Rotation Of Phenyl Group Research Articles

Probing subglass relaxation in polymers via a geometric representation of probabilities, observables, and relaxation modes for discrete stochastic systems

The dynamics of many physical, chemical, and biological systems can be reduced to a succession of infrequent transitions in a network of discrete states representing low energy regions in configuration space. This enables accessing long-time dynamics and predicting macroscopic properties. Here we develop a new, perfectly general statistical mechanical/geometric formulation that expresses both state probabilities and all observables in the same Euclidean space, spanned by the eigenvectors of the symmetrized time evolution operator. Our formalism leads to simple expressions for nonequilibrium and equilibrium ensemble averages, variances, and time correlation functions of any observable and allows a rigorous decomposition of the dynamics into relaxation modes. Applying it to subglass segmental relaxation in atactic polystyrene up to times on the order of 10 micros, we probe the molecular mechanism of the gamma and delta processes and unequivocally identify the delta process with rotation of a single phenyl group around its stem.

Solvent effects on the absorption, circular dichroism and Raman spectroscopy of meso-tetrakis [3-methoxy-4-(N-carbazyl)n-hexyloxyphenyl] porphyrin in water–THF solution

Solutions of meso-tetrakis [3-methoxy-4-(N-carbazyl)n-hexyloxyphenyl] porphyrin (4C6-TPP) in a water–organic mixture (tetrahydrofuran, THF) generate aggregates with absorption spectra characterized by broader red shifts of the Soret band relative to the monomeric porphyrin band. For the aggregated form several phenyl modes enhance and some modes weaken, the ν 2, ν 3 and ν 11 bands in the Raman spectra are slightly downshifted compared with the monomeric form. These data suggest that the phenyl groups of 4C6-TPP in the aggregate rotate to an orientation more coplanar with the porphyrin core. The increased conjugation of the meso-phenyl rings with the porphyrin core brings about the red shift of the Soret band. The circular dichroism (CD) band of the aggregate undergoes some changes with time, as indicated by the transition from a negative band to no signal resulted from the rotation of phenyl groups. All of the results imply that the location of the phenyl groups plays a key role in the determination of the morphology of the aggregate.

Excited state intramolecular proton transfer in 5-hydroxy flavone: A DFT study

Potential energy surfaces (PES) for the ground and excited state intramolecular proton transfer (ESIPT) processes in 5-hydroxy-flavone (5HF) were studied using DFT-B3LYP/6-31G(d) and TD-DFT/6-31G(d) level of theory, respectively. Our calculations suggest the non-viability of ground state intramolecular proton transfer (GSIPT) in 5HF. Excited states PES calculations support the existence of ESIPT process in 5HF. ESIPT in 5HF has been explained in terms of HOMO, LUMO electron density of the enol and keto tautomer of 5HF. PES scan by phenyl group rotation suggests that the twisted form, i.e., phenyl group rotated by 18.7° out of benzo-γ-pyrone ring plane is the most stable conformer of 5HF.

Controlling electronic states and photochemical reactivities of C 60 by chemical functionalization: Ortho-substituent effect of 1,6- N-(phenyl)aza-[60]fulleroids and 1,2-( N-phenyl)aziridino-[60]fullerenes

The reaction rate of the photochemical rearrangement 1,6- N-(2-substituted)aza-[60]fulleroids ( 1) → 1,2-( N-substituted)aziridino-[60]fullerenes ( 2) differed ca. 3.1-fold as the ortho-substituent of the N-phenyl group changed from 2-methylphenyl ( b), 2-ethylphenyl ( c), to 2- iso-propylphenyl ( d). The retardation of the rearrangement is due to the contribution of charge-separated triplet state in the excited states in addition to the normal triplet state, which was confirmed by transient absorption measurements. A semi-empirical calculations on the ground state conformation of 1 and 2 showed that 1 with phenyl ( a) and 2,6-dimethylphenyl ( e) N-substituents and 2a, e have C 2 v symmetry but the rotation of the phenyl group along the N–Ph bond occurs with the increase of steric hindrance between the phenyl group and C 60 moiety ( 1b– d and 2b– d). These results indicate that the rate of the rearrangement, i.e., the nature of the excited states, is probably controlled by the conformation of the molecules at their excited states rather than that of their ground states. The generation of charge-separated triplet state in the excited state can be explained by the occurrence of phenyl group rotation along the N–Ph bond.

