Water-olefin complexes. A matrix isolation study

Unanticipated Hydrophobicity Increases of Squalene and Human Skin Oil Films Upon Ozone Exposure

The C-H and O-H oscillators on the surfaces of thin films of human-derived skin oil and squalene are probed under ambient conditions (300 K, 1 atm total pressure, 40% RH) using second-order vibrational spectroscopy and contact angle goniometry before and after exposure to ppb amounts of ozone. Skin oil and squalene are found to produce different vibrational sum frequency generation spectra in the C-H stretching region, while exposure to ozone results in surface spectra for both materials that is consistent with a loss of C-H oscillators. The measured contact angles show that the hydrophobicity of the films increases following exposure to ozone, consistent with the reduction in C═C···H₂O ("πH") bonding interactions that is expected from C═C double bond loss due to ozonolysis and indicating that the polar functional groups formed point toward the films' interiors. Implications for heterogeneous indoor chemistry are discussed.

Unravelling the microhydration frameworks of prototype PAH by infrared spectroscopy: naphthalene–(water)1–3

Microhydration structures of the prototypical PAH, naphthalene, are probed by IR spectroscopy in helium droplets. The sequential water addition produces an extended hydrogen-bonded hydration network bound via π hydrogen bond to the aromatic ring.

Dominance of unique Pπ phosphorus bonding with π donors: evidence using matrix isolation infrared spectroscopy and computational methodology

Albeit the first account of hypervalentπ interactions has been reported with halogenπ interactions, the feasibility of their extension to other hypervalent atoms as possible Lewis acids is still open. In this work, the role of phosphorus as an acceptor from the π electron cloud (Pπ pnicogen or phosphorus bonding) in PCl3-C2H2 and PCl3-C2H4 heterodimers is explored, by combining matrix isolation infrared spectroscopy with ab initio and DFT computational methodologies. The respective potential energy surfaces of the PCl3-C2H2 and PCl3-C2H4 heterodimers reveal unique minima stabilized by a concert of reasonably strong to weak interactions, of which Pπ phosphorus bonding was energetically dominant. Heterodimers, trimers and tetramers bound primarily by this unique phosphorus bond were generated at low temperatures. The dominance of phosphorus bonding in the PCl3-C2H2 and PCl3-C2H4 heterodimers over other interactions (such as Hπ, HCl, HP, Clπ and lone pair-π interactions) was confirmed and substantiated using extended quantum theory of atoms in molecules, natural bond orbital, electrostatic potential mapping and energy decomposition analyses. The following inferences in correlation with results from non-covalent-interaction analysis offer a complete understanding of the nature of the Pπ phosphorus bonding interactions. The significance of electrostatic forces kinetically favoring the formation of phosphorus bonded heterodimers, in addition to thermodynamic stabilization, is demonstrated experimentally.

Infrared spectroscopic characterization of hydrogen-bonded propylene oxide − ethanol and propylene oxide − 2-fluoroethanol complexes isolated in solid neon matrices

The infrared absorption spectra of hydrogen-bonded complexes of propylene oxide with either ethanol or 2-fluoroethanol have been recorded in neon matrices. Mixtures of propylene oxide and ethanol or propylene oxide and 2-fluoroethanol vapors were mixed with an excess of neon gas and deposited onto a KBr substrate at 4.2 K. The results indicate that hydrogen-bonded complexes were formed with propylene oxide as the hydrogen bond acceptor and either ethanol or 2-fluoroethanol as the hydrogen bond donors. The features assigned to the O−H stretch were red-shifted by 175 and 193 cm−1 for the ethanol- and 2-fluoroethanol-containing complexes, respectively. The difference in red shifts can be accounted for due to the greater acidity of 2-fluroethanol. Deuterium isotope experiments were conducted to help confirm the assignment of the O–H stretch for the complexes. As well, structures and infrared spectra were calculated using B3LYP/6-311++G(2d,2p) calculations and were used to compare with the experimental spectra. A “scaling equation” rather than a scaling factor was used and is shown to greatly increase the utility of the calculations when comparing with experimental spectra. An examination of the O–H stretching red shifts for many hydrogen-bound complexes reveals a relationship between the shift and the difference between the acidity of the hydrogen bond donor and the basicity of the hydrogen bond acceptor (the enthalpy of proton transfer). Both hydrogen-bonded complexes and proton-bound complexes appear to have a maximum in the reduced frequency value that corresponds to complexes where the hydrogen/proton are equally shared between the two bases.

