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Structure and conformation of β-chloroalkyl radicals
Volume 21, number
CHEMICAL
1
1 Augast
PHYSICS LETTERS
STRUCTURE AND CONFORMATION
1973
OF ~&I-ILOROALKYL RADICALS
Ian II ELSON, Kuang S. CHEN and J K KOCH1 Chernrstry Department,
hdrana Received
Chrersrty,
Bloomrngton,
Indrana, USA
16 March 1973
35C~ and ‘H hfs mdlcstes Analysis of isotropy ESR g-values and temperaturedependcnt uls exist m stable conformations consisting of chlorme m asymmetrlc bridges
that p-chloroalkyl
radl-
1 Introduction
and mterestmg concept of brrdgmg and anThe /3-haloalkyl radrcals play a central role 111the contemporary chunenc assistance u-r free radrcal processes [ 1 ] Indeed, recent ESR studres of the fl-chloroethyl radrcal m solulion suggest a preferred conformatron, u-rwhich the P-chlorme atom echpses the p-orbrtal at the radical center and 1s optrmally located to interact wtth rt [2,3] The magmtude of the mteractron, however, 1s msufficrent to cause the chlorme to occupy a symmetrical posrtron between the two carbon atoms and render them equrvalent None the less, homoconJugation and hyperconJugatron in tlus context may be construed as brrdgmg m the broadest sense [3,4] Brrdgmg in the chloroethyl radical rmphes restrrcted rotatron about the C,-Cp bond as well as drstortron prrmarrly at Cp as shown by I*,
and rt appeared to us that methyl groups on C, and Co would optmuze brrdgmg by chlorme In order to probe these factors, we exammed the ESR spectra of a senes of methyl analogs and measured the proton and chlorme hyperfine sphttmgs, then temperature dependence as well as the rsotroprcg-values lrsted m table 1
2 DIstortIon
of chloroalkyl
raduzals at Cg
The unusually small value of the P-proton splrtting m Q-chloroetkyl radical can be ascribed to drstortton at C,, wluch moves Hp closer to the nodal plane (see 1) [3] The trend contmues regularly with mcreasmg methyl substttntion as shown in fig 1, and mdrcates that I 1s the stable conformatron for all of these radicals More srgnificantly, the rsotroprcg-values of these radicals mcrease monotorucally wrth mcreasmg methyl substrtution 2s shown In fig 2, and attam an unusually hrgh value m th: tetramethyl denvatrve The change 111g-values l
See footnote
72
3 m ref [ 3 ]
Volume 21. number 1
CHEhIICAL
PHYSICS LETTERS
Table 1 ESR parameters for P-chloroalkyl Radical
Temp ec,
Couphng
constants
I August 1973
radicals
(G)
“Cl
H,
Hp
&H,
w ?O 00003
a)
CH,CH2CI
-135
18 74
21 57
9 86
-
2 00199
CH3CHCH2Cl
-127
19 89
20 70
7 79
24 03
2 00306
(CH3),CCH2CI
-129
2124
6 18
21 24
2 00359
(CH3),CCH(C~3~Ci
-121 -129
19 43 17 30
5 72 -
21 OG 3-O89
2 0039.5 2 00442
(CHJ),CC(CH,),CI
-
a) pCH3 unresolved
<9’
/.
2 0040“=“‘. 2 0030-
e’ 0--_-_-0-0-0-0
2 ooze-
l
Fig 1 Hyperfine
couplmg constants for P-protons (not l of chloroalbyl rsdxnls m solution
methyl) 0 and 435C1
x = CH,
/ J
H.
