Hydrogen Atoms Of Methyl Groups Research Articles

Электронное влияние ароматического кольца в положении 2 на H/D обмен метильных протонов в пиримидине

The H/D exchange of hydrogen atoms of methyl groups in pyrimidines containing a phenyl or substituted pyrazole ring in position 2, namely, 2-phenyl-4-methyl-5-ethoxycarbonyl-, 2-phenyl-4-methyl-5-acetyl and 2-(3,5 dimethylpyrazolyl-1-)-4-methyl-5-acetylpyrimidines was studied. It has been shown that the rate of deuterium exchange depends on the electronic influence of the lateral aromatic ring associated with pyrimidine. When a phenyl group is introduced into position 2 of the pyrimidine ring almost immediately after adding deuterated sodium methoxide (CD3ONa) to the CD3OD solution, the signals from the protons of the ester or acetyl groups disappear.

Density Functional Theory Study of the Structure of the Pillared Hofmann compound Ni(3-Methy-4,4'-bipyridine)[Ni(CN)4] (Ni-BpyMe or PICNIC-21).

We use dispersion-corrected density functional theory (DFT) to investigate the structure of the pillared Hofmann compound Ni(3-Methy-4,4'-bipyridine)[Ni(CN)4] (Ni-BpyMe for short, or PICNIC-21). We model the disorder found in experimental X-ray structure refinement via an ensemble of supercells with ordered ligand orientation configurations. The ensemble-averaged structure agrees very well with experiment, except for the positions of the methyl group hydrogen atoms. While the dihedral angles between the bipyridal rings of each BpyMe ligand of the averaged structure is 90°, the local dihedral angles are about 80°. DFT screening of configurations where the crystallographic a/b ratio is constrained to equal 1 fail to find the configurations that are most stable when a/b is set to its distorted experimental value of a/b = 0.86, demonstrating the difficulty of solving pillared Hofmann structures purely theoretically without experimental input. The waviness of the Ni(CN)2 sheets is explained as a tendency to maximize dispersion interactions between these sheets and the methyl pyridine rings. This waviness leads to greater residual pore space and greater adsorbate uptake at low pressure compared with the analogous pillared compound Ni-Bpene (PICNIC-60).

Crystal Structure, Packing, and Analysis of Hirshfeld Surfaces of 3a-(p-tolyl)-3,3a-dihydrobenzo[d]pyrrolo[2,1-b]thiazol-1(2H)-one

Crystal and molecular structures of 3a-(p-tolyl)-3,3a-dihydrobenzo[d]pyrrolo[2,1-b]thiazole-1(2H)-one obtained by condensing 4-oxo-4-(p-tolyl)butane acid with 2-aminothiophenol are studied using XRD spectroscopy. The crystal of compound 1 occurs in the form of two crystallographically independent molecules. The crystal has no hydrogen bonds and stacking interactions, but the analysis of Hirshfeld surfaces shows that the molecules are weakly stabilized by non-covalent interactions between oxygen atoms and hydrogen atoms of methyl groups as well as by the interactions between hydrogen atoms and carbon atoms of the benzothiazole fragment.

Unusual effect of temperature on direct fluorination of high temperature vulcanized silicone rubber and properties of the fluorinated surface layers

In order to investigate the effects of fluorination temperature on surface layer characteristics (chemical composition and structure, morphology, and thickness) and surface layer properties, high temperature vulcanized silicone rubber samples were fluorinated in a laboratory vessel using a F 2 /N 2 mixture with 12.5% F 2 by volume, at 0.1 MPa and the different temperatures of 25, 55, and 85 °C, for the same time of 30 min. ATR-IR analysis indicates that the substitution of fluorine atoms for hydrogen atoms of methyl groups is dominant for the fluorination at the different temperatures, while a substitution for methyl groups cannot be excluded. SEM cross-section and surface images show the fluorinated layers with nanostructured surfaces. The surface layer fluorinated at 55 °C appears to have the lowest degree of fluorination, but it clearly has the largest thickness and surface nanostructure size among the fluorinated layers. This unusual phenomenon is caused by two competing effects, the thermal activation of the substitution reaction and fluorine diffusion through the surface layer and the steric hindrance by the fluorinated methyl groups to the fluorination and fluorine diffusion. Contact angle measurements reveal that the fluorinated surfaces have high hydrophobicity and oleophobicity and thus an extremely low surface energy. Surface potential decay measurements show a much more rapid decay of potential on the fluorinated samples, compared to the unfluorinated sample. There is no significant difference in surface hydrophobicity or surface conduction between the fluorinated samples, due to the combined effect of compositional and structural changes. The difference in surface oleophobicity between the fluorination at 55 °C and the fluorination at 25 or 85 °C provides further evidence for the difference in their surface chemistry, because surface geometry of the fluorinated samples has little effect on the oleophobicity.

