Concentration-dependent isotope effects in electron transfer-mediated reactions photocyanation of biphenyl and dimethylbiphenyls

Article abstract of DOI:10.1021/jp027757s

The magnitude of the hydrogen-deuterium isotope effect (IE) in the photosensitized cyanation of biphenyl is found to depend on the concentration of biphenyl and on that of cyanide, as well as on the ionic strength of the medium. These concentration dependences are shown to originate from intermolecular electron exchange between biphenyl and its radical cation, in competition with addition of cyanide. Similar observations are made for the three symmetrically substituted dimethylbiphenyls upon deuteration of the methyl groups. The overall magnitude of the IE is determined by the IE of the back electron transfer step within the initially formed ion pair, as well as by the IE of the electron exchange equilibration. The experimentally determined equilibrium IEs for the biphenyls are in agreement with the results of semiempirical and ab initio calculations.

Full text of DOI:10.1021/jp027757s

J. Phys. Chem. A 2003, 107, 7675-7683
7675
Concentration-Dependent Isotope Effects in Electron Transfer-Mediated Reactions
Photocyanation of Biphenyl and Dimethylbiphenyls
Roel Gronheid, Jacob Baggerman, and Gerrit Lodder*
Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden UniVersity,
P.O. Box 9502, 2300 RA Leiden, The Netherlands
ReceiVed: December 18, 2002; In Final Form: July 8, 2003
The magnitude of the hydrogen-deuterium isotope effect (IE) in the photosensitized cyanation of biphenyl
is found to depend on the concentration of biphenyl and on that of cyanide, as well as on the ionic strength
of the medium. These concentration dependences are shown to originate from intermolecular electron exchange
between biphenyl and its radical cation, in competition with addition of cyanide. Similar observations are
made for the three symmetrically substituted dimethylbiphenyls upon deuteration of the methyl groups. The
overall magnitude of the IE is determined by the IE of the back electron transfer step within the initially
formed ion pair, as well as by the IE of the electron exchange equilibration. The experimentally determined
equilibrium IEs for the biphenyls are in agreement with the results of semiempirical and ab initio calculations.
Introduction
determined to be 0.62.10
In the past decades, radical cations have been recognized as
K
+
•
+•
1
important intermediates in biological systems as well as in
M + MD
{
\
} M + MH
(2)
H
D
2
organic synthesis. This has triggered numerous investigations
of the physical properties and chemical reactivity of a wide range
In this study, the implications of electron exchange between
neutral and radical cation isotopologues on the overall IE of a
nucleophilic aromatic photosubstitution involving such species
were investigated by varying the substrate and nucleophile
concentrations. Biphenyl (BP) was chosen as the model substrate
3
of organic radical cations, using (time-resolved) optical,
4
5
3,6
electrochemical, chemical, and theoretical methods.
Isotope effects (IEs) have proven to be a useful tool to gain
7
insight into reaction mechanisms. The most convenient way
to measure IEs is by performing competition experiments with
the substrate and its isotopomer or isotopologue in one reaction
vessel. With this intraexperimental method, the reaction condi-
tions are by definition the same for the two isotopically
substituted substrates.
+•
for several reasons: (i) Its radical cation (BP ) can be easily
photogenerated by selective excitation of 1,4-dicyanonaphtha-
11
lene (DCN) in the presence of BP. In this way, exciton
exchange as depicted in eq 1 cannot occur. (ii) Although initially
+•
ion pairs (IPs) are formed, the quantum yield of separated BP
is high.
In our laboratories, it has been shown that in the analysis of
such intraexperimentally determined IEs transfer of “reactivity”
12,13
Substantial separation of the ions is required, because
otherwise the radical ion reacts with its counterion (i.e., back
electron transfer (ET)) or with a nucleophile within the solvent
8,9
has to be taken into account. For instance, in the photocya-
nation of naphthalene, the dependence of the magnitude of the
IE on the naphthalene concentration is the consequence of
exchange of an exciton between a singlet excited and a ground
+•
cage preventing the electron exchange of eq 2. (iii) BP readily
14,15
reacts with cyanide, yielding photocyanation products (eq 3).
This allows determination of the IE on the product formation.
9
state naphthalene molecule (eq 1). The IE increases from ∼1
+•
(
iv) Understanding the kinetics of ET to BP is desirable,
at low naphthalene concentrations to 1.35 at high concentrations.
The equilibrium constant K > 1 leads to preponderance of the
C10H8 singlet excited state and to preferential reaction of the
hydrogenated compound and thus to a normal IE (>1).
because BP is a versatile cosensitizer for the generation of
16
otherwise inaccessible radical cations.
In addition to the effect of replacement of the hydrogen atoms
in BP by deuterium, the effect of deuteration of methyl
substituents at aromatic ring positions in three dimethylbiphenyls
(DMBPs) has also been investigated. Generation of the radical
cations of DMBPs under photoinduced ET conditions by use
of appropriate electron acceptors also gives high quantum yields
of separated ions and thus also allows determination of the IEs
K > 1.4
S1
8
S1
C H + C D
{
} C D + C H
8
(1)
1
0
8
10
10
8
10
Concentration-dependent IEs may also occur in reactions in
which intermediates exchange an electron before reacting to
product. A molecule MH can donate an electron to the radical
cation of its isotopologue MD, yielding neutral MD and the
radical cation of MH (eq 2). For such a reaction step to contribute
to the overall IE, its equilibrium constant (K) has to differ
significantly from 1. This can indeed be expected to be the case;
for instance, the effect of deuterium substitution on the electron
exchange has been studied for thianthrene, and its K was
1
2
on the electron exchange equilibria. The three symmetrically
substituted DMBPs were chosen to reduce the number of
nonequivalent cyanation positions.