,
,I
X
Fig 2 Isotroplcg
r”
I-?
for chloroalhyl
iadl&s
/JX values
< l
and alkyl
ndlcals 0 II-Isolution
(Ag) for fl-chloro-substituted radicals 1s II-I strkmg contrast to those of the hydrocarbon analogs (Cl33 replacing Cl) whose g-values are almost mvarlant with methyl substltutlon (fig 2) The mcrease in Ag IS also most readily accounted for by the drstortion at Cp which moves Cl closer to the odd electron orbrtal (see I), and increases the spm-orbit couphng mteraction accordmg tot
where $’ IS the spm-orbrt couplmg constant (587 cm-l) of the chlorme p orbital, do = 19 5” rs the undistorted configuration, and lo and En2 are the energies of the odd electron and non-bondmg orbrtals, respectrvely P;,‘~~ IS the odd electron density on the p orbltals of chlorme III the configuratlon 8 = O”, and has a lower hrtxt of 0 I based on the observed splrttmg and the atomic sphttmg of chlonne An alternatrve change m the drhedrul angIe cpbetween the odd electron orbltal and the Cp -Cl bond causes little or no alteration m the g-values [6] _
3 Hmdered
rotatron
The ESR spectrum t The derivation
in chloroalkyl
radmls
of the /3-chloroethyl
of eq (1) - to be pubhshed
radrcak
shows a selective
- IS based on Stone’s theory
temperature-dependent
vartatron
tn linewrdths
[S]
73
Volume
21, number
CHEhlICAL
1
I
I
0-o
21 0
-O-
t
20
s
1
9-0-d
o-
z 2
-H---O_
O/O
z
H m
190-
0
---O. Cl -1
18
/o
o-
o/o
17
1 August 1973
PHYSICS LET-I ERS
o/o
OH 0’
0
/’
/O
2
Do-
_o
o0 -8 I
-eo
I -l?G
I
-100
T(‘C) Fig 3 Temperature
dependence
of the 35Cl hyperIine
couphng
Lonstnnts
of chloroalkyl
rachcnls m solution
wthm the multlpllcltles ofMfH (MI = 0 hnes broadened) and M f’ (MI = 53/2 are brcader than t1/2 lines) [3] Furthermore, the o-proton and chlorme hyperfine sphttmgs show a temperature dependence (fig 3) which 1s con-
sistent [7] with a torsional motion shown below,
m which II IS the most stable conformatIon We estunate an actlvatlon energy of about 5 kcal/mole from the hne broadenmg As methyl groups are substituted at C, and Cp of CH2CH2Cl, two effects are noted the Cl sphttmg reaches a maxunum at the a,adlmethyl analog (fig l), which also shows mmunal temperature dependence (fig 3) and the spectra exhibit no selective lme broadenmg Clearly, the cqardlmethyl radical IS m a “locked” conformation III
CHEMICAL PHYSICS LETTERS
Volume 2 1, number 1
I August 1973
Further substl?utlon of CH, at C, causes an increase m the torslonal angle I,Oas mantfested m N by the Iower values of aC, and apH (fig 1) Fmally, the decreased value of +, m the tetramethyl analog suggests a further mcrease m rp Conformations III, IV and V are largely attributed to stenc effects [CH3 x Cl], since slmllar differences are observed m the hydro-carbon analogs shown below
IVa
[
(17
94G)
= agH
= (9 ZOG)
1
We mfer from the slopes m fig 3 that barriers to hmdered rotation [7.8] increase with CH3 stibstltutior.. ever, it should be noted that considerable dlstortlon at C, persists even m the tetramethyl analog
How-
Acknowledgement We wish to thank the National Science terest anew and provldmg samples
Foundation
for financial
support
and P S Skell for stm?ulatmg
out m-
