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MP2 Calculations (mp2 + calculation)
Selected AbstractsDiels,Alder Reactions of Symmetrically 1,4-Disubstituted Dienes: Theoretical Study on the Influence of the Configuration of the Double Bonds on the Regio- and EndoselectivityEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 18 2008Samira Baki Abstract It was experimentally shown in Diels-Alder reactions that symmetrically 1,4-disubstituted dienes exhibit high regio- and endoselectivity induced by the (E,Z) configuration of the double bonds. In order to understand the origin of this selectivity, the transition states associated with the reaction between a series of such dienes (R = OMe, CH3, NH2, F, CN) on substituted ethylene (R, = CO2Me, CN, F, OMe) were determined by DFT calculations. If the regioselectivity confirmed by a single-point MP2 calculation is predicted to be good in most cases, its direction remains difficult to predict; the R, group of the dienophile can orient itself toward either the (Z) or the (E) side of the diene. Neither primary nor secondary frontier orbital interactions appear to be able to rationalize these results. They are more likely explained by a delicate balance between steric and electrostatic interactions. This hypothesis is further supported by the calculated reactivity of the (E,E) and (Z,Z) corresponding dienes. In any case, a standard DFT calculation, confirmed by a single-point MP2 step, is likely to provide a reliable guideline for a synthesis strategy that allows the control of up to four stereogenic centres in a single reaction step.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Theoretical Studies on Metal,Metal Interaction and Intrinsic 1,3[,*(d),(s/p)] Excited States of Dinuclear d10 Complexes with Bridging Phosphane LigandsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 5 2006Qing-Jiang Pan Abstract To explore the metal,metal interaction and spectroscopic properties, the ground- and excited-state structures of [M2(dpm)2]2+ [M = Ag (2), Cu (3), dpm = bis(diphosphanyl)methane] and their solvated species [M2(dpm)2]2+·(MeCN)2 were optimized by the MP2 and CIS methods, respectively. In the ground states, the calculated M,M distances and their corresponding M,M stretching frequencies for 2 and 3 indicate the presence of metallophilic attraction; there is strong N,Cu/Ag coordination in acetonitrile, which is different from the case in previous studies of [Au2(dpm)2]2+ (1). CIS calculations show that 2 and 3 have 1,3[,*(d),(s/p)] as their lowest-energy excited state, as is also the case for 1, confirmed by unrestricted MP2 calculations. On the basis of the CIS-optimized structures, the TD-DFT (B3LYP) method was employed to calculate the emission spectra of such complexes. For 3, the phosphorescent emissions were calculated at 424 and 514 nm in the solid state and acetonitrile, which is comparable to the experimental data of 475 and 480 nm, respectively. The comparison between the gas-phase and solution emissions for 1,3 reveals that the N,M coordination results in a large red-shift of the emission wavelength. Taking previous studies into account , we found that the M,M distances are linearly correlated with the M,M stretching frequencies for the dinuclear d10 complexes. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Predicting the tautomeric equilibrium of acetylacetone in solution.JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 4 2010Abstract This study investigates how the various components (method, basis set, and treatment of solvent effects) of a theoretical approach influence the relative energies between keto and enol forms of acetylacetone, which is an important model system to study the solvent effects on chemical equilibria from experiment and theory. The computations show that the most popular density functional theory (DFT) approaches, such as B3LYP overestimate the stability of the enol form with respect to the keto form by ,10 kJ mol,1, whereas the very promising SCS-MP2 approach is underestimating it. MP2 calculations indicate that in particular the basis set size is crucial. The Dunning Huzinaga double , basis (D95z(d,p)) used in previous studies overestimates the stability of the keto form considerably as does the popular split-valence plus polarization (SVP) basis. Bulk properties of the solvent included by continuum approaches strongly stabilize the keto form, but they are not sufficient to reproduce the reversal in stabilities measured by low-temperature nuclear magnetic resonance experiments in freonic solvents. Enthalpic and entropic effects further stabilize the keto form, however, the reversal is only obtained if also molecular effects are taken into account. Such molecular effects seem to influence only the energy difference between the keto and the enol forms. Trends arising due to variation in the dielectric constant of the solvent result from bulk properties of the solvent, i.e., are already nicely described by continuum approaches. As such this study delivers a deep insight into the abilities of various approaches to describe solvent effects on chemical equilibria. