Ring Twist Research Articles

Spectroscopic investigations and potential energy surfaces of the ground and excited electronic states of 1,3-benzodioxan

The laser-induced fluorescence spectra (both fluorescence excitation and dispersed fluorescence) of jet-cooled 1,3-benzodioxan along with its ultraviolet absorption spectra have been recorded and analyzed in order to determine the vibrational quantum levels in both the ground and S(1)(pi,pi(*)) electronic excited states. A detailed energy map of the vibrational levels involving the six lowest frequency vibrations was established and utilized to better understand the structural and conformational differences between the ground and excited electronic states. The energies of more than a dozen vibrational excited states involving the out-of-plane ring twisting (nu(47)) and the out-of-plane ring bending (nu(48)) modes were determined for both S(0) and S(1) electronic states. Ab initio and density functional theory (DFT) calculations were also carried out to complement the experimental work. The data allowed one-dimensional potential energy functions in terms of the ring-twisting coordinate to be calculated. These show the molecule to have a twisting angle of 33 degrees and a barrier to planarity of 4300+/-500 cm(-1) for the S(0) ground state and an angle of 24 degrees and a barrier of 1500+/-200 cm(-1) for the S(1)(pi,pi(*)) excited state.

Molecular Dynamics Simulations of PIP2 and PIP3 in Lipid Bilayers: Determination of Ring Orientation, and the Effects of Surface Roughness on a Poisson-Boltzmann Description

Molecular dynamics (MD) simulations of phosphatidylinositol (4,5)-bisphosphate (PIP2) and phosphatidylinositol (3,4,5)-trisphosphate (PIP3) in 1-palmitoyl 2-oleoyl phosphatidylcholine (POPC) bilayers indicate that the inositol rings are tilted ∼40° with respect to the bilayer surface, as compared with 17° for the P-N vector of POPC. Multiple minima were obtained for the ring twist (analogous to roll for an airplane). The phosphates at position 1 of PIP2 and PIP3 are within an Ångström of the plane formed by the phosphates of POPC; lipids in the surrounding shell are depressed by 0.5–0.8 Å, but otherwise the phosphoinositides do not substantially perturb the bilayer. Finite size artifacts for ion distributions are apparent for systems of ∼26 waters/lipid, but, based on simulations with a fourfold increase of the aqueous phase, the phosphoinositide positions and orientations do not show significant size effects. Electrostatic potentials evaluated from Poisson-Boltzmann (PB) calculations show a strong dependence of potential height and ring orientation, with the maxima on the −25 mV surfaces (17.1 ± 0.1 Å for PIP2 and 19.4 ± 0.3 Å for PIP3) occurring near the most populated orientations from MD. These surfaces are well above the background height of 10 Å estimated for negatively charged cell membranes, as would be expected for lipids involved in cellular signaling. PB calculations on microscopically flat bilayers yield similar maxima as the MD-based (microscopically rough) systems, but show less fine structure and do not clearly indicate the most probable regions. Electrostatic free energies of interaction with pentalysine are also similar for the rough and flat systems. These results support the utility of a rigid/flat bilayer model for PB-based studies of PIP2 and PIP3 as long as the orientations are judiciously chosen.

Numerical analysis of piston ring pack operation

In the paper a model of a piston ring pack motion on an oil film has been analysed. The local oil film thickness can be compared to height of the combined roughness of mating surfaces of piston rings and cylinder liner. Equations describing the mixed lubrication problem based on the empirical mathematical model formulated in works of Patir, Cheng [6, 7] and Greenwood, Tripp [3] have been combined [12] and used in this paper. A model of a gas flow through the labyrinth seal of piston rings has been developed [13, 15]. In addition models of ring twist effects and axial ring motion in piston grooves have been applied [14, 15]. In contrast to the previous papers of the author, an experimental verification of the main parts of developed mathematical model and software has been presented. A relatively good compatibility between the experimental measurements and calculated results has been achieved. In addition this study presents the simulation results for an automobile internal combustion engine

