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ABSTRACT: The wavelength dependences of C-Y and O-H bond fission following ultraviolet photoexcitation of 4-halophenols (4-YPhOH) have been investigated using a combination of velocity map imaging, H Rydberg atom photofragment translational spectroscopy, and high level spin-orbit resolved electronic structure calculations, revealing a systematic evolution in fragmentation behaviour across the series Y = I, Br, Cl (and F). All undergo O-H bond fission following excitation at wavelengths λ ≲ 240 nm, on repulsive ((n∕π)σ∗) potential energy surfaces (PESs), yielding fast H atoms with mean kinetic energies ∼11 000 cm(-1). For Y = I and Br, this process occurs in competition with prompt C-I and C-Br bond cleavage on another (n∕π)σ∗ PES, but no Cl∕Cl∗ products unambiguously attributable to one photon induced C-Cl bond fission are observed from 4-ClPhOH. Differences in fragmentation behaviour at longer excitation wavelengths are more marked. Prompt C-I bond fission is observed following excitation of 4-IPhOH at all λ ≤ 330 nm; the wavelength dependent trends in I∕I∗ product branching ratio, kinetic energy release, and recoil anisotropy suggest that (with regard to C-I bond fission) 4-IPhOH behaves like a mildly perturbed iodobenzene. Br atoms are observed when exciting 4-BrPhOH at long wavelengths also, but their velocity distributions suggest that dissociation occurs after internal conversion to the ground state. O-H bond fission, by tunnelling (as in phenol), is observed only in the cases of 4-FPhOH and, more weakly, 4-ClPhOH. These observed differences in behaviour can be understood given due recognition of (i) the differences in the vertical excitation energies of the C-Y centred (n∕π)σ∗ potentials across the series Y = I < Br < Cl and the concomitant reduction in C-Y bond strength, cf. that of the rival O-H bond, and (ii) the much increased spin-orbit coupling in, particularly, 4-IPhOH. The present results provide (another) reminder of the risks inherent in extrapolating photochemical behaviour measured for one molecule at one wavelength to other (related) molecules and to other excitation energies.
The Journal of chemical physics 04/2013; 138(16):164318. · 3.09 Impact Factor
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Stephanie J Harris,
Daniel Murdock,
Yuyuan Zhang, Thomas A A Oliver,
Michael P Grubb,
Andrew J Orr-Ewing,
Gregory M Greetham,
Ian P Clark,
Michael Towrie,
Stephen E Bradforth,
Michael N R Ashfold
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ABSTRACT: This article explores the extent to which insights gleaned from detailed studies of molecular photodissociations in the gas phase (i.e. under isolated molecule conditions) can inform our understanding of the corresponding photofragmentation processes in solution. Systems selected for comparison include a thiophenol (p-methylthiophenol), a thioanisole (p-methylthioanisole) and phenol, in vacuum and in cyclohexane solution. UV excitation in the gas phase results in RX-Y (X = O, S; Y = H, CH3) bond fission in all cases, but over timescales that vary by ∼4 orders of magnitude - all of which behaviours can be rationalised on the basis of the relevant bound and dissociative excited state potential energy surfaces (PESs) accessed by UV photoexcitation, and of the conical intersections that facilitate radiationless transfer between these PESs. Time-resolved UV pump-broadband UV/visible probe and/or UV pump-broadband IR probe studies of the corresponding systems in cyclohexane solution reveal additional processes that are unique to the condensed phase. Thus, for example, the data clearly reveal evidence of (i) vibrational relaxation of the photoexcited molecules prior to their dissociation and of the radical fragments formed upon X-Y bond fission, and (ii) geminate recombination of the RX and Y products (leading to reformation of the ground state parent and/or isomeric adducts). Nonetheless, the data also show that, in each case, the characteristics (and the timescale) of the initial bond fission process that occurs under isolated molecule conditions are barely changed by the presence of a weakly interacting solvent like cyclohexane. These condensed phase studies are then extended to an ether analogue of phenol (allyl phenyl ether), wherein UV photo-induced RO-allyl bond fission constitutes the first step of a photo-Claisen rearrangement.
