Conformational Studies of Phenyl- and (1-Pyrenyl)triarylmethylcarbenium Ions: Semiempirical Calculations and NMR Investigations under Stable Ion Conditions

Article abstract of DOI:10.1021/jo9715480

Highly stable crowded carbenium ions such as (1-pyrenyl)diphenylmethylcarbenium ion (2) and 1,6- (3) and 1,8-bis(diphenylmethylenium)pyrene [dication] (4) and their dibrominated analogues (3Br and 4Br) have been studied at low and at ambient temperatures.

Full text of DOI:10.1021/jo9715480

J . Org. Chem. 1998, 63, 1827-1835
1827
Con for m a tion a l Stu d ies of P h en yl- a n d
(1-P yr en yl)tr ia r ylm eth ylca r ben iu m Ion s: Sem iem p ir ica l
Ca lcu la tion s a n d NMR In vestiga tion s u n d er Sta ble Ion Con d ition s
Poul Erik Hansen,* J ens Spanget-Larsen, and Kenneth K. Laali
Department of Life Sciences and Chemistry, Roskilde University, P.O. Box 260, DK-4000 Roskilde,
Denmark, and Department of Chemistry, Kent State University, Kent, Ohio 44242-0001
Received August 19, 1997
Highly stable crowded carbenium ions such as (1-pyrenyl)diphenylmethylcarbenium ion (2) and
1,6- (3) and 1,8-bis(diphenylmethylenium)pyrene [dication] (4) and their dibrominated analogues
(3Br and 4Br ) have been studied at low and at ambient temperatures. 2 shows a conventional
two-ring flip (ph,ph), whereas the disubstituted pyrene derivatives show one-ring flips (py) and
two-ring flips (ph,ph) with a higher rotational barrier in agreement with AM1 calculations. A series
of calculations show that the AM1 method gives the transition-state energies in best agreement
with experiment. The propeller-shaped geometry of these molecules is reflected in characteristic
low-frequency resonances of the phenyl rings. At low temperature, 3 and 4 exist as rotational
isomers with C2 and Cs (or Ci) symmetry. In 4, steric interaction makes the two rotamers slightly
different in energy (1 kcal). For 4 or 4Br , the 13C chemical shift differences between the C2 and
the Cs species of 4 or 4Br correlate roughly with the calculated charge differences between the C2
and the Cs species. The charge at the C+ carbon is most extensively delocalized in 2, whereas in
3 and 4 with two C+ groups the pyrene moieties are less effective in charge delocalization.
In tr od u ction
methyl cations. Since there is no degeneracy in these
carbenium ions, flipping of the rings can be followed by
DNMR; the rotational patterns can be determinants in
the stability and reactivity of these ions.
Our computational studies began with the trityl cation
for which the X-ray structure is known;3 elegant dynamic
NMR studies on substituted trityl cations are also
available.15,16
By use of semiempirical MO and force field calcula-
tions, the minimum energy conformations of trityl cation
and the preferred ring rotations were evaluated and
compared with experiment. Subsequently, the conforma-
tions and the charge distribution pattern in the more
crowded (1-pyrenyl)diphenylmethyl cations and their
related dications (see Figure 1) were probed along with
DNMR studies of the persistent ions.
Several classes of sterically crowded triarylcarbenium
ions are known. Now a textbook example, triphenyl-
methyl (trityl) cation, is probably the most thoroughly
studied member and the first long-lived carbocation to
be studied.1,2 The importance of quinoidal resonance
contributors in trityl cation and its propeller-shaped
structure are well established.3
In comparison, the PAH-substituted carbenium ion
analogues and the mode of charge delocalization in such
species are not as well studied, except for triazulenyl-
methyl cations whose dynamics have been a subject of
intense recent work by the Asao group.4-6
Carbenium ions R to PAHs are interesting models of
PAH epoxide ring opening en route to PAH-DNA adduct
formation.7-9
Their charge delocalization mode(s) and dynamics are,
therefore, important in relation to mechanistic carcino-
genesis and PAH activation by electrophilic pathways.
The present paper deals with generation, dynamic
NMR, and semiempirical calculations (MMX,10 AM1,11
PM3,12 MNDO,13 MINDO/314) on (1-pyrenyl)diphenyl-
Resu lts
Th eor etica l Stu d ies. To gain some insight into the
factors influencing conformational interconversion pro-
cesses, their activation enthalpies were studied by a
(10) Gajewski, J . J .; Gilbert, K. E.; McKeIvey, J . Adv. Mol. Model.
1990, 2, 65.
(11) (a) Dewar, M. J . S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J . J .
P., J . Am. Chem. Soc. 1985, 107, 3902. (b) Dewar, M. J . S.; Zoebisch,
E. G. THEOCHEM 1988, 49, 1.
* To whom correspondence should be addressed. Tel.: +45 46742432.
Fax: +45 46743011. E-mail: POULERIK@virgil.ruc.dk.
(1) Carbonium Ions, Bethell, D., Gold, V., Eds.; Academic Press:
London, 1967.
(2) Superacids; Olah, G. A., Prakash, G. K. S., Sommer, J .; J .
Wiley: New York, 1985.
(3) Gomes de Mesquita, A. H.; MacGilvary, C. H.; Eriks, K. Acta
Crystallogr. 1965, 18, 437.
(4) Ito, S.; Fujita, M.; Morita, N.; Asao, T. Chem. Lett. 1995, 475.
(5) Ito, S.; Morita, N.; Asao, T. Bull. Chem. Soc. J pn. 1995, 68, 2011.
(6) Ito, S.; Morita, N.; Asao, T. Bull. Chem. Soc. J pn. 1995, 68, 1409.
