Pyridyl-and pyridylperoxy radicals-a matrix isolation study

Article abstract of DOI:10.1071/CH14149

The three isomeric pyridyl radicals 2a-c were synthesised using flash vacuum pyrolysis in combination with matrix isolation and characterised by infrared spectroscopy. The IR spectra are in good agreement with spectra calculated using density functional theory methods. The reaction of the pyridyl radicals 2 with molecular oxygen leads to the formation of the corresponding pyridylperoxy radicals 3a-c. The peroxy radicals 3 are photolabile, and irradiation results in syn-anti isomerisation of 3a and 3b and ring expansion of all three isomers of 3.

Full text of DOI:10.1071/CH14149

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2014, 67, 1324–1329
Full Paper
http://dx.doi.org/10.1071/CH14149
Pyridyl- and Pyridylperoxy Radicals – A Matrix
Isolation Study
A
A
A B
,
Andre´ Korte, Artur Mardyukov, and Wolfram Sander
A
Lehrstuhl fur Organische Chemie II, Ruhr-Universitat Bochum,
¨
¨
D-44781 Bochum, Germany.
B
Corresponding author. Email: wolfram.sander@rub.de
The three isomeric pyridyl radicals 2a–c were synthesised using flash vacuum pyrolysis in combination with matrix
isolation and characterised by infrared spectroscopy. The IR spectra are in good agreement with spectra calculated using
density functional theory methods. The reaction of the pyridyl radicals 2 with molecular oxygen leads to the formation of
the corresponding pyridylperoxy radicals 3a–c. The peroxy radicals 3 are photolabile, and irradiation results in syn–anti
isomerisation of 3a and 3b and ring expansion of all three isomers of 3.
Manuscript received: 14 March 2014.
Manuscript accepted: 7 May 2014.
Published online: 19 June 2014.
Introduction
The loss of a second hydrogen atom from the pyridyl
radicals 2 leads to the isomeric didehydropyridines (pyri-
dynes).[9] These species have been investigated by matrix iso-
lation spectroscopy,[10,11] gas phase ion techniques,[12,13] and by
theoretical methods.[14,15] Compared with the ring opening,
these processes are energetically less favourable and therefore
pyridynes are not discussed as intermediates in the combustion
of pyridine (1). Despite the interest in the chemistry of pyridyl
radicals 2, the only matrix isolation and spectroscopic charac-
terisation of these species was reported in 1972 by Kasai and
McLeod using electron paramagnetic resonance (EPR) spec-
troscopy for their detection.[16]
In the presence of molecular oxygen, pyridyl radicals 2
rapidly react to form pyridylperoxy radicals 3, which subse-
quently split off oxygen atoms to pyridyloxy radicals or rear-
range to a second generation of rather unstable radicals.[17–19]
The 2-pyridylperoxy radical 3a was generated by pulse radi-
olysis of 2-chloro- or 2-bromopyridine in the presence of air.[19]
This technique allowed recording of a transient spectrum of
3a with a maximum absorption at 440 nm. The mechanism of
the rearrangements of 3 was studied in detail by Fadden and
Hadad using density functional theory (DFT) methods.[18] In
pyridine flames, up to 80 products of the combustion of 1 were
detected by synchroton photoionisation techniques, which
demonstrates the complexity of these processes.[20]
To investigate reactions of radicals with O2 under the
conditions of matrix isolation, argon matrices can be doped
with small amounts (0.5–2 %) of O2. As long as the argon matrix
is kept at temperatures below 20 K, diffusion of O2 is prevented
and no reaction occurs. At temperatures above 25 K, the diffu-
sion of O2 becomes rapid, and the oxygenation can be directly
monitored by IR or UV-vis spectroscopy. To induce this type of
bimolecular reaction in matrices, it is mandatory to generate the
radicals by flash vacuum pyrolysis (FVP) rather than by photo-
lysis of a matrix-isolated precursor. The latter technique results
in radical pairs which are trapped in close proximity or even in
Nitrogen-containing heterocycles are important constituents of
coal, which on combustion produce large amounts of NOx.