A deuterated deep-cavity cavitand confirms the importance of C–H⋯X–R hydrogen bonds in guest binding

A deuterated cavitand host was examined for its affinity to a series of guests; for halogenated, preorganized guests binding was significantly stronger than the corresponding protium host.

FTIR and ab Initio Study of the 1/1 Complex between Water and Carbon Dioxide in Solid Nitrogen

Infrared spectra of the 1/1 complex between water and carbon dioxide isolated in a nitrogen matrix are reported. Isotopic substitution of water (HOD, D2O) suggests the existence of two very weak complexes with a nearly T-shaped structure where the C atom is bonded to the water oxygen: one of C2v symmetry and another one of Cs symmetry, in which a slight distortion results in a very weak interaction between one hydrogen atom of water and one oxygen atom of carbon dioxide. This result is supported by force constant calculations, consistent with a double, symmetrical, and unsymmetrical force field, and ab initio calculations. At the MP2/aug-cc-PVTZ level corrected from basis set superposition error the C2v structure is a saddle point with an imaginary frequency for H2O rocking, in contrast to a Cs structure, which would have nonequivalent hydrogen atoms. Nevertheless, the energies of both structures, with a stabilization energy of 1.97 kcal mol-1, differ by less than 0.01 kcal mol-1 from each other. A comparison of calculated and observed frequency shifts allows us to suggest the presence in the matrix of both C2v and Cs complexes.

Observation of the π···H Hydrogen-Bonded Ternary Complex, (C2H4)2H2O, Using Matrix Isolation Infrared Spectroscopy

FTIR absorption spectra of water-containing ethene:Ar matrices, with compositions of ethene up to 1:10 ethene:Ar, have been recorded. Systematically increasing the concentration of ethene reveals features in the spectra consistent with the known 1:1 ethene:water complex, which subsequently disappear on further increase in ethene concentration. At high concentrations of ethene, new features are observed at 3669 and 3585 cm(-1), which are red-shifted with respect to matrix-isolated nu(3) and nu(1) O-H stretching modes of water and the 1:1 ethene:water complex. These shifts are consistent with a pi...H interaction of a 2:1 ethene:water complex of the form (C(2)H(4)...H-O-H...C(2)H(4)). The analogous (C(2)D(4))(2)H(2)O complex shows little shifting from positions associated with (C(2)H(4))(2)H(2)O, while the (C(2)H(4))(2)D(2)O isotopomer shows large shifts to 2722.3 and 2617.2 cm(-1), having identical nu(3)(H(2)O)/nu(3)(D(2)O) and nu(1)(H(2)O)/nu(1)(D(2)O) values when compared with monomeric water isotopomers. Features at 3626.1 and 2666.2 cm(-1) are also observed and are attributed to (C(2)H(4))(2)HDO. DFT calculations at the B3LYP/6-311+G(d,p) level for each isotopomer are presented, and the predicted vibrational frequencies are directly compared with experimental values. The interaction energy for the formation of the 2:1 ethene:water complex from the 1:1 ethene:water complex is also presented.

Calculation of isotope effects from first principles

Various means of calculating the effect of changing the mass of a given atom upon a chemical process are reviewed. Of particular interest is the deuterium isotope effect comparing the normal protium nucleus with its heavier deuterium congener. The replacement of the bridging protium in a neutral hydrogen bond such as the water dimer by a deuterium strengthens the interaction by a small amount via effects upon the vibrational energy. In an ionic H-bond such as the protonated water dimer, on the other hand, the reverse trend is observed in that replacement of the bridging protium by dimer weakens the interaction. In addition to the stability of a given complex, the rate at which a proton transfers from one group to another is likewise affected by deuterium substitution, viz. kinetic isotope effects (KIEs). The KIE is enlarged as the temperature drops, particularly so if the calculation of KIE includes proton tunneling. The KIE is also sensitive to any angular distortions or stretches present in the H-bond of interest. KIEs can be computed either by the standard transition state theory which is derived via only two points on the potential energy surface, or by more complete formalisms which take account of larger swaths of the surface. While more time intensive, the latter can also be applied to provide insights important in interpretation of experimental data.