(7) Nashed, N. T.; Rao, T. V. S.; J erina, D. M. J . Org. Chem. 1993,
58, 6344.
(12) Stewart, J . J . P. J . Comput. Chem. 1989, 10, 209, 221; 1990,
11, 543.
(13) (a) Dewar, M. J . S.; Thiel, W. J . Am. Chem. Soc. 1977, 99, 4899,
4907. (b) Dewar, M. J . S.; McKee, M. L. J . Am. Chem. Soc. 1977, 99,
5231. (c) Dewar, M. J . S.; Rzepa, H. S. J . Am. Chem. Soc. 1978, 100,
58, 777, 784.
(14) (a) Bingham, R. C.; Dewar, M. J . S.; Lo, D. H. J . Am. Chem.
Soc. 1975, 97, 1285, 1294, 1302, 1307. (b) Dewar, M. J . S. Science 1975,
187, 1037.
(8) Nashed, N. T.; Bax, A.; Loncharich, R. J .; Sayer, J . M.; J erina,
D. M. J . Am. Chem. Soc. 1993, 115, 1711.
(9) Nashed, N. T.; Sayer, J . M.; J erina, D. M. J . Am. Chem. Soc.
1993, 115, 1723.
(15) Schuster, I. I.; Colter, A. K.; Kurland, R. J . J . Am. Chem. Soc.
1968, 90, 4679.
(16) Rakshys, J . W., J r.; McKinley, S. V.; Freedman, H. H. J . Am.
Chem. Soc. 1970, 92, 3518; 1971, 93, 6522.
S0022-3263(97)01548-X CCC: $15.00 © 1998 American Chemical Society
Published on Web 02/20/1998

1828 J . Org. Chem., Vol. 63, No. 6, 1998
Hansen et al.
F igu r e 1. Numbering of molecules. The endo ring (e) atoms
are marked with a prime and the exo ring (x) atoms with a
double prime.
number of theoretical procedures. Entropy effects were
not considered, and the cations 1-4 were treated as free,
unsolvated species. Solvent effects and effects due to
complex formation with counterions are thus neglected.
Interpretation of the present results should be made with
due caution, particularly for the doubly charged species
3 and 4.
F igu r e 2. AM1 ground-state structure and different transi-
tion states corresponding to one-ring, two-ring, and three-ring
flip for (a) the trityl cation 1 and (b) the (1-pyrenyl)diphenyl-
methyl cation 2, with indication of the predicted activation
enthalpies (kcal/mol).
Ta ble 1. Ca lcu la ted Activa tion En th a lp ies (k ca l/m ol) for
Con for m a tion a l In ter con ver sion in th e Tr ityl Ca tion (1)
The conformational transition-state (TS) structures
were generally estimated by the assumption of idealized
geometries: The flipping rings were fixed in conforma-
tions perpendicular to the plane of the cation center, and
the remaining rings were kept coplanar with this plane.
All other geometrical parameters were optimized by the
respective calculational methods. The energy gradients
obtained for the idealized TS structures were in all cases
close to zero (single-point calculations without restraints),
consistent with the assumption that the structures
correspond to saddle points on the potential energy
hypersurface. The individual TSs are identified by
indication of the rings involved in the flipping process;
e.g., the notation (ph,ph) refers to a TS where two phenyl
rings are perpendicular to and the remaining ring
coplanar with the plane of the cation center.
Th e Tr ityl Ca tion (1). Several calculational proce-
dures were tested for their ability to reproduce the
available experimental results for the trityl cation, 1. The
methods applied were the MMX molecular mechanics
method10 and the semiempirical MO procedures AM1,11
PM3,12 MNDO,13 and MINDO/3,14 using commercially
available software.17,18 The predicted conformational
activation enthalpies for one-ring (ph), two-ring (ph,ph),
and three-ring (ph,ph,ph) flip in 1 are given in Table 1.
The results diverge considerably; the AM1 and MMX
results seem most consistent with the experimental
evidence:15,16 They favor the two-ring flip process (ph,ph)
and predict an activation enthalpy reasonably close to
the experimental estimate of 9 kcal/mol (1 kcal ) 4.184
one-ring
two-ring
three-ring
flip (ph)
flip (ph,ph)
flip(ph,ph,ph)
AM1
PM3
MNDO
MINDO/3
MMX
11.6
6.6
13.0
9.4
10.7
7.9
4.2
1.4
10.7
30.2
26.1
19.0
6.2
18.1
23.8
kJ ). The TS structures predicted by AM1 are indicated
in Figure 2. This method predicts that the two-ring
process is favored over the one-ring process by about 1
kcal/mol; in the case of PM3, this situation is reversed.
MNDO and MINDO/3 predict relatively low activation
energies, apparently as the result of a different balance
between steric and conjugative effects; steric effects are
“harder” in these methods, compared to the later devel-
opments AM1 and PM3.11-14 In the following, we shall
refrain from further application of the MNDO and
MINDO/3 methods.
Th e (1-P yr en yl)d ip h en ylm eth yl Ca tion (2). Calcu-
lations on 2 were performed with AM1, PM3, and MMX.
Under the present assumptions of idealized TS structures
(see above), the TSs for two-ring flip (py,ph) involving
the py (1-pyrenyl) group and one of the phenyl groups
have the same activation energy, irrespective of whether
it is the e-ph (endo-phenyl) or x-ph (exo-phenyl) ring that
is involved in the process. The predicted activation
enthalphies are given in Table 2, and TS structures
computed with AM1 are indicated in Figure 2. In the
case of MMX, it was not possible to compute TS struc-
tures corresponding to e-ph or x-ph one-ring flipping;
these structures where the py group is coplanar with one
of the ph groups are highly hindered (see Figure 2).