Experimental Section
Materials. BP and BP-d10 were both purified by repeated
crystallization from acetonitrile. 3,3′-DMBP was purified by
Kugelrohr distillation, and 4,4′-DMBP was used as received.
All compounds were obtained from Aldrich. The acetonitrile
*
To whom correspondence should be addressed. E-mail:
lodder@chem.leidenuniv.nl.
1
0.1021/jp027757s CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/29/2003

7676 J. Phys. Chem. A, Vol. 107, No. 39, 2003
Gronheid et al.
(
high-performance liquid chromatography (HPLC) grade; Rath-
For reference and gas-liquid chromatography reponse cali-
burn) for the irradiations was used as received. All materials
for the syntheses (Acros or Aldrich) were used as received. The
1
to a literature procedure. The same procedure was used to
make the 18-crown-6/KPF6 complex ( C -KPF6).
bration purposes, 4-cyanobiphenyl was prepared from 4-bro-
2
1
mobiphenyl by the same cyanation method as used above in
18
6
1
8-crown-6/KCN complex ( C -KCN) was prepared according
the preparation of DCN. H NMR: δ (ppm) 7.4-7.7 (m). MS
17
m/z (rel. int.): 179 (100), 152 (11). 4-Cyanobiphenyl-d9 was
prepared accordingly from 4-bromobiphenyl-d9.
18 6
A mixture of 2- and 3-cyano-4,4′-DMBP was prepared from
2
2
a mixture of 2- and 3-bromo-4,4′-DMBP by the same
cyanation method as used above in the preparation of DCN. A
mixture of 2- and 3-cyano-4,4′-DMBP-d9 was prepared accord-
1
ingly. H NMR: δ (ppm) 2.4 (s, 5.5H), 2.6 (s, 0.4H), 7.1-7.6
(m, 7H) (7% 3-cyanobiphenyl).
Irradiations. Air-saturated solutions of 2.0 mM DCN and
1
8
6
18 6
The ET sensitizer DCN was prepared from 1,4-dibromonaph-
varying concentrations of C -KCN, C -KPF6, and BP-h10/
BP-d10 (the latter two always present in ∼1:1 ratio) in
acetonitrile were prepared. They were illuminated in 10 mL
Pyrex tubes in a Rayonet Photochemical Reactor RPR 200 fitted
with seven 350 nm lamps, which was placed in a cool room (4
°C). The temperature of all reaction mixtures remained constant
at 12 °C during the irradiations, and for all kinetic measure-
ments, samples were brought to this temperature prior to
irradiation. At appropriate time intervals (up to the desired
conversion, 5-15%, depending on ionic strength and cyanide
and BP concentrations), ∼1 mL aliquots were taken from the
solution and injected in a test tube containing ∼1 mL of water
and ∼1 mL of diethyl ether. The layers were agitated vigorously,
and the organic layer was removed with a pipet, after which it
was analyzed on analytical GC and/or GC-MS. In the case of
BP, the IEs are determined on the formation of o-cyanobiphenyl,
because a minor impurity caused a larger statistical error when
the IEs were determined on the p-isomer. In the case of the
DMBPs, the IEs are determined on the formation of the major
product. In all cases, the trends in the variation of the IE were,
however, the same for all isomers of the cyanation products.
Fluorescence. The fluorescence spectra for the Stern-Volmer
plots were measured at λexc ) 315 nm. Air-saturated stock
solutions of DCN (2.3 mM) and BP (100 mM) in acetonitrile
were mixed and diluted to make samples of constant DCN
mc o Mn c) e. ntration (∼23 µM) and varying BP concentrations (0-80
18
thalene according to method C described in ref 19. The product
was purified by crystallization from acetonitrile and sublimation,
after which the sample was still contaminated with ∼5% 1,5-
dicyanonaphthalene. Further purification of the mixture was not
1
attempted, since it gave efficient photocyanation. H NMR: δ
(
ppm) 7.9 (q, 2.5H), 8.0 (s, 2H), 8.4 (q, 2.5H). MS m/z (rel.
int.): 178 (100), 151 (22), 124 (8).
,2′-DMBP was prepared from 2,2′-diformylbiphenyl by a
Wolff-Kishner reduction: 10 g of 2,2′-diformylbiphenyl and
5 mL of hydrazine monohydrate were stirred in 300 mL of
2
0
2
2
ethylene glycol for 1 h. To the mixture, 25 g of KOH was added,
and it was refluxed for 2 h, after which it was cooled to room
temperature and 100 mL of water was added. Subsequently,
the mixture was extracted with 300 mL of diethyl ether and the
organic layer was washed with water and dried on MgSO4. After
the solvent was removed under reduced pressure, the crude
product was purified by column chromatography on silica gel
(
eluent: pet. ether (40/60):diethyl ether ) 20:1) and Kugelrohr
1
distillation. H NMR: δ (ppm) 2.1 (s, 6H), 7.0-7.3 (m, 8H).
