Christian Tanner

Excited state hydrogen atom transfer in ammonia-wire and water-wire clusters

We review experimental and theoretical investigations of excited-state hydrogen atom transfer (ESHAT) reactions along unidirectionally hydrogen bonded solvent ‘wire’ clusters. The solvent wire is attached to the aromatic ‘scaffold’ molecule 7-hydroxyquinoline (7HQ), which offers an O–H and an N hydrogen bonding site, spaced far enough apart to form two- to four-membered wires. S 1  ← S 0 photoexcitation renders the O–H group more acidic and the quinolinic N more basic. This provides a driving force for the enol → keto tautomerization, probed by the characteristic fluorescence of the 7-ketoquinoline in the molecular beam experiments. For 7-hydroxyquinoline·(NH3)3, excitation of ammonia-wire vibrations induces the tautomerization at ∼200 cm−1. Different reaction pathways have been explored by excited-state ab initio calculations. These show that the reaction proceeds by H-atom transfer along the wire as a series of Grotthus-type translocation steps. There is no competition with a mechanism involving successive proton translocations. The rate-controlling S 1 state barriers arise from crossings of a π π* with a Rydberg-type πσ* state and the proton and electron movements along the wire are closely coupled. The excited state reactant, H-transferred intermediates and product structures are characterized. The reaction proceeds by tunnelling, as shown by deuteration of the solvent molecules (ND3) in the wire. The first step of the reaction exhibits intra/intermolecular vibrational mode selectivity. Substitution of NH3 by one, two or three H2O molecules in the wire leads to increasing threshold with each additional H2O molecule, up to >2000 cm−1 for the 7-hydroxyquinoline·(H2O)3 water-wire cluster. No 7-ketoquinoline fluorescence is observed upon insertion of even a single H2O molecule. The calculations show that insertion of each H2O molecule into the solvent wire introduces a high barrier, which blocks any further H-atom transfer. Contents PAGE 1. Introduction 458 2. 7-Hydroxyquinoline 460 2.1. Ground and excited state acid/base properties 460 2.2. O–H bond breaking in the ππ* and πσ* states 461 3. The 7-hydroxyquinoline · (NH3)n ammonia-wire clusters: an overview 463 3.1. Structures and electronic origins 463 3.2. Resonant two-photon ionization spectra 466 4. Excited-state hydrogen atom transfer in 7-hydroxyquinoline · (NH3)3 468 4.1. UV–UV depletion and fluorescence action spectra 468 4.2. Two reaction paths: ESHAT vs. ESPT 470 4.3. The enol → HT1 reaction coordinate 474 4.4. Vibrational mode selectivity 474 5. Solvent e.ects on excited state H-atom transfer: mixed ammonia/water clusters 479 5.1. Comparison of 7HQ· (H2O)3 and 7HQ · (NH3)3 479 5.2. The mixed 7HQ · (NH3)2 · H2O and 7HQ · NH3 · (H2O)2 clusters 482 5.3. Discussion 484 6. Conclusion 485 References 487

Exploring excited-state hydrogen atom transfer along an ammonia wire cluster: competitive reaction paths and vibrational mode selectivity.

The excited-state hydrogen-atom transfer (ESHAT) reaction of the 7-hydroxyquinoline(NH(3))(3) cluster involves a crossing from the initially excited (1)pipi(*) to a (1)pisigma(*) state. The nonadiabatic coupling between these states induces homolytic dissociation of the O-H bond and H-atom transfer to the closest NH(3) molecule, forming a biradical structure denoted HT1, followed by two more Grotthus-type translocation steps along the ammonia wire. We investigate this reaction at the configuration interaction singles level, using a basis set with diffuse orbitals. Intrinsic reaction coordinate calculations of the enol-->HT1 step predict that the H-atom transfer is preceded and followed by extensive twisting and bending of the ammonia wire, as well as large O-H...NH(3) hydrogen bond contraction and expansion. The calculations also predict an excited-state proton transfer path involving synchronous proton motions; however, it lies 20-25 kcal/mol above the ESHAT path. Higher singlet and triplet potential curves are calculated along the ESHAT reaction coordinate: Two singlet-triplet curve crossings occur within the HT1 product well and intersystem crossing to these T(n) states branches the reaction back to the enol reactant side, decreasing the ESHAT yield. In fact, a product yield of approximately 40% 7-ketoquinoline.(NH(3))(3) is experimentally observed. The vibrational mode selectivity of the enol-->HT1 reaction step [C. Manca, C. Tanner, S. Coussan, A. Bach, and S. Leutwyler, J. Chem. Phys. 121, 2578 (2004)] is shown to be due to the large sensitivity of the diffuse pisigma(*) state to vibrational displacements along the intermolecular coordinates.

