Br Bond Dissociation Research Articles

The Dynamics of Br Formation from the Primary and Secondary C–Br Bond Dissociation of Oxalyl Bromide near 265 and 234 nm

The Dynamics of Br Formation from the Primary and Secondary C–Br Bond Dissociation of Oxalyl Bromide near 265 and 234 nm

A quantum chemical study of the gas-phase homolytic S–Br bond dissociation enthalpies of sulfenyl bromides

Abstract This study reports the gas-phase homolytic S–Br bond dissociation enthalpies (at 298 K) for a set of 44 sulfenyl bromide derivatives (RSBr) obtained using the high-level G4 thermochemical protocol. For the species in this set, we find that the BDEs range from 178.0 kJ mol–1 (F3CSSBr) to 239.5 kJ mol–1 (HC(=NH)SBr). We have additionally reported molecule and radical stabilization enthalpies for all species, allowing for insights to be gained as to the effect of substituents in both the sulfenyl bromide reactants and the sulfur-centered radical products. In addition, we have also reported the BDEs of six sufinyl bromides (RS(=O)Br) and six sulfonyl bromides (RSO2Br). For the sulfinyl bromides, the BDEs range from 156.6 kJ mol–1 (F3CS(=O)Br) to 173.8 kJ mol–1 (tBuS(=O)Br), whilst for the sulfonyl bromides, the BDEs range from 211.0 kJ mol–1 (F3CSO2Br) to 226.2 kJ mol–1 (FSO2Br).

Resonance enhanced multiphoton ionization − time of flight (REMPI-TOF) detection of Br ( 2 P j ) atoms in the photodissociation of 4-bromo-2,3,5,6-tetrafluoropyridine at 234 nm: Effect of low-lying πσ* states

Abstract The photodissociation dynamics of 4-bromo-2,3,5,6-tetrafluoropyridine (BTFP) have been studied in a supersonic molecular beam around 234 nm, which prepares the molecule in its ππ* state. The dynamics of the C Br bond dissociation have been investigated using resonance-enhanced multiphoton ionization coupled with time-of-flight mass spectrometer (REMPI-TOFMS) by detecting the nascent spin orbit states of the primary bromine atoms, namely, Br (2P3/2) and Br*(2P1/2). State specific polarization dependent TOF profiles were obtained, from which the translational energy distributions and recoil anisotropy parameters, βi, were extracted using forward convolution method. A strong polarization dependence of TOF profiles suggests anisotropic distributions of the Br and Br* fragments. Two components, namely, the fast and the slow, are observed in the translational energy distribution of Br and Br* atoms, formed from different potential energy surfaces. The average translational energies released into the Br and Br* channels for the fast component are 17.3 ± 2.0 and 11.1 ± 2.0 kcal/mol, respectively. Similarly, for the slow component, the average translational energies imparted into the Br and Br* channels are 2.6 ± 1.0 and 1.5 ± 1.0 kcal/mol, respectively. The relative quantum yields of Br and Br* are 0.87 ± 0.15 and 0.13 ± 0.06. The anisotropy parameters for Br and Br* are characterized by a similar value of 0.60 ± 0.05. The energy partitioning into the translational modes is interpreted with the help of various models. The experimental studies revealing the nature of translational energies distribution for Br and Br* and its average value along with the theoretical calculations employing Time-Dependent Density Functional Theory (TD-DFT) and Multi Configuration Quasi Degenerate second order Perturbation Theory (MCQDPT2) suggest that the initially prepared ππ* state crosses over to a nearby σ* repulsive state along the C Br bond, mainly, πσ* state from where the dissociation takes place. The results also indicate the process of fast internal conversion to ground state from the initially prepared ππ* state, which eventually forms slow Br atoms after C-Br bond dissociation process on the ground state. During the course of experiments, the absorption spectrum was also obtained with its absolute absorption cross section. Non-adiabatic curve crossing plays an important role in the C Br bond dissociation of BTFP.

