MO Method Research Articles

Dynamic NMR study of the oxaphosphetane complexation with lithium during the Wittig reaction

AbstractDynamic NMR spectroscopy at very low temperatures (148–182 K) reveal the dynamic behavior of the (2‐tri(3‐furyl)‐3‐methyl‐4‐cyclopropenyl‐oxaphosphetane) generated during a Wittig reaction between tri(3‐furyl)ethylphosphonium iodine and cyclopropylaldehyde. The possibility of formation of different adducts between Li+ ions and oxaphosphetane or betainic intermediates was checked calculating the formation enthalpies using the MNDO, AM1, and PM3 semiempirical MO methods. The observed species are interpreted as oxaphosphetane complexes with lithium ions present in solution. Quantum mechanical calculations confirm the spectroscopic results. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

A Deeper Look into Thiophene Coordination Prior to Oxidative Addition of the C−S Bond to Platinum(0): A Computational Study Using DFT and MO Methods

The reaction of thiophene with a zerovalent platinum bisalkylphosphine fragment yields a highly stable thiaplatinacycle derived from cleavage of the C−S bond. Calculations on the [Pt(dmpe)] model system using Density Functional Theory are consistent with experimental results obtained with [Pt(dippe)] in that the reaction is exothermic overall and furthermore predict that the initial η2-coordination of thiophene through the C═C double bond is energetically more favorable than coordination through the sulfur atom (ΔG = 9.3 kcal/mol). There are three well-defined transition states along the pathway to the oxidative addition product from both of these coordination modes. Two of these lead to a higher energy η2-C,S-coordinated intermediate, while the third one leads to cleavage of the C−S bond from the η2-C,S complex. As the reaction was carried out in a polar solvent (THF), the effect of solvation was taken into account by using the polarizable continuum model. The thermodynamic preference for the initial coo...

Theoretical Analysis of Concerted and Stepwise Mechanisms of Diels−Alder Reactions of Butadiene with Silaethylene and Disilene

The concerted and the stepwise mechanisms of the Diels-Alder reactions of butadiene with silaethylene and disilene were studied by ab initio MO methods. For the reaction of butadiene and silaethylene, an asymmetric concerted process that is almost stepwise and two stepwise processes were located. For the first step of the stepwise process, the C-Si bond formation is more favorable than the C-C bond formation. The activation energy barrier of the concerted transition state is only 0.89 kcal/mol lower than that of the first-step transition state of the C-Si bond formation for the stepwise process by the CASPT2 calculation level. For the reaction of butadiene and disilene, the activation energy barrier of the concerted-type transition state constrained with Cs symmetry is about 9 kcal/mol higher than that of the stepwise transition state by the CASSCF method. The energy barrier of the first step of the stepwise reaction disappears at the CASPT2/6-311++G(d,p) calculation level including the nondynamical correlation energy, although the reaction of the butadiene with disilene occurs through the stepwise-like process.

Nonisothermal crystallization kinetics of linear metallocene polyethylenes

AbstractThe effect of weight‐average molecular weight (Mw) on the nonisothermal crystallization kinetics of linear metallocene polyethylene (m‐PE) was studied with modulated differential scanning calorimetry. Six linear m‐PEs of molecular weights in the range 122–934 kg/mol were prepared by gas‐phase polymerization. The cooling rate (R) was varied in the range 2–20°C/min, and it significantly affected the crystallization behavior. Mw had a weak influence on both the peak crystallization temperature and the crystallization onset temperature. All m‐PEs showed primary and secondary crystallizations. At both low and high R's, the crystallinity showed a significant drop (∼ 30%) when Mw was increased from 122 to 934 kg/mol. At low R's (< 10°C/min), the rate parameters in the modified Avrami method [primary rate constant (kR)] and Mo method [F(T)] of analyses agreed in suggesting that an increased Mw slowed the rate of crystallization. The Mw dependency of kR followed the Arrhenius type (kR = kRoe281/Mw, where kRo is a rate‐dependent constant). However, at higher R's, kR approached a constant value. The order parameters obtained by the different methods of analysis (n and α) were independent of Mw, which suggests that the crystal type remained the same. Hoffman–Lauritzen theory was used for data analysis, and activation energy per segment showed a significant decrease, from 225.0 to 11.8 kJ/mol, when Mw was increased from 152 to 934 kg/mol. Finally, all methods of analysis suggested a significant effect of Mw on slowing the overall crystallization process. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Enumeration of Topology-Distinct Structures and Possible Stable Structures of Protonated Water Clusters, H3O+(H2O)n−1 (n≤5)

