Too large barriers

[YuraM]

Good afternoon,

For few pure organic transformations in the catalytic cycle I estimated the barriers about 35 - 37 kcal/mol in terms of Gibbs free energy of activation at room temperature. This catalytic reaction is known to occur at 100 C in benzene.

To me the barriers are too high. I used the simple TST theory rate constant expression, k = (kb*T/h)*e**(Gact/RT) and with 37 kcal/mol I got k=1.65E-9. This rate constant for bimolecular reaction is too small as you see.

Are these barriers too high in your opinion? Any ways to reduce the barriers?

Thank you in advance!

[Shorku]

Maybe you should suggest more appropriate computational model? You have not even mentioned the way you got your estimates

[ Post made via Android ]

[amge]

Соотнесение расчетных барьеров с экспериментальными для би- и более молекулярных реакций (в отличие от мономолекулярных) не однозначно. Люди задуряются и вносят различные поправки (которые, кстати, как раз понижают барьер, причем не слабо). Апофеоз задурения - в приложенной статье.

[YuraM]

Guys, thank you for your replies! Regarding the method = optimization in gas phase (PRIRODA/PBE/L1) - believe that the geometries are usually reasonable. SP: PBE-D3/def2-tzvpp and SMD for solvent. Hybrid functionals give the barrier larger by 3-4 kcal/mol (and I do not want it, it is already large). May be diffuse functions will improve upon the situation? However, I doubt that. Also I think that SMD model might be not so good for organic solvents..

Regarding the cheating, as in the article. To correct everything for 1M conditions is in my opinion OK, but it will only reduce the barrier by 1.89 kcal/mol (for bi-molecular reaction). Another formula they use to reduce the barrier is suspicious, I need to read the literature. But Thank you very much for the article!!!

From what I see so far the barriers are usually overestimated comparing to the experiments, but sometimes it is very hard to identify the origin of this overestimation - either your method does not perform well or you have found the wrong TS.