Sum-frequency spectroscopy and ellipsometry have been used to study monolayers of the nonionic surfactants poly(ethylene glycol) monododecyl ethers (C12E m ; m = 2−8) at the air−water interface. SF spectra were acquired for areas of 30−70 Å2 per molecule, as determined from literature adsorption isotherms. These spectra show an increase in conformational disorder with increasing area per molecule and, surprisingly, an apparent decrease in the angle of tilt of the methyl group. There was no systematic variation in the SF spectra as a function of the length of the poly(ethylene glycol) chain at fixed areas of 45 and 62 Å2 per molecule. For m ≥ 4, the ellipsometric data suggested that the density of the hydrocarbon region of the chain was independent of m at fixed coverage but that the density decreased for m = 2 and 3. This study suggests that the value of m does not, per se, affect the structure of a monolayer of C12E m at the air−water interface for m = 4−8.

The ring-closure reaction of M(C0)5L-L (M = W, Mo) where L-L denotes a bidentate ligand was chosen for its comparative independence of solvent properties to demonstrate the repulsive (or intrinsic) part of the activation volume. The ring-closure reaction rate constant was determined in supercritical CO;? and in supercritical ethane over the range between 1000 and 3500 psi at several temperatures (35, 42, 48, and 60 "C). The activation volume was observed to be as high as +7000 cm3/mol just above the critical point in each of these solvents. The ring-closure activation volume was also determined in liquid C02 and in several other liquid solvents for comparison. These studies demonstrate a huge repulsive contribution to the activation volume in these two supercritical fluids.

Infrared emission from the ν2 bending mode and ν3 C–H stretching mode of HCN have been observed following 193 nm pulsed excimer laser photolysis of 1,3,5-triazine. Using a simple harmonic oscillator analysis, the number of ν2 bending quanta produced in HCN from photolysis of sym-triazine was found to be 70 times larger than the number of ν3 C–H stretching quanta. The combination of a high density of bending vibrational states in HCN and favorable geometry changes which occur in going from 1,3,5-triazine to three HCN molecules, appear to give an unusually pure distribution which maximizes vibrational energy in the HCN bending mode.

Significant line-narrowing has been observed in the laser-induced fluorescence spectra of naphthalene and perylene in direct supersonic expansions of simple supercritical carriers such as CO2, N2O, and even SF6, under conditions which minimize aerosol and cluster formation. Clusters form readily in expansions of CO2 and N2O unless nozzle temperatures exceed about 200°C. Results with more complex carriers, such as n-pentane, are much poorer. Selective detection of perylene at a limit of 2 ng (S/N = 3) in a mixture of ten polycyclic aromatic hydrocarbons and in a coal tar extract has been achieved with a capillary introduction system. Challenges with coupling supersonic jet detection to capillary supercritical fluid chromatography lie with the nozzle.

In vibrationally resonant sum-frequency generation (VR-SFG) spectra, the resonant signal contains information about the molecular structure of the interface, whereas the nonresonant signal is commonly treated as a background and has been assumed to be negligible on transparent substrates. The work presented here on model chromatographic stationary phases contradicts this assumption. Model stationary phases, consisting of functionalized fused-silica windows, were investigated with VR-SFG spectroscopy, both with and without experimental suppression of the nonresonant response. When samples are moved from CD(3)OD to D(2)O, the VR-SFG spectrum was found to change over time when the nonresonant signal was present but not when the nonresonant signal was suppressed. No effect was seen when the solvent was changed and pressurized to 900 psi. These results suggest that the response to the new solvent manifests primarily in the nonresonant response, not the resonant response. Any structural changes caused by the new solvent environment appear to be minor. The nonresonant signal is significant and must be properly isolated from the resonant signal to ensure a correct interpretation of the spectral data. Curve-fitting procedures alone are not sufficient to guarantee a proper interpretation of the experimental results.