(17) PCMODEL for Windows; Serena Software, 1993, Box 3076,
Bloomington, IN 47402-3076.
(18) HyperChem for Windows, Release 4.5, 1995, Hypercube, Inc.,
419 Philip St., Waterloo, Ontario, Canada N2L 3X2.

Studies of Phenyl- and (1-Pyrenyl)triarylmethylcarbenium Ions
J . Org. Chem., Vol. 63, No. 6, 1998 1829
F igu r e 4. AM1 structures for stable conformers of the 1,6-
and 1,8-bis(diphenylmethylenium) pyrene dications 3 and 4
(relative energies in kcal/mol). The molecular symmetry is C2
for 3A and 4A, Ci for 3B, and Cs for 4B.
F igu r e 3. AM1 structures of initial ground state, transition
state, and the product ground state of (a) two-ring (ph,ph) flip
and (b) two-ring (py,x-ph) flip in the (1-pyrenyl)diphenylmethyl
cation 2. Since flipping of the 1-pyrenyl group is not involved
in the (ph,ph) process, the endo-phenyl remains endo, and the
exo-phenyl remains exo; equivalent positions within each
phenyl group are equilibrated (as exemplified by the marked
hydrogen atoms) but positions on different phenyl groups are
not. The (py,x-ph) process involves pyrenyl flipping, and the
exo-phenyl is transformed into endo and vice versa; in com-
bination with the corresponding (py,e-ph) process (Figure 2)
all equivalent positions on the two phenyls are equilibrated.
Ta ble 3. Rela tive AM1 En th a lp ies (k ca l/m ol) for
Con for m er s of th e 1,6- a n d
1,8-Bis(d ip h en ylm eth ylen iu m )P yr en e Dica tion s (3 a n d 4)
a n d for Low -En er gy In ter con ver sion a l Mod el Tr a n sition
Sta tesa
2
3
4
conformer A
conformer B
TS (py,e-ph)
TS (py,x-ph)
TS (py)
(0)
(0)
0.1
7.5
8.4
8.4
(0)
1.1
8.7
8.7
9.6
11.4
11.4
12.7
4.8
Ta ble 2. Ca lcu la ted Activa tion En th a lp ies (k ca l/m ol) for
Con for m a tion a l In ter con ver sion s in th e
(1-P yr en yl)d ip h en ylm eth yl Ca tion (2)
TS (ph,ph)
13.3
13.9
a
one-ring flips
two-ring flips
Corresponding data for the (1-pyrenyl)diphenylmethyl cation
three-ring flip
(py,ph,ph)
(2) are included for comparison.
(py) (e-ph) (x-ph) (py,ph) (ph,ph)
AM1
PM3
MMX 21.7
12.7
8.2
19.5
13.2
32.3
25.2
11.4
8.7
8.8
4.8
1.5
10.8
29.5
24.5
18.0
sponding to two-ring (ph,ph). Hence, according to AM1
and PM3, the lowest TS of 2 corresponds to two-ring
(ph,ph) where py does not flip, well separated from higher
TSs with py flipping. This prediction is possibly signifi-
cant: The two-ring (ph,ph) flip does not equilibrate
different ph group positions (endo remains endo, and exo
remains exo) but equilibrates the two ortho and the two
meta positions on each ph group; see Figure 3. Equili-
bration of endo and exo ph requires py flip, see Figure 3,
but this requires additional energy. AM1 computes
activation enthalpies close to 5 and 11 kcal/mol for the
two processes; according to this prediction, it should be
possible to observe the two different equilibration pro-
cesses by variable-temperature NMR.
Th e 1,6- a n d 1,8-Bis(d ip h en ylm eth ylen iu m )p y-
r en e Dica tion s 3 a n d 4. The dications 3 and 4 are
rather large species (196 valence orbitals) and were
studied only by the AM1 method. For both compounds,
two ground-state minima corresponding to different
conformers A and B were located. The stable conformers
3A and 3B (C2 and Ci symmetry) and 4A and 4B (C2 and
Cs symmetry) are indicated in Figure 4.
AM1 and PM3 clearly predict that two-ring (ph,ph)
flipping requires the lowest activation enthalpy for 2. The
computed TS for this process is stabilized by 4-6 kcal/
mol relative to the corresponding process in 1 (Table 1).
This is easily explained by the greater conjugative effect
of the pyrene π-system that becomes coplanar with the
C+ center during the (ph,ph) process in the case of 2
(Figure 2). According to AM1, the formal net charge of
the cation center is reduced from +0.385 in the (ph,ph)
TS of 1 to +0.244 in the corresponding TS of 2, indicating
the increased delocalization of the positive charge. How-
ever, the MMX results do not agree with this trend. The
MMX force field includes an SCF treatment of the
π-system, but inspection of MMX charge distributions
indicates that the method hardly distinguishes between
the conjugative effects of a phenyl group and a 1-pyrenyl
group. The MMX method thus tends to underestimate
the conjugative stabilization of the TS correponding to
two-ring (ph,ph) flipping in 2.
The second lowest TS energy is predicted by AM1 and
PM3 to correspond to two-ring (py,ph) or one-ring (py).