Energy Transfer in the Nanostar: The Role of Coulombic Coupling and Dynamics

We present a theoretical investigation of energy transfer in the phenylene ethynelene dendrimer known as the nanostar. Data from extensive molecular dynamics simulations are used to model the dynamical effects caused by torsional motion of the phenyl groups. We compare rate constants for energy transfer between the two-ring chromophore and the three-ring chromophore obtained via the Förster model, the ideal dipole approximation (IDA), and the transition density cube (TDC) method, which has as its limit an exact representation of the Coulombic coupling. We find that the rate constants obtained with the TDC are extremely sensitive to the phenyl group rotation, whereas the constants computed with the Förster model and the IDA are not. The implications of these results for the interpretation of recent pump-probe experiments on the nanostar are discussed in detail. Finally, we predict the temperature dependence of the rate constant for energy transfer.

NMR Studies of Hindered Rotation and Magnetic Anisotropy: The Diels–Alder Adducts of Phencyclone with N‐Phenylmaleimide and N‐(2‐Trifluoromethylphenyl)maleimide. Ab Initio Calculations for Optimized Structures

N‐Phenylmaleimide, 2, and N‐(2‐trifluoromethylphenyl)maleimide, 3, were separately added to phencyclone, 1, to yield the corresponding phencyclone Diels–Alder adducts, 4 and 5. The resulting adducts (and some precursors) have been characterized by one‐ and two‐dimensional 1H and 13C NMR at 300 and 75 MHz, and by 19F NMR at 282 MHz, at ambient temperatures. The NMR data are consistent, for both adducts, with: (a) hindered rotation of the bridgehead unsubstituted phenyl groups about the C(sp2)–C(sp3) bonds, based on slow exchange limit (SEL) spectra and (b) endo adduct configuration based on magnetic anisotropic effects in the 1H NMR. The NMR spectra of the phencyclone adduct, 4, of N‐phenylmaleimide, indicate free rotation on the NMR timescales (fast exchange limit, FEL spectra) about the N‐phenyl bond. The spectra for the adduct, 5, of N‐(2‐trifluoromethylphenyl)maleimide are interpreted as consistent with SEL regimes, for the N‐aryl rotations, with a single rotamer present in which the trifluoromethyl group is directed “out of” the adduct cavity, and away from the phenanthrenoid moiety. This conclusion is based, in part, on NMR data suggesting the apparent slow N‐aryl bond rotation in a pair of atropisomers corresponding to the acetic acid addition products from the N‐(2‐trifluoromethylphenyl)maleimide. Evidence of magnetic anisotropic effects due to the phenanthrenoid moiety and proximal carbonyls is discussed. 1H, 13C, and 19F assignments are presented and interpreted. Molecular modeling calculations at the Hartree–Fock level, 6‐31G* basis set, were performed to provide geometry optimizations for energy‐minimized structures of selected compounds.

Creeping friction dynamics and molecular dissipation mechanisms in glassy polymers.

The dissipation mechanism of nanoscale kinetic friction between an atomic force microscopy tip and a surface of amorphous glassy polystyrene has been studied as a function of two parameters: the scanning velocity and the temperature. Superposition of the friction results using the method of reduced variables revealed the dissipative behavior as an activated relaxation process with a potential barrier height of 7.0 kcal/mol, corresponding to the hindered rotation of phenyl groups around the C-C bond with the backbone. The velocity relationship with friction F(v) was found to satisfy simple fluctuation surface potential models with F proportional to const-ln(v) and F proportional to const-ln(v)2/3.

Steps to demarcate the effects of chromophore aggregation and planarization in poly(phenyleneethynylene)s. 1. Rotationally interrupted conjugation in the excited states of 1,4-bis(phenylethynyl)benzene.

A series of photophysical measurements and semiempirical calculations were carried out with 1,4-bis(phenylethynyl)benzene in search of evidence on the effects of phenyl group rotation and chromophore aggregation of oligo- and poly(phenyleneethynylene)s. It is suggested that planarization gives rise to relatively modest shifts of ca. 20-30 nm, which preserve the vibronic structure of the monomer and retain a high emission quantum yield. In contrast, it is proposed that aggregation gives rise to larger shifts and loss of vibronic structure.