A new potential model for acetonitrile: Insight into the local structure organization

Thorough understanding of the microscopic organization and dynamics of individual constituents is a crucial step in the description and the prediction the properties of electrolyte solutions based on dipolar aprotic solvents such as acetonitrile. For this aim, a new potential (force field) model for acetonitrile was developed on the basis of comprehensive approach comprising quantum chemical calculations, ab initio molecular dynamics simulations and empirical parameterization. The developed potential model is able to reproduce the experimental thermodynamic and dynamic properties of neat acetonitrile in the range of temperatures between 228.15 and 348.15K.The local structure of neat liquid acetonitrile then was analyzed in a framework of the nearest neighbor approach. It was shown that the distance standard deviations relative to the average distance between the nearest neighbors have a non-linear behavior that was traced back to the changes in the mutual orientation between acetonitrile molecules. The closest neighbors have a dominant antiparallel dipoles orientation with respect to a reference acetonitrile molecule, while for the further nearest neighbors perpendicular and parallel mutual orientation is observed. The nearest neighbors approach in combination with angular distribution functions was used for the estimation of the Kirkwood factor. Our results show that in order to reproduce the corresponding experimental values derived in the framework of the Onsager-Kirkwood-Fröhlich theory, it is necessary to take into account the mutual orientation of the 5–6 nearest neighbors. Although the atomic charges, on N and the methyl group hydrogen atoms, are negative, the values of the N⋯H distance and the N⋯HC (methyl group), are compatible with a weak hydrogen bond between the two atoms.

Isomeric Effect on the Pharmacokinetic Behavior of Anticancer CuII Mixed Chelate Complexes: Experimental and Theoretical Approach

We present herein the results of a pharmacokinetic (PK) study of CuII mixed‐chelate coordination compound CasIII‐La {[Cu(5,6‐dimethyl‐1,10‐phenanthroline)(acetylacetonate)(H2O)]NO3}, which belongs to the antineoplastic family of compounds known as Casiopeínas. The main interest in this molecule lies in the big difference in its lethal dose and PK parameters in Wistar rats compared with its structural isomer CasIII‐Ea {[Cu(4,7‐dimethyl‐1,10‐phenanthroline)(acetylacetonate)(H2O)]NO3}. For instance, the half‐lives of these compounds are 9.6 ± 1.15 and 48 ± 4.6 h for CasIII‐La and CasIII‐Ea, respectively. The differences in the half‐lives have been attributed to their different capacities to cross cellular membranes. Molecular dynamics studies of a fraction of a cell membrane and Casiopeínas were developed to identify the possible intermolecular interactions responsible for the retention of the compound inside the cellular membrane. Also, quantum theory of atoms‐in‐molecules studies was developed to determine the capacity of the hydrogen atoms of methyl groups to participate in intermolecular interactions. In addition, peripheral human blood lymphocyte and macrophage samples were used to determine the cross‐membrane effect on cytotoxicity.

Crystal structure of S,S′-dimethyl-N-nitroimidodithiocarbonate

The molecule of S,S′-dimethyl-N-nitroimidodithiocarbonate has a planar geometry (with the exception of hydrogen atoms of methyl groups), stabilized by the shortened S…O contact. The bond lengths in the >С=N–NO2 nitrimine moiety, in contrast to nitroguanidine derivatives, make it possible to consider the molecular structure of the compound within the traditional valence formula. The crystal structural feature of the compound is a low energy of its crystal lattice.

IR spectroscopic and quantum-chemical investigation of perchlorate anion solvation in acetonitrile

The solvation of anions in solutions of tetrabutylammonium perchlorates in acetonitrile is studied by means of quantum-chemical calculations and Fourier-transform infrared spectroscopy. It is shown that perchlorate-anions form stable solvate complexes with acetonitrile molecules due to ion-dipole interactions between atoms of oxygen in the composition of ClO4− anion and hydrogen atoms of methyl groups of solvent molecules. It is established that the energy of formation of anionic solvate complexes ClO4−(CH3CN)n (n = 1–4) is comparable in value that of cation solvate complexes Li+(CH3CN)n (n = 1–4) and is about one third of the latter. This requires us to consider the solvation of perchlorate anions along with the solvation of metal cations in acetonitrile. Spectroscopic signs of perchlorate anion solvation by acetonitrile molecules are analyzed.

