Effect Of Methyl Substitution Research Articles

Effect of methyl and hydroxyl substitution to the single-crystal formation of some hydroxypyridinecarboxylic acids and their copper(II)complexes

Effect of methyl and hydroxyl substitution to the single-crystal formation of some hydroxypyridinecarboxylic acids and their copper(II)complexes

The Structural Effect of Methyl Substitution on the Binding of Polypyridyl Ru–dppz Complexes to DNA

Polypyridyl ruthenium complexes have been intensively studied and possess photophysical properties that are both interesting and useful. They can act as probes for DNA, with a substantial enhancement in emission when bound, and can induce DNA damage upon photoirradiation. Therefore, the synthesis and characterization of DNA binding of new complexes is an area of intense research activity. While knowledge of how the binding of derivatives compares to that of the parent compound is highly desirable, this information can be difficult to obtain. Here we report the synthesis of three new methylated complexes, [Ru(TAP)2(dppz-10-Me)]Cl2, [Ru(TAP)2(dppz-10,12-Me2)]Cl2, and [Ru(TAP)2(dppz-11-Me)]Cl2 (TAP = 1,4,5,8-tetraazaphenanthrene; dppz = dipyrido[3,2-a:2′,3′-c]phenazine), and examine the consequences for DNA binding through the use of atomic-resolution X-ray crystallography. We find that the methyl groups are located in discrete positions with a complete directional preference. This may help to explain the qu...

Size effects on cation heats of formation. IV. Methyl and ethyl substitutions in methyl, methylene, acetylene and ethene

An empirical relation between the heat of formation of molecular ions and cation size is used to study the effects of methyl and ethyl substitution of hydrogen atoms on the cations of the CnHm hydrocarbons methyl, methylene, acetylene and ethene. The results provide tests of the graphical method, revealing regularities and irregularities in the empirical size relation used, as well as its value as a predictive tool for determining cation and neutral heats of formation. Of the 36 CnHm cations studied, only 5 have heats of formation listed in the renowned ATcT tables. Some CnHm cation heats of formation are questioned or eliminated, mainly in cases where multiple choices are available in the literature. Proposals are made for investigating or re-investigating the ionisation energies and the heats of formation of several of the molecules studied where no data previously exist or where our analysis suggests that more reliable values are needed. The relative effects of methyl and ethyl substitution on the thermodynamic stability of the series of alkyl-substituted CnHm cations are discussed.

Methyl substitution effect on the jet-cooled laser-induced fluorescence spectrum of cyclohexoxy radical.

Understanding the structure and properties of cyclohexoxy radical and its substitutes is important because of their presence in combustion processes, in atmospheric chemistry, and as intermediates in the hydrocarbon reactions. In this work, jet-cooled laser-induced fluorescence (LIF) spectra of five dimethyl substituted cyclohexoxy radicals are obtained for the first time. The correlation between the spectral variations and the radical structural changes is studied with the assistance of theoretical calculations at the B3LYP/6-31+G(d) and CASSCF/6-31+G(d) levels. The results show that the spectral characters of the dimethylcyclohexoxy radicals and their dissociation kinetics are predominantly affected by the methyl substitution position related to the C-O group. The spectral effect of the two methyl groups will add up if they locate on asymmetric carbons of the cyclohexoxy ring. Methyl substitution on β carbon weakens the six-member ring of cyclohexoxy and results in unimolecular dissociation via β C-C bond cleavage on the methyl group side and forms vinoxy variants. This study clearly shows that the LIF spectra can be used to identify cyclohexoxy and the isomers of its methyl substitutes. The results will help to understand the photochemistry of cyclic hydrocarbons in the atmospheric and combustion processes.

Metal-catalyzed C-C bond cleavage in alkanes: effects of methyl substitution on transition-state structures and stability.

