# What is the approximate lifetime of an SN2 transition state complex?

Imagine some SN2 reaction involving an aliphatic sp3 carbon with a halide leaving group (e.g. iodide or bromide). Given the rise of femtosecond spectroscopy (e.g. using a mode-locked Ti:sapphire laser) and even attosecond spectroscopy, I'm curious if there are any studies that have measured the properties and approximate lifetime of the SN2 transition state complex? I guess that this is in the $\approx$ picosecond regime alongside the timescales for a bond stretching dynamics (e.g. between oxygen and hydrogen on a water molecule), however, is there a more precise value that is now known?

I am be curious about results from quantum mechanical simulations (using e.g. Jaguar from the Schrodinger computational chemistry suite) for SN2 reactions?

I'll specify that I'm referring to a "concerted" SN2 reaction, not, for example, the kind of reaction described in: [Chandrasekhar, J., Jorgensen, W. L. Energy profile for a non-concerted SN2 reaction in solution. J. Am. Chem. Soc. 107, pp. 2974 - 2975 (1985).].

Also, provided that non-concerted SN2 reactions exist (example) perhaps my question is underspecified. Perhaps, I should ask instead if there are any interesting long-lived SN2 interaction complexes, or tables for transition state lifetimes from ultrafast spectroscopy studies?

• The time scale of one molecular vibration. – Lighthart Mar 3 '14 at 22:59
• @Lighthart I'm worried that my question is underspecified because it seems that there are non-concerted $S_N2$ reactions (pubs.acs.org/doi/abs/10.1021/ja00296a024?journalCode=jacsat). However, if the reaction is completely concerted, I'm sure the period of vibration for the relevant bond (femtoseconds to picoseconds depending on the bond type and mass of the leaving group) is the right order-of-magnitude for the lifetime of the transition state complex (as you say). – RGrey Mar 4 '14 at 0:36
• How is that of any concern. You cannot isolate it. – evil999man Mar 10 '14 at 13:27

26-60 femtoseconds depending upon impact angle for $\ce{F- + CH3Cl}$ according to Mugnai et al. (Table IV.)