References [l] L haplan. Bridged free radlcJls (Dehhcr, New Yorh, 1972) [2] A J Bo\rlcs, A Hudson and R A Jachson, Chem Phys Letters 5 (1970) 552, I BIddIe and A Hudson, Chcm Phys Letters 18 (1973) 45 [3] T Kawamura, D J Edge and J K Koch], J Am Chem Sot 94 (1972) 1752 [4] R Hoffman, L Radom. J A Pople, P Schlcyer, W J Hehrc and L SAlem, J Am Chem [S] A J Stone,Proc Roy Sot ~271 (1963)424, hlol Phys 6 (1963)509 [6] D J Edge and J K Koch], J Am Chem Sot 95 (1973), to be pubhshed [7] P J Kruslc, P hleabm and J P Jesson, J Phys Chem 75 (1971) 3438 [8] R W Fessenden, J Glum Phys 61 (1964) 1570
Sot
94 (1972) 6221
75
CHEMICAL
1
1 Augast
PHYSICS LETTERS
STRUCTURE AND CONFORMATION
1973
OF ~&I-ILOROALKYL RADICALS
Ian II ELSON, Kuang S. CHEN and J K KOCH1 Chernrstry Department,
hdrana Received
Chrersrty,
Bloomrngton,
Indrana, USA
16 March 1973
35C~ and ‘H hfs mdlcstes Analysis of isotropy ESR g-values and temperaturedependcnt uls exist m stable conformations consisting of chlorme m asymmetrlc bridges
that p-chloroalkyl
radl-
1 Introduction
and mterestmg concept of brrdgmg and anThe /3-haloalkyl radrcals play a central role 111the contemporary chunenc assistance u-r free radrcal processes [ 1 ] Indeed, recent ESR studres of the fl-chloroethyl radrcal m solulion suggest a preferred conformatron, u-rwhich the P-chlorme atom echpses the p-orbrtal at the radical center and 1s optrmally located to interact wtth rt [2,3] The magmtude of the mteractron, however, 1s msufficrent to cause the chlorme to occupy a symmetrical posrtron between the two carbon atoms and render them equrvalent None the less, homoconJugation and hyperconJugatron in tlus context may be construed as brrdgmg m the broadest sense [3,4] Brrdgmg in the chloroethyl radical rmphes restrrcted rotatron about the C,-Cp bond as well as drstortron prrmarrly at Cp as shown by I*,
and rt appeared to us that methyl groups on C, and Co would optmuze brrdgmg by chlorme In order to probe these factors, we exammed the ESR spectra of a senes of methyl analogs and measured the proton and chlorme hyperfine sphttmgs, then temperature dependence as well as the rsotroprcg-values lrsted m table 1
2 DIstortIon
of chloroalkyl
raduzals at Cg
The unusually small value of the P-proton splrtting m Q-chloroetkyl radical can be ascribed to drstortton at C,, wluch moves Hp closer to the nodal plane (see 1) [3] The trend contmues regularly with mcreasmg methyl substttntion as shown in fig 1, and mdrcates that I 1s the stable conformatron for all of these radicals More srgnificantly, the rsotroprcg-values of these radicals mcrease monotorucally wrth mcreasmg methyl substrtution 2s shown In fig 2, and attam an unusually hrgh value m th: tetramethyl denvatrve The change 111g-values l
See footnote
72
3 m ref [ 3 ]
Volume 21. number 1
CHEhIICAL
PHYSICS LETTERS
Table 1 ESR parameters for P-chloroalkyl Radical
Temp ec,
Couphng
constants
I August 1973
radicals
(G)
“Cl
H,
Hp
&H,
w ?O 00003
a)
CH,CH2CI
-135
18 74
21 57
9 86
-
2 00199
CH3CHCH2Cl
-127
19 89
20 70
7 79
24 03
2 00306
(CH3),CCH2CI
-129
2124
6 18
21 24
2 00359
(CH3),CCH(C~3~Ci
-121 -129
19 43 17 30
5 72 -
21 OG 3-O89
2 0039.5 2 00442
(CHJ),CC(CH,),CI
-
a) pCH3 unresolved
<9’
/.
2 0040“=“‘. 2 0030-
e’ 0--_-_-0-0-0-0
2 ooze-
l
Fig 1 Hyperfine
couplmg constants for P-protons (not l of chloroalbyl rsdxnls m solution
methyl) 0 and 435C1
x = CH,
/ J
H.