Physical Chemistry Chemical Physics 04/2013; · 3.57 Impact Factor
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ABSTRACT: H (Rydberg) atom photofragment translation spectroscopy and high level ab initio electronic structure calculations are used to explore the photodissociation dynamics of three para-substituted thiophenols (p-YPhSH; Y = CH3, F, MeO). UV excitation in the wavelength range 305 > λphot > 240 nm results in S-H bond fission and formation of p-YPhS radicals in their ground (X 2B1) and first excited (A 2B2) electronic states; the X/A product branching ratio, Γ, varies with para-Y substituent and excitation wavelength. Excitation at λphot < 265 nm results in direct population of the dissociative 11πσ* potential energy surface (PES). Γ falls across the series: p-CH3PhSH > p-FPhSH > p-MeOPhSH. Branching is ultimately determined at the conical intersection (CI) between the 11πσ* and ground (S0) PESs at extended RS-H bond length, but is sensitively dependent on the orientation of the S-H bond (relative to the ring plane) in the S0 molecules prior to photoexcitation. Excitation to λphot > 265 nm populates quasi-bound levels of the respective 11πσ* states, which predissociate rapidly by tunneling under the lower diabats of the 11ππ*/11πσ* CI at short RS-H. Less extreme X/A product branching ratios are measured, implicating intramolecular vibrational redistribution within the photoexcited 11ππ* molecules prior to their sampling the region of the 11πσ*/S0 CI.
The Journal of Physical Chemistry A 10/2012; · 2.95 Impact Factor
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ABSTRACT: This article reports the striking interplay between the molecular structure and the photodissociation dynamics of catechol (a key dihydroxybenzene), identified using a combination of electronic spectroscopy, hydrogen (Rydberg) atom photofragment translational spectroscopy, density functional theory and second order approximate coupled cluster methods. We describe how the non-planar (C(1) symmetry) ← planar (C(s) symmetry) geometry change during S(1) (1(1)ππ*) ←S(0) excitation in catechol, as well as the presence of internal hydrogen bonding, can perturb the photodissociation dynamics relative to that of phenol (a monohydroxybenzene), particularly with respect to O-H bond fission via the lowest dissociative (1)πσ* state. For λ(phot) > 270 nm, O-H bond fission (of the non hydrogen bonded hydroxyl moiety) is deduced to proceed via H atom tunnelling from the photo-prepared 1(1)ππ* state into the lowest (1)πσ* state of the molecule. The vibrational energy distribution in the resulting catechoxyl product changes notably as λ(phot) is tuned on resonance with either the v' = 0, m(2)' = 1(+) or m(2)' = 2(+) torsional levels of the photo-prepared 1(1)ππ* state: the product state distribution is highly sensitive to the degree of OH torsional excitation (m(2)) prepared during photo-excitation. It is deduced that such torsional excitation can be redistributed very efficiently into ring puckering (and likely also in-plane ring stretch) vibrations as the molecule tunnels to its repulsive 1(1)πσ* state and dissociates. These observations can be rationalised by consideration of the photo-prepared nuclear wavefunctions. Analysis of the product vibrational energy distribution also reveals that the O-H bond strength of the non hydrogen bonded O-H moiety in catechol, D(0)(H-catechoxyl) ≤ 27 480 ± 50 cm(-1), ∼2500 cm(-1) lower than that of the sole O-H bond in bare phenol. As a consequence, the vertical excitation energy of the 1(1)πσ* state in catechol is reduced relative to that in phenol, yielding a particularly broad distribution of product vibrations for λ(phot) < 270 nm. This study highlights the interplay between molecular geometry and redistribution of vibrational energy during ultraviolet photolysis of phenols.
Physical Chemistry Chemical Physics 03/2012; 14(10):3338-45. · 3.57 Impact Factor
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ABSTRACT: To explore how the solvent influences primary aspects of bond breaking, the gas and solution phase photochemistries of phenol and ofpara-methylthiophenol are directly compared using, respectively, H (Rydberg) atom photofragment translation spectroscopy and femtosecond transient absorption spectroscopy. Approaches are demonstrated that allow explicit comparisons of the nascent product energy disposals and dissociation mechanisms in the two phases. It is found, at least for the case of the weakly perturbing cyclohexane environment, that most aspects of the primary reaction dynamics of the isolated molecule are reproduced in solution. Specifically, in the gas phase, both molecules can undergo fast X-H (X = O, S) bond dissociation upon excitation with short wavelengths (193 < lambda(pump) < 216 nm), following population of the dissociative S2 (1 1(pi sigma*)) state. Product electronic branching, vibrational and translational energy disposals are determined. Photolysis of phenol and para-methylthiophenol in solution at 200 nm results in formation of vibrationally excited radicals on a timescale shorter than 200 fs. Excitation of para-methylthiophenol at 267 nm reaches close to the S1 (1 1(pipi*))/S2 (11(pi sigma*)) conical intersection (CI): ultrafast dissociation is observed in both the isolated and solution systems-again indicating direct dissociation on the S2 potential energy surface. Comparing results for this precursor at different excitation energies, the extent of geminate recombination and the derived H-atom ejection lengths in the condensed phase photolyses are in qualitative agreement with the translational energy release measured in the gas phase studies. Conversely, excitation of phenol at 267 nm prepares the system in its S1 state at an energy well below its S1/S2 CI; the slow O-H bond fission inferred in the gas phase experiments is observed directly in the time-resolved studies in cyclohexane solution via the appearance of phenoxyl radical absorption after -1 ns, with only S1 excited state absorption discernible at earlier delay times. The slow O-H bond fission in solution provides additional evidence for a tunnelling dissociation mechanism, where the H atom tunnels beneath the lower diabats of the S2/S1 CI. Finally, the photodissociation of phenol clusters in solution is considered, where evidence is presented that the O-H dissociation coordinate is impeded in H-bonded dimers.