MS m/z (rel. int.): 182 (100), 167 (38).
For the deuteration of 4,4′-DMBP, 330 mg (13.8 mmol) of
dry sodium hydride (95%) was suspended in 5 mL of dimethyl
sulfoxide-d6. This was stirred for 15 min after which 2.5 g (13.7
mmol) of 4,4′-DMBP was added and the mixture was stirred
overnight at 75 °C. The mixture was cooled to room temper-
ature, ∼5 mL of D2O was added, and stirring was continued
for 15 min. The suspension was extracted with diethyl ether,
and the organic layer was dried on MgSO4 and concentrated.
This procedure was repeated twice. After the final exchange
step, the crude product was purified by column chromatography
on silica gel (eluents: pet. ether (40/60):diethyl ether ) 15:1).
Time-Resolved Experiments. The setup used to measure the
IE on the formation of free BP radical cations (BP ) has been
described in ref 23. The relative amount of free BP-h10 and
BP-d10 formed immediately after the laser pulse was deter-
+
•
+
•
+
•
+
•
mined from a fit of decay of BP for both isotopologues to
1
H NMR: δ (ppm) 7.0 (d, 4H), 7.24 (d, 4H) (100% methyl
deuteration according to NMR). MS m/z (rel. int.): 188 (100),
+
•
[
BP ]
+
•
0
1
70 (40).
,3′-DMBP and 2,2′-DMBP were deuterated by the same
[BP ] )
t
+
•
1
+ kt[BP ]
3
0
procedure as described above and were purified by Kugelrohr
distillation. 3,3′-DMBP-d6: 1H NMR: δ (ppm) 2.3 (s, 0.3H),
where k is the decay rate of BP . The IE on the efficiency of
+• 24
+
•
+•
7
.0-7.4 (m, 8H) (96% methyl deuteration according to NMR).
formation of free BP is determined as the quotient of [BP ]0
for BP-h10 and -d10.
1
MS m/z (rel. int.): 188 (100), 170 (43). 2,2′-DMBP-d6:
H
NMR: δ (ppm) 2.0 (s, 0.2H), 7.0-7.4 (m, 8H) (98% methyl
deuteration according to NMR). MS m/z (rel. int.): 188 (100),
Equipment. The 200 MHz NMR spectra were recorded on
a JEOL JNM FX-200 spectrometer, and the 300 MHz NMR
spectra were recorded on a Bruker WM300 spectrometer. GC
1
70 (97).

Photocyanation of Biphenyl and Dimethylbiphenyls
J. Phys. Chem. A, Vol. 107, No. 39, 2003 7677
SCHEME 1: Postulated Mechanism for the Photocyanation of a BP-h10/BP-d10 Mixture via ET (Only the para Mode of
-
+•
Attack of CN at BP Is Depicted)
analyses of the reaction mixtures were performed with a
Hewlett-Packard 6890 series GC equipped with a WCOT fused
silica CP-SIL-5CB (25 m) column and a flame ionization
detector (FID). With this technique, the isotopologues of the
reactants as well as the reaction products were almost baseline
separated. The FID was calibrated, using synthesized or com-
mercially available (isotopologues) materials. Isomers were
assumed to have equal response factors. An HP Chemstation
was used for analysis of the analytical GC data. All GC-MS
spectra were recorded on a Packard model 438A GC equipped
with a CP-SIL-5CB (25 m) column, coupled with a Finnigan
ITD 700 mass spectrometer. Fluorescence spectra were recorded
on a SPEX Fluorolog II equipped with a SPEX Torin TA300
photomultiplier.
Ab initio calculations were performed using the GAUSSIAN
2
6
98 (revision A5) set of programs on the same computer. The
calculations of the neutral BPs were performed on the B3LYP/
6-31G(d,p) level of theory. For the radical cations, the corre-
sponding unrestricted method was employed. Charge distribu-
2
7
tions were calculated using the NBO 3.1 program, and for all
reported charges, the hydrogens are summed into the heavy
atoms. All charges on carbon atoms are reported as the sum of
the charge on the carbon atom and the charges on the hydrogen
atoms connected to it.
Results and Discussion
Mechanism. The mechanism of photocyanation of unsub-
stituted aromatic compounds, such as BP, in the presence of
1
4,17,28
Calculations. Semiempirical RHF and ROHF calculations
on the BPs and BP radical cations, respectively, using the PM3
electron acceptors is of the SR+NAr type.
Singlet excited
DCN acquires an electron from BP to yield an IP.11 Back ET
within the IP yields the ground states of DCN and BP, and
separation of the IP gives two free radical ions.
2
5
parametrization set as implemented in MOPAC 93, were
performed on an IBM RS/6000 platform. After preoptimization,
the final structures were obtained by optimization with the
eigenvector following method. With the thus obtained geom-
etries, frequency calculations were performed, and the effect
of isotopic substitution on the zero point energies (ZPEs) was
determined.