Neutron diffuse scattering in deuterated para-terphenyl,C18D14

Neutron diffuse scattering is used to explore the short-range order (SRO) in deuterated para-terphenyl,C18D14. The crystal shows SRO because the central of the three phenyl groups of eachmolecule can twist positively or negatively and these twists are correlated overthe local scale. The presence of incipient Bragg peaks at at 200 K shows that these flips are negatively correlated along thea direction (nearest neighbour correlation coefficient of∼–0.3)and b direction (nearest neighbour correlation coefficient of∼−0.87) and appear essentiallyuncorrelated along c. Diffuse peak anisotropy indicates that the range of the correlations alongb is found tobe ∼3 timesthat along a. These correlations persist, although weaker, at room temperature.A Monte Carlo simulation was used to impose a correlation structure on the population ofcentral ring twists that was deduced from Bragg scattering. By then allowing displaciverelaxation of the structure, the observed diffuse scattering was well reproduced.Modelling the displacive motions of molecules showed that the positions of nearestab-planeneighbour molecules are strongly positively correlated, particularly for motions approximately parallelto a, while the displacive correlations are weaker between molecules stacked alongc. The apparent contradiction that the displacements are most strongly correlated alonga while the occupancies are most strongly correlated alongb is explained in terms of the connectivity of molecular interactions.

Highly Diastereo‐ and Enantioselective Aldol Reactions in Common Organic Solvents Using N‐Arylprolinamides as Organocatalysts with Enhanced Acidity

AbstractA broad set of N‐arylprolinamides 1–8 with increasing NH acidity and steric crowding has been synthesized and its catalytic activity explored for enantioselective aldol reactions. In DMF containing 10 mol‐% of TFA, all arylamides are found to catalyze the reaction between cyclohexanone and a variety of electrophilic aldehydes leading to aldols in excess of 90 % yield and >95 % enantioselectivity. The perfluorophenyl catalyst 8 is found to perform best with a broad substrate scope as compared to all other N‐arylamides 1–7. It is shown that 8 can indeed be employed in highly nonpolar as well as polar solvents including brine to afford high yields of aldols with excellent diastereo‐ as well as enantioselectivity. The results observed for 8 are amongst the best reported so far for prolinamides that do not contain additional stereogenic center(s) and hydrogen‐bonding site(s). The molecular structures of 7 and 8, determined by X‐ray crystallography and presumed to reflect the most stable conformations, reveal a notable difference in the conformations of the N‐aryl rings; the aryl ring exhibits tendency to lie coplanar with the amide functionality in the case of 7, while the perfluorophenyl ring twists almost orthogonally with respect to the plane of the amide of functionality in 8. The superior performance of the latter is attributed to, in addition to the enhanced NH acidity, the tendency of the perfluorophenyl ring to lie orthogonally to the amide group, which may facilitate a stronger binding of the electrophilic aldehyde via hydrogen bonding in the transition state.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Vibrational spectral analysis of 2,6 Bis (p-methoxy benzylidene) cyclohexanone using density functional theory

The geometry optimization and the computation of vibrational wavenumbers have been carried out for 2,6 Bis (p-methoxy benzylidene) cyclohexanone using density functional theory at B3LYP/6-31G(d) level. The optimized geometry reveals that the central cyclohexanone ring possesses a nearly 'half chair' conformation. The steric interaction causes the twisting of phenyl rings and the twisting of both Ph1 and Ph2 are energetically favored by 6.66 kJ mol$^{-1}$ and 6.86 kJ mol$^{-1}$ respectively. The relaxed PES scan studies have been conducted to expose the changes in the geometrical parameters caused by ring twisting. The stretching vibrational modes involving aromatic hydrogen atoms participating in steric interaction have been observed to possess higher vibrational wavenumbers. The unusual lowering of C=O stretching vibrational wavenumber has been noticed, attributable to the mesomeric interaction of the group -CH=C-(C=O)-C=CH-. The methylene groups involving steric interaction have been found to possess different spectral behavior compared to free CH$_{2}$ groups. The computed geometry and vibrational spectrum of cyclohexanone at B3LYP/6-31G(d) level have been used to analyze, the spectral distinction between chaired cyclohexanone and half chaired central cyclohexanone ring in BMBC and also the bond angle effect and conjugation effects on carbonyl stretching modes.

Conformations of substituted benzophenones

The inclination of the two aryl rings (ring twists) in a series of benzophenone molecules has been examined. For each structure the dihedral angle (between the planes of the two sets of six aromatic C atoms) relates to both the steric considerations of the single molecule and the packing forces related to the crystal structure. Six new benzophenone structures are incorporated into the study including 2,2'-dihydroxy-4,4'-dimethoxybenzophenone (I), C(15)H(14)O(5), that appears to have the smallest reported twist angle, 37.85 (5) degrees , of any substituted benzophenone reported to date. Three further benzophenones, 4,4'-bis(diethylamino)benzophenone (II), C(21)H(28)N(2)O, 3,4-dihydroxybenzophenone (III), C(13)H(10)O(3), and 3-hydroxybenzophenone (IV), C(13)H(10)O(2), have similar ring twists [49.83 (5), 49.84 (5) and 51.61 (5) degrees , respectively] that are comparable with the value of 54 degrees found for the orthorhombic form of unsubstituted benzophenone. 4-Chloro-4'-hydroxybenzophenone (V), C(13)H(9)ClO(2), has a ring twist of 64.66 (8) degrees that is close to the value of 65 degrees found in the metastable monoclinic form of unsubstituted benzophenone and 2-amino-2',5-dichlorobenzophenone (VI), C(13)H(9)Cl(2)NO(2), has a large ring twist of 83.72 (6) degrees . Comparisons with a further 98 substituted benzophenone molecules from the Cambridge Structural Database (CSD) have been made.