Phosphine Ligand Exchange in Tetrakis(trimethylphosphine)(hydrido)osmium Anilides, Phenoxides, and Thiophenoxides. Examples of Anion Dissociation and of Labilization by Ligand π-Base Effects

The series of phenoxide complexes cis-L4Os(H)OC6H4Z (L = PMe3; Z = H, OMe, CF3, NH2, CN), anilides cis-L4Os(H)NHC6H4Z (Z = H, OMe, CF3), and thiophenoxides cis-L4Os(H)-SC6H4Z (Z = H, OMe) have been prepared by treatment of fac-L3Os(H)(η2-CH2PMe2) with the corresponding neutral arene. 1H NMR spectra of coordinated phenoxides and thiophenoxides show rapid phenyl group rotation, while that of the anilides is slow. Rates and stereochemistry of substitution of P(CH3)3 (L) by P(CD3)3 (L‘) were determined by 31P NMR in benzene at 80 °C. Anilides substitute first only in the mutually trans sites (a sites) with cleanly first-order kinetics and subsequently into the site trans to the anilide (site c). The latter shows non-first-order behavior that is accurately modeled by iterative kinetics calculations using a mechanism of L dissociation only from the a sites presumably to give a quasi-trigonal-bipyramidal intermediate stabilized by π-electron donation from the anilide lone pair. A three-point Hammett plot against...

Calculation of the 13C NMR chemical shift of ether linkages in lignin derived geopolymers: : Constraints on the preservation of lignin primary structure with diagenesis

Methodology for the calculation of 13C NMR shieldings on molecular organic fragments, representative of monomers in a type III kerogen, is presented. Geometry optimization of each molecular fragment was carried out using Density Functional Theory employing the generalized gradient approximation. NMR shieldings were calculated using the Individual Gauge for Localized Orbital Method. Convincing agreement was obtained between calculated and experimentally derived isotropic chemical shielding values over a broad frequency range. Shielding calculations employing the localized orbitals/local origin method resulted in nearly identical results. NMR chemical shift static powder patterns also exhibit excellent agreement with experimental values. These quantum mechanical calculations were applied to determine the extent of lignin primary structure preservation with diagenesis. Specifically, the calculations were used to assess whether inhomogeneous spectral broadening due to both functional group variation and local configurational variability may inhibit the detection of otherwise significant quantities of alkyl-aryl ethers in lignin derived geopolymers. Determination of the chemical-shielding tensor principle axis values reveals a strong correlation between anisotropy and asymmetry with local configuration effects such as dihedral rotation, phenyl group rotation, and bond angle variation. These results indicate that a range of 9 ppm in the isotropic chemical shift can be ascribed to local configuration. Consequently, an upper limit of 5% alkyl-aryl-linkages may go undetected using NMR spectroscopy on lignin-derived geopolymers at the liginite–sub-bituminous transition. It is concluded that the primary structure of lignin does not persist in kerogens even at relatively low thermal maturities.

NMR and X-ray structural study of saturated ( p-chlorophenyl)pyrrolo[1,2-a][3,1]benzoxazin-1-ones prepared from aroylisobutyric acid and cyclic amino alcohols. High energy barriers for hindered rotation of bridgehead phenyl groups

From 2-methyl-3-(p-chlorobenzoyl)propionic acid with stereoisomeric cyclic saturated or partly saturated cis and trans 1,3-amino alcohols and bicyclic amino alcohols, tri- and tetracyclic methyl substituted (p-chlorophenyl)pyrrolo[1,2-a][3,1]benzoxazin-1-ones and methylene bridged derivatives were prepared. For comparison, the bicyclic oxazolone and oxazinone analogs were also prepared. In each case isomeric pairs, which differ in the mutual positions of the aryl and methyl groups, were formed. For the methylene bridged derivatives, the isomers were separated. For evaluation of the structure in solution 1H, 13C{1H}, NOE difference, COSY and HMQC NMR methods were used, and for the crystal structure determinations X-ray diffraction measurements were used. An unusually high free energy barrier of the restricted rotation of the bridgehead p-chlorophenyl group was measured for a cis-, a diendo- and a diexo-fused compound.

Azomethine dyes revisited. Photobleaching of azomethine dyes under photoreducing conditions

This paper presents the results of a study on the bleaching process of azomethine dyes (AMDs) during their irradiation in the presence of an electron donor N-phenylglycine (NPG). The bleaching process and singlet oxygen formation for the dyes under study occurred with very low quantum yields. Experimental results showed that the bleaching of azomethine dyes may be due to both singlet and triplet states. The prominence of the triplet state was suggested by an analysis of double reciprocal plots for bleaching quantum yields and [NPG]. Additional support for this mechanism was given by results from laser flash experiments with a cyclized form of the dye. In these experiments, a transient optical absorption was attributed to a triplet state, and this state was quenched by NPG with a rate constant of 1.2 × 107 M–1 s–1. A similar experiment performed for a branched dye shows a broad, weak transient absorption which may also indicate a small amount of triplet-state formation. Changes in the dye structure affected the rate of photobleaching. The introduction of heavy atoms into a dye molecule only slightly increased the color-loss efficiency. The decrease or restriction of the freedom of the phenyl-group rotation did not increase the rate of the bleaching process. Significant influence of the azomethine dye structure on photobleaching rates was observed only when there was a strong electron withdrawing group in the R2 position. The most significant increase of the bleaching rate was observed when the branching of the dye was limited, especially when the rotation of substituents around the CN bond was prevented by structural constraints.

