F. Lahmani

Proton-transfer reaction dynamics

In this article we discuss the progress made in understanding intermolecular and intermolecular reactions of proton (or hydrogen-atom) transfer. Femtosecond real-time probing, together with spectroscopic studies, in molecular beams are presented with selected examples of reactions. Reaction rates, tunneling dynamics and the nature of the reaction coordinate are examined and related to two-state multidimensional potential energy surfaces.

Luminescence and radiationless transitions from single vibronic levels of the isolated pyrazine molecule in the S1(n,π*) state

Luminescence spectra, fluorescence, and phosphorescence yields were measured for pyrazine excited to six different vibronic levels of the S 1(n,π*) state in pure pyrazine vapor and in gaseous pyrazine‐SF6 mixtures. The vibrational analysis of the fluorescence spectrum is given. As the total gas pressure increases from 10−2 to several torr, the fluorescence yield decreases and reaches a constant pressure‐independent value almost identical with that observed in condensed phases. The amount of this effect depends on the energy of the excited vibronic level. The phosphorescence yield, equal to zero at the low‐pressure limit, increases with gas pressure. The deviation from the Stern‐Volmer law for the fluorescence quenching is explained by the nonexponential decay of the fluorescence, evidenced by direct measurements of the decay curves. The results are discussed in terms of the kinetic and quantum‐mechanical models of radiationless transitions in intermediate‐size molecules.

Enantiodifferentiation in jet-cooled van der Waals complexes of chiral molecules

Abstract Laser-induced fluorescence excitation spectroscopy in a supersonic expansion is used to evidence intermolecular chiral discrimination at the microscopic level. Measurements have been performed in (R)-(+)- and (S)-(−)-α-methyl-2-naphthalenemethanol (a naphthalene compound made chiral by the asymmetric carbon in the 2 position) in the presence of alcoholic solvents (methanol, 1-propanol, and (S)-(+)-chloro-1-propanol). While identical spectral shifts are observed for 1:1 complexes of (R)-(+)- and (S)-(−)-α-methyl-2-naphthalenemethanol with non-chiral solvents, complexation of each enantiomer with the chiral (S)-(+)-2-chloro-1-propanol molecule gives rise to different solvent shifts and allows differentiation between the two forms in the racemic mixture.

Solvation effects in jet-cooled 7-hydroxyquinoline

An investigation of the laser-induced fluorescence of jet-cooled 7-hydroxy quinoline and its complexes with water and methanol is presented. The free molecule exhibits two electronic transitions which are due to rotational isomers. The 1:1 complexes present a moderate red spectral <500 cm−1 shift and a free-molecule-like fluorescence. Higher clusters with H2O and CH3OH lead to an unusually large red-shift (≈2000 cm−1 and to a strong modification of the vibrational pattern in excitation and dispersed emission spectra. No excited state tautomerization was found in these clusters. The structures of the hydrogen-bonded clusters are discussed in the context of these data.

Excited-state intramolecular proton transfer in jet-cooled 1-hydroxy-2-acetonaphthone

Abstract The fluorescence excitation and dispersed emission spectra of the intramolecularly hydrogen bonded 1-hydroxy-2-acetonaphthone (HAN) isolated molecule have been investigated in a free jet expansion. The excitation spectrum is well resolved until 900 cm −1 above the origin located at 388.6 nm. Above this excess energy no fluorescence is observed indicating the onset of an efficient nonradiative process. The emission from the vibrationless level of the S 1 state exhibits a dual character. The short wavelength fluorescence is highly structured and its vibrational structure correlates with that of the S 1 state. The long wavelength part shifted by 2200 cm −1 is more intense and diffuse. Deuteration of the hydroxy proton removes totally the short wavelength part of the emission while the long wavelength part remains unchanged. The results are interpreted in terms of excited state intramolecular proton (or hydrogen atom) transfer from the initial enol form to the tautomeric keto conformer through a small energy barrier between two closely lying local potential minima.

