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MO Calculations (mo + calculation)
Kinds of MO Calculations Selected AbstractsThe Conformation of Alkyl Benzyl Alcohols Studied by ab initio MO Calculations , A Comparison with IR and NMR Spectroscopic DataEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 11 2004Osamu Takahashi Abstract Ab initio MO calculations were carried out for the conformations of a series of alkyl-substituted benzyl alcohols C6H5CH2CHOH,R (R = CH3, C2H5, iPr, tBu) at the MP2/6-311G(d,p)//MP2/6-31G(d) level. It was found that the conformation where the OH group is gauche to the phenyl group is the most stable. The geometry where both the OH and R groups are close to phenyl is the second most stable. This finding has been interpreted on the grounds of the attractive OH/, and CH/, hydrogen bonds and a repulsive van der Waals interaction between vicinal CH groups. NMR nuclear Overhauser effects, spin-coupling data, and IR spectroscopic data are consistent with the conclusion given by the MO calculations. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] The Energetics of Large Deformations of a Single Polyimide Molecular Chain: DFT and MO CalculationsMACROMOLECULAR THEORY AND SIMULATIONS, Issue 9 2008Akinori Fujinami Abstract The large-deformation energetics of a single molecular chain of the rod-like polyimide PMDA-PDA was investigated using DFT, ab initio MO and semi-empirical MO methods. The force/displacement curves were calculated from tensile testing simulations along the axis of the molecular chain, allowing a discussion of the distribution and change of local strain of the molecular chain. The deformation behavior of a single PMDA-PDA molecular chain under finite deformations as functions of bending angle and dihedral angle between PMDA and PDA groups are compared. It is found that the semi-empirical MO calculations provide sufficient accuracy to express the energetics of large deformations except for compressive deformation. [source] Systematic Studies on Photoluminescence of Oligo(arylene-ethynylene)s: Tunability of Excited States and Derivatization as Luminescent Labeling Probes for ProteinsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 14 2006Yong-Gang Zhi Abstract Functionalized oligo(phenylene-ethynylene)s (OPEs) with different conjugation lengths, p -X(C6H4C,C)nSiMe3 (n = 1,4; X = NH2, NMe2, H) were synthesized by Sonogashira coupling of (phenylene-ethynylene)s and 1-iodo-4-(trimethylsilylethynyl)benzene, followed by desilylation of the p -substituted (trimethylsilylethynyl)benzenes with potassium hydroxide. The photoluminescent properties for the OPE series with different chain lengths and their solvatochromic responses were examined. The absorption maxima were red-shifted with increasing numbers of ,(C6H4C,C), units (n), and a linear plot of the absorption energy maxima vs. 1/n was obtained for each series. The emission spectra in dichloromethane showed a broad and structureless band, the energies of which (in wavenumbers) also fit linearly with 1/n. Both the absorption and emission wavelength maxima of the NH2 - and NMe2 -substituted OPEs exhibited significant solvent dependence, whereas the parent OPEs (X = H) showed only minor shifts of the ,max values in different solvents. Substituent effects upon the photoluminescent characteristics of the OPEs and the tunability of the excited states were examined with the p -X(C6H4C,C)nSiMe3 (n = 2, 3; X = NH2, NMe2, H, SMe, OMe, OH, and F) series. The H- and F-substituted counterparts exhibited high-energy vibronically structured emissions attributed to the 3(,,*) excited states of the (arylene-ethynylene) backbone. For compounds bearing NH2 and NMe2 groups, a broad red-shifted emission with a remarkable Stokes shift from the respective absorption maximum was observed, which can be assigned to an n , ,* transition. The n , ,* assignment was supported by MO calculations on the model compounds p -X(C6H4C,C)2SiH3 (X = NH2, H). Functionalization of the oligo(arylene-ethynylene)s with the N -hydroxysuccinimidyl (NHS) moiety enabled covalent attachment of the fluorophore to HSA protein molecules. A series of fluorescent labels, namely p -X(C6H4C,C)nC6H4NHS, (n = 1, X = NH2, NMe2, SMe, OMe, OH, F; n = 2, X = NH2, NMe2) and p -Me2NC6H4C,C(C4H2S)C,CC6H4NHS were synthesized, and their conjugates with HSA (human serum albumin) were characterized by MALDI-TOF mass spectrometry, UV/Vis absorption spectroscopy, and gel electrophoresis. