Equatorial Methyl Research Articles

Fundamental Studies of NOx Emission Characteristics in Dimethyl Ether (DME)/Air Non-premixed Flames

The NOx emission characteristics of dimethyl ether (DME) in laminar co-axial jet and counterflow non-premixed flames were investigated using experimental and numerical approaches, respectively. The flame structure and NOx emissions of DME were compared to those of C2H6, which has equivalent methyl structures but lacks an oxygen atom. Experimental results showed that, in the co-axial jet case, the combustion of DME had the characteristics of a partial premixed flame. Additionally, it had a shorter flame and lower NOx emissions compared to the C2H6 flame. It is thus concluded that the major cause of low NOx emissions from DME co-axial jet flames may be the short flame length because of the lower stoichiometric air/fuel ratio. The activation of reburning NO chemistry because of the characteristics of the partially premixed flame may also play a role. The computational results of the DME counterflow non-premixed flame revealed that the EINO decreased by approximately 50% relative to that of the C2H6 flame. Although the overall NOx reaction path of the DME flame is similar to that of the C2H6 flame, it is concluded that the DME non-premixed flame has a distinct NO reduction mechanism. This is associated with reburning NO chemistry in the fuel-rich region because of fast pyrolysis and oxidation reactions in comparison to that of the C2H6 flame.

The quenching of isopropyl group rotation in van der Waals molecular solids

X-ray diffraction experiments are employed to determine the molecular and crystal structure of 3-isopropylchrysene. Based on this structure, electronic structure calculations are employed to calculate methyl group and isopropyl group rotational barriers in a central molecule of a ten-molecule cluster. The two slightly inequivalent methyl group barriers are found to be 12 and 15 kJ mol(-1) and the isopropyl group barrier is found to be about 240 kJ mol(-1), meaning that isopropyl group rotation is completely quenched in the solid state. For comparison, electronic structure calculations are also performed in the isolated molecule, determining both the structure and the rotational barriers, which are determined to be 15 kJ mol(-1) for both the isopropyl group and the two equivalent methyl groups. These calculations are compared with, and are consistent with, previously published NMR (1)H spin-lattice relaxation experiments where it was found that the barrier for methyl group rotation was 11+/-1 kJ mol(-1) and that the barrier for isopropyl group rotation was infinite on the solid state NMR time scale.

Towards the complete analysis of the rotational spectrum of (CH 3) 3SnCl

The rotational spectrum of the symmetric top trimethyl tin chloride (CH 3) 3SnCl has been studied using a pulsed molecular beam Fourier transform microwave spectrometer in the frequency range from 3 to 24 GHz. The spectrum is exceedingly complicated by the internal rotation motions of the three equivalent methyl tops, the high number of Sn- and Cl-isotopes and the quadrupole hyperfine structure of the chlorine nucleus. In this paper, we present the microwave spectrum, ab initio calculations, permutation inversion (PI) group-theoretical considerations, Stark-effect measurements and finally the K = 0 assignments and fits of the different torsion–rotation species. Based on the Stark-effect measurements, the dipole moment is μ = 3.4980(30) D. Due to Δ K = ± 1 -mixing effects we observe linear Stark-effect behavior and additional quadrupole splitting for some K = 0 torsion–rotation transitions in (CH 3) 3SnCl, which can be group-theoretically explained. The symmetric rotor fit of A 1 and A 2 torsion-rotation states leads to an effective B-constant of 1680.040124(92) MHz for the main isotopologue (CH 3) 3 120Sn 35Cl. A global fit of 182 K = 0 torsion–rotation transitions yields a V 3 torsional barrier of 1.774(6) kJ.