Since coal is a highly complex mixture of compounds difficult
to study directly, pyridine (1) has served as a simple model to
investigate the combustion of nitrogen-containing hetero-
cycles. Loss of a hydrogen atom of 1 results in the formation of
the three isomeric pyridyl radicals 2 that, depending on the
conditions, could either react with molecular oxygen to give the
pyridyloxy radicals 3 (Scheme 1) or degrade further to small,
highly unsaturated fragments. The thermal decomposition of
pyridine has been investigated by a variety of methods,
including shock tube[1,2] and laser IR pyrolysis.[3] In the shock
tube pyrolysis cyanoacetylene was found as the principal
nitrogen containing product at 1300 K, whereas at 1800 K HCN
predominated.[1] Other products observed were acetylene and
hydrogen. These studies suggest that the primary step in the
thermal decomposition of pyridine is the loss of one hydrogen
atom to form pyridyl radicals 2, that subsequently ring-open to
form highly unsaturated secondary radicals. Both the 2-pyridyl
radical 2a and the 3-pyridyl radical 2b are involved in the for-
mation of the stable small molecules observed in these experi-
ments, whereas it was concluded that the 4-pyridyl radical 2c
results in polymerisation and soot formation.
The thermal decomposition of pyridine (1) to give the
isomeric pyridyl radicals 2 was also investigated in a series
of theoretical studies.[4–7] The CH dissociation energy of
the ortho CH bond in 1 is ,105 kcal molꢀ1, and thus is 5–
6 kcal molꢀ1 lower than that of the two other CH bonds and of
that of benzene.[2,8] The key step of the thermal decomposition
of 1 is therefore the formation of the 2-pyridyl radical 2a. The
next step of the decomposition with an estimated activation
barrier of 40.3 kcal molꢀ1 is the rupture of the C(5)–N bond in
2a to form a nitrile radical.[7] Breaking the C(3)–C(4) bond to
produce an alkyne radical is calculated to have a considerably
higher barrier.
Journal compilation Ó CSIRO 2014
www.publish.csiro.au/journals/ajc

Pyridyl Radicals
1325
ϩO2
quartz tube heated electrically with a tantalum wire. For
broadband irradiations mercury high-pressure arc lamps with
quartz optics, dichroic mirrors, and cut-off filters with 50 %
transmission at the specified wavelength were used.
All DFT calculationswereperformed withthe Gaussian09[22]
program package. The geometries were optimised using the
unrestricted Becke, three-parameter, Lee–Yang–Parr (UB3LYP)
method[23–26] in combination with the cc-pVTZ basis set.[27]
2-Iodopyridine (4a), 3-iodopyridine (4b), and 4-iodopyridine
(4c) were purchased from Sigma–Aldrich and used without
further purification. 2,20-Azopyridine (5a) and 4,40-azopyridine
(5c) were prepared according to literature procedures.[28]
O2
O2
N
N
2a
3a
ϩO2
ϪH
N
N
N
N
2b
2c
3b
3c
1
O2
ϩO2
3,30-Azopyridine (5b)
N
To a solution of 124mg (1.0mmol) of 3-nitropyridine in 20 mL of
ethanol, a solution of 160 mg (4.1mmol) of NaOH in water was
added dropwise. After addition of 260 mg (4.1 mmol) of zinc
powder the resulting suspension was heated to reflux for 16h. The
solution was filtered while hot and the solvent evaporated. After
chromatography (EtOAc/EtOH, 5 : 1) the crude product was
crystallised from hot water. The product was obtained as yellow-
orange needles with a melting point of 139–1408C; yield 36mg
(38 %). dH (200 MHz, CDCl3, 308C) 9.26(d, J 2.0, 1H), 8.79(dd, J
4.9, 1H), 8.30 (ddd, J 8.2, 2.3, 1.6, 1H), 7.59 (dd, J 8.2, 4.9, 1H).
m/z (70 eV) 184 [Mþ], 106 (36 %), 78 (70), 51 (64), 28 (100).
Scheme 1. Primary processes in the combustion of pyridine (1).