Ground- and excited state proton transfer and tautomerization in 7-hydroxyquinoline.(NH3)n clusters: Spectroscopic and time resolved investigations

Mass-selected S1↔S0 two color resonant two photon ionization (2C-R2PI) spectra, fluorescence spectra and fluorescence decay times are measured for supersonically cooled 7-hydroxyquinoline (7HQ)⋅(NH3)n clusters with n=4–10. For n=4, the S1←S0 2C-R2PI spectrum shows a 20 cm−1 broad electronic origin at 27 746 cm−1, followed by an intermolecular vibrational progression with band widths that increase up to ≈45 cm−1. In contrast, the 2C-R2PI spectra of the mixed 7HQ⋅(NH3)3H2O and 7HQ⋅(NH3)2(H2O)2 clusters exhibit narrow bands of 1–2 cm−1 width. The large band widths of 7HQ⋅(NH3)4 are due to a fast (k>1012 s−1) excited state process which is blocked when replacing one or more NH3 molecules by H2O in the cluster. For the n=5–10 clusters, the 2C-R2PI spectra display two broad absorption bands peaking at 25 000 and 27 000 cm−1. The latter is characteristic of the 7-quinolinate (7Q−) anion, implying that ground state proton transfer from 7HQ to the ammonia cluster occurs for n⩾5. Excitation at 27 000 cm−1 leads to ...

H atom transfer along an ammonia chain: Tunneling and mode selectivity in 7-hydroxyquinoline⋅(NH3)3

Excitation of the 7-hydroxyquinoline(NH(3))(3) [7HQ(NH(3))(3)] cluster to the S(1) (1)pi pi(*) state results in an O-H-->NH(3) hydrogen atom transfer (HAT) reaction. In order to investigate the entrance channel, the vibronic S(1)<-->S(0) spectra of the 7HQ.(NH(3))(3) and the d(2)-7DQ.(ND(3))(3) clusters have been studied by resonant two-photon ionization, UV-UV depletion and fluorescence techniques, and by ab initio calculations for the ground and excited states. For both isotopomers, the low-frequency part of the S(1)<--S(0) spectra is dominated by ammonia-wire deformation and stretching vibrations. Excitation of overtones or combinations of these modes above a threshold of 200-250 cm(-1) for 7HQ.(NH(3))(3) accelerates the HAT reaction by an order of magnitude or more. The d(2)-7DQ.(ND(3))(3) cluster exhibits a more gradual threshold from 300 to 650 cm(-1). For both isotopomers, intermolecular vibrational states above the threshold exhibit faster HAT rates than the intramolecular vibrations. The reactivity, isotope effects, and mode selectivity are interpreted in terms of H atom tunneling through a barrier along the O-H-->NH(3) coordinate. The barrier results from a conical intersection of the optically excited (1)pi pi(*) state with an optically dark (1)pi sigma(*) state. Excitation of the ammonia-wire stretching modes decreases both the quinoline-O-H...NH(3) distance and the energetic separation between the (1)pi pi(*) and (1)pi sigma(*) states, thereby increasing the H atom tunneling rate. The intramolecular vibrations change the H bond distance and modulate the (1)pi pi(*)<-->(1)pi sigma(*) interaction to a much smaller extent.

Time-dependent density functional theory as a tool for isomer assignments of hydrogen-bonded solute.solvent clusters.

Can isomer structures of hydrogen-bonded solute x solvent clusters be assigned by correlating gas-phase experimental S0 <--> S1 transitions with vertical or adiabatic excitation energies calculated by time-dependent density functional theory (TD-DFT)? We study this question for 7-hydroxyquinoline (7HQ), for which an experimental database of 19 complexes and clusters is available. The main advantage of the adiabatic TD-B3LYP S0 <--> S1 excitations is the small absolute error compared to experiment, while for the calculated vertical excitations, the average offset is +1810 cm(-1). However, the empirically adjusted vertical excitations correlate more closely with the experimental transition energies, with a standard deviation of sigma = 72 cm(-1). For the analogous correlation with calculated adiabatic TD-DFT excitations, the standard deviation is sigma = 157 cm(-1). The vertical and adiabatic TD-DFT correlation methods are applied for the identification of isomers of the 7-hydroxyquinoline.(MeOH) n , n = 1-3 clusters [Matsumoto, Y.; Ebata, T.; Mikami, N. J. Phys. Chem. B 2002, 106, 5591]. These confirm that the vertical TD-DFT/experimental correlation yields more effective isomer assignments.

Intermolecular vibrations of 1-naphthol⋅NH3 and d3-1-naphthol⋅ND3 in the S0 and S1 states

Hydrogen-bonded complexes of the photoacid 1-naphthol with NH3 and ND3 were investigated by resonant two-photon ionization, spectral hole burning, and fluorescence spectroscopies. Although the intermolecular vibrations are weak in both absorption and emission, with typical Franck–Condon factors <2% relative to the electronic origin, all six intermolecular modes were identified, namely the hydrogen bond stretch σ, the ammonia torsion τ, two in-plane wags β1 and β2, and two out-of-plane rocking motions ρ1 and ρ2. Several ammonia torsional excitations were observed, with spacings in good agreement with the S0- and S1 state effective torsional barriers derived by Humphrey and Pratt [J. Chem. Phys. 104, 8332 (1996)]. The β1, β2, and ρ2 vibrational excitations exhibit large (2–8 cm−1) torsional splittings, which indicate strong anharmonic coupling with the ammonia internal rotation. The observed Franck–Condon factors of the intermolecular stretching vibration imply a contraction of the O–H⋅⋅⋅N hydrogen bond by ...