Abstract A rooted digraph is regarded as a protonated water (PW) cluster, and a hydrogen bond (H-B) matrix can represent a rooted digraph. All possible topology-distinct patterns corresponding to PW clusters containing up to 8 water molecules were enumerated by using H-B matrices. From close investigation of the structural patterns obtained, we found several restrictions that should be satisfied for a structure of the PW clusters to be stable. The generated H-B matrices of the restrictive rooted digraph were used as the theoretical framework to obtain all the topology-distinct local minima of H3O+(H2O)n−1 (n≤5), at the level of MP2/6-31G∗∗ of ab initio MO method. For PW clusters up to tetramers, the local minimum structures that we generated are same as those in the literatures. For PW pentamers, we found some new local minimum structures, which had not been obtained previously.

Nonisothermal Crystallization Kinetics of Polypropylene/Polyethersulfone Blend

The nonisothermal crystallization kinetics of PP and PP/PES (80/20 wt%) blend was investigated by using differential scanning calorimetry (DSC). It was observed that the crystallization peak temperature (Tp) and the half time (t1/2) of crystallization of PP/PES blend are slightly but consistently lower than those of PP at various cooling rates. The nonisothermal crystallization data were analyzed by using Avrami equation, Ozawa and Mo method. The validity of the different kinetics models to the nonisothermal crystallization process of two samples is discussed. The Mo method can successfully explain the overall nonisothermal crystallization process of PP and PP/PES blend. The activation energy (ΔE) for nonisothermal crystallization of PP and PP/PES blend is determined by using the Kissinger method. The result shows that the ΔE value of PP is slightly higher than that of PP/PES blend.

Difference in the Potential Energy Surfaces from the Fragment MO Method and Conventional Ab Initio SCF-MO Method. A Case of a Surface for Ring Rotation of (−)-Epicatechin Gallate Using the STO-3G Basis Set

Abstract Difference in the potential energy surfaces (ΔE) obtained from the fragment MO (FMO) method and conventional ab initio Hartree–Fock SCF-MO (HFMO) method was investigated using the best fragmentation pattern of (−)-epicatechin gallate. In the lower energy region (<23.06 kcal mol−1 from the minimum), the average ΔE at each grid point was 1.1 kcal mol−1. The FMO method is applicable to quantum MD calculation of large molecular system at around the standard temperature.

Nonisothermal crystallization, melting behavior, and morphology of polypropylene/linear bimodal polyethylene blends

AbstractThe nonisothermal crystallization, melting behavior, and morphology of isotactic polypropylene (PP)/linear bimodal polyethylene (LBPE) blends were studied with differential scanning calorimetry, scanning electron microscopy, and polarized optical microscopy. The results showed that PP and LBPE were miscible to a certain extent, and there was no obvious phase separation in the blends. The modified Avrami analysis, Ozawa equation, and Mo method were used to analyze the nonisothermal crystallization kinetics of the blends. The values of the Avrami exponent indicated that the crystallization nucleation of the blends was homogeneous, the growth of spherulites was three‐dimensional, and the crystallization mechanism of PP was not affected much by LBPE. The crystallization activation energy was estimated by the Kissinger method. The results obtained with the modified Avrami analysis, Mo method, and Kissinger method agreed well. The addition of a minor LBPE phase favored an increase in the overall crystallization rate of PP, showing some dilution effect of LBPE on PP. The PP spherulites decreased obviously with increasing content of LBPE. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Solid-state photocycloaddition of 6,6′-dimethyl-4,4′-[bis(methylenoxy)phenylene]-di-2-pyrones with benzophenone