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010 [source] Assessment of theoretical methods for the calculation of methyl cation affinitiesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 2 2008Y. Wei Abstract The methyl cation affinity (MCA; 298 K) of a variety of neutral and anionic bases has been examined computationally with a wide variety of theoretical methods. These include high-level composite procedures such as W1, G3, G3B3, and G2, conventional ab initio methods such as CCSD(T) and MP2, as well as a selection of density functional theory (DFT) methods. Experimental results for a variety of small model systems are well reproduced with practically all these methods, and the performance of DFT based methods are far superior in comparison to their MP2 analogs for these small models. For larger model, systems including motifs frequently encountered in organocatalysts, the performance deteriorates somewhat for DFT methods, while it improves significantly for MP2, rendering the former methods unreliable for common organic bases. Thus, MP2 calculations performed in combination with basis sets such as 6-31+G(2d, p) or larger, appear to offer a practical and reliable approach to compute MCAs of organic bases. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2008 [source] An ab initio theoretical study of electronic structure and properties of 2,-deoxyguanosine in gas phase and aqueous mediaJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 5 2002S. K. Mishra Abstract Molecular geometries of two structural forms of 2,-deoxyguanosine (keto-N9R and keto-N7R, R = the sugar moiety) considering both the C2,-endo and C3,-endo conformations of the sugar ring and those of the complexes of these species with two water molecules each were optimized employing the ab initio RHF procedure. A mixed basis set consisting of the 6-311+G* basis set for the nitrogen atom of the amino group and the 4-31G basis set for all the other atoms was used. The RHF calculations were followed by correlation correction of the total energy at the MP2 level. Both the structural forms of 2,-deoxyguanosine were solvated using the polarized continuum model (PCM) of the self-consistent reaction field (SCRF) theory and the corresponding RHF optimized geometries at the RHF and MP2 levels. Geometry optimization was also performed in aqueous media using the Onsager model at the RHF level using the above-mentioned mixed basis set, and subsequently, using the reoptimized geometries, single-point MP2 calculations were performed. It is found that both the keto-N9R and keto-N7R forms of 2,-deoxyguanosine as well as their complexes with two water molecules each would occur, particularly at the water,air interface. Though the normal Watson,Crick-type base pairing would not be possible with the keto-N7R form of 2,-deoxyguanosine(G*), two other (G*-C and G*-T) base pairing schemes may occur with this form of the nucleoside, which may cause mutation. The present calculated geometry of the keto-N9R form of the anti -conformation of 2,-deoxyguanosine including the dihedral angle ,CN agree satisfactorily with the available crystallographic results. The present results also agree satisfactorily with those obtained by other authors earlier for the keto-N9R form of 2,-deoxyguanosine using B3LYP and MP2 methods employing the 6-31G* basis set. Using transition state calculations, it is shown that tautomerism of guanine and other similar molecules where the tautomers would coexist would be facilitated by the occurrence of the H+ and OH, fragments of water molecules. Further, this coexistence of the two tautomers appears to make the C8 carbon atom located between the N7 and N9 nitrogen atoms susceptible to attack by the OH, group. Thus, an explanation is obtained for the efficient formation of the reaction product 8-hydroxy-2,-deoxyguanosine, which serves as a biomarker for oxidative damage to DNA in biological systems. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 530,540, 2002; DOI 10.1002/jcc.10046 [source] Conformational properties of thiophene oligomersJOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 4 2000Salvatore Millefiori The molecular geometries and the torsional potentials about the inter-ring C-C bond in ,-oligothiophenes (,-nTh, n=2,4) have been calculated by means of conventional ab initio and density functional theory (DFT) calculations employing the hybrid B3LYP and BH&HLYP functionals. The position and the energetics of the critical points in the potential energy curve generated by rotation about the inter-ring CC bond are shown to be dependent on the computational method. DFT calculations, in comparison with MP2 calculations, favour conjugative interactions, while steric and coulombic interactions are equally treated by both methods. On oligomerization the electron delocalisation increases slightly, the p-charge being preferentially confined within the rings, although it is sufficient to move the molecular structure towards