Benzyl group conformation in 4-benzyl-4-hydroxypiperidines

The high-resolution 1H and 13C NMR spectra of eight 4-benzyl-4-hydroxypiperidines 1–8 were recorded in CDCl3 and analyzed. In 2, the conformation of the equatorial benzyl group at C(4) was established as an equilibrium mixture of A [the phenyl group is gauche with respect to OH and C(5)] and B [the phenyl group is gauche with respect to OH and C(3)], whereas in 3-alkyl-4-benzyl-4-hydroxypiperidines 3–8, the favored conformation of the benzyl group at C(4) is A. In 1, the axial benzyl group at C(4) adopts the gauche conformations A′ [the phenyl group is gauche with respect to OH and C(3)] and B′ [the phenyl group is gauche with respect to OH and C(5)], in which the phenyl ring of the benzyl group is gauche with respect to the OH group. The HF/DFT B3LYP/6-3G* hybrid calculations of model systems 1′–3′ also support these conformations. The 13C data reveal that the equatorial methyl group at C(3) exerts a shielding influence on the methyl-bearing carbon and the magnitude of the α effect was found to be approximately −1.5 ppm. The parameters of the 13C substituent in the benzyl group show that the the α effect of the equatorial benzyl group is considerably higher in 3-ethyl tertiary alcohol 7 than in 3-methyl tertiary alcohol 3 and 4-benzyl-t(4)-hydroxypiperidine 2. This may be explained if we take into account the different conformations of the ethyl group in t(4)-hydroxy-3-ethyl-2,6-diphenylpiperidine 12 and 3-ethyl tertiary alcohol 7.

Synthesis and dynamic NMR studies of Pt(η 5-C 5Me 5)(CO){C(O)NR 2} complexes

The formation of Pt(η 5-C 5Me 5)(CO){C(O)NR 2} (R = Me, Et) complexes was established by spectroscopic analysis. The infrared spectra of these complexes showed a sharp absorption due to the presence of coordinated carbonyl group in the region 2017–2013 cm −1. The N, N-dialkylcarbamoyl ligands showed a characteristic C O stretching absorption in the range 1609–1616 cm −1. The proton NMR spectra of these complexes revealed the expected singlet arising from five equivalent methyl groups on the cyclopentadienyl ring with satellites due to coupling to 195Pt. The N-methyl and N-ethyl protons exhibited very broad resonances due to restricted rotation about the N–C bond at room temperature. On cooling to −30 °C, the N, N-dimethyl protons for complex Pt(η 5-C 5Me 5)(CO){C(O)NMe 2} showed two sharp singlets at δ 2.86 and 3.09 ppm as expected for the static structure. For the N, N-diethyl derivative, Pt(η 5-C 5Me 5)(CO){C(O)NEt 2}, the methyl protons exhibited only a single triplet at δ 1.06 ppm at −10 °C due to coupling with the methylene protons. This single resonance arises through accidental overlap as the methylene protons of the ethyl groups are inequivalent at this temperature and each exhibited a quartet at δ 3.33 and 3.70 ppm due to coupling with the methyl protons. The singlet resonances for the methyl and ring carbons of the η 5-C 5Me 5 group found in 13C{ 1H} NMR spectra are illustrative of the chemical equivalence of all the carbon atoms caused by free rotation of the ring in these complexes. The signals attributable to the carbonyl and carbamoyl carbon atom resonances are found downfield as two singlets each with a large coupling constant to platinum. The platinum coupling constants of the downfield resonances could not be identified for Pt(η 5-C 5Me 5)(CO){C(O)NMe 2} due to presence of impurities.

Spectroscopic and Theoretical Investigation of (R)-3-Methylcyclopentanone. The Effect of Solvent and Temperature on the Distribution of Conformers

Temperature-dependent electronic circular dichroism (CD) spectra are reported for (R)-(+)-3-methylcyclopentanone (R3MCP) in 34 solvents. Analysis of these data yielded the enthalpy and entropy differences between axial methyl and equatorial methyl conformers, the dominant conformers for R3MCP. The weakly absorbing n-->pi* transition exhibited a decrease in lambdamax as the solvent polarity increased. Vibrational CD fine structure of the n-->pi* transition was observed in the gas phase in addition to several of the solvents. Vapor-phase CD spectra were compared to both the solution-phase spectra and theoretical calculations of the axial and equatorial methyl conformers. Optical rotation (OR) measurements for R3MCP in cyclohexane solution in the visible region showed excellent agreement with OR DFT calculations of the equatorial and axial methyl conformers. Density functional calculations with B3LYP and the 6-13G* and aug-cc-pVDZ basis sets, which incorporate the solvent dielectric constant, yielded trends in thermodynamic quantities as a function of polarity and solvent dipole moments that are only weakly or not observed in experiments.

