Staggered Forms Research Articles

Effect of Hole Arrangement on Failure Mechanism of Multiple-Hole Fiber Metal Laminate under On-Axis and Off-Axis Loading.

Mechanical joints are commonly required in structures made of fiber metal laminate (FML), which pose a threat due to multi-site stress concentrations at rivet or bolt holes. Thus, for a reasonably designed FML joint, it is essential to characterize the failure mechanism of multiple-hole FML; however, little information about this has been found in open literature. In the present work, influences of hole arrangement and loading strategy (on-axis or off-axis) on the failure mechanism of multiple-hole FML were investigated, by performing finite element analyses and energy dissipation analyses with elastoplastic progressive damage models that took curing stress into account. Six types of specimens with holes arranged in parallel and staggered forms were designed, whose geometrical parameters were in strict accordance with those specified for composites joints. It indicated that the stress distribution, gross/net notched strength, critical fracture path, and damage evaluation process were only slightly influenced by the hole number and hole arrangement. On the other hand, they were strongly influenced by the loading strategy, due to the transition of failure domination. Results presented here can provide evidence for introducing design regulations of composite joints into the more hybrid FML, and for reasonably determining its multiple-hole strength merely based on the sing-hole specimen.

A Combined Experimental and Theoretical Study on the Circular Dichroism of Staggered and Eclipsed Forms of Dimethoxy[2.2]-, [3.2]-, and [3.3]Pyridinophanes and Their Protonated Forms.

The circular dichroisms (CDs) of dimethoxy[2.2]-, [3.2]-, and [3.3]pyridinophanes and their protonated forms were investigated experimentally and theoretically. Characteristic multisignate Cotton effects (CEs), typical for planar chiral cyclophane derivatives, were observed. The CD spectral pattern was quite comparable for the staggered forms of [2.2]-, [3.2]-, and [3.3]cyclophanes, but significantly differed for the eclipsed forms. More interestingly, the patterns resembled, but the CE signs were practically opposite between staggered and eclipsed [2.2]pyridinophanes. Upon protonation, the signs of most CEs were inverted in both forms of cyclophanes, due to the reversal of dipole moment in the pyridine against the pyridinium moiety. Such a change in CD spectrum upon protonation was not apparent in [3.2]pyridinophane, and the CD spectral behavior was more complex in [3.3]pyridinophanes. The variation of CD caused by the protonation/deprotonation process was temperature-dependent and hence utilized as a thermal sensor. The protonated forms of the homologous pyridinophanes with different tether lengths in staggered and eclipsed forms served as a model system for systematically studying the cation-π interaction and its effects on chiroptical properties. A steady increase of electronic interaction became apparent for the smaller-sized cyclophanes from the increased excitation energy and electronic coupling element of the charge-transfer (CT) band, while the observed CE at the CT band was a more complex function of the original transition dipole of donor/acceptor pair and linker atoms, as well as the strength of the electronic interaction.

Picosecond Rotational Interconversion Adjacent to a C═O Bond Studied by Two-Dimensional Infrared Spectroscopy

Molecular conformations around the C═O group of carbonyl compounds like ketones and aldehydes play an important role in determining their reaction properties in solutions, including reaction rate, mechanism, steric structure, and chirality of products. Investigating different rotational conformers and their rapid exchange at room temperature will provide information on the rotational barrier and insights into how different rotamers may contribute to fundamental reactions in chemistry. We applied two-dimensional infrared (2D IR) spectroscopy and polarization-dependent IR transient grating technique to the study of 4,4-dimethyl-2-pentanone in CCl(4). Spectroscopic evidence showed that the internal rotation around the single carbon-carbon bond adjacent to the C═O group takes place on a picosecond time scale. DFT calculations suggested the presence of three different rotational conformations, one eclipsed and two staggered forms. Spectral simulation utilized the stochastic Liouville equation with a three-state jump model and incorporated the polarization factors that take into account the different direction of transition dipole moment in the three rotamers. The effects of the intramolecular vibrational energy redistribution process on the waiting time dependence of the 2D absorptive spectra were also included. Through comprehensive simulation of the observed spectral features, the exchange time constants between the three rotamers were determined: 5.4 ps from the eclipsed to staggered forms and 1.7 ps for the reverse direction.