Photocycloaddition reactions of 6,6′-dimethyl-4,4′-[bis(methylenoxy)phenylene]-di-2-pyrones ( 4a– c) with benzophenone ( 2a) by mixing in the solid state (solid solution) afforded the corresponding oxetane derivatives ( 5a– c; 1:2 adducts) with high site- and regioselectivity across the C5–C6 and C5′–C6′ double bonds in 4 via the triplet excited state of benzophenone. The oxetane formation proceeded more effectively in the solid state than in solution. The reaction mechanism was inferred by MO methods to be initiated by electrostatic interaction between the C6 position of 4a– c and the carbonyl oxygen of 2a in their ground states. The solid-state interaction may be enhanced by the electron density at the carbonyl oxygen of the triplet 2a. The transition state (TS) analysis of the [2+2] cycloaddition reactions also suggested some triplet complexes and high regioselectivity. The hydrogen-bonding interaction between 2a and 4a– c and the triplet reaction mechanism were also explained by the IR analyses and the quenching experiments, respectively.

Ab initiostudy onN,N′,N″‐triaminoguanidine

AbstractElectronic structure calculations and second‐order delocalizations inN,N′,N′′‐triaminoguanidine (TAG) have been studied by employingab initioMO and density functional methods. There are total 10 rotational isomers on the potential energy (PE) surface ofTAG. The effect of three amino groups substitution on guanidine (Gu) has been studied in terms of the primary and the secondary electron delocalizations inTAGby employing Natural Population Analysis (NPA). An increased electron delocalization is observed in protonated triaminoguanidine (TAGP) due to the three strong intramolecular hydrogen bonds and hence accounts for its extra stability. The increase in the electron delocalization upon protonation inTAGcan be compared to that in guanidine. The absolute proton affinity (APA) ofTAGis less than that ofGu. HOMA and NICS studies have been carried out to understand electron delocalization inTAGP. Copyright © 2007 John Wiley & Sons, Ltd.

Crystallization Behavior of Rare‐earth Doped Luminous Pigment/Polyamide 6 Composites

The crystallization behavior of well‐dispersed rare‐earth doped luminous pigment/polyamide 6 (PA6) composites prepared through in situ polymerization was investigated by DSC. The rare‐earth doped luminous pigments could accelerate the forming of γ form crystals and also had a great effect on the crystallinity and crystallization rate of PA6 composites. The Ozawa, Jeziorny, and Mo methods were used to analyze the non‐isothermal crystallization kinetics. It was found that the Ozawa method was unsuitable for non‐isothermal crystallization of PA6 composites. The results of Jeziorny analysis showed that the crystallization rates of PA6 composites increased when the luminous pigment content was larger than 5 wt.%. Mo's analysis also showed that the presence of the pigment shortened the crystallization time and accelerated the crystallization rate. Polarized optical microscopy showed that the spherulites became smaller with increasing of the luminous pigment amount due to the heterogeneous nucleation.

A Comparative Relativistic DFT and Ab Initio Study on the Structure and Thermodynamics of the Oxofluorides of Uranium(IV), (V) and (VI)

All the possible uranium(VI, V, IV) oxides, fluorides and oxofluorides were studied theoretically by using density functional theory (DFT) in the generalised gradient approximation (GGA), and three different relativistic methods (all-electron scalar four component Dyall RESC method (AE), relativistic small-core ECPs, and zeroth order regular approximation ZORA). In order to test different correlation methods, for the two former relativistic methods hybrid DFT, and, for the AE method, MP2 molecular orbital calculations were performed as well. Single-point AE-CCSD(T) energies were calculated on MP2 geometries as well. Energies of the uranium(VI) and (V) oxofluorides dissociation, uranium(VI) fluoride hydrolysis and oxofluoride disproportionation were calculated and compared against the available experimental thermochemical data. AE-CCSD(T) energies were the closest to the experiment. For GGA DFT methods, all the relativistic methods used yield similar results. For thermochemistry, the best quantitative agreement with the experimental and CCSD(T) values for both U=O and U-F bond strengths was obtained with hybrid DFT methods, provided that a reliable basis set was used. Both the GGA DFT and MP2 MO methods show overbinding of these bonds; moreover, this overbinding was found to be not uniform but strongly dependent on the coordination environment of the uranium atom in each case. U=O vibrational frequencies given by hybrid DFT, however, are systematically overestimated, and are better reproduced by GGA DFT; MP2 values usually fall in-between. Reaction enthalpies, U=O frequencies and complex geometries given by the PBE, MPBE, BPBE, BLYP and OLYP GGA functionals are quite similar, with OLYP performing slightly better than the others but still not as good as hybrid DFT. The geometries of the molecules are found to be influenced by the following factors: the inverse transinfluence (ITI) of the oxygen ligand and, for U(V), and U(IV), the Jahn-Teller distortion.

