We have calculated vibrational energy levels for an ammonia molecule adsorbed on a Ni(111) surface. An exact kinetic energy operator for the gas phase molecule is combined with a potential energy function that is calculated with the plane-wave density-functional theory. The resulting eigenvalue problem is solved variationally. The vibrational energy levels for a gas phase molecule are also calculated. The calculated adsorption-related shift is +193 cm−1 for the symmetric bend, −54 cm−1 for the asymmetric bend, −68 cm−1 for the symmetric stretch, and −63 cm−1 for the asymmetric stretch, in good agreement with the experimental values +190, −47, −67, and −84 cm−1, respectively.
We have computed vibrational high-frequency overtone spectra of the water-ammonia complex, H(2)O-NH(3), and its isotopomers. The complex has been modeled as two independently vibrating monomer units. The internal coordinate Hamiltonians for each monomer unit have been constructed using exact gas phase kinetic energy operators. The potential energy and dipole moment surfaces have been calculated with the explicitly correlated coupled cluster method CCSD(T)-F12A and the valence triple-ζ VTZ-F12 basis around the equilibrium geometry of the complex. The vibrational eigenvalues have been calculated variationally and the eigenfunctions obtained have been used to compute the intensities of the absorption transitions. In H(2)O-NH(3), the water molecule acts as the proton donor and its symmetry is broken. The hydrogen-bonded OH bond oscillator undergoes a large redshift and intensity enhancement compared to the free hydrogen bond. Broken degeneracy of the asymmetric vibrations, quenched inversion splittings, and blueshift of the symmetric bending mode are the most visible changes in the ammonia unit.
We have studied the vibrational high-frequency spectrum of the water trimer computationally. We expand an earlier study [J. Chem. Phys. A 2009, 113, 9124-9132] where we approximated the water trimer as three individually vibrating water monomer units. Some intramolecular potential energy coupling terms are now included in the previous model. The six OH bond lengths and the three HOH bending angles are used as the internal coordinates. The kinetic energy operator is a sum of the kinetic energy operators of the monomer units. We use the coupled cluster method with single, double, and perturbative triple excitations method [CCSD(T)] with augmented correlation consistent polarized valence triple-ζ (aug-cc-pVTZ) basis set to calculate the potential energy surface (PES). The counterpoise correction is included in the one-dimensional part of the PES. We calculate the vibrational energy eigenvalues using the variational method. The corresponding eigenfunctions are used to obtain the absorption intensities.
We have computationally studied adsorption and vibrational energy levels of the ammonia molecule adsorbed on the fcc (111) transition metal surfaces Ni(111), Cu(111), Rh(111), Pd(111), Ag(111), Ir(111), Pt(111), and Au(111). Vibrational Hamiltonians are obtained by combining an exact kinetic energy operator for the isolated ammonia molecule with plane-wave density functional theory (DFT) potential energy surfaces. The resulting eigenvalue problems are solved variationally. This procedure gives us the anharmonic vibrational energy levels of the adsorbed ammonia molecule. The local density of the states (LDOS) analysis reveals that ammonia adsorbs to all studied surfaces through its lone pair orbital. It makes the symmetric bending potential asymmetrical around the planar structure, quenches inversion splittings, and blue shifts the symmetric bend wavenumber, in agreement with experimental observations. In this work, it has been observed that the magnitude of this blue shift depends almost linearly on the adsorption energy. The asymmetric bend and the stretches red shift, which indicates loosening of the NH bonds upon adsorption.
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