Conformational origin of glassy-state relaxation and ductility in aromatic polycarbonates

A new two-state conformational transition is proposed to explain the large, low-temperature mechanical loss peak seen in glassy polycarbonates. Restricted Hartree Fock ab initio calculations at the 6–31G7 level for diphenyl carbonate (DPC), a key model compound of bisphenol-A polycarbonate, reveal two inequivalent trans-trans carbonate-ring conformations both of which will exist in solution, melt or glassy states. These calculations appear to be the first high level ones (with full geometry optimization) reported for DPC, and the findings are consistent with earlier ab initio results for phenyl formate and other smaller model compounds and also with single-crystal X-ray data for DPC and oligomers. In addition to a trans-trans conformer of DPC with both phenyl rings on the same side of the carbonate unit (called the ‘syn’ conformer) which is seen in the crystalline state of DPC, an ‘anti’ conformer of lower energy is found, which has its two phenyl rings located on opposite sides of the plane of the carbonate unit. Analysis of these calculated ground state geometries and energies as well as experimental single crystal X-ray results indicates that the ‘anti’ conformer has the lowest energy in the gas phase and solution, while the ‘syn’ conformation is stabilized relative to the ‘anti’ in the bulk, probably because of aromatic ring interactions between neighbour chain segments. In the glassy state of either DPC or polycarbonate, one expects a nearly random mixture of ‘syn/anti’ conformers, and the prominent low-temperature mechanical loss peak observed in many polycarbonates is consistent with a molecular level two-state process consisting of ‘syn/ anti’ carbonate conformer conversions. These conformational transitions must involve rotation and translation of both the carbonate units and, most importantly, the neighbouring phenyl groups. The possible influence of these conformational changes and the accompanying correlated molecular motions on polymer ductility and ageing is briefly discussed.     

A density functional theory study of the conformational properties of 1,2-ethanediamine: protonation and solvent effects

 The structures and the conformational energies of nonprotonated, monoprotonated and diprotonated 1,2-ethanediamine have been investigated through density functional theory. The relative performance of local and gradient-corrected functionals is discussed. The existence of hydrogen-bond formation has been determined with electron localisation function calculations. Proton affinities for nonprotonated and monoprotonated 1,2-ethanediamine have been calculated and are in agreement with experimental data. The influence of solvation has been accounted for through the self-consistent isodensity polarisable continuum model. The results for the nonprotonated conformers show that solvation stabilises those conformers which have the lone pair in an antiperiplanar conformation. Solvation of the monoprotonated conformer stabilises significantly the “anti” conformation, which is unstable in the gas phase. For the di-protonated species, solvation stabilises slightly the gauche conformer, which is unstable in the gas phase. Received: 28 September 1999 / Accepted: 2 May 2000 / Published online: 27 September 2000     

Stereodynamics of tribenzylamine

¹H DNMR studies in conjunction with EFF calculations for tribenzylamine indicate a preference for that conformer having C₃ symmetry and the presence of minor rotamers having one phenyl group anti to the lone pair.     

A density functional theory study on pelargonidin

A complete conformational analysis on the isolated and polarizable continuum model (PCM) modeled aqueous solution cation, quinonoidal, and anion forms of pelargonidin, comprising the diverse tautomers of the latter forms, was carried out at the B3LYP/6-31++G(d,p) level. The results indicate that the most stable conformer of cationic and quinonoidal forms of pelargonidin are completely planar in the gas phase, whereas that of the anionic form is not planar. In contrast, PCM calculations show that the plane of the B ring is slightly rotated with regard to the AC bicycle in the most stable conformer of the cation and quinonoidal form. The most stable conformers of the cation, both in gas phase and aqueous solution, display anti and syn orientations for, respectively, C2-C3-O-H and C6-C5-O-H dihedral angles, whereas syn and anti orientation of hydroxyls at 7 and 4' positions are nearly isoenergetic. The most stable tautomer of quinonoidal pelargonidin is obtained by deprotonating hydroxyl at C5 in gas phase but at C7 according to PCM. Also, the most stable tautomer of the anion is different in gas phase (hydrogens are abstracted from hydroxyls at C5 and C4') and PCM simulation (C3 and C5). Tautomeric equilibria affect substantially the geometries of the AC-B backbone providing bond length variations that basically agree with the predictions of the resonance model. Most of the conformers obtained display an intramolecular hydrogen bond between O3 and H6'. Nevertheless, this interaction is not present in the most stable anions. Ionization potentials and O-H bond dissociation energies computed for the most stable conformers of cation, quinonoidal, and anion forms are consistent with an important antioxidant activity.     

