8
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Transition state - multidimensional surfaces
The picture below shows a simple potential energy surface with a transition state at the maximum point on the reaction path, which is at the 'saddle point'. The reaction path is the solid black line. (...
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Finding chair and boat transition states during ring-forming reactions
As you have figured out, this is a non-trivial problem.
It's okay to find a boat conformation that might be a bit higher in energy and see if it can map to a TS. Keep in mind that for 6-membered ...
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8
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How to estimate the temperature needed to overcome an activation energy barrier?
You could convert the rate constant($k$) to half-life($t_{1/2}$) which would give you an idea of the time scale required for the reaction to finish at a certain temperature.
The equation to obtain ...
- 5,685
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How to estimate the temperature needed to overcome an activation energy barrier?
From the Eyring equation, we can simply calculate the $k$ value for it.
\begin{align}
k &= \frac{k_\mathrm{b} T}{h}\exp\left(\frac{-\Delta G^\ddagger}{RT}\right)\\
k_\mathrm{b} &= \pu{1.38E-9 ...
- 2,037
7
votes
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When are transition state's energy barrier "reasonable" at a certain temperature?
Your question is really about what you consider to be a reasonable rate constant and so is somewhat subjective. The fastest a bimolecular reaction can be in solution is given by how fast the reactants ...
- 30.8k
7
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Ambimodal transition states in some organic reaction pathways
Why do these ambimodal transition states occur?
I don't know that there is a general reason for that. Potential energy surfaces are complicated. Very often, there is an abundance of local minima, ...
- 837
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NMR and dienes to isolate certain isotopes?
There are at least two glaring problems:
Look at the energy scales on which NMR operates (for example, take the resonance frequency $\nu$ and calculate the associated energy $h\nu$)... and compare it ...
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5
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References for Transition State Theory and Quantum Transtion State Theory
There are many sources that you can check out, in particular the reviews of Truhlar:
Current Status of Transition-State Theory by Truhlar et al.
This article was used in my undergraduate course of ...
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5
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Why some reaction have no transition state?
There is nothing special in having an activation barrier between reactants and products although, obviously, very many reactions do. But some reactions, such as electron transfer, are observed ...
- 30.8k
4
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Kinetic vs thermodynamic control of HBr to 1,3-Butadiene
A transition state does not ‘represent the energies of […] reactants and […] products’. A transition state is a certain arrangement of atoms somewhere between the most stable atom arrangement the ...
- 68.4k
4
votes
Intrinsic Reaction Coordinate - Converting To Reaction Coordinate with Units of Distance
Intrinsic reaction coordinate (IRC) is not some internal coordinate of a molecular system (such as, bond length, bond angle, etc.), rather it is a curvilinear coordinate that describes the intrinsic ...
- 19.1k
4
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Transition state optimisation on the surface of periclase
To study surface reactions I recommend the growing string method (GSM) for surfaces. Here is a nice quote from the paper which developed it (Ref. 1)
GSM’s efficacy was confirmed by comparison with ...
- 824
4
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Why does transition state loses its ability to vibrate? (Transition State Theory)
Only some of the statements you quote are true, certainly a transition state cannot be isolated since it lasts for less than a picosecond. In fact there is hardly any direct measurement of transition ...
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4
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NMR and dienes to isolate certain isotopes?
Look up the difference of the isotopic masses between hydrogen and deuterium (a factor of about 2),* and the two isotopes of uranium (difference here $\approx1.5\%$), e.g. in NIST's table of atomic ...
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3
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What is the reasoning behind the assumptions of Transition State Theory?
This is deeper discussed in Rate Constant Units and Eyring Equation, but I am going to post a very short take-home message here.
First, the three assumptions you have cited are not complete. They ...
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Where does the RT term come from in the derivation for the activation enthalpy from the Eyring equation?
Using vibrational partition functions to define the reaction rate constant produces an equation of the form $\displaystyle k=aT^be^{-\Delta U_0^\mathrm{O}/(RT)}$ where $a,b$ are constants independent ...
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3
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Entropy of Activation and Temperature Dependence
The equation
$$\ln\left(\frac{k}{T}\right) = \frac{-\Delta H^{\ddagger}}{RT} + \frac{\Delta S^{\ddagger}}{R} + \ln\left(\frac{k_\mathrm{B}}{h}\right)$$
does not assume that $\Delta S^{\ddagger}$ is ...
- 22.5k
3
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Explaining boiling using internal energy
You are wrong when you say the potential energy of particles decrease when they move apart.
The forces between the particles are attractive not repulsive.
Recall the definition of potential energy. ...
- 1,108
3
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Transition state - multidimensional surfaces
First, the question may pose itself why $3N-6$ at all? And for that, we should take a step back and ask ourselves: Why $3N$?
These are the degrees of freedom of a molecule. If you break a molecule ...
- 68.4k
3
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Unix/MobaXterm Project Assistence (Knowing the Error in my Output File)
I copied your input into a new file, added some empty lines at the end and changed the input line to be "#p opt=qst3 hf/6-31..." instead of redundant (and maybe erroneous(?) "opt ... opt=qst3"). I ...
3
votes
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Unix/MobaXterm Project Assistence (Knowing the Error in my Output File)
I already tested the TS search calculation of this reaction based on your proposed structure coordinates. The calculation has been terminated quickly at link9999.exe sub-routine program in which its ...
3
votes
Why only molecules with three or more atoms undergo unimolecular reaction?
The statement is essentially correct. Unimolecular reactions refer to the Lindemann scheme and its improvements called RRKM theory. The Lindemann model is $\ce{A + M <=> A^* +M ;\; A^*\...
- 30.8k
3
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Why only molecules with three or more atoms undergo unimolecular reaction?
I'm guessing this quote is given in the context of classical transition rate theory. Although the statement may seem very strong at first, it is generally true but has some important caveats.
Energy ...
- 1,929
3
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Transition state and free energy
First you are correct, there is no fundamental difference in reactions being described as reversible or irreversible, unlike in thermodynamics. A reaction will be called irreversible (a)if the product ...
- 30.8k
2
votes
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Explain the difference between reaction intermediate and reactive intermediate
A reaction intermediate is nothing more than a plain old intermediate, which you probably understand. There is no stipulation as to how stable the intermediate must be. The IUPAC Gold Book defines ...
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2
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In Statistical Transition State Theory, why is the concentration of the transition state for the forward and reverse reaction identical?
In transition state theory, it is assumed that the forward and the reverse reaction occur via the same transition state. This is implied by assuming a quasi-equilibrium between the reactants and the ...
- 44.3k
2
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When are transition state's energy barrier "reasonable" at a certain temperature?
Using this website, input the activation energy and temperature and it gives half life. A reasonable half-life is up to the user, but more than 1 day is going to be super slow. Usually assume the ...
- 824
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When are transition state's energy barrier "reasonable" at a certain temperature?
If you don't have any idea what $k$ value would be appropriate, arguably the best option is to identify a catalyst that is experimentally verified to do the reaction of interest and is assumed to ...
- 1,352
2
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Is Gibbs Free Energy of formation equal to Gibbs Free Energy of transition state?
The Arrhenius equation
$$k=A\exp\left(-\frac{E_a}{RT}\right)$$
places all of the $T$-dependence in the exponential factor. The pre-exponential factor is not assumed to be temperature-dependent. By ...
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2
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Can rate constant depend upon the concentration of reactants (or other species involved in the reaction)?
It seems possible that you mix the terms. There is the reaction rate constant -- in your equation $k_r$, and there is the rate of reaction -- in your equation $r$ and sometimes expressed by $v$.
...
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