Criteria for pericyclic and pseudopericyclic character of electrocyclization of (Z)-1,2,4,6-heptatetraene and (2Z)-2,4,5-hexatriene-1-imine

The electrocyclic reaction mechanisms of (Z)-1,2,4,6-heptatetraene and (2Z)-2,4,5-hexatriene-1-imine were studied by ab initio MO methods. The activation energy barrier height of the electrocyclic reaction of (Z)-1,2,4,6-heptatetraene is extremely a low energy barrier of 8.58 kcal/mol by a MRMP method. The activation energy barrier height of the electrocyclic ring closure of the trans-type of (2Z)-2,4,5-hexatriene-1-imine is lower by 3.18 kcal/mol than that of (Z)-1,2,4,6-heptatetraene. These low energy barriers come from some orbital interactions relating to allene group. For the reaction of (Z)-1,2,4,6-heptatetraene, the interactions of the vertical and side π orbitals of the allene group with another terminal π orbital are important at the transition state. The interaction of the vertical π orbital of allene group with a lone pair orbital of N atom is dominant at the transition state of the reaction of the trans-type of (2Z)-2,4,5- hexatriene-1-imine. The electrocyclic mechanism of the cis-type of (2Z)-2,4,5-hexatriene-1-imine was also discussed.

Fragmentation Position Dependency of the Total Energy and Atomic Charge Difference between the Fragment MO Method and Conventional Ab Initio SCF-MO Method. A Case of (−)-Epicatechin Gallate with STO-3G Basis Set

Abstract The fragmentation pattern dependence of the total energy and atomic charge difference between the fragment MO method and conventional ab initio Hartree–Fock SCF-MO method (HFMO) was investigated using 44 patterns of (−)-Epicatechin gallate. Both differences were sensitive to the fragmentation pattern. A “best” pattern for which results comparable to HFMO can be achieved, exists.

Two‐acceptor levels in the band gap of boron‐doped diamond semiconductors analyzed by soft x‐ray absorption spectroscopy and DV–Xα calculations

AbstractTo clarify the electronic structure of semiconducting boron‐doped diamond, especially the two‐acceptor levels observed in the soft x‐ray absorption spectra in the C K region, the density of states (DOS) of a B‐doped diamond cluster model, BC146H148, were calculated using discrete variational (DV)‐Xα MO methods. The results were compared to the measured x‐ray spectra. In the calculations, the localized acceptor levels (lowest unoccupied molecular orbitals: (LUMOs)) appeared just above the highest occupied molecular orbitals (HOMOs) of the C atom that was directly bonded to the B atom, and the other C atoms that were bonded to the C atoms next to the B atom. However, a chemical shift of the LUMOs was not observed between the C atoms. On the other hand, a chemical shift of the occupied C 1s orbitals was observed between these C atoms. Therefore, it is concluded that the two‐acceptor levels in the C atoms result from the chemical shift of the inner C 1s orbitals due to the local structure differences between the C atoms in B‐doped diamond. Copyright © 2007 John Wiley & Sons, Ltd.

