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Faculty of Science - Department of Chemistry - Division of Quantum Chemistry and Physical Chemistry
KatholiekeUniversiteit
Leuven
Structure-Activity Relationships -
Mechanism development
Luc Vereecken
Research group on reaction kineticsDepartment of Chemistry
Quantum Chemistry and Physical ChemistryK.U.Leuven, Belgium
Structure activity relationships : WP2
Introduction
Task 2.1 : alkoxy decomposition and isomerisation
Task 2.2 : Site-specific NO3 and OH addition on alkenes
Task 2.3 : O3 cycloaddition on alkenes
Task 2.4 : H-abstraction by OH from hydrocarbons
Mechanism development : WP3 - WP5
Task 3.1 : OH + -pinene
Task 3.2 : O3 + -pinene, -humulene, -caryophyllene
Task 5 : Oxygenates + OH : T,P-dependent mechanism
Chemical mechanisms for modeling
Introduction - SARs
Large, explicit mechanisms (e.g. MCM)100s to 1000s of reactions/compounds
But no direct experimental or theoreticaldata on many of these
Use of SAR’s, predictive correlations
Increasing demand for ever-better accuracy Policy-supporting predictions, what-if analyses: - Smog-episodes, chemical weather, climate - Emission control (compounds and quantities)
Need for accurate Structure Activity Relationships
SAR’s and correlations
Structure-Activity Relationship or Predictive Correlation:
Good predictive accuracyEasy to useContinuous development
Working model:
Independent, additive site-specific rate coefficientsktot = ksite (even for different types of reaction)
Most rate coefficients depend primarily on local effects Inductive, hyperconjugative effects don’t carry very far H-bonds, resonances, … must be treated explicitly
Linear models are easy to work with
Addition of OH-radicals on (poly-)alkenes
Introduction
OH-addition on (poly-)alkenes
AlkenesThe rate of addition depends mainly on the substituents of the radical site Cb after addition : X3 X4
OH
Ca CbOH .
Ca Cbk i
x3
x4
x1
x2 x4x2
x1
x3kprim = 0.4510-11 cm3 s‑1
ksec = 3.010-11 cm3 s‑1
ktert = 5.510-11 cm3 s‑1
Conjugated Alkenes : some contribution from second radical siteksec/prim = 3.010-11 cm3 s‑1
ksec/sec = 3.810-11 cm3 s‑1
ksec/tert = 5.110-11 cm3 s‑1
ktert/prim = 5.710-11 cm3 s‑1
ktert/sec = 8.310-11 cm3 s‑1
ktert/tert = 9.910-11 cm3 s‑1
C CH C CR
R
ROH
kC CH C C
OH R R
R
.
C CH C C
R R
R
OH.
resonance
sec/tert
OH-addition on (poly-)alkenes
0 5 10 15 20 250
5
10
15
20
25
k SA
R /
10
-11 c
m3 s
-1
kexp
/ 10-11 cm3 s-1
Non-cyclic compounds: Average deviation 9%
All compounds: Average deviation 13% Max. deviation 54%
Can this be improved ? Yes
Residual errors mostly due to H-abstraction contributions
Publication submitted to J. Phys. Chem. A
OH-addition on (poly-)alkenes
0 10 20 300
10
20
30
40
Addition Addition+abstraction
k SA
R /
10-1
1 cm
3 mol
ec-1
s-1
kexp / 10-11 cm3 molec-1 s-1
Linear and mono-cyclic compounds
OH-addition on (poly-)alkenes
0 10 20 300
10
20
30
40
Addition Addition+abstraction
k SA
R /
10-1
1 cm
3 mol
ec-1
s-1
kexp / 10-11 cm3 molec-1 s-1
+ bicyclic and (near-)conjugated compounds
H-abstraction by OH-radicals
Introduction
H-abstraction by OH radicals
-15
-14
-13
-12
-11
-10
60 70 80 90 100 110D(C-H) / kcal mol-1 ( B3LYP-DFT/6-31G(d5d,p) )
log
(k a
bs
tr /
cm
3 m
ole
c-1 s
-1 )
p
er H
AlkanesAldehydesAlcoholsEthersketones/aldehydesAcidsAlkenesAlkadienesHyperconjugationVinoxy resonance
super-allyl
allyl
vinoxy
hyperconjugation
H-abstraction by OH radicals
Excellent correlation with bond strengthRate coefficient of abstraction determined by D(CH)Correlation is non-linear (data can be fitted by quadratic eq.)
