I am investigating the mechanism of hydrogen transfer between a hydroperoxyl radical and an aromatic hydroxyl group donating it. The problem is that I cannot find transition state structure. So far, I've tried with
TS with initial geometries either based on similar reactants I'm studying or taken from the literature. In general - the angle between the hydroperoxyl radical and the hydroxyl group of the donor is about 165°, and the distances from the hydrogen atom are 1.358A and 1.076A, respectively.
These failed despite changing the optimization algorithm, by:
maxstep(even to 1);
- calculating force constants using
calcfcor only derivatives for the atoms involved (available in
- calculation of
calcfccoupled with an imposed hessian of -1 for both bonds
The other way I tried to achieve my goal was to freeze the two bond and the angle, optimize the system, and only then reactivate them and run TS (with combinations of the above changes). No satisfactory results.
The last thing I did was to perform the TS task at a lower level of theory. By default I use
M052X/6-311+G(d,p), although I have checked
6-31G(d,p), but still without success.
And now, after a long introduction, I come to the substance: I ran
scan at the desired level of theory. I set up the geometry similarly to the one shown above, although I reduced the bond length between the hydrogen atom and the hydroxyl group to 0.94 (typical for my systems in their ground state) simultaneously increasing the distance between the hydroperoxy oxygen and this hydrogen atom to about 2.0Å.
I ran the simulation for 50 steps every -0.05Å and.... here a certain problem arises. Following the literature and previous works, in such transition states the hydrogen atom is perpendicular to the oxygen of the aromatic hydroxyl group, while the hydroperoxy radical is parallel to the system in sandwich form. To be precise, it lies above/below it. Therefore, the initial structure was constructed this way. However, as a result of the scanning, the hydroperoxy radical shifts to completely the same plane, influencing the hydrogen to follow the same geometry.
The resulting energy scan suggests a barrier-free reaction, but due to the mentioned glitch I can't believe the outcomes. I tried lowering the
maxstep for the scan, and also doing an
calcfc (although I'm not sure if either of those two affect the
scan algorithm) though with the effect.
I appreciate a solution to this issue. Or what are other options I can take to find a TS or energy scan that really reflects the barrierless path of the process (which I think is more true in this particular case).
Thanks in advance.