An ab initio approach to analyze fermi resonance in ammonia clusters

Anharmonic vibrational coupling among N-H stretching fundamental ($\nu_1$ and $\nu_3$) and N-H bending overtone ($2\nu_4$) vibrations in (NH$_3$)$_n$ ($n$ = 1 to 5) are analyzed based a full dimensional Hamiltonian including third and quartic terms. In particular, we examine Fermi resonance between the symmetric N-H stretching ($\nu_1$) and N-H bending overtone ($2\nu_4$) vibrations. As the cluster size increases, enhancement of the hydrogen bond strength makes $\nu_1$ red-shifted while $2\nu_4$ blue-shifted. These shifts result in the crossing of the frequencies of $\nu_1$ and $2\nu_4$ levels, and their energy order reverses between $n$ = 3 to n = 4. Because the nature of Fermi resonance, although the zero-order $\nu_1$ and $2\nu_4$ levels are shifted, the resultant mixed levels do not show remarkable changes in frequency. Instead, the major component of each mixed level largely changes and this causes significant redistribution of the intensity. Our results offer a solution to resolve puzzles on the intensity distribution and assignments of the Fermi mixing bands in the previously reported infrared spectra of (NH$_3$)$_n$

Vibrational coupling in solvated form of eigen proton: tuning the coupling via isotopologues

Experimental studies have shown that features in the vibrational spectra of H$_3$O$^+$ can be modulated not only by the type messengers, but also by the number of messengers. Recently, we compared the experimental H$_3$O$^+$Ar$_m$, $m$=1-3 spectra with accurate theoretical simulations and obtain the peak position and absorption intensity by solving the quantum vibrational Schrodinger equation using the potential and dipole moment obtained ab initio methods.\footnote{J-W Li, M. Morita, T. Takahashi and J-L Kuo, J. Phys. Chem. A, 119, 10887 (2015)} In this work, we studied isotopolgues of this ionic cluster to glean into the details of the vibrational couplings manifested in the spectra region of 1500-3800 cm\wn

BINDING BETWEEN NOBEL GAS ATOMS AND PROTONATED WATER MONOMER AND DIMER

H$_3$O$^+$ and H$_5$O$_2^+$, Eigen and Zundel forms of the excess proton, are the basic moieties of hydrated proton in aqueous media. Using vibrational pre-dissotion spectra, vibrational spectra of messenager-tagged species are often measured; however, only neat species have been studied in detail by theoretical and computational means. To bridge this gap, we carry out extensive CCSD(T)/aug-cc-pvTZ calculations to investigate the binding between commonly used noble gas (NG) messenagers (He, Ne and Ar) with H$_3$O$^+$ and H$_5$O$_2^+$ to get an accurate estimate on the binding energy which yields the upper limits of vibrational temperature of NG-tagged clusters. The binding sites of NG and low-lying transition states have also been searched to give a better description on the energy landscape. In addition, a few exchange/correlation functionals have been tested to access the accuracy of these methods for future and more sophisticated theoretical studies

VIBRATIONAL COUPLING IN SOLVATED FORM OF EIGEN PROTON

The most simple solvated proton, the hydronium ion H$_3$O$^+$ has been studied experimentally in its bare case as well as with the messenger techniques. Recent studies have shown that features in the vibrational spectra can be modulated not only by the different messengers, but also by the number of messengers. Theoretical molecular dynamics simulations have shed some light on the H$_3$O$^+$(H$_2$)$_n$ clusters, but understanding on the effect of microsolvaton by the messengers toward the spectra is still far from complete. We compare the experimental H$_3$O$^+$Ar$_m$ m=1-3 spectra with accurate theoretical simulations and oobtain the peak position and absorption intensity by solving the quantum vibrational Schrodinger equation using the potential and dipole moment obtained from DFT methods. One of the main goals of the study is to glean into the vibrational couplings induced by the microsolvation by the argon on the spectra region of 1500-3800 cm$^{-1}$, and to provide assignment on the peaks observed in these regions

Interaction between graphene and SiO2 surface

With first-principles DFT calculations, the interaction between graphene and SiO2 surface has been analyzed by constructing the different configurations based on {\alpha}-quartz and cristobalite structures. The single layer graphene can stay stably on SiO2 surface is explained based on the general consideration of configuration structures of SiO2 surface. It is also found that the oxygen defect in SiO2 surface can shift the Fermi level of graphene down which opens out the mechanism of hole-doping effect of graphene absorbed on SiO2 surface observed in experiments.Comment: 17 pages, 7 figure

First principles molecular dynamics study of filled ice hydrogen hydrate

We investigated structural changes, phase diagram, and vibrational properties of hydrogen hydrate in filled-ice phase C2 by using first principles molecular dynamics simulation. It was found that the experimentally reported 'cubic' structure is unstable at low temperature and/or high pressure. The 'cubic' structure reflects the symmetry at high (room) temperature where the hydrogen bond network is disordered and the hydrogen molecules are orientationally disordered due to thermal rotation. In this sense, the 'cubic' symmetry would definitely be lowered at low temperature where the hydrogen bond network and the hydrogen molecules are expected to be ordered. At room temperature and below 30 GPa, it is the thermal effects that play an essential role in stabilizing the structure in 'cubic' symmetry. Above 60 GPa, the hydrogen bonds in the framework would be symmetrized and the hydrogen bond order-disorder transition would disappear. These results also suggest the phase behavior of other filled-ice hydrates. In the case of rare gas hydrate, there would be no guest molecues rotation-nonrotation transition since the guest molecules keep their spherical symmetry at any temperature. On the contrary methane hydrate MH-III would show complex transitions due to the lower symmetry of the guest molecule. These results would encourage further experimental studies, especially NMR spectroscopy and neutron scattering, on the phases of filled-ice hydrates at high pressures and/or low temperatures.Comment: typos correcte

Hydrogen bond topology and the ice VII/VIII and Ih/XI proton ordering phase transitions

Ice Ih, ordinary ice at atmospheric pressure, is a proton-disordered crystal that when cooled under special conditions is believed to transform to ferroelectric proton-ordered ice XI, but this transformation is still subject to controversy. Ice VII, also proton disordered throughout its region of stability, transforms to proton-ordered ice VIII upon cooling. In contrast to the ice Ih/XI transition, the VII/VIII transition and the crystal structure of ice VIII are well characterized. In order to shed some light on the ice Ih proton ordering transition, we present the results of periodic electronic density functional theory calculations and statistical simulations. We are able to describe the small energy differences among the innumerable H-bond configurations possible in a large simulation cell by using an analytic theory to extrapolate from electronic DFT calculations on small unit cells to cells large enough to approximate the thermodynamic limit. We first validate our methods by comparing our predictions to the well-characterized ice VII/VIII proton ordering transition, finding agreement with respect to both the transition temperature and structure of the low-temperature phase. For ice Ih, our results indicate that a proton-ordered phase is attainable at low temperatures, the structure of which is in agreement with the experimentally proposed ferroelectric Cmc2_1 structure. The predicted transition temperature of 98 K is in qualitative agreement with the observed transition at 72 K on KOH-doped ice samples