A DFT study of the dissociation, ionization, and UV/Visible spectra of methyl hypobromite

The energetics and the molecular structures of CH3OBr in the ground electronic, S0, second excited, S2, sextet excited, S6, and cationic ground, D0+ states have been investigated theoretically. The HBr elimination of CH3OBr is found to have two dissociation pathways involving two transition states with an energy difference of 5.28kcal/mol. The B3LYP optimization of the S2 state, which is found to be 1.95eV above the S0 state, shows a tendency of O−Br bond dissociation, resulting in CH3O and Br formation. The optimization of the S6 state, which is found to be 2.35eV above the S0 state, shows a tendency of C−O bond dissociation, forming CH3 and OBr. The ionization energy of the anticipated dissociation products was calculated and found to agree well with the values reported experimentally. For CH3OBr, which has no experimental value for the ionization energy, our ionization energy calculations predicted a value of 9.98eV. Utilizing the excited states calculations and the ionization energy calculations, a design of a pump–probe experiment has been provided in order to explore the ultrafast dissociation dynamics of CH3OBr through S2 and S6 states. The vertical excitation energy calculations were carried out to explore the UV/Visible spectrum of CH3OBr, and the frequency calculations were performed to explore its IR spectrum. The excitation and ionization of CH3OBr result in significant molecular changes including bond lengths, bond angles, dihedral angles, atomic charges, dipole moments, and rotational constants. The excitation to the S2 results in a significant elongation of the O−Br bond, which reflects its tendency for the dissociation through this bond. The excitation to the S6 state results in a remarkable elongation in the C−O bond, reflecting its tendency to dissociate along this bond. The calculations of atomic charges and dipole moment indicate noticeable changes due to excitation and ionization. The electron density contour of HOMO’s indicates the specific site of the HOMO from which the excitation and ionization takes place. The calculations of rotational constants A, B, and C indicated the structural changes through the three mutually perpendicular axes x, y, and z. The effect of ionization to D0+ state on CH3O, CH3, and OBr was investigated following the same thread used for the mother molecule, CH3OBr. CH3O was found to suffer dramatic changes in the molecular structure. It was found to form the CH2OH isomer upon ionization, in which the C−O bond has more double bond character. Moreover, the ionization induces a significant change in the planarity of the CH3O, which changes to the fully planar CH2OH isomer.

Homolytic C–Br bond dissociation energies obtained by means of the G4 thermochemical protocol

Abstract Knowledge of the energies required to induce homolytic cleavage of the C–Br bonds of brominated organic molecules, a process that affords carbon-centered radicals and Br • , is of fundamental importance. Although some data pertaining to the strength of C–Br bonds can already be found in the literature, the chemical diversity of the species for which bond dissociation energies (BDEs) are available is somewhat limited. In this data article, we report a comprehensive set of homolytic C–Br BDEs, obtained using the G4 thermochemical protocol, for brominated organic molecules with wide structural diversity. The species in this set have C–Br BDEs (at 298 K) that differ by as much as 188.3 kJ mol –1 , with α-bromoalanine having the lowest BDE (214.1 kJ mol –1 ) and 1-bromobut-1-yne having the largest (402.4 kJ mol –1 ). Of particular relevance to biological systems are the BDEs of 8-bromoguanine (345.3 kJ mol –1 ), 8-bromoadenine (345.6 kJ mol –1 ), 5-bromocytosine (348.8 kJ mol –1 ) and 5-bromouracil (350.3 kJ mol –1 ).

Dynamics of C⿿Br bond dissociation in methyl 2-bromopropionate at 235 nm: A resonance-enhanced multiphoton ionization study

The dynamics of the C⿿Br bond dissociation on UV excitation of methyl 2-bromopropionate mainly to the 1(nϿ*) state, repulsive in the C⿿Br bond, has been investigated, employing resonance-enhanced multiphoton ionization. Both the ground state and spin-orbits excited bromine atoms were detected, with the former being the major channel. Bromine fragments show bimodal translational energy distributions, with slow and fast (major) bromine atoms arising mainly from the ground and excited electronic states, respectively. The measured recoil anisotropy suggests isotropic angular distributions of bromine atoms. Molecular orbital calculations reveal an important role of avoided curve crossing on C⿿Br bond dissociation dynamics.

