Spectroscopic and computational studies of matrix-isolated iso-CXBr3 (X=F, Cl, Br): Structure, properties, and photochemistry of substituted iso-tribromomethanes

Abstract

Iso-polyhalomethanes are important reactive intermediates in the condensed and gas-phase chemistry of halomethanes. Building upon our recent study of iso-bromoform, in this work the substituted iso-tribromomethanes (iso-CXBr3; X = F, Cl, Br) were characterized by matrix isolation infrared and UV/Vis spectroscopy, supported by ab initio calculations, to further probe the structure, spectroscopy, properties, and photochemistry of these important intermediates. Selected wavelength laser irradiation of CXBr3 samples in an inert rare gas (typically Ar; mixing ratio 1:500) held at ∼5 K yielded iso-CXBr3 (XBrC-Br-Br or Br2C-Br-X). The observed infrared and UV/Vis absorptions are in excellent agreement with computational predictions, and the energies of various stationary points on the CXBr3Potential Energy Surfaces (PESs) were characterized computationally using DFT, MP2, and CCSD (T) methods in combination with triple-zeta quality basis sets. These calculations show that the isomers are minima on the PESs that lie ∼200 kJ/mol above the global CXBr3 minimum, yet are bound by some 50-70 kJ/mol in the gas-phase with respect to the CXBr2 + Br asymptote. Laser irradiation of the isomers resulted in back photoisomerization to CXBr3, and intrinsic reaction coordinate (IRC) calculations confirmed the existence of a first order saddle point connecting the two isomers. Calculations of important stationary points on the CXBr3 PESs show that in the gas-phase the isomerization barrier lies energetically near the threshold for simple bond fission. The iso-CXBr3species are significantly stabilized in the condensed phase, due to the high degree of ion-pair character, as revealed by Natural Resonance Theory analysis.

Global sampling of the photochemical reaction paths of bromoform by ultrafast deep-UV through near-IR transient absorption and ab initio multiconfigurational calculations

“…60 Recently, Reid and co-workers trapped iso-CHBr 3 following 220-nm irradiation of CHBr 3 in Ar and Ne matrices. 61,62 The absorption spectrum of iso-CHBr 3 exhibits a 245-nm band, and a more intense 398-nm band (molecular extinction coefficient, ε = 18600 M −1 cm −1 ) with a ∼560-nm shoulder. 61 This is consistent with early observations of the absorption of color centers developed in frozen hydrocarbon solutions containing bromoform following their exposure to near-UV light.…”

Case of the Missing Isomer: Pathways for Molecular Elimination in the Photoinduced Decomposition of 1,1-Dibromoethane

“…For example, in condensed phases simple halomethanes are known upon irradiation to form isomers (the iso-halons) that feature a halogenhalogen bond and are best represented as ion-pairs between a halide anion and halocarbocation. (23,25) These are unique systems where isomerization is intimately linked to electron transfer. (23) Such isomerization can occur in condensed phases from simple geminate recombination upon cleavage of a C-X bond, yet our recent studies have shown that a well-defined reaction path links the two isomers, (23) and photoisomerization is a facile event.…”

On the electronic spectroscopy of the iso-polyhalomethanes

“…Time-dependent DFT (TDDFT) methods are now widely used for modeling electronically excited states, and it is well appreciated that local exchange functionals perform poorly for states involving significant charge transfer [28]. Thus, in this work we employed the CAM-B3LYP functional, which has previously been shown to model well the electronic spectrum of the iso-halons [16], [19]. The vibrational frequencies and mass-weighted displacements (l-matrices) derived from the (TD) DFT calculations were used as input into a multidimensional Franck-Condon routine in the PGOPHER program suite [29], which incorporated the full effects of Duschinsky mixing.…”

“…The electronic spectra of the iso-polyhalomethanes universally exhibit a broad, very intense absorption band in the near-UV to Visible region, with weaker band(s) lying at lower energy [2], [3], [15], [16], [18]. On the basis of Time-Dependent Density Functional Theory (TDDFT) calculations, we have assigned the strong feature to the S0 → S3 transition, with the overlapping S0 → S1, S2 transitions lying to lower energy [15], [19]. The calculated oscillator strength of the S0 → S3 feature is in good agreement with experiment, while the S0 → S1, S2 absorptions are calculated to be much weaker than observed experimentally, suggesting that these bands borrow intensity from the S0 → S3 transition.…”

“…In condensed phases, these isomers are readily formed in the geminate recombination of the nascent radical pair formed following photolytic cleavage of the carbon-halogen bond in the parent halon. Building upon the initial matrix studies of Maier and co-workers [2], [3], [18], and subsequent studies in solution [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], we have recently examined the vibrational and electronic spectroscopy of a series of iso-polyhalomethanes in cryogenic matrices [1], [14], [15], [16], [17], [19]. Recently, in collaboration with groups at UW-Madison, we have probed on the ultrafast timescale the formation and vibrational relaxation of iso-CH2ClI in solution and cryogenic matrices [17].…”