Both of these processes involve flipping of the 1-pyrenyl
group, and their TSs are predicted to be fairly close in
energy and 6-7 kcal/mol above the lowest TS corre-
Conformational interconversion around one cation
center transforms conformer A into conformer B and vice
versa. Conformers 3A and 3B have essentially similar
energy, but conformer 4A is predicted to be about 1 kcal/
mol more stable than 4B, probably as a result of slight

1830 J . Org. Chem., Vol. 63, No. 6, 1998
Ta ble 4. 13C Ch em ica l Sh iftsa of 2-4Br
C-3 C-4 C-5 C-6
127.9 129.9 141.9 135.7
Hansen et al.
compd T (°C)
C-1
146.5
C-2
142.8
C-7
130.8
C-8
135.0
C-9
136.6
C-10
2
2
2
2
300
225
205
194
128.4
144.0
143.7
143.5
141.8
141.9
141.7
126.9
126.8
126.6
128.5
188.3
128.2
140.4
140.3
140.2
133.8
133.6d
133.4
129.2
129.0
128.9
133.0
132.8d
132.7
134.6
134.4
134.25
127.3
127.2
127.1
3
3
300
215
141.8d
139.8g
142.2d
139.6g
131.0
129.4
134.1
132.3
134.3
133.3
142.2
139.6
131.0
129.4
134.1
132.2
134.4
133.3
3Br
3Br
300
215
144.9 (br) 139.5
127.4
125.5
134.7
133.8
132.7
130.8
149.9 (br) 139.5
127.4
125.5
134.7
133.8
132.7
130.8
143.1
137.3
143.1
137.3
4
4
300
215
143.1
143.3
131.8
130.19,
129.99
137.7
136.15,
135.93
137.7
136.15,
135.93
131.8
130.19
143.3
143.1
140.7
131.2
129.50,
129.51
131.2
129.50,
129.51
140.82,h
141.8k
141.8k
140.74h
4Br
4Br
300
205
140.6
144.7
127.5
130.7
129.20
(br)
130.7
129.20
(br)
127.5
144.7k
140.6k
137.99,i
137.88
136.9
136.9
137.99,i
137.88
142.00,i
142.55
125.64,i
125.47
125.64,i
125.47
142.00,i
135.26,i
135.04
135.26,i
135.04
142.55k
b
d
g
h
aIn ppm. br, broad. c Hidden. May be interchanged. e Second leg is hidden. f May be interchanged. May be interchanged. Two
i
j
decimals are given so that the separation between the two resonances can be estimated. Are pairwise coalesced at 225 K. Broad but
not coalesced at 225 K. Assignment tentative.
k
steric interference of the two endo-phenyl groups in the
latter. TS structures for conformational rearrangement
around one cation center were estimated assuming
idealized geometries as explained above. Calculated
relative energies for low-lying TSs are included in Table
3, and the corresponding molecular structures are indi-
cated in Figure 5.
In the (1-pyrenyl)diphenylmethyl monocation system,
2, two-ring (ph,ph) flip was clearly predicted as the
process with lowest AE, 4.8 kcal/mol; see above. The
corresponding TS was predicted to be stabilized by the
conjugative effect of the 1-pyrenyl group py, which
becomes coplanar with the cation center during this
process. However, in the dications 3 and 4 the (ph,ph)
rotation process is much less favored, with predicted
activation energies between 13 and 14 kcal/mol. The py
moiety in 3 and 4 carries a diphenylmethylenium sub-
stituent and is formally positively charged; stabilization
of the (ph,ph) TS due to the conjugative effect of the py
group is thus virtually eliminated.
At least three processes in the dication system are
predicted with lower activation enthalpy than (ph,ph),
all involving py flipping. During these rearrangements,
one or both ph groups becomes coplanar with the cation
center, and conjugation with these formally uncharged
groups leads to more efficient TS stabilization. In fact,
the activation enthalpies for these processes are predicted
to be slightly lower than for the corresponding processes
in the monocation 2 (Table 3). The situation is thus
predicted to be significantly different for the dication and
the monocation systems. For the dications, the lowest
activation enthalpies are predicted for (py,ph) or (py)
flippings. All dynamic processes considered lead to
equilibration of equivalent positions in the pyrene ring
systems of the A and B conformers. Moreover, a combi-
nation of (py,e-ph) and (py,x-ph) processes, which are
predicted to have rather similar activation enthalpies
(Table 3), lead to equilibration of equivalent positions in
the phenyl rings.
NMR Stu d ies. Sa m p le P r ep a r a tion . The carbe-
nium ions were generated from the corresponding alco-
hols at dry ice/acetone temperature with FSO3H/SO2ClF
1
(a few drops of CD2Cl2 served as the NMR lock). The H
and 13C NMR spectra were recorded between 225 and 194
K. The room-temperature reaction of the alcohols with
CF3COOD produced the same carbocations, which were
stable at room temperature. This allowed their NMR
study at ambient temperature (300 K) for comparison
with the low-temperature spectra.
For all the compounds 2-4Br , the presence of a carbon
resonance at ∼200 ppm is clearly indicative of carbenium
ion formation, all having a persistent green color.
Assign m en t of NMR Sp ectr a . The assignments of
the 13C NMR spectra are given in Table 4. The assign-
ments are based on chemical shifts of the parent com-
pounds, substituent effects, and HETCOR spectra.19 For
4 and 4Br the resonances can be divided into two sets
according to intensity, and the more intense is assigned
to the lower energy conformation of isomers A (Figure
5). These splittings appear to be very similar in the two
spectra and therefore provide a relation between reso-
nances observed in the spectra of 4 and 4Br .
1
The assignments of the H NMR spectra were based
on H,H coupling constants, substituent effects on chemi-
cal shifts, and in some cases COSY20 and HETCOR19 2D
spectra. Assignment of all the proton resonances belong-
ing to the phenyl rings was in most cases not possible
due to severe overlap of resonances. However, some very
diagnostic features are visible especially for the low-
frequency resonances of H-2′ and H-3′. These resonances
are discussed as structural gauges (see later).
(19) Bax, A. J . Magn. Reson. 1983, 53, 517.
