S. Chervenkov

Mass-analyzed threshold ionization spectroscopy of ortho fluorinated 2-phenylethanol: identification of an additional gauche conformer.

The cationic ground state of the ortho fluorinated 2-phenylethanol has been investigated by combination of mass-analyzed threshold ionization (MATI) spectroscopy and quantum chemistry ab initio density functional theory (DFT) calculations employing the hybrid functional M05 with cc-pVDZ basis set. The MATI spectra measured via vibronic bands in the S1 intermediate state of the most stable gauche conformer stabilized by an intramolecular OH···π hydrogen bond are structureless. The MATI spectrum recorded via a small band blueshifted by 3 cm−1 from the 0(0)(0) electronic origin of the gauche conformer features well-resolved peaks and is assigned to a cationic gauche structure without an OH···π bond. The ab initio calculations are qualitatively consistent with the experimental observations and show that the presumable conformer giving rise to the observed MATI spectrum retains its structure during ionization, whereas the lowest-energy gauche conformer undergoes a significant structural change resulting in a break of the OH···π bond, thus leading to unfavorable Franck−Condon factors for the threshold ionization.

Specific and nonspecific interactions in a molecule with flexible side chain: 2-phenylethanol and its 1:1 complex with argon studied by high-resolution UV spectroscopy

Using high-resolution resonance-enhanced two-photon ionization spectroscopy in combination with genetic-algorithm-based computer-aided rotational fit analysis and ab initio quantum chemistry calculations we determined the conformational structure and transition moment orientation in 2-phenylethanol and its 1:1 clusters with argon. The results clearly demonstrate that the gauche structure of 2-phenylethanol, which is stabilized by the intramolecular pi-hydrogen bond between the folded side chain and the benzene ring, is the most abundant in the cold molecular beam. In this conformer the transition moment is rotated by 18 degrees from the short axis of the aromatic ring. Two distinct 1:1 complexes of 2-phenylethanol with argon in a cis- and trans-configuration with respect to the side chain have been found. Employing the Kraitchman [Am. J. Phys. 21, 17 (1953)] analysis we have found that the structure of the 2-phenylethanol moiety and the orientation of the transition moment do not change after the complexation with argon within the experimental accuracy. From the measured band intensities we conclude that in addition to the dispersion interaction of the argon atom with the aromatic ring a hydrogen-bond-type interaction with the terminal -OH group of the side chain stabilizes the cis-structure of the 1:1 complex of 2-phenylethanol with argon.

Mass-Analyzed Threshold Ionization Spectroscopy of 2-Phenylethanol: Probing of Conformational Changes Caused by Ionization

The vibrational structure of the ionic ground state of different conformers of the biologically relevant molecule 2-phenylethanol has been investigated by combination of two-photon two-color mass-analyzed threshold ionization spectroscopy (MATI) and quantum chemical calculations at M05, MP2, and coupled cluster (CC) levels of theory with extended basis sets. MATI spectra recorded via gauche vibronic bands are with poor structure and increasing background, whereas the ones measured via vibronic bands of the anti conformers feature well-resolved vibronic structure in the cation. Ab initio computations predict three stable conformers for the 2-phenylethanol cation out of five initial neutral structures. None of the theoretical structures in the cation features a nonclassical OH...pi hydrogen bond in conjunction with the analysis of the MATI spectra. This provides clear evidence that the OH...pi hydrogen bond stabilizing the lowest-energy gauche conformer in the neutral breaks upon ionization.

Identification of Conformational Structures of 2-Phenylethanol and Its Singly Hydrated Complex by Mass Selective High-Resolution Spectroscopy and ab Initio Calculations†

The flexible prototype molecule 2-phenylethanol (2-PE) and its singly hydrated complex have been investigated in a cold supersonic beam by a combination of high-resolution two-color R2PI spectroscopy and quantum chemistry ab initio calculations. The existence of two monomer structures separated by a high potential energy barrier, gauche and anti ones, was proven. Higher energy conformers are supposed to relax to the observed ones during the jet expansion process. We have identified the conformational structure of the complex between 2-PE and water, which corresponds to water binding to the most stable gauche conformer. No detectable structural changes of the host 2-PE molecule have been observed upon attachment of a single water molecule. A conformational relaxation mechanism is suggested also for the 2-PE x H2O complex.

High-resolution ultraviolet spectroscopy of p-fluorostyrene-water: Evidence for a σ-type hydrogen-bonded dimer

Ab initio calculations predict four stable conformational structures of the singly hydrated cluster of p-fluorostyrene: two out of plane with pi- and two in plane with sigma-type intermolecular hydrogen bonding between p-fluorostyrene and water. We employed mass-selective resonance-enhanced two-photon ionization high-resolution (70-MHz FWHM laser bandwidth) spectroscopy to partially resolve the rotational structure of the 0(0) (0) origin band of the S(1) <--S(0) electronic transition. A computer-aided fit based on genetic algorithms was used to analyze the experimental high-resolution spectrum and to determine the observed conformational structure. The good agreement between the experimental and the simulated spectra of the 0(0) (0) band and the assignment of the other prominent bands as inter- and intramolecular vibrational progressions clearly demonstrates that the anti in-plane conformer is the most abundant one in the molecular beam. The existence of the sigma-type hydrogen bond between p-fluorostyrene and water manifests that the electron attracting effect of fluorine dominates over the releasing mesomeric effect of the vinyl group and thus a pi-type hydrogen bonding with the aromatic ring is not favored in this case.

