form Py1+CNPy2. Semi‐empirical molecular orbital calculations show that both pyridines are bound to the carbon atom of the cyanide cation. Making the assumption that the effective temperatures of the activated cyanide‐bound dimers are similar to those of the corresponding Cl+‐ and H+‐bound dimers, relative +CN cation affinities are estimated to be 1.5 kcal mol−1 (3‐MePy), 1.7 kcal mol−1 (4‐MePy), 2.6 kcal mol−1 (3‐EtPy), 3.5 kcal mol−1 (3‐n‐BuPy) and 3.6 kcal mol−1 (3,5‐diMePy), all expressed relative to pyridine (1 kcal = 4.184 kJ). A linear relationship between the relative +CN affinity and relative proton affinity (PA) is derived as Δ +CN affinity (kcal mol−1) = 0.78 (ΔPA), with the assumption that the +CN dimer effective temperature is 600 K. The estimated uncertainty is 0.5 kcal mol−1. Relative +CN affinities of pairs of pyridines are smaller by ca. 1 kcal mol−1 than the corresponding Cl+ affinities. Dimers in which one of the pyridines is meta‐chlorine‐ or para‐alkyl‐substituted have the covalently bound, ring‐carbon‐substituted structure, in which the +CN group is attached to the pyridine nitrogen and the second pyridine molecule is bound to a ring carbon. The fragmentation of these isomeric dimers yields the corresponding monomers, in addition to other minor ions, but the distribution of the cyanide cation between the two pyridines does not correlate with Cl+ affinity or proton affinity. In the special cases of the 3‐methylpyridine–3‐n‐butylpyridine and the 4‐methylpyridine–pyridine cyanide cation adducts, both the loosely bound dimer and the covalently bound adduct are generated and distinguished by their fragmentation behavior. Evidence for the formation of the covalently bound, ring‐carbon‐substituted structure was also obtained in semi empirical AMI calculations.

Spectroscopic investigation of concentrated solutions of gallium(III) chloride in mesitylene and benzene

The metal–arene interaction in concentrated binary mixtures of GaCl3 and mesitylene or benzene has been investigated using multinuclear (1H, 13C, 71Ga) NMR, Raman and UV/VIS spectroscopy as well as liquid X-ray scattering. The effects of hydrolysis and radiolysis of such mixtures have also been studied using UV/VIS, EPR and IR spectroscopy. Gallium(III) chloride has been found to be monomeric in mesitylene and to form η6 complexes with this arene. A structural determination of this complex in solution yielded a Ga–C distance of 2.20 A and a GaCl3 moiety with C3v symmetry and Ga–Cl distances of 2.10 A. In benzene, GaCl3 exists as a mixture of the Cl2Ga(µ-Cl)2GaCl2 dimer and one or several other species, containing terminal GaCl3 groups of C3v symmetry, e.g. the dimer isomer Cl2Ga(µ-Cl)GaCl3 and monomeric GaCl3. The complex with benzene is weaker than with mesitylene and the co-ordination mode cannot be unambiguously elucidated. Hydrolysis of the GaCl3–C6H6 system produced a darkly coloured precipitate showed by IR spectroscopy to be a mixture of poly(p-phenylene) and other polymeric species, probably meta-substituted, chlorinated and hydroxylated polyphenylenes. On the basis of 71Ga NMR results, it is suggested that the formation of this polymer proceeds via protonated Wheland intermediates, rather than direct oxidation of benzene by GaIII. Radiolysis of the mixtures of GaCl3 with mesitylene and benzene yielded radicals, according to EPR spectroscopy. In benzene solution the formation of radicals is accompanied by reduction of GaIII to GaI. No formation of solid polymers could be detected.

Actobindin binds with high affinity to a covalently cross-linked actin dimer.

Actobindin, a 9.8-kDa protein purified from Acanthamoeba castellanii, contains two actin-binding sites that can simultaneously bind two actin monomers. However, actobindin inhibits actin polymerization to a greater extent than can be explained by its affinity for actin monomers (site-specific KD = 3.3 microM). This paradox would be resolved if actobindin could interfere with the nucleation phase of polymerization by using both binding sites to bind simultaneously to an actin oligomer because the interaction with oligomer would be thermodynamically favored over that with actin monomer. We now show that a covalently cross-linked actin dimer prepared from cross-linked F-actin binds to actobindin with high affinity (apparent KD = 11 nM) in accordance with theoretical predictions for simultaneous binding of two actin subunits per single actobindin and consistent with the hypothesis that actobindin might bind to native actin oligomers and prevent them from nucleating polymerization. Furthermore, the interaction with cross-linked dimer exhibits specificity in that an isomeric cross-linked actin dimer with more rapid electrophoretic mobility binds weakly to actobindin. However, only this isomeric dimer is produced when cross-linking reagents are added to actin undergoing polymerization in the presence of actobindin. Therefore, if actobindin inhibits polymerization by interacting with a native dimer whose conformation is similar to that of the cross-linked dimer with slower electrophoretic mobility, then actobindin must either block the cross-linking sites or convert the dimer to a different conformation.

Quantum chemical modeling of chiral catalysis. Part 16. On the isomerism of dimers of chiral oxazaborolidines used in the catalytic enantioselective reduction of ketones

Relative stabilities of several dimeric isomers of oxazaborolidines were studied by means of ab initio MO (RUF) methods (1,3,2-oxazaborolidine 1′ used as a model of oxazaborolidines). The most stable of the isomers found was 24 kJ kJ −1 (6-31G//6-31G) lower in energy than two separate monomeric oxazaborolidine ( 1′) molecules. Formation of several other dimeric isomers was found to be energetically advantageous.

