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Vibrational Analysis (vibrational + analysis)
Selected Abstracts,Green earths': vibrational and elemental characterization of glauconites, celadonites and historical pigmentsJOURNAL OF RAMAN SPECTROSCOPY, Issue 8 2008Francesca Ospitali Abstract ,Green earths' are employed since antiquity as pigments in the creation of artworks. The minerals responsible for the colour belong to four groups: (1) the clayey micas celadonite and glauconite, undoubtedly the most common; (2) smectites; (3) chlorites; (4) serpentines. Whereas there have been several studies on clayey materials, mineralogical analyses in the field of cultural heritage are mainly limited to the identification of the green earth without specific characterization of the mineralogical species. This work shows a preliminary characterization by the multi-techniques approach of some raw minerals (glauconite, celadonite and ferroceladonite). Vibrational analyses have been correlated with elemental analyses, thanks to the hyphenated instrumentation of scanning electron microscopy with EDS and Raman structural and chemical analyser (SEM-EDS-SCA) probes, which permitted collection of EDS and Raman spectra on the same microscopic area. Micro-Raman and Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopies were able to distinguish between celadonite and glauconite. The use of different lasers revealed resonance effects in the Raman spectra. In addition to pure minerals, archaeological samples and commercial green earths were also analysed, thereby enabling a more precise classification of the green pigments in heterogeneous samples such as wall paintings. Some commercially available green earths were found to contain organic dyes. Copyright © 2008 John Wiley & Sons, Ltd. [source] Density functional study on the structural conformations and intramolecular charge transfer from the vibrational spectra of the anticancer drug combretastatin-A2JOURNAL OF RAMAN SPECTROSCOPY, Issue 4 2009L. Padmaja Abstract Combretastatin-A2 (CA2), a potential anticancer drug in advanced preclinical development, is extracted from the medicinal plant Combretum caffrum. The NIR-FT Raman and FT-IR spectral studies of the molecule were carried out and ab initio calculations performed at the B3LYP/6-31G(d) level to derive the equilibrium geometry as well as the vibrational wavenumbers and intensities of the spectral bands. The vibrational analysis showed that the molecule has a similar geometry as that of cis-stilbene, and has undergone steric repulsion resulting in twisting of the phenyl ring with respect to the ethylenic plane. Vibrational analysis was used to investigate the lowering of the stretching modes, and enhancement of infrared band intensities of the C,H stretching modes of Me2 may be attributed to the electronic effects caused by back-donation and induction from the oxygen atom. Analysis of phenyl ring modes shows that the CA2 stretching mode 8 and the aromatic C,H in-plane bending mode are equally active as strong bands in both IR and Raman spectra, which can be interpreted as the evidence of intramolecular charge transfer (ICT) between the OH and OCH3groups via conjugated ring path and is responsible for bioactivity of the molecule. Copyright © 2008 John Wiley & Sons, Ltd. [source] Vibrational analysis of Ni(II)- and Cu(II)-octamethylchlorin by polarized resonance Raman and Fourier transform infrared spectroscopyJOURNAL OF RAMAN SPECTROSCOPY, Issue 6-7 2001Robert J. Lipski We measured the polarized resonance Raman spectra of Cu(II)-2,2,7,8,12,13,17,18-octamethylchlorin in CS2 at various excitation wavenumbers in a spectral region covering the Qy, Qx and Bx optical absorption bands. Additionally, we measured the FTIR-Raman spectrum of the highly overcrowded spectral region between 1300 and 1450 cm,1. The spectral decomposition was carried out by a self-consistent global fit to all spectra obtained. The thus identified Raman and IR lines were assigned by comparison with the resonance Raman spectra of Cu(II)-octaethylporphyrin, by utilizing their depolarization ratio dispersions and by a normal mode analysis. The latter was based on a modified transferable molecular mechanics force field of Ni(II)-octaethylporphyrin [E. Unger, M. Beck, R.J. Lipski, W. Dreybrodt, C.J. Medforth, K.M. Smith and R. Schweitzer-Stenner, J. Phys. Chem. B103, 10229 (1999)]. A comparison of normal mode patterns obtained for Cu(II)-octamethylchlorin and Cu(II)-octaethylporphyrin revealed that some modes are significantly distorted by the reduction of the pyrrole ring, in accordance with results