Michel Broquier

Excited state hydrogen transfer dynamics in substituted phenols and their complexes with ammonia: ππ*-πσ* energy gap propensity and ortho-substitution effect.

Lifetimes of the first electronic excited state (S(1)) of fluorine and methyl (o-, m-, and p-) substituted phenols and their complexes with one ammonia molecule have been measured for the 0(0) transition and for the intermolecular stretching σ(1) levels in complexes using picosecond pump-probe spectroscopy. Excitation energies to the S(1) (ππ*) and S(2) (πσ*) states are obtained by quantum chemical calculations at the MP2 and CC2 level using the aug-cc-pVDZ basis set for the ground-state and the S(1) optimized geometries. The observed lifetimes and the energy gaps between the ππ* and πσ* states show a good correlation, the lifetime being shorter for a smaller energy gap. This propensity suggests that the major dynamics in the excited state concerns an excited state hydrogen detachment or transfer (ESHD/T) promoted directly by a S(1)/S(2) conical intersection, rather than via internal conversion to the ground-state. A specific shortening of lifetime is found in the o-fluorophenol-ammonia complex and explained in terms of the vibronic coupling between the ππ* and πσ* states occurring through the out-of-plane distortion of the C-F bond.

Gas-Phase Folding of a Two-Residue Model Peptide Chain: On the Importance of an Interplay between Experiment and Theory

In order to assess the ability of theory to describe properly the dispersive interactions that are ubiquitous in peptide and protein systems, an isolated short peptide chain has been studied using both gas-phase laser spectroscopy and quantum chemistry. The experimentally observed coexistence of an extended form and a folded form in the supersonic expansion was found to result from comparable Gibbs free energies for the two species under the high-temperature conditions (< or = 320 K) resulting from the laser desorption technique used to vaporize the molecules. These data have been compared to results obtained using a series of quantum chemistry methods, including DFT, DFT-D, and post-Hartree-Fock methods, which give rise to a wide range of relative stabilities predicted for these two forms. The experimental observation was best reproduced by an empirically dispersion-corrected functional (B97-D) and a hybrid functional with a significant Hartree-Fock exchange term (M06-2X). In contrast, the popular post-Hartree-Fock method MP2, which is often used for benchmarking these systems, had to be discarded because of a very large basis-set superposition error. The applicability of the atomic counterpoise correction (ACP) is also discussed. This work also introduces the mandatory theoretical examination of experimental abundances. DeltaH(0 K) predictions are clearly not sufficient for discussion of folding, as the conformation inversion temperature is crucial to the conformation determination and requires taking into account thermodynamical corrections (DeltaG) in order to computationally isolate the most stable conformation.

Role of the charge-transfer state in the electronic absorption of protonated hydrocarbon molecules.

The vibrationally resolved electronic spectra of isolated protonated polycyclic aromatic hydrocarbons (PAHs)--naphthalene, anthracene, and tetracene--have been recorded via neutral photofragment spectroscopy. The S1←S0 transitions are all in the visible region and do not show a monotonic red shift as a function of the molecular size, as observed for the neutral analogues. Comparison with ab initio calculations indicates that this behavior is due to the nature of the excited state, which has a pronounced charge-transfer character for protonated linear PAHs with an even number of aromatic rings.

Effect of protonation on the electronic structure of aromatic molecules: naphthaleneH+.

Protonated naphthalene, the smallest protonated polycyclic aromatic hydrocarbon cation, absorbs in the visible, around 500 nm, which corresponds to an unusually large red shift with respect to the neutral naphthalene counterpart.

Unraveling the Mechanisms of Nonradiative Deactivation in Model Peptides Following Photoexcitation of a Phenylalanine Residue

The mechanisms of nonradiative deactivation of a phenylalanine residue after near-UV photoexcitation have been investigated in an isolated peptide chain model (N-acetylphenylalaninylamide, NAPA) both experimentally and theoretically. Lifetime measurements at the origin of the first ππ* state of jet-cooled NAPA molecules have shown that (i) among the three most stable conformers of the molecule, the folded conformer NAPA B is ∼50-times shorter lived than the extended major conformer NAPA A and (ii) this lifetime is virtually insensitive to deuteration at the NH(2) and NH sites. Concurrent time-dependent density functional theory (TDDFT) based nonadiabatic dynamics simulations in the full dimensionality, carried out for the NAPA B conformer, provided direct insights on novel classes of ultrafast deactivation mechanisms, proceeding through several conical intersections and leading in fine to the ground state. These mechanisms are found to be triggered either (i) by a stretch of the N(Phe)H bond, which leads to an H-transfer to the ring, or (ii) by specific backbone amide distortions. The potential energy surfaces of the NAPA conformers along these critical pathways have been characterized more accurately using the coupled cluster doubles (CC2) method and shown to exhibit barriers that can be overcome with moderate excess energies. These results analyzed in the light of the experimental findings enabled us to assign the short lifetime of NAPA B conformer to a number of easily accessible exit channels from the initial ππ* surface, most importantly the one involving a transfer of electronic excitation to an nπ* surface, induced by distortions of the backbone peptide bond.

