Matrix photoionization and radiolysis of dichloromethane and dibromomethane. Infrared and ultraviolet absorption spectra and photolysis of CH2Cl2+ and CH2Br2+

A combined experimental and theoretical study of the ultraviolet photolysis of CH2I2 in water is reported. Ultraviolet photolysis of low concentrations of CH2I2 in water was experimentally observed to lead to almost complete conversion into CH2(OH)2 and 2HI products. Picosecond time-resolved resonance Raman spectroscopy experiments in mixed water/acetonitrile solvents (25%-75% water) showed that appreciable amounts of isodiiodomethane (CH2I-I) were formed within several picoseconds and the decay of the CH2I-I species became substantially shorter with increasing water concentration, suggesting that CH2I-I may be reacting with water. Ab initio calculations demonstrate the CH2I-I species is able to react readily with water via a water-catalyzed O--H-insertion and HI-elimination reaction followed by its CH2I(OH) product undergoing a further water-catalyzed HI-elimination reaction to make a H2C=O product. These HI-elimination reactions produce the two HI leaving groups observed experimentally and the H2C=O product further reacts with water to produce the other final CH2(OH)2 product observed in the photochemistry experiments. These results suggest that CH2I-I is the species that reacts with water to produce the CH2(OH)2 and 2HI products seen in the photochemistry experiments. The present study demonstrates that ultraviolet photolysis of CH2I2 at low concentration leads to efficient dehalogenation and release of multiple strong acid (HI) leaving groups. Some possible ramifications for the decomposition of polyhalomethanes and halomethanols in aqueous environments as well as the photochemistry of polyhalomethanes in the natural environment are briefly discussed.

Section: Introductionmentioning

confidence: 99%

Efficient dehalogenation of polyhalomethanes and production of strong acids in aqueous environments: Water-catalyzed O–H-insertion and HI-elimination reactions of isodiiodomethane (CH2I–I) with water

Kwok

Zhao

Guan

et al. 2004

“…Radiolysis of an aqueous solution 6 as well as UV-irradiation 7-10 and photoionization 11 of lowtemperature matrices all produce products with a strong absorption band centered at 380 nm ͑3.3 eV͒ and a weak band with absorption maximum 570 nm ͑2.2 eV͒. These bands have been assigned to trapped electrons, 7 the radical cation CH 2 I 2 ϩ , 6,8,11 and the isomer CH 2 I-I. 9,10 Several groups have studied the ultrafast dissociation dynamics of diiodomethane.…”

Section: Introductionmentioning

confidence: 99%

Photodissociation of diiodomethane in acetonitrile solution and fragment recombination into iso-diiodomethane studied with ab initio molecular dynamics simulations

Odelius

Kadi

Davidsson

et al. 2004

Photodissociation of diiodomethane (CH2I2) in acetonitrile solution has been studied with ab initio molecular dynamics simulations, which show how the iso-diiodomethane photoproduct (CH2I-I) can be formed. The first excited state, described by the "restricted open-shell Kohn-Sham" density functional method, is dissociative and photoexcitation of diiodomethane induces a breaking of one of the C-I bonds. In the simulations, we observe that energy dissipation to the surrounding solvent is essential in the formation of a stable iso-diiodomethane molecule. The caging effect of the solvent results in a recombination of the CH2I and I fragments into iso-diiodomethane on a picosecond time scale. The molecular dynamics simulations enable us to study the cage effect as well as the relaxation of intermediates and the distribution of energy. The CH2I fragment is formed vibrationally excited along the C-I stretching mode. After recombination of the CH2I and I fragments, iso-diiodomethane shows a strong vibration excitation in the CH2 group, which could be used as a fingerprint of the proposed mechanism.

“…For instance, the excitation of diiodomethane in condensed phases with ultraviolet light, 21-24 direct photoionization, 25 or radiolysis 26,27 all form photoproducts that have very similar characteristic ϳ385 nm ͑strong͒ and ϳ570 nm ͑weak͒ absorption bands and these transient absorption bands have been attributed to a variety of species such as trapped electrons, 21 the CH 2 I 2 radical cation, 25,27 and the isomer of diiodomethane ͑CH 2 I-I͒. 23,24 Recently, several research groups have examined the solution phase photodissociation reaction of diiodomethane using femtosecond transient absorption spectroscopy experiments.…”

Section: Introductionmentioning

confidence: 99%

“…However, these results were given three different interpretations that varied due to their assignments of which the photoproduct species was responsible for the transient absorption being monitored during the experiment. [28][29][30] The conflicting assignments for the ϳ385 nm and ϳ570 nm transient absorption bands, observed in both the photochemistry experiments [21][22][23][24][25][26][27] and the femtosecond transient absorption experiments, 28-30 prompted a nanosecond transient resonance Raman investigation of the intense ϳ385 nm transient absorption band in order to unambiguously identify the photoproduct species responsible for it. 37 These experimental results and a comparison to the results of density functional theory computations for several proposed photoproduct species clearly demonstrated that the iso-CH 2 I-I species is mainly responsible for the ϳ385 nm transient absorption band.…”

Section: Introductionmentioning

confidence: 99%

Picosecond time-resolved resonance Raman observation of the iso-CH2I–I photoproduct from the “photoisomerization” reaction of diiodomethane in the solution phase

Kwok

Parker

et al. 2000

We report a preliminary picosecond Stokes and anti-Stokes time-resolved resonance Raman ͑267 nm pump and 400 nm probe excitation wavelengths͒ investigation of the initial formation and vibrational cooling of the iso-CH 2 I-I photoproduct species produced after ultraviolet excitation of diiodomethane in room temperature solutions. A comparison of the picosecond resonance Raman spectra with previously reported nanosecond transient resonance Raman spectra and density functional theory computations shows that the iso-CH 2 I-I photoproduct species is predominantly responsible for the ϳ385 nm transient absorption band observed from several picoseconds to nanoseconds after ultraviolet excitation of diiodomethane in the solution phase. Similar results were obtained in both nonpolar solution ͑cyclohexane solvent͒ and polar solution ͑acetonitrile͒ solvent. The picosecond resonance Raman spectra confirm that the iso-CH 2 I-I photoproduct species is formed vibrationally hot within several picoseconds and then subsequently undergoes vibrational cooling on the 4-50 ps time scale. This is consistent with the absorption bands changes occurring over similar times in a recent femtosecond transient absorption study. We discuss a possible qualitative scenario for the formation of the iso-CH 2 I-I species that is in agreement with the available gas phase experimental results for the ultraviolet photodissociation reaction of diiodomethane and gas phase collisional deactivation studies of the CH 2 I radical. The proposed hypothesis is consistent with the lack of distinct resonance Raman bands in the first few picoseconds of our solution phase spectra of the iso-CH 2 I-I photoproduct as well as previously reported femtosecond transient absorption bands that are broad and weak in the 300-500 nm region over the 0.3-3 ps time scale.