Spectroscopic and computational studies of matrix-isolated iso-CHBr3: Structure, properties, and photochemistry of iso-bromoform

Strong-Field-Induced Coulomb Explosion Imaging of Tribromomethane

The Coulomb explosion of tribromomethane (bromoform, CHBr₃) induced by 28 fs near-infrared laser pulses is investigated by three-dimensional coincidence ion momentum imaging. We focus on the fragmentation into three, four, and five ionic fragments measured in coincidence and present different ways of visualizing the three-dimensional momentum correlations. We show that the experimentally observed momentum correlations for 4- and 5-fold coincidences are well reproduced by classical Coulomb explosion simulations and contain information about the structure of the parent molecule that could be used to differentiate structural isomers formed, for example, in a pump-probe experiment. Our results thus provide a clear path toward visualizing structural dynamics in polyatomic molecules by strong-field-induced Coulomb explosion imaging.

Ultrafast Solution-Phase Photophysical and Photochemical Dynamics of Hexaiodobismuthate(III), the Heart of Bismuth Halide Perovskite Solar Cells

The ultrafast relaxation pathways in a hexaiodide bismuth(III) complex, BiI₆^3-, excited at 530 nm in acetonitrile solution are studied by means of femtosecond transient absorption spectroscopy supported by steady-state absorption/emission measurements and DFT computations. Radiationless relaxation out of the Franck-Condon, largely metal-centered (MC) triply degenerate ³T_1u state (46 ± 19 fs), is driven by vibronic coupling due to the Jahn-Teller effect in the excited state. The relaxation populates two lower-energy states: a ligand-to-metal charge transfer (LMCT) excited state of ³π I(5p_π) → Bi(6p) nature and a luminescent "trap" ³A_1u(³P₀) MC state. Coherent population transfer from the initial ³T_1u into the ³π LMCT state occurs in an oscillatory, stepwise manner at ∼190 and ∼550 fs with a population ratio of ∼4:1. The ³π LMCT state decays with a 2.9 ps lifetime, yielding two short-lived reaction intermediates of which the first one reforms the parent ground state with a 15 ps time constant, and the second one decays on a ∼5 ps timescale generating the triplet product species, which persists to the longest 2 ns delay times investigated. This product is identified as the η² metal-ligated diiodide-bismuth adduct with the intramolecularly formed I-I bond, [(η²-I₂)Bi(II)I₄]^3-, which is the species of interest for solar energy conversion and storage applications. The lifetime of the "trap" ³A_1u state is estimated to be 13 ns from the photoluminescence quenching of BiI₆^3-. The findings give insight into the excited-state relaxation dynamics and the photochemical reaction mechanisms in halide complexes of heavy ns² metal ions.

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

A great deal of attention has been given to the decomposition chemistry of halons (halomethanes) due to their role in stratospheric ozone depletion. Knowledge of certain aspects of dissociation of halons such as the competition between radical and molecular pathways and their mechanistic details is limited. Halon molecules can isomerize to an iso form containing a halogen-halogen bond and such iso-halon forms have been identified as intermediates in condensed phase chemistry. Recently, a quantum chemistry study of role of iso-halons in the gas phase decomposition of halomethanes has been reported. In the present work, we have investigated the ground state dissociation chemistry of select halon molecules - CF₂Cl₂, CF₂Br₂, CHBr₃, and CH₂BrCl using electronic structure theory calculations and direct chemical dynamics simulations. Classical trajectories were generated on-the-fly using density functional PBE0/6-31G* level of theory at a fixed total energy. Simulation results showed that molecular products, in general, were dominant for all the four molecules at the chosen energy. A variety of mechanisms such as direct dissociation via multicenter transition states, decomposition via isomerization, radical recombinations, and roaming pathways contributed to the formation of molecular products. Atomic level mechanisms are presented, and the role of iso-halons in the gas phase chemistry of halomethanes is clearly established.

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

Femtosecond transient absorption measurements powered by 40 fs laser pulses reveal that ultrafast isomerization takes place upon S₁ excitation of both CH₂I₂ and CHBr₃ 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 S₁ Franck-Condon points lead to S₁/S₀ conical intersections, which directly connect the parent CHBr₃ and CH₂I₂ molecules to their isomeric forms. Changes in the chemical bonding picture along the S₁/S₀ 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.

An ab initio study on noble gas inserted halogenated acetylene: HNgCCX (Ng = Kr and Xe; X = halogen)

Although HNgCCX (Ng = Kr and Xe; X = F and Cl) have been identified in cryogenic matrices, similar Br and I analogues have not been prepared so far. In this paper, the nature of HNgCCX (Ng = Kr and Xe; X = F, Cl, Br and I) have been investigated by ab initio methods. The main characteristic absorption peak of HNgCCX is the v_H-Ng, which decreases as X varies from F to I. Moreover, the H-Xe bond is stronger than the H-Kr bond. The v_C≡C and v_C-X exhibit red- and blue-shift characters, respectively, especially the C-X bond is abnormal blue-shift halogen bond. AIM results show that the H-Ng bond is essentially covalent bond and the covalent character of H-Xe bond is underestimated, and the trend of the covalent character is C-Cl > C-Br > C-F > C-I. Although HNgCCX is instable thermodynamically with respect to Ng + HCCX, it is kinetically stable with respect to the two-/three-body channels due to the relatively larger energy barriers. The three-body channels of HNgCCX is the main decomposition channel, and the kinetically stability of HXeCCX is more than its Kr analogues. This study is helpful for the preparation of new HNgCCX in cryogenic matrices.

