Density functional theory , transition structural studies

All semiempirical calculations were performed on a DEC 7620 computer. Chem-3D Plus on a Macintosh Ilfx was used as a graphical interface for drawing and visualizing all structures and for preparing input files for MOPAC [1]. The transition state structures were localized, optimized, and verified as explained in our previous work [2]. All Density Functional Theory (DFT) computational studies were performed with B3LYP [3] and 6-31G(d) [4] basis set as incorporated into GAUSSIAN [5] computational package. 

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... 

Numerous quantum mechanic calculations have been carried out to better understand the bonding of nitrogen oxide on transition metal surfaces. For instance, the group of Sautet et al have reported a comparative density-functional theory (DFT) study of the chemisorption and dissociation of NO molecules on the close-packed (111), the more open (100), and the stepped (511) surfaces of palladium and rhodium to estimate both energetics and kinetics of the reaction pathways [75], The structure sensitivity of the adsorption was found to correlate well with catalytic activity, as estimated from the calculated dissociation rate constants at 300 K. The latter were found to agree with numerous experimental observations, with (111) facets rather inactive towards NO dissociation and stepped surfaces far more active, and to follow the sequence Rh(100) > terraces in Rh(511) > steps in Rh(511) > steps in Pd(511) > Rh(lll) > Pd(100) > terraces in Pd (511) > Pd (111). The effect of the steps on activity was found to be clearly favorable on the Pd(511) surface but unfavorable on the Rh(511) surface, perhaps explaining the difference in activity between the two metals. The influence of... 

Currently, the density functional theory (DFT) method has become the method of choice for the study of reaction mechanism with transition-metals involved. Gradient corrected DFT methods are of particular value for the computational modeling of catalytic cycles. They have been demonstrated in numerous applications for several elementary processes, to be able to provide quantitative information of high accuracy concerning structural and energetic properties of the involved key species and also to be capable of treating large model systems.30... 

Our attempt in this chapter is to demonstrate the application of density functional theory (DFT) to real-life problems in transition metal organometallic chemistry through examples. Organometallic chemistry is an area where use of DFT to predict the structure, bonding, and reactivity has become complementary to experimental studies. A major part of the organometallic chemistry can be viewed profitably as... 

Ab initio calculations and density functional theory studies of the gas-phase addition of HF to CH2=CH2 have revealed the possibility of forming trimolecular (two HF and one ethylene) and dimolecular (one FIF and one ethylene) complexes and transition-state structures and of the catalytic effect of the second molecule of the reagent. An energetically favourable pathway was selected on the basis of the computed potential-energy surface for these two reactions. ... 

To understand the unusual structures found in bare post-transition element clusters and underlying reasons for differences in the structures between analogous boranes and post-transition element clusters, we carried out extensive density functional theory (DFT) studies of bare germanium clusters during the past decade [18-20, 69-72]. In three cases (Fig. 5), these studies predict lowest energy structures for Ge clusters that are different from the well-established structures of the isovalent boranes and carboranes C2B 2H . Thus, the lowest energy... 

Density functional theory computational studies have been used to determine die importance of secondary orbital interactions for the stability of transition-state structures for die 4 + 2-cycloaddition of furan with cyclopropene.175 Kinetic studies of die 2 + 4-cycloaddition of 2-cyclopropylidene acetates with furan and dimethylful-vene suggest a mechanism involving diradicals or zwitterions as intermediates.176 Cyclopropene, produced by die reaction of allyl chloride with sodium bis(bimediyl-silyl)amide, reacts with 1,3-diphenylisobenzofuran to produce both endo- and exo-Diels-Alder cycloadducts isolated for the first tune.177... 

These studies, which employed density functional theory (DFT) methods (B3LYP/LANL2DZ/Gaussian 98) proposed that the reactions of all alkali metal phenoxides with C02 followed a similar ground mechanism that comprised three intermediates and three transition states. In step 1, C02 must first be activated by an alkali metal phenoxide. In the case of the sodium phenoxide [24a], C02 can only attack at the polarized O-Na bond to form a Ph0Na/C02 complex as the first intermediate (structure 4). The calculation definitely rules out a direct C-C bond formation at the aromatic ring. 

This chapter mainly focuses on the reactivity of 02 and its partially reduced forms. Over the past 5 years, oxygen isotope fractionation has been applied to a number of mechanistic problems. The experimental and computational methods developed to examine the relevant oxidation/reduction reactions are initially discussed. The use of oxygen equilibrium isotope effects as structural probes of transition metal 02 adducts will then be presented followed by a discussion of density function theory (DFT) calculations, which have been vital to their interpretation. Following this, studies of kinetic isotope effects upon defined outer-sphere and inner-sphere reactions will be described in the context of an electron transfer theory framework. The final sections will concentrate on implications for the reaction mechanisms of metalloenzymes that react with 02, 02 -, and H202 in order to illustrate the generality of the competitive isotope fractionation method. 

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