Faraday Discussions 01/2012; 157:141-63; discussion 243-84. · 5.00 Impact Factor
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09/2011; , ISBN: 9780470749593
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ABSTRACT: H (Rydberg) atom photofragment translational spectroscopy (HRA-PTS) and complete active space with second order perturbation theory (CASPT2) methods have been used to explore the competing N-H and O-H bond dissociation pathways of 4- and 5-hydroxyindoles (HI) and methoxyindoles (MI). When 4-HI was excited to bound (1)L(b) levels, (λ(phot) ≤ 284.893 nm) O-H bond fission was demonstrated by assignment of the structure within the resulting total kinetic energy release (TKER) spectra. By analogy with phenol, dissociation was deduced to occur by H atom tunnelling under the barrier associated with the lower diabats of the (1)L(b)/(1)πσ*((OH)) conical intersection (CI). No evidence was found for a significant N-H bond dissociation yield at these or shorter excitation wavelengths (284.893 ≥ λ(phot) ≥ 193.3 nm). Companion studies of 4-MI revealed different reaction dynamics. In this case, N-H bond fission is deduced to occur at λ(phot) ≤ 271.104 nm, by direct excitation to the (1)πσ*((NH)) state. Analysis of the measured TKER spectra implies a mechanism wherein, as in pyrrole, the (1)πσ*((NH)) state gains oscillator strength by intensity borrowing from nearby bound states with higher oscillator strengths. HRA-PTS studies of 5-HI, in contrast, showed no evidence for O-H bond dissociation when excited on (1)L(b) levels. The present CASPT2 calculations assist in rationalizing this observation: the area underneath the (1)L(b)/(1)πσ* CI diabats in 5-HI is ~60% greater than the corresponding area in 4-HI and O-H bond dissociation by tunnelling is thus much less probable. Only by reducing the wavelength to ≤ 255 nm were signs of N-H and/or O-H bond dissociation identified. By comparison with companion 5-MI studies, we deduce little O-H bond fission in 5-HI at λ(phot) > 235 nm and that N-H bond fission is the dominant source of H atoms in the wavelength region 255 > λ(phot) > 235 nm. The very different dissociation dynamics of 4- and 5-HI are traced to the position of the -OH substituent, and its effect on the overall electronic structure.
Physical Chemistry Chemical Physics 08/2011; 13(32):14646-62. · 3.57 Impact Factor
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ABSTRACT: The dynamics of reactions of CN radicals with cyclohexane, d(12)-cyclohexane, and tetramethylsilane have been studied in solutions of chloroform, dichloromethane, and the deuterated variants of these solvents using ultraviolet photolysis of ICN to initiate a reaction. The H(D)-atom abstraction reactions produce HCN (DCN) that is probed in absorption with sub-picosecond time resolution using ∼500 cm(-1) bandwidth infrared (IR) pulses in the spectral regions corresponding to C-H (or C-D) and C≡N stretching mode fundamental and hot bands. Equivalent IR spectra were obtained for the reactions of CN radicals with the pure solvents. In all cases, the reaction products are formed at early times with a strong propensity for vibrational excitation of the C-H (or C-D) stretching (v(3)) and H-C-N (D-C-N) bending (v(2)) modes, and for DCN products there is also evidence of vibrational excitation of the v(1) mode, which involves stretching of the C≡N bond. The vibrationally excited products relax to the ground vibrational level of HCN (DCN) with time constants of ∼130-270 ps (depending on molecule and solvent), and the majority of the HCN (DCN) in this ground level is formed by vibrational relaxation, instead of directly from the chemical reaction. The time-dependence of reactive production of HCN (DCN) and vibrational relaxation is analysed using a vibrationally quantum-state specific kinetic model. The experimental outcomes are indicative of dynamics of exothermic reactions over an energy surface with an early transition state. Although the presence of the chlorinated solvent may reduce the extent of vibrational excitation of the nascent products, the early-time chemical reaction dynamics in these liquid solvents are deduced to be very similar to those for isolated collisions in the gas phase. The transient IR spectra show additional spectroscopic absorption features centered at 2037 cm(-1) and 2065 cm(-1) (in CHCl(3)) that are assigned, respectively, to CN-solvent complexes and recombination of I atoms with CN radicals to form INC molecules. These products build up rapidly, with respective time constants of 8-26 and 11-22 ps. A further, slower rise in the INC absorption signal (with time constant >500 ps) is attributed to diffusive recombination after escape from the initial solvent cage and accounts for more than 2/3 of the observed INC.