co-planarization and to increase the barrier through the perpendicular conformation. The IR and Raman spectra on the relevant rotamers of ,-2Th have been computed at HF/6,31G* and B3LYP/6,31G* levels. The comparison with the experiment is excellent. It has been found that small twisting from the planar conformation has no apparent effects, while 90° twisting and isomerization to the syn-gauche form produce significant frequency and intensity variations which could be useful probes in conformational studies. The simulated IR and Raman spectra of the ,-2Th rotamers are consistent with a small,-electron delocalisation between the rings. [source] Conformational analysis, Part 41.JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 6 2006-unsaturated carbonyl compounds, A modelling, LIS/NMR investigation of the conformations of Abstract A novel Lanthanide Induced Shift Analysis (LISA) is presented. In this analysis both the paramagnetic and the diamagnetic lanthanide induced chemical shifts are normalised separately in contrast to previous techniques in which only the paramagnetic shifts were normalised. This procedure is used together with molecular mechanics (MMFF94) and ab initio (RHF/6-31G, RHF/6-311G** and B3LYP/6-311G**) calculations to investigate s - cis/s - trans isomerism in some ,,, -unsaturated aldehydes, ketones and esters. In tiglic aldehyde 1 and trans -cinnamaldehyde 4 the s - trans conformer predominates with energy differences ,E (s - cis,s - trans) of 1.64 and 1.76,kcal/mol. In methyl vinyl ketone 2 and trans -cinnamyl methyl ketone 5 the populations of the s - cis and s - trans isomers are almost equal (,E 0.24 and 0.0,kcal/mol) and in methyl crotonate 3 and methyl trans -cinnamate 6 the s - cis conformer is more stable (,E ,0.72 and ,0.41,kcal/mol). These results are in agreement with both the MMFF94 and ab initio calculated energies for the compounds except tiglic aldehyde 1 in which all the calculated values are too large and cinnamyl ketone 5. In this compound the ab initio calculations predict the s - cis form to be more stable than the s - trans in contrast to both the MM calculations and the observed result which give both forms of equal energy. Also in both the MM and ab initio calculations phenyl substitution in the ketone (2 vs. 5) considerably stabilises the s - cis form. This is not observed in practise. In phenyl acetate 7 the B3LYP calculations give two equally stable structures, one planar one non-planar. The MMFF94 and MP2 calculations and the LIS analysis support the existence of only the non -planar conformer in solution, which is also the conformation of phenyl acetate in the crystal. Copyright © 2006 John Wiley & Sons, Ltd. [source] Rapid Prediction of the Hydrogen Bond Cooperativity in N -methylacetamide ChainsCHEMPHYSCHEM, Issue 18 2009Xiao-Nan Jiang Abstract A method is proposed to rapidly predict the hydrogen bond cooperativity in N -methylacetamide chains. The parameters needed are obtained from the fittings to the hydrogen bonding energies in the formamide chains containing 2 to 8 monomeric units. The scheme is then used to calculate the individual hydrogen bonding energies in N -methylacetamide chains containing 2 to 7 monomeric units. The cooperativity predicted is in good agreement with those obtained from MP2/6-31+G** calculations by including the BSSE correction. Our scheme is further employed to predict the individual hydrogen bonding energies in larger N -methylacetamide chains containing up to 200 monomeric N -methylacetamide units, to which the MP2 method cannot be applied. Based on our scheme, a cooperative effect of over 170,% of the dimer hydrogen bonding energy in long N -methylacetamide chains is predicted. The method is also applied to heterogeneous chains containing formamide, acetamide, N -methylformamide, and N -methylacetamide. The individual hydrogen bonding energies in these heterogeneous chains are also in good agreement with those obtained from MP2 calculations with the BSSE correction, further demonstrating that our method is reasonable. [source] Carbon Nanorings: A Challenge to Theoretical ChemistryCHEMPHYSCHEM, Issue 12 2006I. García Cuesta Dr. Abstract High-level quantum-chemical methods show that the binding in the inclusion complex of hexamethylbenzene (HMB) in 6-cycloparaphenilacetylene (6-CPPA) cannot be explained only in terms of electrostatic interactions,caused by the polarization associated to curved ,-conjugated systems,and the inclusion of dispersion forces is definitely needed. The theoretical description of van der Waals interactions is notoriously complicated and in fact some DFT methods cannot even predict the existence of the relatively small supramolecular nanoring studied here. However, ab initio MP2 calculations agree with experimental data and show that, in the considered complex, the HMB fragment is placed at the center of the 6-CPPA ring. The binding energy, which is not available experimentally, is calculated to be around ,14 kcal,mol,1 with a lower limit of ,19 kcal,mol,1. [source] | ||||||||||||||||||||||