Br Bond Fission Research Articles

Probing ultrafast C-Br bond fission in the UV photochemistry of bromoform with core-to-valence transient absorption spectroscopy.

UV pump–extreme UV (XUV) probe femtosecond transient absorption spectroscopy is used to study the 268 nm induced photodissociation dynamics of bromoform (CHBr3). Core-to-valence transitions at the Br(3d) absorption edge (∼70 eV) provide an atomic scale perspective of the reaction, sensitive to changes in the local valence electronic structure, with ultrafast time resolution. The XUV spectra track how the singly occupied molecular orbitals of transient electronic states develop throughout the C–Br bond fission, eventually forming radical Br and CHBr2 products. Complementary ab initio calculations of XUV spectral fingerprints are performed for transient atomic arrangements obtained from sampling excited-state molecular dynamics simulations. C–Br fission along an approximately symmetrical reaction pathway leads to a continuous change of electronic orbital characters and atomic arrangements. Two timescales dominate changes in the transient absorption spectra, reflecting the different characteristic motions of the light C and H atoms and the heavy Br atoms. Within the first 40 fs, distortion from symmetry to form a quasiplanar CHBr2 by the displacement of the (light) CH moiety causes significant changes to the valence electronic structure. Displacement of the (heavy) Br atoms is delayed and requires up to ∼300 fs to form separate Br + CHBr2 products. We demonstrate that transitions between the valence-excited (initial) and valence + core-excited (final) state electronic configurations produced by XUV absorption are sensitive to the localization of valence orbitals during bond fission. The change in valence electron-core hole interaction provides a physical explanation for spectral shifts during the process of bond cleavage.

Competitive Bond Rupture in the Photodissociation of Bromoacetyl Chloride and 2- and 3-Bromopropionyl Chloride: Adiabatic versus Diabatic Dissociation

Competitive bond dissociation mechanisms for bromoacetyl chloride and 2- and 3-bromopropionyl chloride following the (1) [n(O)→π*(C=O)] transition at 234-235 nm are investigated. Branching ratios for C−Br/C−Cl bond fission are found by using the (2+1) resonance-enhanced multiphoton ionization (REMPI) technique coupled with velocity ion imaging. The fragment branching ratios depend mainly on the dissociation pathways and the distances between the orbitals of Br and the C=O chromophore. C−Cl bond fission is anticipated to follow an adiabatic potential surface for a strong diabatic coupling between the n(O)π*(C=O) and np (Cl)σ*(C−Cl) bands. In contrast, C−Br bond fission is subject to much weaker coupling between n(O)π*(C=O) and np (Br)σ*(C−Br). Thus, a diabatic pathway is preferred for bromoacetyl chloride and 2-bromopropionyl chloride, which leads to excited-state products. For 3-bromopropionyl chloride, the available energy is not high enough to reach the excited-state products such that C−Br bond fission must proceed through an adiabatic pathway with severe suppression by nonadiabatic coupling. The fragment translational energies and anisotropy parameters for the three molecules are also analyzed and appropriately interpreted.

Photoproduct Channels from BrCD2CD2OH at 193 nm and the HDO + Vinyl Products from the CD2CD2OH Radical Intermediate

