Thermochemistry, Bond Energies, and Internal Rotor Potentials of Dimethyl Tetraoxide

Abstract

Thermochemical properties of dimethyl tetraoxide (CH(3)OOOOCH(3)), the dimer of the methylperoxy radical, are studied using ab initio and density functional theory methods. Methylperoxy radicals are known to be important intermediates in the tropospheric ozone cycle, and the self-reaction of methylperoxy radicals, which is thought to proceed via dimethyl tetraoxide, leads to significant chain radical termination in this process. Dimethyl tetraoxide has five internal rotors, three of them unique; the potential energy profiles are calculated for these rotors, as well as for those in the CH(3)OO, CH(3)OOO, and CH(3)OOOO radicals. The dimethyl tetraoxide internal rotor profiles show barriers to rotation of 2-8 kcal mol(-1). Using B3LYP/6-31(d) geometries, frequencies, internal rotor potentials, and moments of inertia, we determine entropy and heat capacity values for dimethyl tetraoxide and its radicals. Isodesmic work reactions with the G3B3 and CBS-APNO methods are used; we calculate this enthalpy as -9.8 kcal mol(-1). Bond dissociation energies (BDEs) are calculated for all C-O and O-O bonds in dimethyl tetraoxide, again with the G3B3 and CBS-APNO theoretical methods, and we suggest the following BDEs: 46.0 kcal mol(-1) for CH(3)-OOOOCH(3), 20.0 kcal mol(-1) for CH(3)O-OOOCH(3), and 13.9 kcal mol(-1) for CH(3)OO-OOCH(3). From the BDE calculations and the isodesmic enthalpy of formation for dimethyl tetraoxide, we suggest enthalpies of 2.1, 5.8, and 1.4 kcal mol(-1) for the CH(3)OO, CH(3)OOO, and CH(3)OOOO radicals, respectively. We evaluate the suitability of 10 different density functional theory (DFT) methods for calculating thermochemical properties of dimethyl tetraoxide and its radicals with the 6-31G(d) and 6-311++G(3df,3pd) basis sets, using a variety of work reaction schemes. Overall, the best-performed DFT methods of those tested were TPSSh, BMK, and B1B95. Significant improvements in accuracy were made by moving from atomization to isodesmic work reactions, with most DFT methods yielding errors of less than 2 kcal mol(-1) with the 6-311++G(3df,3pd) basis set for isodesmic calculations on the dimethyl tetraoxide enthalpy. These isodesmic calculations were basis set consistent, with a considerable reduction in error found by using the 6-311++G(3df,3pd) basis set over the 6-31G(d) basis set. This was not the case, however, for atomization and bond dissociation work reactions, where the two basis sets returned similar results. Improved group additivity terms for the O-O-O moiety (O/O2 central atom group) are also determined.

Thermochemical Properties (ΔfH°(298 K), S°(298 K), Cp(T)) and Bond Dissociation Energies for C1–C4 Normal Hydroperoxides and Peroxy Radicals

“…Goldsmiths et al also reported the standard entropy of methoxy, ethoxy, and n -propoxy radicals as (in cal mol –1 K –1 ) 54.4 ± 0.3, 66.8 ± 1.0, and 75.7 ± 1.5, respectively, which also agree with group additivity calculation. Burke et al recommended 66.11 cal mol –1 K –1 as the standard entropy for ethoxy radical and suggested 74.5 cal mol –1 K –1 for the n -propoxy radical; they calculated 75.29 cal mol –1 K –1 for the n -propoxy radical based on group additivity. In this study, we report the standard entropy of methyl hydroperoxide, ethyl hydroperoxide, n -propyl hydroperoxide, and n -butyl hydroperoxide as (in cal mol –1 K –1 ) 66.27, 77.08, 87.35, and 97.71, respectively, which also follow group additivity.…”

“…There are a number of calculated values for the enthalpy of formation data on smaller (C1–C6) alkyl peroxides and the corresponding peroxy radicals, and these values are widely used and applied in modeling combustion , and atmospheric chemistry . They are also used in understanding oxidation chemistry of organic chemicals to form hydroperoxides for synthesis…”

Theoretical study of the geometrical and electronic structures and thermochemistry of spherophanes

“…The B3LYP method has proven its accuracy in determining geometrical parameters [29,30], electronic energies and thermochemical properties [31][32][33][34], and is used widely for studying structures and the stability of large molecules, fullerenes, and supramolecules [22,29,35,36]. The basis sets 6-31G and 6-31G(d) have also been recommended for purposes such as the present study.…”

Dynamics of the CH3 + OH reaction

“…(A1). Spectroscopic and potential constants are listed in Table I 22,28–48. The H m, i – H m, j interaction in CH 3 can be estimated in terms of the Lennard–Jones parameters for CH 3 ⋅⋅⋅CH 3 , ε H m / k B = 144 K and σ H m = 3.8 Å 49, where i , j are deleted for convenience.…”