which Boldt et al. reported earlier for Ni(II)-octaethylchlorin [N.J. Boldt, F.J. Donohoe, R.R. Birge and D.F. Bocian, J. Am. Chem. Soc.109, 2284 (1987)]. In contrast to conclusions drawn from this study, however, the results of our vibrational analysis and several further lines of evidence suggest that the normal modes of corresponding chlorines and porphyrins are still comparable, because they display contributions from the same local coordinates. Thus, the classical normal mode classification developed for metalloporphyrins is also applicable to metallochlorins. Finally, we performed a preliminary analysis of the absorption spectrum and the resonance excitation profiles and depolarization ratio dispersions of some Raman lines. The results show that the electronic properties of Cu(II)-octamethylchlorin can still be described in terms of Gouterman's four orbital model [M. Gouterman, J. Chem. Phys.30, 1139 (1959)]. In regions of the Q bands, Raman scattering of A1 modes is determined by interferences between Franck, Condon coupling and interstate Herzberg, Teller coupling between Qx(Qy) and Bx(By) states. The B2 modes are resonance enhanced by Herzberg, Teller coupling between Qx and Qy and between Qx(Qy) and By(Bx). Franck, Condon coupling of A1 modes with large contributions from C,Cm stretching vibrations is comparatively strong for Qx. This is interpreted as reflecting the expansion of the chlorin macrocycle by an electronic transition into this excited state. Copyright © 2001 John Wiley & Sons, Ltd. [source] Car,Parrinello Molecular Dynamics Study of the Blue-Shifted F3CH,,,FCD3 System in Liquid N2CHEMPHYSCHEM, Issue 6 2006Pawel Rodziewicz Dr. Abstract Fluoroform, as confirmed by both experimental and theoretical studies, can participate in improper H-bond formation, which is characterized by a noticeable increase in the fundamental stretching frequency ,(CH) (so-called blue frequency shift), an irregular change of its integral intensity, and a CH bond contraction. A Car,Parrinello molecular dynamics simulation was performed for a complex formed by fluoroform (F3CH) and deuterated methyl fluoride (FCD3) in liquid nitrogen. Vibrational analysis based on the Fourier transform of the dipole moment autocorrelation function reproduces the blue shift of the fundamental stretching frequency ,(CH) and the decrease in the integral intensity. The dynamic contraction of the CH bond is also predicted. The stoichiometry of the solvated, blue-shifted complexes and their residence times are examined. [source] Infrared spectra of water molecule encapsulated inside fullerene studied by instantaneous vibrational analysis,INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 10 2009Kiyoshi Yagi Abstract Instantaneous vibrational analysis (IVA) is proposed for computing the infrared spectrum of dynamically fluctuating system, and applied to a water molecule encapsulated into fullerene (H2O@C60). A molecular dynamics simulation is first carried out to generate an ensemble of configurations averaging the rotational and translational motion of H2O inside fullerene. At each configuration, instantaneous vibrational frequencies of the water molecule are computed by the vibrational configuration interaction method, which are then employed to construct the line-shape of the spectrum. The vibrational spectrum in the OH stretching region is computed at a temperature of 10 and 100 K based on a direct potential energy surface incorporating the electronic structure theory. It is found that the vibrational frequency of the symmetric stretching mode is blue-shifted compared to that of isolated water, whereas that of the asymmetric stretching mode exhibits no shift in average. The relation between IVA and instantaneous normal mode analysis is presented, and their performances are compared. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] Optical spectra and covalent chemistry of fulleropyrrolidinesINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 14 2007B. S. Razbirin Abstract Low-temperature vibronic spectra of two fulleropyrrolidines (1-methyl-3,4-FP and 1-methyl-2(4-pyridine)-3,4-FP) embedded in crystalline toluene matrix have been studied. Two-component composition of the spectra has been established and charge-transfer-excitation origin of the structureless component has been suggested. Fine-structured Shpol'skii spectra were observed for 1-methyl-3,4-FP, which made possible to perform the vibrational analysis of its vibronic spectra. General similarities of the absorption spectra of fulleropyrrolidines and C60 molecules along with significant difference in their details have been discussed. A detailed interpretation of the C60 spectra serves as a basis for analyzing the spectra of the derivatives. Quantum-chemical study is based on the effectively-unpaired-electron concept for the fullerene molecule. Computations have been performed for the singlet states of the molecules in unrestricted Hartree,Fock approximation implemented in AM1 semiempirical quantum chemical codes of the CLUSTER-Z1 software. The population of the HOMO and LUMO of the molecules under study alongside with the lowering of the molecules symmetry have been proposed to explain the spectral features observed. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Theoretical studies of mechanisms of cycloaddition reaction between difluoromethylene carbene and acetoneINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 2 2007Xiu Hui Lu Abstract Mechanisms of the cycloaddition reaction between singlet difluoromethylene carbene and acetone have been investigated with the second-order Mřller,Plesset (MP2)/6-31G* method, including geometry optimization and vibrational analysis. Energies for the involved stationary points on the potential energy surface (PES) are corrected by zero-point energy (ZPE) and CCSD(T)/6-31G* single-point calculations. From the PES obtained with the CCSD(T)//MP2/6-31G* method for the cycloaddition reaction between singlet difluoromethylene carbene and acetone, it can be predicted that path B of reactions 2 and 3 should be two competitive leading channels of the cycloaddition reaction between difluoromethylene carbene and acetone. The former consists of two steps: (i) the two reactants first form a four-membered ring intermediate, INT2, which is a barrier-free exothermic reaction of 97.8 kJ/mol; (ii) the intermediate INT2 isomerizes to a four-membered product P2b via a transition state TS2b with an energy barrier of 24.9 kJ/mol, which results from the methyl group transfer. The latter proceeds in three steps: (i) the two reactants first form an intermediate, INT1c, through a barrier-free exothermic reaction of 199.4 kJ/mol; (ii) the intermediate INT1c further reacts with acetone to form a polycyclic intermediate, INT3, which is also a barrier-free exothermic reaction of 27.4 kJ/mol; and (iii) INT3 isomerizes to a polycyclic product P3 via a transition state TS3 with an energy barrier of 25.8 kJ/mol. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Explicitly correlated SCF study of anharmonic vibrations in (H2O)2INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4-5 2002Donald D. Shillady Abstract Modeling solvation in high-pressure liquid chromatography (HPLC) requires calculation of anharmonic vibrational frequencies of solvent clusters for a statistical partition function. An efficient computational method that includes electron correlation is highly desirable for large clusters. A modified version of the "soft Coulomb hole" method of Chakravorty and Clementi has recently been implemented in a Gaussian-lobe-orbital (GLO) program (PCLOBE) to include explicit electron,electron correlation in molecules. The soft Coulomb hole is based on a modified form of Coulomb's law: An algorithm has been developed to obtain the parameter "w" from a polynomial in the effective scaling of each primitive Gaussian orbital relative to the best single Gaussian of the H1s orbital. This method yields over 90% of the correlation energy for molecules of low symmetry for which the original formula of Chakravorty and Clementi does not apply. In this work, all the vibrations of the water dimer are treated anharmonically. A quartic perturbation of the harmonic vibrational modes is constrained to be equal to the exact Morse potential eigenvalue based on a three-point fit. This work evaluates the usefulness of fitting a Morse potential to a hydrogen bond vibrational mode and finds it to be slightly better than using MP2 vibrational analysis for this important dimer. A three-point estimate of the depth, De, of a Morse potential leads to a correction formula for anharmonicity in terms of the perturbed harmonic frequency: When scaled by 0.9141, the harmonic Morse method leads to essentially the same results as scaling the BPW91 local density method by 0.9827. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002 [source] Theoretical characterizations of HAsXH (X = N, P, As, Sb, and Bi) isomers in the singlet and triplet statesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 15 2008Chin-Hung Lai Abstract The lowest singlet and triplet potential energy surfaces for all group 15 HAsXH (X = N, P, As, Sb, and Bi) systems have been explored through ab initio calculations. The geometries of the various isomers were determined at the QCISD/LANL2DZdp level and confirmed to be minima by vibrational analysis. In the case of nitrogen, the global minimum is found to be a triplet H2NAs structure. For the phosphorus case, singlet trans -HAsPH is found to be global minima surrounded by large activation barriers, so that it should be observable. For arsenic, theoretical investigations demonstrate that the stability of HAsAsH isomers decreases in the order singlet trans -HAsAsH > triplet H2AsAs > singlet cis -HAsAsH > triplet HAsAsH > singlet H2AsAs. For antimony and bismuth, the theoretical findings suggest that the stability of HAsXH (X = Sb and Bi) systems decreases in the order triplet H2AsX , singlet trans -HAsXH > singlet cis -HAsXH > triplet HAsXH > triplet H2XAs > singlet H2AsX > singlet H2XAs. Our model calculations indicate that the relativistic effect on heavier group 15 elements should play an important role in determining the geometries as well as the stability of HAsXH molecules. The results obtained are in good agreement with the available experimental data and allow a number of predictions to be made. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2008 [source] Dynamic Charge Equilibration-Morse stretch force field: Application to energetics of pure silica zeolitesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 16 2002Jan Sefcik Abstract We present the Dynamic Charge Equilibration (DQEq) method for a self-consistent treatment of charge transfer in force field modeling, where atomic charges are designed to reproduce electrostatic potentials calculated quantum mechanically. Force fields coupled with DQEq allow charges to readjust as geometry changes in classical simulations, using appropriate algorithms for periodic boundary conditions. The full electrostatic energy functional is used to derive the corresponding forces and the second derivatives (hessian) for vibrational calculations. Using DQEq electrostatics, we develop a simple nonbond force field for simulation of silica molecular sieves, where nonelectrostatic interactions are described by two-body Morse stretch terms. Energy minimization calculations with the new force field yield accurate unit cell geometries for siliceous zeolites. Relative enthalpies with respect to quartz and third-law entropies calculated from harmonic vibrational analysis agree very well with available calorimetric data: calculated SiO2 enthalpies relative to ,-quartz are within 2 kJ/mol and entropies at 298 K are within 3 J/mol K of the respective experimental values. Contributions from the zero point energy and vibrational degrees of freedom were found to be only about 1 kJ/mol for the free energy of mutual transformations between microporous silica polymorphs. The approach presented here can be applied to interfaces and other oxides as well and it is suitable for development of force fields for accurate modeling of geometry and energetics of microporous and mesoporous materials, while providing a realistic description of electrostatic fields near surfaces and inside pores of adsorbents and catalysts. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 1507,1514, 2002 [source] Synthesis and vibrational analysis of N-(2,-Furyl)-ImidazoleJOURNAL OF RAMAN SPECTROSCOPY, Issue 8 2009A. E. Ledesma Abstract The N-(2,-furyl)-imidazole (1) has been prepared and characterized using infrared, Raman and multidimensional nuclear magnetic resonance spectroscopies. Theoretical calculations have been carried out by employing the Density Functional Theory (DFT) method, in order to optimize the geometry of their two conformers in the gas phase and to support the assignments of the vibrational bands of 1 to their normal modes. For a complete assignment of the compound, DFT calculations were combined with Scaled Quamtum Mecanic Force Field (SQMFF) methodology in order to fit the theoretical wavenumber values to the experimental one. Furthermore, Natural Bond Orbital (NBO) and topological properties by Atoms In Molecules (AIM) calculations were performed to analyze the nature and magnitude of the intramolecular interactions. The result reveals that two conformers are expected in liquid phase. Copyright © 2009 John Wiley & Sons, Ltd. [source] Structural conformations and electronic interactions of the natural product, oroxylin: a vibrational spectroscopic studyJOURNAL OF RAMAN SPECTROSCOPY, Issue 12 2008Jose P. Abraham Abstract The oroxylin, 5,7-dihydroxy 6-methoxy flavone is a potent natural product extracted from ,Vitex peduncularis'. Density functional theory (DFT) at