log (k298K) = -0.00328D2 + 0.3869D - 19.392
Resonance stabilization shifts curve: e.g. vinoxy stabilisationlog (k298K) = -0.00315D2 + 0.3840D – 21.860
Dependence similar for all compoundsAngle and curvature similar for all resonances:
Hyperconjugation, allyl, super-allyl, vinoxy.In 1st order approximation: use same value for all
Different resonance stabilizations have different shift
Correlation will break down for oxygenates/H-bonding at low TAt room temperature: Carboxylic acids are already different
Addition of NO3-radicals on (poly-)alkenes
Introduction
NO3-addition on (poly-)alkenes
Addition of NO3 radicals: double interactionThe rate of addition depends on substitution on both carbons:
Fprim = 1.2810-8 cm3/2 s‑1/2 fprim = 1.2810-8 cm3/2 s‑1/2
Fsec = 7.2710-7 cm3/2 s‑1/2 fsec = 3.3010-7 cm3/2 s‑1/2
Ftert = 3.8510-5 cm3/2 s‑1/2 ftert = 7.0210-7 cm3/2 s‑1/2
N
O
OO N O
O
OF
fRadical site: factor FAddition site: factor f
kadd = F f
kadd,site = F f kadd,tot = ksite
Open questions: - Corrections for allyl-resonance stabilization of radical - H-abstraction (e.g. with allyl-resonance stabilization)
NO3-addition on (poly-)alkenes
1E-16 1E-15 1E-14 1E-13 1E-12 1E-11 1E-101E-16
1E-15
1E-14
1E-13
1E-12
1E-11
1E-10
k NO
3(SA
R)
/ cm
3 s-1
kNO3
(exp) / cm3 s-1
Regular compounds Bicyclic compounds Conjugated alkenes (lin & cyc) Other
Average deviation 1.2
NO3-addition on (poly-)alkenes
1E-16 1E-15 1E-14 1E-13 1E-12 1E-11 1E-101E-16
1E-15
1E-14
1E-13
1E-12
1E-11
1E-10
k NO
3(SA
R)
/ cm
3 s-1
kNO3
(exp) / cm3 s-1
Regular compounds Bicyclic compounds Conjugated alkenes (lin & cyc) Other
Average deviation 2.2
NO3-addition on (poly-)alkenes
1E-16 1E-15 1E-14 1E-13 1E-12 1E-11 1E-101E-16
1E-15
1E-14
1E-13
1E-12
1E-11
1E-10
k NO
3(SA
R)
/ cm
3 s-1
kNO3
(exp) / cm3 s-1
Regular compounds Bicyclic compounds Conjugated alkenes (lin & cyc) Other
Systematic underestimation
NO3-addition on (poly-)alkenes
1E-16 1E-15 1E-14 1E-13 1E-12 1E-11 1E-101E-16
1E-15
1E-14
1E-13
1E-12
1E-11
1E-10
k NO
3(SA
R)
/ cm
3 s-1
kNO3
(exp) / cm3 s-1
Regular compounds Bicyclic compounds Conjugated alkenes (lin & cyc) Other
NO3-addition on (poly-)alkenes
k(alkene+NO3)
0
5E-15
1E-14
1.5E-14
2E-14
2.5E-14
0 2 4 6 8
# carbons in 1-alkene
rate
coe
ffici
ent c
m3
s-1
ktot(NO3+alkene)
ktot - kabstr(est) ~ kadd(SAR)
Possible influence of H-abstraction: e.g. series of 1-alkenes- Could be sizable for large hydrocarbons- Affected by addition followed by HNO3 elimination ?