We present the results of our product branching studies of the OH + C(2)D(4) reaction, beginning at the CD(2)CD(2)OH radical intermediate of the reaction, which is generated by the photodissociation of the precursor molecule BrCD(2)CD(2)OH at 193 nm. Using a crossed laser-molecular beam scattering apparatus with tunable photoionization detection, and a velocity map imaging apparatus with VUV photoionization, we detect the products of the major primary photodissociation channel (Br and CD(2)CD(2)OH), and of the secondary dissociation of vibrationally excited CD(2)CD(2)OH radicals (OH, C(2)D(4)/CD(2)O, C(2)D(3), CD(2)H, and CD(2)CDOH). We also characterize two additional photodissociation channels, which generate HBr + CD(2)CD(2)O and DBr + CD(2)CDOH, and measure the branching ratio between the C-Br bond fission, HBr elimination, and DBr elimination primary photodissociation channels as 0.99:0.0064:0.0046. The velocity distribution of the signal at m/e = 30 upon 10.5 eV photoionization allows us to identify the signal from the vinyl (C(2)D(3)) product, assigned to a frustrated dissociation toward OH + ethene followed by D-atom abstraction. The relative amount of vinyl and Br atom signal shows the quantum yield of this HDO + C(2)D(3) product channel is reduced by a factor of 0.77 ± 0.33 from that measured for the undeuterated system. However, because the vibrational energy distribution of the deuterated radicals is lower than that of the undeuterated radicals, the observed reduction in the water + vinyl product quantum yield likely reflects the smaller fraction of radicals that dissociate in the deuterated system, not the effect of quantum tunneling. We compare these results to predictions from statistical transition state theory and prior classical trajectory calculations on the OH + ethene potential energy surface that evidenced a roaming channel to produce water + vinyl products and consider how the branching to the water + vinyl channel might be sensitive to the angular momentum of the β-hydroxyethyl radicals.

Time-sliced Velocity Map Imaging Study on Photodissociation of Neopentyl Bromide andTert-pentyl Bromide at 234 nm

We present a first velocity map imaging study on the 234 nm photodissociation dynamics of two carbon-chain branched alkyl bromides, neopentyl bromide (denoted as NPB) and tert-pentyl bromide (denoted as TPB). Unlike the 234 nm photodissociation of the unbranched n-C5H11Br molecule where only a direct fission of the C—Br bond is involved, the branched NPB and TPB molecules exhibit one and two more independent dissociation pathways with much energy being decayed via an extensive excitation of the bending modes of the parent molecules prior to the C—Br bond fission. This observation strongly suggests that the dissociation coordinate for the two carbon-chain branched molecules is no longer solely ascribed to the C—Br stretching mode but rather a combination of the bending-stretching modes.

C–Br bond fission mechanism of 1-bromo-3-chloropropane at 234 and 265 nm

The photodissociation of 1-bromo-3-chloropropane was studied at 234 and 265nm using ion velocity imaging technique. Bromine fragments in this study were produced via direct dissociation of C–Br bond. The speed and angular distributions of Br∗ and Br were measured. The appearance of chlorine fragments at 234nm was assigned to a secondary dissociation process. The contributions of three excited states: 1Q1, 3Q0 and 3Q1 were estimated at each wavelength. Comparing with previous results, the study reveals more details about the dynamics of molecules with multichromophores.

Near-UV photolysis of substituted phenols, I: 4-fluoro-, 4-chloro- and 4-bromophenol