Phosphorescence excitation spectrum of the T1(n,π*)←S transition of 4H-pyran-4-one

The phosphorescence excitation (PE) spectrum of 4H-pyran-4-one (4PN) vapor at 40-50 degrees C was recorded near 366 nm. The most intense vibronic feature in this region of the spectrum is the T(1)(n,pi*)<--S(0) origin band. The value of nu(0) for the 0(0)(0) transition was determined to be 27 291.5 cm(-1) by comparing the observed spectrum to a simulation in the T(1)<--S(0) origin-band region. Attached to the origin band in the PE spectrum are several Deltav=0 sequence bands involving low-frequency ring modes. From the positions of these bands, together with the known ground-state combination differences, fundamental frequencies for nu(18') (ring bending), nu(13') (ring twisting), and nu(10') (in-plane ring deformation) in the T(1)(n,pi*) excited state were determined to be 126, 269, and 288 cm(-1), respectively. These values represent drops of 15%, 32%, and 43%, compared to the respective fundamental frequencies in the S(0) state. The changes in these ring frequencies indicate that the effects of T(1)(n,pi*)<--S(0) excitation extend beyond the nominal carbonyl chromophore and involve the conjugated ring atoms as well. The delocalization may be more extensive for T(1)(n,pi*) than for S(1)(n,pi*) excitation.

Structure and Charge Distribution, Electronic and Vibrational (Hyper)Polarizabilities of Some TICT Molecules

Dipole moment, vibrational and electronic isotropic polarizabilities and first hyperpolarizabilities have been determined for a set of intramolecular charge transfer molecules qt the HF/6-31G level. The vibrational contributions to the (hyper)polarizabilities have been computed using the formulation of Bishop and Kirtman. The study have been mainly focused in the effect of the inter ring twist angle upon these quantities and upon the inter ring bond character and the BLA. It has been found that tuning the twist angle affects substantially both electronic and geometrical structures. The results show that ground state dipole moment, electronic (hyper)polarizabilities increases with the dihedral inter ring angle however the vibrational quantities Δαv and βμv show a maximum value at ∼78°.

Toward Quantitative Prediction of Molecular Fluorescence Quantum Efficiency: Role of Duschinsky Rotation

It is a highly desirable but difficult task to predict the molecular fluorescence quantum efficiency from first principles. The molecule in the excited state can undergo spontaneous radiation, conversion of electronic energy to nuclear motion, or chemical reaction. For relatively large molecules, it is impossible to obtain the full potential energy surfaces for the ground state and the excited states to study the excited-state dynamics. We show that, under harmonic approximation by considering the Duschinsky rotation effect, the molecular fluorescence properties can be quantitatively calculated from first principles coupled with our correlation function formalism for the internal conversion. In particular, we have explained the peculiar fluorescence behaviors of two isomeric compounds, cis,cis-1,2,3,4-tetraphenyl-1,3-butadiene and 1,1,4,4-tetraphenyl-butadiene, the former being nonemissive in solution and strongly emissive in aggregation or at low temperature, and the latter being strongly emissive in solution. The roles of low-frequency phenyl ring twist motions and their Duschinsky mode mixings are found to be crucial, especially to reveal the temperature dependence. As an independent check, we take a look at the well-established photophysics of 1,4-diphenylbutadiene for its three different conformers. Both the calculated radiative and nonradiative rates are in excellent agreement with the available experimental measurements.

Low-frequency vibrations specific for conformers of 1-aminoindan studied by UV-UV hole-burning spectroscopy

The UV-UV hole-burning spectra of the jet-cooled 1-aminoindan were measured for the first time. Complicated spectral features observed in the laser-induced fluorescence excitation spectrum due to two conformers, R and B, were firmly separated. On the basis of fluorescence measurements and B3LYP/cc-pVTZ calculations, low-frequency ring twisting and ring puckering modes were assigned. These modes are coupled in the S1 state due to the Duschinsky rotation. The Duschinsky matrix was calculated from the normal modes predicted by quantum chemical calculations. The coupling between the twisting and puckering modes for conformer B is stronger than that for conformer R. The twisting mode was observed at 0+99 cm(-1) in the S1 state for conformer B, while not for conformer R. The Franck-Condon activity of the twisting mode substantially differs between the two conformers. The transition to the twisting level for conformer B would be allowed by the Duschinsky rotation. The fluorescence lifetime of conformer vibronic levels was also measured and differed for each conformer.