The role of chirality in the competition between inter and intramolecular hydrogen bonds: jet-cooled van der Waals complexes of (±)2-naphthyl-1-ethanol with (±)1-amino-2-propanol and (±)2-amino-1-butanol

The structure of jet-cooled complexes of (±) 2-naphtyl-1-ethanol with (±) 1-amino-2-propanol and (±) 2-amino-1-butanol has been studied by laser-induced fluorescence, UV and IR fluorescence dip spectroscopy, combined with DFT calculations. Chiral discrimination has been evidenced both in the electronic and vibrational spectra. The homochiral complex with 1-amino-2-propanol displays two distinct kinds of structure: an O-addition complex in which the chromophore adds to the alcohol group of 1-amino-2-propanol, and a N-addition complex in which the chromophore binds to the amino group of 1-amino-2-propanol. In contrast, the heterochiral complex with 1-amino-2-propanol, or both complexes with 2-amino-1-butanol, are of O-addition type. Deeper analysis of the O-addition structures, within the frame of the Natural Bond Orbital theory, has shown that the difference in the vibrational spectrum of the homo- and heterochiral complexes can be rationalised in terms of a different shift from linearity of the intermolecular H-bond, which results in a difference in electron density transfer along the H bond. Last, the study of the vibrational spectrum of the electronic excited state shows that the hydrogen bond network is reinforced upon electronic excitation.

Binding energy of hydrogen-bonded complexes of the chiral molecule 1-phenylethanol, as studied by 2C-R2PI: comparison between diastereoisomeric complexes with butan-2-ol and the singly hydrated complex

The relative gas phase binding energy of the two diastereoisomeric complexes of R-1-phenylethanol with R- and S-butan-2-ol formed in a supersonic expansion has been obtained from fragmentation measurements following two-colour resonance two-photon ionisation of the complex. The homochiral species (Rr) is found to be more stable than the heterochiral complex (Rs) by about 0.7 kcal mol−1. The present study also points out two possible reasons for an underestimation of the absolute binding energy when using a photofragmentation technique: (i) when the adiabatic ionisation threshold of the chromophore (here, 1-phenylethanol) is not accessible by photoionisation because of a too large geometry change between the neutral species and the ion, only lower limits of the binding energies can be drawn; and (ii) when the solvent (here, butan-2-ol) exists under different conformers, the most stable form of the complex can involve a conformation different from the most stable one in the isolated fragment. The lowest-energy fragmentation channel in the ion nevertheless corresponds to the formation of the most stable form of the fragments. In the present case, this latter effect may contribute to an anomalous low binding energy of the complexes of 1-phenylethanol with butan-2-ol compared to the 1:1 hydrate.

Chiral recognition between lactic acid derivatives and an aromatic alcohol in a supersonic expansion: electronic and vibrational spectroscopy

Jet-cooled diastereoisomeric complexes formed between a chiral probe, (+/-)-2-naphthyl-1-ethanol, and chiral lactic acid derivatives have been characterised by laser-induced fluorescence and IR fluorescence-dip spectroscopy. Complexes with non chiral alpha-hydroxyesters and chiral beta-hydroxyesters have also been studied for the sake of comparison. DFT calculations have been performed to assist in the analysis of the vibrational spectra and the determination of the structures. The observed 1 : 1 complexes correspond to the addition of the hydroxy group of the chromophore on the oxygen atom of the hydroxy in alpha-position relative to the ester function. Moreover, (+/-)-methyl lactate and (+/-)-ethyl lactate complexes with (+/-)-2-naphthyl-1-ethanol show an enantioselectivity in the size of the formed adducts: while fluorescent 1 : 1 complexes are the most abundant species observed when mixing (S)-2-naphthyl-1-ethanol with (R)-methyl or ethyl lactate, they are absent in the case of the SS mixture, which only shows 1 : 2 adducts. This property has been related to steric hindrance brought by the methyl group on the hydroxy-bearing carbon atom.

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.

Localization of electronic and vibrational energy in the jet-cooled m-cyanophenol/o-cyanophenol dimer: laser induced fluorescence and fluorescence-dip IR spectra

The mixed o-cyanophenol/m-cyanophenol dimer has been studied by laser-induced fluorescence and infrared fluorescence dip spectroscopy, in the region of the hydride stretch. DFT calculations have been carried out in conjunction with the experimental work. The most stable form of the dimer contains o-cyanophenol in its trans form and the cis isomer of m-cyanophenol. It exhibits a planar structure bound by a double hydrogen bond, similar to that observed in the homo (o-cyanophenol)2 or (m-cyanophenol)2 dimers. Its vibrational spectrum shows two strong bands assigned to the v(OH) stretch modes localized on each sub-unit. Electronic excitation localized on each moiety has been identified on the basis of dispersed fluorescence spectra. The lower-energy electronic state corresponds to the excitation of the cis m-cyanophenol sub-unit, while that localized on the trans o-cyanophenol moiety occurs 215 cm−1 higher in energy. No electronic energy transfer has been evidenced following excitation of the mixed dimer, up to 409 cm−1 excess energy.