B3LYP/6-311G(d,p) level has been used to compute energies of different conformers of oroxylin to find out their stability, the optimized geometry of the most stable conformer and its vibrational spectrum. The conformer ORLN-1 with torsion angles 0, 180, 180 and 0 degrees, respectively, for H13O12C6C5, H14O10C4C5, H13O12C6C5 and H14O10C4C5 is found to be most stable. The optimized geometry reveals that the dihedral angle , between phenyl ring B and the chrome part of the molecule in , 19.21° is due to the repulsive force due to steric interaction between the ortho-hydrogen atom H29 of the B ring and H18 of the ring C (H29·H18 = 2.198 Ĺ). A vibrational analysis based on the near-infrared Fourier transform(NIR-FT) Raman, Fourier transform-infrared (FT-IR) and the computed spectrum reveals that the methoxy group is influenced by the oxygen lone pair-aryl pz orbital by back donation. Hence the stretching and bending vibrational modes of the methoxy group possess the lowest wavenumber from the normal values of methyl group. The carbonyl stretching vibrations have been lowered due to conjugation and hydrogen bonding in the molecules. The intramolecular H-bonding and nonbonded intramolecular interactions shift the band position of O10H14 and O12H13 stretching modes, which is justified by DFT results. Copyright © 2008 John Wiley & Sons, Ltd. [source] Vibrational analysis of Ni(II)- and Cu(II)-octamethylchlorin by polarized resonance Raman and Fourier transform infrared spectroscopyJOURNAL OF RAMAN SPECTROSCOPY, Issue 6-7 2001Robert J. Lipski We measured the polarized resonance Raman spectra of Cu(II)-2,2,7,8,12,13,17,18-octamethylchlorin in CS2 at various excitation wavenumbers in a spectral region covering the Qy, Qx and Bx optical absorption bands. Additionally, we measured the FTIR-Raman spectrum of the highly overcrowded spectral region between 1300 and 1450 cm,1. The spectral decomposition was carried out by a self-consistent global fit to all spectra obtained. The thus identified Raman and IR lines were assigned by comparison with the resonance Raman spectra of Cu(II)-octaethylporphyrin, by utilizing their depolarization ratio dispersions and by a normal mode analysis. The latter was based on a modified transferable molecular mechanics force field of Ni(II)-octaethylporphyrin [E. Unger, M. Beck, R.J. Lipski, W. Dreybrodt, C.J. Medforth, K.M. Smith and R. Schweitzer-Stenner, J. Phys. Chem. B103, 10229 (1999)]. A comparison of normal mode patterns obtained for Cu(II)-octamethylchlorin and Cu(II)-octaethylporphyrin revealed that some modes are significantly distorted by the reduction of the pyrrole ring, in accordance with results which Boldt et al. reported earlier for Ni(II)-octaethylchlorin [N.J. Boldt, F.J. Donohoe, R.R. Birge and D.F. Bocian, J. Am. Chem. Soc.109, 2284 (1987)]. In contrast to conclusions drawn from this study, however, the results of our vibrational analysis and several further lines of evidence suggest that the normal modes of corresponding chlorines and porphyrins are still comparable, because they display contributions from the same local coordinates. Thus, the classical normal mode classification developed for metalloporphyrins is also applicable to metallochlorins. Finally, we performed a preliminary analysis of the absorption spectrum and the resonance excitation profiles and depolarization ratio dispersions of some Raman lines. The results show that the electronic properties of Cu(II)-octamethylchlorin can still be described in terms of Gouterman's four orbital model [M. Gouterman, J. Chem. Phys.30, 1139 (1959)]. In regions of the Q bands, Raman scattering of A1 modes is determined by interferences between Franck, Condon coupling and interstate Herzberg, Teller coupling between Qx(Qy) and Bx(By) states. The B2 modes are resonance enhanced by Herzberg, Teller coupling between Qx and Qy and between Qx(Qy) and By(Bx). Franck, Condon coupling of A1 modes with large contributions from C,Cm stretching vibrations is comparatively strong for Qx. This is interpreted as reflecting the expansion of the chlorin macrocycle by an electronic transition into this excited state. Copyright © 2001 John Wiley & Sons, Ltd. [source] Adsorption of 6-mercaptopurine and 6-mercaptopurine-ribosideon silver colloid: A pH-dependent surface-enhanced Raman spectroscopy and density functional theory study.BIOPOLYMERS, Issue 6 2005Abstract Surface-enhanced Raman spectroscopy (SERS) has been applied to characterize the interaction of 6-mercaptopurine-ribose (6MPR), an active drug used in chemotherapy of acute lymphoblastic leukemia, with a model biological substrate at therapeutic concentrations and as function of the pH value. Therefore, a detailed vibrational analysis of crystalline and solvated (6MPR) based on Density Functional Theory (DFT) calculations of the thion and thiol tautomers has been performed. 