Thermochemical Properties of Hydroxycyclohexadienyl Peroxy Isomers from Reaction of O2 with the Benzene-OH adduct
Authors: Chen Chiung-Chu , Bozzelli Joseph W. , Krasnoperov Lev N. 
Published: June 2015

The addition of OH radical to benzene and other aromatic
species to form a cyclohexadienyl radical
(
C•HD-OH

), is the first
step for conversion (oxidation) of aromatic species in atmospheric
chemistry. Reaction of this

C•HD-OH

intermediate, with
ground-state molecular oxygen forms a number of
hydroxyl-cyclohexadienyl peroxy
(
CHD-OH
-OO•) isomers,
which further react to form aerosols. Ab initio and density functional
calculations were used to study the structures and thermochemical
properties (Δ

f

H
o
298, S
o(T)
and C

p
(T) of the hydroxyl-cyclohexadienyl peroxy isomers
(
CHD-OH
-OO•)
((i) para-cis-(Z) and para-trans-(E)
(OO• relative to the OH group);
(ii) ortho-cis-(Z) and ortho-trans-(E) at
pseudo-equatorial (e') and pseudo-axial (a') positions). The
thermochemical properties are important for modeling studies on the
kinetics of the aerosol formation. Molecular structures and vibration
frequencies were determined at the B3LYP/6-31G(d,p) level. Energy
calculations were performed at the CBS-lq, CBS-4,
G3(MP2)//B3LYP/6-31G(d,p), and
BLYP/6-311++G(2d,p)//BLYP/6-31G(d,p) levels. Enthalpies of
formation (Δ

f

H
o
298) were calculated at
each level using group balance isodesmic reactions. An analysis is
performed for entropy from inclusion of internal rotor analysis versus
use of torsion frequencies and conformer statistics. Standard entropy,
S
o(T), and heat capacity, C

p
(T), from vibrational,
translational, and external rotation contributions were calculated
using statistical mechanics based on the vibration frequencies and
structures. Hindered rotational contributions to S
o(T) and
C

p
(T) were calculated from the energy levels, where the internal
rotation potential was calculated at the B3LYP/6-31G(d) level. This
analysis of the hindered internal rotor contributions yields higher
entropy than the values calculated when the
rigid-rotor-harmonic-oscillator approximation (RRHO) applied for the
calculated torsion frequencies. Differences between the uses of RRHO
for torsions with correction for statistical mix of conformations,
versus hindered internal rotor analysis are as high as
3.5 cal mol
–1 K
–1 in entropy and
3.6 cal mol
–1 K
–1 in heat capacity at 50 ≤ T/K ≤ 5000. A number of intramolecular interactions are
found to stabilize the

CHD-OH
-OO• isomers:
(i) intramolecular hydrogen bonding (between the OH and the
peroxy OO• groups, OO–H–O),
(ii) a negative hyperconjugation where the lone pair on oxygen atom in
OH group is hyperconjugated with the antibonding σ
* orbital of adjacent C=C bond, (iii) an
anomeric interaction in which antibonding σ
* orbital of
C–O bond in a pseudo-axial position with
a neighboring double bond π
C–C. The eclipsing
steric repulsion between OH and OO• groups
counteract the stabilizing hydrogen bonding interaction. The
importance of these effects are evaluated in determining the
thermodynamic stability of the

CHD-OH
-OO•
isomers. Evaluations of data from the isodesmic reaction at
G3(MP2)//B3LYP/6-31G(d,p) level, results in the enthalpy of
formation of the most stable isomers of

CHD-OH
-OO• in a range of
− 0.47 to −3.62 kcal mol
–1 at 298 K (for
(Z)-o-II, −0.47 kcal mol
–1 and
(E)-o-II, −3.62 kcal mol
–1 ). The reaction
energies (Δ
r
H
o
298) of
hydroxycyclohexadienyl radical
(
C•HD-OH

) + O2 ↔ 
CHD-OH
-OO•
isomers were determined to be − 10.63 ∼ − 13.67
(− 10.56 ∼ − 13.34) kcal mol
–1. Entropy
(S
o
298) of

CHD-OH
-OO• was in range of
93.17 ∼ 96.18
(90.82 ∼ 95.55) cal mol
–1 K
–1. Data in
parentheses are based on RRHO/torsion frequency approximation with
correction for the mix of conformers. The literature values of
reaction energy (Δ
r
H
o
298 
K) and entropy (Δ
r
S
o
298 
K) were reported
−10.5 ± 1.3 kcal mol
–1 and
−33.9 ± 1.4 cal mol
–1 K
–1, respectively, based
on second-law analysis at temperature between 265 and
345 K.