Isomers of the acetic acid-water complex trapped in an argon matrix.

The complexes of acetic acid (AcOH) with water have been studied using FTIR matrix isolation spectroscopy and DFT/B3LYP, DFT/B3LYP-D, and MP2 calculations with the aug-cc-pVTZ basis set. The AcOH/H2O/Ar matrices were prepared in two different ways. In one set of experiments, the vapor above a solid AcOH sample, cooled to 203 K, was diluted with H2O/Ar mixture in the vacuum chamber of the cryostat, and the mixture was solidified on the target. In the second set of experiments, the matrix was prepared by simultaneous deposition of AcOH/Ar and H2O/Ar mixtures at room temperature. The first method of matrix preparation strongly favors the formation of the "acyclic" higher energy AcOH-H2O complex I(B) compared to the second one. Warming of matrices containing the higher energy complex, I(B), from 11 to 39 K, results in the decrease of I(B) concentration and formation of the lowest energy cyclic complex I(A). The calculations indicate that I(B) is formed by an O-H···O hydrogen bond between the carbonyl oxygen and a water O-H group and, additionally, by a weak interaction between one of the methyl group hydrogen atoms and the water oxygen atom. The cyclic complex I(A) has a six-membered ring involving two O-H···O bonds. An activation energy of 0.94, 1.71, and 1.38 kcal mol(-1) was calculated for the I(B) → I(A) rearrangement at the B3LYP, B3LYP-D, and MP2 levels of theory, respectively. Van't Hoff plots for the association of H2O and AcOH leading to formation of the complexes I(A) and I(B) are presented and discussed. Evidence is also given for the formation of the AcOH-(H2O)2 and (AcOH)2-H2O complexes in the matrices. A potential atmospheric impact of the enhanced formation of the higher energy I(B) complex at low temperatures is discussed.

Structure and some properties of [UO2CrO4{CH3CON(CH3)2}2

A new complex [UO2CrO4{CH3CON(CH3)2}2] (I) was studied by thermal analysis, IR spectroscopy, and X-ray crystallography. The crystals are monoclinic: a = 13.8108(11) A, b = 8.6804(7) A, c = 13.0989(10) A, β = 104.777(1)°, V = 1518.4(2) A3, space group P21/c, Z = 4, R = 2.39%. The structure of I contains infinite chains of the [UO2CrO4{CH3CON(CH3)2}2] composition running along [001]; the complex belongs to the AT11M1 2 crystal-chemical group (A = UO 2 2+ , T11 = CrO 4 2− , M1 = CH3CON(CH3)2) of uranyl complexes. The chains are linked into a three-dimensional framework due to hydrogen bonds between oxygen atoms of chromate ions and hydrogen atoms of methyl groups of the dimethylacetamide.

Molecular structure of trimethyl[(3-indole)ethoxy]silane cooled in a supersonic jet

Conformations of He-jet-cooled trimethyl[(3-indole)ethoxy]silane (TIES) have been studied using a laser spectroscopy technique in combination with quantum-chemical computations. Six probable conformers of the molecule were computed, of which only two conformations were observed. Based on an analysis of fluorescence excitation spectra, fluorescence spectra, shapes of rotational band contours at the electronic S0–S1 transition of TIES, and theoretical computations, the above conformers were assigned to steric structures. Twisted structures have the lowest energy due to intramolecular hydrogen bonds $$ C - H \cdots O <_C^{Si} $$ between hydrogen atoms of methyl groups and an oxygen atom and C–H···π between H and the π-electron cloud of the indole ring.

Ground and ionic states of 1,2,5-thiadiazoles: An UV-photoelectron spectroscopic and theoretical study

Electronic structures of 1,2,5-thiadiazoles (X 2C 2N 2S, where X = H, F, Cl, CH 3, CN) have been investigated in the gas phase by He I and He II UV-photoelectron spectroscopy, as well as by calculations at the B3LYP and SAC-CI levels using aug-cc-pV(T+d)Z basis sets. The ground state geometry of the neutral molecules has been obtained from quantum-chemical calculations at the B3LYP level. Thiadiazoles are predicted to be planar, not considering methyl-group hydrogen atoms, with C 2v symmetry. Equilibrium structures of the four lowest energy cationic states have been determined at the UB3LYP level, and calculations predicted that the sequence of ionic states is 2B 1(ground state) < 2B 2 < 2A 2 < 2A 1. Photoelectron spectroscopy and theory applied to thiadiazoles has provided information on the valence occupied levels, on the ionisation energies, and on the fundamental vibrations of the cations.