Organized photodimerization of unsaturated carboxylates. Selectivity control by normal and reversed micelles

Aromatic unsaturated carboxylates such as cinnamate and 2-indenecarboxylate form, in the presence of long-chain alkylammonium ion, short rod-like micelles with an aggregation number of an order of 103 in water and premicelle aggregates including 2–3 surfactant molecules in carbon tetrachloride. It was found that these aggregates undergo efficient and stereoselective photodimerizations. Photoreactions in aqueous rod-like micellar solutions resulted in the predominant formation of anti-head-to-head dimers (anti-HH), while the reaction in small aggregated premicelles yielded thermodynamically least stable syn-head-to-head dimers (syn-HH). The addition of a small amount of methanol to the premicelle aggregate shifted the major product from syn-HH to anti-HH. Control experiments showed that the photoreaction of the corresponding methyl ester afforded mixtures of four isomeric dimers, syn- and anti-HH and HT (head-to-tail), of statistical distribution. These stereochemical selectivities are discussed in terms of the organized olefin orientation and correlated to the results of light scattering and NMR probe techniques.

Oxidation chemistry of 5,6-dihydroxy-2-methylindole

In aqueous buffer at neutral pHs. 5,6-dihydroxy-2-methylindole ( 2) is rapidly oxidised by the peroxidase/H 2O 2 system to give in the early phases a complex mixture of oligomers, the major of which were isolated as the O-acetyl derivatives. Five of these were formulated as the 3,3′-biindolyl 3, the isomeric dimers 4–6 and trimer 9, arising by coupling of the indole ring at the nuclear positions, whereas the remaining ones were assigned structures 7–8, marked by methylene bridges between the indole moieties, and notably 10, featuring a tetrahydrocyclopentadiindole ring system.

Semiconductor photocatalysis: quantised CdS-catalysed photoformation of 1-benzyl-1,4-dihydronicotinamide (BNAH) from 1-benzylnicotinamide (BNA+)

Freshly pepared CdS suspensions (CdS-0) consisting of quantised particles and their loose aggregates catalyse the photoreduction of 1-benzylnicotinamide (BNA+) in water under visible-light irradiation using triethylamine (TEA) as a sacrificial electron donor. Isomeric dimers of (BNA)2 are also found as one-electron reduction products from BNA+, whereas the photocatalysis of commercially available CdS powder (Aldrich, CdS-Ald) leads only to the formation of the dimers. The formation of cadmium metal (Cd0) on the lattice is indispensable for the photoformation of BNAH, suggesting that the eventual two-electron reduction on irradiation of CdS-0 with TEA in water, should proceed through hydrogen atom transfer from Cd0—H formed on the lattice of CdS-0.

Polymer-based electrochromic devices—I. Poly(3-methylthiophenes)

The basic aspects of electrochromism induced by a p-doping/undoping process of conjugated polymers are briefly described. The most important requisites for an electrochromic device are discussed, and polymer electrochromic performance data as well as the test results of a polymer-based variable light transmission electrochromic device are reported. The colour contrast control of conjugated polymers by “tailoring” their conjugation length is discussed and spectroelectrochemical data of poly(3-methylthiophenes) electrosynthesized from 3-methylthiophene isomeric dimers and isomeric tetramers are reported.

Excitation energy relaxation and dimer formation of fluorene in Langmuir—Blodgett monolayer films

Picosecond time-resolved fluorescence has been studied with Langmuir—Blodgett monolayer films containing fluorene as chromophore, in the concentration range of 0.1–29 mol%. Two types of dimer emissions of fluorene were observed. They were assigned as the conformational isomers of fluorene dimers. The dominant energy relaxation pathway is an efficient energy transfer from monomers to dimers.

Thymine photoproduct formation and inactivation of intact spores of Bacillus subtilis irradiated with short wavelength UV (200–300nm) at atmospheric pressure and in vacuo

Vacuum exposure renders the survival of spores of Bacillus subtilis approximately five times more sensitive to ultraviolet light irradiation than exposure under atmospheric conditions. The photoproduct formation in spores irradiated under ultrahigh vacuum (UHV) conditions is compared to the photoproduct formation in spores irradiated at atmospheric pressure. Compared to irradiation at atmospheric pressure, where only the “spore photoproduct″ 5-thyminyl-5,6-dihydrothymine (TDHT) can be detected, two additional photoproducts, known as the c,s and t,s isomers of thymine dimer (T???T) are produced in vacuo. The spectral efficiencies for photoproduct formation in spores under atmospheric and vacuum conditions are compared. Since there is no increased formation of TDHT after irradiation in vacuum, TDHT cannot be made responsible for the observed vacuum effect. “Vacuum specific” photoproducts may cause a synergistic response of spores to the simultaneous action of ultraviolet light (UV) and UHV. Three different mechanisms are discussed for the enhanced sensitivity of B. subtilis spores to UV radiation in vacuum. The experiments described contribute valuable research information on the chance for survival of microorganisms in outer space.

Characterization of methyl derivatives of cyclopentadiene monomer and dimer by 13C NMR spectroscopy

AbstractThe assignment of the carbons of different isomers of methylcyclopentadiene monomers (Me‐CPD) and dimers (diMe‐DCPD) has been performed directly in a mixture, without previous separation of these products, by analysing data from quantitative 13C NMR spectra of the mixtures and applying a J‐modulated spin–echo sequence. Semi‐empirical rules for the influence of methyl groups on the chemical shifts of carbons in pentenic and norbornyl ring systems have been formulated and utilized to assign the absorptions. The same method can then be applied to the carbon assignments in cyclopentadiene–methylcyclopentadiene codimers (Me‐CPD), and it is possible to develop additivity rules and predict chemical shifts for diMe‐DCPD based on the chemical shifts for the carbons of these codimers.