Development of eclipsed and staggered forms in some hydrogen bonded complexes

AbstractIntermolecular hydrogen bonding in X3CH···NH3 (X = H, F, Cl, and Br) complexes has been studied by B3LYP, B3PW91, MP2, MP3, MP4, and CCSD methods using 6‐311++G(d,p) and AUG‐cc‐PVTZ basis sets. These complexes could exist in both eclipsed (EC) and staggered (ST) forms. The differences between binding energies of EC and ST forms are negligible and all EC and ST shapes correspond to minimum stationary states. The order of stabilities of them is in an agreement with the results of atoms in molecules (AIM) and natural bond orbital (NBO) analyses. On the basis of low differences between binding energies, ST forms are more stable than EC forms in all complexes with the exception of Br3CH···NH3, which behaves just opposite. Although the differences between binding energies are negligible, they are consistent with the results of AIM analysis. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Conformational analysis of the sea anemone and sea pansies neuropeptide Antho-RFamide (Glu1-Gly2-Arg3-Phe4-NH2) by molecular-mechanics calculations

Conformation energy-minimization for the sea anemone neuropeptide Antho-RFamide (Glu1-Gly2-Arg3-Phe4-NH2) was computed by molecular mechanics (MM) using an initial investigation of staggered forms examining the linkage bonds characterized by the torsion angles φ, ψ and ω and the Antho-R Famide side groups characterized by the torsion angles Χ1 , Χ2 , Χ3 R subsequently. The energy-map for each monopeptide of the Antho-RFamide was drawn in the range between 21807 and 180° by the step 20°. Conformation facilities for monopeptides were decided from these maps. Conformation facilities for monopeptides were examined from the best choice and the results were used in the examination of dipeptides. (Glu1 -Gly2 ) and (Arg3-Phe4-NH2) dipeptides of the (Glu1-Gly2-Arg3-Phe4-NH2) neuropeptide were examined separately. The most convenient alternation of these was determined and used in the conformation analysis of the whole molecule.

Molecular dynamics simulations of maltose in water

The conformational preferences and flexibility of α-maltose were calculated using a modified GROMOS potential energy function. Six molecular dynamics (MD) simulations of α-maltose with explicit inclusion of water were run for up to 1000 ps each, starting from different conformations of the glycosidic linkage, the hydroxymethyl groups, and the hydroxyl groups. Comparison of calculated ensemble-averaged optical rotations with experimental data demonstrated an excellent agreement. Reasonable agreement was also found between experimental and calculated time-averaged NMR parameters. The global minimum centered at ϕ/ ψ 3 = −49°/−36° was populated to more than 90% of the time. The second minimum with ≈ 11 kJ/mol higher potential energy at ϕ/ ψ = − 29°/ − 173° represented the inverted conformation. Transitions between both minimum regions were only found from the local into the global minimum within the total simulation time of 5400 ps. The overall ratio between the staggered populations of the hydroxymethyl groups is gg: gt: tg = 70:23:6 for the reducing glucose, and gg: gt: tg = 55:38:6 for the nonreducing residue. Average lifetimes of the rotamers of the hydroxymethyl groups range between 5 ps for tg and 700 ps for gg conformers. The lowest energy barriers between the rotameric populations were estimated to be between 12 kJ/mol and 16 kJ/mol. Cluster analysis techniques employed to analyze the large amount of data in multidimensional space revealed correlations between the glycosidic linkage and the staggered forms of the hydroxymethyl groups. Interresidue hydrogen bonds that were found in less than 5% of all conformations occurred when the glycosidic linkage adopted conformations with relatively high ϕ and ψ values. Several additional minima that had been found from in vacuo studies are not stable once water is included since the importance of intramolecular hydrogen bonds is drastically reduced. Methodological aspects and the efficiency of MD simulations for reaching conformational equilibria are discussed.

Rotamers of the 1-phenylethylamine ligand in trans- bis(imidato)bis(1-phenylethylamine)copper(II) complexes, where imidato = 3,3-dimethylglutarimidato, hydantoinato and succinimidato ligands

Three [CuN 4] complexes, red trans-[Cu(3,3-dmgluim) 2( R-phenea) 2] ( 1), blue-violet trans-[Cu(hyd) 2( R-phenea) 2] ( 2) and blue trans-[Cu(succim) 2( S-phenea) ( R-phenea)] ( 3) (3,3-dmgluim=3,3-dimethylglutarimidato, hyd=hydantoinato, succim = succinimidato, phenea = 1-phenylethylamine), were prepared and their crystal structures were determined. Crystal data are as follows: 1 is monoclinic, space group P2 1, a = 14.854(1), b = 17.105(3), c-5.988(2) Å, β=91.65(1)°, U=1520.8(5) Å 3, Z=2, R=0.033, and 2318 reflections; 2 is monoclinic, space group P2 1, a = 19.946(2), b = 6.035(1), c = 10.025(1) Å,β= 104.383(7)°, U= 1168.9(3) Å 3, Z=2, R =0.035, and 1864 reflections; 3 is triclinic, space group P 1 , a = 12.434(4), b = 12.670(3), c = 8.765(2) Å, α=94.83(2), β= 112.87(2) γ=73.15(2)°, U= 1219.9(6) Å 3, Z=2, R=0.054, and 3678 reflections. In 1 the coordination geometry is planar, and the specific intermolecular hydrogen bonds are formed along the c axis between H(NH 2) of phenea and O(CO) of the six- membered cyclic imidate. In 2 and 3 the coordination geometries are tetrahedrally distorted. The intramolecular hydrogen bonds are formed between H(NH 2) of phenea and O(CO) of the five-membered cyclic imidate. Three different staggered forms of and one eclipsed form of rotamers in the phenea ligands have been found.