Self-assembly of 2,6-dimethylpyridine on Cu(1 1 0) directed by weak hydrogen bonding

Sequential stages of formation of a self-assembled monolayer of flat-lying 2,6-dimethylpyridine molecules on a single crystal Cu(1 1 0) surface have been observed by low-temperature scanning tunneling microscopy (LT-STM). At an adsorption temperature of 10 K, all of the molecules are randomly distributed at low coverage upon adsorption. The isolated molecules align their molecular axes parallel to the 〈0 0 1〉 azimuth of the Cu lattice. The nitrogen atom in the molecule is located at the four-fold hollow site. Upon annealing to 100 K, the molecules associate to form head-to-head dimers. The dimer units involve a pair of weak hydrogen bonds between methyl group-hydrogen atoms and N moieties on adjacent molecules, forming a core structure for further growth. In a later stage of self-assembly, single head-to-tail weak hydrogen bonds between ring C–H bonds and N moieties form in chains on the periphery of the central cores, leading to larger domains with a c(6 × 2) overlayer structure.

Ab initio and DFT investigation of fluorinated methyl hydroperoxides: Structures, rotational barriers, and thermochemical properties

Ab initio and density functional theory (DFT) calculations have been performed on CH 2FOOH, CHF 2OOH, CF 3OOH, CF 2ClOOH, and CFCl 2OOH. Geometries of stable conformers are optimized at the MP2(FULL)/6-31G(d,p) and B3LYP/6-31G(d,p) levels of theory. The enthalpies of formation ( Δ H f , 298 ° ) and the ROO H, RO OH, and R OOH bond dissociation enthalpies (BDEs) are estimated for each of the studied hydroperoxides using the calculated reaction enthalpies ( Δ H rxn , 298 ° ) of the adopted isodesmic reactions. The results show that the progressive fluorine substitution of hydrogen atoms in methyl group results in an increase in each of BDE(O H), BDE(O O), and BDE(C O). This has been explained in terms of the stabilizing influence of fluorine-substituted methyl groups. However, the replacement of F by Cl when going from CF 3OOH to CFCl 2OOH leads to a decrease in both BDE(O O) and BDE(C O). Potential energy barriers for internal rotation about C O and O O are calculated at the B3LYP/6-31G(d,p) level for each of the studied hydroperoxides.

Neutron diffraction of p-hydroxyacetanilide (Paracetamol): libration or disorder of the methyl group at 100 K

A single-crystal pulsed neutron diffraction study of p-hydroxyacetanilide (the analgesic Paracetamol, C 8H 9NO 2) was carried out at 100 K. The structure, in good agreement with previous X-ray determination, is monoclinic, space group P2 1 a , a = 12.763(6), b = 9.250(4), c = 7.082(2) A ̊ , β = 115.52(3)°, D x = 1.34 g cm −3, Z = 4. The present neutron data allowed full anisotropic refinement of all atoms including the hydrogens, yielding an agreement factor wR( F) = 0.063, for 2882 unique reflections and 181 refined parameters. Large, very anisotropic thermal ellipsoids are found on the methyl group hydrogen atoms; librational analysis was carried out along with attempts to model possible disorder associated with these large thermal vibrations.

Solute-solute correlations in aqueous solutions of tetramethylammonium chloride

Neutron diffraction with hydrogen/deuterium isotope substitution on the methyl group hydrogen atoms of the solute is used to investigate the distribution of the tetramethylammonium (TMA) ion, (CH3)4N+, in aqueous solution of 4·0 molal tetramethylammonium chloride at room temperature. The centre-centre correlation function for TMA ions is calculated from the HH correlation function for the solute by deconvolution of a spherical form factor representing the distribution of methyl hydrogen atoms. The centre-centre function shows a broad distribution of TMA … TMA distances, but indicates that close contacts occur frequently. This suggests that the charge interaction of TMA is not important in determining the distribution and that the TMA ion behaves more like an apolar solute.

Neutral solvent/crown ether interactions 3. Reorientation of the hydrogen bonds in the low temperature (?150�C) structure of 18-crown-6�2(CH3NO2)

The title complex was crystallized from a saturated solution of 18-crown-6 in nitromethane at 5°C and cooled to −150°C prior to X-ray diffraction data collection. At −150° C 18-crown-6·2(CH3NO2) is monoclinic,P21/n witha=9.290(2),b=7.864(6),c=13.627(8) A, β=1000.84(4)° andD calc=1.31 g cm−3 for Z=2. Leastsquares refinement using 1521 independent observed reflections [F o≥5σ(F o)] led to a final conventionalR value of 0.041. The complex at −150°C is isostructural with its room temperature structure with the exception of the orientation of the methyl hydrogen atoms and their crown ether oxygen interactions. The methyl group hydrogen atoms were fully refined isotropically. The crown ether resides around a center of inversion and hasD 3d symmetry. There is one methyl hydrogen...crown interaction at 2.35(3) A, one apparently bifurcated hydrogen bond utilizing a second methyl hydrogen atom (2.55(3), 2.65(3) A) and the third hydrogen atom is actually directed away from the crown ring (closest H...O contact=2.67(3) A).