Methyl substituents at C-C bonds influence hydrogenolysis rates and selectivities of acyclic and cyclic C2-C8 alkanes on Ir, Rh, Ru, and Pt catalysts. C-C cleavage transition states form via equilibrated dehydrogenation steps that replace several C-H bonds with C-metal bonds, desorb H atoms (H*) from saturated surfaces, and form λ H2(g) molecules. Activation enthalpies (ΔH(‡)) and entropies (ΔS(‡)) and λ values for (3)C-(x)C cleavage are larger than for (2)C-(2)C or (2)C-(1)C bonds, irrespective of the composition of metal clusters or the cyclic/acyclic structure of the reactants. (3)C-(x)C bonds cleave through α,β,γ- or α,β,γ,δ-bound transition states, as indicated by the agreement between measured activation entropies and those estimated for such structures using statistical mechanics. In contrast, less substituted C-C bonds involve α,β-bound species with each C atom bound to several surface atoms. These α,β configurations weaken C-C bonds through back-donation to antibonding orbitals, but such configurations cannot form with (3)C atoms, which have one C-H bond and thus can form only one C-M bond. (3)C-(x)C cleavage involves attachment of other C atoms, which requires endothermic C-H activation and H* desorption steps that lead to larger ΔH(‡) values but also larger ΔS(‡) values (by forming more H2(g)) than for (2)C-(2)C and (2)C-(1)C bonds, irrespective of alkane size (C2-C8) or cyclic/acyclic structure. These data and their mechanistic interpretation indicate that low temperatures and high H2 pressures favor cleavage of less substituted C-C bonds and form more highly branched products from cyclic and acyclic alkanes. Such interpretations and catalytic consequences of substitution seem also relevant to C-X cleavage (X = S, N, O) in desulfurization, denitrogenation, and deoxygenation reactions.

Selective coordination of multidentate ligands in manganese(II) complexes: Syntheses, structures and phenoxazinone synthase mimicking activity

The present report describes the syntheses, structures and catalytic activity mimicking the function of phenoxazinone synthase of four new mononuclear manganese(II) complexes, [Mn(L1)Cl2] (1), [Mn(L2)Cl2] (2), [Mn(L3)Cl2] (3) and [Mn(L4)Cl2]·H2O (4), derived from the multidentate ligands L1–L4, which are the 1:2 condensation products of triamines (or tetraamine) and 2-pyridinecarboxaldehyde (or 6-methyl-2-pyridinecarboxaldehyde). X-ray crystallography reveals the preferential coordination ability of all the ligands (cyclic isomers over the acyclic analogues) to a common metal center (Mn2+) and in one case (2) methanol addition across the –CN– bond. All the complexes show moderate phenoxazinone synthase activity and the experimental data suggest that a correlation can be drawn between E1/2 and Vmax of the complexes, namely the lower the E1/2 value the higher the catalytic activity. The effect of methyl substitution on the pyridine ring (in 3) on the matrix parameters and on the phenoxazinone synthase activity is also noticeable. The ESI-MS positive spectrum of a mixture of a representative complex (1 or 4) and o-aminophenol shows a peak corresponding to [Mn(L)(OAP)]+ (OAPH=o-aminophenol), suggesting that the catalytic activity proceeds through the formation of a stable complex–substrate intermediate.

Theoretical study, and infrared and Raman spectra of copper(II) chelated complex with dibenzoylmethane

There are some discrepancies in both the vibrational assignments and in the metal–ligand (M–L) bond strengths predicted in the previous studies on the copper (II) chelated complex of dibenzoylmethane, Cu(dbm)2. Also, there is a lack of theoretical structure, Raman spectrum and full vibrational assignment for Cu(dbm)2 in the literatures. Density functional theory (DFT) at the B3LYP level and also MP2 calculations using different basis sets, besides Natural Bond Orbital (NBO) and Atoms-in-Molecules (AIM) analyses, have been employed to investigate the effect of methyl substitution with the phenyl group on the stabilities of bis(acetylacetonate) copper (II), Cu(acac)2, and Cu(dbm)2 complexes and the electron delocalization in their chelated rings. Measured solid phase infrared and Raman bands for Cu(dbm)2 complex have been interpreted in terms of the calculated vibrational modes and detailed assignment has been presented. We concluded that, theoretically, the results of charge transfer studies, and experimentally, in-phase symmetric OCuO stretching mode of these complexes are very useful measures for M–L bond strength. The electron delocalization in the chelated rings and the M–L bond strength in Cu(dbm)2 are concluded to be higher than those in Cu(acac)2. The calculated geometries and vibrational results are in good agreement with the experimental data.

Influence of acrylated guar gum on alkyd coating properties

Purpose – The purpose of this paper is to evaluate the efficiency of acrylated guar gum (AGG) as an additive in alkyd resin for improved mechanical properties and to optimize the results of such an addition. Design/methodology/approach – For studying the effect of AGG on coating properties, guar gum was modified to various degrees of esterification and various compositions of alkyd systems were made by incorporating different concentrations of AGG. The mechanical and solvent absorption of the unmodified and modified alkyd systems were characterized. Findings – The incorporation of AGG into alkyd coating showed significant improvement of mechanical properties over the unmodified one. The modification caused an additional crosslink site through its unsaturation which led to increased crosslink density without phase separation of additive from the alkyd system which was confirmed by SEM scans. Research limitations/implications – The reactive additive, AGG used in the present study was synthesised using acryloyl chloride. Besides, it could also be synthesised from methacryloyl chloride and the effect of methyl substitution on water and solvent absorption could be studied. Practical implications – The method developed provided a simple and practical solution to improving the mechanical properties of alkyd coatings. Originality/value – The method for enhancing mechanical properties of cured alkyd system was novel and could find numerous applications in surface coatings.

