Supersonic expansions of pure and seeded rare gases have been investigated experimentally, measuring the translational and rotational temperatures. The lowest achievable translational temperature in the jet depends on both gas properties as well as on experimental boundary conditions like nozzle shape and nozzle–skimmer distance. We show that there is a limit to the lowest temperature achieved, under practical conditions, set by condensation in the jet. A large cluster binding energy enhances the formation of clusters and they release their condensation energy into the beam. The spatial confinement of the jet extends to long distances, and is sensitive to the shape of the nozzle. The confined jet forms a narrow cone of high intensity, and results in increased collision probability and cluster formation.

Infrared spectroscopy has been used to characterize the solvent effect on the OH stretching vibrations νOH of phenol, 1-naphthol, 2-naphthol, 1-hydroxypyrene, and ethanol. We distinguish the dielectric (nonspecific) effect of the solvent on ΔνOH, the observed red-shifts in νOH, from the much larger red-shift caused by direct hydrogen (H)-bonding interactions with the solvents. To isolate the solvent dielectric constant ε effect on νOH, the OH oscillator was also studied when it is already H-bonded with an invariant oxygen base, dimethyl sulfoxide. We find that ΔνOH depends importantly on ΔPA, the difference between the proton affinities of the conjugate base of the proton donor and the proton acceptor. For a given H-bonded complex, νOH tends to vary inversely with ε, exhibiting different slopes for polar and nonpolar solvents, i.e., solvents comprising molecules with and without a permanent dipole moment, respectively. We use a two-state valence-bond-based theory to analyze our experimental data. This demonstrates that the OH oscillator acquires a more ionic-like character in the vibrational excited state, i.e., charge transfer; this results in a stronger H-bond in a more anharmonic potential for the OH vibration. The theory distinguishes between nonpolar and polar solvents and successfully accounts for the observed 1/ε and ΔPA variations.

To assess the potential use of O−H stretching modes of aromatic alcohols as ultrafast local probes of transient structures and photoacidity, we analyze the response of the O−H stretching mode in the 2-naphthol-acetonitrile (2N−CH 3 CN) 1:1 complex after UV photoexcitation. We combine femtosecond UV-infrared pump−probe spectroscopy and a theoretical treatment of vibrational solvatochromic effects based on the Pullin perturbative approach, parametrized at the density functional theory (DFT) level. We analyze the effect of hydrogen bonding on the vibrational properties of the photoacid−base complex in the S 0 state, as compared to O−H stretching vibrations in a wide range of substituted phenols and naphthols covering the 3000−3650 cm −1 frequency range. Ground state vibrational properties of these phenols and naphthols with various substituent functional groups are analyzed in solvents of different polarity and compared to the vibrational frequency shift of 2N induced by UV photoexcitation to the 1 L b electronic excited state. We find that the O−H stretching frequency shifts follow a linear relationship with the solvent polarity function F 0 = (2ε 0 − 2)/(2ε 0 + 1), where ε 0 is the static dielectric constant of the solvent. These changes are directly correlated with photoacidity trends determined by reported pK a values and with structural changes in the O···N and O−H hydrogen-bond distances induced by solvation or photoexcitation of the hydrogen-bonded complexes.

Infrared spectroscopy measurements were used to characterize the OH stretching vibrations in a series of similarly structured fluoroethanols, RCH2OH (R = CH3, CH2F, CHF2, CF3), a series which exhibits a systematic increase in the molecule acidity with increasing number of F atoms. This study, which expands our earlier efforts, was carried out in non-hydrogen-bonding solvents comprising molecules with and without a permanent dipole moment, with the former solvents being classified as polar solvents and the latter designated as nonpolar. The hydrogen bond interaction in donor-acceptor complexes formed in solution between the fluorinated ethanol H-donors and the H-acceptor base DMSO was investigated in relation to the solvent dielectric and to the differences ΔPA of the gas phase proton affinities (PAs) of the conjugate base of the fluorinated alcohols and DMSO. We have observed that νOH decreases as the acidity of the alcohol increases (ΔPA decreases) and that νOH varies inversely with ε, exhibiting different slopes for nonpolar and polar solvents. These 1/ε slopes tend to vary linearly with ΔPA, increasing with increasing acidity. These experimental findings, including the ΔPA trends, are described with our recently published two-state Valence Bond-based theory for acid-base H-bonded complexes. Lastly, the correlation of the alcohol's conjugate base PAs with Taft σ* values of the fluorinated ethyl groups CH(n)F(3-n)CH2- provides a connection of the inductive effects for these groups with the acidity parameter ΔPA associated with the H-bonded complexes.

No abstract