Two new Ni(II) Schiff base complexes: X-ray absolute structure determination, synthesis of a 15N-labelled complex and full assignment of its 1H NMR and 13C NMR spectra

The Ni(II) complex of the Schiff base of ( S)- N-(2-benzoyl-4-chlorophenyl)-1-benzylpyrrolidine-2-carboxamide and glycine ( 1) [GKCl] and the hemihydrate of the Ni(II) complex of the Schiff base of ( S)- N-(2-benzoylphenyl)-1-benzylpyrrolidine-2-carboxamide and 2-aminoisobutiric acid ( 2) [Me 2GK] were prepared and their absolute structures determined. The conformations of the complexes and their hydrogen bonding are discussed in detail. In complex 2, the repulsion between the benzyl group and an equatorial methyl group should affect the conformation of the benzyl group, distribution of the π-electron density in this group and distortion of the internal coordination sphere, while for complex 1, only minor conformational changes were expected. 1H and 13C NMR data for the 15N-labelled complex 2 were acquired and fully assigned in order to study the influence of π-electron density of the benzyl group to the long-range 13C– 15N and 13C– 13C spin–spin interactions.

Structure of (Z)-(5R)-methyl-2-(4-phenylbenzylidene)cyclohexanone as chiral component of liquid-crystalline systems

The spatial structure of (Z)-(5R)-methyl-2-(4-phenylbenzylidene)cyclohexanone prepared by photochemical isomerization of the E-isomer was studied by analyzing the magnitudes and temperature dependence of the proton spin-spin coupling constants obtained by 1H NMR spectroscopy and the results of molecular modeling using semiempirical quantum chemical AM1 and PM3 methods and the density functional theory (DFT). Comparison of the results obtained for the Z-and E-isomers shows that in both cases the conformational equilibrium for both isomers is characterized by significant preference of the chair conformer having an equatorial methyl group, namely, − ΔH (chair a ⇌ chair e) = 1.98–2.12 and 1.36–1.54 kcal mole−1 for the Z-and E-isomers, respectively. Distinctions in the non-planarity of the enone fragment and cyclohexanone ring in the Z-and E-isomers under study following from the results of mathematical modeling were confirmed by the experimental values of the geminal spin-spin coupling constants of protons of the methylene groups in α,α ′-positions with respect to the enone group. Quantum chemical calculations of the Z-isomer revealed the existence of intramolecular hydrogen bond between the carbonyl oxygen and the nearest aromatic proton in ortho-position of the benzene ring. Possible reasons for different helical twisting power of (Z)-(5R)-methyl-2-(4-phenylbenzylidene)cyclohexanone and the E-and Z-arylidene derivatives of 1R, 4R-isomenthone in the mesophase are discussed based on the results of molecular structure studies for these compounds.

Full Non-rigid Group of Sponge and Pina

The non-rigid molecule group theory (NRG) in which the dynamical symmetry operations are defined as physical operations is a new field in chemistry. Smeyers and Villa computed the r-NRG of the triple equivalent methyl rotation in pyramidal trimethylamine with inversion and proved that the r-NRG of this molecule is a group of order 648, containing a subgroup of order 324 without inversions (see J. Math. Chem. 28(4) (2000) 377–388). In this work, a computational method is described, by means of which it is possible to calculate the symmetry group of molecules. We study the full non-rigid group (f-NRG) of Sponge and Pina molecules with C 2 and C i point groups, respectively. It proved that these are groups of order 162 and 13122 with 54 and 3240 conjugacy classes, respectively. The character tables of these groups are also computed.