The experimental techniques of H (Rydberg) atom photofragment translational spectroscopy and resonance-enhanced multiphoton ionisation time-of-flight spectroscopy have been used to investigate the dynamics of H atom loss processes from gas phase 4-fluorophenol (4-FPhOH), 4-chlorophenol (4-ClPhOH) and 4-bromophenol (4-BrPhOH) molecules, following excitation at many wavelengths, lambda(phot), in the range between their respective S(1)-S(0) origins (284.768 nm, 287.265 nm and 287.409 nm) and 216 nm. Many of the Total Kinetic Energy Release (TKER) spectra obtained from photolysis of 4-FPhOH show structure, the analysis of which reveals striking parallels with that reported previously for photolysis of bare phenol (M. G. D. Nix, A. L. Devine, B. Cronin, R. N. Dixon and M. N. R. Ashfold, J. Chem. Phys., 2006, 125, 133318). The data demonstrates the importance of O-H bond fission, and that the resulting 4-FPhO co-fragments are formed in a select fraction of their available vibrational state density. All spectra recorded at lambda(phot)> or = 238 nm show a feature centred at TKER approximately 5500 cm(-1). These H atom fragments show no recoil anisotropy, and are rationalised in terms of initial S(1)<-- S(0) (pi* <--pi) excitation and subsequent dissociation via two successive radiationless transitions: internal conversion to ground (S(0)) state levels carrying sufficient O-H stretch vibrational energy to allow efficient transfer to (and round) the Conical Intersection (CI) between the S(0) and S(2)((1)pi sigma*) Potential Energy Surfaces (PESs) at larger R(O-H), en route to H atoms and ground state 4-FPhO products. The vibrational energy disposal in the 4-FPhO products indicates that parent mode nu(16a) promotes non-adiabatic coupling at the S(0)/S(2) CI. Spectra recorded at lambda(phot)< or = 238 nm reveal a faster (but still isotropic) distribution of recoiling H atoms, centred at TKER approximately 12 000 cm(-1), attributable to H + 4-FPhO products formed when the optically excited (1)pi pi* molecules couple directly with the (1)pi sigma* PES. Parent mode nu(16b) is identified as the dominant coupling mode at the S(1)((1)pi pi*)/S(2)((1)pi sigma*) CI, and the resulting 4-FPhO radical co-fragments display progressions in nu(18b) (the C-O in-plane wagging mode) and nu(7a) (an in-plane ring breathing mode involving significant C-O stretching motion). Analysis of all structured TKER spectra yields a C-F bond dissociation energy: D(0)(H-OC(6)H(4)F) = 29 370 +/- 50 cm(-1). The photodissociation of 4-ClPhOH shows many similarities, though the 4-ClPhO products formed together with faster H atoms at shorter wavelengths (lambda(phot)< or = 238 nm, by coupling through the S(1)/S(2) CI) show activity in an alternative ring breathing mode (nu(19a) rather than nu(7a)). Spectral analysis yields D(0)(H-OC(6)H(4)Cl) = 29 520 +/- 50 cm(-1). H atom formation via O-H bond fission is (at best) a very minor channel in the photolysis of 4-BrPhOH at all wavelengths investigated. Time-dependent density functional theory calculations suggest that this low H atom yield is because of competition from the alternative C-Br bond fission channel, and that the analogous C-Cl bond fission may be responsible for the weakness of the one photon-induced H atom signals observed when photolysing 4-ClPhOH at longer wavelengths.

C–Br bond fission dynamics in ultraviolet photodissociation of 1,2-dibromoethane

The photodissociation of 1,2-dibromoethane was studied at 234 and 267 nm using velocity map imaging technique. The speed distributions for the Br( 2P 3/2) and Br ∗( 2P 1/2) were determined and mostly fitted by two Gaussian curves indicating different product channels. The direct C–Br bond fission dominates the process and the recoil anisotropies suggest a transition mainly parallel to the C–Br bond. Vibrational structures are shown with ∼550 cm −1 spacing originating from C–Br stretching excitation of C 2H 4Br photofragment. The three-body dissociation channel is also found. The total product quantum yields were Φ 234nm(Br ∗) = 0.14 and Φ 267 nm (Br ∗) = 0.17, respectively.

Photodissociation of CH2Br2, 1,1- and 1,2-C2H4Br2 at 248 nm: A simple C–Br bond fission versus a concerted three-body formation

The photodissociation of CH2Br2 (DBM), 1,1- and 1,2-C2H4Br2 (DBE) at 248 nm was investigated using product translational spectroscopy. The results show that DBM and 1,1-DBE undergo a simple C–Br bond fission with fragments recoiling anisotropically. When the laser energy increased from 10 to 40 mJ/pulse, the nascent products, CH2Br and CH3CHBr, absorbed an additional photon to produce the secondary Br atom. In contrast, 1,2-DBE dissociates into the triple products Br (fast) + Br (slow) + C2H4 in a concerted reaction. This is confirmed from the measured anisotropy of the products, which indicates that the reaction is completed in a fraction of rotational period. Following an asynchronous concerted reaction, the simulation for the triple products was derived with the P(Et) distributions coupled by asymmetric angular distributions. The results are discussed in terms of the weakness of the C–Br bond strength in the β-bromoethyl radical that a rapid scission of the second C–Br bond occurred asynchronously with the cleavage of the first C–Br bond along the nσ* potential energy surface.