Electronic Spectra of Phenylcyclopropane and Cumene Cation Radicals: Interplay of Experiment and Theory

The structures of phenylcyclopropane (1+•) and cumene (2+•) cation radicals were calculated using both CASSCF and B3LYP computational methods. Both methods predict that 1+• adopts a delocalized, bisected structure and that the barrier to phenyl group rotation is substantial (11−14 kcal/mol). In contrast, the spin and charge in 2+• is largely localized in the phenyl ring and there is no strongly preferred ground state conformation. The CASPT2 method was used to predict the electronic spectra of 1+• and 2+•. The large differences in the spectra of 1+• and 2+• can be traced to significant σ−π interaction in 1+•. The results are rationalized in terms of a simple valence bond configuration mixing model. In general, the calculated transition energies were in good accord with the experimental values; however, the relative UV−vis peak intensities for 1+• were not well reproduced if a static structure was assumed. Better agreement was obtained by taking into account libration of the phenyl group in 1+•.

Studies of Hindered Rotation and Magnetic Anisotropy by 1H, 19F and 13C NMR in Models for Drugs. I. The Diels-Alder Adduct of Phencyclone with N-2, 2, 2-Trifluoroethylmaleimide, and Precursors

To gain understanding of hindered rotations and magnetic anisotropy in drugs, compounds of much intrinsic importance, we have investigated as a model compound the Diels-Alder adduct of phenycyclone with N-2, 2, 2-trifluoroethylmaleimide for study by 1H, 19F and 13C NMR. The N-2, 2, 2-trifluoroethylmaleamic acid and maleimide precursors were examined by 1H and 19F NMR. The phencyclone adduct is shown by 1H and 13C NMR (at 300 and 75 MHz, respectively) to exhibit slow exchange limit spectra at ambient temperatures, arising from hindered rotation of the bridgehead unsubstituted phenyl groups.

Conformations of 2,4-Diphenylpentane: A Quantum Chemistry and Gas-Phase Molecular Dynamics Simulation Study

We report on the relative conformer energies and rotational energy barriers for meso and racemic 2,4-diphenylpentane (DPP) from ab initio electronic structure calculations. It is found that dispersion interactions between phenyl rings strongly influence the conformational geometries, necessitating the inclusion of electron correlation in both geometry optimizations and energy calculations. Furthermore, basis set superposition contributions to the phenyl−phenyl interactions, estimated by extracting the phenyl rings from the optimized DPP geometries and computing the basis set superposition error for the resulting benzene dimer configurations, are significant. An atomistic molecular mechanics force field is parametrized to reproduce our best values for the conformational energies and rotational energy barriers in DPP obtained from ab initio calculations. Conformational energy contour maps are presented for the DPP enantiomers, and their salient features are discussed. Gas-phase molecular dynamics simulations of DPP have been performed using the quantum chemistry based force field. Important entropic contributions to the conformer populations, due primarily to restricted phenyl group rotation, are discussed.

Studies of Hindered Rotation and Magnetic Anisotropy by 1H, 13C and 19F NMR. The Diels-Alder Adduct of N- Pentafluorophenylmaleimide and Phencyclone: A Model for Drugs.

The potent Diels-Alder diene, phencyclone, 1, reacts with N-pentafluorophenylmaleimide, 2, to form an adduct, 3, characterized by 1H, 13C, and 19F NMR at 300, 75 and 282 MHz, respectively. The one-dimensional (1D) and two-dimensional (2D) 1H and 13C NMR spectra of 3 at ambient temperatures imply a slow exchange limit (SEL) regime with respect to rotation of the unsubstituted bridgehead phenyl groups about severely hindered C(sp2)-C(sp3) bonds. Major non-bonded interactions are expected between the ortho protons of the C6H5 groups and H-1, 8 of the phenanthrenoid moiety of 3. 19F 1D and 2D (COSY) NMR spectra show that the SEL regime also obtains for rotation about the N-C6F5 bond of 3, with five separate fluorine signals seen, consistent with a preferred conformation in which the C6F5 may lie roughly perpendicular to the plane of the pyrrolidinedione moiety, and may be in the mirror symmetry plane of 3. The results are considered relevant to hindered aryl rotations in numerous Pharmaceuticals. Selected spectral data for 2 and precursors are also presented.