Electronic spectra of molecules with two C3 v internal rotors: Torsional analysis of the [formula omitted] LIF spectrum of biacetyl

The laser-induced fluorescence excitation spectrum of the biacetyl A 1 A u ( S 1)– X 1 A g ( S 0) transition in the region from 22 182 to 22 800 cm −1 shows a complicated absorption line spectrum, which is believed to arise from a long progression in the torsional vibrations of the two equivalent methyl tops in this molecule. In this paper, we discuss three topics: (i) a numerical calculation of these energy levels using a kinetic and potential energy formalism and constants from the literature [M.L. Senent, D.C. Moule, Y.G. Smeyers, A. Toro-Labbé, F.J. Peqalver, J. Mol. Spectrosc. 164 (1994) 66], (ii) a qualitative description of the calculated energy level pattern using local-mode ideas applied to the two equivalent methyl rotors together with G 36 permutation-inversion group symmetry species, and (iii) a least-squares refinement of the torsional potential parameters based on results of some of our high-resolution rotational analyses, followed by a comparison of energy levels calculated from the refined parameters with a low-resolution spectrum taken in the region from 0 to 500 cm −1 above the A– X band origin. Concerning (ii), we find that the two vibrational levels with one quantum of torsional excitation are best described by a normal mode formulation, but that many levels with more than one quantum of torsional excitation are better described by a local-mode formulation. Concerning (iii), we obtain low-order molecular constants quite similar to those reported by Senent et al., but higher-order constants which are quite different. Our calculated spectrum agrees well with the low-resolution spectrum, but full confirmation of the present interpretation for levels with three or more quanta of torsion excited will require high-resolution studies of additional bands.

Full non-rigid group theory and symmetry of melamine

The non-rigid molecule group (NRG) theory in which the dynamic symmetry operations are defined as physical operations is a new field in chemistry. Smeyers, in a series of papers, applied this notion to determine the character table of restricted NRG of some molecules. For example, Smeyers and Villa computed the r-NRG of the triple equivalent methyl rotation in pyramidal trimethylamine with inversion and proved that the r-NRG of this molecule is a group of order 648, containing two subgroups of order 324 without inversion [5]. In this work, a simple method is described, through which it is possible to calculate character tables for the symmetry group of molecules. We study the full NRG of melamine, and prove that it is a groups of order 48, with 27 and 10 conjugacy classes. Also, we compute the symmetry of melamine and prove that it is a non-abelian groups of order 6. The method can be generalized to apply to other non-rigid molecules.

Exploiting PdII and TiIII Chemistry To Obtain γ-Dioxygenated Terpenoids: Synthesis of Rostratone and Novel Approaches to Aphidicolin and Pyripyropene A

In nature there are several terpenoids with a characteristic gamma-dioxygenated system on the A ring, and many of them show interesting pharmacological properties. We have developed a novel strategy for the synthesis of these terpenoids involving three stages: (a) the selective epoxidation of commercial polyenes, (b) titanium(III)-catalyzed cyclization of the epoxypolyprenes thus obtained, and (c) Pd-mediated remote functionalization of the equatorial methyl group attached at C-4 on ring A of the cyclic terpenoid thus formed. This strategy has proved to be useful for the synthesis of the natural labdane rostratone (1) and related terpenoids, as well as for advanced synthetic approaches toward the pharmacologically active products aphidicolin (2) and pyripyropene A (3).

