Impact of molecular conformation on barriers to internal methyl rotation: the rotational spectrum of m-methylbenzaldehyde

The ground state spectrum of m-methylbenzaldehyde (m-MBA) was measured with a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer. The methyl rotor on m-MBA introduces an internal rotation barrier, which leads to splitting of the torsional energy level degeneracy into A and E states. Ab initio calculations predict a low torsional barrier for both the O-cis and O-trans conformers, resulting in a large doublet splitting up to several gigahertz in the frequency spectrum. The rotational constants, distortion terms, and V(3) values for both species have been determined from the ground state rotational spectrum using the BELGI-C(s) fitting program. There are significant differences in the torsional potential for the O-cis and O-trans m-MBA conformers. Molecular orbitals and resonance structures for each conformer are analyzed to understand the difference in torsional barrier height as well as the irregular shape of the O-trans torsional potential.     

Nuclear magnetic resonance studies of internal rotation: IV—rotational barriers and conformation of phthalic amides

The rotational barriers about the C? N bond of the N,N,N′,N′-tetramethyldiamides of phthalic, isophthalic and terephthalic acids have been determined by the iterative total line shape NMR method. Some evidence about the conformation of these compounds in solution has also been obtained by infrared and dipole moment studies.     

The enantiomers of N,N-dimethylthiobenzamides: Chromatographic behaviour and rotational barriers

N,N-Dimethylthiobenzamides bearing hydrogen atoms in both orthopositions were shown by ¹H NMR in the presence of an optically active alcohol to have non-coplanar π-systems. The barrier to rotation about the C(sp²-C(sp²) bond amounts to 43 ± 2 kJ/mol. Liquid chromatography on triacetylcellulose served for the enrichment of enantiomers of 2-substituted thiobenzamides. In several cases considerable enantioselectivity (relative retention up to 8.7) was observed. The barriers to enantiomerization (Tables 3, 4 and 5), determined by thermal racemization, were discussed in terms of non-bonding interactions, electrostatic repulsions and buttressing effects in the transition state of rotation.     

Calculation of rotational barriers in amorphous polymers

The Monte Carlo method was used for generation of amorphous polyethylene configurations on a diamond lattice. Chain building was performed on the tetrahedral lattice of edges 36, 62 and 43 Å with periodic boundary conditions imposed.32 chains were generated, each with a length of 100 CH₂-groups (resulting density = 0.81 g·cm⁻³). Small spherical volumes with a radius of 10 Å were chosen at random from the total volume for the calculation of rotational barriers. The rotating bond was chosen to be close to the center of this sphere. We employed the method of molecular mechanics in order to calculate the rotational barriers. The calculation was made for 578 rotating bonds and the obtained distribution of rotational barriers is approximated by the corresponding Γ-distribution.     

Separation of the mineral oil aromatic hydrocarbons of three and more aromatic rings from those of one or two aromatic rings

An analytical method was developed for the quantitation of the mineral oil aromatic hydrocarbons in cosmetic raw materials separating those of one or two aromatic rings from those of three and more aromatic rings. Normal phase high performance liquid chromatography was used with donor‐acceptor complex chromatography. The composition of both fractions and the quantities of respective compounds were determined by comprehensive two dimensional gas chromatography with time of flight mass spectrometry and by liquid chromatography coupled to gas chromatography with flame ionization detection.     

NMR and rotational angles in solution conformation of polypeptides

Professor San-Ichiro Mizushima and Professor Yonezo Morino's classical contributions provided unique means and firm basis for understanding of conformational states and internal rotation in polypeptide molecules. Now the NMR spectroscopy is the best choice to study molecular conformation, mechanism of action and structure-functional relationships of peptide and proteins in solution under conditions approaching those of their physiological environments. Crucial details of spatial structure and interactions of these molecules in solution are revealed by using proton-proton and carbon-proton vicinal coupling constants, proton nuclear Overhauser effect and spectral perturbation techniques. The results of NMR conformational analysis are presented for valinomycin “bracelet”, gramicidin A double helices, honey-bee neurotoxin apamin, scorpion insectotoxins and snake neurotoxins of long and short types.     

