Generation and reactivity of the phenyl cation in cryogenic argon matrices: Monitoring the reactions with nitrogen and carbon monoxide directly by IR spectroscopy

Article abstract of DOI:10.1021/jo0603678

The phenyl cation 1 has been prepared by co-deposition of iodobenzene 6 or bromobenzene 7 with a microwave-induced argon plasma and characterized by IR spectroscopy in cryogenic argon matrices. The cation can clearly be identified by its strongest absorpt

Full text of DOI:10.1021/jo0603678

Generation and Reactivity of the Phenyl Cation in Cryogenic Argon
Matrices: Monitoring the Reactions with Nitrogen and Carbon
Monoxide Directly by IR Spectroscopy
Michael Winkler and Wolfram Sander*
Lehrstuhl f u¨ r Organische Chemie II der Ruhr-UniVersit a¨ t Bochum, UniVersit a¨ tsstrasse 150,
4
4780 Bochum, Germany
wolfram.sander@rub.de
ReceiVed February 22, 2006
The phenyl cation 1 has been prepared by co-deposition of iodobenzene 6 or bromobenzene 7 with a
microwave-induced argon plasma and characterized by IR spectroscopy in cryogenic argon matrices.
-
1
The cation can clearly be identified by its strongest absorption at 3110 cm that is rapidly bleached
upon visible light irradiation. This characteristic band is observed neither in the conventional
photochemistry of 6 or 7 nor in discharge experiments with alkyl halides or chlorobenzene. The latter
finding is in line with energetic considerations. According to density functional theory (DFT) computations,
the strongest absorption of 1 is caused by a C-H stretching vibration that involves almost entirely the
ortho-hydrogens. This is confirmed by isotopic labeling experiments. Co-deposition of halobenzene/N
mixtures leads to a decrease of the 3110 cm absorption, whereas several new signals are detected in
2
-
1
-
1
the 2200-2400 cm range of the IR spectrum. Annealing of a matrix that contains 1 and 1% N
2
leads
-
1
to an increase of a broad band at 2260 cm that is assigned to the benzenediazonium ion 2. A sharp
-
1
signal at 2327 cm that had previously been assigned to the N-N stretching vibration of 2 is due to
molecular nitrogen. The mechanism that triggers the IR activity of N is not yet understood. Annealing
2
-
1
of a matrix that contains 1 and 0.5% CO leads to an increase of a broad band at 2217 cm that is
considerably stronger than the 2260 cm- absorption of 2. This signal is assigned to the C-O stretching
vibration of the benzoyl cation 12, in excellent agreement with previous investigations of 12 in superacidic
media. Some consequences of the measured frequencies with regard to bonding in 2 and 12 are discussed.
1
Introduction
upon relaxation of the cation from the geometry of its tritiated
precursor). The cations formed in this way can be trapped by
various reagents both under gas-phase conditions at different
The phenyl cation 1 is of fundamental importance in diazo
1
chemistry and has been the subject of numerous experimental
2,3
pressures and in solution.
2
and theoretical studies. According to its high reactivity and
low stability, formation of 1 in solution requires precursors with
1,2
excellent leaving groups such as nitrogen or helium. The latter
has been introduced into organic compounds by the nuclear
decay approach which employs the spontaneous â-decay of
3
+ 2,3
covalently bound tritium resulting in transmutation to He .
Under these conditions, the cations are formed with a rather
high excess energy (resulting partly from the energy released
Aryl cations bearing appropriate substituents were generated
4
in solution by solvolysis of perflouroalkylsulfonic aryl esters
(
(
1) Zollinger, H. Diazo Chemistry; VCH: Weinheim, 1994; Vol. 1.
2) Dicoordinated Carbocations; Rappoport, Z., Stang, P. J., Eds.;
5
or by solvolytic cyclization of suitable dienynes, but dediazo-
niation of benzenediazonium ions 2 is the most common and
general route to these species. The mechanism of dediazoniation
Wiley: Chichester, 1997.
3) Speranza, M. Chem. ReV. 1993, 93, 2933.
(
1
0.1021/jo0603678 CCC: $33.50 © 2006 American Chemical Society
Published on Web 07/29/2006
J. Org. Chem. 2006, 71, 6357-6367
6357

Winkler and Sander
In contrast to radical cations or small halogenated carboca-
17
has been subjected to intense research, and even the bare
existence of aryl cations in the condensed phase has been
debated controversially.1,2 There are strong indications that this
process is considerably more complex in solution than in the
+
18
tions (e.g., CCl ), the isolation of unsubstituted (closed-shell)
3
carbocations in cryogenic noble gas matrices has been described
in the literature only incidentally. By co-condensation of benzyl
bromide 4 with microwave-excited argon, Andrews et al.
obtained two novel species with characteristic UV absorptions
at 353 and 263 nm that were assigned to the benzyl cation 3
gas phase,1 where, according to calculations, the extrusion
of nitrogen is a strongly endothermic elementary reaction.
Attempts to generate and identify (singlet) aryl cations directly
by the techniques of laser-flash photolysis (LFP) failed so far
,2,6
7
19
and the tropylium ion 5, respectively. These isomers can be
8
owing to their short lifetime (e0.5 ns) and, probably, their lack
interconverted photochemically.
of strong UV absorptions above 250 nm.9 In 1,1,1,3,3,3-
hexafluoro-2-propanol, Steenken et al. were able to study several
cations by time-resolved spectroscopy, among them the benzyl
1
0
cation 3. Starting from various substituted arenediazonium
ions, however, no signals of intermediate aryl cations could be
detected. Still, these authors demonstrated that aryl cations are
formed and can be scavenged by aromatics giving cyclohexa-
1
1
dienyl cations. In superacidic media, products indicative of
heterolytic dissociation were identified, but aryl cations could
not be detected directly.12 For para-amino-substituted aryl
cations, which have triplet ground states, it was shown that their
lifetime in solution is below 15 ps;13 these systems are efficiently
formed by photoheterolysis of haloanilines in polar or protic
solvents, and their chemistry has been investigated in a series
of elegant studies by Albini et al.14 Triplet aryl cations
stabilized by several methoxy groups were studied by EPR and
UV/vis spectroscopy in LiCl matrices at 77 K.16
The mechanism of energy transfer during co-deposition of
organic substrates with excited argon from an open-ended
discharge tube has been a matter of debate. Whereas Jacox et
al. favored a Penning ionization by collision with argon
2
0
metastables, Andrews et al. presented compelling evidence
for a direct photoionization that takes place on the surface of
15
21
the forming matrix rather than in the gas phase.