O
O
ϩO2
FVP
Ph
N N Ph
7
6
hν
Results and Discussion
O
The pyridyl radicals 2a–c were synthesised by FVP of the cor-
responding iodopyridines 4 or azopyridines 5 at temperatures
between 620 and 7008C with subsequent trapping of the pro-
ducts in argon at 10 K (Scheme 3). Both types of precursors
resulted in the formation of pyridine (1), acetylene, HCN, and
diacetylene as the major products. These products were identi-
fied by comparison of the matrix IR spectra obtained after the
FVP with literature data. The pyridyl radicals 2 were generally
formed in small amounts only.
The IR spectra recorded after FVP of 4a or 5a and trapping
of the products in an argon matrix exhibited a series of
weak absorptions at 1044, 941, 735, and 565 cmꢀ1 assigned to
the 2-pyridyl radical 2a by comparison with results from DFT
calculations (Fig. 1). Both precursors 4a and 5a produce the
same new bands, which confirms the assignment. Radical 2a
was also formed by 254 nm photolysis of matrix-isolated 4a.
Annealing of such a matrix at 30 K for several minutes results in
the recombination of radical 2a with the iodine atoms to give
back precursor 4a. Annealing of matrices containing 2a pro-
duced by FVP of 4a, on the other hand, does not lead to radical
recombination since the diffusion of both 2a and iodine atoms is
slow under these conditions.
O
C
O
hν
O
9
8
Scheme 2. Synthesis of the phenyl radical 2 and phenylperoxy radical 7.
the same matrix cage. Annealing of these matrices results in
recombination of the radical pair, rather than in bimolecular
reactions with O2. In contrast, FVP of the precursor with
subsequent trapping of the products in matrices results in
radicals that are trapped in individual matrix cages. Annealing
now preferentially leads to bimolecular reactions with O2.
This technique was successfully used in our laboratory for
the synthesis and matrix isolation of the phenylperoxy radical
7.[21] FVP of azobenzene produces phenyl radicals 6 which were
trapped in argon matrices doped with 0.5–1 % O2. Under these
conditions only small fractions of 6 react with O2. Annealing of
the matrix at 30–35 K allows the diffusion of O2 and formation
of the peroxy radical 7 (Scheme 2). Photolysis of 7 results in
insertion of one oxygen atom into the ring to give lactone 8,
which on prolonged irradiation opens the ring to ketene 9.
The same technique should also be applicable for the
synthesis of pyridyl radicals 2 and pyridylperoxy radicals 3.
Here, we describe the matrix isolation and spectroscopic char-
acterisation of the three isomeric radicals 2 and their oxygen
trapping products 3.
The 3-pyridyl radical 2b wasgeneratedinasimilarwaybyFVP
of 4b and 5b attemperatures above 6008C. In addition tounreacted
starting material and products such as acetylene, diacetylene, and
HCN, a band at 671.7 cmꢀ1 and several weaker absorptions are
assigned to 2b (Fig. 2). These bandsrapidly decrease in intensity if
the matrix is irradiated with visible light (l $ 420 nm). Simulta-
neously, bands in the region between 3300 and 3100 cmꢀ1 appear,
indicating the formation of terminal alkynes.
Experimental
Matrix isolation experiments were performed by standard
techniques using closed-cycle helium cryostats. Matrices were
deposited at 3 K and annealing experiments performed between
25 and 35 K. FTIR spectra were recorded in the range between
400 and 4000 cmꢀ1 with a resolution of 0.5 cmꢀ1. FVP was
carried out by slowly subliming the precursor through an 8 cm
The 4-pyridyl radical 2c was obtained by FVP of 4c or 5c.
The pyrolysis of 5c produces radical 2c in moderate yields and
only small amounts of side products. The most intense absorp-
tion in the IR spectrum of 2c is the out-of-plane CH deformation
mode at 750 cmꢀ1 (Fig. 3). These absorptions are in good
agreement with the IR spectrum of 2c, calculated at the

1326
A. Korte, A. Mardyukov, and W. Sander
hν or Δ
ϪI
3a
3b
2a
2b
2c
I
O2
O2
N
N
N
N
4a: ortho
4b: meta
4c: para
ϩO2
N
O2
Δ
3c
N
N
ϪN2
N
N
N
N
5a: ortho, ortho'
5b: meta, meta'
5c: para, para'
ϩ"H"
1
N
Scheme 3. Synthesis of the pyridyl radicals 2 and pyridylperoxy radicals 3.
