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

Abstract

Iso-polyhalomethanes are known reactive intermediates that play a pivotal role in the photochemistry of halomethanes in condensed phases. In this work, iso-bromoform (iso-CHBr(3)) and its deuterated isotopomer were characterized by matrix isolation infrared and UV/visible spectroscopy, supported by ab initio and density functional theory calculations, to further probe the structure, spectroscopy, and photochemistry of this important intermediate. Selected wavelength laser irradiation of CHBr(3) isolated in Ar or Ne matrices at ~5 K yielded iso-CHBr(3); the observed infrared and UV/visible absorptions are in excellent agreement with computational predictions, and the energies of various stationary points on the CHBr(3) potential energy surface were characterized computationally using high-level methods in combination with correlation consistent basis sets. These calculations show that, while the corresponding minima lie ~200 kJ/mol above the global CHBr(3) minimum, the isomer is bound by some 60 kJ/mol in the gas phase with respect to the CHBr(2) + Br asymptote. The photochemistry of iso-CHBr(3) was investigated by selected wavelength laser irradiation into the intense S(0) → S(3) transition, which resulted in back photoisomerization to CHBr(3). Intrinsic reaction coordinate calculations confirmed the existence of a first-order saddle point connecting the two isomers, which lies energetically below the threshold of the radical channel. Subsequently, natural bond orbital analysis and natural resonance theory were used to characterize the important resonance structures of the isomer and related stationary points, which demonstrate that the isomerization transition state represents a crossover from dominantly covalent to dominantly ionic bonding. In condensed phases, the ion-pair dominated isomerization transition state structure is preferentially stabilized, so that the barrier to isomerization is lowered.

Roaming-mediated ultrafast isomerization of geminal tri-bromides in the gas and liquid phases

“…This isomerization pathway could be termed 'roamingmediated', because it involves the migration of the incipient bromine fragment and leads to the iso product species; however, this could not be confirmed without comparison to the same reaction in the gas phase. The isomeric form occupies a shallow minimum on the potential energy surface (PES)-somewhat below the C-Br bond radical dissociation asymptote [12][13][14][15] -and has been proposed 16 to be accessible on the way towards molecular bromine from photoexcited bromoform in the gas phase. This isomerization pathway in the gas phase can only occur through the internal rearrangement of atoms, that is, directly and without the involvement of any other species.…”

Photoisomerization and Photoinduced Reactions in Liquid CCl4 and CHCl3

“…A recent computational study of CHBr3 isomerization showed significant charge-separation character in iso-CHBr2-Br, which exhibits mixed covalent and ionic character, with (CHBr2) + Br -contributing the major (62%) resonance structures. 26 We therefore undertook calculations of the electronic character of the iso-forms of CCl4 and CHCl3, following an NBO analysis similar to that reported by George et al 26 and using the computational methods outlined in Section 2.2. Table 1: Calculated wavelengths of the electronic absorptions of the Cl-solvent (Cl-CCl4 and Cl-CHCl3) complexes and iso forms (iso-CCl3-Cl and iso-CHCl2-Cl) considered to give rise to the transient UV and visible bands evident in Fig.…”

“…Our assignments are similar in spirit to the earlier literature, but more consistent with recent experimental and computational studies of other halogenated methanes such as bromoform (CHBr3). [24][25][26] Iso species of this type are important as reactive agents in solution:…”

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

“…4,[25][26][27] Cage-enhanced geminate recombination has significant yield to the iso products and these products have been observed in variety of different experiments. 3,25,26,[28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] Ultraviolet excitation of CH 2 BrI yields iso-CH 2 Br-I or iso-CH 2 I-Br following C-I or C-Br bond cleavage, respectively. These products are predicted to have transitions in the visible or near UV with very large oscillator strength (∼0.5).…”