Photodissociation of CH₂I₂ and Subsequent Electron Transfer in Solution

Abstract

We studied photoinduced reactions of diiodomethane (CH(2)I(2)) upon excitation at 268 nm in acetonitrile and hexane by subpicosecond-nanosecond transient absorption spectroscopy. The transient spectra involve two absorption bands centered at around 400 (intense) and 540 nm (weak). The transients probed over the range 340-740 nm show common time profiles consisting of a fast rise (<200 fs), a fast decay ( approximately 500 fs), and a slow rise. The two fast components were independent of solute concentration, whereas the slow rise became faster (7-50 ps) when the concentration in both solutions was increased. We assigned the fast components to the generation of a CH(2)I radical by direct dissociation of the photoexcited CH(2)I(2) and its disappearance by subsequent primary geminate recombination. The concentration-dependent slow rise produced the absorption bands centered at 400 and 540 nm. The former consists of different time-dependent bands at 385 and 430 nm. The band near 430 nm grew first and was assigned to a charge-transfer (CT) complex, CH(2)I(2) (delta+)I(delta-), formed by a photofragment I atom and the solute CH(2)I(2) molecule. The CT complex is followed by full electron transfer, which then develops the band of the ion pair CH(2)I(2) (+)I(-) at 385 nm on the picosecond timescale. On the nanosecond scale, I(3) (-) was generated after decay of the ion pair. The reaction scheme and kinetics were elucidated by the time-resolved absorption spectra and the reaction rate equations. We ascribed concentration-dependent dynamics to the CT-complex formation in pre-existing aggregates of CH(2)I(2) and analyzed how solutes are aggregated at a given bulk concentration by evaluating a relative local concentration. Whereas the local concentration in hexane monotonically increased as a function of the bulk concentration, that in acetonitrile gradually became saturated. The number of CH(2)I(2) molecules that can participate in CT-complex formation has an upper limit that depends on the size of aggregation or spatial restriction in the neighboring region of the initially photoexcited CH(2)I(2). Such conditions were achieved at lower concentrations in acetonitrile than in hexane.

Solvent dependent branching between C-I and C-Br bond cleavage following 266 nm excitation of CH2BrI

“…If this band corresponds to the 340 nm gas phase transition of CH 2 I, the absence of the stronger 280 nm transition is surprising, although the probe only extended to 290 nm and the contribution of the ground state bleach could complicate the spectrum as this lies near the peak of the CH 2 I 2 spectrum in acetonitrile (λ max ≈ 288 nm, ε max ≈ 1300 M −1 cm −1 ). 74 In the transient spectra reported here we observe a persistent UV absorption band in acetonitrile. There is an initial peak at ∼325 nm that shifts to the red, stabilizing at 356 nm within 50 ps.…”

UV photodissociation dynamics of CHI2Cl and its role as a photolytic precursor for a chlorinated Criegee intermediate

“…21,22 The photochemistry of dihalomethanes has also been studied in condensed phases using ultrashort laser pulses, with transient absorption measurements leading to the suggestion that the CH2I-Y isomer is formed on short timescales. [26][27][28][29][30] In contrast to the dihalomethanes, little work has been done on trihalomethanes.…”

Ultrafast structural dynamics of in-cage isomerization of diiodomethane in solution

“…Please do not adjust margins From the TA and TRR studies, it was suggested that the cold CH 2 I-I isomer is formed by the vibrational cooling of the hot CH 2 I-I isomer. [22][23][24][25][26][27][28] However, since the spectroscopic signals at the optical frequencies are not directly related to the global molecular structures at the atomic level, the structure of the hot isomer, or the precursor of the cold isomer, remains to be determined. SADS1 contains the structural information necessary for this purpose, and to shed light on the identity of the precursor of the isomer, we performed the structural fitting analysis of SADS1.…”

“…[13][14][15][16][17] Since the formation of the isomer is rarely observed in the gas phase, [18][19][20][21] the chemical dynamics of the isomer formation has been an important topic of many experimental investigations to understand the role of the solvent cage. In this regard, diiodomethane, CH 2 I 2 , is a prototypical example and its photodissociation is known to produce a unique isomer species, isodiiodomethane (CH 2 I-I), [22][23][24][25][26][27][28][29][30][31][32][33][34] which has no analog in the gas phase ( Figure 1). 19,[35][36][37] The photodissociation dynamics of CH 2 I 2 and the formation of CH 2 I-I isomer in solution have been mainly investigated with ultrafast transient absorption (TA) spectroscopy 22-24, 38, 39 and transient resonance Raman (TRR) spectroscopy, [25][26][27][28] which successfully identified the transient spectrum assigned to the isomer and the time scale of the isomer formation.…”

“…The studies using TA spectroscopy uncovered that the excited molecule rapidly dissociates into CH 2 I• and I• fragments and recombines to form the CH 2 I-I isomer with a time constant of ~5 ps. [22][23][24] It is also known that a part of I• radicals escape the solvent cage and form CH 2 I• + I• within about 120 fs. 38,39 Here CH 2 I• + I• denotes the CH 2 I• and I• photofragments that are completely separated and not in the same solvent cage.…”