Advanced

Photochemistry of diiodomethane in solution studied by femtosecond and nanosecond laser photolysis. Formation and dark reactions of the CH2I-I isomer photoproduct and its role in cyclopropanation of olefins

Tarnovsky, Alexander LU ; Sundström, Villy LU ; Åkesson, Eva LU and Pascher, Torbjörn LU (2004) In The Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory 108(2). p.237-249
Abstract
Femtosecond and nanosecond photolysis of CH2I2 in acetonitrile at an excitation wavelength of 266-nm yield the ground-state CH2I-I isomer product in similar to70% quantum yield. High isomer formation quantum yields (> 70%) were measured also in n-hexane, dichloromethane, methanol, and ethanol using nanosecond photolysis. The CH2I-I product is formed biphasically within similar to15 ps after excitation and can survive on a nanosecond to microsecond time scale before it decays via a mixture of first- and second-order processes. At 21 degreesC, the first-order rate constants are 1.8 x 10(5) s(-1) in n-hexane, with the measured apparent activation energy of 41 +/- 2 kJ mol(-1) (5-55 degreesC), and 4.3 x 10(6) s(-1) in acetonitrile. These... (More)
Femtosecond and nanosecond photolysis of CH2I2 in acetonitrile at an excitation wavelength of 266-nm yield the ground-state CH2I-I isomer product in similar to70% quantum yield. High isomer formation quantum yields (> 70%) were measured also in n-hexane, dichloromethane, methanol, and ethanol using nanosecond photolysis. The CH2I-I product is formed biphasically within similar to15 ps after excitation and can survive on a nanosecond to microsecond time scale before it decays via a mixture of first- and second-order processes. At 21 degreesC, the first-order rate constants are 1.8 x 10(5) s(-1) in n-hexane, with the measured apparent activation energy of 41 +/- 2 kJ mol(-1) (5-55 degreesC), and 4.3 x 10(6) s(-1) in acetonitrile. These rate constants correspond to intramolecular decomposition Of CH2I-I into a CH2I. radical and an I atom, with the additional formation of I- in acetonitrile. In methanol and ethanol, the CH2I-I isomer decays faster with the pseudo-first-order rate constants of 2.0 x 10(8) and 1.3 x 10(8) s(-1), respectively, and as in acetonitrile, I-3(-) and I-2(-) ion products dominate the absorption of photolyzed CH2I2 samples at long times. The thermodynamics of the CH2I2 system and its implication for the CH2I-I formation mechanism are discussed. A possible role of the CH2I-I isomer as a methylene transfer agent in photocyclopronation of olefins is investigated. Vibrationally relaxed CH2I-I is demonstrated to react with cyclohexene yielding an I-2 leaving group with the second-order rate constants similar to4.4 x 10(5) M-1 s(-1) in n-hexane, 3.4 x 10(6) M-1 s(-1) in dichloromethane, and 4.2 x 10(6) M-1 s(-1) in acetonitrile. (Less)
Please use this url to cite or link to this publication:
author
organization
publishing date
type
Contribution to journal
publication status
published
subject
in
The Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory
volume
108
issue
2
pages
237 - 249
publisher
The American Chemical Society
external identifiers
  • wos:000187951600001
  • scopus:1642567334
ISSN
1520-5215
DOI
10.1021/jp035406n
language
English
LU publication?
yes
id
e3d59fac-1b0a-4c4e-87ac-90ea5e25df8d (old id 139133)
date added to LUP
2007-07-04 08:39:44
date last changed
2017-12-17 03:52:42
@article{e3d59fac-1b0a-4c4e-87ac-90ea5e25df8d,
  abstract     = {Femtosecond and nanosecond photolysis of CH2I2 in acetonitrile at an excitation wavelength of 266-nm yield the ground-state CH2I-I isomer product in similar to70% quantum yield. High isomer formation quantum yields (> 70%) were measured also in n-hexane, dichloromethane, methanol, and ethanol using nanosecond photolysis. The CH2I-I product is formed biphasically within similar to15 ps after excitation and can survive on a nanosecond to microsecond time scale before it decays via a mixture of first- and second-order processes. At 21 degreesC, the first-order rate constants are 1.8 x 10(5) s(-1) in n-hexane, with the measured apparent activation energy of 41 +/- 2 kJ mol(-1) (5-55 degreesC), and 4.3 x 10(6) s(-1) in acetonitrile. These rate constants correspond to intramolecular decomposition Of CH2I-I into a CH2I. radical and an I atom, with the additional formation of I- in acetonitrile. In methanol and ethanol, the CH2I-I isomer decays faster with the pseudo-first-order rate constants of 2.0 x 10(8) and 1.3 x 10(8) s(-1), respectively, and as in acetonitrile, I-3(-) and I-2(-) ion products dominate the absorption of photolyzed CH2I2 samples at long times. The thermodynamics of the CH2I2 system and its implication for the CH2I-I formation mechanism are discussed. A possible role of the CH2I-I isomer as a methylene transfer agent in photocyclopronation of olefins is investigated. Vibrationally relaxed CH2I-I is demonstrated to react with cyclohexene yielding an I-2 leaving group with the second-order rate constants similar to4.4 x 10(5) M-1 s(-1) in n-hexane, 3.4 x 10(6) M-1 s(-1) in dichloromethane, and 4.2 x 10(6) M-1 s(-1) in acetonitrile.},
  author       = {Tarnovsky, Alexander and Sundström, Villy and Åkesson, Eva and Pascher, Torbjörn},
  issn         = {1520-5215},
  language     = {eng},
  number       = {2},
  pages        = {237--249},
  publisher    = {The American Chemical Society},
  series       = {The Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory},
  title        = {Photochemistry of diiodomethane in solution studied by femtosecond and nanosecond laser photolysis. Formation and dark reactions of the CH2I-I isomer photoproduct and its role in cyclopropanation of olefins},
  url          = {http://dx.doi.org/10.1021/jp035406n},
  volume       = {108},
  year         = {2004},
}