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Sum frequency generation (SFG) spectroscopy is a powerful experimental technique to probe surface structures. This paper presents a new theoretical mode of nonempirical analysis of SFG spectra for interfacial structures, which considerably generalizes our previous effort (Chem. Phys. 2000, 258, 371), which involved several empirical elements. The method is based on a time correlation function for the frequency-dependent hyperpolarizability, which can be straightforwardly evaluated via molecular dynamics simulations and which explicitly takes into account, for example, intramolecular vibrations and electronic polarization. The new theory is capable of precisely describing a number of factors significant for the spectrum, such as the dielectric local field correction, vibrational dephasing, inter/intramolecular vibrational coupling, etc., and is fairly nonempirical and rigorous, within the dipole approximation. The results for the water-vapor interface reproduce the experimental spectra fairly well, although with some discrepancies; possible reasons for these are suggested.

The energetically unfavorable termination of the hydrogen-bonded network of water molecules at the air/water interface causes molecular rearrangement to minimize the free energy. The long-standing question is how water minimizes the surface free energy. The combination of advanced, surface-specific nonlinear spectroscopy and theoretical simulation provides new insights. The complex χ((2)) spectra of isotopically diluted water surfaces obtained by heterodyne-detected sum frequency generation spectroscopy and molecular dynamics simulation show excellent agreement, assuring the validity of the microscopic picture given in the simulation. The present study indicates that there is no ice-like structure at the surface--in other words, there is no increase of tetrahedrally coordinated structure compared to the bulk--but that there are water pairs interacting with a strong hydrogen bond at the outermost surface. Intuitively, this can be considered a consequence of the lack of a hydrogen bond toward the upper gas phase, enhancing the lateral interaction at the boundary. This study also confirms that the major source of the isotope effect on the water χ((2)) spectra is the intramolecular anharmonic coupling, i.e., Fermi resonance.

Gas-liquid interfacial structures of NaCl and NaI aqueous solutions are investigated via molecular dynamics simulations using a flexible and polarizable water model we have developed. The new five-site model of water aims at suitably describing interfacial properties, including vibrational sum frequency spectroscopy where both flexibility and polarization are crucial. The performance of the water model is systematically examined and demonstrated by a number of properties of bulk and interface, including density, vaporization energy, dipole moment, diffusion coefficient, radial distribution function, infrared and Raman spectra of the O-H stretching region, surface potential, and surface excess of ions. The orientational structure of surface water is investigated in detail in connection with the issue of surface solvation of anions. These investigations will be utilized to analyze the sum frequency generation spectra in relation to the orientational structure at the molecular level.

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We performed ab initio molecular orbital (MO) calculations of the response kernel (∂Q a /∂V b ), which represents the response of the intramolecular charge polarization by external electrostatic field, on the basis of the coupled perturbed Hartree−Fock equation. The response kernels of some organic molecules including pyrazine, pyrazinyl radical, acetone, and 2-hydroxypropyl radical were calculated along the present formulation. The results revealed that the hydrogen abstraction of pyrazine causes the product radical to be remarkably deformable in the partial charge distribution, while the hydrogen abstraction of acetone does not induce such enhancement of the charge sensitivity. The augmented sensitivity does not appear in the usual polarizability for a uniform field but emerges for a local fluctuated field. To elucidate the remarkable difference, we performed the normal mode analysis and decomposition based on the intrinsic soft MO pairs or localized orbitals. As a result, the enhancement in the aromatic species is attributed to the softest normal mode due to the π−σ mixing that facilitates the deformation of the π-electron orbitals. In the nonaromatic species, on the other hand, this effect is not dominant and is canceled by the breakdown of hyperconjugation. We suggest that the particular sensitivity of aromatic radicals is the origin of anomalously slow diffusion in solution.

A new flexible and polarizable water model based on the charge response kernel (CRK) theory is developed for the analysis of sum frequency generation (SFG) spectroscopy. The CRK model well describes several bulk water properties and SFG spectrum by molecular dynamics (MD) calculations. While the flexible and polarizable MD simulation generally adopts the short-range damping of intermolecular interaction, it is found that the same procedure is not adequate for the calculation of transition dipole in strongly hydrogen bonding environment. Accordingly, the improved calculation of the nonlinear susceptibility of water surface results in the positive imaginary part in the 3000-3200 cm(-1) region, which is consistent with recent phase-sensitive experiments. The mechanism of the positive region is attributed to the anisotropic local field effect induced by the orientational correlation of surface water.

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