Issue information

N‐Brominated compounds are widely used in synthetic and industrial chemistry. In addition, N‐brominated species are implicated in the development of numerous inflammatoryrelated diseases. In this study by Robert J. O'Reilly and Amir Karton (DOI: 10.1002/qua.25024), accurate NBr bond dissociation energies for a diverse set of molecules have been obtained by means of the high‐level W2 composite protocol. This article focuses on the effect of structure on the strength of NBr bonds toward the formation of free radicals. Additionally, to facilitate the study of other N‐brominated species, a large number of more economical theoretical procedures have been assessed for the computation of accurate NBr bond energies.

A dataset of highly accurate homolytic NBr bond dissociation energies obtained by Means of W2 theory

Homolytic NBr bond dissociation constitutes the initial step of numerous reactions involving N‐brominated species. However, little is known about the strength of NBr bonds toward homolytic cleavage. We herein report accurate bond dissociation energies (BDEs) for a set of 18 molecules using the high‐level W2 thermochemical protocol. The BDEs (at 298 K) of the species in this set range from 162.2 kJ mol−1 (N‐bromopyrrole) to 260.6 kJ mol−1 ((CHO)2NBr). In order to compute BDEs of larger systems, for which W2 theory is not applicable, we have benchmarked a wide range of more economical theoretical procedures. Of these, G3‐B3 offers the best performance (root‐mean‐square deviations = 2.9 kJ mol−1), and using this method, we have computed NBr BDEs for four widely used N‐brominated compounds. These include (BDEs are given in parentheses): N‐bromosuccinimide (281.6), N‐bromoglutarimide (263.2), N‐bromophthalimide (274.7), and 1,3‐dibromo‐5,5‐dimethylhydantoin (218.2 and 264.8 kJ mol−1). © 2015 Wiley Periodicals, Inc.

C—Br Bond Dissociation Dynamics of Cyclopentyl Bromide near 234 nm#

C—Br Bond Dissociation Dynamics of Cyclopentyl Bromide near 234 nm^#

The theoretical assessment and prediction of C[sbnd]Br bond dissociation enthalpies

The CBr bond dissociation enthalpies (BDE) of various bromides were assessed and predicted using quantum chemical methods, including composite high-level ab initio and a number of density functional theory (DFT) methods. The systematic evaluation of the DFT CBr BDEs against available experimental ones indicated that the PBE1KCIS method gave the highest accuracy (the root mean square error is 7.5kJ/mol). Then we used this method to examine the substituent effects, as well as the remote substituent effects on CBr BDEs of different kinds of CBr bonds. In addition, the CBr BDEs of typical five- and six-membered heterocyclic bromides at the same level were also investigated.

The nascent OH detection in photodissociation of 2-(bromomethyl)hexafluoro-2-propanol at 193 nm: Laser-induced fluorescence study

Photodissociation of 2-(bromomethyl)hexafluoro-2-propanol (BMHFP) and 3-bromo-1-propanol (BP), involving σ C – Br * ← n Br transition at 193 nm, has been investigated by measuring laser-induced fluorescence spectra of the expected OH product. The OH channel is a minor dissociation pathway with a quantum yield of 0.17 ± 0.05 in BMHFP, whereas it was not observed in BP. Partitioning of the available energy into translation, rotation, and vibration of the photoproducts has been measured by state selective detection of the nascent OH product in BMHFP. OH is produced mostly in the ground vibrational level ( v″ = 0), with a rotational distribution being characterized by a temperature of 465 ± 25 K. But, a significant fraction of the available energy of 30.2 kcal mol −1 is partitioned into translation of OH (14.6 kcal mol −1). The OH( v″ = 0, J″) populations in the spin-orbit states as well as in the Λ-doublet states are statistical. A plausible mechanism of OH formation on excitation of BMHFP at 193 nm is suggested, with the primary reaction channel being elimination of Br atom by direct C–Br bond dissociation from a repulsive surface. The Br radical is detected using (2 + 1) resonance-enhanced multiphoton ionization (REMPI) at ∼234 nm. It is produced in both the ground ( 2P 3/2) and the excited ( 2P 1/2) spin-orbit states with the relative quantum yield of the latter to be 0.36. The co-fragment of Br undergoes secondary C–O bond dissociation to produce OH and F 3C–C( CH 2)–CF 3, with the reaction having a barrier located in the exit channel. In this two-step three-body dissociation process, a major fraction of the available energy is released into translation (〈 f T〉 ∼ 0.75), resulting from an impulsive C–Br bond dissociation in the primary step and presence of an exit barrier in the secondary process. Experimental results combined with theoretical calculations provide a clear picture of the dynamics of OH formation from BMHFP at 193 nm. In addition, the energetics of another channel, competing with OH, have been calculated from the primary product F 3C–C(CH 2)(OH)–CF 3. In contrast to BMHFP, the OH product could not be observed from the photolysis of 3-bromo-1-propanol (another saturated halogenated propanol) at 193 nm under the detection limit of the present experimental condition, although it has a higher absorption cross-section at 193 nm.