6MPR adopts the thion tautomeric form in the polycrystalline state. The SERS spectra of 6MPR and 6-mercaptopurine (6MP) recorded on silver colloid provided evidence that the ribose derivative shows different adsorption behavior compared with the free base. Under acidic conditions, the adsorption of 6MPR on the metal surface via the N7 and possibly S atoms was proposed to have a perpendicular orientation, while 6MP is probably adsorbed through the N9 and N3 atoms. Under basic conditions both molecules are adsorbed through the N1 and possibly S atoms, but 6MP has a more tilted orientation on the silver colloidal surface while 6MPR adopts a perpendicular orientation. The reorientation of the 6MPR molecule on the surface starts at pH 8 while in the case of 6MP the reorientation starts around pH 6. Under basic conditions, the presence of the anionic molecular species for both molecules is suggested. The deprotonation of 6MP is completed at pH 8 while the deprotonation of the riboside is finished at pH 10. For low drug concentrations under neutral conditions and for pH values 8 and 9, 6MPR interacts with the substrate through both N7 and N1 atoms, possibly forming two differently adsorbed species, while for 6MP only one species adsorbed via N1 was evidenced. © 2005 Wiley Periodicals, Inc. Biopolymers 78: 298,310, 2005 This article was originally published online as an accepted preprint. The "Published Online" date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com [source] Ab initio vibrational calculations on ara-T molecule: Application to analysis of IR and Raman spectraBIOPOLYMERS, Issue 4 2001Belén Hernández Abstract The FTIR and FT-Raman spectra are reported for the arabinonucleoside ara-T (1-,- D -arabinofuranosylthymine), which shows antiviral activity. The accurate knowledge of the vibrational modes is a prerequisite for the elucidation of drug,nucleotide and drug,enzyme interactions. The FTIR and FT-Raman spectra of ara-T were recorded from 4000 to 30 cm,1. A tetradeuterated derivative (deuteration at N3, and hydroxyl groups O,2, O,3, and O,5) was synthesized and the observed isotopic shifts in its spectra were used for the vibrational analysis of ara-T. The theoretical frequencies and the potential energy distribution (PED) of the vibrational modes of ara-T were calculated using the ab initio Hartree,Fock/3-21G method. An assignment of the vibrational spectra of ara-T is proposed considering the scaled PED and the observed band shifts under deuteration. The scaled ab initio frequencies were in reasonable agreement with the experimental data. © 2001 John Wiley & Sons, Inc. Biopolymers (Biospectroscopy) 62: 193,207, 2001 [source] Theoretical Study on the Vibrational Spectra and Thermodynamic Properties for Nitro Derivatives of Benzene and AnilinesCHINESE JOURNAL OF CHEMISTRY, Issue 8 2008Gui-Xiang WANG Abstract Nitro derivatives of benzene and anilines were optimized to obtain their molecular geometries and electronic structures at a DFT-B3LYP/6-31G, level. Their IR spectra were obtained and assigned by vibrational analysis. Comparing the calculated IR spectra with these of experiments known, all the IR data obtained in this paper were considered to be reliable. Based on the frequencies scaled by 0.96 and the principle of statistic thermodynamics, the thermodynamic properties were evaluated, which were linearly related with the number of nitro and amino groups as well as the temperature, obviously showing good group additivity. [source] Theoretical Study on the Mechanism of the Cycloaddition Reaction between Methylidenesilene and EthyleneCHINESE JOURNAL OF CHEMISTRY, Issue 3 2006Xiu-Hui Lu Abstract The mechanism of a cycloaddition reaction between singlet methylidenesilene and ethylene has been investigated with MP2/6-31G, and B3LYP/6-31G, methods, including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. Energies of the involved conformers were calculated by CCSD(T)//MP2/6-31G, and CCSD(T)//B3LYP/6-31G, methods, respectively. The results show that the dominant reaction pathway of the cycloaddition reaction is that a complex intermediate is firstly formed between the two reactants through a barrier-free exothermic reaction of 13.3 kJ/mol, and the complex is then isomerized to a four-membered ring product P2.1 via a transition state TS2.1 with a barrier of 32.0 kJ/mol. [source] | ||||||||||