X‐Ray Structure Analyses of syn/anti‐Conformers of N‐Furfuroyl‐, N‐Benzoyl‐, and N‐Picolinoyl‐Substituted (2R)‐Bornane‐10,2‐sultam Derivatives

The synthesis and the X‐ray structure of the three new N‐(arylcarbonyl)‐substituted derivatives 2a – 2c of (2R)‐bornane‐10,2‐sultam are presented and discussed. Direct comparison of the solid‐state analyses shows that the dipole‐directed SO₂/C?O anti‐/syn‐conformations may be very sensitive to weak electronic/electrostatic repulsions of the heteroatom lone pairs. The optimum interactions are reached when the lone pair of the β‐positioned heteroatom is oriented in the O(3)?C(11)? N(1) plane. Such rare syn‐conformations may be observed with at least up to 1.8 kcal/mol higher energy as compared to their ground states. Additionally, these anti/syn‐conformations are also very sensitive to external influences such as, for example, the crystal‐packing forces.     

Off-center oxygen-arene interactions in solution: a quantitative study

[reaction: see text] Triptycenes with C1-MeO/RCOO (R = H, Me, Et, i-Pr, CF3) and C9-XC6H4CH2 (X = Me, H, F, CN, CF3) have been prepared to determine lone pair-arene interactions in the off-center configuration. The ratios of the syn and anti conformers were determined by low-temperature NMR spectroscopy. The syn conformer allows the attached arene and the MeO/ester to interact with each other while the anti conformer does not. The free energies of interaction have been derived from the syn/anti ratios. Compound 7 in the ester series with X = H and R = CF3 is the only compound that shows a slightly repulsive interaction (0.08 kcal/mol). Compound 2e in the MeO series with X = CF3 exhibits an attractive interaction (-0.47 +/- 0.05 kcal mol). All other compounds show smaller attractive interactions.     

Theoretical investigation of isotopic scrambling mechanisms in 2-chloroethyl methyl sulfide

Ab initio and semiempirical molecular orbital calculations have been applied to study the concerted and stepwise isotopic scrambling mechanisms of 2-chloroethyl methyl sulfide in the gas phase and in aqueous solution. The calculations reveal the structural details of the reactants, transition structures, and intermediates involved in this reaction and provide relative energy estimates. The concerted mechanism is found to be competitive with the stepwise mechanism in the gas phase, but the stepwise mechanism is favored in aqueous solution as no true transition structure for the concerted mechanism could be found using the solvation models. A combined approach of evaluating solvation energies with the generalized-Bom-plus-surface-tensions SM _x solvation models of Cramer and Truhlar at ab initio optimized geometries is found to deliver the best agreement with experimentally determined reaction barriers. Together with the recent experimental results of McManus and co-workers, the present study provides insights into the controlling factors involved in the elementary reaction steps of sulfur mustards and a solid foundation for investigations into more complex reactions of related compounds.     

Surprising Stability of Larger Criegee Intermediates on Aqueous Interfaces

Criegee intermediates have implications as key intermediates in atmospheric, organic, and enzymatic reactions. However, their chemistry in aqueous environments is relatively unexplored. Herein, Born–Oppenheimer molecular dynamics (BOMD) simulations examine the dynamic behavior of syn ‐ and anti ‐CH₃CHOO at the air–water interface. They show that unlike the simplest Criegee intermediate (CH₂OO), both syn ‐ and anti ‐CH₃CHOO remain inert towards reaction with water. The unexpected high stability of C₂ Criegee intermediates is due to the presence of a hydrophobic methyl substituent on the Criegee carbon that lowers the proton transfer ability and inhibits the formation of a pre‐reaction complex for the Criegee–water reaction. The simulation of the larger Criegee intermediates, (CH₃)₂COO, syn ‐ and anti ‐CH₂C(CH₃)C(H)OO on the water droplet surface suggests that strongly hydrophobic substituents determine the reactivity of Criegee intermediates at the air–water interface.     

