Chapter II: Observation and assignment of the A–X electronic transition of chlorine-substituted hydrocarbon peroxy radicals of chlorine-substituted hydrocarbon peroxy radicals
Technique 3: Time-Resolved Broadband Cavity-Enhanced Absorption Spectroscopy The final chapter of this thesis, Chapter 9, describes a separate set of experiments carried out
3.4 Assignment of the spectra of peroxy radicals with C4 backbones
We did not attempt to calculate the structures of the peroxy radicals formed from chlorine- initiated oxidation of 2-butene, 3-methyl-1-butene, or 2-methyl-3-buten-2-ol (MBO-232) because the number of possible conformers and isomers becomes prohibitively large with
these molecules. We note, however, that the peroxy radicals formed from these three alkenes (shown in Figure 3) share the same characteristics as those for the C1 and C2 peroxy radicals studied in this work. The origin region is crowded with several overlapping peaks between 7000–7750 cm−1. Intensity decreases around 8000 cm−1, with evidence for some structure that likely corresponds to excitation of C–O–O bending modes in the excited state.
Then, above 8000 cm−1, intensity rises again in the region typically assigned to excitation of the O–O stretch.
In 2-butene, we observe three distinct peaks in the origin region, centered at 7365, 7479, and 7594 cm−1. This pattern is repeated in the O–O stretch region, with three peaks appearing at 8275, 8401, and 8532 cm−1. The three O–O vibrations probably correspond to the same conformer or isomer as the three absorbers in the origin region.
The origin region of 3-methyl-1-butene has several overlapping features, but the most notable is the sharp, intense feature centered at 7533 cm−1. This spectral feature is reminiscent of the sharp features observed in the origin region of allylic peroxy radical. Hydrogen abstraction of the hydrogen atom on the C3 carbon atom would lead to a stable tertiary radical site, which could ultimately form the allylic peroxy radical.
The spectrum of MBO-232 is highly congested, leading to absorption throughout the entire spectral region covered in this experiment.
4 Conclusions
The chlorine-substituted peroxy radicals studied in this work have characteristic spectra in the near-infrared. The appearance of several bands in the origin region of each spectrum indicated the presence of multiple conformers and isomers that form upon chlorine-initiated oxidation of alkenes. Much like previous work on fluorine-substituted peroxy radicals, we observed a red-shift in the position of the A–X electronic transition upon β-substitution of the peroxy radical with an electron-withdrawing halogen—however, not to the extent of the more-electrophilic fluorine atom.12
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Figure 1. Pulsed cavity ringdown instrument schematic.
Figure 2. Near-infrared cavity ringdown spectrum of the peroxy radicals formed from simple alkenes. The top panel shows the experimental spectrum collected during chlorine-initiated oxidation of ethene. The bottom panel shows the experimental spectrum collected during chlorine-initiated oxidation of propene. The origin regions have been fit as a sum of Gaussian functions.
20
10
0
Absorbance (ppm)
8500 8000
7500 7000
Wavenumbers (cm-1) 30
15
0
ethene
propene
7280 7353
7410 7520
7280 7328
7387 7446
7485 7536
7590 7636
7704 7783
Figure 3. Near-infrared cavity ringdown spectrum of the peroxy radicals formed from alkenes with C4 backbones. The panels show the peroxy radicals formed by chlorine-initiated oxidation of (top) 2- butene, (middle) 3-methyl-1-butene, (bottom) MBO-232.
30
15
0
8500 8000
7500 7000
Wavenumbers (cm-1) 40
20
0
Absorbance (ppm)
40
20
0
2-butene
3-methyl-1-butene
MBO-232
Figure 4. Geometries of the four unique conformers of the chloro- ethyl peroxy radical. The labels indicate the C–C–O–O and Cl–C–C–
O dihedral angles, respectively, as trans (T) or gauche (G).
Figure 5. Experimental spectrum of the A–X electronic transition of chloro-ethyl peroxy radical (offset in the y-direction by 20 ppm) along with the computed Franck-Condon spectra of the four local minimum structures. The origin transitions were set to the values obtained at EOMEE-CCSD/aug-cc-pVDZ. The Franck-Condon spectrum was simulated from normal modes computed at TD- B3LYP/aug-cc-pVDZ, with frequencies scaled by 0.970. Intensities were weighted by Boltzmann factors computed using the G2 method.
40
30
20
10
0
Abs (ppm)
8500 8000
7500 7000
Wavenumbers (cm-1)
TT GT TG GG
TT 0-0 GT 0-0 TG 0-0 GG 0-0 GT 13
GT 17 GG17 GG 12
TG 12
TT 13 GT 12
GT 14
GT 13
GT 19
Figure 6. Geometries of the seven unique conformers of the 1-chloro propyl peroxy radical (top) and the seven unique conformers of the 2-chloro propyl peroxy radical (bottom). The labels indicate the C–
C–O–O, Cl–C–C–O, and Cl–C–C–C dihedral angles, respectively, as trans (T) or gauche (G).
