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ESR Detection of the diphenylmethyl radical
Tetrahedron Letters No.2, pp. 145-148, 1968.
Pergamon Press Ltd.
Printed in Great Britain.
ESR DETECTION OF THE DIPHENYLMETHYL RADICAL D. R. Dalton, S. A. Liebman and H. Waldman Department of Chemistry, Temple University Philadelphia, Pennsylvania 19122 and
R. S. Sheinson Department of Chemistry, Massachusetts Institute of Technology Cambridge, Massachusetts 02139 (Received in USA 1 Septembrrl96'j') Electron spin resonance (esr) spectroscopic phenylphosphazine
(2), azobisdiphenylmethane
(1) examination of diphenylmethylenetri-
(3) or diphenyldiasomethane
(4) during
thermolysis, in solvents capable of hydrogen donation, e.g. decalin and diphenylmethane, unequivocally demonstrates the presence of a radical species. Figure 1 records the derivative of the esr spectrum of the hitherto unobserved diphenylmethyl
radical, (CSHS)2CH*
the corresponding diphenylmethylene
, g F 2.0030.
This radical is presumably generated -via
produced on fragmentation of the above compounds.
Figure 1. The derivative of the resonance absorption spectrum of a solution of the diphenylmethyl radical (C6H5)2CH* in decalin at 140'
145
146
No.2
In gneral, in contrast to Figure 1, ortho alkylated diarylmethyl radicals reported (5) do not show complications due to overlapping of the doublet halves and the relative hyperfine splittings of the ortho, s
and para hydrogens.
The computer-simulated
spectrum (Figure 2) (6) utilizing the experimental hyperfine
splitting constants presented in Table 1. served to confirm our conclusions regarding the validity of these values.
c Figure 2.
5 Gauss
*
The computed resonance absorption spectrum of the diphenylmethyl radical (C6H5)2CH. (Half spectrum shown.)
Table l.- Hyperfine splitting constants, aH, and spin densities,p various (9,10,11, 12) values of Q. SPIN ortho, para = 3.05 aH
, for
DENSITIES
meta aF1.22
central aH = 8.36
adjacent
Value of Q (ref.) 23a (2b, 12)
0.133
-0.053
0.363
-0.05
27b (11)
0.113
-0.045
0.602'
-0.05
23b (2b,12)
0.133
-0.053
0.514d
-0.05
a. b. c. d.
This value This value Calculated Calculated
of Q of Q spin spin
used for used for density, density,
all positions. ring positions only. 1.00 - c of spin densities in rings; Qcenter = 13.8 1.00 - c of spin densities in rings; Qcenter = 16.2
147
No.2
Table 1 also presents spin densities for various values of Q (7) calculated from the experimentally determined hyperfine splittings (9). It has been noted that introduction of phenyl groups onto a carbon atom bearing an unpaired electron results in a lower electron density at that carbon and a prediction of
Pcenter = 0.66 has been made for the radical reported (13). Our experimentally observed aH and pcenter = 0.60 tends to confirm the original prediction. Finally, since the ratio of the exchange integral between the rings and the central carbon atom (K) and the exchange integral between the ring atoms (J) is directly related to the angle subtended by the odd-electron ps orbital and the pi cloud of the aromatic rings (14) it is possible to infer that the angle of twist in the unsubstituted diphenylmethyl radical is 50-60' for both rings.
This relationship is, in part, substantiated
by examination of models, which also demonstrate that both rings in ortho-substituted diary1 radicals (5) need not participate to the same extent.
Acknowledgement We gratefully acknowledge fruitful discussions with A. M. Troszolo and L. C. Snyder, Bell Telephone Laboratories, Murray Hill, New Jersey. Acknowledgement
is made to the donors of The Petroleum Research Fund, administered by
The American Chemical Society, for partial support of this work.