Photoionization of two substituted methyl radicals: Cyanomethyl and bromomethyl

The photoionization of two substituted methyl radicals is presented. Cyanomethyl (CH 2CN) and bromomethyl (CH 2Br) were generated by flash pyrolysis from chloroacetonitrile and bromonitromethane. Mass-selective threshold photoelectron spectra of both radicals were recorded. The adiabatic ionization energies of CH 2CN and CH 2Br have been determined to be 10.28 eV ± 20 meV and 8.62 eV ± 10 meV. CH 2Br shows a vibrational progression upon ionization that is assigned to the C–Br stretch mode. The photoionization of the precursors as well as the dissociative photoionization of CH 2Br were examined. A 0 K heat of formation of 174.5 kJ mol −1 is derived for CH 2Br and a C–Br bond dissociation energy of 334.1 kJ mol −1.

Photochemistry of methyl bromide on the α-Cr 2O 3(0001) surface

The photochemical properties of the Cr-terminated α-Cr 2O 3(0001) surface were explored using methyl bromide (CH 3Br) as a probe molecule. CH 3Br adsorbed and desorbed molecularly from the Cr-terminated α-Cr 2O 3(0001) surface without detectable thermal decomposition. Temperature programmed desorption (TPD) revealed a CH 3Br desorption state at 240 K for coverages up to 0.5 ML, followed by more weakly bound molecules desorbing at 175 K for coverages up to 1 ML. Multilayer exposures led to desorption at ~ 130 K. The CH 3Br sticking coefficient was unity at 105 K for coverages up to monolayer saturation, but decreased as the multilayer formed. In contrast, pre-oxidation of the surface (using an oxygen plasma source) led to capping of surface Cr 3+ sites and near complete removal of CH 3Br TPD states above 150 K. The photochemistry of chemisorbed CH 3Br was explored on the Cr-terminated surface using post-irradiation TPD and photon stimulated desorption (PSD). Irradiation of adsorbed CH 3Br with broad band light from a Hg arc lamp resulted in both photodesorption and photodecomposition of the parent molecule at a combined cross section of ~ 10 − 22 cm 2. Photodissociation of the CH 3–Br bond was evidenced by both CH 3 detected in PSD and Br atoms left on the surface. Use of a 385 nm cut-off filter effectively shut down the photodissociation pathway but not the parent molecule photodesorption process. From these observations it is inferred that d-to-d transitions in α-Cr 2O 3, occurring at photon energies < 3 eV, do not significantly promote photodecomposition of adsorbed CH 3Br. It is unclear to what extent band-to-band versus direct CH 3Br photolysis play in CH 3–Br bond dissociation initiated by more energetic photons.