1
2,13
Cyanide
attack can occur both at the aromatic radical cation within the
IP and at the free radical cation, to yield the cyanohydrogen
9
radical. Prior to cyanation, the free radical cation may also give
electron exchange with a neutral substrate molecule.10 The
cyanohydrogen radical species rapidly reacts with the radical

7678 J. Phys. Chem. A, Vol. 107, No. 39, 2003
Gronheid et al.
1
- and 2-positions of naphthalene was observed to vary with
9
the cyanide concentration. This is ascribed to attack by cyanide
at the IP at high cyanide concentrations and at the free
naphthalene radical cation at low cyanide concentrations. The
geminate radical IP is somewhat more selective for reaction
with cyanide ion than the free radical ion. In the DCN-sensitized
photocyanation of BP, the ratio of ortho to para substitution
does not vary within experimental error, upon changing the
cyanide concentration. The difference is due to the high
efficiency of ion separation in the case of BP (cf. Scheme 1).1
Almost all of the product is formed by cyanide attack at free
2,13
+
•
BP .
The IEs on this reaction as a function of both the cyanide
and the BP concentration, at constant ionic strength (I ) ∼50
mM; maintained constant using 18-crown-6-KPF6) are depicted
in Figure 2 (the collected data are given in Table 1 of the
Figure 1. Stern-Volmer plots of the quenching of the fluorescence
of DCN by BP-h10 (9) and BP-d10 (b) in acetonitrile.
anion of oxygen by hydrogen atom transfer, yielding the
cyanoaromatic. DCN is regenerated from the radical anion of
31
Supporting Information).
The IEs are seen to vary from 0.94 to 1.10 as a function of
the concentrations of both reactants. Three general trends emerge
from Figure 2: (i) The IE decreases from normal to inverse
with increasing BP concentration. (ii) The IE increases from
inverse to normal with increasing cyanide concentration. (iii)
At low BP concentrations, the variation of the IE with the
cyanide concentration (almost) disappears. The same is true for
the variation of the IE with the BP concentration at high cyanide
concentrations.
9
the electron acceptor by reaction with oxygen. On the basis of
the results of these studies, the detailed mechanism of Scheme
1
is postulated for the photocyanation of BP, and all of the IEs
will be discussed in the framework thereof.
On the basis of Scheme 1, IEs upon BP deuteration may stem
from three reaction steps: (I) ET from BP to the excited DCN,
+
•
(
II) back ET from the DCN radical anion to the BP within
the IP, and (III) electron exchange between neutral BP and its
radical cation. No substantial IE is expected on the addition of
the cyanide anion, since this step is (nearly) diffusion con-
All observations are in agreement with the mechanism
presented in Scheme 1 including electron exchange and a value
of the equilibrium constant for the electron exchange step K <
9
,29,30
trolled,
or on the hydrogen atom abstraction step by the
oxygen radical anion, since this is not a rate-determining step
1
. The IEs are caused by the competition of cyanide and neutral
9
in aerated solutions.
+•
BP in their reaction with BP (eq 4). In the first case,
substitution by cyanide occurs and in the latter electron
exchange.
The importance of step I for the overall IE will be investigated
by measuring the IE on the Stern-Volmer quenching constant.
The importance of steps II and III will be clarified by the study
of the effect of the cyanide and BP concentration on the
magnitude of the IE.
Fluorescence Quenching. BP efficiently quenches the fluo-
rescence of DCN by an ET mechanism.11 To study whether
there is an IE on this ET step, the Stern-Volmer constants for
quenching of singlet excited DCN with BP-h10 and BP-d10 were
measured in acetonitrile. In both cases, linear plots were obtained
(
9
Figure 1) and the Stern-Volmer constants for BP-h10 (kSV )
-
1
-1
3.8 ((0.6) M ) and BP-d10 (kSV ) 91.9 ((2.4) M ) are
identical within experimental error (IE ) 1.02 ( 0.03). In
oxygen-free samples, a value of kSV of 141 M has been
-
1
1
1
measured. The presence of oxygen in our samples reduces
the lifetime of the singlet excited state of DCN. This accounts
for the lower Stern-Volmer constant, since kSV ) kq × τS where
kq is the rate of quenching by DCN and τS is the DCN singlet
lifetime. Competitive quenching of the DCN fluorescence by
oxygen and BP thus lowers the Stern-Volmer constants.
The ET step from BP to singlet excited DCN thus is not
affected by deuteration of BP. The IE being close (or equal) to
unity can simply be understood. The rate of quenching of DCN
(i) The variation of the IE with the BP concentration is the
result of the occurrence of the electron exchange step. The
cyanation at very low BP concentrations has a small normal IE
(vide infra). In that case, the electron exchange between neutral
+
•
BP at BP cannot compete with attack of negatively charged
+
•
cyanide at BP . Upon increasing the concentration of BP,
equilibration by electron exchange becomes important and
11
+•
by BP-h10 has previously been found to be diffusion controlled,
competes with the reaction of BP with cyanide. With K < 1,
and deuterium substitution will hardly affect the rate of diffusion.