(c)
12
8
6
5
3
A
19
18 17
11
9
6
(c)
A
N
HCN
1
N
2a
?
N
1
N
2c
(b)
(a)
A
A
A
(b)
(a)
N
4a
I
N
4c
A
1100
1000
900
800
700
600
ν [cmϪ1
∼
]
1600
1400
1200
1000
800
600
ν [cmϪ1
]
∼
Fig. 1. Matrix IR spectra showing the flash vacuum photolysis (FVP) of
2-iodopyridine 4a. (a) 4a in argon at 10 K. (b) FVP of 4a at 6208C with
subsequent trapping of the products in argon at 10 K. (c) IR spectrum of 2a
calculated at the UB3LYP/cc-pVTZ level of theory.
Fig. 3. Matrix IR spectra showing the flash vacuum photolysis (FVP) of
4-iodopyridine (4c). (a) 4c in argon at 10 K. (b) FVP of 4c at 6208C with
subsequent trapping of the products in argon at 10 K. (c) IR spectrum of 2c
calculated at the UB3LYP/cc-pVTZ level of theory.
20 19 17
12
9
6
5
3
(d)
(c)
A
A
corresponding pyridyl radicals 2 at temperatures between 25 and
35 K. At these temperatures the diffusion of O2 in argon becomes
rapid, and thus bimolecular reactions are possible as long as the
activation barrier is very small or zero. The disappearance of 2
and formation of 3 can be directly monitored by IR spectroscopy.
After several minutes of annealing to allow for the diffusion of
O2, the matrices are generally cooled back to 10 K. To study the
photochemistry of the peroxy radicals 3, the matrices were irra-
diated at 10 K with visible and UV light at various wavelengths.
The relative energies of the regio isomers and conformers of 3 as
well as the transition states connecting the conformers were
calculated at the UB3LYP/cc-pVTZ level of theory (Fig. 4).
N
2b
(b)
(a)
A
A
I
N
4b
1600
1400
1200
1000
800
600
ν [cmϪ1
]
∼
2-Pyridylperoxy Radical 3a
Fig. 2. Matrix IR spectra showing the flash vacuum photolysis (FVP) of
3-iodopyridine (4b). (a) 4b in argon at 10 K. (b) FVP of 4b at 6208C with
subsequenttrapping of the products in argon at 10 K. (c) Difference spectrum
after irradiation (l $ 420 nm) of a matrix containing 2b. Bands pointing
downwards are disappearing during irradiation and are assigned to 2b. (d) IR
spectrum of 2b calculated at the UB3LYP/cc-pVTZ level of theory.
The 2-pyridylperoxy radical 3a was synthesised by annealing
argon matrices containing 2a and 2 % oxygen. New absorptions
at 1599, 1465, 1429, 1301, 1260, 1177, 1125, 1039, 986, 804,
775, and 727 cmꢀ1 are assigned to 3a (Fig. 5). If 18O2 is used in
the experiments, the bands at 1125 and 804 cmꢀ1 show large
isotopic shifts of ꢀ56 and ꢀ12 cmꢀ1, respectively, as expected
for the O–O and C–O stretching vibration (Figs S1 and S2 in
the Supplementary Material). All other bands of 3a show
UB3LYP/cc-pVTZ level of theory. FVP of the deuterated
precursor d4-5c yields the deuterated pyridyl radical d4-2c.
isotopic shifts of less than 1 cmꢀ1
.