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 CH₂FOOH, CHF₂OOH, CF₃OOH, CF₂ClOOH, and CFCl₂OOH. 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 () and the ROOH, ROOH, and ROOH bond dissociation enthalpies (BDEs) are estimated for each of the studied hydroperoxides using the calculated reaction enthalpies () 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(OH), BDE(OO), and BDE(CO). 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 CF₃OOH to CFCl₂OOH leads to a decrease in both BDE(OO) and BDE(CO). Potential energy barriers for internal rotation about CO and OO are calculated at the B3LYP/6-31G(d,p) level for each of the studied hydroperoxides.     

Asymmetric reduction of aromatic ketones: importance of the conformation of the aromatic group

Some aromatic ketones have been reduced by borane or by catecholborane using oxazaborolidine as a catalyst. It has been found that, when borane is used, the enantiomeric excess of alcohol produced decreases as the substitution on the ortho position of benzene ring increases. However, for ketones with 2,6-disubstituted aryl substituents the enantiomeric excess increases when catecholborane is used.     

Geometries and rotational barriers of alkylamides and lithium alkylamides.

The barriers to rotation of methylamide, ethylamide and the corresponding lithium amides have been computed at the $\text{ab}$$\text{initio}$ 4-31G level. The barriers to rotation about the CN bond are higher for amides than for amines, but are lowered by coordination with Li⁺.     

Studies on rotational barriers of N-C axially chiral compounds: increase in the rotational barrier by aromatization

The rotational barriers in axially chiral quinolin-2-one and quinazolin-2-one possessing N-(ortho-tert-butyl)phenyl group were found to significantly increase in comparison with those of corresponding dihydroquinolin-2-one and dihydroquinazolin-2-one. Analysis of transition state structure during N-Ar bond rotation based on DFT calculation indicates that the increase in the rotational barrier is due to considerable distortion of the nitrogen-containing heterocyclic part.     

Synthesis and characterization of new, modified terphenyl ligands: Increasing the rotational barrier for flanking rings

The synthesis and characterization of some new terphenyl ligands, modified by meta alkyl substitution on the central ring are described. The new ligands were designed for potential applications in the stabilization of novel low valent main group species or transition metal heteronuclear multiply bonded compounds. Compounds (1), (3) (Mes = 2,4,6-trimethylphenyl), (5) (Trip = 2,4,6-triisopropylphenyl) and (6) (Dipp = 2,6-diisopropylphenyl) were obtained by addition of two equivalents of the corresponding aryl Grignard reagent to the benzyne intermediate generated by lithiation with BuⁿLi of the starting material 2,4-dichloro-5-isopropylcumene, followed by quenching with iodine. The lithium salts of 2 and 4 were obtained treatment of the parent terphenyl iodides with one equivalent of nBuLi. All compounds were isolated as either colorless crystals or as white powders. They were characterized by ¹H and ¹³C NMR spectroscopy and (in the case of 1 and 3) by X-ray crystallography. DFT calculations were performed on model terphenyl molecules in an attempt to estimate how much the rotation barriers of the flanking aryls can be influenced by substitution by alkyl groups of the two meta positions on central ring.     

Synthesis and rotational barriers of aryl-substituted naphthacenesystems: Framework for a receptor model

Barriers to aryl rotation in ortho and meta aryl-substituted naphthacene systems have been determined.     

Conformation and rotational barriers of 2-substituted 1,3-diphenylallyl anions

The conformations and the rotational barriers of the 2-substituted 1,3-diphenylallylanions $\text{1b}$–$\text{g}$ (Tab. 2) have been determined. Increasing size of the substituents leads to more $\text{exo}$,$\text{endo}$- and $\text{endo}$,$\text{endo}$-conformers at the cost of the $\text{exo}$,$\text{exo}$-species. This trend is connected with decreasing ΔG³-value sof the rotational barriers; the barriers are essentially not affected by ion pair effects, which is in contrast to the parent “allyl anion”.     

Carbolithiation of aromatic rings: cyclohexadienes from N-aroyl-2,2,6,6-tetramethylpiperidines

Benzamides whose nitrogen atom is part of a 2,2,6,6-tetramethylpiperidine ring are dearomatised by alkyllithiums, which attack them regioselectively to yield, after electrophilic quench, substituted cyclohexadienes.     