Because the IE on the quenching step equals one within
experimental error, this step (step I) will therefore not play a
role in the overall IE on the photocyanation of BP. An IE will
therefore exclusively be caused by steps II and/or III.
increasing the BP concentration decreases the relative amount
+
•
of BP-h10 and the IE becomes smaller.
(ii) The increase of the IE with increasing cyanide concentra-
tion is also caused by the electron exchange. At higher cyanide
+
•
concentrations, reaction of BP with cyanide occurs more
efficiently than equilibration by electron exchange. The extent
of equilibration thus depends on the cyanide concentration. With
K < 1, upon increasing the cyanide concentration, electron
exchange equilibration becomes less important and the IE
becomes larger.
Concentration-Dependent IEs. The photocyanation of mix-
tures of BP-h10 and BP-d10 was achieved using 18-crown-6-
KCN as the cyanide donor to free the cyanide nucleophile from
9
,17
its counterion.
In the study of the intraexperimental IEs on
the photocyanation of naphthalene, the ratio of cyanation at the

Photocyanation of Biphenyl and Dimethylbiphenyls
J. Phys. Chem. A, Vol. 107, No. 39, 2003 7679
Figure 2. Intraexperimental IEs on the photocyanation of BP as a function of the cyanide and the BP concentration.
3
4
(
iii) At very low BP concentrations, equilibration by the
been reported to be inverse. An inverse IE means that relatively
more of the BP-h10 than the BP-d10 radical cation is formed,
resulting in a normal IE on the cyanation reaction in the absence
of electron exchange equilibration, as observed!
electron exchange mechanism does not (or hardly) occur,
because this reaction between a charged and a neutral species
is far slower than the cyanide addition, which is a reaction
between two oppositely charged particles. In that situation, the
cyanide concentration thus has no (or little) effect on the IE.
Similarly, at very high cyanide concentrations, the reaction of
+
•
BP with cyanide (cation-anion reaction) is too fast for the
electron exchange (cation-molecule reaction) to compete and
the BP concentration has no (or little) effect on the IE.
The equilibrium constant K (i.e., the equilibrium IE) for the
electron exchange step can be estimated to be ∼0.9 on the basis
of the asymptotic value to which the IE levels off. Such an
inverse IE is in line with the previously reported electron
exchange equilibrium for thianthrene-h8/-d8 and its radical cation
The IE on the formation of free BP+• from BP and DCN
was independently measured using transient absorption spec-
troscopy. This method has previously been used to determine
35,36
+•
(
equilibrium IE ) 0.62)10 and with computations (vide infra).
the IE on back ET reactions.
The amount of free BP that
Deuterium substitution has been found to reduce the electron
affinity of both positively charged and neutral aromatic sub-
strates.1 The reduced electron affinity of deuterium-substituted
isotopologues is due to the larger electron-donating effect of
is formed immediately after the laser pulse is measured for
hydrogenated and deuterated BP. This method gives an IE on
the formation of free BP of 1.06 ( 0.02. In the absence of
0,32
+•
cyanide, the two competing pathways for the IP are the back
ET and separation of the ions. Because the IE on separation
3
3
deuterium (σI,F ) -0.0011) as compared to hydrogen.
+
•
The IE at very low BP concentrations is significantly larger
than 1 (∼1.10). This is unexpected since the Stern-Volmer
quenching experiments show that the hydrogenated and deu-
terated radical cations of BP are formed with equal efficiency.
Thus, the deviation from 1 cannot be due to an IE on the
(i.e., formation of free BP ) is normal, the IE on the back ET
must be inverse, as predicted from the IEs on the photocyana-
tion. The magnitude of the IE also is in excellent agreement
with our photocyanation result of ∼1.10. Thus, the IE on the
photocyanation at low BP concentrations and at high cyanide
concentrations is governed by the back ET step.
+
•
formation of BP . Furthermore, at these very low BP concen-
trations, equilibration by electron exchange cannot occur (vide
supra). Therefore, the only reaction step that can account for
this IE is back ET within the IP, leading to neutral ground state
BP and DCN (eq 5). The back ET step within an IP is usually
exothermic and thus lies in the “Marcus inverted region”, so
the rate of the ET decreases with increasing driving force.
Because the BP-d10 radical cation is more stable than the BP-
h10 radical cation (vide supra), back ET will be less exothermic
and thus faster for BP-d10 than for BP-h10. In agreement with
this, the IEs on the back ET step for benzene and p-xylene have
Salt Effect. The ionic strength of a solution has a significant
3
7
effect on the lifetime of radical ions. Because the lifetime of
+•
BP plays a crucial role in its availability for electron exchange
equilibration prior to cyanation (eq 4), ionic strength is expected
to affect the IE. In Figure 3, the IE is given as a function of the
ionic strength I, increased by addition of 18-crown-6/KPF6,
while keeping the BP and cyanide concentrations constant.
The variation of the IE of cyanation of BP with the ionic
strength provides further evidence for the radical cation
equilibration causing the concentration-dependent IE. For

7680 J. Phys. Chem. A, Vol. 107, No. 39, 2003
Gronheid et al.
Figure 3. Dependence of the IE on the photocyanation of BP on the
ionic strength, varied by addition of 18-crown-6/KPF , at constant BP
22.9 mM) and cyanide (6.2 mM) concentrations.