(20) Bax, A.; Freeman, R. J . Magn. Reson. 1981, 44, 452.

Studies of Phenyl- and (1-Pyrenyl)triarylmethylcarbenium Ions
J . Org. Chem., Vol. 63, No. 6, 1998 1831
Ta ble 4 (Con tin u ed )
C-3a
144.4
C-5a
133.5
C-8a
132.8
C-10a
139.8
C-10b
125.7
C-10c
122.8
C-1′ + C1′′
∼143.3 (br), 142.3 (br),
∼144.1 (br) 140.2 (br)
142.8, 142.1 ∼141.6,
∼139.6
C-2′ + C-2′′ C-3′ + C-3′′ C-4′ + C-4′′
C+
solvent
132.8 (br) 141.5 (br) 201.4
CF3COOD
142.0
141.7
141.7
131.4
131.2
131.0
142.0c
130.7
130.4
131.4
137.3
137.1
126.0
125.8
125.8
123.2
123.0
123.0
130.9,
130.3
140.0
199.5
199.3
199.1
SO2ClF
SO2ClF
SO2ClF
142.8,
∼141.8
142.4,
141.7
142.15, 141.1, 130.8,
139.8,
138.9e
130.2
139.7
139.7,
139.5
142.2, 141.0, 130.6,
∼139.7,c
130.1
138.8
141.5f
139.9g
140.9f
139.5g
141.5
140.9
126.6
124.8
126.6
124.8
144.8
142.5,
143.3
144.9 (br)
143.1,
143.9
144.4
144.0, 142.3, 131.69,
142.3, 141.2 131.66
144.9 (br) 133.4
145.6, 143.1, 131.8
143.1, 139.3
133.1
146.2
144.5
210.0
206.9
CF3COOD
SO2ClF
139.9g
139.5g
144.9 (br) 139.5c
142.1
144.9 (br) 139.5c
142.1
127.1
125.4
127.1
125.4
147.3
145.3
n.o.
205.9
CF3COOD
SO2ClF
137.9
137.9
139.7
138.04,
137.53
139.7
138.04, 139.27,
137.53 138.99
141.2
141.2
126.7
126.7
144.8
144.5
133.0
131.33,
131.22,
130.90
145.9
209.2
206.62, SO2ClF
205.17
CF3COOD
139.27, ∼124.60, ∼124.6, 144.23,c
142.90,k
144.54,k
138.99
∼124
124
143.89,c
143.49,
143.31
142.69,k
144.38,k
142.43,k
143.09,k
142.97k
144.7 (br)
143.26,
143.11,
142.92,
142.55,
142.04,
141.73
144.23,k
143.89k
n.o.
n.o.
n.o.
n.o.
127.1
127.1
n.o.
130.9
146.9
145.74,
145.44,
145.30,
143.13
n.o.
CF3COOD
139.17,i 139.17,i 136.85,j 136.85,i 125.17,i 125.17,i 144.71,
131.58,h
131.41,
131.12
206.00, SO2ClF
204.79
138.87
138.87
136.34
136.34
125.15
125.15
144.21,
143.98,
143.98
F igu r e 5. Model transition-state structures for the most favored conformational interconversions (a) of 3A and 3B and (b) 4A
and 4B (AM1 activation enthalpies in kcal/mol).
The greater simplicity of both the 13C and 1H NMR
spectra at ambient temperature helped in the assignment
process. Other common useful features are intensities
and differential line broadenings.
of the pairs are reduced considerably at 225 K. The
chemical shift difference of C-3′ and C-3′′ is conserved
at 225 K, whereas the resonances of C-4′ and C-4′′ now
coincide. The second resonance of the C-1′,C-1′′ pair is
now clearly observed.
1
Sp ectr a l F ea tu r es. At 194 K the H spectrum of 2
exihibits sharp resonances for the pyrene moiety. Some
of the 1H resonances belonging to the phenyl moieties
are sharp and some are broad. An interesting feature is
the observation of a doublet and a triplet at low fre-
quency, each integrating 2H due to H-2′ and H-3′′ of the
phenyl moieties (Figure 6a). These resonances broaden
at 205 K and move strongly to higher frequency at
225 K.
The ambient-temperature 1H NMR spectrum of 2
shows sharp resonances for the pyrene moiety. A broad
resonance for H-4′,H-4′′ and three broad resonances in
the approximate ratio 1:2:1 for H-2′, H-2′′, H-3′ and H-3′′
are shown as well (Figure 6b). The 13C spectrum shows
two broad resonances for C-1′,C-1′′ and for C-2′,C-2′′,
whereas the C-3′,C-3′′ resonances are close together
(Figure 7b).
The 13C spectrum of 2 at 195 K shows sharp pyrene
resonances. The resonances for the phenyl rings show
two different sharp resonances for C-3′,C-3′′. The same
is true for C-1′,C-1′′ (although one is masked at this
temperature) and for C-4′,C-4′′. However, C-2′ and C-2′′
show two pairs of resonances (Figure 7). The splitting
The H low-temperature spectra of 4 are rather invari-
1
able in the temperature range 205-225 K. The H-4,H-5
resonance of pyrene shows two doublets at high fre-
quency. The H-9,H-10 resonances give rise to likewise
two singlets at low frequency. In the same manner, the
H-2,H-3 and H-6,H-7 resonances are two sets of doublet

1832 J . Org. Chem., Vol. 63, No. 6, 1998
Hansen et al.
between 142 and 146 ppm (Figure 9a). The splittings
characteristic for 4 and 4Br are starting to collapse at
225 K.
In CF3COOD at ambient temperature the picture is
much different for 4Br . In the 1H spectrum the H-2, H-4,
and H-10 resonances of pyrene are sharp and the
resonances belonging to the phenyl groups are somewhat
broadened, whereas in the 13C spectrum only C-4 and
C-10 remain sharp. At 310 K the remaining resonances
are even broader.