Neurosteroid Analogues. 10. The Effect of Methyl Group Substitution at the C-6 and C-7 Positions on the GABA Modulatory and Anesthetic Actions of (3α,5α)- and (3α,5β)-3-Hydroxypregnan-20-one

The planar 5alpha-reduced steroid (3alpha,5alpha)-3-hydroxypregnan-20-one and the nonplanar 5beta-reduced steroid (3alpha,5beta)-3-hydroxypregnan-20-one act at GABA(A) receptors to induce general anesthesia. The structural features of the binding sites for these anesthetic steroids on GABA(A) receptors have not been determined. To determine how structural modifications at the steroid C-6 and C-7 positions effect the actions of these anesthetic steroids, an axial or equatorial methyl group was introduced at these positions. The analogues were evaluated (1) in [(35)S]-tert-butylbicyclophosphorothionate binding experiments, (2) in electrophysiological experiments using rat alpha(1)beta(2)gamma(2L) GABA(A) receptors expressed in Xenopus laevis oocytes, and (3) as tadpole anesthetics. The effects of methyl group substitution in the 5alpha- and 5beta-reduced series of compounds were strikingly similar. In both series, a 6beta-Me group gave compounds with actions similar to or greater than those of the parent steroids. A 6alpha-, 7beta- or 7alpha-Me substituent resulted in reduced potency for inhibition of radioligand binding, GABA(A) receptor modulation and tadpole anesthesia. Because of the similar effects of methyl group substitution in the two series of compounds and previous results from other studies showing that structural modifications in the steroid D ring/side chain region produce similar effects regardless of the stereochemistry of the A,B-ring fusion, we propose that either the 3alpha-hydroxyl groups of planar and nonplanar anesthetic steroids hydrogen bond to different amino acids on GABA(A) receptors or that this critical hydrogen bonding group interacts with membrane lipids instead of the receptor.

Noninvasive MR thermometry using paramagnetic lanthanide complexes of 1,4,7,10‐tetraazacyclodoecane‐α,α′,α″,α‴‐tetramethyl‐1,4,7,10‐tetraacetic acid (DOTMA4–)

Noninvasive techniques to monitor temperature have numerous useful biomedical applications. However, MR thermometry techniques based on the chemical shift, relaxation rates, and molecular diffusion rate of the water 1H signal suffer from poor thermal resolution. The feasibility of MR thermometry based on the strong temperature dependence of the hyperfine-shifted 1H signal from the paramagnetic lanthanide complex thulium-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate (TmDOTA-) was recently demonstrated. The use of paramagnetic lanthanide complexes for MR thermometry can be further enhanced by improving the signal-to-noise ratio (SNR) of the observed signal. In this study, the use of lanthanide complexes of a methyl-substituted analog of DOTA4-, 1,4,7,10-tetramethyl 1,4,7,10-tetra azacyclodoecane-1,4,7,10-tetraacetic acetate (DOTMA4-) was evaluated. DOTMA4- complexes have 12 magnetically equivalent methyl protons, which provide an intense and sharper resonance compared to the corresponding DOTA- complexes. Experiments with paramagnetic Pr3+, Yb3+, Tb3+, Dy3+, and Tm3+ complexes of DOTMA4- showed that the Tm3+ complex is most favorable for MR thermometery because of the high temperature dependence of its chemical shift and its relatively narrow linewidth. The chemical shift of the methyl 1H signal from TmDOTMA- was approximately 60 times more sensitive to temperature than the water 1H shift and was insensitive to changes in concentration, pH, [Ca2+], or the presence of other ions and macromolecules. The application of TmDOTMA- for measuring temperature in a subcutaneously implanted tumor model was demonstrated. Lastly, the feasibility of obtaining 3D images from the methyl 1H resonance of TmDOTMA- was demonstrated in phantom and live animal experiments. Overall, TmDOTMA- appears to be a promising probe for MR thermometry in vivo.

An isotope effect on the comparative quantification of flavonoids by means of methylation-based stable isotope dilution coupled with capillary liquid chromatography/mass spectrometry

Ionization suppression is a serious problem in liquid chromatography/mass spectrometry-based metabolomics, and stable isotope dilution-based comparative quantification is one of the most important methods of overcoming this problem. Herein, the use of [ 13C]-methylation-based stable isotope dilution for comparative quantification of flavonoids is demonstrated. This is in contrast to the equivalent deuterium labeling methylation method, which has an adverse isotope effect on reverse phase chromatography.