6
(
instance, when equilibration occurs by singlet exciton exchange
eq 1), there is no effect of the ionic strength on the IE, since
(
Figure 4. IEs of the photocyanation of 2,2′-DMBP (2), 3,3′-DMBP
in that case no charged species is involved in the equilibration
(9), and 4,4′-DMBP ([) as a function of the DMBP concentration at
9
step.
constant ionic strength (I ) ∼50 mM) and cyanide concentration (∼10
In the present case, the decrease of the IE with increasing
mM).
+
•
ionic strength is due to the increase in stability of BP . At
higher ionic strengths (more inert 18C -KPF6 salt added), the
lifetime of radical cations is higher, because the positively
charged ions are shielded from the negatively charged cyanide
ions by the hexafluorophosphate ions.38 Their shielding results
in a significant reduction of the efficiency of the photocyanation
reaction. Because the electron exchange reaction that causes
the concentration dependence of the IE does not involve an ion-
ion reaction, it is less affected by the addition of an inert salt.
The relative larger importance of the ET step (with K < 1) at
higher salt concentrations leads to a lower IE.
6
Upon irradiation of DCN in the presence of the DMBPs, in
principle, four different monoring cyanation products can be
formed from both 2,2′- and 3,3′-DMBP and two from 4,4′-
DMBP. Both 2,2′- and 3,3′-DMBP do yield three monoring
cyanation products (in a ratio of 1:1.5:23.0 and 1:1.1:3.9,
respectively), and 4,4′-DMBP yields two monocyanation prod-
ucts (in a 1:8.7 ratio). The position of ring cyanation in the
products was not determined. Cyanation of the methyl groups
does not occur. The IEs were measured using the product formed
in the largest amount for each compound. The IEs on the
formation of that photocyanation product as a function of the
DMBP concentration are given in Figure 4 (the collected data
are given in Table 2 of the Supporting Information). The
corresponding data for BP are added for comparison.
At higher salt concentrations, BP+• thus is less reactive and
more selective. This conclusion has practical value to widen
the scope of the BP/DCN donor/acceptor system for the
photochemical generation of radical cations, which are inac-
cessible by direct irradiation of a donor.16 In the presence of
Clearly, the trends in the variation of the IEs with the
concentrations of the substrates are influenced by the presence
of methyl groups. Within experimental error, the IE on the
photocyanation of 2,2′-DMBP equals 1 throughout the explored
concentration range. The IE on the photocyanation of 3,3′-
DMBP slightly decreases with increasing DMBP concentration
and that of 4,4′-DMBP increases. At very low DMBP concen-
trations, the IE for both compounds is ∼1.1. The IEs for 3,3′-
and 4,4′-DMBP level off to ∼1.05 and ∼1.15, respectively.
These trends can also be rationalized using the mechanism
of photocyanation as proposed for BP (see Scheme 1) but now
with an equilibrium constant K > 1 for the electron exchange
process (see eq 2). The 3,3′- and 4,4′-DMBP exhibit normal
equilibrium IEs on the electron exchange step, since they both
level off to a value higher than 1 at high concentrations. The
normal equilibrium IEs are caused by hyperconjugative stabi-
lization of the positive charge on the aromatic ring by the methyl
groups. This stabilization is reduced upon deuteration, resulting
more than one electron donor, the selectivity of their formation
can be altered by changing the ionic strength of the solution,
especially for differently charged electron donors.
Methyl-Substituted BPs. The inverse equilibrium IE on the
electron exchange between BP and BP+ is due to the inductive
electron-releasing effect of the ring deuterium atoms (vide
supra). In the case of deuteration of the methyl group of
methylated BPs, a normal equilibrium IE is expected on the
basis of the larger hyperconjugative stabilization of the cationic
charge by methyl hydrogen atoms than by methyl deuterium
•
3
9
atoms. This effect is much larger than the inductive effect.
This means that now upon increasing the concentration of the
substrate the IE is expected to increase. The magnitude of the
hyperconjugative effect and thus of the equilibrium IEs of
methyl-substituted BPs will depend on the charge on the
adjacent (aromatic) carbon atom and can therefore give informa-
tion about the charge distribution in the radical cations. The
influence of ortho methyl substituents in BP is of special interest.
In neutral BP, the planes of the two aromatic rings are at a
+•
in preferential formation of hydrogenated DMBP .
The magnitudes of the (inverse) IEs on the back ET step are
similar for 3,3′- and 4,4′-DMBP, as reflected in their equal
normal IE (∼1.1) at low DMBP concentrations. This is
remarkable, because xylenes show normal IEs for back ET upon
4
0
41
∼
40° angle according to experiment and calculations, but
+
•
42
41b
in the case of BP , experiments as well as computations
indicate a more planar structure (0-20°). Methyl substituents
at the ortho positions prevent planarity and thus the conjugation
of the two phenyl rings. This may affect the charge distribution
in the radical cation and therefore also the equilibrium IE.
35
methyl deuteration, so an inverse IE for cyanation at low
concentrations was expected. The reason for this dichotomy is
not clear.