In a preliminary communication, we reported the formation
of 1 by co-deposition of iodobenzene 6 or bromobenzene 7 with
22
microwave-excited argon. Matrix-isolated 1 was identified by
(
4) Early attempts at solvolytic generation of aryl cations were not
successful: (a) Streitwieser, A., Jr.; Daffron, A. Tetrahedron Lett. 1976,
435. (b) Subramanian, L. R.; Hanack, M.; Chang, L. W. K.; Imhoff, M.
-1
its strong IR absorption at 3110 cm , and this assignment has
been confirmed by isotopic labeling experiments. A strong
1
-
1
A.; Schleyer, P. v. R.; Effenberger, F.; Kurz, W.; Stang, P. J.; Dueber, T.
E. J. Org. Chem. 1976, 41, 4099. Following theoretical predictions by
Apeloig et al., Sonoda et al. finally achieved the generation and trapping
of 2,6-disubstituted aryl cations in fluorinated solvents: (c) Apeloig, Y.;
Arad, D. J. Am. Chem. Soc. 1985, 107, 5285. (d) Himeshima, Y.; Kobayashi,
H.; Sonoda, T. J. Am. Chem. Soc. 1985, 107, 5286.
absorption in the same spectral range (3125 cm ) has more
recently been observed by Solca and Dopfer for protonated
fluorobenzene in the gas phase, and therefore, this species has
(
5) (a) Hanack, M.; Michel, U. Angew. Chem., Int. Ed. Engl. 1979, 18,
(15) Several computational studies have been devoted to the singlet-
triplet energy splitting of 1 and substituted derivatives: (a) Dill, J. D.;
Schleyer, P. v. R.; Binkley, J. S.; Seeger, R.; Pople, J. A.; Haselbach, E. J.
Am. Chem. Soc. 1976, 98, 5428. (b) Hrusak, J.; Schr o¨ der, D.; Iwata, S. J.
Chem. Phys. 1997, 106, 7541. (c) Nicolaides, A.; Smith, D. M.; Jensen, F.;
Radom, L. J. Am. Chem. Soc. 1997, 119, 8083. (d) Harvey, J. N.; Ashi,
M.; Schwarz, H.; Koch, W. Theor. Chem. Acc. 1998, 99, 95. (e) Ashi, M.;
Harvey, J. N. J. Chem. Soc., Perkin Trans. 2 1999, 1059. (f) Laali, K. K.;
Rasul, G.; Prakash, G. K. S.; Olah, G. A. J. Org. Chem. 2002, 67,
2913.
(16) (a) Ambroz, H. B.; Przybytniak, G. K.; Stradowski, C.; Wolszczak,
M. J. Photochem. Photobiol. A 1990, 52, 369. For a recent review, see:
(b) Kemp, T. J. Proc. React. Kinet. Mec. 2003, 28, 11 and references
therein.
(17) For leading references, see: (a) Molecular Ions: Spectroscopy,
Structure, and Chemistry; Miller, T. A., Bondybey, V. E., Eds.; North-
Holland: Amsterdam, 1980. (b) Knight, L. B. Acc. Chem. Res. 1986, 19,
313. (c) Jacox, M. E.; Thompson, W. E. Res. Chem. Intermed. 1989, 12,
33. (d) Bally, T. In Radical Ionic Systems; Lund, A., Shiotani, M., Eds.;
Kluwer: Dordrecht, 1991. (e) Tang, W.; Zhang, X.-L.; Bally, T. J. Phys.
Chem. 1993, 97, 4373. (f) Truttmann, L.; Asmis, K. R.; Bally, T. J. Phys.
Chem. 1995, 99, 17844. (g) Bondybey, V. E.; Smith, A. M.; Agreiter, J.
Chem. ReV. 1996, 96, 2113.
(18) (a) Prochaska, F. T.; Andrews, L. J. Chem. Phys. 1977, 67, 1091.
(b) Andrews, L.; Keelan, B. W. J. Am. Chem. Soc. 1979, 101, 3500. (c)
Andrews, L. Annu. ReV. Phys. Chem. 1979, 30, 79. (d) Keelan, B. W.;
Andrews, L. J. Phys. Chem. 1979, 83, 2488. (e) Bai, H.; Ault, B. S. Chem.
Phys. Lett. 1992, 188, 126.
8
7
70. (b) Hanack, M.; Holweger, W. J. Chem. Soc., Chem. Commun. 1981,
13. (c) Hanack, M. Pure Appl. Chem. 1984, 56, 1819.
(6) (a) Hashida, Y.; Landells, R. G. M.; Lewis, G. E.; Szele, I.; Zollinger,
H. J. Am. Chem. Soc. 1978, 100, 2816. (b) Maurer, J.; Szele, I.; Zollinger,
H. HelV. Chim. Acta 1979, 62, 1079. (c) Dewar, M. J. S.; Storch, D. M. J.
Chem. Soc., Perkin Trans. 1989, 877. (d) Ussing, B. R.; Singleton, D. A.
J. Am. Chem. Soc. 2005, 127, 2888.
(7) (a) Glaser, R.; Horan, C. J. J. Org. Chem. 1995, 60, 7518. (b) Glaser,
R.; Horan, C. J.; Lewis, M.; Zollinger, H. J. Org. Chem. 1999, 64, 902. (c)
Wu, Z. Y.; Glaser, R. J. Am. Chem. Soc. 2004, 126, 10632.
(8) An early report in which the detection of aryl cations was proposed
turned out to be incorrect: (a) Boettcher, H.; Becker, H. G. O.; Inanov, V.
L.; Kusmin, M. G. Chimia 1973, 27, 437. (b) Zollinger, H. Angew. Chem.,
Int. Ed. Engl. 1978, 17, 141. (c) Scaiano, J. C.; Kim-Thuan, N. Can. J.
Chem. 1982, 60, 2286. (d) Scaiano, J. C.; Kim-Thuan, N. J. Photochem.
1
983, 23, 269.
9) Smith, D. M.; Maksic, Z. B.; Maskill, H. J. Chem. Soc., Perkin Trans.
2002, 906.
10) (a) McClelland, R. A.; Chan, C.; Cozens, F.; Modro, A.; Steenken,
S. Angew. Chem. 1991, 103, 1389; Angew. Chem., Int. Ed. Engl. 1991, 30,
337. See also: (b) McClelland, R. A.; Mathivanan, N.; Steenken, S. J.
(
2
(
1
Am. Chem. Soc. 1990, 112, 4857. (c) McClelland, R. A.; Cozens, F. L.; Li,
J.; Steenken, S. J. Chem. Soc., Perkin Trans. 2 1996, 1531.
(11) Steenken, S.; Ashokkumar, M.; Maruthamuthu, P.; McClelland, R.