1
The low-temperature H spectra of 3 at 215 K show a
high-frequency doublet with small splittings. The same
is found for H-3. Three resonances are outside the main
body of resonances, two doublets, one of which shows
further splittings (see 3Br ) and a triplet (H-3′) at low
frequency.
The 13C spectrum of 3 at 215 K consists of single
resonances for all types of pyrene carbons. The phenyl
ring carbon resonances are a singlet for C-4, C-4′′ and
doublets for C-2, C-2′′ and for C-3,C-3′′ (Figure 9c).
1
The ambient-temperature (300 K) H NMR spectrum
of 3 in CF3COOD shows sharp resonances for all hydro-
gens belonging to the pyrene moiety and for H-4′,H-4′′,
whereas those of H-2′,H-2′′ and H-3′,H-3′′ are broad and
close together. No additional splittings of resonances are
observed. The 13C spectrum of 3 shows 12 sharp singlets.
In the temperature range 195-215 K the 1H NMR
spectrum of 3Br consists of sharp resonances belonging
to the pyrene moiety. The phenyl ring proton part is very
similar to that of 3. At 215 K, the extra splitting of
H-4,H-5 had disappeared and that of one of the doublets
is about to disappear.
The 13C NMR spectrum of 3Br is simple as only the
C-1′, C-1′′ and the C-2′,C-2′′ carbon resonances show two
resonances at 195 and 215K (Figure 9b).
The ambient-temperature spectra for 3Br are similar
to those of 4Br in that they only show a few sharp
resonances (H-4,H-9 and H-5 and H-10 and the corre-
sponding carbon resonances).
F igu r e 6. 1H NMR spectra of 2 at (a) 194 K in SO2ClF and
(b) at 300 K in CF3COOD.
of doublets. The resonances of the phenyl rings show also
a characteristic pattern with a low frequency doublet
(H-2′′) and two low-frequency triplets (H-3′) combined
with a large number of unresolved resonances in the
middle of the spectrum and two triplets to higher
frequency (H-4′ and H-4′′) (Figure 8).
All resonances in the 13C spectrum of 4 at 215 K are
comparatively sharp and are for the pyrene moiety split
into two except for C-4,C-5. The observed splittings are
of variable size and as large as 0.5 ppm. The C+
resonance shows a splitting of 1.45 ppm. Most of the
resonances belonging to the phenyl rings appear to be
doublets judging from the number of resonances, but
considerable overlap makes a detailed analysis difficult.
The C-4′, C-4′′, C-1′, C-1′′, C-2′, C-2′′, and C-2,C-7 carbons
show 13 separate resonances in the 141.8-144.5 ppm
range. The spread is slightly smaller than for 4Br .
The 1H spectrum of 4 in CF3COOD at 300 K shows
sharp resonances for all protons except H-3′ and H-4′ and
no doubling of resonances. The corresponding 13C spec-
trum shows sharp resonances for all carbon resonances
belonging to the pyrene moiety. The C-2′, C-3′, and C-4′
are slightly broadened.
Discu ssion
On the basis of the NMR spectra at different temper-
atures, the dynamics of these crowded triaryl compounds
can be discussed.
For 2, the appearance at 195-215 K of four resonances
for C-2′, C-2′′ shows that the rotation around the
C+-C-1′ and C+-C-1′′ axes is so slow that the C-2′a and
C-2′b resonances are different, likewise for C-2′′a and
C-2′′b. The observation of H-2′ doublet (1H) and the H-3′
triplet (1H) resonances at lower frequencies supports this
and is a common motif for nonaveraging around the
C+-C-1′ and C+-C-1′′ bonds.
Some averaging has started at 225 K. Now only two
resonances are observed for C-2′,C-2′′ and two for
C-3′,C-3′′. For C-4′,C-4′′ a coincidence of resonances
occurs. However, at this temperature two separate
1
The H NMR spectrum of 4Br at 205-215 K is very
1
H-4′,H-4′′ triplets occur in the H spectrum. This shows
similar to that of 4 except that H-2,H-7 is now showing
that the ring is not rotating fast at 225 K. The rotational
picture at 225 K is apparently one in which both phenyl
rings rotate around the C+-C-1′ or C+-C-1′′ axis. As
demonstrated in the theoretical section, the rotation
around the C+-C-1′ or C+-C-1′′ axis is not sufficient to
average the phenyl ring positions. The coalesence point
for phenyl ring flipping can be estimated to approxi-
singlets and the H-4,H-5 resonances are shifted to high
1
frequency. At 225 K the H spectrum is broadened.
The low-temperature 13C NMR spectrum of 4Br is very
similar to that of 3 except that C-3,C-6 resonances are
shifted to low frequency. The C-4′, C-4′′, C-1′, C-1′′, C-2′,
C-2′′, and C-2,C-7 carbons show 15 separate resonances

Studies of Phenyl- and (1-Pyrenyl)triarylmethylcarbenium Ions
J . Org. Chem., Vol. 63, No. 6, 1998 1833
F igu r e 7. 13C NMR spectra of 2 (a) at 194 K in SO2ClF and
(b) at 300 K in CF3COOD.
F igu r e 9. 13C NMR spectrum of (a) 4Br , (b) 3Br , and (c) 3.