The molecular structure of gaseous tetrafluoro- p-benzoquinone and tetramethyl- p-benzoquinone as determined by electron diffraction

The molecular structures of gaseous tetrafluoro-p-benzoquinone ( p-fluoranil) and tetramethyl- p-benzoquinone (duroquinone) have been investigated by electron diffraction. Except for the methyl group hydrogen atoms, the molecules are planar to within experimental error, but small deviations from planarity are completely compatible with the data. Values for the geometrical parameters (r adistances and r α with parenthesized uncertainties of 2σ including estimated uncertainty in the electron wavelength and correlation effects, are as follows. Tetrafluoro-p-benzoquinone: D 2h symmetry (assumed); r(C0) = 1.211(6) Å, r(CC) = 1.339(12) Å, r(C-C) = 1.489(5') Å, r(C-F) = 1.323(5) Å, ∠C-C-C = 116.8(7)° and ∠C-C-F = 116.1(7)°. Tetramethyl- p-benzoquinone: C 2h symmetry (assumed);r(C-H) = 1.102(18) Å, r(CO) = 1.229(8) Å, r(CC) = 1.352(8) Å, r(C sp2-C sp2) = 1.491(11) Å, r(C sp2-C sp3) = 1.504(12) A, ∠C-CO-C = 120.8(8)°. ∠C-C-CH 3 = 116.1(8)°, ∠C-C-H = 110.5(34)° and α 1 = α 2 (methyl torsion = 30° (assumed).

13C NMR investigation of adsorbed hydrocarbons

By means of Fourier transform techniques 13C nmr spectra and longitudinal 13C magnetic relaxation times of various butene isomers sorbed on NaY and NaX zeolites have been studied. In contrast to 1H resonance where only two broad lines can be observed, the 13C spectra show separated sharp lines. The shift Δσ = σ ads- σ free of the carbon-13 lines of adsorbed molecules with respect to the resonance positions of free molecules and the longitudinal relaxation times indicate a peculiar behaviour of the groups  CH and = C ▪ of the adsorbed n-butene and isobutene molecules, respectively. The resonance deviations Δσ to lower fields in these groups decrease in the order isobutene ( δσ = − 10.3 ppm), but-1-ene (δσ ≈ −5.7 ppm) and but-2-enes (Δσ ≈ − 2.2–2.4 ppm), whereby the spectra of cis-but-2-ene and trans-but-2-ene show similar differences as found in the liquid state. When adsorbed on NaY and NaX types (silica/alumina ratios 5 and 3.5) the 13C nmr line positions of but-1-ene molecules are the same within the limits of experimental error, which demonstrates that electrostatic fields have no noticeable influence on the molecular parameters in these systems. Therefore one may conclude that chemical bonds to Na + cations have the dominant effect on the line shift Δσ. CNDO-MO calculations based on such a model have shown that a charge transfer occurs from the π-electrons and the electrons of the methyl group hydrogen atoms to the cation.

The Selectivity of Catalysts in the Hydrogen Exchange Reaction of p-Xylene with Deuterium Oxide

The isotopic exchange reaction of p-xylene with deuterium oxide was studied using rhodium, silver, and nickel-alumina as catalysts at 100 deg C, and the results were compared with those of previous studies. The reaction products were analyzed by infrared and mass-spectrometric methods. On rhodium catalyst, the exchange occurred at both hydrogen atoms of methyl groups and benzene ring of p-xylene, but on silver the exchange did not take place under the same experimental condition. The following order of the selectivity for benzene ring was found to exist among the metals studied: Pt > Pd > Rh > Ni, a new relation between the selectivity and the geometric factor of catalysts. When the surface of nickel-alumina had been previously exposed to diethylsulfide at room temperature, the exchange at methyl groups was more retarded than at the benzene ring. Furthermore, selectivity for the demethylation of p-xylene, low as it was, was confirmed only in the case of nickelalumina under similar experimental conditions. The results suggest that the mechanism of the exchange of benzene ring is not the same for the cases of palladium and nickel-alumina catalysts. (auth)