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] The Conformation of Alkyl Benzyl Alcohols Studied by ab initio MO Calculations , A Comparison with IR and NMR Spectroscopic DataEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 11 2004Osamu Takahashi Abstract Ab initio MO calculations were carried out for the conformations of a series of alkyl-substituted benzyl alcohols C6H5CH2CHOH,R (R = CH3, C2H5, iPr, tBu) at the MP2/6-311G(d,p)//MP2/6-31G(d) level. It was found that the conformation where the OH group is gauche to the phenyl group is the most stable. The geometry where both the OH and R groups are close to phenyl is the second most stable. This finding has been interpreted on the grounds of the attractive OH/, and CH/, hydrogen bonds and a repulsive van der Waals interaction between vicinal CH groups. NMR nuclear Overhauser effects, spin-coupling data, and IR spectroscopic data are consistent with the conclusion given by the MO calculations. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Chemical graph theory and n -center electron delocalization indices: A study on polycyclic aromatic hydrocarbonsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 10 2007Marcos Mandado Abstract Relations between aromaticity indices derived from chemical graph theory and those based on 6-center electron delocalization are investigated for a series of polybenzenoid hydrocarbons. Aromatic stabilization obtained by means of the effective scaled electron delocalization is highly correlated to the resonance energy, RE, obtained both from SCF MO calculations and conjugated ring circuits model. Local aromaticity of benzene rings is discussed using two different criteria, in one of them aromaticity is just given by the cyclic , -electron conjugation of the ring, whereas terms involving more than one ring are also considered in the other one. Indices derived from chemical graph theory and those obtained from the 6-center electron delocalization give rise to the same local aromaticity. Moreover, 6-center electron delocalization provides more quantitative information. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007 [source] Effect of Lewis base coordination on boryl radical reactivity: investigation using laser flash photolysis and kinetic ESRJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 10 2009Jacques Lalevée Abstract The effect of Lewis base coordination on boryl radical reactivity (L,BH where L is triethylamine, 2-picoline, 4-dimethylaminopyridine, quinoline and diphenyl phosphine) is examined. Direct detection of the different boryl radicals, their transient absorption spectra, their interaction with double or triple bonds, oxygen, oxidizing agent, alkyl halides and their hydrogen lability have been studied using laser flash photolysis (LFP), kinetic ESR (KESR), ESR spin trapping and MO calculations. For example, a strong decrease of both the bond dissociation energy (BDE)(BH) (33.8,kcal,mol,1) and the addition rate constant to MA (1300 105 to,>105 M,1,s,1) was noted when going from the triethylamine borane to the quinoline borane complex, in line with the spin delocalization (1.04 vs. 0.19). These structures are also proposed as new highly efficient co-initiators for the acrylate photopolymerization. Copyright © 2009 John Wiley & Sons, Ltd. [source] Computational study of base-induced skeletal conversion via a spirocyclic intermediate in dibenzodithiocinium derivatives by ab initio MO calculationsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 5 2003Keiji Okada Abstract Reaction of 6-methyl-12-oxo-5H,7H -dibenzo[b,g][1,5]dithiocinium salt (1-SO) with methanolic KOH afforded a mixture of dibenzothiepin derivatives 2-SO. In order to clarify the intermediates of the rearrangement, ab initio MO calculations with the HF/6,31G* basis set were performed on the reaction intermediates, the transition states and related compounds. The rearrangement was explained in terms of the usual [2,3]-sigmatropic shift via a spirocyclic intermediate, followed by a 1,3-shift of the sulfonyl group. However, a different type of rearrangement was observed in 6-methyl-5H,7H -dibenzo[b,g][1,5]dithiocinium salt (1-S), giving an unexpected dibenzothiepin derivative 3-S along with a small amount of a ring-opening product 8 under the same reaction conditions. The formation of 3-S and 8 was understandable by the assumption of a cationic intermediate resulting from heterolytic cleavage at the benzyl position. Copyright © 2003 John Wiley & Sons, Ltd. [source] Study of carvedilol by combined Raman spectroscopy and ab initio MO calculationsJOURNAL OF RAMAN SPECTROSCOPY, Issue 10 2002M. P. M. Marques The novel cardioprotective drug carvedilol was studied by both Raman spectroscopy and ab initio molecular orbital methods (using the density functional theory approach). The spectra, acquired both for the solid samples and DMSO solutions as a function of pH, were assigned in view of the calculated wavenumbers and intensities, and also based on the experimental