Comprehensive study of the correlations that exist among the members of the [n]cyclacene series and the Möbius[n]cyclacene series

Cyclacenes are the smallest substructures of carbon nanotubes used in modelling studies. The systematics that exists between Hückel molecular orbital eigenvalues and eigenvectors of cyclacenes are delineated. This study of cyclacenes combines the interconnection of concepts of complementarity theorem, characteristic and matching polynomial recursion equations, embedding, greater than twofold symmetry and doubly degenerate eigenvalues, open-shell singlet character, and pairing theorem. Proof that cyclacenes have more open-shell (diradical) character than do polyacenes is also provided by the sum total of this work. Mirror-plane scission of even-ring cyclacenes gives linear polyacene fragments. This shows that the properties of the successor linear polyacene must be contained in the precursor cyclacene. Corresponding Möbius[n]cyclacene isomers display contrasting and unusual comparative properties. A partial list of contrasting properties include alternant (cyclacenes) versus nonalternant (Möbius[n]cyclacene) polyenes, presence of Hamiltonian circuits in Möbius[n]cyclacene and presence of oscillatory electronic properties in cyclacenes.     

Nuclear resonance reflectivity from a [57Fe/Cr]30 multilayer with the Synchrotron Mössbauer Source

Mössbauer reflectivity spectra and nuclear resonance reflectivity (NRR) curves have been measured using the Synchrotron Mössbauer Source (SMS) for a [⁵⁷Fe/Cr]₃₀ periodic multilayer, characterized by the antiferromagnetic interlayer coupling between adjacent ⁵⁷Fe layers. Specific features of the Mössbauer reflectivity spectra measured with π‐polarized radiation of the SMS near the critical angle and at the `magnetic' maximum on the NRR curve are analyzed. The variation of the ratio of lines in the Mössbauer reflectivity spectra and the change of the intensity of the `magnetic' maximum under an applied external field has been used to reveal the transformation of the magnetic alignment in the investigated multilayer.     

Topology induced anomalous plasmon modes in metallic Möbius nanorings

We report on the theoretical investigation of plasmonic resonances in metallic Möbius nanorings. Half‐integer numbers of resonant modes are observed due to the presence of an extra phase π provided by the topology of the Möbius nanostrip. Anomalous plasmon modes located at the non‐orientable surface of the Möbius nanoring break the symmetry that exist in conventional ring cavities, thus enable far‐field excitation and emission as bright modes. The far‐field resonant wavelength as well as the feature of half‐integer mode numbers is constant to the change of charge distribution on the Möbius nanoring due to the topology of Möbius ring. Owing to the ultra‐small mode volume induced by the remaining dark feature, an extremely high sensitivity as well as a remarkable figure of merit is obtained in our numerical calculations for sensing performance. The topological metallic nanostructure provides a novel platform for the investigation of localized surface plasmon modes exhibiting unique phenomena for potential plasmonic applications.

     

Electronic, energy, and thermal properties of the Möbius strip and related ring nanostructures of NbS3

The electronic structure and energy characteristics of the Möbius strip and related ring nanostructures of niobium trisulfide NbS₃ are investigated using the density functional tight-binding (DFTB) method. The results obtained are analyzed and compared with those obtained for the crystal and planar ribbon structures of NbS₃. The formation of NbS₃ ring structures leads to a considerable increase in the density of Nb 4d states at the Fermi level. The simulation of the behavior of the ring structures in the temperature range T = 0–700 K demonstrates that the Möbius strip has a lower thermal stability as compared to the NbS₃ ring.     

Relationship between 237Np Mössbauer parameters and bond distances in nitrogen coordinated [NpO2(acac)2py]

We now report the ²³⁷Np Mössbauer spectra for [NpO₂(acac)₂py] I. The neptunium(VI) ion in I is coordinated by six oxygen atoms and the one nitrogen atom of pyridine. The isomer shift value obtained for I is ?40 mm/s, which falls in the range for the Np(VI) oxidation state. The relationship between the isomer shift and Np–O bond distance of the neptunyl group for oxygen coordination compounds is in good agreement with the reported structure of I. It is well known that the mean Np–O bond distance of the neptunium compounds is shorter by 0.01 Å than that of analogous uranium compounds. The good agreement in the ²³⁷Np–Mössbauer parameters and the Np–O distance has indicated that the reported analogous [UO₂(acac)₂py] structure may have some inaccuracies. Therefore, we have re-determined the crystal structure of [UO₂(acac)₂py]. The U–O bond distance we have obtained is reasonable.     