A. J. Am. Chem. Soc. 1998, 120, 11925.
(
12) (a) Laali, K.; Szele, I.; Yoshida, K. HelV. Chim. Acta 1983, 66,
1
1
710. (b) Laali, K.; Szele, I.; Zollinger, H. HelV. Chim. Acta 1983, 66,
737.
(19) Andrews, L.; Keelan, B. W. J. Am. Chem. Soc. 1981, 103, 99.
(20) (a) Jacox, M. E. Chem. Phys. 1976, 12, 51. (b) Jacox, M. E.;
Milligan, D. E. Chem. Phys. 1976, 16, 195. (c) Jacox, M. E. ReV. Chem.
Intermed. 1978, 2, 1. See also: (d) Haugsjaa, P. O.; Amme, R. C.; Utterback,
N. G. Phys. ReV. Lett. 1969, 22, 322.
(21) Andrews, L.; Tevault, D. E.; Smardzewski, R. R. Appl. Spec. 1978,
32, 157 and references therein.
(22) Winkler, M.; Sander, W. Angew. Chem. 2000, 112, 2091; Angew.
Chem., Int. Ed. Engl. 2000, 39, 2014.
(
13) Gasper, S. M.; Devadoss, C.; Schuster, G. B. J. Am. Chem. Soc.
995, 117, 5206.
14) (a) Fagnoni, M.; Mella, M.; Albini, A. Org. Lett. 1999, 1, 1299.
b) Guizzardi, B.; Mella, M.; Fagnoni, M.; Freccero, M.; Albini, A. J. Org.
1
(
(
Chem. 2001, 66, 6353. (c) Freccero, M.; Fagnoni, M.; Albini, A. J. Am.
Chem. Soc. 2003, 125, 13182. (d) Milanesi, S.; Fagnoni, M.; Albini, A. J.
Org. Chem. 2005, 70, 603. (e) Fagnoni, M.; Albini, A. Acc. Chem. Res.
2
005, 38, 713 and references therein.
6358 J. Org. Chem., Vol. 71, No. 17, 2006

Phenyl Cation in Cryogenic Argon Matrices
FIGURE 1. Photochemistry of 6 upon long-wavelength irradiation. (a) Difference spectrum; bands pointing upward increase in intensity upon
-
1
irradiation (λ > 295 nm) of a matrix that contains 6 (Ar, 10 K). The strong absorption at 2175 cm (labeled by a dot) is assigned to complexed
HI. (b) IR spectrum of (Z)-hex-3-ene-1,5-diyne (Z)-9.
been described as a weakly bound ion-dipole complex of 1
reactions of an aryl cation directly by IR spectroscopy. Finally,
we describe the reaction of 1 with CO under similar conditions.
2
3
and HF.
Results and Discussion
Photochemistry of Iodobenzene 6. The formation of the
phenyl radical 8 upon irradiation of 6 with UV light (λ ) 254
nm) was observed by EPR spectroscopy already in 1969.24 The
vibrational spectrum of 8 was studied in detail starting from
We also reported our first attempts to study the intermolecular
chemistry of the phenyl cation, particularly, the trapping reaction
with molecular nitrogen to give the benzenediazonium ion 2.
2
5
several precursors, and all IR absorptions assigned to the
radical are also found in our experiments after 254 nm irradiation
of 6 (cf., Figure 2a). In contrast, irradiation of iodobenzene in
argon at 10 K with light of wavelength λ > 295 nm gives rise
to a different photochemistry (Figure 1).
2
2
One major drawback of our early approach has been the low
quality of the matrices that made it impossible to trigger
intermolecular reactions in the usual way by annealing the
matrices to 30-35 K. Instead, we added successively larger
amounts of trapping reagents and followed the changes in the
IR spectra in a series of otherwise identical experiments (this
procedure has been called “titration”). However, the informa-
tion gained in this way is necessarily of a rather indirect
nature.
In this contribution, we describe the matrix isolation of the
phenyl cation in full detail and present additional evidence to
support this assignment. Next, we reconsider the reaction of 1
with molecular nitrogen on the basis of improved experimental
conditions that make it possible for the first time to follow
-
1
A strong absorption at 2175 cm is assigned to complexed
HI; the H-I stretching vibrational frequency is red shifted from
-
1
-1
monomeric HI (2244 ( 10 cm in argon matrices, 2232 cm
2
6
-1
27
in krypton matrices, and 2227 cm in the gas phase ) by
approximately 60 cm , indicating the formation of a complex.
-
1
(
24) Kasai, P. H.; Hedaya, E.; Whipple, E. B. J. Am. Chem. Soc. 1969,
1, 4364.
25) (a) Pacansky, J.; Bargon, J. J. Am. Chem. Soc. 1975, 97, 6896. (b)
Hatton, W. G.; Hacker, N. P.; Kasai, P. H. J. Chem. Soc., Chem. Commun.
990, 227. (c) Radziszewski, J. G.; Nimlos, M. R.; Winter, P. R.; Ellison,
9
(
1
G. B. J. Am. Chem. Soc. 1996, 118, 7400. (d) Friderichsen, A. V.;
Radziszewski, J. G.; Nimlos, M. R.; Winter, P. R.; Dayton, D. C.; David,
D. E.; Ellison, G. B. J. Am. Chem. Soc. 2001, 123, 1977.
(
23) (a) Solca, N.; Dopfer, O. Angew. Chem. 2003, 115, 1575; Angew.
(26) Whereas HCl and HBr in different rare gas hosts show features
similar to the vibrational-rotational spectra in the gas phase, the IR spectrum
of HI is strongly perturbed by the matrix medium, so that assignments
according to gas-phase quantum numbers are inappropriate: (a) Kuntuu,
H. M.; Seetula, J. A. Chem. Phys. 1994, 189, 273. Absorptions at 2253
and 2245 cm have been assigned to two components of R0 of the (rotating)
HI monomer: (b) Engdahl, A.; Nelander, B. J. Phys. Chem. 1986, 90, 6118
and references given therein.
Chem., Int. Ed. Engl. 2003, 42, 1537. (b) Solca, N.; Dopfer, O. J. Am.
Chem. Soc. 2003, 125, 1421. (c) Dopfer, O. J. Phys. Org. Chem., in press.
For a general outline of IR photodissociation spectroscopy, see: (d) Bieske,
E. J.; Dopfer, O. Chem. ReV. 2000, 100, 3963. For additional studies on
protonated fluorobenzene, compare: (e) Olah, G. A.; Mo, Y. K. J. Org.