data obtained for individual compounds which comprise the molecule, namely carbazole and 1,2-dimethoxybenzene. The pH dependence of the Raman pattern of carvedilol was studied, and the pKa value of its secondary amine group was determined (pKa = 8.25) through pH titration experiments. This kind of information is of great significance for the understanding of the biochemical role of carvedilol, which is strongly determined by the acid,base behaviour of the molecule. Copyright © 2002 John Wiley & Sons, Ltd. [source] The Energetics of Large Deformations of a Single Polyimide Molecular Chain: DFT and MO CalculationsMACROMOLECULAR THEORY AND SIMULATIONS, Issue 9 2008Akinori Fujinami Abstract The large-deformation energetics of a single molecular chain of the rod-like polyimide PMDA-PDA was investigated using DFT, ab initio MO and semi-empirical MO methods. The force/displacement curves were calculated from tensile testing simulations along the axis of the molecular chain, allowing a discussion of the distribution and change of local strain of the molecular chain. The deformation behavior of a single PMDA-PDA molecular chain under finite deformations as functions of bending angle and dihedral angle between PMDA and PDA groups are compared. It is found that the semi-empirical MO calculations provide sufficient accuracy to express the energetics of large deformations except for compressive deformation. [source] NMR spectroscopic characterization of inclusion complexes comprising cyclodextrins and gallated catechins in aqueous solution: cavity size dependencyMAGNETIC RESONANCE IN CHEMISTRY, Issue 4 2009Takashi Ishizu Abstract The structure of inclusion complexes of ,-cyclodextrin (,-CD), (,)-gallocatechin gallate (GCg), and (,)-epigallocatechin gallate (EGCg) in D2O was investigated using several NMR techniques. GCg formed a 1:1 inclusion complex with ,-CD in which the A and C rings of GCg were inserted deep at the head of the A ring into the ,-CD cavity from the wide secondary hydroxyl group side. In the 1:1 inclusion complex with GCg and ,-CD, the GCg moiety maintained a conformation in which the B and B, rings of GCg took both pseudoequatorial positions with respect to the C ring. The structure of the inclusion complex of GCg and ,-CD obtained from NMR experiments supported well that determined from PM6 semiempirical SCF MO calculations. However, 1H NMR experiments suggested that EGCg did not form any inclusion complex with ,-CD in D2O. The marked difference between GCg and EGCg in inclusion behavior toward ,-CD may be explained in terms of the stabilization energy calculated with the PM6 method. Copyright © 2008 John Wiley & Sons, Ltd. [source] Structural characterization of poly(diethylsiloxane) in the crystalline, liquid crystalline and isotropic phases by solid-state 17O NMR spectroscopy and ab initio MO calculationsMAGNETIC RESONANCE IN CHEMISTRY, Issue 3 2005Hideaki Kimura Abstract The structure of poly(diethylsiloxane) (PDES) has been characterized using solid-state NMR of 17O. The sample studied had a weight-average molecular weight of 2.45 × 105. The sample was prepared by utilizing the cationic ring-opening polymerization of 17O-enriched hexacyclotrisiloxane. Solid-state NMR of 17O-enriched PDES was measured on the low-temperature ,1 phase, the high-temperature ,2 phase, the two-phase system consisting of the liquid crystal and isotropic liquid phase and the isotropic phase. From these data, the molecular structure and dynamics of PDES in the various phases were characterized via the chemical shifts of 17O, and electric field gradient parameters were determined from NMR and ab initio molecular orbital (MO) calculations. In addition to the solid-state NMR of 1H, 13C and 29Si previously reported on these samples, knowledge of the dynamic behavior of PDES as inferred from the NMR of 17O in the present study was enhanced significantly. Further, the potential of combining the experimental NMR of 17O with ab initio MO calculations to characterize the dynamics of polymers containing oxygen is demonstrated. Copyright © 2004 John Wiley & Sons, Ltd. [source] 15N NMR spectroscopy of 3-substituted 5-trichloromethyl-1,2-dimethyl-1H -pyrazolium chloridesMAGNETIC RESONANCE IN CHEMISTRY, Issue 3 2002Marcos A. P. Martins Abstract 15N NMR data of a series of 3-alkyl[aryl] substituted 5-trichloromethyl-1,2-dimethyl-1H-pyrazolium chlorides (where the 3-substituents are H, Me, Et, n -Pr, n -Bu, n -Pe, n -Hex, (CH2)5CO2Et, CH2Br, Ph and 4-Br-C6H4), are reported. The 15N substituent chemical shifts (SCS) parameters are determined and these data are compared with the 13C SCS values and data obtained by MO calculations. Copyright © 2002 John Wiley & Sons, Ltd. [source] Boryl-substituted 