Theoretical analysis on geometries and electronic structures of antiaromatic pentalene and its N‐substituted derivatives: monomer,oligomers and polymer

The antiaromatic compounds have received a great deal of attention for several decades because of their unusual electronic structures. The electronic structures and properties of antiaromatic pentalene and its six nitrogen heterocyclic derivatives were systematically studied by the density functional theory at the Becke, three‐parameter, Lee–Yang–Parr level with 6‐31G* basis set. The results indicated that all the monomers have stable singlet states and remarkable bond‐length alternations. From the dimer to polymer in those molecules, pentalene(P), cyclopenta[b]pyrrole(CPP), cyclopenta[d]imidazole(CPI), pyrrolo[2,3‐b]pyrrole(PP1) and pyrrolo[3,2‐d]imidazole(PI) are stable diradical structures; pyrrolo[3,2‐b]pyrrole (PP2) and imidazo[4,5‐d]imidazole(II) are stable singlet ground states. The electronic properties including bond length, bond‐length alternation, electron density at bond critical points, Wiberg bond index and nucleus‐independent chemical shift were analyzed. It was found that in diradical molecules the bond‐length alternations are diminished, the charge tends to equilibrate, the π‐electron delocalization and conjugation are strengthened. The electronic properties of singlet ground state molecules have nearly no variations from monomers to polymers. The band structure analysis shows that diradical structure molecules have small band gaps (<1.0 eV), wide bandwidth and small effective masses of holes and electrons which suggest that diradical structure molecules are very good candidates for conductive materials. Copyright © 2011 John Wiley & Sons, Ltd.     

Asymmetry in the nuclear magnetic shielding tensor

The applicability of Green's function (GF) and Feynman path-integral quantum Monte Carlo (QMC) methods for the simulation of cyclic networks with (4n + 2) and 4n (n = 1, 2, 3, …) electrons is analysed. Both QMC techniques are employed in simulations on the basis of the simple Hückel Hamiltonian which is exclusively defined by nearest-neighbour hopping elements. In addition we have used the Pariser-Parr-Pople (PPP) Hamiltonian to perform GF QMC simulations. The electronic energies E derived by the QMC methods are compared either with Hückel molecular orbital (HMO) results or exact configuration interaction data where (π) electronic correlations are fully taken into account. A sign problem occurs in QMC simulations of 4n annulenes. This leads to an error in the total energy in the standard formulations of the employed QMC techniques, which is enhanced with decreasing ring size. A simple modification in the QMC formalisms is suggested to avoid the numerical uncertainties caused by the sign problem in 4n annulenes. Renormalization of the kinetic hopping integrals t by t cos (π/M) with M abbreviating the number of atomic sites leads to ground state energies as well as any other quantity close to the values derived by conventional diagonalization techniques. Substitution of t against t cos (π/M) conserves a common sign of all matrix elements containing the hopping. The occurrence of negative probabilities, which lead to numerical problems in the QMC simulations, is thereby prevented. The transformation suggested in 4n rings has a formal connection to so-called Möbius rings.     

Dimer statistics on the Möbius strip and the Klein bottle

Closed-form expressions are obtained for the generating function of close-packed dimers on a 2M×2N simple quartic lattice embedded on a Möbius strip and a Klein bottle. Finite-size corrections are also analyzed and compared with those under cylindrical and free boundary conditions. Particularly, it is found that, for large lattices of the same size and with a square symmetry, the number of dimer configurations on a Möbius strip is 70.2% of that on a cylinder. We also establish two identities relating dimer generating functions for Möbius strips and cylinders.     

Möbius and super-Möbius gauge fixing for the closed string amplitudes on the desk

We systematically study the gauge fixing of the Möbius and the super-Möbius transformations for the N-point closed-string amplitudes on the disk. Using the Faddeev-Popov method, we obtain the explicit formulas for the Koba-Nielsen factors for these amplitudes.     

Tetrasubstituted bisadamantylidenes—Highly twisted alkenes

A number of 1,3,1′,3′‐tetrasubstituted bisadamantylidenes are examined with 4 different density functional methods (BP86‐D3, ωB97X‐D, B3LYP, and B3LYP‐D3) and the 6‐311G(d) basis set. With increasing steric bulk, these substituted bisadamantylidenes become ever more twisted about the central carbon‐carbon double bond. This manifests in significantly reduced singlet‐triplet gaps from that of typical alkenes, with the tetra‐t‐butylbisadamantylidene 17 , twisted to almost 90°, having a gap of only 2.2 kcal mol^?1. While its large strain energy may preclude the synthesis of 17 , other less‐strained tetrasubstituted analogues are more feasible synthetic targets and still possess small singlet‐triplet gaps, which should be testable by variable temperature NMR and EPR spectroscopy. The twisted alkenes also result in a low rotational energy barrier, and 4 examples with a low barrier to cis‐trans isomerization are presented. These too should be testable by experiment.     