Syntheses and structural characterizations of cobalt(II) complexes with N4-donor Schiff base ligands: Influence of methyl substitution on structural parameters and on phenoxazinone synthase activity

The present report deals with the syntheses, crystal structures and phenoxazinone synthase mimicking activity of three new mononuclear complexes, [Co(L1)Cl(H2O)]Cl·H2O (1), [Co(L1)(NCS)2] (2) and [Co(L2)Cl2] (3), derived from the tetradentate ligands (L1=N,N′-bis(pyridin-2-ylmethylene)-2,2-dimethylpropane-1,3-diamine and L2=N,N′-bis(6-methylpyridin-2-ylmethylene)-2,2-dimethylpropane-1,3-diamine). Single crystal X-ray crystallography reveals that the geometry of the cobalt(II) center in these complexes is found to be distorted-octahedron in which the equatorial positions are occupied by tetradentate Schiff base ligands and remaining two apical positions are bonded to either solvent molecule or counter ion. The crystal packing of the complexes is mainly stabilized by the hydrogen bonding and π–π stacking interactions. All the complexes show moderate phenoxazinone synthase activity and their comparative catalytic activity has been described. The effect of methyl substitution in pyridine ring (in 3) on the structural factor and on the phenoxazinone synthase activity has been explored. The ESI-MS positive spectrum of a mixture of the representative complex (1 or 2) and o-aminophenol shows a peak corresponding to [Co(L1)(OAP)]+ (OAPH=o-aminophenol) that supports the complex substrate adduct formation in the catalytic cycle.

Engineering short, strong hydrogen bonds in urea di-carboxylic acid complexes

The persistence of the acid⋯amide heterodimer and the effect of methyl substitution on the short strong O–H⋯O hydrogen bond is investigated in urea and methylurea di-carboxylic acid molecular complexes. Temperature dependent structural changes are also reported utilising X-ray and neutron diffraction in tandem.

Chemical structure dependent electron transport in 9,10-bis(2-phenyl-1,3,4-oxadiazole) derivatives of anthracene

In this work, we present a detailed analysis on electron transport studies of 9,10-bis(2-phenyl-1,3,4-oxadiazole) derivatives of anthracene (OXD-PH, OXD-PTOL and OXD-OTOL). The effect of methyl substitution at ortho (OXD-OTOL) and para position (OXD-PTOL) on the phenyl ring on the electron transport properties was studied and the results were compared with the anthracene derivative without any substitution at the phenyl ring. Electron transport was found to be highly dependent on the methyl substitution and electron mobilities in OXD-PTOL and OXD-OTOL were found to be lower than in OXD-PH. Mobilities were also found to be different for OXD-PTOL and OXD-OTOL, which indicates that the substitution at different places did not have a similar effect on charge transport properties. Thickness dependent trap states were observed for all three molecules with thickness dependent electron mobilities. Electron mobility was found to increase in all three molecules with the decrease in thickness, which favors their use for organic electronic devices and all three molecules had a better electron transport in comparison to Alq3. These results were explained by the DFT calculation which showed a dihedral structure. The dihedral angle was found to reduce in the anionic form of these molecules. Therefore, these molecules are likely to favor a proper stacking in the solid state form.

Ionization/dissociation processes of methyl-substituted derivates of cyclopentanone in intense femtosecond laser field

Ionization and dissociation of 2- and 3-methyl cyclopentanones have been investigated in molecular beams by their irradiation with intense 394 and 788nm laser fields with pulse duration of 90fs and intensity of 3×1013–4×1014W/cm2. The analysis of the resulting mass spectra allowed us to discern the effects of methyl substitution and its position on the outcome of ionization/dissociation processes induced by the intense femtosecond laser field. Generalized Keldysh-Faisal-Reiss (g-KFR) and ab initio G3(MP2,CCSD)//B3LYP/6–31G∗/RRKM theoretical calculations helped to uncover the formation mechanism of major ionic fragments observed in the mass spectra including C5H10+, C4H6O+, C3H3O+, C3H4O+, C3Hx+, and C2Hx+.