The Journal of chemical physics 06/2011; 134(24):244503. · 3.09 Impact Factor
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ABSTRACT: When phenol is photoexcited to its S(1) (1(1)ππ∗) state at wavelengths in the range 257.403 ≤ λ(phot) ≤ 275.133 nm the O-H bond dissociates to yield an H atom and a phenoxyl co-product, with the available energy shared between translation and well characterised product vibration. It is accepted that dissociation is enabled by transfer to an S(2) (1(1)πσ∗) state, for which the potential energy surface (PES) is repulsive in the O-H stretch coordinate, R(O-H). This S(2) PES is cut by the S(1) PES near R(O-H) = 1.2 Å and by the S(0) ground state PES near R(O-H) = 2.1 Å, to give two conical intersections (CIs). These have each been invoked-both in theoretical studies and in the interpretation of experimental vibrational activity-but with considerable controversy. This paper revisits the dynamic mechanisms that underlie the photodissociation of phenol and substituted phenols in the light of symmetry restrictions arising from torsional tunnelling degeneracy, which has been neglected hitherto. This places tighter symmetry constraints on the dynamics around the two CIs. The non-rigid molecular symmetry group G(4) necessitates vibronic interactions by a(2) modes to enable coupling at the inner, higher energy (S(1)/S(2)) CI, or by b(1) modes at the outer, lower energy (S(2)/S(0)) CI. The experimental data following excitation through many vibronic levels of the S(1) state of phenol and substituted phenols demonstrate the effective role of the ν(16a) (a(2)) ring torsional mode in enabling O-H bond fission. This requires tunnelling under the S(1)/S(2) CI, with a hindering barrier of ∼5000 cm(-1) and with the associated geometric phase effect. Quantum dynamic calculations using new ab initio PESs provide quantitative justification for this conclusion. The fates of other excited S(1) modes are also rationalised, revealing both spectator modes and intramolecular vibrational redistribution between modes. A common feature in many cases is the observation of an extended, odd-number only, progression in product mode ν(16a) (i.e., the parent mode which enables S(1)/S(2) tunnelling), which we explain as a Franck-Condon consequence of a major change in the active vibration frequency. These comprehensive results serve to confirm the hypothesis that O-H fission following excitation to the S(1) state involves tunnelling under the S(1)/S(2) CI-in accord with conclusions reached from a recent correlation of the excited state lifetimes of phenol (and many substituted phenols) with the corresponding vertical energy gaps between their S(1) and S(2) PESs.
The Journal of chemical physics 05/2011; 134(19):194303. · 3.09 Impact Factor
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Stuart J Greaves,
Rebecca A Rose, Thomas A A Oliver,
David R Glowacki,
Michael N R Ashfold,
Jeremy N Harvey,
Ian P Clark,
Gregory M Greetham,
Anthony W Parker,
Michael Towrie,
Andrew J Orr-Ewing
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ABSTRACT: Solvent collisions can often mask initial disposition of energy to the products of solution-phase chemical reactions. Here, we show with transient infrared absorption spectra obtained with picosecond time resolution that the nascent HCN products of reaction of CN radicals with cyclohexane in chlorinated organic solvents exhibit preferential excitation of one quantum of the C-H stretching mode and up to two quanta of the bending mode. On time scales of approximately 100 to 300 picoseconds, the HCN products undergo relaxation to the vibrational ground state by coupling to the solvent bath. Comparison with reactions of CN radicals with alkanes in the gas phase, known to produce HCN with greater C-H stretch and bending mode excitation (up to two and approximately six quanta, respectively), indicates partial damping of the nascent product vibrational motion by the solvent. The transient infrared spectra therefore probe solvent-induced modifications to the reaction free energy surface and chemical dynamics.