Dissociation Channels of the 1-Propenyl Radical and Its Photolytic Precursor cis-1-Bromopropene

The primary photodissociation dynamics of cis-1-bromopropene upon excitation at 193 nm and the unimolecular dissociation dynamics of the nascent 1-propenyl radical are investigated in a crossed laser−molecular beam apparatus. The lowest-energy dissociation barrier of the 1-propenyl radical is experimentally determined for the first time and is found to be 31.5 ± 2.0 kcal/mol, in substantial agreement with the theoretical calculations of Davis et al. There are three dissociation channels of the 1-propenyl radical seen in these experiments: C−C bond fission to give C2H2 + CH3, C−H bond fission to give propyne + H, and isomerization to the allyl radical followed by H atom loss to give allene + H. The data show that the C−C fission channel dominates the product branching for the dissociative radicals with the lowest internal energies. The branching to the H + allene channel at higher energies suggests that the calculated isomerization barrier is too high with respect to the H + propyne barrier. The data for the precursor molecule, cis-1-bromopropene, indicate that there are at least three competing primary product channels, HBr elimination and two C−Br bond fission pathways leading to 1-propenyl + Br formation. In addition, there is a product channel described by the total reaction C3H5Br → Br + H2 + C3H3. We also show evidence that the Br atom is formed in the excited (2P1/2) spin−orbit state from the C−Br bond fission product channels.

Nonadiabatic transition-state theory: A Monte Carlo study of competing bond fission processes in bromoacetyl chloride

Nonadiabatic Monte Carlo transition-state theory is used to explore competing C–Cl and C–Br bond fission processes in a simple model of [n,π1*(CO)] photoexcited bromoacetyl chloride. Morse potentials are used to represent bond stretching coordinates, and the positions and magnitudes of nonadiabatic coupling between excited state potentials are modeled using ab initio data. The main effect of nonadiabaticity is to favor C–Cl fission over C–Br, despite a larger barrier to C–Cl dissociation. The absolute values of the rate constants are smaller than observed experimentally, but the calculated branching ratios are close to the experimental value. For C–Cl fission, it is shown that the minimum energy crossing point is not sufficient to describe the rate constant, suggesting that care must be taken when using alternative models which make this assumption.

Photodissociation dynamics of CH 2BrCl at 234 nm

The photodissociation dynamics of CH 2BrCl has been studied at 234 nm utilizing the molecular beam-imaging system. In the photolysis, the C–Br bond fission is observed exclusively. The internal energy of the Br( 2 P 3/2 ) channel consists of two parts which is caused by the interaction between σ *(C–Br) and σ *(C–Cl) surfaces. This separation is newly resolved and studied to unravel the effect of another chromophore. The energy separation of two peaks is 25 kJ mol −1 and the ratio of fast to slow one is 3.2. The fast part is interpreted in terms of a rigid impulsive model with no vibrational excitation and the slow part by a soft one with a little vibrational excitation, respectively. The energy gap is suggested to be due to an umbrella motion.

Photodissociation of CH≡CCH2X (X=Br and Cl) by translational spectroscopy

The photodissociation of CH≡CCH2X (X=Br and Cl) at 248 and 193 nm was studied using an angle-resolved beam apparatus. Both the translational energy distribution P(Et) and the anisotropy parameters β of the photoproducts were derived from the time-of-flight spectra. Excitation at 248 nm, CH≡CCH2Br underwent exclusively a direct C–Br bond fission with anisotropic recoiling fragments. By increasing the photon energy to 193 nm, three competing primary dissociation channels were observed: (1) Br+C3H3 (fast, 26%), (2) Br+C3H3 (slow, 44%), and (3) HBr+C3H2 (30%), with average translational energies of 34, 12, and 9 kcal/mol, respectively. From the measured β values, all reactions were shown to proceed extremely fast within a fraction of rotational period. On the other hand, excitation at 193 nm, CH≡CCH2Cl underwent a predominant C–Cl bond fission with a minor channel producing the HCl+C3H2 fragments. The results are discussed in terms of either a direct or an indirect dissociation process arising from the transition to the (π,π*) state.