Quantum mechanical studies of the protonation and N [sbnd]Br bond dissociation of the biologically important N-bromosuccinimide

N-Bromosuccinimide (NBS) is a brominating and oxidizing agent that is used as a source of bromine. The proton affinities, the tautomeric forms and N Br bond dissociation of NBS have been computed using the B3LYP functional as implemented in the density functional approach. The electronic structures of all possible tautomeric forms of NBS have been thoroughly investigated. The keto form of NBS has been shown to be more stable than any other tautomeric forms. The geometries and relative energies for various stationary structures were determined. The results indicate clearly that O-site protonation is strongly favored over N-site protonation for the studied compound in case of mono- and di-protonation. The bond dissociation energies (BDEs), involving the formation of the bromine radical, cation, and anion, of the N Br bond have been investigated. The N Br BDE of the Br radical formation is lower than that of the Br anion or cation. These conclusions are in good agreement with the experimental results.

The dynamics of Br(2Pj) formation in the photodissociation of vinyl and perfluorovinyl bromides

The photodissociation dynamics of vinyl bromide and perfluorovinyl bromide have been investigated at 234 nm using a photofragment ion imaging technique coupled with a state-selective [2+1] resonance-enhanced multiphoton ionization scheme. The nascent Br atoms stem from the primary C-Br bond dissociation leading to the formation of C2H3(X) and Br(2Pj;j=1/2,3/2). The obtained translational energy distributions have been well fitted by a single Boltzmann and three Gaussian functions. Boltzmann component has not been observed in the perfluorovinyl bromide. The repulsive 3A'(n,sigma *) state has been considered as the origin of the highest Gaussian components. Middle translational energy components with Gaussian shapes are produced from the 1A"(pi,sigma*) and/or 3A"(pi,sigma*) which are very close in energy. Low-energy Gaussian components are produced via predissociation from the 3A'(pi,pi*) state. The assignments have also been supported by the recoil anisotropy corresponding to the individual components. It is suggested that intersystem crossing from the triplet states to the ground state has been attributed to the Boltzmann component and the fluorination reduces the probability of this electronic relaxation process.

A new-type photoreaction of a carbonyl compound: Part 2. Photoinduced ω-bond dissociation in p-halomethylbenzophenone studied by time-resolved EPR technique and laser flash photolysis

Photodissociation of the carbon-X (X = Br and Cl) bonds in p-bromo- and p-chloromethylbenzophenone (BMBP and CMBP) in solution were investigated by time-resolved EPR and laser flash photolysis techniques. BMBP and CMBP were found to undergo ω-bond cleavage to yield the p-benzoylbenzyl radical (BBR) at 295 K, and the quantum yields ( Φ BBR) were determined. The CIDEP signal originated from BBR formed upon decomposition of CMBP was obtained while that for BMBP was absent. By using triplet sensitization of acetone, the efficiencies ( α BBR) of the C X bond fission in the triplet states of BMBP and CMBP were determined. The agreement between the Φ BBR and α BBR values for CMBP indicates that the C Cl bond dissociation occurs only in the triplet state. In contrast to CMBP, the cleavage of the C Br bond in BMBP upon direct excitation was concluded to be the event only in the excited singlet state without triplet formation, whereas the triplet state was also reactive for ω-bond dissociation. The rate of C Br bond dissociation seemed to be greater than that of intersystem crossing from the S 1 to the T 1 state. Schematic energy diagrams of the excited states of BMBP and CMBP were shown, and the reaction profiles were discussed from the viewpoint of the C X bond enthalpies.

Bond Dissociation Energies of CF3-X Bonds (X = C, O, N, S, Br): Ab Initio Molecular Orbital Calculation and Application to Evaluation of Fire Suppression Ability

The C−X (X = C, O, N, S, Br) bond dissociation enthalpies (BDEs) of perfluoroalkanes, perfluoroethers, perfluorotrimethylamine, perfluorodimethyl sulfide, and trifluoromethyl bromide were evaluated by the G2, G2MP2, and G2MS methods. Perfluoroethers had high BDEs of the C−O bond because of the strong negative hyperconjugation between the CF3 group and lone pair orbitals of oxygen. The BDEs of CF3−X bonds could be used as an index of the fire suppression ability.