,
,I
X
Fig 2 Isotroplcg
r”
I-?
for chloroalhyl
iadl&s
/JX values
< l
and alkyl
ndlcals 0 II-Isolution
(Ag) for fl-chloro-substituted radicals 1s II-I strkmg contrast to those of the hydrocarbon analogs (Cl33 replacing Cl) whose g-values are almost mvarlant with methyl substltutlon (fig 2) The mcrease in Ag IS also most readily accounted for by the drstortion at Cp which moves Cl closer to the odd electron orbrtal (see I), and increases the spm-orbit couphng mteraction accordmg tot
where $’ IS the spm-orbrt couplmg constant (587 cm-l) of the chlorme p orbital, do = 19 5” rs the undistorted configuration, and lo and En2 are the energies of the odd electron and non-bondmg orbrtals, respectrvely P;,‘~~ IS the odd electron density on the p orbltals of chlorme III the configuratlon 8 = O”, and has a lower hrtxt of 0 I based on the observed splrttmg and the atomic sphttmg of chlonne An alternatrve change m the drhedrul angIe cpbetween the odd electron orbltal and the Cp -Cl bond causes little or no alteration m the g-values [6] _
3 Hmdered
rotatron
The ESR spectrum t The derivation
in chloroalkyl
radmls
of the /3-chloroethyl
of eq (1) - to be pubhshed
radrcak
shows a selective
- IS based on Stone’s theory
temperature-dependent
vartatron
tn linewrdths
[S]
73
Volume
21, number
CHEhlICAL
1
I
I
0-o
21 0
-O-
t
20
s
1
9-0-d
o-
z 2
-H---O_
O/O
z
H m
190-
0
---O. Cl -1
18
/o
o-
o/o
17
1 August 1973
PHYSICS LET-I ERS
o/o
OH 0’
0
/’
/O
2
Do-
_o
o0 -8 I
-eo
I -l?G
I
-100
T(‘C) Fig 3 Temperature
dependence
of the 35Cl hyperIine
couphng
Lonstnnts
of chloroalkyl
rachcnls m solution
wthm the multlpllcltles ofMfH (MI = 0 hnes broadened) and M f’ (MI = 53/2 are brcader than t1/2 lines) [3] Furthermore, the o-proton and chlorme hyperfine sphttmgs show a temperature dependence (fig 3) which 1s con-
sistent [7] with a torsional motion shown below,
m which II IS the most stable conformatIon We estunate an actlvatlon energy of about 5 kcal/mole from the hne broadenmg As methyl groups are substituted at C, and Cp of CH2CH2Cl, two effects are noted the Cl sphttmg reaches a maxunum at the a,adlmethyl analog (fig l), which also shows mmunal temperature dependence (fig 3) and the spectra exhibit no selective lme broadenmg Clearly, the cqardlmethyl radical IS m a “locked” conformation III
CHEMICAL PHYSICS LETTERS
Volume 2 1, number 1
I August 1973
Further substl?utlon of CH, at C, causes an increase m the torslonal angle I,Oas mantfested m N by the Iower values of aC, and apH (fig 1) Fmally, the decreased value of +, m the tetramethyl analog suggests a further mcrease m rp Conformations III, IV and V are largely attributed to stenc effects [CH3 x Cl], since slmllar differences are observed m the hydro-carbon analogs shown below
IVa
[
(17
94G)
= agH
= (9 ZOG)
1
We mfer from the slopes m fig 3 that barriers to hmdered rotation [7.8] increase with CH3 stibstltutior.. ever, it should be noted that considerable dlstortlon at C, persists even m the tetramethyl analog
How-
Acknowledgement We wish to thank the National Science terest anew and provldmg samples
Foundation
for financial
support
and P S Skell for stm?ulatmg
out m-
References [l] L haplan. Bridged free radlcJls (Dehhcr, New Yorh, 1972) [2] A J Bo\rlcs, A Hudson and R A Jachson, Chem Phys Letters 5 (1970) 552, I BIddIe and A Hudson, Chcm Phys Letters 18 (1973) 45 [3] T Kawamura, D J Edge and J K Koch], J Am Chem Sot 94 (1972) 1752 [4] R Hoffman, L Radom. J A Pople, P Schlcyer, W J Hehrc and L SAlem, J Am Chem [S] A J Stone,Proc Roy Sot ~271 (1963)424, hlol Phys 6 (1963)509 [6] D J Edge and J K Koch], J Am Chem Sot 95 (1973), to be pubhshed [7] P J Kruslc, P hleabm and J P Jesson, J Phys Chem 75 (1971) 3438 [8] R W Fessenden, J Glum Phys 61 (1964) 1570
Sot
94 (1972) 6221
75