7-Hydroxyquinoline·(NH 3 ) 3 : A Model for Excited State H-Atom Transfer Along an Ammonia Wire

The 7-hydroxyquinoline.(NH 3 ) 3 cluster has been used as a model to study excited state H-atom transfer along a hydrogen-bonded NH 3 ...NH 3 ...NH 3 wire. Excitation of the supersonically cooled cluster to its vibrationlessS 1 state produces no reaction, whereas excitation of ammonia-wire vibrations induces fluorescence of the excited 7-ketoquinoline.(NH 3 ) 3 cluster, revealing the excited state enol → keto tautomerization. This reaction is assumed to occur via successive H transfers along the ammonia wire in a Grotthus-like process. Ab initio calculations of the reaction path show that proton and electron movement along the wire are closely coupled. The rate-controlling S 1 state barriers arise from crossing of a ππ* and a Rydberg-type πσ* electronic excited state.

Structural study of the hydrogen-bonded 1-naphthol⋅(NH3)2 cluster

The structure of the 1-naphthol⋅(NH3)2 cluster was investigated by rotational coherence spectroscopy (RCS), mass selective one- and two-color resonant two-photon ionization (R2PI) experiments and ab initio calculations. RCS measurements yielded rotational constants of 1-naphthol⋅(NH3)2 as A=1197, B=500, and C=413 MHz, as well as those for several isotopomers. The counterpoise-corrected second-order Moller–Plesset perturbation theory (MP2) method predicts two isomers A and B. Both structures have hydrogen bonded naphthol–OH⋯NH3⋯NH3 chains, with the second NH3 bent above the proximal aromatic ring and pointing towards the π-electron system and have nearly the same binding energy. The experimental rotational constants agree better with those calculated for structure B. The B3LYP and PW91 density functional methods also predict two isomers A, B with the rotational constants of B in acceptable agreement with experiment. Based on two-color R2PI experiments using low ionization frequency to suppress cluster frag...

Ammonia-chain clusters: Vibronic spectra of 7-hydroxyquinoline⋅(NH3)2

Mass- and isomer-selected S1←S0 resonant two-photon ionization and S1→S0 fluorescence spectra were measured for the 7-hydroxyquinoline⋅(NH3)2 [7HQ⋅(NH3)2] and d2-7-hydroxyquinoline⋅(ND3)2 clusters cooled in supersonic expansions. UV/UV hole burning measurements prove that a single cluster isomer is formed. Ab initio self-consistent field and density functional calculations predict that the most stable cluster form has an “ammonia wire” hydrogen bonded to the –OH and N groups of the cis-7HQ rotamer. The experimental S0 and S1 frequencies are in very good agreement with the calculated normal mode frequencies for both the normal and deuterated ammonia-wire clusters. S1←S0 excitation leads to contractions of the –O–H⋯N and NH3⋯NH3 hydrogen bonds, as well as smaller displacements for the NH3⋯N(quinoline) stretch and the in plane rotation (or bend) of the ammonia dimer relative to 7HQ. The coupling of these modes to the S1←S0 electronic excitation indicates that hydrogen bond contractions in the excited state a...

Spectral tuning by switching C–H⋯O hydrogen bonds: Rotation-induced spectral shifts of 7-hydroxyquinoline∙HCOOH isomers

Spectral tuning effects on visible chromophores by hydrogen bonds are central to the chemistry of vision and of photosynthesis. A model for large spectral tuning effects by hydrogen bond switching is provided by the 7-hydroxyquinoline x HCOOH complex, which forms two isomers, CTN1 and CTN2, both with an HCOOH[...]N hydrogen bond but with different (quinoline)C-H[...]O=C hydrogen bonds. A 180 degrees rotation of the HCOOH moiety around the O-H[...]N hydrogen bond exchanges the C-H[...]O hydrogen bonds, rotates the dipole moment of HCOOH, and leads to an approximately 850 cm(-1) shift of the electronic spectrum. Mass-selected S1<--S0 resonant two-photon ionization, UV-UV holeburning, S1-->S0 fluorescence spectra, and photoionization efficiency curves of the two 7-hydroxyquinoline x HCOOH isomers were measured in supersonic expansions. Comparison to ab initio calculations allow us to determine the H-bond connectivity and structure of the two isomers and to assign their inter- and intramolecular vibrations. The Franck-Condon factors of the intermolecular shear vibration chi in the S1<--S0 spectra indicate that the weak C-H[...]O hydrogen bond contracts markedly in the CTN1 isomer but expands in the CTN2 isomer. These changes of H-bond lengths agree with the spectral shifts. In contrast, the strong O-H[...]N hydrogen bond undergoes little change upon S1[...]S0 excitation.