Theoretical Insights into Halogenated Uranium Cyanide/Isocyanide Compounds

Two kinds of halogenated uranium cyanide/isocyanide compounds, XUCN and XUNC (X = halogen) formed by the insertion of uranium atom into X-C(N) bonds of XCN (or XNC), were investigated by DFT and ab initio methods. Although XNC is less stable thermodynamically than XCN, XUNC is more stable than XUCN and is expected to be prepared and characterized in matrix isolation experiments. The C-N stretching vibration mode (ν_C-N) is the primary fingerprint for the identification of these isomers due to its red-shift character with respect to the relevant precursor. Atoms-in-molecule (AIM) analysis illustrates that both X-U and U-C(N) bonds in XUCN and XUNC show closed-shell interaction character, although partial covalent character contributes to them, and can be denoted as X^-U²⁺(CN)^- and X^-U²⁺(NC)^-, respectively. Charge decomposition analysis (CDA) further reveals that the isocyanide exhibits better donation performance than the cyanide, which should be the root cause of the difference between XUCN and XUNC.

Time-Domain Simulations of Transient Species in Experimentally Relevant Environments

Simulating the spectroscopic properties of short-lived thermal and photochemical reaction intermediates and products is a challenging task, as these species often feature atypical molecular and electronic structures. The complex environments in which such species typically reside in practice add further complexity to the problem. Herein, we tackle this problem in silico using ab initio molecular dynamics (AIMD) simulations, employing iso-CHBr3, namely H(Br)C-Br-Br, as a prototypical system. This species was chosen because it features both a nonconventional C-Br-Br bonding pattern, as well as a strong dependence of its spectral features on the local environment in which it resides, as illustrated in recent experimental reports. We simulate the UV-vis and IR spectra of iso-CHBr3 in the gas phase, as well as in a Ne cluster (64 atoms) and in a methylcyclohexane cage (14 solvent molecules) representative of the previously characterized matrix isolated and solvated iso-CHBr3 species. We exclusively perform fully quantum mechanical static and dynamic simulations. By comparing our condensed phase simulations to their experimental analogues, we stress the importance of (i) conformational sampling, even at cryogenic temperatures, and (ii) using a fully quantum mechanical description of both solute and bath to properly account for the experimental observables.

Computational insights into CH3MX (M = Cu, Ag and Au; X = H, F, Cl, Br and I)

The thermodynamically stability of CH₃MX with respect to CH₃X + M is CH₃CuX > CH₃AuX > CH₃AgX. Some stable CH₃MX have not been identified experimentally because their vibrational fingerprints (ν_C−M and v_M−X) are too low to be detected.

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

It is well known that ultraviolet photoexcitation of halomethanes results in halogen-carbon bond cleavage. Each halogen-carbon bond has a dominant ultraviolet (UV) absorption that promotes an electron from a nonbonding halogen orbital (nX) to a carbon-halogen antibonding orbital (σ*C-X). UV absorption into specific transitions in the gas phase results primarily in selective cleavage of the corresponding carbon-halogen bond. In the present work, broadband ultrafast UV-visible transient absorption studies of CH2BrI reveal a more complex photochemistry in solution. Transient absorption spectra are reported spanning the range from 275 nm to 750 nm and 300 fs to 3 ns following excitation of CH2BrI at 266 nm in acetonitrile, 2-butanol, and cyclohexane. Channels involving formation of CH2Br + I radical pairs, iso-CH2Br-I, and iso-CH2I-Br are identified. The solvent environment has a significant influence on the branching ratios, and on the formation and stability of iso-CH2Br-I. Both iso-CH2Br-I and iso-CH2I-Br are observed in cyclohexane with a ratio of ~2.8:1. In acetonitrile this ratio is 7:1 or larger. The observation of formation of iso-CH2I-Br photoproduct as well as iso-CH2Br-I following 266 nm excitation is a novel result that suggests complexity in the dissociation mechanism. We also report a solvent and concentration dependent lifetime of iso-CH2Br-I. At low concentrations the lifetime is >4 ns in acetonitrile, 1.9 ns in 2-butanol and ~1.4 ns in cyclohexane. These lifetimes decrease with higher initial concentrations of CH2BrI. The concentration dependence highlights the role that intermolecular interactions can play in the quenching of unstable isomers of dihalomethanes.

Solvent-dependent photochemistry of 2,2,2-tribromoethyl-(2'-phenylacetate)

Photolysis (254 nm) of the title compound 1 produces a variety of stable products, which vary significantly with the nature of the solvent. Solvents that serve as efficient H atom donors (methanol, ethanol, isopropyl alcohol) favor products arising from a net reduction of one or more of the C-Br bonds. These include 2,2-dibromoethyl-(2'-phenylacetate) 2 and 2-bromoethyl-(2'-phenylacetate) 3. In the presence of nucleophiles, products such as 2-(2'-phenylacetoxy)acetic acid 5a and/or its ester derivatives are produced. Phenylacetic acid 6 is formed in some cases but under the conditions studied appears to be a minor product. The results are interpreted in terms of a general mechanism that features formation of an iso-tribromo intermediate 9 and/or a geminate radical-atom pair.