Ground state and excited state properties of ethylene isomers studied by a combined Feynman path integral-ab initio approach

Ground state and excited state properties of ethylene, C₂H₄, and several ethylene isomers have been studied by Feynman path integral Monte Carlo (PIMC) simulations. The PIMC treatment of the atomic nuclei has been combined with different electronic Hamiltonians in order to analyse the influence of the nuclear degrees of freedom on electronic quantities. Electronic expectation values at the minimum of the potential energy surface (PES) have been compared with PIMC based ensemble averaged values. Ensemble averaged quantities have been derived by Hamiltonians of the ab initio type and a tight-binding (TB) one-electron model. The combined influence of anharmonicities in the interatomic potential and the quantum fluctuations of the atomic nuclei lead to ensemble averaged bondlengths r _g which are significantly larger than the parameters r _e, at the minimum of the PES. The implications of this bond length elongation for the electronic properties of ethylene are discussed. The occupied canonical molecular orbitals (CMOs) of ethylene are destabilized under the influence of the nuclear degrees of freedom while virtual CMOs are stabilized. These shifts of one-electron energies suggest a comparison of electronic excitation energies at the minimum of the PES with PIMC based ensemble averages. The quantum fluctuations of the nuclei cause a strong redistribution in the intensities of electronic transitions. Transitions, which are dipole allowed in the planar D_2h geometry of ethylene, lose intensity under the influence of nuclear quantum effects, and vice versa for electronic excitations that are dipole forbidden under D_2h symmetry. This ‘vibrational borrowing’ is enhanced with decreasing atomic masses. The Feynman centroid density has been used to calculate the anharmonic vibrational wavenumbers of C₂H₄ and C₂D₄. The results of the present PIMC simulations have been employed to emphasize general problems of electronic structure calculations based on a single nuclear configuration (i.e. the configuration at the minimum of the PES).     

Miscibility behavior of dipalmitoylphosphatidylcholine with a single-chain partially fluorinated amphiphile in Langmuir monolayers

Surface pressure-area, surface potential-area, and dipole moment-area isotherms were obtained for monolayers made from a partially fluorinated surfactant, (perfluorooctyl)undecyldimorpholinophosphate (F8H11DMP), dipalmitoylphosphatidylcholine (DPPC), and their combinations. Monolayers, spread on a 0.15 M NaCl subphase, were investigated at the air/water interface by the Wilhelmy method, ionizing electrode method, and fluorescence microscopy. Surface potentials were analyzed using the three-layer model proposed by Demchak and Fort. The contribution of the dimorpholinophosphate polar head group of F8H11DMP to the vertical component of the dipole moment was estimated to be 4.99 D. The linear variation of the phase transition pressure as a function of F8H11DMP molar fraction (X(F8H11DMP)) demonstrated that DPPC and F8H11DMP are miscible in the monolayer. This result was confirmed by deviations from the additivity rule observed when plotting the molecular areas and the surface potentials as a function of X(F8H11DMP) over the whole range of surface pressures investigated. Assuming a regular surface mixture, the Joos equation, which was used for the analysis of the collapse pressure of mixed monolayers, allowed calculation of the interaction parameter (xi=-1.3) and the energy of interaction (Delta epsilon =537 Jmol(-1)) between DPPC and F8H11DMP. The miscibility of DPPC and F8H11DMP within the monolayer was also supported by fluorescence microscopy. Examination of the observed flower-like patterns showed that F8H11DMP favors dissolution of the ordered LC phase domains of DPPC, a feature that may be key to the use of phospholipid preparations as lung surfactants.