NO3-addition on (poly-)alkenes
Addition to conjugated alkadienes: Substitution effect different than for OH-addition
(partial stabilisation of radical electron by allyl-resonance)
Underestimation seems different for linear and cyclicLinear: underestimation by 0.3Cyclic: underestimation by 0.1
Different addition scheme across -bonds ?
NO
O
ON
OO
O
NO
O
O
Allyl-resonance Interaction across -bonds
Decomposition of alkoxy radicals
Introduction
Alkoxy radical decomposition
Decomposition barrier depends mostly on , -substituents
A first version of this SAR was published as: J. Peeters, G. Fantechi, L. Vereecken, J. Atmos. Chem. 48, 59 (2004)
k(T) = × 1.8×1013 exp(-Eb/RT) s-1
Eb / kcal mol-1 = 17.5 + 2.1 n-alkyl + 3.1 n-alkyl + 8.0 n,-hydroxy + 8.0 n-oxo + 12.0 n-oxo
curvature for small Eb < 7 kcal mol-1 : Eb
' / kcal mol‑1 = Eb + 0.027 (9.0-Eb)2
Alkoxy radical decomposition
0
2
4
6
8
10
12
14
16
18
20
0 2 4 6 8 10 12 14 16 18 20
Eb(SAR) (E'b for < 7 kcal/mol)
Eb(e
xptl
; a
b in
itio
)
B3LYP-DFT/6-31G(d(6), p)
B3LYP-DFT/6-31G(d(5), p)
B3LYP-DFT/SVP
G2(MP2/SVP)
Experimental
Alkoxy radical decomposition
Current developments (in progress) :
- More quantum chemical methods6-31G(d,p), 6-311++G(2df,2pd), aug-cc-pVTZMPW1K, BB1K, MPWKCIS1K, (CC, Gx, QCI)
- Multi-rotamer TST with (modified) Arrhenius fit SAR for Ea, A, (n)
- More substituents (preliminary) / kcal mol-1:-OR : -9.1 -OR : -9.0-OOR : -7.5 -OOR : =C : +21.1 =C : +4.6-C=C : -5.0 -C=C : -9.6-ONO2 : -3.1 -ONO2 : -2.7 -ONO : -4.2 -ONO : -6.2
Alkoxy radical decomposition
Future work:
- Use multi-rotamer TST for alkoxy isomerisation (H-shift)L. Vereecken, J. Peeters, J. Chem. Phys. 119, 5159 (2003)
- Perform URESAM calculations on these systems: Pressure dependence
SAR for Troe Parameters: Fc, k0, …
O3 cycloaddition
No results yet, but see literature
Conclusions - I
OH-addition SAR: Very good accuracyCan only be improved by explicitly incorporating H-abstraction
H-Abstraction correlationVery good correlation with bond strengthCurvature and slope similar, delocalisation shifts curve
NO3 addition SARVery good accuracy for most compounds (1.2, 2.2)Conjugated alkenes are underpredicted delocalisation effects
Alkoxy decomposition SAR:Being extended (substituents and methodology)Data serves as basis for alkoxy isomerisation SAR
Four site-specific predictive SARs:
Part II: Mechanism developmentTerpenes and sesquiterpenes
Introduction - Mechanism development
OH-initiated oxidation of -pinene using traditional chemistry:
Chemistry of -pinene + OH
OHCH2OH
OO-pinene
promptring opening
70%
+OH
+O2
90%
NO
NO2
CH2OH
O
CH2OH
+ acetone
Prediction of 60 % acetone formation
Experiment: acetone yields 8% (Aschmann et al, 1998)2% (Orlando et al., 2000)13% (Wisthaler et al., 2001)
?
