A. Zehnacker-Rentien

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.

Chirality influence on the aggregation of methyl mandelate

The methyl ester of mandelic acid is investigated by a wide range of techniques to unravel its aggregation pattern and the influence of relative chirality of the aggregating monomers. Matrix isolation confirms that a single monomer conformation prevails. The electronic spectrum of the dimers is strongly affected by the relative monomer chirality. Vibrational effects are more subtle and can be explained in terms of the most stable homo- and heteroconfigurational dimer structures, when compared to results of MP2 and DFT-D computations. Selective IR/UV double resonance techniques and wide-band FTIR spectroscopy provide largely consistent spectroscopic fingerprints of the chirality discrimination phenomena. The dominant homochiral dimer has two intermolecular O–H⋯OC hydrogen bonds whereas the more strongly bound heterochiral dimer involves only one such hydrogen bond. This is a consequence of the competition between dispersion and intramolecular or intermolecular hydrogen bonding. Aromatic interactions also play a role in trimers and larger clusters, favoring homochiral ring arrangements. Analogies and differences to the well-investigated methyl lactate system are highlighted. Bulk phases show a competition between different hydrogen bond patterns. The enantiopure, racemic, and 3 : 1 crystals involve infinite hydrogen-bonded chains with different arrangements of the aromatic groups. They exhibit significantly different volatility, the enantiopure compound being more volatile than the racemic crystal. The accumulated experimental and quantum-chemical evidence turns methyl mandelate into a model system for the role of competition between dispersion forces and hydrogen bond interactions in chirality discrimination.

Chiral recognition in cinchona alkaloid protonated dimers: mass spectrometry and UV photodissociation studies.

Chiral recognition in protonated cinchona alkaloid dimers has been studied in mass spectrometry experiments. The experimental setups involved a modified 7T FT-ICR (Fourier transform-ion cyclotron resonance) mass spectrometer (MS) and a modified Paul ion trap both equipped with an electrospray ionization source (ESI). The Paul ion trap has been coupled to a frequency-doubled dye laser. The fragmentation of protonated dimers made from cinchonidine (Cd) and the two pseudoenantiomers of quinine, namely, quinine (Qn) and quinidine (Qd), has been assessed by means of collision-induced dissociation (CID) as well as UV photodissociation (UVPD). Whereas CID fragmentation of the dimers only leads to the evaporation of the monomers, UVPD results in the additional loss of a neutral radical fragment corresponding to the quinuclidinyl radical. The effect of the excitation wavelength and of complexation with H(2)SO(4) has been studied to cast light on the reaction mechanism. Complexation with H(2)SO(4) modifies the photoreactivity of the dimers; only evaporation of the monomeric fragments, quinine, and cinchonidine is observed. Comparison between the mass spectra of the cinchona alkaloid (CdQnH(+)) or (CdQdH(+)) dimers resulting from the UVPD of (CdQnH(2)SO(4)H(+)) and that of bare (CdQnH(+)) helps propose a fragmentation mechanism, which is thought to involve fast proton transfer from the quinuclidine part of a molecular subunit to the quinoline ring. CID and UV fragmentation experiments show that the homochiral dimer is more strongly bound than the heterochiral adduct.

Conformational Analysis of Quinine and Its Pseudo Enantiomer Quinidine: A Combined Jet-Cooled Spectroscopy and Vibrational Circular Dichroism Study

Laser-desorbed quinine and quinidine have been studied in the gas phase by combining supersonic expansion with laser spectroscopy, namely, laser-induced fluorescence (LIF), resonance-enhanced multiphoton ionization (REMPI), and IR-UV double resonance experiments. Density funtional theory (DFT) calculations have been done in conjunction with the experimental work. The first electronic transition of quinine and quinidine is of π-π* nature, and the studied molecules weakly fluoresce in the gas phase, in contrast to what was observed in solution (Qin, W. W.; et al. J. Phys. Chem. C2009, 113, 11790). The two pseudo enantiomers quinine and quinidine show limited differences in the gas phase; their main conformation is of open type as it is in solution. However, vibrational circular dichroism (VCD) experiments in solution show that additional conformers exist in condensed phase for quinidine, which are not observed for quinine. This difference in behavior between the two pseudo enantiomers is discussed.

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.

The o-cyanophenol dimer as studied by laser-induced fluorescence and IR fluorescence dip spectroscopy: a study of a symmetrical double hydrogen bond

Abstract The o -cyanophenol dimer has been characterised under supersonic-jet conditions by laser-induced fluorescence and IR–UV depletion spectroscopy associated with density functional theory (DFT) calculations. The ground state structure consists of a symmetrical, planar, doubly hydrogen-bonded bridge linking the phenolic OH groups to the N atom of the CN substituents. The origin of the S 0 →S 1 transition is red shifted by 1094 cm −1 with respect to the monomer. Active vibrations in both electronic states are measured from the excitation and dispersed fluorescence spectra and are discussed on the basis of harmonic frequency DFT calculations. The IR depletion spectrum exhibits a very strong dip at 3322 cm −1 , which is assigned to the out-of-phase combination (b u ) of the ν (OH) stretching modes. This main band is accompanied by three bands of lower intensity at 3391, 3406 and 3444 cm −1 . Their attribution is discussed in terms of the coupling with the intermolecular modes or of possible Fermi resonances.

Vibrational study of the S0 and S1 states of 2-naphthyl-1-ethanol/(water)2 and 2-naphthyl-1-ethanol/(methanol)2 complexes by IR/UV double resonance spectroscopy

2-naphthyl-1-ethanol/(water)2 and 2-naphthyl-1-ethanol/(methanol)2 complexes have been formed in a supersonic expansion and studied by laser induced fluorescence (LIF) and IR/UV double resonance spectroscopy in the region of the O–H stretch. Comparison of the measured frequencies with DFT calculations has led to the attribution of the observed 2-naphthyl-1-ethanol/(water)2 and 2-naphthyl-1-ethanol/(methanol)2 complexes to bridged structures. In these structures the solvent dimer is inserted in between the OH group of the chromophore which acts as a hydrogen bond donor, and the electron π system of the aromatic ring. Electronic excitation results in a red shift of all the νOH stretch frequencies.

Fluorescence spectroscopy of jet-cooled chiral (±)-indan-1-ol and its cluster with (±)-methyl- and ethyl-lactate

The laser-induced fluorescence excitation, dispersed fluorescence, and IR-UV double resonance spectra of chiral (+/-)-indan-1-ol have been measured in a supersonic expansion. Three low energy conformers of the molecule have been identified, and the ground state vibrational modes of each conformer are tentatively assigned with the aid of quantum chemistry calculations. The frequencies of the nu(OH) and nu(CH) stretch modes of the two most abundant conformers have been measured by fluorescence dip IR spectroscopy and have been used for their assignment. The dispersed fluorescence spectra clearly indicate the coupling of low-frequency modes, as was seen in other substituted indanes such as 1-aminoindan and 1-amino-2-indanol. (R)- and (S)-indan-1-ol distinctly form different types of clusters with (R)- and (S)- methyl- and ethyl-lactate. Both hetero- and homochiral clusters are characterized by complex spectra which exhibit a progression built on low-frequency intermolecular modes.