Chem. 1973, 38, 3212. (f) Hrusak, J.; Schr o¨ der, D.; Weiske, T.; Schwarz,
H. J. Am. Chem. Soc. 1993, 115, 2015. (g) Schr o¨ der, D.; Oref, I.; Hrusak,
J.; Weiske, T.; Nikitin, E. E.; Zummack, W.; Schwarz, H. J. Phys. Chem.
A 1999, 103, 4609 and references therein.
-1
(27) Weidlein, J.; M u¨ ller, U.; Dehnicke, K. Schwingungsfrequenzen I;
Georg Thieme Verlag: Stuttgart, 1981.
J. Org. Chem, Vol. 71, No. 17, 2006 6359

Winkler and Sander
FIGURE 2. (a) Photochemistry of 6 upon short-wavelength irradiation. Difference spectrum; bands of the phenyl radical 8 pointing upward
increase in intensity upon irradiation (λ ) 254 nm) of a matrix that contains 6 (Ar, 10 K). (b) IR spectrum of a matrix obtained after co-deposition
of 6 with excited argon. (c) The same matrix after irradiation with visible light (λ > 400 nm). (d) Difference spectrum (c)-(b); bands pointing
downward decrease in intensity upon long-wavelength irradiation. (e) Calculated IR spectrum of 1 (B3LYP/aug-cc-pVTZ).
In the system C2H2/HI, Abrash and Pimentel observed a complex
with an absorption at 2179 cm . Additional bands formed
upon irradiation of 6 coincide with the IR spectrum of (Z)-hex-
excited argon is shown in Figure 2b. Apart from unreacted
iodobenzene, the spectrum shows the formation of phenyl
radicals as well as some acetylenic byproducts, which give rise
to a broad and poorly structured absorption around 3320-3330
-
1 28
3
-ene-1,5-diyne (Z)-9, although some of the signals are split
-
1
-1
because of complexation with HI. No attempts were made to
clarify the structure of (Z)-9-HI, but knowledge of its IR data
is important to exclude formation of these byproducts during
co-deposition of 6 with excited argon. Formation of (Z)-9-HI
is reversible, and 254 nm irradiation leads back to 8. Although
we cannot strictly rule out a direct ring opening of 6, there are
indications that 8 is formed first and subsequently yields (Z)-
cm . Another absorption is found at 3110 cm and assigned
2
2
to 1 as proposed previously. Subsequent irradiation of the
matrix with light of longer wavelength causes the latter signal
and all absorptions of 8 to vanish, whereas the bands of 6
increase in intensity.
Photolability is a characteristic feature of cations with high
18,30
electron affinity in cryogenic matrices,
although the pho-
9
-HI. Thus, the complex forms more rapidly, if 8/I is first
tophysical and chemical processes accompanying the photo-
bleaching are not well understood. Increasing the energy and
bandwidth of the radiation (λ > 400 nm; λ > 320 nm; λ > 305
nm) causes a faster disappearance of the cation absorption. Most
probably, irradiation with UV/vis light induces electron-transfer
processes that lead to electroneutralization. In addition, recom-
binations with nearby iodine atoms occur because of local
warming of the matrix upon irradiation. Accordingly, the IR
signals of 1 and 8 both decrease, whereas those of 6 increase
in intensity during photolysis.
generated by 254 nm irradiation. Interestingly, no ring-opening
reaction is observed under iodine-free conditions, e.g., when 8
is produced from dibenzoylperoxide. In the absence of iodine,
the ring opening of 8 requires an activation energy of 67.1 kcal
-
1
mol (UB3LYP/aug-cc-pVTZ) and the resulting hexa-1,3-
-
1
diene-5-yne-1-yl radical is 61.9 kcal mol less stable than the
2
9
phenyl radical.
Several points regarding the assignment of the 3110 cm-
peak to 1 are worth mentioning. It is evident from Figure 2
that, apart from the phenyl radical absorptions, there are no
additional strong signals in the IR spectrum that decrease in
intensity upon long wavelength irradiation. Therefore, the band
1
Matrix Isolation of the Phenyl Cation 1. The IR spectrum
of a matrix obtained after co-deposition of 6 with microwave-
-
1
at 3110 cm is caused by a species that has its strongest IR
absorption in the high-frequency part of the aromatic C-H
stretching regime. This characteristic feature matches exactly
our expectations based on DFT calculations for 1 (Figure 2e).
A second signal of slightly lower intensity is expected around
(
(
28) Abrash, S. A.; Pimentel, G. C. J. Phys. Chem. 1989, 93, 5828.
29) This reaction of 8 is of importance in combustion processes and
has been studied in great detail; e.g.: (a) Walch, S. P. J. Chem. Phys. 1995,
03, 8544. (b) Huang, J.-H.; Han, K.-L.; Deng, W.-Q.; He, G.-Z. Chem.
1
Phys. Lett. 1997, 273, 205. (c) Madden, L. K.; Moskaleva, L. V.; Kristyan,
S.; Lin, M. C. J. Phys. Chem. A 1997, 101, 6790. (d) Wang, H.; Laskin,
A.; Moriarty, N. W.; Frenklach, M. Proc. Combust. Inst. 2000, 28, 1545.
(30) Knight, L. B. In Chemistry and Physics of Matrix-Isolated Species;
Andrews, L., Moskovitz, M., Eds.; Elsevier: New York, 1989.
6360 J. Org. Chem., Vol. 71, No. 17, 2006

Phenyl Cation in Cryogenic Argon Matrices
FIGURE 3. (a) IR spectrum of 6. (b) IR spectrum of a matrix containing 6 after irradiation with λ ) 254 nm. (c) IR spectrum of a matrix
containing 6 after irradiation with λ > 295 nm. (d) IR spectrum of a matrix obtained after co-deposition of 6 with microwave-excited argon. (e)
The same matrix after long-wavelength irradiation (λ > 400 nm).
TABLE 1. Appearance Potentials (AP) of 1 from 6, 7, and 10a
-
1
7
00 cm , very close to the strongest band of 8. As shown in
Figure 2, a photolabile group of absorptions is indeed found in
this (less-characteristic) spectral region. Because it is difficult
to decide which of these overlapping bands belongs to 1, the
main focus of this work is on the characteristic high-frequency
absorption of the phenyl cation.
Another important point relates to the absence of the 3110
cm- signal in the conventional photochemistry (Figure 3) and
in high-temperature experiments with halobenzenes (using
1
31
different setups for pyrolysis). Thus, formation of this entity
relies upon high energies as provided in the discharge experi-
ments (vide infra), whereas the 3110 cm absorption is not
observed after photochemical or thermal excitation of 6 or 7
with energies below the ionization threshold.