Pyridylperoxy Radicals
According to DFT calculations (UB3LYP/cc-pVTZ) the
bimolecular reaction of 2a with oxygen is exothermic by
43.1kcal molꢀ1, and the anti-conformer of 3a is 1.8 kcal molꢀ1
The pyridylperoxy radicals 3a–c were generated by annealing
argon matrices doped with 1–2 % oxygen containing the

Pyridyl Radicals
1327
more stable than the syn-conformer (Scheme 4, Fig. 4). The
twoconformersareseparatedbyanactivationbarrierof4.46 kcal
molꢀ1. The IR spectrum of 3a is in very good agreement with the
spectrum calculated for anti-3a (Table S7, Supplementary Mate-
rial), whereas the less stable syn-3a is not observed (Table S8,
Supplementary Material). Visible light irradiation (. 415 nm),
however, results in the formation of syn-3a and disappearance of
the anti-isomer (Fig. 6). Subsequent annealing of the matrix for
several minutes at 30 K again yields the spectrum assigned to
anti-3a. This clearly demonstrates that the activation barrier for
the isomerisation of syn-3a to anti-3a is small enough to be
overcome at temperatures as low as 30 K, and that the anti-
isomer is thermodynamically more stable than the syn-isomer, in
agreement with the predictions from the DFT calculations.
Prolonged visible light irradiation (l . 415 nm) results in
the disappearance of 3a and formation of a new product with
a strong absorption at 1701 cmꢀ1 which is red-shifted by
ꢀ23cmꢀ1 if 18O2 is used in the experiment. By comparison with
DFT calculations the new product is assigned to the cyclic
carbamate radical 10 (Fig. 7). Radical 10 is also photolabile,
and continuous photolysis with l . 415 nm leads to new bands in
the region between 2350 and 2050cmꢀ1 which indicates ring-
opening and formation of alkynes or nitriles. Due to the complex-
ity of these spectra we were not able to identify these products.
3-Pyridylperoxy Radical 3b
The reaction of 2b with oxygen was carried out in a 0.5–2 %
O2-doped argon matrix. During annealing at 35 K several IR
absorptions assigned to the 3-pyridylperoxy radical 3b increase
in intensity (Fig. 8). Two absorptions at 799 and 796 cmꢀ1 are
assigned to the out-of-plane deformation modes of the two
conformers syn-3b and anti-3b, respectively. The assignment
of the IR bands of 3b were confirmed by 18O2 isotopic labelling
and comparison to the results of DFT calculations. Both
26 25
24 23
20 18
16 14
1110 (c)
A
O
O
N
(b)
syn-3a
A
9.0
8.28
8.09
TS-3b
TS-3c
O
O
O
O
O
N
O
N
anti-3a
O
6.0
3.0
0
O
5.01
(a)
N
4.46
A
syn-3b
4.34
26 25
24 23
20 18
1200
16 14
1110
N
3.49
TS-3a
anti-3b
1600
1400
1000
800
O
O
N
3c
ν [cmϪ1
]
∼
1.84
syn-3a
O
N
0
O
Fig. 6. Matrix IR spectra showing the photochemical conversion of anti-3a
into syn-3a. (a) Spectrum calculated for anti-3a at the UB3LYP/cc-pVTZ
level of theory. (b) Difference spectrum showing the l . 415 nm photo-
chemistry. Peaks pointing downwards are disappearing; peaks pointing
upwards are appearing. (c) Spectrum of syn-3a calculated at the UB3LYP/
cc-pVTZ level of theory.
anti-3a
Fig. 4. Relative energies of the syn and anti conformers of the pyridyl-
peroxy radicals 3a–c and of the transition states connecting the conformers
calculated at the UB3LYP/cc-pVTZ level of theory. Values are given in
kcal molꢀ1 relative to the most stable isomer anti-3a.
A
(c)
(b)
O
O
N
(c)
20
16
10
18
A
10
N
1
O
N
O
anti-3a
A
A
(b)
A
A
O
N
O
HCN
anti-3a
(a)
I
N
4a
26 25 24 23
20 18
1200
11 10
800
(a)
2000
1800
1600
1400
1000
ν [cmϪ1
]
∼
1200
1100
1000
900
]
800
700
ν [cmϪ1
∼
Fig. 7. Matrix IR spectra showing the long-time photochemistry
(l . 415 nm, 7 h) of anti-3a in argon/2 % O2. (a) Spectrum of anti-3a
calculated at the UB3LYP/cc-pVTZ level of theory. (b) Difference spectrum
after prolonged irradiation with visible light (l . 415 nm, 7 h). (c) Spectrum
of the carbamate radical 10 calculated at the UB3LYP/cc-pVTZ level of
theory.