Formulation of polymer-based lubricant for metal forming

A new metal forming lubricant based on polymeric materials has been developed. The lubricant has been developed through emulsion copolymerization. Copolymers with the common composition of stearyl methacrylate and 2-hydroxyethyl methacrylate acid phosphate were made with three secondary polymers: methyl methacrylate, butyl acrylate, and 2-hydroxyethyl methacrylate. The performance of the developed lubricant was evaluated using the ring compression and twist compression tests. The performance of the polymeric lubricant was compared against a zinc-phosphate-coating-based lubricant, commonly used in forging, and three other commercial lubricants. The polymeric lubricant exhibited at least equal performance in all tests conducted at room temperature. Preliminary test results show that the developed lubricant can be used effectively for non-severe deformation processes such as cold heading and extrusion processes with low surface expansions.

An ab initio study of stable conformation and thermal isomerization of p-aminoazobenzene

In this work, the stable conformation and thermal isomerization of p-aminoazobenzene (AAB) were investigated by using ab initial MP2/6-31+G ∗ and DFT-B3LYP/6-31+G ∗ calculations. For comparison reason, corresponding results of azobenzene (AB) were also obtained by the same theoretical methods. Compared with AB, the MP2 optimized geometries of AAB show little variation in the NN bond length and most bond angles, but show obvious variations in the CN bond length and CCNN dihedral angle that link with the NH 2-substituted phenyl ring. Phenyl ring twist of both AB and AAB show two equivalent nonplanar minima in the potential energy surface (PES) of the NNCC dihedral angles, but the potential energy barriers at 0° (the planar structure) are low. For AAB, the energy barrier is even lower for CCNN dihedral angle of the NH 2-substituted phenyl ring. The S 0 PES of AB obtained by DFT method shows that the energy barrier for CNN inversion is lower than that for CNNC rotation. On the contrary, the energy barriers for AAB along both rotation and inversion pathways are almost equal. Calculation indicates that the molecular structures at the highest points on the PESs of CNNC rotation and CNN inversion of the unsubstituted phenyl ring are identical. On the basis of the calculation, it is believed that even the relative stability of trans- and cis-isomers is not obviously affected by the donor substitution, the thermal isomerization of AAB can go through both the inversion and rotation pathways.

Mass analyzed threshold ionization spectroscopy of indazole cation

We have recorded the two-color resonant two-photon mass analyzed threshold ionization (MATI) spectra of indazole via four intermediate states. The adiabatic ionization energy of this molecule is determined to be 67 534 ± 5 cm −1. The observed MATI bands include in-plane ring bending as well as out-of-plane ring twisting and bending vibrations of the indazole cation. Comparing the present data with those of indole and 7-azaindole leads to a better understanding about the influence of the nitrogen atom in the aza-aromatic bicyclic system.

2,4,6-Tris(pyrimidin-2-ylsulfanyl)-1,3,5-triazine

The title compound, C15H9N9S3, was synthesized by the reaction of cyanuric chloride with the potassium salt of 2-mercapto­pyrimidine. The dihedral angles between the three pyrimidinyl rings and the central triazine plane are 94.1 (4), 105.5 (3) and 126.6 (5)°. The three S atoms are essentially coplanar with the triazine ring, while the orientations of the three pyrimidinyl rings are different, with one ring twisting in the opposite direction to the other two.

Non‐linear absorption of 2,5‐dialkynyl thiophenes

AbstractThree diarylalkynyl‐substituted thiophenes were synthesized and the optical power limiting (OPL) effect at 532 nm was investigated for solutions of these compounds. The relationship between the OPL and parameters obtained from molecular‐orbital‐based calculations is discussed. Semi‐empirical calculations indicate that the compounds can have a broad distribution of conformations due to inter‐ring twisting, but that the second hyperpolarizability (γ) can be significant despite a ring twist. The calculations imply that substitution by alkyl groups can lead to enhanced γ values. The measurements and calculations show a greater increase of the OPL and γ effects from compound 5 to 6 than from 6 to 7. For these compounds, which differ mainly by the length of the π‐electron system, the magnitude of non‐linear absorption seems to be well correlated with several properties of the electronic ground state estimated by standard ab initio molecular orbital calculations, as well as with γ values from the semi‐empirical calculations. Copyright © 2004 John Wiley &amp; Sons, Ltd.