Evidence for a C–H⋯π type weak interaction: 1 : 1 complex of styrene with acetylene studied by mass selective high-resolution UV spectroscopy and ab initio calculations

The 1 : 1 complex of styrene with acetylene has been studied by mass selective low- and high-resolution UV resonance-enhanced two-photon ionisation (R2PI) spectroscopy combined with genetic-algorithm-based computer-aided fit of the spectra with partial rotational resolution, and high level ab initio quantum chemistry calculations. Two stable conformeric geometries of the 1 : 1 complex of styrene and acetylene have been theoretically found: one with acetylene binding to styrene as a proton donor, and one with acetylene acting as a proton acceptor. From the analysis of the vibronic structure of the S1<-- S0 spectrum and the fit of the highly resolved spectrum of the 0 origin band of the complex it is shown that the favoured conformation is the one in which acetylene binds to the benzene ring of styrene through formation of a non-conventional hydrogen bond of C-H...pi type with no marked change of the transition moment orientation of styrene. The styrene moiety remains planar and the acetylene molecule is tilted by a small angle of 4 degrees relative to the C6 symmetry axis of the benzene ring, most likely due to the reduced symmetry of the benzene ring pi electrons rather than to a direct interaction with the vinyl group.

High-resolution mass-selective UV spectroscopy of pseudoephedrine: evidence for conformer-specific fragmentation.

Using resonance-enhanced two-photon ionization spectroscopy with mass resolution of jet-cooled molecules, a low-resolution S(1) ← S(0) vibronic spectrum of pseudoephedrine was recorded at the mass channels of three distinct fragments with m/z = 58, 71, and 85. Two of the fragments, with m/z = 71 and 85, are observed for the first time for this molecule. The vibronic spectra recorded at different mass channels feature different patterns, implying that they originate from different conformers in the cold molecular beam, following conformer-specific fragmentation pathways. Highly resolved spectra of all prominent vibronic features were measured, and from their analysis based on genetic algorithms, the molecular parameters of the conformers giving rise to the respective bands have been determined. Comparing the experimental results with those obtained from high-level ab initio quantum chemistry calculations, the observed prominent vibronic bands have been assigned to originate from four distinct conformers. The conformers are separated into two groups that have different fragmentation pathways determined by the different intramolecular interactions.

Competition between π and σ hydrogen bonds and conformational probing of 2-orthofluorophenylethanol by low-and high-resolution electronic spectroscopy

The flexible model molecule 2-orthofluorophenylethanol has been investigated by laser-induced fluorescence, and low- and high-resolution resonance-enhanced two-photon ionization spectroscopy in combination with high-level ab initio quantum chemistry calculations. One dominant conformation has been identified in the cold molecular beam corresponding to the most stable theoretically predicted gauche structure stabilized by an intramolecular OH...pi hydrogen bond. A tentative assignment of a higher-lying gauche conformer present in the molecular beam separated by high potential barriers from the most stable one has been made. The missing other higher-energy theoretically predicted conformations most likely relax to the most stable ones during the process of the adiabatic expansion. The good agreement between the experimental and theoretical results demonstrates that even in the case of a substitution with an electronegative atom at the ortho position, bringing about a significant redistribution of the electron density in the benzene ring and providing a convenient binding site for the formation of a competing OH...F sigma hydrogen bond, the nonclassical OH...pi bond remains the preferred binding motif for the most stable conformer.

High resolution UV resonance enhanced two-photon ionization spectroscopy with mass selection of biologically relevant molecules in the gas phase

The high resolution Doppler-free resonance-enhanced two-photon ionization (R2PI) spectroscopy with mass selection of jet-cooled (2-12 K) molecular species is a powerful experimental method providing comprehensive information on both isolated molecules and molecular clusters. We have demonstrated for the first time that this technique can be applied to large molecules and provides detailed information on their conformational structure. It allows rotationally resolved (FWHM = 70 MHz) spectra of the vibronic bands of the S1←S0 electronic transition of the studied molecular systems to be measured. A specially designed computer-assisted fitting routine based on genetic algorithms is used to determine their rotational constants in the ground and excited electronic states, respectively, and the transition moment ratio. To interpret the experimental information and to discriminate and unambiguously assign the observed approach to the study of the neurotransmitter molecule, ephedrine. The results elucidate the role of the intramolecular hydrogen bonds stabilizing the respective conformations and affecting their intrinsic properties.

Water binding sites in 2-para- and 2-ortho-fluorophenylethanol: A high-resolution UV experiment and ab initio calculations

The singly hydrated complexes of the flexible prototype molecules 2-para-fluorophenylethanol and 2-ortho-fluorophenylethanol have been investigated by combination of high-resolution resonance-enhanced two-photon ionization spectroscopy in a cold supersonic beam and quantum chemistry ab initio calculations. We have identified the conformational structures of the above complexes, which correspond to water binding to the most stable gauche monomers conformers in both cases. No structural changes of the host molecules upon the attachment of a single water molecule have been found. For the 2-ortho-fluorophenylethanol-water complex we have observed an additional structure with one of the higher-in-energy gauche conformers of the monomer. This corroborates the assumption that the complexation with water stabilizes the higher-energy conformer of the monomer, precluding it from relaxation to the lowest-energy geometry.