Science 02/2011; 331(6023):1423-6. · 31.20 Impact Factor
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ABSTRACT: A recent review (Ashfold et al., Phys. Chem. Chem. Phys., 2010, 12, 1218) highlighted the important role of dissociative excited states formed by electron promotion to σ* orbitals in establishing the photochemistry of many molecular hydrides. Here we extend such considerations to molecular halides, with a particular focus on iodobenzene. Two experimental techniques (velocity mapped ion imaging (VMI) and time resolved infrared (IR) diode laser absorption) and electronic structure calculations have been employed in a comprehensive study of the near ultraviolet (UV) photodissociation of gas phase iodobenzene molecules. The VMI studies yield the speeds and angular distributions of the I((2)P(3/2)) and I*((2)P(1/2)) photofragments formed by photolysis in the wavelength range 330 ≥λ≥ 206 nm. Four distinct dissociation channels are observed for the I((2)P(3/2)) atom products, and a further three channels for the I*((2)P(1/2)) fragments. The phenyl (Ph) radical partners formed via one particular I* product channel following excitation at wavelengths 305 ≥λ≥ 250 nm are distributed over a sufficiently select sub-set of vibrational (v) states that the images allow resolution of specific I* + Ph(v) channels, identification of the active product mode (ν(10), an in-plane ring breathing mode), and a refined determination of D(0)(Ph-I) = 23,390 ± 50 cm(-1). The time-resolved IR absorption studies allow determination of the spin-orbit branching ratio in the iodine atom products formed at λ = 248 nm (ϕ(I*) = [I*]/([I] + [I*]) = 0.28 ± 0.04) and at 266 nm (ϕ(I*) = 0.32 ± 0.05). The complementary high-level, spin-orbit resolved ab initio calculations of sections (along the C-I bond coordinate) through the ground and first 19 excited state potential energy surfaces (PESs) reveal numerous excited states in the energy range of current interest. Except at the very shortest wavelength, however, all of the observed I and I* products display limiting or near limiting parallel recoil anisotropy. This encourages discussion of the fragmentation dynamics in terms of excitation to states of A(1) total symmetry and dissociation on the 2A(1) and 4A(1) (σ* ← n/π) PESs to yield, respectively, I and I* products, or via non-adiabatic coupling to other σ* ← n/π PESs that correlate to these respective limits. Similarities (and differences) with the available UV photochemical data for the other aryl halides, and with the simpler (and more thoroughly studied) iodides HI and CH(3)I, are summarised.
Physical Chemistry Chemical Physics 02/2011; 13(18):8075-93. · 3.57 Impact Factor
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ABSTRACT: Gas-phase H (Rydberg) atom photofragment translational spectroscopy and solution-phase femtosecond-pump dispersed-probe transient absorption techniques are applied to explore the excited state dynamics of p-methylthiophenol connecting the short time reactive dynamics in the two phases. The molecule is excited at a range of UV wavelengths from 286 to 193 nm. The experiments clearly demonstrate that photoexcitation results in S-H bond fission--both in the gas phase and in ethanol solution-and that the resulting p-methythiophenoxyl radical fragments are formed with significant vibrational excitation. In the gas phase, the recoil anisotropy of the H atom and the vibrational energy disposal in the p-MePhS radical products formed at the longer excitation wavelengths reveal the operation of two excited state dissociation mechanisms. The prompt excited state dissociation motif appears to map into the condensed phase also. In both phases, radicals are produced in both their ground and first excited electronic states; characteristic signatures for both sets of radical products are already apparent in the condensed phase studies after 50 fs. No evidence is seen for either solute ionisation or proton coupled electron transfer--two alternate mechanisms that have been proposed for similar heteroaromatics in solution. Therefore, at least for prompt S-H bond fissions, the direct observation of the dissociation process in solution confirms that the gas phase photofragmentation studies indeed provide important insights into the early time dynamics that transfer to the condensed phase.