Cis-trans isomerization of iodine-doped polyacetylene

A quantitative analysis of the cis—trans isomerization of the lightly doped polyacetylene with iodine at room temperature is reported. The cis—trans ratio was determined on the basis of the infrared absorption measurement. The result clearly demonstrates the promotion of the isomerization with increase in the dopant concentration. The degree of the amount of the isomerization, however, is rather small compared with the previous study based on the nuclear magnetic resonance measurement.     

Correlation between Mössbauer isomer shifts and XPS binding energies

A series of low-spin pentacyanoferrates (‖) of the type Na₃[Fe-(CN)₅L] (L = phosphine or phosphite) have been characterized by⁵⁷Fe Mössbauer spectroscopy and X-ray photoelectron spectroscopy. The Fe?P bond in Na₃[Fe(CN)₅L] is discussed in terms of a correlation between the isomer shift and the binding energy. It has been demonstrated that the isomer shift increases with the decreasing Fe2p_3/2 binding energy. The Mössbauer isomer shift reflects the total electron density at the nuclear site, while the chemical shift in the binding energy is concerned with the electron density on the atom of interest. The increasing Fe-to-P back donation leads to an increase of the s-electron density at the iron nucleus, i.e. a decrease of the isomer shift. The increasing tendency in the Fe2p_3/7 binding energy is interpreted by the increasing Fe-to-P back donation.     

Newby shift ofK=0 rotational bands in odd-odd rare-earths

A data set of 29 experimentally determined Newby shifts in rare-earth nuclei is examined for the reliability of each values. Using this data set, Newby shifts are obtained which are free from the Coriolis and the particle-particle coupling effects. These new empirical values help resolve the failure of a recently proposed rule for the sign of the Newby shift in the {5/2[413]p − 5/2[642]n} configuration of¹⁶⁰Tb and the {5/2[402]p − 5/2[512]n} configuration of¹⁷⁴Lu. Also the Newby shifts are significantly modified in two other cases namely the {1/2[411]p − 1/2[521]n} configuration in¹⁶⁸Tm and the {1/2[541]p − 1/2[521]n} configuration in¹⁷²Lu. Only marginal changes are seen in the rest of the cases in the rare-earth nuclei.     

Degenerate thermal isomerizations of bicyclo[3.1.1]hept‐2‐ene: strategy and analysis

Bicyclo[3.1.1]hept‐2‐ene was first prepared and well identified in 1972. In 1974, the degenerate thermal isomerization involving 1‐d‐ and 3‐d‐bicyclo[3.1.1]hept‐2‐ene was approached successfully, as one of the two deuterium‐labeled structures was selected, heated, and equilibrated. There has been no further study of this degenerate isomerization. Here, a detailed outline of reaction trajectories for d₂‐labeled bicyclo[3.1.1]hept‐2‐enes is given that will establish the four independent kinetic parameters needed for 20 linking paths between six d₂‐species. The use of racemates, eliminating chiral separations and dissections, provides degenerate isomerization paths providing this method with general utility. Copyright © 2014 John Wiley & Sons, Ltd.     

Introducing boranorcaradienes with more stability than their corresponding borepins: Reversal of tautomerization via substituents at theoretical levels

In contrast to typical borepins, which appear about 40.0 kcalmol^?1 more stable than their corresponding tautomeric boranorcaradienes, we have found 2 species, which have reversed this trend and pushed the equilibrium in favor of their corresponding boranorcaradienes. They are namely 1a,9b‐dihydro‐1H‐borireno[2,3‐h]pyridazino[4,3‐f]cinnoline and 1a,9b‐dihydro‐1H‐borireno[2,3‐h]pyrimido[5,4‐f]quinazoline which stand up among 14 isomeric systems probed, at B3LYP/AUG‐cc‐pVTZ, M06‐2X/AUG‐cc‐pVTZ, MP2/AUG‐cc‐pVTZ, and HF/AUG‐cc‐pVTZ. Energy barriers are calculated in gas‐phase, where the possibility of a rapid interconversion of the isomers is ruled out. Generally, dibenzoboranorcaradienes assume C_s symmetry with planar geometry and dihedral angle of zero degree. In contrast, their corresponding borepins show a high tendency for puckering with dihedral angle of ~66°. The preference of the latter for puckered non‐planar geometries is evidenced by natural bonding orbitals calculations and visually through their frontier molecular orbitals. Main interactions appear to be hyperconjugations of σ and π bonds across the rings. Position and number of nitrogen atoms on the fused rings seem to affect the energy gap, dipole moment, symmetry, dihedral angle, the chemical shift, NICS, bond lengths, and charge distribution.     

