Ea in ROOH_sec;C_rad/H/NonDeC - Amrit Jalan's Thesis - Is the entry in the database correct?

[DetlevCM]

The reaction ROOH + R. = ROO. + RH has an exact match in the RMG database, indicated with '[AJ]'. It was suggested that this was calculated by Amrit Jalan - however reading his thesis, unless I am making a mistake, a positive activation energy is suggested while the database contains a negative activation energy? (Thesis link: https://dspace.mit.edu/handle/1721.1/91059 - page 195)

Entry in the database (same applies for the entry above with ROOH_pri):

entry(
    index = 551,
    label = "ROOH_sec;C_rad/H/NonDeC",
    kinetics = ArrheniusEP(
        A = (2.51e-11, 'cm^3/(mol*s)'),
        n = 6.77,
        alpha = 0,
        E0 = (-8.6, 'kcal/mol'),
        Tmin = (500, 'K'),
        Tmax = (1000, 'K'),
    ),
    rank = 3,
    shortDesc = u"""[AJ]CBS-QB3 calculations with 1DHR corrections, reverse rates computed using DFT_QCI_thermo""",
)

And in Jalan's Thesis (page 195) I find:

Table 6.1: Predicted Arrhenius parameters (Units: mol, cm 3 , sec, kcal/mol) for CH3OO* + RH* H-abstraction reactions with R=CH3CH2CH2, CH3C(OOH)CH3, CH3COCH2_, CH3CH2C_O.

Reaction A n Ea CH3CH2CH3 -> CH300H + CH3CH_CH3 1.3 x10-8 6.2 8.5 CH3CH(OOH)CH 3 -> CH3C(O)CH3 + OH + CH300H 1.1 x10-13 7.2 2.9 CH3C(O)CH3 -> CH300H + CH2C(O)CH3 4.4 x10-12 7.1 8.9 CH3CH2CHO -> CH300H + CH3CH2C_O 1.9 x10-3 4.5 4.4

The units for A are sec- 1 for unimolecular reactions and in cm 3 mol-1 sec-1 for bimolecular reactions, with Ea in kcal/mol. The rate coefficient is k = A(T/1[K])" exp(-Ea/RT).

[nickvandewiele]

I have not analyzed the numbers in detail, but the comment by @ajalan mentions that the reaction rates for this reaction were stored in the reverse direction from what he computed in the first place:

shortDesc = u"""[AJ]CBS-QB3 calculations with 1DHR corrections, reverse rates computed using DFT_QCI_thermo""",

The procedure computes the reverse rate coefficient by dividing the forward rate coefficient (i.e. from P195) by the equilibrium constant of the reaction, computed from the thermochemistry of the reactants/products, using the thermo library DFT_QCI_thermo.

As the n factor in the modified Arrhenius expression is often fixed to the same value for an entire reaction family, the fitting procedure that derives the modified Arrhenius parameters may lead to a negative Evans-Polanyi activation energy E0.

[DetlevCM]

Thanks @nickvandewiele - I did overlook something...

Having said that, working out reverse Arrhenius parameters I get rather different values ("but point" after table):

The forward reaction in RMG is ROOH + R. = ROO. + RH |____ | A (mol/cm^3) | n | Ea(kcal/mol) |____ | ----------------- | ------ | ------------------- | RMG-Py | 2.51x10^-11 | 6.77 | -8.6 | rev._____ | 1.850x10^-4 | 5.12 | 4.18

Now to the but: I would think the values in Amrit Jalan's thesis are for the gas phase and I am producing a liquid phase mechanism which will have liquid phase corrections applied to the parameters, so the difference in the values must not be an indication of a problem.

However, because the reaction is stored in the reverse form, there must be an assumption that the thermodynamics are well estimated, as these will influence k_equilibrium which in turn influences the ratio of k_forward to k_reverse....