Faraday Discussions 01/2011; 150:439-58; discussion 505-32. · 5.00 Impact Factor
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ABSTRACT: Resolved sets of photoproducts arising from the photodissociation of axial and equatorial conformers of 3-pyrroline have been observed using H(Rydberg) atom photofragment translational spectroscopy following excitation in the wavelength range of 250-213 nm. 3-pyrroline (alternatively 2,5-dihydropyrrole) is a five membered partially saturated heterocycle in which the bonding around the N atom is pyramidal (sp(3) hybridized) and the N-H bond can lie either axial or equatorial to the ring. Careful analysis of total kinetic energy release data derived from H atom time-of-flight measurements reveals excitation of the 3-pyrrolinyl cofragment consistent with N-H bond fission in both the axial and equatorial conformers. This allows determination of the energy difference between the ground state conformers to be 340±50 cm(-1) and the N-H bond strength for axial and equatorial conformers as 31,610±50 and 31,270±50 cm(-1), respectively.
The Journal of chemical physics 11/2010; 133(19):194303. · 3.09 Impact Factor
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ABSTRACT: This article reports a comprehensive study of the mechanisms of H atom loss in aniline (C(6)H(5)NH(2)) following ultraviolet excitation, using H (Rydberg) atom photofragment translational spectroscopy. N-H bond fission via the low lying (1)pi sigma(*) electronic state of aniline is experimentally demonstrated. The (1)pi sigma(*) potential energy surface (PES) of this prototypical aromatic amine is essentially repulsive along the N-H stretch coordinate, but possesses a shallow potential well in the vertical Franck-Condon region, supporting quasibound vibrational levels. Photoexcitation at wavelengths (lambda(phot)) in the range 293.859 nm > or = lambda(phot) > or = 193.3 nm yields H atom loss via a range of mechanisms. With lambda(phot) resonant with the 1(1)pi pi(*) <-- S(0) origin (293.859 nm), H atom loss proceeds via, predominantly, multiphoton excitation processes, resonantly enhanced at the one photon energy by the first (1)pi pi(*) excited state (the 1(1)pi pi(*) state). Direct excitation to the first few quasibound vibrational levels of the (1)pi sigma(*) state (at wavelengths in the range 269.513 nm > or = lambda(phot) > or = 260 nm) induces N-H bond fission via H atom tunneling through an exit barrier into the repulsive region of the (1)pi sigma(*) PES, forming anilino (C(6)H(5)NH) radical products in their ground electronic state, and with very limited vibrational excitation; the photo-prepared vibrational mode in the (1)pi sigma(*) state generally evolves adiabatically into the corresponding mode of the anilino radical upon dissociation. However, as the excitation wavelength is reduced (lambda(phot) < 260 nm), N-H bond fission yields fragments with substantially greater vibrational excitation, rationalized in terms of direct excitation to 1(1)pi pi(*) levels, followed by coupling to the (1)pi sigma(*) PES via a 1(1)pi pi(*)/(1)pi sigma(*) conical intersection. Changes in product kinetic energy disposal once lambda(phot) approaches approximately 230 nm likely indicate that the photodissociation pathways of aniline proceed via direct excitation to the (higher) 2(1)pi pi(*) state. Analysis of the anilino fragment vibrational energy disposal-and thus the concomitant dynamics of (1)pi sigma(*) state mediated photodissociation-provides a particularly interesting study of competing sigma(*) <-- pi and pi(*) <-- pi absorption processes and develops our appreciation of the photochemistry of aromatic amines. It also allows revealing comparisons with simple amines (such as ammonia and methylamine) as well as the isoelectronic species, phenol. This study yields a value for the N-H bond strength in aniline, D(0)(H-anilino) = 31630+/-40 cm(-1).