Figure 7. Experimental spectrum of the A–X electronic transition of chloro-propyl peroxy radical (offset in the y-direction by 30 ppm) along with the computed Franck-Condon spectra of the 14 local minimum structures (spectra for the 1-chloro propyl peroxy radical conformers are offset in the y-direction by 10 ppm). The origin transitions were set to the values obtained at TD-B3LYP/aug-cc- pVDZ. The Franck-Condon spectrum was simulated from normal modes computed at TD-B3LYP/aug-cc-pVDZ, with frequencies scaled by 0.970. Intensities were weighted by Boltzmann factors computed using the G2 method, with additional weighting of the 2- chloro propyl peroxy radical spectrum by 1/6th to account for preferred addition of the chlorine atom to the primary carbon.
50
40
30
20
10
0
Abs (ppm)
8500 8000
7500 7000
Wavenumbers (cm-1)
1-chloro propyl peroxy GTG
GTG' TTG TGG' GGG' TGT GGT 2-chloro propyl peroxy
TTG' GTG GTG' TG'G TGT GGG' GGT
Table 1. Origin frequencies for the chloro-ethyl peroxy radical. Experimental origins for all peaks with centers < 7550 cm−1 are listed. The theoretical origins are listed for the four conformers, along with the A state vibrations of the C–O–O bend and O–O stretch modes.
Energies and vibrations are given in cm−1. experimental
origina
theoretical originb
bendc stretchc
A 7280 TT 7229 379 1003
B 7353 GT 7362 430 980
C 7410 TG 7390 298 1007
D 7520 GG 7542 373 990
aThe origin region was fit with the sum of four Gaussian functions, each with FWHM = 50 cm−1. Reported here are the center frequencies.
bThe origin for each conformer was computed as the adiabatic energy difference between the ground state energies of the X and A states, using EOM-EE/CCSD/aug-cc-pVDZ. The adiabatic energy difference was corrected by comparison to the same calculations for the ethyl peroxy radical, as described in the text.
cThe vibrational frequencies for the A state of each conformer were calculated using TD-DFT B3LYP/aug-cc- pVDZ. Vibrations were scaled by 0.970.26
Table 2. Ground state degeneracies (g) and energy differences for conformers of chloro- ethyl and chloro-propyl peroxy radicals. Energy differences are given in cm−1 and include zero-point energy corrections unless otherwise noted.a
chloro-ethyl peroxy radical
Conformer g B3LYP/aug-cc-pVDZ CCSD/aug-cc-pVDZa G2
TT 1 253 355 346
GT 2 0 0 0
TG 2 214 377 322
GG 1 73 96 91
1-chloro propyl peroxy radical
Conformer g B3LYP/aug-cc-pVDZ G2
GTG 2 0 0
GTG’ 2 282 164
TTG 2 268 298
TGG’ 2 425 414
GGG’ 2 197 148
TGT 2 233 299
GGT 2 327 237
2-chloro propyl peroxy
Conformer g B3LYP/aug-cc-pVDZ G2
TTG’ 2 214 386
GTG 2 0 0
GTG’ 2 187 207
TG’G 2 555 645
TGT 2 379 508
GGG’ 2 591 522
GGT 2 211 187
aCCSD energies are not corrected for zero point energies.
Table 3. Comparison of X and A state bond lengths (in Å) and adiabatic A–X electronic transitions (cm−1) calculated for the chloro-ethyl peroxy radical Cs conformer. Energies were not corrected for zero-point energies for ease in comparison, as CCSD frequency calculations were too computationally-expensive.
HF B3LYP MP2 CCSD TD-
B3LYP
EOM- CCSD
2A'' dO–O 1.301 1.323 1.315 1.335 1.313 1.325
2A' dO–O 1.362 1.390 1.393 1.412 1.373 1.406
E (A–X) 4593 7486 6846 6894 8894 7558
Scaled E (A–X) 7304 7229
Table 4. Origin frequencies for the chloro-propyl peroxy radical. Experimental origins for all peaks with centers <7800 cm−1 are listed. The theoretical origins are listed for the 1-chloro and 2-chloro propyl peroxy conformers. Energies are given in cm−1.
experimental origina
1-chloro propyl peroxy
conformer
1-chloro propyl peroxy origin
2-chloro propyl peroxy conformer
2-chloro propyl peroxy
origin
A 7280 GTG 7382 TTG’ 7282
B 7328 GTG’ 7480 GTG 7294
C 7387 TTG 7484 GTG’ 7353
D 7446 TGG’ 7531 TG’G 7359
E 7485 GGG’ 7533 TGT 7427
F 7536 TGT 7602 GGG’ 7469
G 7590 GGT 7682 GGT 7520
H 7636
I 7704
J 7783
aThe origin region was fit with the sum of ten Gaussian functions, each with FWHM = 35 cm−1. Reported here are the center frequencies.
bThe origin for each conformer was computed as the adiabatic energy difference between the ground state energies of the X and A states, using TD-DFT B3LYP/aug-cc-pVDZ. The adiabatic energy difference was corrected by comparison to the same calculations for the ethyl peroxy radical, as described in the text.
6 Supporting Information