The total energy available, Eavl, to CH2I + I* (2P1/2) fragments in channel (2)
following photodissociation of CH2I2 at 248 nm can be expressed as Eavl = hν – D0 – EI* = ET + Eint
where hν is the photon energy, D0 is the dissociation energy,[21] EI* is the I* (2P1/2) – I
(2P3/2) energy,[35] ET is the total translational energy of the fragments, or TKER, and Eint
is the internal excitation of the CH2I fragment. The dominant TKER feature in Figure 2c
arising from photodissociation of CH2I2 at 248 nm via channel (2) has an average total
(Eavl ~ 14800 cm-1). The remaining 86% of the available energy is deposited into the nascent CH2I photofragments with an average internal energy Eint = 12,700 cm-1 (36.3
kcal mol-1). The distribution over translational energies also reveals the corresponding
spread of internal energies for the CH2I fragments.
The present findings are consistent with the prior IR emission from CH2I
fragments produced by channel (1) or (2).[21] The observation of significant IR emission over a broad range of wavelengths indicated extensive CH2I internal excitation, with
vibrational excitation nearing a quasicontinuum of states. The CH2I internal energy
(~85% of Eavl) estimated from IR emission arising from both I and I* channels can be compared with the present experimental result (~86% of Eavl) for channel (2) alone to demonstrate that the CH2I fragments acquire similar percentages of the available energy
in channels (1) and (2).
A simple impulsive model can be applied to CH2I2 dissociation to predict the
fraction of available energy being channeled into product translation.[21,36] An
instantaneous impulse coming from the C-I bond rupture causes translational energy to be imparted to products based on the reduced mass of the atoms directly involved with the dissociating bond (C-I) relative to the system as a whole. This simple model predicts that translational energy will account for 16% of the available energy, leaving the remaining 84% as CH2I internal energy. This is quite similar to what is seen experimentally (86%).
Moreover, as observed experimentally here and elsewhere,[19] this fraction does not change significantly over the photolysis wavelength from 313 nm to 248 nm, remaining close to 80-85%. This simple impulsive model seems to work well to describe the CH2I2
photodissociation dynamics for the many different excited electronic states accessed in this wavelength range as well as for both I and I* product channels (1) and (2).
Interestingly, the internal energy distribution of the CH2I fragments is broader
upon photolysis of CH2I2 at 248 nm than the narrow distribution observed upon
photolysis at 313 nm (shown in Figure 2a and discussed in Appendix I) and the 277-305 nm wavelength region probed previously.[19] The TKER distribution and corresponding internal energy distribution of the CH2I fragments resulting from 248 nm photolysis is
almost four times as broad (fwhm 2800 cm-1) as that from 313 nm photolysis (fwhm
~700 cm-1). A prior photodissociation study of CH2I2 at 266 nm showed a similarly
broad distribution (fwhm ~ 3100 cm-1), although this study did not distinguish between I and I* cofragments in channel (1) and (2).[17]
The photodissociation of CH2I2 at 248 nm is of renewed interest because it is
being used in laboratory settings to generate the smallest Criegee intermediate, CH2OO.[1-4] The subsequent reaction of CH2I photofragments with O2 generates
CH2OO + I in a near thermoneutral process.[5] The internal energy distribution of the
CH2I fragments determined in this work should enable future modeling of the internal
excitation of the CH2OO intermediates.
The CH2I fragment is generated with an average internal energy of ~36 kcal mol-1
with an 8 kcal mol-1 breadth (fwhm) in the CH
2I + I* channel (0.46 quantum yield).[22]
Since a similar partitioning of available energy to CH2I fragments is evident in the I*
channel (this work) and combined I/I* channels (prior work),[21] one can infer that the CH2I + I channel will have the same partitioning. As a result, the average internal energy
of the CH2I fragments in the I channel is expected to be close to ~54 kcal mol-1, likely
with a similar breadth. [A small fraction of the CH2I fragments may even have enough
internal energy to dissociate into CH2 + I (Hrxn= 62.6 kcal mol-1)[21], which would no
longer be relevant to the formation of CH2OO via the CH2I + O2 reaction.] This bimodal
distribution of internally ‘hot’ CH2I fragments is expected to give rise to highly excited
CH2OO intermediates.
Moreover, the degree of CH2OO internal excitation produced in the new
laboratory scheme with 248 nm photolysis of CH2I2, in particular channel (1), is
comparable to that acquired in alkene-ozone reactions in the atmosphere, which are highly exothermic reactions with up to 60 kcal mol-1 released to products.[9] This
suggests that the CH2OO intermediates generated via the alternative laboratory scheme at
248 nm may undergo unimolecular and bimolecular processes similar to those observed upon ozonlysis of alkenes. Notably, there should be adequate internal excitation of the Criegee intermediate to isomerization to dioxirane (barrier height of 19.2 kcal mol-1),[37] and release OH radicals.[4,8,9]
The simplest Criegee intermediate has also been generated using 351 nm photolysis of CH2I2.[38] CH2I2 has an order of magnitude smaller absorption cross
section at 351 nm than 248 nm,[39] which in principle can be overcome by the high photon flux of excimer lasers. The internal energy distribution of the CH2OO
intermediate is also expected to be much lower, since the CH2I fragments will have
substantially less internal excitation. The fraction of available energy channeled into CH2I internal energy is similar at 351 nm and 248 nm, but the amount of CH2I internal
excitation is substantially different. Since only the CH2I + I product channel is open,[22]
CH2I fragments will be generated with ~25 kcal mol-1 of internal energy, assuming 85%
partitioning of the available energy. This is significantly less than the CH2I internal
excitation from 248 nm photodissociation via channels (1) and (2). The lower internal excitation of CH2I and correspondingly lower internal energy of CH2OO should make it
easier to collisionally stabilize the Criegee intermediates for subsequent study.