The rotational picture for 4 at 215 K is different from
that of 2. The interesting observation is the occurrence
of two sets of resonances in a ca. 1:1 ratio. These two
sets can be explained by the existence of two different
isomers, 4A and 4B (Figure 5). The rotational barrier
is high enough to prevent averaging of these two species
at low temperature, and the phenyl rings are close
enough to interfere with each other. At ambient tem-
perature, averaging takes place and only resonances
corresponding to one species are found. The pyrenyl ring
rotation is not sufficiently fast to give sharp resonances
for H-2′,H-2′′ and for H-3′,H-3′′ at ambient temperature.
The low-temperature spectra for 4Br at 205 K are very
similar to those of 4, but at 225 K some rotation occurs
to average the pairs of resonances. However, the low-
frequency motif mentioned for 2 occurred in the same
F igu r e 8. 1H NMR spectra of 4.
mately 215 K, which gives an activation energy of less
than 40 kJ /mol (10 kcal/mol).
At 300 K the pyrenyl ring is also rotating, but not fast
enough to fully average C-3′ and C-3′′ or C-2′ and C-2′′.

1834 J . Org. Chem., Vol. 63, No. 6, 1998
Ta ble 5. 1H C-h em ica l Sh ifts a n d 3J (H,H) of 2-4Br in CF 3COOD a t 300 K
Hansen et al.
H-2
H-3
H-4
H-5
H-6
H-7
H-8
H-9
H-10
H-2′
H-3′
H-4′
7.90
br
8.30
7.27, 7.32
8.29
br
H-2′′
H-3′′
H-4′′
7.90
br
8.17
7.27, 7.37
8.29
br
2
a
3
a
3Br
a
7.78
8.63
8.11
8.30
8.29
br
8.10
8.10
8.51
8.71
7.93
9.32
7.84
br
8.42
7.58
8.06
8.31
7.75
8.53
8.33
8.02
7.71
9.27
7.93
9.32
7.84
br
7.42
vbrb
∼7.8
vbr
7.85
br
7.63
brb
7.88
vbr
7.85
br
7.53
br
∼7.8
vbr
7.85
br
7.53
br
7.47
vbr
7.85
br
8.53
8.33
8.25
9.24
8.61
br
8.11
8.30
8.29
br
8.25
9.24
8.61
br
4
a
4br
a
8.20
8.36
8.40
s
8.70
8.30
8.75
s
9.23
s
8.75
s
9.23
s
8.70
8.30
8.20
8.36
8.40
s
7.46
s
7.42
s
7.46
s
7.42
s
∼7.7
7.84
br
7.80
br
8.27
7.36, 7.42
8.26
∼7.7
vbr
7.68
br
7.84
br
7.80
br
8.27
7.36, 7.42
8.26
vbr
7.68
br
br
br
a
b
Coupling constants. br, broad; vbr, very broad. c s, singlet.
position, indicating that the rotation of the phenyl rings
is slow. The ambient temperature spectra of 4Br are for
most resonances broad for unknown reasons.
For 3 and 4 the picture is in general similar taking into
consideration the different positions of the substituents.
A comparison of the charge distribution in 3A and 3B
shows very little difference between the two species,
whereas a comparison of the charges of 4A and 4B
reveals small differences that in most cases are propor-
tional to the splittings for the pyrenyl resonances of 4
and 4Br . However, this is not so for the C+ carbons as
the C+ resonances are separated in the former case by
1.45 ppm and in the latter by 1.23 ppm and the charge
difference is as small as 0.0015.
The low-temperature spectra of 3 at 215 K are in many
respects similar to those of 4 except that no doubling of
resonances is seen in the 13C spectrum. For 3 and 3Br
1
only small splittings are observed in the H spectra at
low temperature. These splittings disappear at higher
temperature. Both compounds can exist in forms with
C2 and Ci symmetry (Figure 5). As the substituents are
far away from each other, the interference is minimal
and the ring conformations are very similar in the A and
B isomer. This combined with a lower rotational barrier
(see Table 3) makes the observation of the two species
difficult. That the geometry of the three compounds 2-4
indeed are slightly different is seen from the positions of
the H-2′ and H-3′ chemical shifts (Table 5).
Exp er im en ta l Section
Syn th esis of th e P r ecu r sor Alcoh ols. The crowded
mono- and diols were prepared from the corresponding mono-
and dibenzoyl derivatives of pyrene (or its dibromopyrene) by
reaction with PhLi. The benzoyl derivatives were prepared
by conventional Friedel-Crafts-type benzoylation of pyrene
(or its dibromo derivative). The synthetic details have been
previously published.21
FSO3H (Fluorochem or Aldrich) was doubly distilled in
an all-glass distillation unit and stored in Nalgene bottles.
SO2ClF (high purity grade) was purchased from Aldrich in
glass ampules, stored in a Schlenk tube in the freezer, and
used without further purification.
Sta ble Ion Gen er a tion a n d NMR. The procedure for
stable ion generation in FSO3H/SO2ClF was analogous to our
previously reported methods.22
The ions prepared at ambient temperature were made by
mixing with CF3COOD.
The ambient temperature spectra of 3 show features
identical to those of 4.
For 3Br , the low-temperature spectra are very similar
to those of 3. The 13C spectrum of 3Br , which is by far
the most simple of all spectra, shows two separate
resonances for C-2′ but not for C-2′′ (they may be
interchanged). The resonances for C-1′,C-1′′ have merged
into a broad resonance and so have the resonances of
C-4′,C-4′′ (Figure 9b). This pattern is not consistent with
a two-ring (ph,ph) flip but is in line with a two-ring
(py,ph) flip.
At ambient temperature, all the carbocations except
for 2 show only a single resonance for the phenyl
ring resonances (C-1′,C-1′′, C-2′,C-2′′, C-3′,C-3′′ and
C-4′,C-4′′) showing that averaging around the C+-C-1
bond in 2 is fast. As judged from the theoretical calcula-
tions, this can be either one-ring (py) flip or more likely
due to two-ring (py,ph) flips.