Computational study of urea and its homologue glycinamide: conformations, rotational barriers, and relative interactions with sodium chloride

Conformational behaviors of urea and glycinamide have been investigated using the B3LYP functional with the 6-311+G* and 6-311+G** basis sets. Urea monomers have nonplanar minima at all the levels studied, even in the aqueous phase. In the case of glycinamide, the intramolecular hydrogen bond formed from the amide to the amine is important for stabilizing the global minimum. Bond rotations and nitrogen inversion barriers for glycinamide conformations have also been reported. The DFT calculated results suggest that urea conformers interact preferentially with the {111} surface of sodium chloride and such interactions can be responsible for the change in the habit of sodium chloride. Glycinamide conformers have a lower affinity toward the {111} surface of sodium chloride in water. The pyramidality of nitrogens in urea conformers does not influence the relative trends of interaction energies with sodium chloride surfaces. The mode of interactions predicted at the LDA/PWC/DND level for urea and glycinamide with sodium chloride for both slab and cluster models shows that the amide functionality (-CONH2) interacts with both Na(+) and Cl(-) ions on the {100} surface; however, the carbonyl oxygen of these additives predominantly interacts with the sodium ions on the {111} surface.     

Formation of aromatic rings through enamine annulation

Condensation of pyrrolidine enamine of ketones with 1, 4-diacetoxy-2-butanone provides a new, concise synthetic route to substituted benzenes, dihydroindenes, tetrahydronaphthalenes, and di- and octahydrophenanthrenes. The reaction produced modest yields with regiocontrol of the secondary amine substituent.     

A study of aromatic three membered rings

The resonance energies (REs) of neutral three membered ring analogs of the cyclopropenyl cation, computed using block localized wave function (BLW) methods, reveal considerable variations. The RE's of cyclopropenes substituted with exocyclic double bonded groups C?X, (X = O, NH, CH₂, S, PH, SiH₂) increase with the electronegativity of X in the same row (SiH₂ < PH < S and CH₂ < NH < O). The extra cyclic resonance energies (ECREs) (an energetic measure of aromaticity based on comparisons with the RE's of acyclic models) of these derivatives range from +10.5 kcal/mol for cyclopropenone (X = O) (somewhat aromatic; the benzene ECRE is 29.3 kcal/mol) to ?2.4 kcal/mol (slightly antiaromatic) for X = SiH₂. Additional disubstitution of the C?C double bond by X′ groups (X′ = CH₃, NH₂, OH, SiH₃, PH₂, SH) increases the REs considerably, but has only small effects on the ECREs. Even the ECRE of deltic acid (X = O, X′ = OH) is only increased to +13.3 kcal/mol. The conclusion based on ECRE's, that all 12 of the three membered rings are only marginally aromatic/antiaromatic, is supported by the satisfactorily plot (R² = 0.92) of ECRE against values of NICS(0)_πzz (a superior nucleus chemical independent shift magnetic index of aromaticity), which range only from ?6.1 ppm (diatropic) for deltic acid (cf., ?35.5 ppm for benzene and ?14.2 ppm for the parent cyclopropenium ion) to +8.9 ppm (paratropic) for the silicon derivative, X = SiH₂, X′ = SiH₃. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Conformational analysis and rotational barriers of alkyl- and phenyl-substituted urea derivatives

Potential energy surfaces (PES) for rotation about the N-C(sp(3)) or N-C(aryl) bond and energies of stationary points on PES for rotation about the C(sp(2))-N bond are reported for methylurea, ethylurea, isopropylurea, tert-butylurea, and phenylurea, using the B3LYP/DZVP2 and MP2/aug-cc-pVDZ methods. The analysis of alkylureas reveals cis and (less stable) trans isomers that adopt anti geometries, whereas syn geometries do not correspond to stationary points. In contrast, the analysis of phenylurea reveals that the lowest energy form at the MP2 level is a trans isomer in a syn geometry. The fully optimized geometries are in good agreement with crystal structure data, and PESs are consistent with the experimental dihedral angle distribution. Rotation about the C(sp(2))-N bond in alkylureas and phenylurea is slightly more hindered (8.6-9.4 kcal/mol) than the analogous motion in the unsubstituted molecule (8.2 kcal/mol). At the MP2 level of theory, the maximum barriers to rotation for the methyl, ethyl, isopropyl, tert-butyl, and phenyl groups are predicted to be 0.9, 6.2, 6.0, 4.6, and 2.4 kcal/mol, respectively. The results are used to benchmark the performance of the MMFF94 force field. Systematic discrepancies between MMFF94 and MP2 results were improved by modification of several torsional parameters.