Peroxy ringclosure in isoprene / terpenes :
-pinene
-pinene
peroxy ring closure+ O2
OO
OO
OH
OH+ OHring opening+ O2
OO
OHOO
OO
OO+ OHring opening+ O2
OHperoxy ring closure+ O2
Chemistry of unsaturated (per)oxy radicals
Ring closure in -pinene + OHO
+NO
-NO2
+O2
+ CH2OHOH
CH2OH
OO
-pinene
promptring opening
30%
70%
+OH
CH2OH
OO
ringclosure
OO
CH2OH
+NO
-NO2
+O2
OO
CH2OHO syn: 7.09 kcal/mold
anti: 7.90 kcal/mold
10.2 kcal/mols
OO
O
OO
OHO
+ CH2OH
+NO
-NO2
CH2OH
O
CH2OH
O
CH2OH
O
O+NO
-NO2
+O2
2.8 kcal/mols
12.3 kcal/mols
O
CH2OH
O
+NO
-NO2
+O2O
O
O
+ CH2OH
CH2OH
O
+NO -NO2
+O2
CH2OH
O
O
anti; 5.70 kcal/mold
syn: 8.28 kcal/moldanti: 3.78 kcal/mold
syn: 3.98 kcal/mold
10.2 kcal/mols
O
O
+ CH2OH
O
O OH
OO
+NO
-NO2
+O2
O
HO
OHO
O
6 ring closure
5 ring closure
anti: 8.7 kcal/mold
syn: 5.05 kcal/mold
O
OHO
O
CH2OH + O2 CH2O + HO2
spont.
OHO
O+ acetone
HO
OHO
O
7.1 kcal/mols
0 kcal/mole
11 kcal/mold HO
OHO
O
*
HO
OHO
O
HO
OHO
- CO
collision
activated
56 kcal/mold + Etherm
+NO
-NO2
+O2
HO
OHO
HO
- CH2O
11.3 kcal/mols
HO
OO
HO
HO
OHO
O
+O2-HO2
2.76 kcal/mold
+NO
-NO2
+O2
- CO2
5.5 kcal/mols
8.15 kcal/mold
HO
OHO
HO
HO
OHO
HO
11.26 kcal/mold
H-shift
+ O2
- HO2
O O
OHO
O O
OHO
O
+O2
O O
OO
HO
O
+NO
-NO2
+O2
O O
OHO
HO
O O
OHO
HO
O O
OHO
HO
-HO2
3 kcal/mole
9.5 kcal/mole
7 kcal/mole
O O
OHO
- CH2O
11.3 kcal/mols
syn: 5.15 kcal/mold
anti: 1.9 kcal/mols
syn: 4.04 kcal/mold
syn: 4.10 kcal/mold
anti: 2.96 kcal/mold
Mininum C
Mininum B
16.19 kcal/mold
O
CH2OH
O
HO
H-shift
ringclosesyn: 13.3 kcal/mold
activated
46 kcal/mold + Etherm
ether ringopenanti: 18.3 kcal/mold
- acetone
CH2OH
+O2
-pinene + OH
O
+NO
-NO2
+O2
+ CH2OHOH
CH2OH
OO
-pinene
promptring opening
30%
70%
+OH
+O2
Nopinone: 25 %
CH2OH
OO
ringclosure
+NO
-NO2
OO
CH2OH
+NO
-NO2
+O2
OO
CH2OHO
OO
OHO
O
OHO
Ospont.
