Direct photoisomerization of CH₂I₂vs. CHBr₃ in the gas phase: a joint 50 fs experimental and multireference resonance-theoretical study

Abstract

Femtosecond transient absorption measurements powered by 40 fs laser pulses reveal that ultrafast isomerization takes place upon S excitation of both CHI and CHBr in the gas phase. The photochemical conversion process is direct and intramolecular, i.e., it proceeds without caging media that have long been implicated in the photo-induced isomerization of polyhalogenated alkanes in condensed phases. Using multistate complete active space second order perturbation theory (MS-CASPT2) calculations, we investigate the structure of the photochemical reaction paths connecting the photoexcited species to their corresponding isomeric forms. Unconstrained minimum energy paths computed starting from the S Franck-Condon points lead to S/S conical intersections, which directly connect the parent CHBr and CHI molecules to their isomeric forms. Changes in the chemical bonding picture along the S/S isomerization reaction path are described using multireference average coupled pair functional (MRACPF) calculations in conjunction with natural resonance theory (NRT) analysis. These calculations reveal a complex interplay between covalent, radical, ylidic, and ion-pair dominant resonance structures throughout the nonadiabatic photochemical isomerization processes described in this work.

Structural Dynamics of Bismuth Triiodide in Solution Triggered by Photoinduced Ligand-to-Metal Charge Transfer

“…The halogen− halogen bond in isomeric species of various halocarbons has the character of polar covalent bond and therefore can be affected by the polar environment. 36 According to a study on CHBr 3 , it was theoretically shown that the structure of iso-CHBr 2 −Br varies depending on the solvent polarity, especially with the Br−Br bond length being proportional to the solvent polarity. 37 Accordingly, the I−I bond of iso-BiI 2 −I (3.444 Å) is long in acetonitrile, which is a polar solvent, and especially much longer than the I−I bond of an isomer, iso-CHI 2 −I, (2.922 Å) formed from the photoreation of CHI 3 in nonpolar cyclohexane, which was recently investigated using TRXL.…”

“…Especially, the I–I bond of iso -BiI 2 –I is significantly longer than that of other trihalide isomers. The halogen–halogen bond in isomeric species of various halocarbons has the character of polar covalent bond and therefore can be affected by the polar environment . According to a study on CHBr 3 , it was theoretically shown that the structure of iso -CHBr 2 –Br varies depending on the solvent polarity, especially with the Br–Br bond length being proportional to the solvent polarity .…”

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

“…In the case of haloalkanes, geminate recombination can yield an isomer species as well as the parent molecule. [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).…”

“…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. As UV excitation of CH 2 I 2 induces the n(I) → σ * (C-I) transition, one of the two C-I bonds in CH 2 I 2 dissociates.…”

Competing Molecular and Radical Pathways in the Dissociation of Halons via Direct Chemical Dynamics Simulations

“…However, no electronic structure studies characterizing such multicenter TSs have been reported. Alternate proposed pathways for molecular product formation involve iso-halons containing a halogen–halogen bond − and subsequent dissociation. These iso-halons have been identified as reactive intermediates in the condensed phase chemistry of halomethanes .…”