-
1
Ph-X
APminb
APmaxc
APmeand
APmatrixe
ref
Ph-Cl (10)
Ph-Br (7)
Ph-I (6)
11.8
11.7
10.6
13.2
12.2
11.5
12.7
11.9
11.1
>11.7
>10.9
>10.1
34
35
36
This point can be made more quantitative. The appearance
potentials (AP) of 1 from iodobenzene 6, bromobenzene 7, or
chlorobenzene 10 were determined in various gas-phase studies
using different experimental techniques.32 All values reflect the
increasing ionization energy and C-X bond dissociation energy
in the series 6 < 7 < 10. Compared with the gas phase,
ionization energies in argon matrices are usually lowered by
a
All energies are given in electronvolts.32 Lowest values published
b
in the literature. c Highest values published in the literature. Mean value
d
e
of all appearance potentials considered. Estimated lower limit for the AP
of 1 on top of the forming argon matrix.
subtracted 1 eV to account for the polarizable environment.
These values are listed in Table 1.
The output of the windowless argon resonance lamp is
dominated by the principle argon emission doublet at 104.8 and
06.7 nm (11.6-11.8 eV) under the high-pressure conditions
employed in our experiments. Similarly, the excitation energy
of the argon metastables is 11.55 and 11.73 eV for the P2 and
P0 states, respectively.
transfer occurs via a Penning or a photoionization mechanism,
3
3
0
.5-1 eV. As a lower limit of the energy required to produce
phenyl cations from halobenzenes on the surface of the forming
matrix, we took the mean value of the gas-phase appearance
potentials published in the literature (denoted APmean) and
1
21
3
(31) This absorption has been detected in neither flash vacuum pyrolysis
3
20,37
experiments nor under the conditions of carrier gas pyrolysis (pulsed or
conventional). Either 8 or acetylenic fragmentation products of unknown
constitution are formed in these experiments.
Regardless of whether the energy
(
32) Data compilation: NIST Chemistry WebBook, NIST Standard
Reference Database Number 69; Lindstrom, P. J., Mallard, W. G., Eds.;
National Institute of Standards and Technology: Gaithersburg, MD, March
(34) Lowest AP: (a) Ripoche, X.; Dimicoli, I.; Botter, R. Int. J. Mass
Spectrom. Ion Processes 1991, 107, 165. Highest AP: (b) Majer, J. R.;
Patrick, C. R. J. Chem. Soc., Faraday Trans. 1962, 58, 17. See ref 32 for
details.
2
003 (http://webbook.nist.gov).
33) Gedanken, A.; Raz, B.; Jortner, J. J. Chem. Phys. 1973, 58, 1178.
(
J. Org. Chem, Vol. 71, No. 17, 2006 6361

Winkler and Sander
TABLE 2. Calculated (B3LYP/aug-cc-pVTZ) and Measured
Frequencies of the Strongest Absorptions of Various Phenyl Cation
Isotopomers
stretching vibrational frequencies in the benzenediazonium
ion 2, the borabenzene-nitrogen adduct 11, and the benzoyl
cation 12.
1
Icalcd/km mol-1
ν˜ exptl/cm-1
isotopomer
ν˜ calcd/cm-
Reaction with N2. Because 1 is accessible in cryogenic argon
matrices only in low concentrations, the investigation of its
intermolecular chemistry is restricted to unreactive (selective)
reactants that give rise to very strong and characteristic IR
chromophores. The reaction with dinitrogen is a barrierless
1
3226
3226
2397
2399
3215
190
190
98
110
189
3110
3110
4-D1-1
a
2
,4,6-D3-1
a
D5-1
1
3
C6-1
3098
7
-1
process (Figure 7), which is exothermic by 28.3 kcal mol
a
Not assigned because of the high complexity of the IR spectrum in
(
2
B3LYP). The IR intensity of the N-N stretching vibration of
this spectral range.
-
1
amounts to 247 km mol (Table 3) so that the benzenedia-
zonium ion should be easily detectable by IR spectroscopy, even
if only modest concentrations can be achieved by the intermo-
lecular reaction of 1 with N2.
the energy available to produce 1 is 11.5-11.8 eV. Comparison
with Table 2 indicates that 6 and 7 qualify as precursors for 1,
whereas the appearance potential of 1 from 10 in the gas phase
Our early approach to the trapping chemistry of 1 employed
the addition of successively larger amounts of nitrogen under
otherwise identical conditions. This titration procedure was
necessitated by the observation that matrices obtained by
deposition of microwave-excited argon are of rather low quality,
so that subsequent annealing led to strong baseline shifts in the
IR spectra. The titration with N2 (cf., ref 22) gave rise to an
(
and very probably also on the matrix surface) exceeds the
38
energy available from the discharge. The IR spectra of matrices
obtained after co-deposition of 6, 7, and 10 with excited argon
are given in Figure 4. In agreement with the energetic
considerations outlined above, the photolabile absorption at 3110
-
1
cm is found with iodo- and bromobenzene but is absent from
the IR spectrum when 10 is used as a precursor instead.
-1
intense and sharp signal at 2327 cm and a broad and weaker
Assignment of the 3110 cm- band to 1 is further supported
by isotopic labeling. The displacement vector of the normal
mode associated with this absorption is shown in Figure 5. The
vibration involves mainly the ortho-hydrogens with small
contributions from a Cmeta-H stretching mode, whereas the
Cpara-H moiety does not participate at all.
1
-1
absorption at 2260 cm . On the basis of the high intensity and
the good agreement between calculated and measured shifts
14
15
upon substitution of N2 by N2, we assigned the former band
22
to the N-N stretching vibration of 2. However, a signal with
the same frequency could be observed in subsequent experi-
ments with alkyl halide/N2 mixtures. The carrier of this trace
is, therefore, not the benzenediazonium ion 2, but polarized
dinitrogen. The mechanism that triggers the IR activity of N2
is not clear, but activation under discharge conditions or in the
presence of suitable impurities is not without precedence.4
In nitrogen matrices, a very weak absorption at 2328-2329
Accordingly, co-deposition of 4-D1-7 with excited argon
yields 4-D1-1 without changing the frequency and intensity of
-1
the 3110 cm feature (Figure 6, Table 2). In contrast, no signal
is observed in this spectral range when 2,4,6-D3-1 is generated
from the corresponding 2,4,6-D3-7 isotopomer. The C-D
stretching vibration of 2,4,6-D3-1 as well as that of the fully
deuterated cation D5-1 is of much lower intensity and could
not be identified with certainty because many overlapping bands
0,41
-
1
cm has been observed in discharge experiments with CO /
2
4
1
N mixtures. In the gas phase, the N stretching vibration has
been detected by Raman spectroscopy at 2330 cm .