HO
OHO
O
HO
OHO
+NO
-NO2
+O2
HO
OHO
O
HO
OO
HO
- COCH2OH
O
H-shift
+O2 -HO2
-pinene + OH
Peroxy ring closure path forms dicarbonyl dihydroxy compound
-pinene + OH
CH2OH
O
CH2OH
+ acetone
CH2OH
O
CH2OH
O
O
O
O
O O
OO
HO
About 4 %
Chemistry with oxy ring closure finds low acetone yield comparable to experimental findings
Compounds formed are highly oxygenated cyclic esters, formates
0%
10%
20%
30%
40%
50%
60%
70%
80%
10 31.6 100 316 1000 3162 10000 31620 100000 316226 1000000
-pinene + OH
10ppt
100ppt
1ppb
10ppb
100ppb
1ppm[NO]
Peroxy chemistry ROO + R’OO/HOO(pre- and post ring closure)
Peroxy ring closuredi-OH-di-carbonyl
Oxy ring closure
Degradation mechanism depends on [NO], [HO2/RO2]
-pinene + OH
.
bpineneRC3
1
5
4
3
2
8
7
610
9
1. +O2
2. +NO -NO2
3. 1,6-H-shift+ 27% nitrate
1. +O2
2. +NO -NO2
3. breaking(3-4)+ 27% nitrate
1
5
4
3
2
8
7
610
9
OH
bpineneOH3RC10
.
1. +O2
2. +NO -NO2
3. 1,5-H-shift+13% nitrate .
1
5
4
3
2
8
7
610
9
OHHO
bpineneOH3OH10RC4
1. +O2
2. +NO -NO2
3. breaking(3-4)+ 28% nitrate
1. +O2
2. +NO -NO2
3. 1,5-H-shift+ 28% nitrate
.
1
5
4
3
2
8
7
610
9
OHHO
R7RC10
1. +O2
2. -HO2.
1
5
4
3
2
8
7
610
9
OHO
R7O10
.
1
5
4
32
8
7
610
9
OH
HO
O
R6RC3
1. +O2
2. +NO -NO2
3. breaking(2-3) -HCOOH
.
1
5
4
2
8
7
610
9
HO
O
S1RC2
1. +O2
2. +NO -NO2
.
1
5
4
2
8
7
610
9
HO
O
O
S1RO2
. 1
5
4
2
8
7
610
9
HO
O
O
S1RC1
High NO concentration, at laboratory conditions
. 1
5
4
32
8
7
610
9
OH
HO
O
R6RC1
.
1
54
32
8
7
610
9
OH
HO
O
OH1. +O2
2. +NO -NO2
3. 1,7-H-shift+ 14% nitrate
1
5
4
32
8
7
610
9
O
HO
OR6O3
1. +O2
2. -HO2.
1
5
4
3
2
8
7
610
9
O
.
P1RC4
1. +O2
2. +NO -NO2
3. 1,7-H-shift+ 13% nitrate
1
54
32
8
7
610
9
.
O
OH
P1OH4RC3
1. +O2
2. +NO -NO2
3. breaking(2-3) -CO2
1
54
2
8
7
610
9
.
OH
R1RC2
1. +O2
2. +NO -NO2
1
54
2
8
7
610
9
.
OH
R1RO2
O
1
54
32
8
7
610
9
. O
OH
P1OH4RC1
1
54
2
8
7
610
9
.
OH
R1RC1
O
1. +O2
2. +NO -NO2
3. breaking(1-2) -CH2O + 13% nitrate
54
2
8
7
610
9
.
OH
S5RC2
O
1. +O2
2. +NO -NO2
3. breaking(2-7) -CO2
54
8
7
610
9
.
OH
T1RC7
1. +O2
2. +NO -NO2
3. breaking(7-8)5
4
8
7
10
9
.OH
O6
U1RC8
1. +O2
2. +NO -NO2
3. 1,5-H-shift+ 9% nitrate 5
4
8
7
10
9
.
OH
O6
V1RC7
HO
1. +O2
2. +NO -NO2
3. breaking (6-7) -CO2 5
4
8
10
9
.
OH
6
W1RC6
HO
. 1
5
4
3
2
8
7
610
9
bpineneRC1
1. +O2
2. +NO
-NO2
3. +O2
4. -HO2.