By a comparison of the merging of lines with equal
separation, a comparison of activation energies can be
made in the compounds 2-4. The activation energy is
largest for 3 and 4 followed closely by 3Br and 4Br and
the disubstituted ones are higher in energy than 2 in
agreement with calculations for 2-3 and 4 (Table 3).
Ch a r ge Distr ibu tion s. The chemical shift values for
the C+ carbons are indicative of the degree of charge
delocalization. The calculations indicate that this should
be the largest for 2. This can also be inferred from the
13C chemical shifts. The C+ chemical shifts for 3, 3Br , 4
and 4Br are slightly lower than that found for 1
underlining the large capacity of the pyrenyl ring for
charge delocalization. This is furthermore seen for the
aromatic carbons as the calculated charge differences
between the carbenium ion and the parent alcohol, ∆qC’s,
show that extensive charge delocalization occurs into both
the pyrenyl and phenyl rings in an alternating fashion.
The NMR spectra were recorded on a Bruker-250 MHz or a
GE-GN-300 MHz instrument. CD2Cl2 served as both internal
lock and reference for the low-temperature measurements. For
details see ref 22. At ambient temperature in CF3COOD, the
CF3 resonance served as reference.
NMR Da ta of Sta r tin g Ca r bin ols. 1H NMR (CDCl3). 2:
δ 3.45 (s, 1H) OH; 7.43 (d, J ) 8.25, 1H) H-2; 7.85 (d, J )
9.55, 1H) H-9; 7.94 (d, J ) 7.5, 1H) H-6; 7.97 (d, J ) 7.80, 1H)
H-8; 7.96 (br, 1H) H-3; 7.96 + 8.00 (m, 2H) H-4 + H-5; 8.13
(m, 1H) H-7.
3: δ 3.70 (s, 1H) OH; 7.33 (m, 20H) H-2′,H-3′,H-4′; 7.42 (d,
J ) 8.3, 2H) H-2,H-7; 7.81 (d, J ) 9.4, 2H) H-4,H-9; 7.90 (d, J
) 8.1, 2H) H-3, H-8; 8.41 (J ) 9.3, 2H) H-5, H-10.
3Br : δ 3.79 (s, 2H) OH; 8.32 (m, 20H) H-2′,H-3′,H-4′; 7.80
(s, 2H) H-2,H-7; 8.22 (d, J ) 10.0, 2H) H-4,H-9; 8.49 (d, J )
9.6, 2H) H-5, H-10.
4: 3.39 (s,2H) OH; 7.32 (m, 20H) H-2′,H-3′H-4′; 7.37, (d, J
) 8.1, 2H) H-2, H-7; 7.93 (d, J ) 8.1, 2H) H-3,H-6; 7.97 (s,
2H) H-4,H-5; 8.21 (s, 2H) H-9,H-10.
4BR: 3.35 (s, 2H) OH; 7.30 (m, 20H) H-2′,H-3′,H-4′; 7.71
(s, 2H) H-2,H-7; 8.19 (s, 2H) H-9,H-10; 8.50 (s, 2H) H-4,H-5.
13C NMR (CDCl3). 2: 83.73, COH; 123.60, C-10; 124.78,
C-10c; 125.28, C-8; 125.32, C-6; 126.03, C-3; 126.78, C-7;
126.97, C-2; 127.31, C-9; 127.36, C-4′, 127.70, C-5; 127.87, C-4;
(21) Lund, H.; Berg, A. Kgl. Videnskab. Selskab 1941, 18, 1.
(22) Laali, K. K.; Hansen, P. E. J . Org. Chem. 1991, 56, 6795.

Studies of Phenyl- and (1-Pyrenyl)triarylmethylcarbenium Ions
J . Org. Chem., Vol. 63, No. 6, 1998 1835
127.95, C-3′, 128.11, C-2′; 129.53, C-10a; 130.33, C-5a; 131.26,
C-8a; 140.02, C-1; 142.61, C-1′.
3: 83.72, COH; 123.11, C-10b; 123.75, C-2; 125.55, C-3;
127.00, C-5; 127.18, C-4′; 127.83, C-3′, 128.01, C-2′, 130.40,
C-10a; 130.76, C-3a; 139.47, C-1; 147.39, C-1′.
3Br : 83.38,OH; 120.40, C-3; 125.43, C-5; 126.83, C-10b;
127.72, C-4′; 127.85, C-3′; 128.36, C-2′; 128.84, C-4; 129.14,
C-3a; 129.19, C-10a, 132.42, C-2; 141.62, C-1; 146.65, C-1′.
4: 83.70, COH; 123.28, C-9; 126.00, C-10b; 126.28, C-2′;
127.80, C-4′; 127.89, C-3′; 127.93, C-3; 127.99, C-4; 128.08, C-2′;
128.70, C-8a; 131.5, C-3a; 140.33, C-1; 147.42, C-1′.
4Br : 83.26, COH; 119.84, C-3; 126.24, C-9; 126.85, C-10b;
127.53, C-4′; 127.67, C-3′; 127.85, C-4; 128.19, C-2′; 128.42,
C-8a; 129.62, C-3a; 132.54, C-2; 141.73, C-1; 146.49, C-1′.
Ack n ow led gm en t. We are grateful to the NCI of
NIH (R15CA63595 to K.K.L.) and to NATO (CRG
930113 to K.K.L. and P.E.H.) for research support in
the PAH arenium ions area. We would also like to
thank Professor A. Berg for donating the carbinols.
J O9715480