Electronic Spectra of the Protonated Indole Chromophore in the Gas Phase

The electronic spectroscopy of cold protonated indole was investigated experimentally and theoretically. Two isomers were observed by experiment: The first isomer corresponds to the lowest-energy isomer in the calculations, absorbing at ~350 nm and protonated on the C3 atom of the pyrrole ring. According to our calculations, the absorptions of the other isomers protonated on carbon atoms (C2, C4, C5, C6, and C7) are in the visible region. Indeed, the absorption of the second observed isomer starts at 488 nm and was assigned to protonation on the C2 carbon of the pyrrole ring. Because good agreement was obtained between the calculated and experimental transitions for the observed isomers, reasonable ab initio transition energies can also be expected for the higher-energy isomers protonated on other carbon atoms, which should also absorb in the visible region. Protonation on the nitrogen atom leads to a transition that is blue-shifted with respect to that of the most stable isomer.

Protonated benzene dimer: an experimental and ab initio study.

The excitation spectrum of the protonated benzene dimer has been recorded in the 415-600 nm wavelength range. In contrast to the neutral iso-electronic benzene dimer, its absorption spectrum extends in the visible spectral region. This huge spectral shift has been interpreted with ab initio calculations, which indicate that the first excited states should be charge transfer states.

IR-UV investigation of the structure of the 1-phenylethanol chromophore and its hydrated complexes

The structure of the 1-phenylethanol molecule and its hydrated complexes has been investigated by means of laser-induced fluorescence and IR fluorescence-dip spectroscopy in the region of the OH vibration, coupled with DFT calculations. The isolated chromophore has a gauche conformation with the OH group slightly interacting with the aromatic cycle. In the singly hydrated complex, the water molecule acts as a proton acceptor from the OH group of the chromophore and is involved as a donor in the OH–π interaction with the aromatic ring. The 1:2 water complex consists of a water dimer acting as an acceptor from the OH group of 1-phenylethanol and as a donor to its aromatic ring.

Electronically excited states of protonated aromatic molecules: benzaldehyde

The photofragmentation spectrum of protonated benzaldehyde has been recorded in the 435-385 nm wavelength range. The first excited state is a pipi* state, strongly red shifted compared to the pipi* state of neutral benzaldehyde. The spectrum presents well resolved vibronic bands in contrast to some other protonated aromatic molecules like benzene or tryptophan in which the excited state dynamics is so fast that no vibrational structure can be observed. The bands can be assigned on the basis of a Franck-Condon analysis using ground and excited state frequencies calculated at the CC2/TZVP level.

Photolysis of methane revisited at 121.6 nm and at 118.2 nm: quantum yields of the primary products, measured by mass spectrometry.

Methane photolysis has been performed at the two Vacuum UltraViolet (VUV) wavelengths, 121.6 nm and 118.2 nm, via a spectrally pure laser pump-probe technique. The first photon is used to dissociate methane (either at 121.6 nm or at 118.2 nm) and the second one is used to ionise the CH(2) and CH(3) fragments. The radical products, CH(3)(X), CH(2)(X), CH(2)(a) and C((1)D), have been selectively probed by mass spectrometry. In order to quantify the fragment quantum yields from the mass spectra, the photoionisation cross sections have been carefully evaluated for the CH(2) and CH(3) radicals, in two steps: first, theoretical ab initio approaches have been used in order to determine the pure electronic photoionisation cross sections of CH(2)(X) and CH(2)(a), and have been rescaled with respect to the measured absolute photoionisation cross section of the CH(3)(X) radical. In a second step, in order to take into account the substantial vibrational energy deposited in the CH(3)(X) and CH(2)(a) radicals, the variation of their cross sections near threshold has been simulated by introducing the pertinent Franck-Condon overlaps between neutral and cation species. By adding the interpolated values of CH quantum yields measured by Rebbert and Ausloos [J. Photochem., 1972, 1, 171-176], a complete set of fragment quantum yields has been derived for the methane photodissociation at 121.6 nm, with carefully evaluated 1σ uncertainties: Φ[CH(3)(X)] = 0.42 ± 0.05, Φ[CH(2)(a)] = 0.48 ± 0.05, Φ[CH(2)(X)] = 0.03 ± 0.08, Φ[CH(X)] = 0.07 ± 0.01. These new data have been measured independently of the H atom fragment quantum yield, subject to many controversies in the literature. From our results, we evaluate Φ(H) = 0.55 ± 0.17 at 121.6 nm. The quantum yields for the photolysis at 118.2 nm differ notably from those measured at 121.6 nm, with a substantial production of the CH(2)(X) fragment: Φ[CH(3)(X)] = 0.26 ± 0.04, Φ[CH(2)(a)] = 0.17 ± 0.05, Φ[CH(2)(X)] = 0.48 ± 0.06, Φ[CH(X)] = 0.09 ± 0.01, Φ(H) = 1.31 ± 0.13. These new data should bring reliable and essential inputs for the photochemical models of the Titan atmosphere.