Size Effects on Cation Heats of Formation. II. Methyl Substitutions in Oxygen Compounds

A relation between the heat of formation of molecular ions and cation size is used to study the effects of methyl substitution on a series of species containing oxygen. These include methanol, the hydroxymethyl radical, formaldehyde and related isomers, acetaldehyde and related isomers, ketene, formic acid, and acetic acid. This size-dependent relation is found to be valid in most cases, enabling a choice to be made among conflicting reported ion and neutral heat of formation values, but it holds much less well in methyl-substituted formaldehyde and its carbene isomers. The cations formed in methanol, hydroxymethyl, hydroxycarbene, and formic acid by methyl substitution at carbon sites were found to be more stable than those substituted at oxygen sites. Suggestions are made for investigating or reinvestigating the ionization energies and the heats of formation of several of the molecules studied in cases where multiple choices are available in the literature or where our analysis suggests that more reliable values are needed.

Molecular Structure, NMR, HOMO, LUMO, and Vibrational Analysis of O-Anisic Acid and Anisic Acid Based on DFT Calculations

This work deals with the vibrational spectroscopy of O-Anisic acid (OAA) and Anisic acid (AA). The fundamental vibrational frequencies and intensity of vibrational bands were evaluated using density functional theory (DFT) with standard B3LYP/6-31G** method and basis set combinations. The vibrational spectra were interpreted, with the aid of normal coordinate analysis based on a scaled quantum mechanical force field. The infrared and Raman spectra were also predicted from the calculated intensities. The effects of carbonyl and methyl substitutions on the structure and vibrational frequencies have been investigated. Comparison of simulated spectra with the experimental spectra provides important information about the ability of the computational method to describe the vibrational modes. The13C and1H NMR chemical shifts of the DFA and CA molecules were calculated using the gauge-invariant-atomic orbital (GIAO) method in DMSO solution using IEF-PCM model and compared with experimental data.

Baeyer–Villiger oxidation with peracid generated in situ by CaLB-CLEA catalyzed perhydrolysis

Candida antarctica lipase B, immobilized as cross linked enzyme aggregates (CLEAs) was used to mediate the Baeyer-Villiger oxidation of cyclohexanone to epsilon-caprolactone, and the reaction was compared with the one using Novozym (R) 435 as catalyst. The conversion was dependent on the initial concentration of cyclohexanone, and was about 90% after 48 h at concentrations of up to 0.25 M but was decreased at higher concentrations. Caprolactone concentrations up to 0.6 M had no effect on the reaction efficiency. Among the cyclic ketones tested, the highest degree of conversion was achieved for cyclopentanone (88%) and the lowest for cyclooctanone (about 2%). The effect of methyl substitution and position of substitution on the cycloketone was studied using methylcyclohexanone and it has shown to influence the conversion efficiency. Both hydrogen peroxide and the reaction by-product acetic acid had a deleterious effect on the stability of the biocatalyst. (C) 2012 Elsevier B.V. All rights reserved. (Less)

Substituent Effect on the Intermolecular Arrangements of One-Dimensional Molecular Assembly on the Si(100)-(2×1)-H Surface

The effect of methyl substitution in styrene molecules on the spatial arrangement of molecules in a one-dimensional (1-D) molecular assembly on the Si(100)-(2×1)-H surface has been studied using a scanning tunneling microscope (STM) at 300 K. Styrene molecules form well-defined 1-D molecular assemblies through a chain reaction mechanism along the dimer row direction, where the phenyl rings are separated by distances equal to that of the interdimer distance in a row and aligned parallel to each other. We observed that the substitution in a phenyl ring has no observable effect on the adsorption sites, configurations, and stacking of phenyl rings along the dimer row. In contrast, the methyl substitution at α site (α-methylstyrene) results in a 1-D assembly where the adsorption sites are similar to that of styrene but the adsorbed molecules are arranged in alternate geometrical configurations along the dimer row. In the case of β-methylstyrene, the adsorption sites (diagonal silicon atoms in a dimer row) and ...

Influence of the CH/π hydrogen bond on the enhancement of circular dichroism (CD) amplitude of 1,3-cyclohexadiene compounds

Introduction of methyl group(s) at bridgehead carbons at positions 10 and 5 of steroidal 1,3-dienes has been known to influence the Cotton effect of the π–π* transition at ca. 260 nm. Thus the positive sign of the circular dichroism (CD) curve of 5α-estra-1,3-dien-17β-ol becomes negative in 5α-androsta-1,3-dien-17β-ol, even though the helical sense of the diene chromophore remains practically unchanged. In order to investigate the effect of methyl substitution on the CD of 1,3-diene steroids, we carried out time dependent density functional theory (TD-DFT) calculations at the M06-2X/6-311++G(d,p)//MP2/6-31G(d,p) level. It has been shown that replacement of the bridgehead hydrogen by a methyl group greatly influenced the sign and amplitude of CD spectra of the π–π* transition. We suggest that the change in the CD curves by methyl substitution originates from CH/π hydrogen bonds between the diene π-system and a CH3 group homoallylic to it.