2
2
-
1 42
-
1
occur in the 2220-2320 cm range of the IR spectrum in
The quality of cryogenic argon matrices produced by the slow
spray-on technique is often improved by maintaining the
temperature of the spectroscopic window during condensation
between 25 and 30 K and reducing it afterward (down to 3-10
13
experiments with D5-6, D5-7, and 2,4,6-D3-7. In C6-1 (from
1
3
-1
C6-7), the photolabile absorption is found at 3098 cm . The
-
1
measured red shift of 11.8 cm agrees favorably with the shift
of 11.0 cm calculated at the B3LYP/aug-cc-pVTZ level of
theory.
-
1
43
K) for photochemical investigations. On the other hand, a
higher temperature (lower viscosity) clearly supports aggregation
or intermolecular reactions. To improve the quality of the
matrices, we studied the co-deposition of 6/N2/Ar mixtures (ca.
1:10:400) with excited argon (the final N2 concentration is
approximately 1%) at various temperatures of the spectroscopic
window during deposition (between 3 and 25 K) and followed
The high frequency of the Cortho-H stretching vibration may
seem counterintuitive at first glance. The small CCC angle at
the ortho-carbon atoms implies a high degree of s-character for
2
the σ(C(sp )-H) bonding orbital. In addition, the Cortho-H bond
is strongly polarized (atomic NPA charges39 of 1 are also given
in Figure 5). Both effects outweigh the release of electron
density from the C-H bonding region by σ(Cortho-H) f n*-
-
1
the intensity changes of the 3110 cm absorption. For tem-
peratures above 22 K, the yield of 1 decreases rapidly, as
1
6b
(Cipso) hyperconjugation. Electrostatic arguments are also em-
expected for a “hyper-reactive” species. In the temperature
-
1
ployed in the next section to rationalize the N-N and C-O
range 17-20 K, the intensity of the 3110 cm signal is only
modestly reduced compared with the lowest-temperature experi-
ments. It turned out, however, that the matrices obtained under
these conditions can be subsequently annealed without signifi-
cant baseline shifts (Figure 8).
(35) Lowest AP: (a) Rosenstock, H. M.; Stockbauer, R.; Parr, A. C. J.
Chem. Phys. 1980, 73, 773. Highest AP: (b) Burgers, P. C.; Holmes, J. L.
Org. Mass Spectrom. 1982, 17, 123. See ref 32 for details.
(36) Lowest AP: (a) Gefen, S.; Lifshitz, C. Int. J. Mass Spectrom. Ion
Processes 1984, 58, 251. Highest AP: (b) Howe, I.; Williams, D. H. J.
Am. Chem. Soc. 1969, 91, 7137. See ref 32 for details.
(40) Fredin, L.; Nelander, B.; Ribbegard, G. J. Mol. Spectrosc. 1974,
53, 410.
(41) DiLella, D. P.; Tevault, D. E. Chem. Phys. Lett. 1986, 126, 38.
(42) (a) Bendtsen, J. J. Raman Spectrosc. 1974, 2, 133. (b) Gilson, T.
R.; Beattie, I. R.; Black, J. D.; Greenhalgh, D. A.; Jenny, S. N. J. Raman
Spectrosc. 1980, 9, 361.
(
37) Moore, C. E. Atomic Energy LeVels; National Bureau of Standards
Circular No. 467; U. S. Government Printing Office: Washington, 1949;
Vol. I.
(
38) At lower pressure, high-energy emissions of Ar+ (13-15 eV)
become relevant; cf. ref 21.
39) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88,
99.
(
(43) Dunkin, I. R. Matrix-Isolation Techniques; Oxford University
Press: Oxford, 1998.
8
6362 J. Org. Chem., Vol. 71, No. 17, 2006

Phenyl Cation in Cryogenic Argon Matrices
FIGURE 4. (a) IR spectrum of a matrix obtained after co-deposition of 6 with excited argon. (b) The same matrix after long-wavelength irradiation
(
λ > 400 nm). (c) IR spectrum of a matrix obtained after co-deposition of 7 with excited argon. (d) The same matrix after long-wavelength
irradiation. (e) IR spectrum of a matrix obtained after co-deposition of 10 with excited argon. (f) The same matrix after long-wavelength irradiation.
1
5
-1
N2, a broad feature is observed at 2180 cm that may
1
5
tentatively be assigned to N2-2. The measured red shift of 80
cm agrees well with the shift of 79 cm-1 calculated at the
B3LYP level. However, this absorption overlaps with a side
product that is also formed in experiments without nitrogen
addition. Thus, to identify the N-N stretching frequency of
-1
1
5
N2-2 with certainty, additional improvements of the experi-
mental technique are necessary to achieve significantly higher
concentrations of the diazonium ion.
Numerous investigations have been devoted to the structures44
and the vibrational spectra (measured as Nujol mulls) of
1,45
benzenediazonium salts. Depending on the counterion, N-N
-
1
stretching frequencies between 2250 and 2310 cm have been
reported. Attempts to combine these data with information from
44
X-ray structure determinations to clarify the influence of anions
and to derive the intrinsic properties (bond strength, charge
distribution) of 2 have been disputed controversially.4 Ac-
cording to the dative bond model of diazonium ions developed
by Glaser et al. on the basis of topological electron density
4a
4
6
analysis, 2 is best described in terms of a phenyl cation and
a polarized N2 moiety that, as a whole, remains essentially
uncharged. The σ-dative charge transfer from N2 to 1 is largely
compensated by π(C6H5) f π*(N2) back-bonding. Whereas the
FIGURE 5. Structure of the phenyl cation and displacement vector
of the b symmetric C-H stretching vibration that gives rise to the
strongest IR absorption of 1 (B3LYP/aug-cc-pVTZ). NPA charges
2
(B3LYP/cc-pVTZ//aug-cc-pVTZ) are given for comparison.
The absorption at 2327 cm- is of considerably lower
1
(
44) (a) Sorriso, S. In The chemistry of diazonium and diazo groups,
intensity than in our earlier titration experiments at 10 K, in
line with the lower nitrogen concentration employed here. On
annealing to 30 K, the 3110 cm signal vanishes rapidly (Figure
b), whereas a broader absorption at 2260 cm increases
Part 1; Patai, S., Ed.; Wiley: Chichester, 1978 and references given therein.
For the benzenediazonium tetrafluoroborate, see: (b) Cygler, M.; Przybylska,
M.; MacLeod Elofson, R. Can. J. Chem. 1982, 60, 2852.
22
-1
(45) (a) Aroney, M.; Le Fevre, R. J. W.; Werner, R. L. J. Chem. Soc.