Dissociation of acetyl bromide: an experimental and theoretical study

We examine the ground state dissociation pathways of acetyl bromide, CH3C(O)Br, using infrared multiphoton dissociation and ab initio molecular orbital theory. Experiments reveal stable products C2H6, CH3Br, and CO which are consistent with the CBr bond fission pathway. However, production of C2H4 is consistent with ketene (CH2CO) formation, which has been a suggested intermediate in the dissociation of CH3C(O)Br. Calculations indicate that the activation energy for the molecular dissociation of CH3C(O)Br into ketene and HBr exceeds the CBr bond energy of 64 kcal mol−1 by about 30 kcal mol−1, implying that ketene may be formed by secondary chemistry rather than by molecular dissociation of acetyl bromide.

Cyclometallation of azine phosphines of type Z,E-PPh2CH2C(But)N–NCHR (R = an aromatic or heterocyclic group) involving X–Y (X = C, N or O; Y = H, I or Br) bond fission by platinum(II)

The phosphino hydrazone Z-PPh2CH2C(But)NNH21 is shown to be a convenient ‘reagent’ for converting aryl or heterocyclic aldehydes into azines which can then be cycloplatinated readily with C–H, C–I, C–Br, N–H or O–H bond fission. Condensation of 1 with benzaldehyde derivatives (RCHO) gave mixed azines of type Z,E-PPh2CH2C(But)N–NCHR [R = Ph 2a, C6H2(OMe)3-3,4,5 2b, C6H4NO2-4 2c, C6H4I-2 2d, C6H4Br-2 2e, C6H4Cl-2 2f or C6H2(OH-2)(OMe)2-4,6 2g]. The azines 2a–2f reacted with [PtMe2(cod)](cod = cycloocta-1,5-diene) at 20 °C to give dimethylplatinum(II) complexes [[graphic omitted]CHR}]3a–3f respectively, containing six-membered chelate rings. Treatment of 2g with [PtMe2(cod)] effected O–H bond fission to give the O-cyclometallated methylplatinum(II) complex [[graphic omitted](OMe)2-4,6]}]4. When the dimethylplatinum(II) complex 3a was heated in toluene at 100 °C, it underwent C–H bond fission with elimination of CH4 to give the cyclometallated methylplatinum(II) complex [[graphic omitted]C6H4)}]5a. Similarly, 3b and 3c were converted into the corresponding cyclometallated methylplatinum(II) complexes 5b and 5c, respectively. The square-planar dimethylplatinum(II) complex 3d in benzene at 20 °C underwent oxidative addition of the C–I bond to give the cyclometallated iododimethylplatinum(IV) complex mer,cis-[[graphic omitted]H4)}]6a. When the bromo-mixed azine complex 3e was heated in benzene at 75 °C for 16 h the cyclometallated bromodimethylplatinum(IV) complex mer,cis-[[graphic omitted]H4)}]6b formed. In contrast the chloro-mixed azine complex 3f underwent the C–H bond fission and not C–Cl bond fission to give the cyclometallated square-planar methylplatinum(II) complex [[graphic omitted]H3Cl-6)}]5d. The methylplatinum(II) complexes 5a and 5d oxidatively added Mel to give the mer,cis-iododimethylplatinum(IV) complexes 6a and 6c, respectely. Treatment of 1 with 2-carbaldehydes of pyrrole, thiophene or furan gave the corresponding phosphines 7a–7c, which with [PtMe2(cod)] at 20 °C gave the dimethylplatinum(II) complexes 8a–8c, respectively. When 8a, derived from the pyrrole azine phosphine, was heated at 50 °C for 10 min, the N-cyclometallated methylplatinum(II) complex [[graphic omitted])}]9 and methane were formed via a N–H bond fission. When the dimethylplatinum(II) complexes 8b or 8c, derived from the thiophene or furan azine phosphines, were heated at 100 °C in toluene they underwent C–H bond fission to give the cyclometallated methylplatinum(II) complexes [[graphic omitted]H2X)}](X = S 10a or O 10b), which oxidatively added Mel to give the iododimethylplatinum(IV) complexes mer,cis-[[graphic omitted]H2X)}](X = S 11a or O 11b), respectively. Proton, 31P-{1H} and some 13C-{1H} NMR data have been obtained.