Although the IEs for 3,3′- and 4,4′-DMBP are similar at low
DMBP concentrations, they deviate significantly at higher
concentrations. The IE for 3,3′-DMBP becomes smaller with
increasing DMBP concentration, because the electron exchange
equilibrium IE is smaller than the IE on the formation of the
IEs on back ET within an IP are also very different upon
deuteration of a methyl substituent instead of the aromatic ring.
For several methyl-substituted benzenes, normal IEs on the back
35,36
ET step are reported.

Photocyanation of Biphenyl and Dimethylbiphenyls
J. Phys. Chem. A, Vol. 107, No. 39, 2003 7681
TABLE 1: PM3 and B3LYP/6-31G(d,p) Calculated ZPE
Differences (in cal/mol) for the Electron Exchange Equilibria
of Eq 6 and the Equilibrium IE (K) at 285 Ka
PM3
ZPE
B3LYP/6-31G(d,p)
experiment
IE
∆
IE
∆ZPE
IE
BP
+65
+134
-46
0.89
0.79
1.08
1.26
+117
+19
+58
-39
0.81
0.97
0.90
1.07
0.9
2
3
4
,2′-DMBP
,3′-DMBP
,4′-DMBP
1.00
1.05
1.2
-129
a
The (estimated) experimental values are added for comparison.
+
•
free DMBP . Similarly, the IE for 4,4′-DMBP becomes larger
with increasing DMBP concentration, because the electron
exchange equilibrium IE is larger.
Figure 5. Changes in NBO charge density in the B3LYP/6-31G(d,p)
optimized structures upon ionization of BP and the DMBPs.
Taking the IE upon deuteration of the methyl groups as a
measure of the charge distribution within the radical cations of
the BPs, the positions in order of diminishing charge are 4 >
π-systems, moving the two rings in a plane and (ii) steric
interaction of the methyl groups with the ortho hydrogen atoms
of the phenyl rings, forcing the rings out of a plane. Semiem-
pirical calculations, such as PM3, often give a poor description
of steric interactions, and this may well cause the deviation of
the calculated from the experimental IE for 2,2′-DMBP.
Surprisingly, the results of the DFT ab initio calculations
deviate more from the experimental values than the PM3
calculations, but in this case, the IE for 2,2′-DMBP is calculated
correctly. At this level of theory, the radical cations are
calculated not to be planar. The torsion angle between the two
3
. This is in agreement with the results of semiempirical and
ab initio calculations of the magnitudes of the equilibrium IEs
and of the changes in the atomic charges, as shown in the next
section.
For 2,2′-DMBP, one may conclude that apparently there is
no IE on either the back ET step or the electron exchange
equilibrium. The first possibility accounts for a unity IE at low
DMBP concentrations and the second for a unity IE at high
concentrations. However, an equilibrium IE of one is unlikely,
because a significant charge increase is calculated at the 2- and
+•
+•
phenyl rings is ∼20° for BP , 3,3′-, and 4,4′-DMBP and 50°
2
′-positions in 2,2′-DMBP upon ionization (vide infra). Alter-
+•
for 2,2′-DMBP . Overestimation of π-delocalization by the
natively, the equilibrium IE may be significant but is not
expressed because the electron exchange is slow. In that case,
equilibration cannot compete with cyanation. The inability of
density functional theory method is probably the reason for the
poor results for the equilibrium IEs in the higher level
45
calculations.
+
•
2
,2′-DMBP to acquire a (near) planar geometry causes the
Calculations additionally provide the opportunity to check
whether the magnitude of the equilibrium IEs in the DMBPs
correlates with the changes in charge distribution upon ioniza-
tion. The changes in the atomic charges calculated using the
NBO method are presented in Figure 5, along with the results
for the parent BP.
charge to be mainly localized on one of the two phenyl rings.
The molecule behaves more as a monosubstituted benzene than
as a BP in the electron exchange. This may slow the exchange.
+
•
A third possibility is that because 2,2′-DMBP is not planar
+•
(
and thus less stable than BP+• and the two other DMBPs )
the cyanation occurs before dissociation of the IP occurs. As
argued before, the dissociation is crucial for the electron
exchange equilibration. The results presented here do not allow
For all four compounds, the positive charge increase is largest
at the 4-position and significantly lower at the 1-, 2- (or 6-),
and 3- (or 5-) positions. The charge differences are 4 . 1 ≈ 2
a choice between the explanations.
>
3 in decreasing order. Clearly, the IEs cannot give information
Calculations. Semiempirical3
2c,43
and ab initio44 molecular
about the charge at the 1-position, since substitution at that
carbon atom is not possible. The magnitude of the equilibrium
IE of 2,2′-DMBP does not correlate with the charge increase.
On the basis of the calculated charge increase, an equilibrium
IE between 1.05 (that of 3,3′-DMBP) and 1.2 (that of 4,4′-
DMBP) is expected. This prediction not only deviates from the
experimental value but also deviates from the calculated values.