Conformational analysis of levanbiose by molecular mechanics

A relaxed conformational energy map for levanbiose, O-β- d-fructofuranosyl-(2 → 6)-β- d-fructofuranoside, was computed with the molecular mechanics program MM2(87). All torsion angles of the three linkage bonds were driven by 30° increments while two primary alcohol groups were held at three staggered forms. The steric energy of all other parameters was optimized. The side groups were retained at the same relative positions on the two rings in this first part of the study so our results are directly applicable to the study of polymeric levan with identical repeating units. The low-energy dimers did not lead to viable polymers. The interresidue linkage torsion angles defined by C-6O-′2C-2′C-′1 (φ) and O-5C-5C-6O′2 (ω) have minima at +60° and −60°, respectively, with accessible minima at other staggered forms. As observed in inulobiose, the preferred torsion angle at central linkage bond defined by C-5C-6O-2′C-2′2 (ψ) was antiperiplanar. An analysis of all conformations of staggered side groups showed that the C-1 and C-1′ groups had little effect but the C-6′ group showed a preference for X -6′(O-5′C-5′C-6′O-6′) = −60°. The fructofuranose rings were started at the low-energy 4 3 T conformation (angle of pseudorotation, φ 2 = 265°) that was retained except when the linkage conformations created severe inter-residue conflict.

1H and 13C NMR spectra and rotational isomerism of 1,3‐dibromobutane

AbstractThe 1H and 13C NMR spectra of 1,3‐dibromobutane were assigned using selective proton decoupling 1D and selective proton‐flip 2D experiments. The chemical shifts and spin‐spin coupling constants were obtained from complete spectral analysis of the experimental 1H and 13C spectra. The spectral parameters obtained are discussed in terms of conformational and stereospecific substituent effects. The rotamer populations were obtained from the vicinal coupling constants, and the rotamers in which the CH3 and CH2Br groups are situated trans to each other contribute as much as 69% of the staggered forms.

Conformational study of methylethers of the type CH 3OCH 2R: an NMR and vibrational spectroscopic study

Analysis of 13C NMR spectra of methylethers of the type CH 3OCH 2R, with R = CH 3 (I), CH 2CH 3 (II), CH(CH 3) 2 (III) and C(CH 3) 3 (IV) has shown that the trans form strongly predominates on the OCH, bond, the gauche forms appearing only in those cases where close contact between methyl groups can be avoided. Analysis of both 1H and 13C NMR spectra indicates that all staggered forms are populated on the CH 2R bond. In vibrational spectra, the strong Raman bands near 800 cm −1 were found to be very sensitive to conformational structure. For I, III and IV, the bands of various conformers in this range are separated, permitting Δ H to be determined.

Molecular Conformations of 4-Aminomethyl-1-cyclohexanecarboxylic Acids in Aqueous Solution

Abstract The structures of cis- and trans-4-aminomethyl-1 -cyclohexanecarboxylic acids were studied by proton and carbon-13 NMR, and by semiempirical molecular orbital methods (CNDO/2 and MNDO). Both isomers exist in zwitter ionic forms in aqueous solution. It was found that the most stable conformations for them are all staggered forms: the diequatorial conformer in the trans form and the axial carboxyl group in the cis form. These preferred conformers are similar to those in crystalline states. In these conformers the deformation is very small in the cyclohexane ring of the trans form. The atomic distance between the nitrogen of amino group and the carbon of carboxyl group in the preferred conformer of trans form is 0.65 nm and that in cis form is 0.55 nm. This distance of 0.65 nm will play an important role in the antifibrinolytic effect.

The barrier to internal rotation in metallocenes

It has been shown by calculation that the eclipsed forms of ferrocene and ruthenocene are more stable than their staggered forms. The main contribution to the energy difference is the induction energy of the metal in the potential field of the rings. Direct ring-ring electrostatic energy also favours the eclipsed forms by a small amount. The calculated barrier in ferrocene is in good agreement with an experimental estimate from electron diffraction.

Hindered Internal Rotation of N, N-Dimethylpropylamines

The N-methyls in 2, 3-disubstituted N, N-dimethylpropylamines (I-VIII) have given two peaks with an equal intensity in an acid solution owing to the hindered internal rotation about the C-NMe2H+ bond. The energy barriers between two staggered forms have been estimated to be 0.7 (benzylthio)-7 (thiocyanato) kcal mole-1 from the temperature dependence of the peak separations. The high-field shift and the largest separation of the N-methyls in the benzylthio derivative (I) have been ascribed to an anisotropic effect of the benzene ring. 1, 3-Disubstituted 2-N, N-dimethylaminopropanes (IX) and 2-substituted N, N-dimethylethylamines (XII-XV) have shown a single peak for the N-methyls. A restricted rotation about the CH2-CH bond in 2, N, N-trimethylaminoethyl disulfide (X) has been studied by the temperature dependence of the vicinal coupling constants.