The Journal of chemical physics 06/2010; 132(21):214307. · 3.09 Impact Factor
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ABSTRACT: This article reports a comprehensive study of the mechanisms of H atom loss in aniline (C6H5NH2) following ultraviolet excitation, using H (Rydberg) atom photofragment translational spectroscopy. N–H bond fission via the low lying 1πσ∗ electronic state of aniline is experimentally demonstrated. The 1πσ∗ potential energy surface (PES) of this prototypical aromatic amine is essentially repulsive along the N–H stretch coordinate, but possesses a shallow potential well in the vertical Franck–Condon region, supporting quasibound vibrational levels. Photoexcitation at wavelengths (λphot) in the range 293.859 nm ≥ λphot ≥ 193.3 nm yields H atom loss via a range of mechanisms. With λphot resonant with the 11ππ∗←S0 origin (293.859 nm), H atom loss proceeds via, predominantly, multiphoton excitation processes, resonantly enhanced at the one photon energy by the first 1ππ∗ excited state (the 11ππ∗ state). Direct excitation to the first few quasibound vibrational levels of the 1πσ∗ state (at wavelengths in the range 269.513 nm ≥ λphot ≥ 260 nm) induces N–H bond fission via H atom tunneling through an exit barrier into the repulsive region of the 1πσ∗ PES, forming anilino (C6H5NH) radical products in their ground electronic state, and with very limited vibrational excitation; the photo-prepared vibrational mode in the 1πσ∗ state generally evolves adiabatically into the corresponding mode of the anilino radical upon dissociation. However, as the excitation wavelength is reduced (λphot<260 nm), N–H bond fission yields fragments with substantially greater vibrational excitation, rationalized in terms of direct excitation to 11ππ∗ levels, followed by coupling to the 1πσ∗ PES via a 11ππ∗/1πσ∗ conical intersection. Changes in product kinetic energy disposal once λphot approaches ∼ 230 nm likely indicate that the photodissociation pathways of aniline proceed via direct excitation to the (higher) 21ππ∗ state. Analysis of the anilino fragment vibrational energy disposal—and thus the concomitant dynamics of 1πσ∗ state mediated photodissociation—provides a particularly interesting study of competing σ∗←π and π∗←π absorption processes and develops our appreciation of the photochemistry of aromatic amines. It also allows revealing comparisons with simple amines (such as ammonia and methylamine) as well as the isoelectronic species, phenol. This study yields a value for the N–H bond strength in aniline, D0(H−anilino) = 31630±40 cm−1.
The Journal of Chemical Physics 06/2010; 132(21):214307-214307-12. · 3.33 Impact Factor
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ABSTRACT: The last few years have seen a surge in interest (both theoretical and experimental) in the photochemistry of heteroaromatic molecules (e.g. azoles, phenols), which has served to highlight the importance of dissociative excited states formed by electron promotion to sigma* molecular orbitals. Such excited states--which, for brevity, are termed pi sigma* states in this Perspective article--may be populated by direct photo-excitation (though the transition cross-sections are intrinsically small), or indirectly, by non-adiabatic coupling from an optically 'bright' excited state (e.g. an excited state resulting from pi* <--pi excitation). The analogous pi sigma* excited states in prototypical hydride molecules like H(2)O and NH(3) have long been recognised. They have served as test-beds for developing concepts like Rydbergisation, conical intersections (CIs) between potential energy surfaces, and for investigating the ways in which non-adiabatic couplings at such CIs influence the eventual photofragmentation dynamics. This Perspective article seeks to highlight the continuity of behaviour revealed by the earlier small molecule studies and by the more recent studies of heteroaromatic systems, and to illustrate the photochemical importance of pi sigma* excited states in many broad families of molecules. Furthermore, the dynamical influence of such excited states is not restricted to closed shell species; the Article concludes with a brief consideration of the consequences of populating sigma* orbitals in free radical species, in molecular cations, and in dissociative electron attachment processes.
Physical Chemistry Chemical Physics 02/2010; 12(6):1218-38. · 3.57 Impact Factor
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ABSTRACT: The photophysics of gas phase pyrazole (C(3)N(2)H(4)) and 2H-1,2,3-triazole (C(2)N(3)H(3)) molecules following excitation at wavelengths in the range 230 nm>or=lambda(phot)>or=193.3 nm has been investigated using the experimental technique of H (Rydberg) atom photofragment translational spectroscopy. The findings are compared with previous studies of pyrrole (C(4)N(1)H(5)) and imidazole (C(3)N(2)H(4)), providing a guide to H atom loss dynamics in simple N-containing heterocycles. CASPT2 theoretical methods have been employed to validate these findings. Photoexcitation of pyrazole at the longest wavelengths studied is deduced to involve pi( *)<--pi excitation, but photolysis at lambda(phot)</=214 nm is characterized by rapid N-H bond fission on a (1)pisigma( *) potential energy surface. The eventual pyrazolyl radical products are formed in a range of vibrational levels associated with both the ground ((2)A(2)) and first excited ((2)B(1)) electronic states as a result of nonadiabatic coupling at large N-H bond lengths. The excitation energy of the lowest (1)pisigma( *) state of pyrazole is found to be significantly higher in energy than that of pyrrole and imidazole. Similar studies of 2H-1,2,3-triazole reveal that the lowest (1)pisigma( *) state is yet higher in energy and not accessible following excitation at lambda(phot)>or=193.3 nm. The N-H bond strength of pyrazole is determined as 37 680+/-40 cm(-1), significantly greater than that of the N-H bonds in pyrrole and imidazole. The correlation between the photochemistry of azoles and the number and position of nitrogen atoms within the ring framework is discussed in terms of molecular symmetry and orbital electron density. A photodissociation channel yielding H atoms with low kinetic energies is also clearly evident in both pyrazole and 2H-1,2,3-triazole. Companion studies of pyrazole-d(1) suggest that these slow H atoms arise primarily from the N-H site, following pi( *)<--pi excitation, and subsequent internal conversion and/or unintended multiphoton absorption processes.
