I am working on the Ethanimine formation in the Interstellar medium by reaction CH2+CH2NH---->CH3CHNH. CH2 is taken in both singlet and triplet form. when I add water molecule in reaction complex for simulation of ice. sometimes I get transition state which has lower energy than reaction complex(RC). I want to know the physical meaning of that kind of transition state and how to find rate coefficient for these reactions.
One more question , is "Hammond–Leffler postulate" is applicable in Interstellar medium? Can I use it to verify my TS structure is correct or not?
The reactions with intermediate complexes and submerges barriers are my favourite topic.
You terminology is wrong. TS is not equal with the barrier. In a quantum chemical calculation you get only raw energy of barriers or local minima. These quantities are related to the topology of the potential energy surface (PES is calculated in Qchem). Transition state is a dividing surface in the phase space that separates the reactants and products. In conventional transition state theory (TST) the transition state is located at the barrier (but it is not necessary to locate here). Sometimes there is no barrier, or the barrier is submerged. Then it is not trivial where is (or even exist) the transition state regarding to the potential energy surface.
If you can not find barrier, or you find a submerged barrier....your elementary reaction is guided by attractive forces. The potential energy surface guides the reactants to each others. This guiding effect can be very efficient at low temperatures (like in space). If you has a submerges barrier, you must find a van der Waals complex before your submerges barrier. In this case the potential energy surface is attractive, but there are a chance to the formation of this complex. If this complex lives long, you can predict the rate of the reaction with statistical rate theories (RRKM, SACM, special version of TST (see Klippenstein's double TST (2TST) version).
According to the attractive potential energy surface you may get negative activation energy for the reaction (activation energy is not the height of the barrier, it is the local slope of the Arrhenius plot!). In some case the sign of the activation energy can be changed. At low temperatures the activation energy is negative, but at high temperature it's value is positive.
And the answers for the other questions:
Can I use it to verify my TS structure is correct or not?
You can verify only the goodness of your barrier with a frequency calculation at the barrier. The saddle point is correct, if you have ONLY one imaginary frequency, the others are real.
...how to find rate coefficient for these reactions?"
you can use RRKM theory for the vdW complex. If you are an expert in TST calcualtion you can use Klippenstein's 2TST or Krasnoperov's modified TS theroy.
thank you very much for your very informative answer Péter Szabó .
My suggestion is that check the applied level of theory first before do too much kinetics! I recommend to calculate the standard reaction entalpy and compare with highly accurate literature value. Also, worthy to do IRC in both directions. Good luck!
Thank you Milán Szőri for your answer. I have tried IRC but it does connect reactant and product at HF/6-31+G(d,p) level of theory but does not connect Reactant and product at B3LYP/6-311++G(d,p) level. I tried Constraint optimization on my TS guess and if I get correct negative frequency whose normal mode is along reaction coordinate than I perform relax TS search. many times It failed with "error in internal coordinate". In some cases, it is Successful and in one case (when I add 4 water molecules to my RC) I get submerge TS. Should I try to on the higher level of theory like MP2 or I should change optimization step size and try tight convergence criteria?
As you wrote "does not connect Reactant and product at B3LYP/6-311++G(d,p) level". Does it mean that IRC gave you unwanted reactants and product or it crashed before you get close enough to local mimium of the PES?
To make the level of theory story short: B3LYP can often give more local minima than HF or post-HF, so I prefer to have MP2 or CCSD PES. Always use Tight criteria (pay attention to symmetry as well...). Sometimes the applied basis set provides totally different feature than the others. Worthy to test several different types of basis set and compare your results with literature! Good luck!
Thank You Milán Szőri. I will try for the same. when i apply B3LYP/6-311++G(d,p) IRC always gives me reactants at both sides. I will use MP2 as it is the most I can afford computationally.
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