RFFERENCES
1. ESR spectra were obtained on a Varian V4500-lOA, X-band Spectrometer, equipped with a variable temperature probe. 2. (a) H. Staudinger and G. Luscher, -Helv. -Chim. -s=t Acta W. R. Hasek, J. &. &.,
Chem. *.,
75 (1922); (b) W. E. Parham and
76, 935 (1954); (c) W. Kirmse, L. Homer
6l&, 19 (1958); (d) V. Franeen and H. J
in "Organophosphorus
5
and H. Hoffmann,
Joschek, ibid, 644, 7 (1960); (e) G
Wittig
Compounds", IUPAC Symposium, Heidelberg, 1964, Butterworths, London,
1964, p.245; (f) A. W. .Johnson, "Ylid Chemistry", Organic Chemistry Monographs, Vol. 7, Academic Press, N. Y., 1966, pp 35 and 238.
148
3.
No.2
L.
I.
Smith
N. Y., 4.
S.
5.
(a)
1955,
p.
G. Cohen D. B.
Fleurke 6.
and K. L.
Chesnut R.
We gratefully
van
simulated
spectrum. given
in
is
Table
able
spin
77,
p,
Vol.
2457
2,
John
Wiley
and
Sons,
the
densities:
443
(1961);
of
the
Computation
for
their
1.
than
Center,
help
that
a greater
ortho
and para
hydrogens
= iG.63016;
ortho
central
(b)
Delong,
K.
9
(1966)
Massachusetts,
be noted
(1955)
35, =
284
assistance
1 for
in
Massachusetts
obtaining
5% variation
in
was not
the
the
experiment-
acceptable.
= + 0.1198;
meta
HMO-SCF
= -
0.0458
= + 0.1189. the
emperical
constant
8.
C.
Heller
and H. M. McConnell,
9.
It
has
been
spin
spin
density,
pointed
out
density,
p,
of
systems.
these
usually
the
at
the
central
IQ1 is
assigned
L.
C.
Snyder
11.
9.
M. McConnell
12.
(a)
2147
(1963);
A.
L.
14.
A.
D. McLachlan,
Amos, 5.
27
(10)
(d)
Ruchachenko,
1.
Chesnut,
J-
47, =
ay = QP,
where
Q is
a semi-
Z.
Radicals”,
Chem. phys,
32, -
(1960)
f or
ll),
141 has 2, g, -
ibid, (1962);
it
treated
4670
(1965)
=, 35 (c)
Consultants (1960).
is
positions
107
hydrocarbon to
the
positions used
the
absolute
of
an un-
positions 16.2.
thenpcenter
elsewhere
(10). value
in
ring
a value
only,
aH and radicals
results
for
IQ1 assumes
been
Naturforschung,
1488
all
and
ring
between
relating
for if
0.514
the
relationships
odd-alternate
whereas,
ibid,
1489
1535
suggestions
(Zb,
Chem. m.,
K. Mobius,
“Stable
12)
becomes
of
and G. K. Fraenkel,
, ibid,
11,
atom,
atom
32, -
neutral
IQ1 = 23 carbon
of
for
(10,
Adjustment
and D.
Fraenkel
13.
equation
semi-emperical
equivalent
carbon
value
and T.
B. M. Karplus
and G. K.
the
Phys.,
various
numerous
= 13.8.
10.
the
central
the
IQ/ center
through
Chem
Utilizing
pat
p
and
are
low
only,
related
J.
that
become
there
Q in
are
(8)
Nevertheless,
39,
Collective
Chem. w.,
Recueil,
to
aH andp,
the
J,
Cambridge,
It
calculationsgive
7.
Synthesis,
Am. Chem. &.,
Sloan,
the
Technology,
aH values
J. ---
Hardelveld,
acknowledge of
para
H. Wang, and G. J.
Institute
al
Organic
315
and C.
and
Howard,
If
= 0.602
(12).
(1958) 1312
(1961);
(b)
R. W. Fessenden 2&, Bureau,
1102
I.
and
Berna P.
H
,
Schuler,
(1965).
