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

Abstract

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.

Structural Determination of a Transient Isomer ofCH2I2by Picosecond X-Ray Diffraction

“…Using this protocol an iodine-iodine bond length of 3:02 0:02 A for CH 2 I-I isomer and an interatomic distance of 3:50 0:02 A for the CH 2 I 2 parent molecule were recovered, where the uncertainty represents the change required to double the minimum residue. These values are in good agreement with theoretical models where the predicted CH 2 I-I and CH 2 I 2 iodine-iodine separations were 3.03 and 3.58 Å respectively [14].…”

“…1). More recent investigations using transient resonance Raman spectroscopy [20 -23] as well as theoretical calculations [14,[24][25][26][27] support this interpretation.…”

“…Dihalomethanes are also of interest in atmospheric chemistry, for reactions in both the gas and the condensed phase, since they may be an important natural source of organohalide compounds emitted into the atmosphere [10 -13]. Figure 1 presents a schematic of the photoreaction of CH 2 I 2 [14]. Following excitation onto the antibonding C-I surface, an iodine atom is repelled and its covalent bond to the central carbon is broken.…”

“…During these simulations, bond lengths and angles for CH 2 I 2 and CH 2 I-I were taken from Ref. [14].…”

Electronic state spectroscopy of diiodomethane (CH2I2): Experimental and computational studies in the 30 000–95 000 cm−1 region

“…5 Yet another reason for the interest in CH 2 I 2 is related to the potential for lasing action in excited I 2 produced by photodissociation of CH 2 I 2 . 6 Over the years, a considerable number of experimental 1, 3-22 and theoretical [23][24][25][26][27] investigations on CH 2 I 2 , largely pertaining to photodissociation dynamics in the UV region (<50 000 cm −1 ), have been carried out. Notwithstanding the vast amount of literature available on the UV region, it is rather surprising that there is very little information available on the vacuum ultraviolet (VUV) spectrum of CH 2 I 2 .…”