1-silacyclobutenes.APPLIED ORGANOMETALLIC CHEMISTRY, Issue 1 2007Formation, molecular structure Abstract The 1,2-hydroboration of the chloro(hexyn-1-yl)- (1a) and chloro(phenylethyn-1-yl)diphenylsilanes (1b) with 9-borabicyclo[3.3.1]nonane afforded selectively the alkenylsilanes 2a, b, in which the boryl and the silyl groups are linked to the same olefinic carbon atom. In case of 2a, treatment with phenylethynyl lithium gave a mixture of the alkyn-1-ylborate 3a and the alkenyl(phenylethynyl)diphenylsilanes 4a. In the case of 2b, only the alkyn-1-ylsilane 4b was identified as an intermediate. Both 4a, b slowly rearranged by intramolecular 1,1-vinylboration into the silacyclobutenes 5a, b. The intermediates were characterized by 1H, 11B, 13C and 29Si NMR spectroscopy in solution, and the molecular structure of the 1-silacyclobutene 5a was determined by X-ray analysis. The gas phase geometries of model molecules corresponding to 5a were optimized by MO calculations using DFT methods [B3LYP/6-311 + G(d,p) level of theory], found to be in reasonable agreement with the results of the crystal structure determination, and NMR parameters were calculated at the same level of theory. Copyright © 2006 John Wiley & Sons, Ltd. [source] Sterically Crowded peri -Substituted Naphthalene Phosphines and their PV DerivativesCHEMISTRY - A EUROPEAN JOURNAL, Issue 25 2010Fergus Abstract Three sterically crowded peri -substituted naphthalene phosphines, Nap[PPh2][ER] (Nap=naphthalene-1,8-diyl; ER=SEt, SPh, SePh) 1,3, which contain phosphorus and chalcogen functional groups at the peri positions have been prepared. Each phosphine reacts to form a complete series of PV chalcogenides Nap[P(E,)(Ph2)(ER)] (E,=O, S, Se). The novel compounds were fully characterised by using X-ray crystallography and multinuclear NMR spectroscopy, IR spectroscopy and MS. X-ray data for 1, 2, n,O, n,S, n,Se (n=1,3) are compared. Eleven molecular structures have been analysed by naphthalene ring torsions, peri -atom displacement, splay angle magnitude, X,,,E interactions, aromatic ring orientations and quasi-linear arrangements. An increase in the congestion of the peri region following the introduction of heavy chalcogen atoms is accompanied by a general increase in naphthalene distortion. P,,,E distances increase for molecules that contain bulkier atoms at the peri positions and also when larger chalcogen atoms are bound to phosphorus. The chalcogenides adopt similar conformations that contain a quasi-linear E,,,PC fragment, except for 3,O, which displays a twist-axial-twist conformation resulting in the formation of a linear O,,,SeC alignment. Ab initio MO calculations performed on 2,O, 3,O, 3,S and 3,Se reveal Wiberg bond index values of 0.02 to 0.04, which indicates only minor non-bonded interactions; however, calculations on radical cations of 3,O, 3,S and 3,Se reveal increased values (0.14,0.19). [source] Electrophilic Attack on Sulfur,Sulfur Bonds: Coordination of Lithium Cations to Sulfur-Rich Molecules Studied by Ab Initio MO MethodsCHEMISTRY - A EUROPEAN JOURNAL, Issue 4 2005Yana Steudel Dr. Abstract Complex formation between gaseous Li+ ions and sulfur-containing neutral ligands, such as H2S, Me2Sn (n = 1,5; Me = CH3) and various isomers of hexasulfur (S6), has been studied by ab initio MO calculations at the G3X(MP2) level of theory. Generally, the formation of LiSn heterocycles and clusters is preferred in these reactions. The binding energies of the cation in the 29 complexes investigated range from ,88 kJ,mol,1 for [H2SLi]+ to ,189 kJ,mol,1 for the most stable isomer of [Me2S5Li]+ which contains three-coordinate Li+. Of the various S6 ligands (chair, boat, prism, branched ring, and triplet chain structures), two isomeric complexes containing the S5S ligand have the highest binding energies (,163±1 kJ,mol,1). However, the global minimum structure of [LiS6]+ is of C3v symmetry with the six-membered S6 homocycle in the well-known chair conformation and three LiS bonds with a length of 256 pm (binding energy: ,134 kJ,mol,1). Relatively unstable isomers of S6 are stabilized by complex formation with Li+. The interaction between the cation and the S6 ligands is mainly attributed to ion,dipole attraction with a little charge transfer, except in cations containing the six sulfur atoms in the form of separated neutral S2, S3, or S4 units, as in [Li(S3)2]+ and [Li(S2)(S4)]+. In the two most stable isomers of the [LiS6]+ complexes, the number of SS bonds is at maximum and the coordination number of Li+ is either 3 or 4. A topological analysis of all investigated complexes revealed that the LiS bonds of lengths below 280 pm are characterized by a maximum electron-density path and closed-shell interaction. [source] | ||||||||||