N. Y.
(?965),
P.
p.49
R.
Peiger ibid,
Pergamon Press Ltd.
Printed in Great Britain.
ESR DETECTION OF THE DIPHENYLMETHYL RADICAL D. R. Dalton, S. A. Liebman and H. Waldman Department of Chemistry, Temple University Philadelphia, Pennsylvania 19122 and
R. S. Sheinson Department of Chemistry, Massachusetts Institute of Technology Cambridge, Massachusetts 02139 (Received in USA 1 Septembrrl96'j') Electron spin resonance (esr) spectroscopic phenylphosphazine
(2), azobisdiphenylmethane
(1) examination of diphenylmethylenetri-
(3) or diphenyldiasomethane
(4) during
thermolysis, in solvents capable of hydrogen donation, e.g. decalin and diphenylmethane, unequivocally demonstrates the presence of a radical species. Figure 1 records the derivative of the esr spectrum of the hitherto unobserved diphenylmethyl
radical, (CSHS)2CH*
the corresponding diphenylmethylene
, g F 2.0030.
This radical is presumably generated -via
produced on fragmentation of the above compounds.
Figure 1. The derivative of the resonance absorption spectrum of a solution of the diphenylmethyl radical (C6H5)2CH* in decalin at 140'
145
146
No.2
In gneral, in contrast to Figure 1, ortho alkylated diarylmethyl radicals reported (5) do not show complications due to overlapping of the doublet halves and the relative hyperfine splittings of the ortho, s
and para hydrogens.
The computer-simulated
spectrum (Figure 2) (6) utilizing the experimental hyperfine
splitting constants presented in Table 1. served to confirm our conclusions regarding the validity of these values.
c Figure 2.
5 Gauss
*
The computed resonance absorption spectrum of the diphenylmethyl radical (C6H5)2CH. (Half spectrum shown.)
Table l.- Hyperfine splitting constants, aH, and spin densities,p various (9,10,11, 12) values of Q. SPIN ortho, para = 3.05 aH
, for
DENSITIES
meta aF1.22
central aH = 8.36
adjacent
Value of Q (ref.) 23a (2b, 12)
0.133
-0.053
0.363
-0.05
27b (11)
0.113
-0.045
0.602'
-0.05
23b (2b,12)
0.133
-0.053
0.514d
-0.05
a. b. c. d.
This value This value Calculated Calculated
of Q of Q spin spin
used for used for density, density,
all positions. ring positions only. 1.00 - c of spin densities in rings; Qcenter = 13.8 1.00 - c of spin densities in rings; Qcenter = 16.2
147
No.2
Table 1 also presents spin densities for various values of Q (7) calculated from the experimentally determined hyperfine splittings (9). It has been noted that introduction of phenyl groups onto a carbon atom bearing an unpaired electron results in a lower electron density at that carbon and a prediction of
Pcenter = 0.66 has been made for the radical reported (13). Our experimentally observed aH and pcenter = 0.60 tends to confirm the original prediction. Finally, since the ratio of the exchange integral between the rings and the central carbon atom (K) and the exchange integral between the ring atoms (J) is directly related to the angle subtended by the odd-electron ps orbital and the pi cloud of the aromatic rings (14) it is possible to infer that the angle of twist in the unsubstituted diphenylmethyl radical is 50-60' for both rings.
This relationship is, in part, substantiated
by examination of models, which also demonstrate that both rings in ortho-substituted diary1 radicals (5) need not participate to the same extent.
Acknowledgement We gratefully acknowledge fruitful discussions with A. M. Troszolo and L. C. Snyder, Bell Telephone Laboratories, Murray Hill, New Jersey. Acknowledgement
is made to the donors of The Petroleum Research Fund, administered by
The American Chemical Society, for partial support of this work.