The Journal of chemical physics 02/2010; 132(6):064305. · 3.09 Impact Factor
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Physical Chemistry Chemical Physics - PHYS CHEM CHEM PHYS. 01/2010; 12(6).
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ABSTRACT: H atom loss following ultraviolet photoexcitation of 2-methyl, 3-furanthiol (2M,3FT) at many wavelengths in the range 269 nm > or = lambda(phot) > or = 210 nm and at 193 nm has been investigated by H (Rydberg) atom photofragment translational spectroscopy. The photodissociation dynamics of this SH decorated aromatic ring system are contrasted with that of thiophenol (Devine et al. J. Phys. Chem. A 2008, 112, 9563), the excited electronic states of which show a different energetic ordering. Ab initio theory and experiment find that the first excited state of 2M,3FT is formed by electron promotion from an orbital comprised of an admixture of the S lone pair and the furan pi system (n/pi) to a sigma* orbital centered on the S-H bond. Photoexcitation at long wavelengths results in population of the (1)(n/pi)sigma* excited state, prompt S-H bond fission, H atoms displaying a (nonlimiting) perpendicular recoil velocity distribution, and partner radicals formed in selected low vibrational levels of the ground state. This energy disposal can be rationalized by considering the forces acting as the excited molecules evolve on the (1)(n/pi)sigma* potential energy surface (PES). Energy conservation arguments, together with the product vibrational state analysis, yield a value of 31320 +/- 100 cm(-1) for the S-H bond strength in 2M,3FT. Excitation at shorter wavelengths (lambda(phot) < or = 230 nm) is deduced to populate one or more (diabatically bound) (1)(n/pi)pi* excited states which decay by coupling to the (1)(n/pi)sigma* PES and/or to high vibrational levels of the electronic ground state.
The Journal of Physical Chemistry A 09/2009; 114(3):1338-46. · 2.95 Impact Factor
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ABSTRACT: The dissociation dynamics of gas phase phenol-d(5) molecules (C(6)D(5)OH) following excitation at numerous wavelengths in the range 275 > or = lambda(phot) > or = 193.3 nm have been investigated using the techniques of H (Rydberg) atom photofragment translational spectroscopy and resonance enhanced multiphoton ionization spectroscopy. The results are compared with those from recent studies of the fully hydrogenated and fully deuterated isotopologues (C(6)H(5)OH and C(6)D(5)OD), and various halo- and methyl-substituted phenols. Analysis of the vibrational energy disposal within the phenoxyl-d(5) dissociation products identifies three distinct O-H bond fission pathways, involving nonadiabatic coupling to dissociative states of (1)pisigma* character, following initial pi* <-- pi excitation. Dissociation at lambda(phot) > 248 nm involves internal conversion (IC) to high vibrational levels of the electronic ground ((1)pipi) state and subsequent coupling to the lowest (1)pisigma* potential energy surface (PES) via a conical intersection (CI) between the (1)pipi/(1)pisigma* PESs at extended O-H bond lengths (R(O-H)). Once lambda(phot) < or = 248 nm, dissociation proceeds directly, via a (1)pipi*/(1)pisigma* CI. Both pathways yield phenoxyl-d(5) products in selected vibrational levels of the ground (X(2)B(1)) electronic state. The detailed energy disposal within the phenoxyl-d(5)(X) products shows many parallels with that deduced from companion studies of other phenol isotopologues and various substituted phenols, but a notable isotope effect is identified, thus providing yet greater insights into the factors controlling the vibrational energy disposal in the phenoxyl products. A hitherto unobserved O-H bond fission channel yielding phenoxyl-d(5) fragments in the electronically excited B(2)A(2) state is identified at the shortest excitation wavelength (lambda(phot) = 193.3 nm) and rationalized in terms of nonadiabatic coupling to, and subsequent dissociation on, the second excited (1)pisigma* PES. Selective deuteration as in phenol-d(5) causes little reduction in the intensity of the "slower" H atom products that are observed from all phenol systems, suggesting that C-H/D bond fission makes at most a minor contribution to this feature.
The Journal of Physical Chemistry A 06/2009; 113(28):7984-93. · 2.95 Impact Factor