Competition between unimolecular C–Br–bond fission and Br2 elimination in vibrationally highly excited CF2Br2

The competition between C–Br–bond fission and three-center elimination of molecular bromine (Br2) in highly excited CF2Br2 molecules has been studied under collision-free conditions. Transient resonantly enhanced multiphoton ionization (REMPI) was used to monitor Br(2P1/2) and Br(2P3/2) formation during and after infrared (IR) multiphoton excitation of CF2Br2; time-resolved laser-induced fluorescence (LIF) spectroscopy was employed for the detection of transient CF2 after Br2 elimination. Direct time-resolved measurements of the sum of afterpulse reaction rates, absolute product yields for the CF2 and Br(2P3/2) channels as well as absorbed energies per excitation pulse were used to characterize parts of the vibrational energy distribution P(E) established after IR multiphoton excitation and to determine rate coefficients and branching ratios for the elimination and dissociation reaction as a function of the average internal energy 〈E〉. The existence of both channels, the dissociation and the elimination channel, has been confirmed. A comparison of the experimental data with statistical adiabatic channel model calculations (SACM) enabled us to determine the threshold energies E0(J=0) for the unimolecular Br2 elimination [E0(J=0)=19 070±500 cm−1] and the C–Br bond fission [E0(J=0)=20 700±500 cm−1], the two possible pathways of the reaction.

Fluence and pressure effects on IR multiphoton dissociation of ethylbromide

The IR multiphoton dissociation of C2H5Br was studied as a function of the laser fluence and substrate pressure using a pulsed TEA CO2 laser at the 10P(20) (10.59μm,944 cm−1) laser line. The effects of added H2, He, N2 and NO were also studied. The reaction products were found to be C2H2, C2H4, C2H6 and CH4. The increase in the partial pressure of the products with increasing sustrate pressure was interpreted in terms of the large contribution from a collision-induced between the reactant molecules excited below the reaction threshold. At a focal fluence of less than 100 J cm−2, C2H4 was the only reaction product detected, whereas C2H2 and CH4 also appeared at higher fluence. The formation of C2H2 and CH4 at higher fluence was attributed to additional laser photon absorption by vibrationally excited ethylene (C2H4*) primarily in the quasi-continuum state. The production of C2H6 was due to CBr bond fission. The observed dissociation data were compared with the theoretical predictions obtained using the energy-grained master equation.

Competing C–Br and C–C bond fission following 1[n(O),π*(C=O)] excitation in bromoacetone: Conformation dependence of nonadiabaticity at a conical intersection