Theoretical study of helical structure caused by chirality of cysteine dimer

In this work we report a theoretical study of the helix structure and chiral discrimination on the interactions between the chiral cysteine–cysteine. Two reasonable geometries on the potential energy hypersurface of the cysteine–cysteine system are considered with the global minimum. Accurate geometric structures, relative stabilities, harmonic vibrational frequencies, and infrared (IR) intensities were investigated. To take into account the water solvation effect, the Onsager model within the self‐consistent reaction field (SCRF) method and the polarized continuum (PCM) method were used to evaluate the interaction energy, ΔG_solv at the same level employed in the gas phase. The results indicate that the polarity of the solvent plays an important role in the structures and relative stabilities of different isomers. Computational results indicate that the global minimum should be conformer I regardless of whether in the gas phase or in aqueous solution, which differs from previous theoretical reports. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Molecular Structure and Conformational Preferences of Trimethyl Phosphorotrithioite,P(SMe)<Subscript>3</Subscript>, Evaluated by Gas-Phase Electron Diffraction and Quantum Chemical Calculations

Free P(SMe)₃ molecule was studied by gas electron diffraction (GED) and by B3PW91/6-311+G* (DFT) and MP2/6-31+G* calculations. Each conformer is characterized by three dihedral angles τ(CSPlp), where lp denotes the direction of the lone electron lone pair on the P atom. DFT calculations indicate that the most stable conformer is an anti,gauche+,gauche- (ag+g-) conformer of C _s symmetry; the next are the ag+g+ (ΔE = 2.5 kJ mol⁻¹), g+g+g+ (ΔE = 5.2 kJ mol⁻¹), and aa+g+ (Δ E = 12.5 kJ mol⁻¹) conformers. The MP2 calculations give the similar order, with the relative energies of 0.3, 4.3, and 10.6 kJ mol⁻¹, respectively. The experimental GED data agree well with the presence of only two conformers: χ(ag+g+) = 80(20)% and χ(ag+g-) = 20(10)%.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 5, 2005, pp. 742–750.Original Russian Text Copyright © 2005 by Belyakov, Khramov, Baskakova, Naumov.     

The influence of hydrophobic amino acid side groups on the acidity of the aromatic imidazole ring of histidine: A theoretical study

In this study we have calculated the acidity constant (pKa) of imidazole ring in Histidine‐Hydrophobic amino acid dipeptides using the quantum chemistry and continuum solvation methods. Density functional theory calculations with the large basis sets are used to determine the Gibbs free energy of deprotonate in the gas and liquid phases. Based on our results ΔG_S values are located between ?69.38 and ?18.82 kcal mol^?1 which are related to His⁺–Gly and His forms, respectively. pKa of the dipeptides in the aqueous phase was obtained from the calculated gas‐phase and solvation free energies through a thermodynamic cycle and the solvation model chemistry of Martin Karplus et al. Solvation effects are treated using a self‐consistent reaction field formalism involving polarized continuum models. According to our calculations pKa values are between 5.50 and 8.19 that are belong to His⁺–ILe and His⁺–Ala forms, respectively. Natural bond orbital analysis of dipeptides reveals that the electron delocalization in imidazole ring is the most effective factor in determination of acidity order for these compounds. Structural analysis confirmed that the orientation of carbonyl group with respect to imidazole ring is an effective factor in imidazole ring stability. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Solvent effect of aqueous media on properties of glycine: Significance of specific and bulk solvent effects,and geometry optimization in aqueous media