Rearrangement as a probe for radical formation: bromomethylcyclopropane on oxygen-covered Mo(1 1 0)

The reactions of bromomethylcyclopropane on oxygen-covered Mo(1 1 0) were studied in order to investigate the lifetimes of radical intermediates, which are important in heterogeneous oxidation catalysis. The methylcyclopropyl radical is known to rearrange on the nanosecond time scale, providing us with a means of probing for radical formation. Surprisingly, no rearrangement occurs subsequent to C–Br bond dissociation, which commences at ∼220 K. Instead, displacement of bromine by oxygen occurs to yield adsorbed methylcyclopropoxide, which is identified using infrared spectroscopy. The C–O bond of methylcyclopropoxide is cleaved at ∼400 K to yield a transient methylcyclopropyl radical. As shown previously, the methylcyclopropyl radical rearranges and the ring-opened butenyl species is trapped on the surface. Addition to oxygen yields 3-buten-1-oxy and addition to the metal affords the butenyl–Mo moiety. Infrared spectroscopy is used to identify these intermediates. The same linear species are formed from the reaction of 4-bromo-1-butene. The 3-buten-1-oxy species is also formed from reactions of 3-buten-1-ol on O-covered Mo(1 1 0). Upon further heating, the 3-buten-1-oxy reacts to form 1,3-butadiene, 1-butene, water, and dihydrogen between 450 and 600 K. Ethene is also evolved at ∼560 K. The primary mechanism for ethene evolution is elimination from metal-bound butenyl. Carbon monoxide is also formed above 900 K, due to reaction of surface carbon and oxygen. The implications of our results for studies where alkyl halides are used as models for radical reactions on surfaces are discussed.

Self-Limiting Heterogeneous Reactions: Bifunctional Hydrocarbon on a Bimetallic Alloy Surface

Here we report the remarkable chemistry exhibited by 6-bromo-1-hexene on a Cu3Pt(111) surface. The dehydrocyclization reaction leading to the formation of benzene takes place at very low coverage; as the surface fills up, decomposition becomes a predominant pathway. Further increase of the surface concentration of 6-bromo-1-hexene leads to the hydrogenation of 5-hexenyl formed on a surface as a result of C−Br bond dissociation. This reaction produces 1-hexene, which is stable on the surface until the molecular desorption of this compound commences at 230 K. Molecular desorption of 6-bromo-1-hexene from the monolayer becomes significant at even higher exposures. All these processes occur very cleanly within a specific coverage range, making the 6-bromo-1-hexene on the Cu3Pt(111) surface extremely attractive for analysis of kinetics of coverage-dependent reactions. Steric requirements for each of the processes described here and the availability of the adsorption sites on the alloy surface are believed to govern the predominant reaction.

Quantum yields and energy partitioning in the ultraviolet photodissociation of 1,2 dibromo-tetrafluoroethane (Halon-2402)

The photodissociation of 1,2 dibromo-tetrafluoroethane (Halon-2402) has been investigated at 193 nm using photofragment translational spectroscopy with vacuum ultraviolet ionization and at 193, 233, and 266 nm using state-selected translational spectroscopy with resonance-enhanced multiphoton ionization. The product branching ratios, angular distributions, and translational energy distributions were measured at these wavelengths, providing insight into the ultraviolet photodissociation dynamics of CF2BrCF2Br. The total bromine atom quantum yields were found to be 1.9±0.1 at both 193 and 233 nm and 1.4±0.1 at 266 nm. The first C–Br bond dissociation energy was determined to be 69.3 kcal/mol from ab initio calculations. The second C–Br bond dissociation energy was determined to be 16±2 kcal/mol by modeling of the bromine quantum yield. In addition, variational Rice–Ramsperger–Kassel–Marcus theory was used to calculate the secondary dissociation rates for a range of dissociation energies above threshold. These results suggest that CF2CF2Br photofragments with sufficient internal energies will undergo secondary dissociation prior to collisional stabilization under atmospheric conditions. Based on the measured translational energy distributions and product branching ratios, a model is proposed to describe the wavelength-dependent bromine quantum yield and the implications of these results to atmospheric chemistry are discussed.