Bis(diphenylphosphino)acetylene-bridged ten-membered dimetallacyclic Cu(I) complexes: 2,2′-bipyridine methyl substitution effect on structure and bonding

Four new bis(diphenylphosphino)acetylene-bridged copper(I) bipyridine complexes, [Cu2(dppa)2(L)2](BF4)2; L=2,2′-bipyridine (1), 4,4′-dimethyl-2,2′-bipyridine (2), 5,5′-dimethyl-2,2′-bipyridine (3) and 6,6′-dimethyl-2,2′-bipyridine (4), have been prepared and characterized by spectroscopic methods. X-ray crystal structure analyses of complexes 2-4 reveal bis-chelate doubly bridged bimetallic ten-membered metallacycles. The structures of 2 and 3 exhibit a centrosymmetric chair-like configuration. In compound 4, the steric effect of the methyl groups at the 6,6′ positions of the bipyridine derivative induces a severe distortion in the configuration of the metallacycle. The Raman shift of the dppa triple bond stretch is significantly affected upon coordination and shifts to higher wavenumbers relatively to the free ligand. This fact was rationalized by considering the coordination effect on the relative contributions of the resonance forms of dppa. Within the series of complexes, the lowest shift of the complex based on 4,4′-Me2bipy (2) is consistent with the relatively highest basic strength of the chelating ligand in this series and the relative value of the para methyl Hammett constant. The largest 31P NMR downfield shift, relative to free dppa, observed for 1 is consistent with the relatively lowest basic strength of the chelating ligand in this series of 2,2′-bipyridine derivatives.

Theoretical study on the effects of nitrogen and methyl substitution on tris-(8-hydroxyquinoline) aluminum: An efficient exciton blocking layer for organic photovoltaic cells

We studied the effect of nitrogen and methyl substitution on tris-(8-hydroxyquinoline) aluminum (Alq(3)) with density functional theory, which has been adopted as an exciton blocking layer (EBL) in organic photovoltaic cells (OPVCs). The substitution of electron withdrawing nitrogen on the phenoxide moiety of Alq(3) lowers the highest molecular orbital (HOMO) level, thus photogenerated excitons can be effectively blocked in OPVC. Additional substitution of methyl on the pyridine moiety makes that Alq(3) has a smaller electron reorganization energy, which results in higher electron mobility with keeping HOMO level almost intact. Therefore, nitrogen and methyl simultaneous substitution shows high performance both in exciton blocking and electron mobility. This is the origins of the short circuit current enhancement in OPVC with 4-hydroxy-8-methyl-1,5-naphthyridine aluminum chelate (Alq(3) with the substitution of both nitrogen and methyl group) EBL.

Structure–Activity Studies on the Fluorescent Indicator in a Displacement Assay for the Screening of Small Molecules Binding to RNA

A series of xanthone and thioxanthone derivatives with aminoalkoxy substituents were synthesized as fluorescent indicators for a displacement assay in the study of small-molecule-RNA interactions. The RNA-binding properties of these molecules were investigated in terms of the improved binding selectivity to the loop region in the RNA secondary structure relative to 2,7-bis(2-aminoethoxy)xanthone (X2S) by fluorimetric titration and displacement assay. An 11-mer double-stranded RNA and a hairpin RNA mimicking the stem loop IIB of Rev response element (RRE) RNA of HIV-1 mRNA were used. The X2S derivatives with longer aminoalkyl substituents showed a higher affinity to the double-stranded RNA than the parent molecule. Introduction of a methyl group on the aminoethoxy moiety of X2S effectively modulated the selectivity to the RNA secondary structure. Methyl group substitution at the C1' position suppressed the binding to the loop regions. Substitution with two methyl groups on the amino nitrogen atom resulted in reducing the affinity to the double-stranded region by a factor of 40%. The effect of methyl substitution on the amino nitrogen atom was also observed for a thioxanthone derivative. Titration experiments, however, suggested that thioxanthone derivatives showed a more prominent tendency of multiple binding to RNA than xanthone derivatives. The selectivity index calculated from the affinity to the double-stranded and loop regions suggested that the N,N-dimethyl derivative of X2S would be suitable for the screening of small molecules binding to RRE.