Dear Oleg B. Gadzhiev,

Thank you very much for providing me lot of information about the valley-ridge inflection (VRI)  point on the potential energy surface (PESs) with references.

I read your 2NO + O2 paper, in which the VRI point along the path was reported. In your case, the IRC calculation directly ends up to the VRI point on the PES which is another transition state.

The situation is little bit different in our case. Let me mention some more interesting results those we have observed for our current project.

1. The optimized products P1 and P2 have different diabatic character (the states have been characterized using CASSCF wave function). The diabatic states of P1 and P2 can be characterized by the distribution of three electrons among two orbitals (say a and b). In the case of P1, a is doubly occupied and b is singly occupied while in the case of P2, a is singly occupied and b is doubly occupied. These two products, P1 and P2,  are separated by a conical intersection between these two diabatic states.

2. The analysis of the state character along the IRC path reveals that the state has mixed character of P1 and P2 in post transition state and pre product formation region.

3. We also have analysed the energy difference between the state characterising orbitals "a" and "b" along the IRC path and found a point where orbital-a and orbital-b is very close in energy. In fact from this point only, we have done the interpolation to P2 (mentioned in the previous question) to get a path for the formation of P2 in the reaction.

3. We believe that the mixed state character and near degeneracy of orbitals a and b along the reaction path is responsible for the the formation of both P1 and P2 in the reaction.

Dear Oleg B. Gadzhiev,

Thank you very much for the helpful reply.

Yes, you are absolutely right, P1 is on the ground state PES while the P2 is on the excited state. I called both of them as the ground state products (which I should not have called) due to the following reason.

let's assume that the ground and excited states have A1 and B1 characters respectively. The minimum geometry on the A1 state has been denoted as P1 and the minimum geometry on the B1 state has been defined as P2. Now let me mention one interesting observation. At the P1, A1 is ground state and B1 is the excited state where as at the P2 geometry, two states flip their energy order, i.e, B1 becomes lower in energy than the A1 state energy.

As I mentioned earlier, along the reaction path, there exist a region with mixed state character of A1 and B1 due to which both the products in the reaction are possible.

We also have enough evidence in the literature that the reaction involves only ground state PES and yet forms the product in the ground and excited states.

So, I thought getting information about obtaining MEPs for two products via single transition state would be enough for calculating MEPs for the reaction which we are interested.

Yeah, dynamical calculations would be another possibility (probably on-the-fly). But,  with the appropriate active-space in CASSCF, on-the-fly dynamics would not be affordable. So, we would like to explore the mechanism in the static picture.

Thanks

Mahesh

Dear Nandhu,

Have you considered spin flip possibilities? Have you considered electronic state principles? However, to enable you understand your movement to P1 and P2;

1. Get your ground state energy for your localized reactants (M)

2. Get that of the excitation (which would help you get the TS)

3. Get that of the product  P1 (N)

4. Get that of the product P2 (Q)

Calculate the relative energies of the products (P1 & P2) from the first principle having known that of the reacting species and represent them on a nice energy profile (PES).

Simulate the two structures (P1 & P2) separately and compute for their energies at the ground state considering both singlet and triplet states. You would have more insight into your problem after doing this I believe.

Thank you.

Dear Ahmed,

Thank you very much for the reply.

Could you please elaborate on spin flip possibilities and electronic state principles?

We obtained minimum geometries for reactant, products (P1 and P2), and Transition state. We also have obtained MEP for P1 and analysed, how the nature of electronic states and molecular orbitals change along the path.

We also have tried to find the transition state on the excited state (probably, that is what you are suggesting in point 2) but, that is too high in energy and not accessible.

Dear Oleg,

Thank you very much for the information.

yes, you are correct, the reaction which we are interested in exploring the mechanism involves the NO2 generation.

The referred paper is the one I was looking for, thanks.

If you find any relevant papers like this, please let me know.

I'll read the paper that you have referred and I'll let you know the update.

Mahesh