-
1
8
1
955, 276. (b) Whetsel, K. B.; Hawkins, G. F.; Johnson, F. E. J. Am. Chem.
slightly in intensity. This signal is not affected by H/D exchange
of the phenyl hydrogens, in agreement with B3LYP calculations
for 2. Furthermore, it is absent in the CH3I/N2 system and in
all experiments without nitrogen addition. In experiments with
Soc. 1956, 78, 3360. (c) Gray, L. S., Jr.; Fassel, V. A.; Kniseley, R. N.
Spectrochim. Acta 1960, 16, 514. See also ref 44a.
(46) (a) Glaser, R. J. Phys. Chem. 1989, 93, 7993. (b) Chen, G. S.; Glaser,
R.; Barnes, C. L. J. Chem. Soc., Chem. Commun. 1993, 1530. Compare
also ref 7.
J. Org. Chem, Vol. 71, No. 17, 2006 6363

Winkler and Sander
1 3
FIGURE 6. IR spectrum of a matrix obtained after co-deposition of different isotopomers of 7 with excited argon. (a) 7. (b) 4-D -7. (c) 2,4,6-D -7.
1
3
(
6
d) C -7.
The data given in Table 4 support this point of view. The
N-N bond in 2 is stronger than in the isoelectronic but neutral
borabenzene-N2 adduct 11, prepared in a nitrogen matrix by
4
7
Maier and co-workers. On the other hand, it is weaker than
+
in the methanediazonium ion because CH3 is strongly polar-
izing, whereas back-bonding effects are (virtually) absent. The
N-N stretching vibration in the latter ion is calculated to be
even blue shifted from dinitrogen.
Reaction with CO. The benzoyl cation 12 has been studied
spectroscopically in weakly coordinating media, and X-ray
4
8
structures of various salts are known. The degree of com-
plexation and intermolecular perturbation by counterions is
difficult to assess, however, and spectroscopic data of free
acylium ions are, therefore, highly desirable. According to DFT
calculations, the addition of 1 to the carbon atom of carbon
FIGURE 7. Energy as a function of the C-N distance in 2. For each
point, the C-N distance was constrained to a certain value and all
other coordinates were optimized at the B3LYP/aug-cc-pVTZ level.
-1
monoxide is exothermic by 64.8 kcal mol , whereas addition
to the oxygen atom is considerably less favorable (Figure 9).
TABLE 3. N-N Stretching Vibrational Frequencies and Bond
Lengths of the Methanediazonium and Benzenediazonium Ions and
Their Isoelectronic but Neutral Boron Counterparts
The C-O stretching frequency of 12 is predicted to be blue
-
1
shifted from CO by 79 cm (Table 4), and the calculated
-
1
RN-N/pma
ν˜ calcd/cm-1 a
I/km mol-1 a
ν˜ exptl/cm-1
intensity (930 km mol ) is approximately 4 times larger than
system
the strongest absorption of 2.
+
CH3-N2
109.08
109.12
109.49
110.17
110.70
2464
2447
2384
2342
2269
1
0
88
247
838
2327b
The blue shift of the C-O stretching mode hints again at
electrostatic effects. The strengthening of the C-O bond in
N2
BH3-N2
+
2260b
2198c
C6H5-N2 (2)
+
+
HCO and the weakening in COH have been investigated in
C5H5B-N2 (11)
a
B3LYP/aug-cc-pVTZ. b Ar, 10 K; this work. c Nitrogen matrix; ref 47.
(
47) Maier, G.; Reisenauer, H. P.; Henkelmann J.; Kliche, C. Angew.
Chem. 1988, 100, 303; Angew. Chem., Int. Ed. Engl. 1988, 27, 295.
48) (a) Olah, G. A.; Tolgyesi, W. S.; Kuhn, S. J.; Moffatt, M. E.; Bastien,
(
latter weakens the N-N bond, the internal polarization of the
N2 group (with NR negatively polarized and Nâ positively
polarized) leads to additional Coulomb interaction which
strengthens the bond.
I. J.; Baker, E. B. J. Am. Chem. Soc. 1963, 85, 1328. (b) Olah, G. A.;
Germain, A.; White, A. M. In Carbonium Ions; Olah, G. A., Schleyer, P.
v. R., Eds.; Wiley: New York, 1976; Vol. 5 and references given therein.
(
c) Oomens, J.; Bakker, J. M.; Sartakov, B. G.; Meijer, G.; von Helden, G.
Chem. Phys. Lett. 2003, 367, 576.
6364 J. Org. Chem., Vol. 71, No. 17, 2006

Phenyl Cation in Cryogenic Argon Matrices
2
FIGURE 8. (a) IR spectrum of a matrix obtained by co-deposition of a mixture of 6/N /Ar (ca. 1:10:500) with excited argon. (b) The same matrix
after short annealing at 30 K. (c) Difference spectrum; bands of 2 and 6 pointing upward increase in intensity upon annealing.
TABLE 4. C-O Stretching Vibrational Frequencies and Bond
Lengths of Acylium Ions and Their Isoelectronic but Neutral Boron
Counterparts
system
RC-O/pm a ν˜ calcd/cm-1 a I/km mol-1 a ν˜ exptl/cm-1
CH3-CO+
111.31
112.61
112.58
112.95
113.82
114.76
2373
2287
2208
2206
2148
2015
226
930
80
494
1408
248
+
2217b
2138b
C6H5-CO (12)
CO
BH3-CO
C5H5B-CO
+
C6H5-OC (13)
a
B3LYP/aug-cc-pVTZ. b Ar, 10 K; this work.
some detail.49 Upon protonation, the C-O localized molecular
orbitals become less polarized in the former but more polarized
in the latter. It was argued that the more even distribution of π
electrons between C and O leads to stronger covalent bonding
+
+ 49
in HCO , whereas the covalent character is lowered in COH .
This interpretation differs considerably from the bonding model
in the diazonium ions mentioned above, according to which
internal polarization results in a strengthening of the N-N bond.
A more detailed comparison of diazonium and acylium ions
seems to be necessary to fully understand the complex interplay
of factors determining the variations in N-N and C-O bond
strengths in these systems. The availability of experimental data
for essentially unperturbed ions should facilitate these investiga-
tions.
FIGURE 9. Energy as a function of the C-CO distance in 12 and the
C-OC distance in 13. For each point, the bond length was constrained
to a certain value and all other coordinates were optimized at the
B3LYP/aug-cc-pVTZ level.
the 3110 cm- signal of 1 decreases concomitantly. The
frequency (and also the higher intensity compared with 2,
assuming similar concentrations of trapping products) is in
excellent agreement with the DFT calculations for 12 outlined
above.