A finite element study of piston tilt effects on piston ring dynamics in internal combustion engines

The motion of the piston and rings strongly influences lubrication and blow-by in a reciprocating engine. Three-dimensional finite element models of single-cylinder and four-cylinder internal combustion (IC) engines are developed to analyse the piston and ring motions. The ring motion in the radial and axial directions and the ring twist, along with the end gap size variation, are studied in detail. Results are compared with those obtained by neglecting piston secondary motions. A comparative analysis of piston and ring motions between the single-cylinder and four-cylinder engines has also been performed. It has been found that the predicted ring motion of a single-cylinder engine is different from that of a multicylinder engine. The conclusion is that piston tilt has a profound effect on the end gap variation, the ring twist and the ring lift.

Symmetry switching of the fluorescent excited state in alpha,omega-diphenylpolyynes.

By using MO calculations based on DFT, absorption, and fluorescence spectroscopy, we have comprehensively studied the low-lying excited singlet states of alpha,omega-diphenylpolyynes (DPY) having 1-6 triple bonds. The a(g) vibrational modes of the C(triple bond)C stretching and of the phenyl ring motion were observed in the fluorescence spectra of diphenylacetylene and 1,4-diphenylbutadiyne. On the other hand, in the fluorescence spectra of the long DPY with the triple-bond number (N) more than two, the phenyl ring motion with a(g) symmetry disappeared and the b(1g) modes of the phenyl ring twisting (approximately 400 cm(-1)) and of the C-H bending (approximately 900 cm(-1)) were detected. The observed fluorescent states of DPY with N < or = 2 and N > or = 3 are assigned to the 1(1)B(1u) (pi(x)pi(x*)) and 1(1)A(u) (pi(x)pi(y*) and/or pi(y)pi(x*)) states, respectively, based on the vibronic structures, the relatively short lifetimes, and the solvatochromic shifts of the fluorescence spectra. Not only the allowed transition of 1(1)B(1u) <-- S(0) but also the forbidden transition of 1(1)A(u) <-- S(0) was detected in the fluorescence excitation spectra of the long DPY with N > or = 3. The low-lying excited state with A(u) symmetry is characteristic in polyyne, which does not exist in polyene. The oscillator strength (f) of the first absorption band in DPY decreases with an increase in N, which is the opposite behavior of the all-trans-alpha,omega-diphenylpolyenes. The N-dependence of the f value is understood by the configuration interaction between the 1(1)B(1u) and 2(1)B(1u) (pi(y)pi(y*)) states, which is consistent with the reduction of the nonlinear optical response of polyyne.

Mass analyzed threshold ionization spectroscopy of indoline cation: Cyclization effect and large amplitude vibrations

We have applied mass analyzed threshold ionization (MATI) spectroscopy to record the vibrational spectra of the indoline cation by ionizing through several vibrational levels in the S1 state. The adiabatic ionization energy is determined to be 59 269±5 cm−1. The present experimental results show that only one stable configuration is involved in the photo-excitation processes, as predicted by ab initio calculations. Comparing the electronic transition and ionization energies of indoline with those of aniline and N-ethylaniline suggests that the cyclization of the five-membered ring leads to a redshift in both quantities. When the intermediate states involve large amplitude vibrations, the intense bands that appeared in the MATI spectra correspond to similar vibrational patterns, indicating that the structure of indoline in the D0 state resembles that in the S1 state. The puckering of the five-membered ring, the ring flopping (butterfly), the out-of-plane ring twisting, and the N-inversion of indoline in the D0 state are observed with frequencies of 92, 187, 235, and 706 cm−1, respectively. These spectral bands may be regarded as the characteristic features of the indoline cation for molecular identification.

DSC investigations of polyamide 6 in hybrid GF/PA 6 yarns and composites

Glass/PA 6 composites were manufactured from specially designed hybrid yarns. The hybrid yarns were produced by three different spinning systems: friction spinning, ring twisting and pneumatic texturing. Each of these systems gives a different structure of the yarn and a different level of blending of the reinforcing and thermoplastic fibres. The crystallization and melting behaviour of PA 6 in the yarns and the composites were studied by differential scanning calorimetry. The different structure of the hybrid yarns leads to differences in the crystallinity of PA 6 in the yarns and the composites. The mechanical properties of the thermoplastic yarns and the composites are influenced, among other factors, by the crystal structure of the polymer.