Thermodynamic characteristics of metal complexation with 3-[4-iodophenylazo]-2,4-pentanedione in an aqueous ethanol solution

Complexation of metals with 3-[4-iodophenylazo]-2,4-pentanedione (L) in an aqueous ethanol solution at temperatures 298 ± 0.5, 318 ± 0.5, and 318 ± 0.5 K has been studied by the potentiometric titration method. The standard thermodynamic functions of the complexation reaction have been found. They change in the following sequence: $$\begin{gathered} \left| {\Delta G^0 } \right|:Fe > Cu > Ni > Co > Mn > UO_2 > Zn > Cd > Mg > Ca, \hfill \\ \left| {\Delta H^0 } \right|:Fe > Cu > UO_2 > Ni > Co > Zn > Cd > Mn > Mg > Ca, \hfill \\ \Delta S^0 :Ca > Mn > Mg > Cd > Co > Ni > Zn > Fe > Cu > UO_2 . \hfill \\ \end{gathered} $$ The effective charges of atoms in the tautomeric forms of L (enol azo, keto azo, hydrazo) have been calculated by the LCAO MO method in the Huckel approximation.

Nonisothermal crystallization kinetics of kenaf fiber/polypropylene composites

AbstractNonisothermal crystallization behavior of model ecocomposites based on polypropylene (PP), maleic anhydride modified polypropylene (PPm), and kenaf fibers were extensively studied. Melting and crystallization behavior of the composites with 20 wt/wt% kenaf fibers were analyzed by differential scanning calorimetry (DSC) in dynamic regime (with heating rate of 10 K/min, and cooling rates of 5, 10, 15, 20, and 40 K/min). It was shown that the kenaf fiber surface acts as a nucleating agent during nonisothermal crystallization of both PP matrices, shifting the onset and peak crystallization temperatures toward higher values. Crystallization behavior was analyzed by Avrami, Jeziorny, Ozawa, Mo, and Kissinger methods. The results confirmed the applicability of the used models, with exception of the Ozawa approach that was rather inapplicable for these composites. POLYM. ENG. SCI., 47:745–749, 2007. © 2007 Society of Plastics Engineers.

New possible ionic structures in Wittig reaction: Analysis of stability and rotation barriers by semiempirical PM3 method

AbstractFor the Wittig reaction between butylidentriphenylphosphorane and benzaldehyde studied previously using NMR spectroscopy, three ionic intermediates have been proposed in order to explain the “stereochemical drift” (different ratio in oxaphosphetane stereoisomers compared with the ratio of the final Z/E olefins). In this paper, the PM3 semiempirical MO method was applied on the proposed ionic intermediates. Two of the ionic intermediates could lead to a shift of the reaction towards the final products, olefins and phosphine oxide, while the third intermediate could be implicated in a retro‐Wittig reaction. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

The Predictive Accuracies of Spectroscopic Parameters in Biological Macromolecules by Fragment MO Method

生体高分子などの巨大系を第一原理計算の精度で高速に計算する近似法の1つに Fragment MO (FMO) 法がある。FMO 法では、分子をフラグメントに切り分けて計算することで、従来の第一原理計算では計算機資源や時間の問題から計算自体が困難な巨大系の全エネルギー計算が可能である。近年の先行研究により、この近似法がエネルギーだけでなく分極率などの分子物性においても十分な精度で計算可能であることが証明されつつある。生体高分子における諸物性の物理化学的測定が可能になってきた現在において、シミュレーションによるその分光学的解析や予測がより重要な意味を持つと考えられる。本研究では、系のエネルギーに加え、IR、ラマンという分光学的基本スペクトルを決定する双極子、分極率の各物性とその空間微分について FMO 法による計算を行い、これらの物性においても FMO 法が有効な近似法であることを紹介する。

Theoretical studies of d– d magnetic interaction in organic superconductors

In the κ-(BETS) 2FeCl 4 crystal as one of organic magnetic-metal crystals, localized spins of d-orbitals in FeCl 4 - anions is expected to play an important role to magnetic properties. To investigate all the magnetic interactions between neighboring two FeCl 4 - anions, various spatial conformation of dimers were assumed and studied. For variety of configurations of FeCl 4 - anion dimers, the values of effective exchange integral ( J) in the Heisenberg spin model were calculated by using ab initio MO methods. It was found that very weak ferromagnetic interaction existed in a certain conformation, though antiferromagnetic interaction was reproduced in most of all conformations.