1
The measured C-O stretching frequency agrees well with
Co-deposition of 6/CO/Ar or 7/CO/Ar mixtures (1:5:500) with
previous IR measurements under Lewis acidic conditions. In
-
1
excited argon gives rise to a broad absorption at 2217 cm
(
-1
the system C6H5COF/SbF5, a very strong vibration at 2212 cm
Figure 10) that is more intense than the 2260 cm- absorption
1
-1
has been attributed to 12, and a similar frequency (2215 cm )
of 2 and further increases on short annealing to 30 K, whereas
48a
has been measured in the system C6H5COCl/SbCl5. In a recent
gas-phase study, Oomens et al. assigned a very broad absorption
(
49) (a) Goldman, A. S.; Krogh-Jespersen, K. J. Am. Chem. Soc. 1996,
-1
at 2196 cm with a strong tailing toward higher frequencies
1
18, 12159. (b) Lupinetti, A. J.; Fau, S.; Frenking, G.; Strauss, S. H. J.
Phys. Chem. A 1997, 101, 9551. See also: (c) Frenking, G.; Fr o¨ hlich, N.
Chem. ReV. 2000, 100, 717.
48c
to the C-O stretching mode of 12. Although it has been noted
that this range is characteristic of acylium ion formation,4 it
8b
J. Org. Chem, Vol. 71, No. 17, 2006 6365

Winkler and Sander
FIGURE 10. (a) IR spectrum of a matrix obtained by co-deposition of a mixture of 6/CO/Ar (ca. 1:5:500) with excited argon. (b) The same matrix
after short annealing at 30 K. (c) Difference spectrum; the absorption of 12 pointing upward increases in intensity upon annealing.
is important to remark that some smaller signals around the same
frequency are also observed in discharge experiments with CH3I/
CO mixtures. Although we consider the assignment of the 2217
Experimental Section
Experimental Methods. 4-D
1
-bromobenzene 4-D
1
-7 was pre-
pared from 1,4-dibromobenzene,50 and 2,4,6-D
-bromobenzene
by procedures described
3
-
1
cm trace to 12 likely, all structural assignments based on a
single IR absorption (especially in potentially very complex
mixtures of charged and neutral compounds) necessarily bear
a high risk of misinterpretation and should be considered with
some caution. The investigation of more complex chemical
reactions of hyper-reactive carbocations in noble-gas matrices
clearly requires significant improvements of the experimental
techniques available today. Attempts in this direction are
currently in progress in our laboratory.
50b,51
3
2,4,6-D -7 was prepared from aniline
in the literature. Matrix isolation experiments were performed by
standard techniques43 with closed cycle helium cryostats. Infrared
spectra were recorded with Bruker IFS 66 and Equinox 55 FTIR
-
1
2
spectrometers with a standard resolution of 0.5 cm using a N (l)
-
1
cooled MCT detector in the range 400-4000 cm . For irradiation
at 254 nm, a low-pressure mercury arc lamp was employed. Broad-
band irradiation was carried out with Osram HBO 500 W/2 or Ushio
USH-508SA mercury high-pressure arc lamps in Oriel housings
equipped with quartz optics and dichroic mirrors in combination
with Schott cutoff filters. IR irradiation from the lamps was
absorbed by a 10-cm path of water. The discharge tube (quartz; 20
cm; 1 cm o. d.) for argon excitation was opened to the high-vacuum
side by a 2 mm orifice and arranged normal to the spectroscopic
window (CsI). A Sairem GMP 03 KE (300 W, 245 MHz) magne-
tron diathermy unit operated at 75 W in combination with an air-
cooled cavity, placed near the middle of the tube, was used to excite
the discharge. Argon was flowed through the quartz tube at rates
necessary to maximize the glow intensity and mixed with the
sample/argon mixtures from a second spray-on line ca. 4 cm in
front of the cold window.
Conclusions
The phenyl cation is accessible in cryogenic argon matrices
by co-deposition of 6 and 7 with a microwave-induced argon
plasma. It is readily identified by its strong IR absorption at
-1
3
110 cm , and various arguments in support of this assignment
are discussed. A careful optimization of all experimental
parameters is essential to study intermolecular reactions of 1
directly by IR spectroscopy. Because only small amounts of
the title cation can be isolated, the approach outlined here is
applicable only to selective reactants such as nitrogen or carbon
monoxide that give rise to very strong and characteristic features
in the IR spectrum. The N-N and C-O stretching frequencies
of the benzenediazonium ion 2 and the benzoyl cation 12 have
Computational Methods. Geometries of all species were fully
optimized at the B3LYP level,52 and analytic second derivatives
were calculated to characterize stationary points as minima or
transition states. Tight convergence criteria for gradients (with
maximum residual forces on nuclei below 0.000015 au) and a full
(99, 590) integration grid, having 99 radial shells per atom and
-
1
been detected at 2260 and 2217 cm , respectively. The
availability of precise stretching frequencies of essentially free
diazonium and acylium ions should be helpful to understand
the complex interplay of factors contributing to variations in
bond strengths in these compounds and to assess the degree of
perturbation by counterions in the corresponding salts.
(
50) (a) Kasai, P. H.; Hedaya, E.; Whipple, E. B. J. Am. Chem. Soc.
1
969, 91, 4364. (b) Streitwieser, A.; Klein, H. S. J. Am. Chem. Soc. 1964,
86, 5170.
(
51) Akawie, R. I.; Scarborough, J. M.; Burr, J. G. J. Org. Chem. 1959,
2
4, 946.
(
52) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (b)
Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
6366 J. Org. Chem., Vol. 71, No. 17, 2006

Phenyl Cation in Cryogenic Argon Matrices
90 angular points per shell, have been used throughout to obtain
accurate values for geometries and low-frequency vibrational modes.
If not mentioned otherwise, Dunning’s aug-cc-pVTZ basis set has
5
been used.53 All energies reported in the text are zero-point
corrected. For NBO analysis, diffuse functions were omitted
(B3LYP/cc-pVTZ//aug-cc-pVTZ). All calculations were carried out
with Gaussian 98.54
(
53) (a) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007. (b) Kendall,
R. A.; Dunning, T. H., Jr.; Harrison, R. J. J. Chem. Phys. 1992, 96, 6796.
54) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
(
Acknowledgment. Financial support by the Deutsche For-
schungsgemeinschaft and the Fonds der Chemischen Industrie
is gratefully acknowledged.
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; Gaussian, Inc.: Pittsburgh,
PA, 1998.
Supporting Information Available: Cartesian coordinates,
vibrational frequencies, and absolute energies of calculated struc-
tures are available as Supporting Information. This material is
available free of charge via the Internet at http://pubs.acs.org.
JO0603678
J. Org. Chem, Vol. 71, No. 17, 2006 6367