These experiments investigate the competition between C–C and C–Br bond fission in bromoacetone excited in the 1[n(O),π*(C=O)] absorption, elucidating the role of molecular conformation in influencing the probability of adiabatically traversing the conical intersection along the C–C fission reaction coordinate. In the first part of the paper, measurement of the photofragment velocity and angular distributions with a crossed laser-molecular beam time-of-flight technique identifies the primary photofragmentation channels at 308 nm. The time-of-flight spectra evidence two dissociation channels, C–Br fission and fission of one of the two C–C bonds, BrH2C–COCH3. The distribution of relative kinetic energies imparted to the C–Br fission and C–C fission fragments show dissociation is not occurring via internal conversion to the ground electronic state and allow us to identify these channels in the closely related systems of bromoacetyl- and bromopropionyl chloride. In the second part of the work we focus on the marked conformation dependence to the branching between C–C fission and C–Br fission. Photofragment angular distribution measurements show that C–Br fission occurs primarily from the minor, anti, conformer, giving a β of 0.8, so C–C fission must dominate the competition in the gauche conformer. Noting that the dynamics of these two bond fission pathways are expected to be strongly influenced by nonadiabatic recrossing of the reaction barriers, we investigate the possible mechanisms for the conformation dependence of the nonadiabatic recrossing with low-level ab initio electronic structure calculations on the C–Br reaction coordinate and qualitative consideration of the conical intersection along the C–C reaction coordinate. The resulting model proposes that C–C bond fission cannot compete with C–Br fission in the anti conformer because the dissociation samples regions of the phase space near the conical intersection along the C–C fission reaction coordinate, where nonadiabaticity inhibits C–C fission, while from the gauche conformer C–C fission can proceed more adiabatically and dominate C–Br fission. A final experiment confirms that the branching ratio changes with the relative conformer populations in accord with this model.

Distance dependence of nonadiabaticity in the branching between C–Br and C–Cl bond fission following 1[n(O),π*(C=O)] excitation in bromopropionyl chloride

These experiments on bromopropionyl chloride investigate a system in which the barrier to C–Br fission on the lowest 1A″ potential energy surface is formed from a weakly avoided electronic configuration crossing, so that nonadiabatic recrossing of the barrier to C–Br fission dramatically reduces the branching to C–Br fission. The results, when compared with earlier branching ratio measurements on bromoacetyl chloride, show that the additional intervening CH2 spacer in bromopropionyl chloride reduces the splitting between the adiabatic potential energy surfaces at the barrier to C–Br fission, further suppressing C–Br fission by over an order of magnitude. The experiment measures the photofragment velocity and angular distributions from the 248 nm photodissociation of Br(CH2)2COCl, determining the branching ratio between the competing primary C–Br and C–Cl fission pathways and detecting a minor C–C bond fission pathway. While the primary C–Cl:C–Br fission branching ratio is 1:2, the distribution of relative kinetic energies imparted to the C–Br fission fragments show that essentially no C–Br fission results from promoting the molecule to the lowest 1A″ potential energy surface via the 1[n(O),π*(C=O)] transition; C–Br fission only results from an overlapping electronic transition. The results differ markedly from the predictions of statistical transition state theories which rely on the Born–Oppenheimer approximation. While such models predict that, given comparable preexponential factors, the reaction pathway with the lowest energetic barrier on the 1A″ surface, C–Br fission, should dominate, the experimental measurements show C–Cl bond fission dominates by a ratio of C–Cl:C–Br=1.0:&amp;lt;0.05 upon excitation of the 1[n(O),π*(C=O)] transition. We compare this result to earlier work on bromoacetyl chloride, which evidences a less dramatic reduction in the C–Br fission pathway (C–Cl:C–Br=1.0:0.4) upon excitation of the same transition. We discuss a model in which increasing the distance between the C–Br and C=O chromophores decreases the electronic configuration interaction matrix elements which mix and split the 1n(O)π*(C=O) and np(Br)σ*(C–Br) configurations at the barrier to C–Br bond fission in bromopropionyl chloride. The smaller splitting between the adiabats at the barrier to C–Br fission increases the probability of nonadiabatic recrossing of the barrier, nearly completely suppressing C–Br bond fission in bromopropionyl chloride. Preliminary ab initio calculations of the adiabatic barrier heights and the electronic configuration interaction matrix elements which split the adiabats at the barrier to C–Br and C–Cl fission in both bromopropionyl chloride and bromoacetyl chloride support the interpretation of the experimental results. We end by identifying a class of reactions, those allowed by overall electronic symmetry but Woodward–Hoffmann forbidden, in which nonadiabatic recrossing of the reaction barrier should markedly reduce the rate constant, both for ground state and excited state surfaces.