Fig. 5. Matrix IR spectra showing the formation of the 2-pyridylperoxy
radical 3a. (a) Flash vacuum photolysis (FVP) of 4a and deposition in argon at
10K. (b) FVP of 4a and deposition in argon containing 2 % oxygen at 10 K.
(c) IRspectrum ofanti-3acalculatedatthe UB3LYP/cc-pVTZlevel of theory.
O
O
λ Ͼ 415 nm
ϩO2
λ Ͼ 415 nm
10 K
N
O
N
O
30K
N
N
O
O
10
2a
anti-3a
syn-3a
Scheme 4. Photochemistry of pyridylperoxy radical 3a.

1328
A. Korte, A. Mardyukov, and W. Sander
(c)
1918 17
15
13
11
9
8
7
A
A
(b)
6
26 25 24 23
18
14
1110
O
O
N
N
HCCH
1
anti-3b
O
O
N
(b)
(a)
3c
A
A
I
HCN
A
N
4b
(a)
600
1200
1100
1000
900
800
700
600
500
1600
1400
1200
1000
800
ν [cmϪ1
∼
]
ν [cmϪ1
]
∼
Fig. 8. Matrix IR spectra showing the formation of the 3-pyridylperoxy
radical 3b. (a) Flash vacuum photolysis (FVP) of 4b and deposition inargon at
10 K. (b) FVP of 4b and deposition in argon containing 2 % oxygen at 10K.
(c)IRspectrum ofanti-3b calculatedattheUB3LYP/cc-pVTZleveloftheory.
Fig. 10. Matrix IR spectra showing the formation of the 4-pyridylperoxy
radical 3c. (a) Flash vacuum photolysis (FVP) of 4a and deposition in argon
containing 2 % oxygen at 10 K. (b) IR spectrum of 3c calculated at the
UB3LYP/cc-pVTZ level of theory.
(c)
26 25
24 23
20 19
15
11
9
8
A
A
(c)
(b)
26 25 24 23
18
11 10
6
A
A
O
O
N
syn-3b
(b)
O
O
O
O
N
anti-3b
(a)
7
N
A
3c
26 25
1600
24 23
20 19 17
1200
ν [cmϪ1
∼
15
11
9
8
(a)
1400
1000
800
600
A
]
2200 2000 1800 1600 1400 1200 1000
800
600
ν [cmϪ1
]
∼
Fig. 9. Matrix IR spectra showing the photochemical conversion of anti-
3b into syn-3b. (a) Spectrum calculated for anti-3b at the UB3LYP/cc-pVTZ
level of theory. (b) Difference spectrum showing the l . 550 nm photo-
chemistry. Peaks pointing downwards are disappearing; peaks pointing
upwards are appearing. (c) Spectrum of syn-3b calculated at the UB3LYP/
cc-pVTZ level of theory.
Fig. 11. Matrix IR spectra showing the photochemistry of 3c. (a) Flash
vacuum photolysis (FVP) of 4a and deposition in argon containing 2 %
oxygen at 10 K. (b) Difference spectrum after irradiation with visible light
(l $ 420 nm). (c) IR spectrum of 3c calculated at the UB3LYP/cc-pVTZ
level of theory.
conformers syn-3b and anti-3b are formed during annealing,
which is in contrast to 3a where only the anti-conformer is
formed under similar conditions. This observation is in accor-
dance with the DFT calculations, which give a slight energy
difference of 0.7 kcal molꢀ1 between the conformers (the anti-
conformer being more stable) and an activation barrier of
3.9 kcal molꢀ1 for the anti–syn isomerisation. Irradiation with
visible light (l . 550 nm) leads to a decrease of anti-3b and a
simultaneous increase of syn-3b (Fig. 9). This isomerisation is
reversed by irradiation with l . 515 nm. Obviously, the two
conformers are formed in photostationary equilibria with the
relative amounts depending on the irradiation wavelength.
Prolonged irradiation results in the formation of intense
bands between 1750 and 1680 cmꢀ1, and simultaneous disap-
pearance of 3b. The new bands in the carbonyl-stretching region
suggest that ring expansion products are formed, but a definitive
assignment was not possible. Experiments carried out with 18O2
show the expected red-shifts of the carbonyl absorptions.