A theoretical study on the conformation of 3-phosphinoxido- and 3-phosphono-1,2,3,4,5,6-hexahydrophosphinine 1-oxides

The title compounds (2 and 4) obtained by the diastereoselective hydrogenation of the corresponding 1,2,3,6-tetrahydrophosphinine oxides (1 and 3) were subjected to a detailed quantum chemical study. The possible chair conformers were calculated at the HF/6-31G* level of theory, according to which, the 1-phenyl-3-P(O)Y2-substituted products (2) exist in the trans1 form, in which all substituents are equatorial. At the same time, the 1-ethoxy-3-dialkylphosphono compounds (4) adopt the cis conformations, in which the 1-ethoxy group is axial and the 3-P(O)(OR)2 moiety is equatorial. The major diastereomer (4–1) is cis3, in which the 5-methyl group is axial, while the minor one is cis1 with an equatorial methyl substituent. It is noteworthy that the rotational position of the exocyclic P(O)Z2 function affected the energy content of the chair conformer to a high extent. The possibility of the involvement of the twist conformers was also considered. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:520–524, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20148

Molecular complexes of organo-metallic molecules with rare gases: the rotational spectrum of difluorodimethylsilane–argon

Abstract The rotational spectrum of the molecular complex difluorodimethylsilane–argon has been investigated by pulsed jet absorption millimeter wave and pulsed supersonic jet Fourier transform microwave spectroscopy. The absolute energy minimum corresponds to a conformation with the argon atom lying in the F–Si–F plane of symmetry of difluorodimethylsilane. This is confirmed by the analysis of the observed splitting due to the internal rotation of two equivalent methyl groups. The V 3 barrier to the internal rotation of the two methyl groups has been determined to be 4.857(5) kJ/mol. The distance of Ar from the center of mass of difluorodimethylsilane is 4.07 A, with Ar–Si line forming an angle of 60.6° with the C–Si–C bisector. The zero point dissociation energy is estimated, from the centrifugal distortion constant D J , to be 2.75 kJ/mol.

33S NMR spectra of sulfonium salts: calculated and experimental.

33S NMR chemical shifts were calculated by the scaled DFT and EMPI approaches for the fluoride, chloride and bromide of trimethylsulfonium ion (1) and S-methyltetrahydrothiophenium ion (2), in addition to the free cations. Experimental values were obtained for the iodides of 1 (delta +48, CS2 = 0 ppm) and 2 (delta +95), and were found to agree with the calculated values well within the standard deviation of 35 ppm (3.5% of the shielding range) established in earlier work for a great variety of sulfur compounds. An earlier literature value of delta +750 for the iodide of 2 is therefore to be replaced. Calculations provide a shift of delta +68 for S-methylthianium ion with equatorial methyl, indicating that the reported value of delta +670 for the iodide is also incorrect.

Palladium‐Mediated C—H Activation in the Field of Terpenoids: Synthesis of Rostratone.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Molecular conformation effects on mesomorphism and twisting ability of chiral cyclohexanones in mesophases

The molecular conformations of 2,6-bis-(4-phenyl)benzylidene-3R-methyl-cyclohexanone and its mono-(4-phenyl)benzylidene methylcyclohexanone isomers were investigated by molecular simulation using the semiempirical AM1 and PM3 methods and by analyzing the experimental spin-spin coupling constants in the PMR spectra. Mesomorphism and the twisting ability of the induced cholesteric mesophases of the title compounds are analyzed using the ratios between conformers with the axial and equatorial methyl groups and other peculiarities of the spatial structure of molecules (differences in anisometricity and in the degree of flattening of the cinnamoyl fragment). The equatorial orientation of the methyl group in the dominant conformations generally favors the formation of mesophases. The twisting ability is higher for chiral compounds with an axial methyl substituent and with the chiral center directly bonded to the enone and arylidene groups.