The factors governing the value of the equilibrium IE of 2,2′-
DMBP are thus more complex than those for the other DMBPs
and are not fully understood. However, it is likely that steric
interactions by the methyl groups play a dominant role. The
magnitudes of the equilibrium IEs of 3,3′- and 4,4′-DMBP on
the other hand do correlate with the relative charge increase at
the 3- and 4-positions. The equilibrium IE is smaller for 3,3′-
than for 4,4′-DMBP (1.05 and 1.2, respectively), indicating that
the charge increase at the 3-position is smaller than at the
orbital calculations have proven to be useful in the evaluation
of electron exchange equilibrium IEs. Generally, however, the
origin of an equilibrium IE is complex, since the total change
in ZPE stems from a number of opposing IEs on the various
4
3a
vibrations.
K
+
•
•
+•
C H + C D
{\
} C D + C H
1
2
10
12 10
12 10
12 10
(6)
K
+
6
+•
C H + C H D
{
\
} C H D + C H
1
4
14
14
8
14
8
6
14 14
In Table 1, the changes in ZPEs for the electron exchange
equilibria depicted in eq 6 are given. From these, the equilibrium
IEs at 285 K, the temperature at which the experiments were
performed, are calculated.
4
-position, in agreement with the calculations.
The PM3 calculations give good predictions of the directions
as well as the magnitudes of the equilibrium IEs, with the
exception of the IE for 2,2′-DMBP. In the PM3 calculations,
Conclusions
+
•
all cations studied are planar except 2,2′-DMBP (for this
cation, the angle between the two rings is 40°). The angle
between the two aromatic rings is determined by two opposing
effects: (i) maximization of the conjugation of the phenylic
The magnitudes of the IEs measured in the photocyanation
reaction of BP initiated by excitation of DCN depend on the
BP as well as on the cyanide concentration. Also, the ionic
strength of the medium affects the IE. These observations can

7682 J. Phys. Chem. A, Vol. 107, No. 39, 2003
Gronheid et al.
be fully understood within a mechanism, which involves electron
exchange of the BP radical cation with neutral BP. Competition
of cyanide addition with electron exchange results in the
observed concentration dependences. At high BP concentrations,
the IE is governed by the equilibrium IE, which is inverse
because of the electron-releasing character of deuterium. This
is confirmed by computation of the IE with semiempirical and
ab initio DFT methods. At low BP concentrations, the IE is
normal, which is caused by an inverse IE on the back ET within
the initially formed IP.
The concentration-dependent IEs observed upon methyl
deuteration in the corresponding photocyanation reaction of 3,3′-
and 4,4′-DMBP are satisfactorily interpreted within the same
mechanism. Here, the equilibrium IEs are normal, due to the
decreased hyperconjugative stabilization of the positive charge
by the methyl group upon deuteration. This interpretation is
confirmed by calculations of the change in charge distribution
upon ionization. The equilibrium IE is highest in the compound
that is methylated at the position of the largest charge increase
(12) Gould, I. R.; Ege, D.; Moser, J. E.; Farid, S. J. Am. Chem. Soc.
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(13) Brancaleon, L.; Brousmiche, D.; Johnson, L. J. Can. J. Chem. 1999,
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Chem. 1958, 23, 311.
(
(
(
(
(
19) Friedman, L.; Shechter, H. J. Org. Chem. 1961, 26, 2522-2524.
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Rabinovitz, M.; Shaw, W. C. J. Org. Chem. 1979, 44, 1936-1941. (b)
Bailey, P. S.; Erickson, R. E. Organic Syntheses; Wiley: New York, 1973;
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(
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(
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(the para position).
(24) Atkins, P. W. Physical Chemistry, 5th ed.; Oxford University
Remarkably, no concentration-dependent IE is found for 2,2′-
Press: Oxford, 1994; p 872.
DMBP. This is not expected from either the calculations of the
equilibrium IE or the change in charge distribution upon
ionization. The steric interactions that are introduced upon
methyl substitution of the ortho position of BP likely play a
crucial role. It is not clear whether the absence of an IE is the
result of slow electron exchange equilibration or due to
cyanation before dissociation of the initially formed IP.
(25) Stewart, J. J. P.; Fujitsu Ltd. MOPAC 93; Fujitsu Ltd.: Tokyo,
Japan, 1993.
(26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M.
W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.6 and A.7;
Gaussian, Inc.: Pittsburgh, PA, 1998.
Acknowledgment. We thank Dr. Fred Brouwer (University
of Amsterdam) for performing the time-resolved transient
absorption experiments. We also acknowledge Prof. Jan Cor-
nelisse (Leiden University) for his useful comments and
continuing interest in our research.
(27) Glendeling, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO
version 3.1.
(
28) (a) Kitagawa, F.; Murase, M.; Kitamura, N. Chem. Lett. 2001, 786-
Supporting Information Available: Tables of IEs on the
photocyanation of BP as a function of the BP and DCN
concentration and 2,2′-, 3,3′-, and 4,4′-DMBP as a function of
the DMBP concentration. This material is available free of
charge via the Internet at http://pubs.acs.org.
787. (b) Kitagawa, F.; Murase, M.; Kitamura, N. J. Org. Chem. 2002, 67,
2
4
524-2531. (c) Kitagawa, F.; Kitamura, N. Phys. Chem. Chem. Phys. 2002,
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6
958.
(
31) At low BP concentrations, the yield of cyanation product is low.
This causes more scatter in the IEs.
32) (a) Stevenson, G. R.; Sturgeon, B. E.; Vines, K. S.; Peters, S. J. J.
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