Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Exploring a Potential Energy Surface

We ll now use Gaussian s reaction path following facility to explore the H CO potential energy surface. There are many minima on this surface—including [Pg.175]


In this exercise, you will explore the bond rupture process by performing a potential energy surface scan. Run potential energy surface scans for these molecules, gradually increasing one of the C-H bond lengths, using the specified model chemistries ... [Pg.186]

In some situations we have performed finite temperature molecular dynamics simulations [50, 51] using the aforementioned model systems. On a simplistic level, molecular dynamics can be viewed as the simulation of the finite temperature motion of a system at the atomic level. This contrasts with the conventional static quantum mechanical simulations which map out the potential energy surface at the zero temperature limit. Although static calculations are extremely important in quantifying the potential energy surface of a reaction, its application can be tedious. We have used ah initio molecular dynamics simulations at elevated temperatures (between 300 K and 800 K) to more efficiently explore the potential energy surface. [Pg.226]

In this section we focused our attention to a rationalization of the factors determining the stereoselectivity through ab initio mixed quantum/classical (QM/MM) Car-Parrinello molecular dynamic simulations. We have used the same basic computational approach used in Section 3 to explore the potential energy surface of the reaction. Since the catalyst system, 1, is relatively large, we have used the combined QM/MM model system B as shown in Figure 3 and described in subsections 2.1 and 3.1. [Pg.240]

Density functional theory and a high-level cib initio procedure (G2+) have been used to explore the potential energy surface for the base-induced elimination reaction of fluoride ion with ethyl fluoride.11 The DFT barriers are smaller and looser than those predicted by the ab initio method but the nature of the transition state cannot be defined with confidence since the predictions are unusually sensitive to the choice of functional and basis set. The results suggest that improvement in density functional methods will require fundamental change in the functionals themselves. [Pg.366]

Given some overall strategy of what you hope to accomplish with the calculations, you need to decide which conformation(s) of your reactant(s) to use. A potential energy surface of nontrivial molecules will have many hills and valleys. The deepest valley is called the global minimum, as mentioned earlier. Without some other basis for selecting a conformation, many chemists will want to use the global minimum conformation. However, most pertinent is the conformation a molecule adopts at the time of reaction. This may or may not be the global minimum. You may need to use your chemical intuition to choose the most relevant conformation. Sometimes, you will need to explore several different conformations. [Pg.386]

In this section we will present results of ab initio molecular dynamics simulations performed for more complex chemical reactions. Catalytic copolymerization of a-olefins with polar group containing monomers, chosen here as an example, is a complex process involving many elementary reactions. While for many aspects of such a process the standard approach by static quantum chemical calculations performed for the crucial reaction intermediates provides often sufficient information, for some aspects it is necessary to go beyond static computations. In the case of the process presented here, MD was priceless in exploring the potential energy surfaces for a few elementary reactions that were especially difficult for a static approach, due to a large number of alternative transition states and thus, alternative reaction pathways.77... [Pg.253]

Marti S, V Moliner (2005) Improving the QM/MM description of chemical processes A dual level strategy to explore the potential energy surface in very large systems. J. Chem. Theory Comput. 1 (5) 1008-1016... [Pg.299]

To explore the relevance of symmetry allowed conformations to chemically real structures it is necessary to locate steric-energy minima in a potential energy surface that spans all possible conformations. A variety of computational techniques [210] are available, commonly combined with experimental results retrieved from structural databases [211]. Such procedures have revealed the occurrence of countless different rotamers and conformers, arising from pseudorotation and conformational inversion under special environmental conditions. In addition, situations of disorder in the crystalline state are symptomatic in many cases, of the stabilization of variable intermediate forms. [Pg.224]

Ab initio molecular orbital theory is utilized to study the hydrogen abstraction reaction of n-bromopropane with hydroxyl radical and chlorine atom. The stability of the trans and gauche isomers of n-bromopropane is explored. The potential energy surface of both reactions is characterized by pre- and post-reactive complexes, as well as transition state structures in both trans and gauche isomeric forms. The importance of these two reactions relies on the ultimate product distribution from both reactions. Differences in the reactivity of 1-bromopropane toward OH and Cl are observed. The reaction of n-bromopropane with OH radical favors the abstraction of hydrogen atoms while the reaction with Cl atoms favors the abstraction of hydrogen atoms at the a and p carbon sites. [Pg.215]

To study CO2 on clean Pd(lll), two different clusters Pdio(7,3) and Pd 15(10,5) were selected to represent mono-coordinated and bi-coordinated adsorption modes respectively. The local/outer separation described above was employed, pseudopotentials and basis sets chosen according to this partition. The hybrid B3LYP density functional method was used to explore the potential energy surface. The different optimizations converged to three unique species corresponding to two coordination models only. For theri -C coordination two different species were found, one being a physisorbed and... [Pg.166]

Van der Waals Complexes a Tool to Explore the Potential Energy Surface in the Electron-transfer Region... [Pg.3033]

Parameterized methods like ZINDO/S are probably the only way to calculate reasonably accurate UV spectra for large molecules. AMI and PM3 have become extremely useful not only because they allow quantum mechanical calculations to be done on molecules which are still too big for ab initio or DFT (chapter 7) methods, but also as adjuncts to these latter methods, since they often allow a relatively rapid survey of a problem, such as an exploration of a potential energy surface one can locate minima and transition states, then use the semiempirical structures (size permitting) as inputs for initial geometries, wavefunctions and hessians (section 2.4) in a higher-level geometry... [Pg.376]

A potential energy surface scan allows you to explore a r ion of a potential energy surface. A normal scan calculation performs a series of single point energy calculations at various structures, thereby sampling points on the potential energy surface. When you request a scan, you specify the variable(s) in the molecular structure which are to vary and the range of values which they should take on. [Pg.288]

The study of reaction paths, in DFT, is not a new 1114,115. Thus we have chosen to explore the potential energy surfaces (PES) introducing the possibility to rationalize the results through the computations of the global hardnesses along the whole reaction path, with the aim to verify if, for the studied processes, the maximum hardness principle (MHP) 53 is satisfied. [Pg.114]


See other pages where Exploring a Potential Energy Surface is mentioned: [Pg.175]    [Pg.177]    [Pg.205]    [Pg.304]    [Pg.290]    [Pg.291]    [Pg.118]    [Pg.175]    [Pg.177]    [Pg.205]    [Pg.304]    [Pg.290]    [Pg.291]    [Pg.118]    [Pg.1]    [Pg.1]    [Pg.171]    [Pg.261]    [Pg.43]    [Pg.387]    [Pg.602]    [Pg.25]    [Pg.374]    [Pg.256]    [Pg.423]    [Pg.435]    [Pg.10]    [Pg.412]    [Pg.426]    [Pg.190]    [Pg.311]    [Pg.93]    [Pg.301]    [Pg.646]    [Pg.33]    [Pg.143]    [Pg.209]    [Pg.279]    [Pg.398]   


SEARCH



Exploration

Explorer)

Exploring Potential Energy Surfaces

© 2024 chempedia.info