Trans‐2‐Aminocyclohexanol derivatives as pH‐triggered conformational switches

A series of trans‐2‐aminocyclohexanol derivatives have been explored as powerful conformational pH triggers. On protonation of the amino group, a conformer with equatorial position of ammonio and hydroxy groups becomes predominant because of an intramolecular hydrogen bond and electrostatic interactions. The energy of these interactions was estimated to be above 10 kJ/mol and in some models exceeded 20 kJ/mol (strong enough to twist a ring in tert‐butyl derivatives). As a result of this conformational flip, all other substituents are forced to change their orientation. If the substituents are designed to perform certain geometry‐dependent functions, for example, as cation chelators or as lipid tails, such acid‐induced transition may be used to control the corresponding molecular properties. The pH sensitivity of conformational equilibria was explored by ¹H nuclear magnetic resonance spectroscopy (NMR), and the titration curves were used for estimation of the pK_a values of protonated compounds that varied from 2.6 to 8.5 (in d₄‐methanol) depending on the structure of amino group. Thus, trans‐2‐aminocyclohexanols can be also used as conformational pH indicators in organic solvents.     

Partitioning in multiconfiguration perturbation theory

A simple and effective formulation of multi‐configuration perturbation theory is reviewed emphasizing the various possibilities for partitioning the Hamiltonian. We study different principles how traditional partitionings (like that of Møller and Plesset or Epstein and Nesbet) can be generalized to the multi‐configurational case. Level shift parameters are introduced with the purpose of fine‐tuning the partitioning. Variational conditions for optimizing level shift parameters in multi‐configuration theory are investigated, both for N‐particle levels and for quasiparticle (one‐electron) energies.     

Correlation Between Structural and Dynamic‐Mechanical Transitions of Different Syndiotactic Polypropylene Polymorphs

Abstract

We prepared several well‐characterized syndiotactic polypropylene (sPP) polymorphs so as to correlate their thermal and dynamic‐mechanical behaviors. A sample was crystallized in pure form I at 100°C; a second sample containing only the trans‐planar mesophase was prepared by directly drawing a quenched sample at 0°C; a third sample, drawn at room temperature, contained both the trans‐planar mesophase and a fraction of the helical form II. By annealing this sample at increasing temperatures, we obtained a series of samples containing either trans‐planar mesophase, or form II and form I crystallinities.

In the dynamic‐mechanical analysis, the sample containing form I crystallinity showed only the amorphous glass transition, at 19°C, before melting at a high temperature. The trans‐planar mesophase transformed, at temperatures higher than 50°C, into the helical forms, and this transition was completed at 80°C. The dynamic mechanical curve of the sample containing only the mesophase showed a peak, centered at 50°C, which could be clearly associated to this transition. The sample containing the trans‐planar mesophase and the helical form II, showed in the dynamic‐mechanical curve a third peak that can be associated with the melting recrystallization of form II into the most stable form I. These results are important, because it was possible to directly correlate the structural transitions of the sPP polymorphs to the dynamic‐mechanical behavior. Moreover, a dynamic‐mechanical analysis could help recognize the presence of the trans‐planar mesophase or of the helical form II in more complex structural organizations.     

UV resonance Raman spectroscopy probes the amide II′p band position in short breast milk peptides with antioxidant activity

UV resonance Raman spectroscopic study of six short proline‐containing peptides with antioxidant activity isolated from human breast milk was performed. The amide II′ proline spectroscopic band was used to estimate relative cis trans isomerization state of proline amide bonds in the different peptides. Antioxidant activity of the peptides was determined using 1,1‐diphenyl‐2‐picryl‐hydrazyl (DPPH) assay and linoleic acid emulsion assay. Although no clear correlation between the amide II′p position and antioxidant activity of the peptides was observed, they both were found to be sensitive to the presence and/or relative position of proline and tyrosine residues in the peptide. Copyright © 2011 John Wiley & Sons, Ltd.     

Lamb–Mössbauer factor of electronically excited molecular states measured by time‐differential Mössbauer emission spectroscopy

The Lamb-Mössbauer Factor (LMF) of molecular crystals is expected to depend on the electronic molecular states by their different intramolecular vibrational frequencies. Revisiting Mössbauer spectra obtained by time differential Mössbauer emission spectroscopy of the low spin compound [⁵⁷Co/Mn(bipy)₃](PF₆)₂ (bipy= 2,2'-bipyridine) a ratio of 1.25 for the LMFs of the low spin ground state and of an excited high spin state decaying in the Mössbauer time window could be evaluated. The difference found is in line with the change of LMF observed for spin crossover compounds where the excited high spin state is populated by the so‐called LIESST effect. The initial population of the high spin state is close to 100%.