Heterodyne-detected sum frequency generation spectroscopy of polyacrylic acid at the air/water-interface.

The conformational and hydration behavior of polyacrylic acid (PAA) at the air/water interface is studied using heterodyne-detected vibrational sum frequency generation spectroscopy (HD-VSFG). The amount of adsorption at the water-air interface and the influence of PAA on the water structure at the interface are found to be highly dependent on the degree of ionization (acid dissociation) of the carboxylic acid groups. At a low degree of ionization PAA is surface active and remains in a dense packing arrangement with intrachain hydrogen bonds. The interfacial water molecules show a net orientation with their OH groups pointing to the surface. Ionization of the carboxylic acid groups leads to an increase of the negative charge and to a break-up of the intrachain hydrogen bonds. The increase in negative charge is accompanied by an increase of the VSFG signal of the interfacial water molecules, as a result of their enhanced orientation. At ionization degrees α > 0.203 PAA loses its enhanced surface propensity and is well dissolved in the bulk. Addition of NaCl to the solution is observed to lead to a recurrence of PAA at the interface.

Large scale polarizability calculations using the approximate coupled cluster model CC2 and MP2 combined with the resolution-of-the-identity approximation.

We present an implementation of static and frequency-dependent polarizabilities for the approximate coupled cluster singles and doubles model CC2 and static polarizabilities for second-order Mo̸ller-Plesset perturbation theory. Both are combined with the resolution-of-the-identity approximation for electron repulsion integrals to achieve unprecedented low operation counts, input-output, and disc space demands. To avoid the storage of double excitation amplitudes during the calculation of derivatives of density matrices, we employ in addition a numerical Laplace transformation for orbital energy denominators. It is shown that the error introduced by this approximation is negligible already with a small number of sampling points. Thereby an implementation of second-order one-particle properties is realized, which avoids completely the storage of quantities scaling with the fourth power of the system size. The implementation is tested on a set of organic molecules including large fused aromatic ring systems and the C(60) fullerene. It is demonstrated that exploiting symmetry and shared memory parallelization, second-order properties for such systems can be evaluated at the CC2 and MP2 level within a few hours of calculation time. As large scale applications, we present results for the 7-, 9-, and 11-ring helicenes.