NMR Studies of Hindered Rotation. The Diels-Alder Adduct of 4-Methyl-1, 2, 4-triazoline-3, 5-dione with Phencyclone: Restricted Motion of Unsubstituted Bridgehead Phenyls.

4-Methyl-1, 2, 4-triazoline-3, 5-dione was produced by lead tetraacetate oxidation of 4-methylurazole and allowed to react with phencyclone, 1-The resulting Diels-Alder adduct, 2, has been characterized by one-and two-dimensional 1H and 13C NMR at 300 and 75 MHz, respectively, at ambient temperatures in different solvents. The NMR data are consistent with hindered rotation of the bridgehead unsubstituted phenyl groups about the C(sp2)-C(sp3) bonds, based on numbers of absorption signals in the 1H and 13C NMR aryl region, together with magnetic anisotropic effects in the 1H spectrum. The spectral simplicity suggests further that stereochemistry at the ring junction nitrogens involves only a single isomer or very rapidly interconverting “exo”/“endo” isomers (if the ring junction nitrogens are pyramidalized).

Further studies of intramolecular motions in crystalline ammonium bromides by CP/MAS NMR

A series of nine compounds consisting of dimethyl, trimethyl and ethyldimethyl ammonium bromides in which the other alkyl group contains a (2-phenyl)ethyl moiety were synthesised and studied by CP/MAS NMR. The results of dynamic NMR studies on the solids suggest that there is a dramatically wide range of molecular motions occurring in this simple series of compounds. A combination of dynamic line shape analyses and T1ρ measurements reveals the considerable extent of intramolecular group motions including rotations of trimethylammonium, ethyldimethylammonium and phenyl groups. Rates of rotation and activation parameters for these molecular motions are derived where appropriate.

Local chain motions in a dilute solution of phenolphthalein poly(ether sulfone)

Abstract13C and 1H relaxation times were measured as a function of temperature in two magnetic fields for dilute solutions of phenolphthalein poly(ether sulfone) (PES‐C) in deuterated chloroform. The spin‐lattice relaxation times were interpreted in terms of segmental motion characterized by the sharp cutoff model of Jones and Stockmayer (J. S. model). The phenyl group rotation is treated as a stochastic diffusion by the J. S. model. The restricted butterfly motion of the phenyl group attached to the cardo ring in PES‐C is mentioned but is not discussed in detail in this work. Correlation times for the segmental motion are in the picosecond range which indicates the high flexibility of PES‐C chains. The correlation time for the phenyl group internal rotation is similar to that of the segmental motion. The temperature dependence of these motions is weak. The apparent activation energy of the motions considered is less than 10kJ/mol. The simulating results for PES are also reasonable considering the differences in structure compared with PES‐C. The correlation times and the apparent activation energy obtained using the J. S. model for the main chain motion of PES‐C are the same as those obtained using the damped orientational diffusion model and the conformational jump model.

NMR spectra of polyimides: Interpretation and local chain motion study

AbstractThree kinds of high‐performance polyimides 1 (poly(ketone‐imide) PKI), 2 (poly(ether‐imide) PEI) and 3 (poly(oxy‐imide) POI) were studied using nuclear magnetic resonance (NMR). The NMR spectra of the polyimides were assigned according to the comprehensive consideration of the substitution effect of different substituting groups, viz. distortionless enhancement by polarization transfer (DEPT), no nuclear Overhauser effect (NNE), analysis of relaxation time, and two‐dimensional correlated spectroscopy (COSY) techniques. The structural units of these three polyimides were determined. Carbon‐13 and proton relaxation times for PEI and PKI were interpreted in terms of segmental motion characterized by the sharp cutoff model of Jones and Stockmayer (JS model) and anisotropic group rotation such as phenyl group rotation and methyl group rotation. Correlation times for the main‐chain motion are in the tens of picosecond range which indicates the high flexibility of polyimide chains. Correlation times for phenyl group and methyl group rotations are more than 1 order of magnitude lower and approximately 1 order of magnitude higher than that of the main chain, respectively.