1572, 1484, 1411, 1220, 1150, 1119, 990, 817, 808, and
508 cmꢀ1 are assigned to 3c (Fig. 10). If 18O2 is used in the
experiments, the bands at 1119 and 808 cmꢀ1 are red-shifted by
ꢀ57 and ꢀ16 cmꢀ1, respectively, and therefore assigned to the
O–O and C–O stretching vibrations of 3c. As expected, only one
conformer of 3c is observed.
Irradiation of 3c with blue light (l . 420 nm) results in the
decrease of all bands assigned to 3c, and formation of a strong
and broad absorption at 2144 cmꢀ1 is observed. When 18O2 is
used in the experiments, new bands at 2118 and 2089 cmꢀ1 are
found, together with the peak of C18O2 at 2310 cmꢀ1. This
suggests that a ketene is formed. Due to the low intensity of the
other IR bands, a definitive assignment of the ketene was not
possible (Fig. 11).
Conclusion
4-Pyridylperoxy Radical 3c
Pyridylradicals 2 can be synthesised by FVP of iodopyridines 4
or azopyridines 5 and trapping of the products in argon matrices.
By-products of the FVP are pyridine 1 and fragmentation
The 4-pyridylperoxy radical 3c was generated by annealing
of O2-doped matrices containing radical 2c. New bands at 1600,

Pyridyl Radicals
1329
products, in particular acetylene, HCN, and diacetylene. The
fragmentation is driven by the formation of the very stable CN
triple bond, and therefore the yield of pyridyl radicals is lower
than that of phenyl radicals 6 from the analogous precursors
iodobenzene or azobenzene. By carefully analysing the spectra
and comparison with results from DFT calculations we were
able to identify all three isomers of 2.
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907. doi:10.1016/S1386-1425(03)00318-4
The bimolecular reaction of radicals 2 with O2 was induced
by annealing 1–2 % O2-doped argon matrices containing 2 for
several minutes at 25–35 K. This suggests that, as expected, the
reaction between radicals 2 and O2 has no thermal activation
barrier. The only products observed are the pyridylperoxy
radicals 3. For 3a and 3b two conformers, syn and anti, are
possible, but only for 3b are both conformers observed after the
thermal reaction between 2b and O2, while for 3a the more
stable anti-isomer is formed exclusively. The conformers form
photostationary equilibria, and visible light irradiation results in
syn–anti isomerisation. For both 3a and 3b the anti-conformers
with the terminal oxygen atom pointing away from the nitrogen
atom have the smaller dipole moment and are thermodynami-
cally more stable by 1.84 and 0.67 kcal molꢀ1, respectively.
For both 3a and 3b, both the syn- and the anti-conformer can be
generated by irradiation in photostationary equilibria. Even by
annealing the matrices at temperatures above 30 K the confor-
mers don’t equilibrate thermally, which suggests a substantial
thermal activation barrier for the isomerisation, in agreement
with the calculations (Fig. 4). On the other hand, the thermal
reaction between 2a and O2 produces syn-3a exclusively, while
under similar conditions 2b reacts to a mixture of syn- and anti-
3b. The reaction of the pyridyl radicals 3 with O2 is exothermic,
and the excess energy released might result in a syn–anti
equilibration. Since the energy difference between syn- and
anti-3b is small (the calculations are not accurate enough to
reproduce these small differences reliably) both conformers
could be formed in similar amounts in the matrix.
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Prolonged visible light irradiation, or UV irradiation, results
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seven-membered heterocycles. Due to the low yields of these
photoproducts, only 10 could be identified as the product of the
rearrangement of 3a, whereas the photoproducts of 3b and 3c
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the same path as that of the phenyl radical 6.
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Supplementary Material
13
The 1H- and C-NMR spectra of 3,30-azopyridine and the
spectroscopic and calculated data of the pyridyl radicals 2a–c
and the pyridylperoxy radicals 3a–c are available on the
Journal’s website.
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Acknowledgements
This work was financially supported by the Cluster of Excellence RESOLV
(EXC 1069) funded by the Deutsche Forschungsgemeinschaft and the Fonds
der Chemischen Industrie.
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