5
the substrate at approximately 9 eV. Data to be presented now
likely, the 2-octyl radical is excited to a Rydberg state. The
methyl radical, a model of the 2-octyl radical, for example,
undergoes two-photon excitation to a Rydberg state at 266.7 nm
in the gas phase. The excited radical pair then reacts internally
to yield the three alkenes and HI/DI.
suggest that another mechanism operates. Octane, a free-radical
product, is formed in trace amounts in methanol. When the
substrate in degassed cyclopentane is photolyzed with 266 nm
laser light, a trace of one of the 7,8-dimethyltetradecanes is the
product (Table 1, column 5). This product can only arise by the
dimerization of the 2-octyl radical. This is even more manifest
in the gas-phase laser-induced photoreaction where 77% of the
product mixture consists of the two dimers. Since the 2-octyl
radical is clearly formed in cyclopentane and the gas phase, it is
reasonable to assume that it is also formed in methanol.
9
There have been numerous studies on the multiphoton
photochemistry of alkyl halides, most notably CH
3
I, in the gas
phase. Numerous resonant and non-resonant ionization and
dissociation mechanisms for CH I have been formulated. Of
significance to the chemistry described here is the mechanism
6
3
If one in general could suppress the formation of an ion pair
and its subsequent carbocation chemistry by diverting its radical
pair precursor by its rapid excitation, this would represent a
unique method for converting an ionic into a free radical
reaction.
This work was supported by the National Science Founda-
tion.
3
involving one-photon dissociation to yield CH · and I· followed
by multiphoton excitation/ionization of the radicals.7
The reaction thus described here commences by a one-photon
excitation to produce the very short-lived n?s* excited state
which dissociates into a radical pair consisting of the 2-octyl
radical (R·) and iodine atom. Under the low power density lamp
conditions the photochemistry is over and the remaining
chemistry is dictated by the behavior of the radical pair which
can reform reactant, react to given alkenes and HI, escape in the
bulk solvent, and, most significantly, undergo electron transfer
Notes and references
1
(a) P. J. Kropp, G. S. Poindexter, N. J. Pienta and D. C. Hamilton, J. Am.
Chem. Soc., 1976, 98, 8135; (b) P. J. Kropp, J. A. Sawyer and J. J. Snyder,
J. Org. Chem., 1984, 49, 1583; (c) P. J. Kropp, Acc. Chem. Res., 1984, 17,
+
2
to yield an ion pair (R I ). The cleavage of the carbon–iodine
bond to form a radical pair and subsequent electron transfer take
1
31; (d) P. J. Kropp and R. L. Adkins, J. Am. Chem. Soc., 1991, 103,
8
place in the picosecond time regime. In the intense field of the
2709.
laser pulse (width 5 ns), however, one of the components in the
radical pair undergoes two-photon excitation to form a radical
pair excited state before the original radical pair has the
opportunity to undergo any of the chemistry noted above. Most
2 F. Gao, D. Boyles, R. Sullivan, R. N. Compton and R. M. Pagni, J. Org.
Chem., 2002, 67, 9361.
3
(a) J. C. Scaiano, L. J. Johnston, W. G. McGimpsey and D. Weir, Acc.
Chem. Res., 1988, 21, 22; (b) R. M. Wilson and K. A. Schnapp, Chem.
Rev., 1993, 93, 223.
4
J. San Filipo Jr and L. J. Romano, J. Org. Chem., 1975, 40, 1514.
Table 2 Photochemical behavior of (R)-2-iodooctane with 266 nm laser
light in methanol
5 Estimated from the values of iodopropanes, -butanes and -pentanes found
in (a) R. A. Boschi and D. R. Salahab, Mol. Phys., 1982, 24, 289; (b) J.
C. Traeger, Org. Mass. Spec., 1981, 16, 193.
Time (min)
([R] + [S])/[R
0
]a
([R] 2 [S])/[R
0
]b
eec
6 B. Zhang, X. Wang, N. Lou, B. Zhang and J. Wei, Spectrochim. Acta A,
2
001, 57, 1759and references cited therein.
0
5
1
1
1
7 (a) Y. Jiang, M. R. Giorgi-Arnazzi and R. B. Bernstein, Chem. Phys.,
1986, 106, 171; (b) S. J. Garret, D. H. Fairbrother, V. P. Holbert, E. Weitz
and P. C. Stair, Chem. Phys. Lett., 1994, 219, 409.
8 (a) M. Lipson, A. A. Denit and K. S. Peters, J. Phys. Chem., 1996, 100,
3580; (b) J. Dreyer and K. S. Peters, J. Phys. Chem., 1996, 100, 15156;
(c) M. Lipson, A. A. Deniz and K. S. Peters, J. Am. Chem. Soc., 1996,
118, 2992; (d) M. Lipson, A. A. Deniz and K. Peters, Chem. Phys. Lett.,
1998, 288, 789.
0.88
0.81
0.79
0.75
0.68
0.60
0.85
0.79
0.76
0.74
0.66
0.59
0.97
0.98
0.96
0.99
0.97
0.98
1
1
2
3
4
a
0
5
0
0
0
Measured by gc/ms. b Determined by polarimetry. c ee = enantiomeric
excess of 2-iodooctane.
9
J. W. Hudgens, T. G. DiGuiseppe and M. C. Lin, J. Chem. Phys., 1983,
9, 571.
7
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