Geometries of the normal (N) and zwitterionic (Z) forms of glycine (gly) and their complexes gly.(H₂O)_n, n = 0–2, were fully optimized in gas phase and aqueous media, and transition states located between the corresponding N and Z forms. The geometry was also optimized and vibrational spectra calculated for the gly.(H₂O)₃ complex of Z glycine. Density functional theory at the B3LYP/AUG‐cc‐pVDZ level was employed for the geometry optimization calculations in gas phase and aqueous media while single point energy calculations were performed at the MP2/AUG‐cc‐pVDZ level in each case. Solvation in bulk water was treated using the polarizable continuum model (PCM). Zero‐point energy correction to total energy and thermal energy correction to enthalpy were obtained at the B3LYP/AUG‐cc‐pVDZ level of theory in both gas phase and bulk aqueous media and these corrections were also considered to be valid for the corresponding single point energy calculations performed at the MP2/AUG‐cc‐pVDZ level of theory. When geometries of the complexes of glycine with water molecules are optimized in aqueous media, the calculated properties are found to be appreciably modified with respect to those obtained by gas phase geometry optimization followed by solvation in aqueous media. For several vibrational frequencies, the agreement between the calculated and experimentally observed results is improved appreciably when both the specific and bulk solvent effects are considered in combination with full geometry optimization in aqueous media. For certain vibrational frequencies, mode assignments have also been modified. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Encapsulation-Controlled Photoisomerization of a Styryl Derivative: Stereoselective Formation of the Anti Z-Isomer in the Cucurbit[7]uril Cavity

The photochemical isomerization of a styrylpyridinium dye ( SP ) bearing an unsymmetrically attached benzo-15-crown-5 ether has been studied in aqueous solution in the absence and presence of cucurbit[7]uril (CB[7]). The detailed analysis of the UV/Vis and NMR spectra showes that the isomeric composition of the photostationary mixtures of SP can be modulated by the host-guest complexation with CB[7]. It was found that steric hindrance caused by encapsulation of SP in the host cavity induces the exclusive formation of the anti conformer of Z- SP in contrast with the mixture of both anti and syn conformers obtained during photoisomerization of the dye without CB[7]. Remarkably, the displacement of anti Z- SP from CB[7] does not lead to the transformation of the anti Z-isomer into the syn Z-isomer pointing out the conformational memory of the system. The results provide an interesting example of the supramolecular stereorecognition by the achiral CB[7] host.     

Vibrational analyses of sulfamoyl halides NH2SO2X (X is F, Cl and Br)

The structural stability of sulfamoyl halides NH(2)-SO(2)X (X is F, Cl and Br) were investigated by DFT-B3LYP/6-311+G** and ab initio MP2/6-311+G** calculations. From the calculations the molecules were predicted to exist only in the anti (XS bond is anti with respect to nitrogen lone pair) conformation with the possibility of very low abundance of the syn (SO(2) and NH(2) groups eclipse each other) form of only the fluoride. The equilibrium constant for the syn<-->anti conformational conversion of sulfamoyl fluoride was calculated to be 0.0172 that corresponds to an equilibrium mixture of about 2% syn and 98% anti at 298.15K. The vibrational frequencies were computed at DFT-B3LYP level for the stable anti conformer of the d(0) and d(2) (ND(2)-SO(2)X) deuterated species of the three molecules. Normal coordinate calculations were then carried out and the potential energy distributions were calculated for the molecules.     

Solvent effect on the syn/anti conformational stability: A comparison between conformational bias Monte Carlo and molecular dynamics methods

The solvent effect on the syn/anti population ratio of the mesityl oxide (MOx) was investigated using a new implementation of conformational bias Monte Carlo (CBMC) and molecular dynamics (MD) methods. It was observed by a previous theoretical work (Theor. Chem. Acc. (2012) 131:1214) that in gas-phase the MOx exists dominantly in syn-form and in aqueous solution in anti-form. The syn/anti free energy difference in the gas phase was used in the intramolecular parametrization and a rotational barrier of approximately 10 kcal mol⁻¹ was found. Molecular systems with barriers of this order of magnitude have been studied by experimental techniques. However, they have not been discussed yet comparing CBMC and MD simulations. In this work, we show that the intramolecular geometrical information such as bond lengths, angles and torsional angles sampled with CBMC and MD methods are equivalent. Nonetheless, only the CBMC simulations sample appropriately the syn/anti population ratio. With the CBMC configurations in gas phase, it was obtained 95% in syn-form and 5% in anti-form regardless the initial conformation. An inversion of the population was found in water, 25% in syn-form and 75% in anti-form. Comparing the gas phase and in-water CBMC sampling, it was observed that the MOx spends typically approximately 110 successive MC cycles in anti-form and approximately 2300 in syn-form in gas phase. While it was much larger with explicit water, approximately 400 times more for anti-form and approximately 6 times more for syn-form. We argue that this strong stabilization of the anti-form in aqueous solution, does not come from the MOx-water hydrogen bonds interactions, because they are the same for both conformations. Instead, the stabilization comes from the dipole-dipole interaction caused by a larger dipole moment of the MOx in the anti-form, 7.2 D, than in the syn-form, 5.2 D. With the MD sampled configurations in both conditions, we observe that the syn/anti conformational change is a very rare event due to the rotational barrier, which is approximately 17 times larger than the thermal energy. Therefore, the MD sampling of the MOx is not appropriated because it is strongly dependent on the initial conformation even for large simulations with 150 ns up to 400 ns for the isolated solute and for solute–solvent systems.     