+ 13% nitrate
1
5
4
3
2
8
7
610
9
bpineneO1
O
52%
48%
8%
92%
S2RC4
64%
36%
Minor H-abstraction channels (Klara Petrov)
Mainly formation of larger (multisubstituted) oxygenates. Larger products should nearly all be reactive to OH, O3, NO3
-pinene + O3
Other mechanisms
Some additional theoretical verification on impact of- ring closure- low-NOx chemistry
Mechanism sufficiently mature for modeling (see BIRA)
Sesquiterpenes + O3
No results yet
Oxygenates + OH
Introduction
General mechanism:
Oxygenates + OH
T,P-dependences:
Oxygenates + OH
Barriers above reactants: Formation of pre-reactive complexes not too important
Positive T-dependence (except at low T: tunneling)No P-dependence
Barriers below reactants: Chemical activation effects
Negative T-dependence at all TPressure dependent
See: Peeters and Vereecken, Int. Symp. Gas Kin. 2006
Specific issues for theoretical work on oxygenate+OH reactions
Oxygenates + OH
- Calculation of tunneling contributionsSmall-curvature corrections most often usede.g. Masgrau et al., J. Phys. Chem. A 106, 11760 (2002)
tunneling contribution 22 at 202 Kfor acetone+OH
- Variational effectsH-abstraction over H-bonds: low and broad TSVariational effects can be important
(kinetic bottleneck not at energy maximum)e.g. Masgrau et al. 2002 (acetone+OH)
variational effects up to order of magnitude
- Specific reaction pathways(See acids)
Acetone + OH
The reaction of acetone + OH shows a curved Arrhenius plot:
Wollenhaupt, Carl, Horowitz, Crowley, J. Phys. Chem. A 104, 2695 (2000)
Gierczak, Gilles, Bauerle, Ravishankara, J. Phys. Chem. A
107, 5014 (2003); Talukdar et al., J. Phys. Chem. A 107, 5021 (2003)
Acetone + OH
Theoretical work shows the general features of the PES:
Vandenberk, Vereecken and Peeters, PCCP 4, 461 (2002)
Similar PESes by Masgrau et al., J. Phys. Chem. A 106, 11760 (2002)Vasvári et al., PCCP 3, 551 (2001)
Hydroxyacetone + OH
The reaction of hydroxyacetone + OH : Negative T-dependence
Dillon, Horowitz, Hölscher, Crowley, Vereecken, Peeters, PCCP, 8, 236, 2006
Hydroxyacetone + OH
Accuracy of barrier heights did not allow for finaltheoretical kinetic predictions.
Glycolaldehyde + OH
The reaction of CH2OHCHO+ OH : No T-dependence
Karunanandan, Hölscher, Dillon, Horowitz, Crowley, Vereecken, Peeters, submitted for publication
-Slowdown relative to CH3CHO: due to charge distribution - Lack of T-dependence: due to specific barrier height:
200 300 400 500 600 700 8002.0
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3.0
k eff (
a.u
.)
Temperature / K
RRKM-MEsimulation
Stringent requirements for theoretical methodologies
Oxygenates + OH
Quantum chemical methods: very high level needed Calculation of energiesBut also for calculation of geometries and frequencies
Mechanism developmentUnexpected mechanisms can exist
Kinetic methodologies: Important effects ofTunneling (SCT or better needed)Variational effectsAnharmonicity effectsMulti-conformer (multi-well) effectsMultiple pathwaysInternal rotors
Conclusions - II
-pinene + OH Very complex reaction mechanismDepends strongly on [NOx] versus [ROO/HOO]Many fast unimolecular reaction steps
reduction of mechanism possibleIn progress
Terpenoids + O3
In progress
Oxygenates + OH :
Very complex kineticsStringent demands on theoretical methodologyT,P-dependence of k(T) or product distribution still difficult
Mechanism development