RFFERENCES
1. ESR spectra were obtained on a Varian V4500-lOA, X-band Spectrometer, equipped with a variable temperature probe. 2. (a) H. Staudinger and G. Luscher, -Helv. -Chim. -s=t Acta W. R. Hasek, J. &. &.,
Chem. *.,
75 (1922); (b) W. E. Parham and
76, 935 (1954); (c) W. Kirmse, L. Homer
6l&, 19 (1958); (d) V. Franeen and H. J
in "Organophosphorus
5
and H. Hoffmann,
Joschek, ibid, 644, 7 (1960); (e) G
Wittig
Compounds", IUPAC Symposium, Heidelberg, 1964, Butterworths, London,
1964, p.245; (f) A. W. .Johnson, "Ylid Chemistry", Organic Chemistry Monographs, Vol. 7, Academic Press, N. Y., 1966, pp 35 and 238.
148
3.
No.2
L.
I.
Smith
N. Y., 4.
S.
5.
(a)
1955,
p.
G. Cohen D. B.
Fleurke 6.
and K. L.
Chesnut R.
We gratefully
van
simulated
spectrum. given
in
is
Table
able
spin
77,
p,
Vol.
2457
2,
John
Wiley
and
Sons,
the
densities:
443
(1961);
of
the
Computation
for
their
1.
than
Center,
help
that
a greater
ortho
and para
hydrogens
= iG.63016;
ortho
central
(b)
Delong,
K.
9
(1966)
Massachusetts,
be noted
(1955)
35, =
284
assistance
1 for
in
Massachusetts
obtaining
5% variation
in
was not
the
the
experiment-
acceptable.
= + 0.1198;
meta
HMO-SCF
= -
0.0458
= + 0.1189. the
emperical
constant
8.
C.
Heller
and H. M. McConnell,
9.
It
has
been
spin
spin
density,
pointed
out
density,
p,
of
systems.
these
usually
the
at
the
central
IQ1 is
assigned
L.
C.
Snyder
11.
9.
M. McConnell
12.
(a)
2147
(1963);
A.
L.
14.
A.
D. McLachlan,
Amos, 5.
27
(10)
(d)
Ruchachenko,
1.
Chesnut,
J-
47, =
ay = QP,
where
Q is
a semi-
Z.
Radicals”,
Chem. phys,
32, -
(1960)
f or
ll),
141 has 2, g, -
ibid, (1962);
it
treated
4670
(1965)
=, 35 (c)
Consultants (1960).
is
positions
107
hydrocarbon to
the
positions used
the
absolute
of
an un-
positions 16.2.
thenpcenter
elsewhere
(10). value
in
ring
a value
only,
aH and radicals
results
for
IQ1 assumes
been
Naturforschung,
1488
all
and
ring
between
relating
for if
0.514
the
relationships
odd-alternate
whereas,
ibid,
1489
1535
suggestions
(Zb,
Chem. m.,
K. Mobius,
“Stable
12)
becomes
of
and G. K. Fraenkel,
, ibid,
11,
atom,
atom
32, -
neutral
IQ1 = 23 carbon
of
for
(10,
Adjustment
and D.
Fraenkel
13.
equation
semi-emperical
equivalent
carbon
value
and T.
B. M. Karplus
and G. K.
the
Phys.,
various
numerous
= 13.8.
10.
the
central
the
IQ/ center
through
Chem
Utilizing
pat
p
and
are
low
only,
related
J.
that
become
there
Q in
are
(8)
Nevertheless,
39,
Collective
Chem. w.,
Recueil,
to
aH andp,
the
J,
Cambridge,
It
calculationsgive
7.
Synthesis,
Am. Chem. &.,
Sloan,
the
Technology,
aH values
J. ---
Hardelveld,
acknowledge of
para
H. Wang, and G. J.
Institute
al
Organic
315
and C.
and
Howard,
If
= 0.602
(12).
(1958) 1312
(1961);
(b)
R. W. Fessenden 2&, Bureau,
1102
I.
and
Berna P.
H
,
Schuler,
(1965).
N. Y.
(?965),
P.
p.49
R.
Peiger ibid,