Condensed-phase effects on the conformational equilibrium of ethylene glycol

Density functional calculations for ethylene glycol (CH₂OHCH₂OH) in the gas and in a dielectric medium are reported. The condensed-phase calculations are based on the self-consistent reaction field approach and the environment has the dielectric constant of liquid methanol. NPT Monte Carlo simulations of ethylene glycol (ETG) in liquid methanol are also reported. The simulations were carried out for three conformers of ETG (tGg′, gGg′, and tTt). Comparison between SCRF results for the conformational equilibrium in the gas and in the dielectric suggests that the tGg′ conformer is slightly stabilized relative to the gGg′ conformer in the solvent. However, the energy difference between them is less the 1.0 kJ/mol, which indicates that frequent interconversions between the tGg′ and gGg′ conformers are expected in the condensed phase. The all-trans conformer (tTt) is higher than the most stable conformer in the gas by 14 kJ/mol. Monte Carlo simulations predict that the tGg′ and gGg′ conformers have very similar energies in the solvent. However, the simulations also show, in agreement with experimental data, that the tTt conformer is stabilized in liquid methanol, relative to the gas phase. The microscopic mechanism leading to the stabilization of the tTt conformer in the liquid is related to the differential hydrogen-bonding formation between the ETG conformers and the methanol molecules. © 1996 John Wiley & Sons, Inc.     

Experimentation with different thermodynamic cycles used for pKa calculations on carboxylic acids using complete basis set and Gaussian‐n models combined with CPCM continuum solvation methods

Complete basis set and Gaussian‐n methods were combined with Barone and Cossi's implementation of the polarizable conductor model (CPCM) continuum solvation methods to calculate pK_a values for six carboxylic acids. Four different thermodynamic cycles were considered in this work. An experimental value of ?264.61 kcal/mol for the free energy of solvation of H⁺, ΔG_s(H⁺), was combined with a value for G_gas(H⁺) of ?6.28 kcal/mol, to calculate pK_a values with cycle 1. The complete basis set gas‐phase methods used to calculate gas‐phase free energies are very accurate, with mean unsigned errors of 0.3 kcal/mol and standard deviations of 0.4 kcal/mol. The CPCM solvation calculations used to calculate condensed‐phase free energies are slightly less accurate than the gas‐phase models, and the best method has a mean unsigned error and standard deviation of 0.4 and 0.5 kcal/mol, respectively. Thermodynamic cycles that include an explicit water in the cycle are not accurate when the free energy of solvation of a water molecule is used, but appear to become accurate when the experimental free energy of vaporization of water is used. This apparent improvement is an artifact of the standard state used in the calculation. Geometry relaxation in solution does not improve the results when using these later cycles. The use of cycle 1 and the complete basis set models combined with the CPCM solvation methods yielded pK_a values accurate to less than half a pK_a unit. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

The infrared spectra and normal coordinate analysis of syn and anti acetaldoxime

The infrared spectra of mixtures of syn and anti acetaldoxime and its deuterated analogues CH₃CHNOD, CD₃CHNOH, CD₃CDNOH and CH₃CDNOH have been recorded. The syn and anti isomers of CH₃CHNOH, CD₃CHNOH, CD₃CDNOH and CH₃CDNOH have been separated by gas chromatography [1]. The infrared spectra of separated isomers in CS₂ solution have been recorded and the assignment of ten in-plane vibrations made. From a normal coordinate analysis the Urey-Bradley force field, the potential energy distribution and additional information about assignments have been obtained.For the anti isomer the simple Urey-Bradley force field gives satisfactory agreement between the calculated and measured frequencies. For the syn isomer it is necessary to take into account the interactions between atoms separated by three bonds.