Nonadiabaticity and the competition between alpha and beta bond fission upon 1[n,π*(C=O)] excitation in acetyl- and bromoacetyl chloride

This work investigates how molecular dissociation induced by local 1[n(O),π*(C=O)] electronic excitation at a carbonyl functional group can result in preferential fission of an alpha bond over a weaker bond beta to the functional group and how nonadiabaticity in the dynamics drives the selectivity. The experiment measures the photofragment velocity and angular distributions from the photodissociation of acetyl chloride and bromoacetyl chloride at 248 nm, identifying the branching between bond fission channels and the mechanism for the selectivity. The anisotropic angular distributions measured shows dissociation occurs on a time scale of less than a rotational period, resulting in primary C–X (X=Cl, Br) bond fission, but no significant C–C bond fission. While the selective fission of the C–Cl over the C–C alpha bond can be predicted from the adiabatic correlation diagram for this special class of Norrish type I cleavage, the preferential fission of the C–Cl alpha bond over the C–Br bond beta to the carbonyl group would not be predicted on the adiabatic potential energy surface. In bromoacetyl chloride, fission of the C–Cl and C–Br bonds occurs with a branching of 1.0:1.1 (approximately 1.0:0.5 from the 1nπ* transition) compared with a predicted statistical branching ratio of 1:30. This preferential α-bond fission is attributed to a dissociation mechanism on the coupled [n,π*(C=O)] and [n(X),σ *(C–X)] electronic states, a model consistent with the lack of C–C fission and the measured kinetic energy and angular distributions. The selectivity results from the relative strengths of the electronic coupling between the initially excited [n,π*(C=O)] bound configuration and the two [n(X),σ *(C–X)] states, the weaker coupling inhibiting the adiabatic crossing over the barrier to C–Br bond fission. The results demonstrate the need to go beyond the Born–Oppenheimer approximation to gain predictive ability in any reactive system where the electronic configuration changes along the reaction coordinate, particularly at barriers due to configuration crossings. In addition, the Cl product angular distribution determines the orientation of the 1[n(O),π*(C=O)] transition dipole moment and shows it is governed by the C2v symmetry of the localized carbonyl electronic orbitals and not by the asymmetric substitution at the carbonyl group. Spectra of the Br atoms from direct dissociation at 193 nm help separate the contribution from the overlapping nσ *(C–Br) transition at 248 nm.

The electronic state-selective photodissociation of CH2BrI at 248, 210, and 193 nm

The primary photodissociation channels of CH2BrI following excitation at 193.3, 210, and 248.5 nm have been studied with the crossed laser-molecular beam technique. Product translational energy distributions and polarization dependences were derived for the primary dissociation processes observed. The data demonstrate bond selective photochemistry as well as some selective formation of electronically excited photofragments in bond fission and concerted dissociation. Excitation at 248.5 nm, which is assigned to excitation of primarily a n(I)→σ*(C–I) transition with some contribution from an overlapping n(Br)→σ*(C–Br) transition, results in both C–I and C–Br bond fission. C–I bond fission is the dominant channel, producing I atoms in both the 2P3/2 and spin-orbit excited 2P1/2 states in a ratio of 1.0:0.75. Excitation at 193.3 nm, assigned to a transition to primarily predissociated Rydberg levels on the I atom, leads to C–Br bond fission, some C–I bond fission, and significant concerted elimination of IBr. Analysis of the product translational energy distributions for the dissociation products indicates that the IBr is formed electronically excited and that the halogen atom products are spin-orbit excited. Excitation at 210 nm, of the transition assigned as n(Br)→σ*(C–Br) based on comparison with CH3Br, results in selective breaking of the stronger C–X bond in the molecule, the C–Br bond, and no fission of the C–I bond. Some concerted elimination of IBr also occurs; the IBr velocity distribution indicates it is probably formed electronically excited as in photolysis at 193.3 nm. The selective breaking of the C–Br bond over the weaker C–I bond is discussed in contrast to previous photolysis studies of polyhalomethanes.