Polycyclic aromatic hydrocarbons (PAHs) or PAH-related molecules are considered to be responsible for the unidentified infrared (UIR) emission features at 3.3, 6.2, 7.7, 8.6 and 11.2 µm. However, the exact identification of PAH or PAH-related molecules is difficult. There have been several investigations on the spectroscopic characterization of PAH molecules. But none of them compared the spectra of isomers of PAHs, which might have help in the identification of the UIR emission features. This work presents the infrared and electronic absorption spectra of isomers of C 16 H 10. The aim of the present work is to compare infrared and electronic absorption spectra of four isomers of C 16 H 10 PAH viz. pyrene, aceanthrylene, acephenanthrylene and fluoranthene, their ions and doubly ions. We also compare the spectra of pyrene in the gas-phase and in H 2 O ice. We have used the density functional theory with B3LYP exchange and correlation functional and 6-311++g** basis set to study the infrared spectra. The time-dependent density functional theory (TDDFT) has been used to obtain the electronic absorption spectra. Significant difference in the CC stretching, CH in-plane bending and CH out-of-plane bending vibration modes is observed for the isomers of C 16 H 10 whereas there is no large difference in the CH stretching vibration band. A significant change in the vibrational band is observed for pyrene in H 2 O ice compared to gas-phase pyrene. Though isomers of C 16 H 10 PAH have the same number of carbon and hydrogen atoms, their spectroscopic characteristics are different. This study should help in identifying the isomers of C 16 H 10 , their ions and doubly cation in the interstellar medium.

We present theoretical vibrational and absorption spectra of aminoacetonitrile, its cation, anion, cyanoprotonated, and aminoprotonated aminoacetonitrile. We used second-order Moller-Plesset perturbation method (MP2) with TZVP basis set to obtain ground state geometries and vibrational spectra. Time dependent density functional theory method was used to obtain absorption spectra. Shifts in vibrational modes for aminoacetonitrile upon ionization and protonation are determined. The C:N stretching mode which is the most important mode in detection of nitriles in space is more intense in aminoacetonitrile ions and its two protonated form and is less IR active for neutral aminoacetonitrile. The nature of electronic transition for these molecules is identified. All the electronic transitions for neutral aminoacetonitrile and its cation are the r ! r* electronic transitions, whereas its anion and protonated aminoacetonitrile display the r ! r* as well as p ! p* transitions.

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Zü ndel ion (H 5 O þ2 ) is one of the two important structures formed during the proton transfer process in aqueous system. This work reports microsolvation of Zü ndel ion using density functional theory based B3LYP method with aug-cc-pVTZ basis set. Interaction of Zü ndel ion with four water molecules in its first solvation shell is studied using many-body analysis approach. A change in many-body energies and their contribution to the binding energy of a complex during the proton transfer process from donor to acceptor water molecule in Zü ndel ion-4H 2 O complex is obtained. For the hydrated Zü ndel ion complex, the contribution from total two-body, three-body, fourbody, five-body, and relaxation energy to the binding energy is 84.7, 14, 6.87, 1.6, and 4%, respectively, at B3LYP/aug-cc-pVTZ level. Relaxation energy and total five-body energy have repulsive contribution to the binding energy of a hydrated Zü ndel ion complex. It is found that the relaxation energy and binding energy of a Zü ndel-4H 2 O complex is the maximum and minimum, respectively, when a shared proton is at equal distance from oxygen atom of donor and acceptor water molecules. A significant change in two-body, three-body, and four-body energies for which Zü ndel ion is one of the many-body terms is observed during the proton transfer process. A change in total two-body, total threebody, total four-body, and relaxation energy is about 2.6, 1.8, 0.4, and 1.1%, respectively, during the proton transfer process. A change in two-body, three-body, and four-body interaction energies between water molecules is very small during the proton transfer process.

The molecular interactions between water and dioxane are studied using hydrogen bonding model and density functional method. The average number of hydrogen bonds obtained using hydrogen bonding model is maximum at 0.83 mole fraction of water in dioxane. This concentration corresponds to five water molecules and a dioxane molecule in a complex [Mashimo et al., J Chem Phys 1992, 96, 6358; Kanse et al., J Ind Chem Soc 2007, 83, 168]. The values of dielectric constant for the mixture obtained by hydrogen bonding model are in excellent agreement with the experimental values. Density functional theory calculations show that the most stable conformer of dioxane-5H 2 O hydrogen bonded complex has six short hydrogen bonds. Number of hydrogen bonds from density functional calculations and hydrogen bonding model are in excellent agreement at 0.83 mole fraction of water. In this hydrogen bonded complex five water molecules bridging the two oxygen atoms of dioxane molecule and thus is a cyclic structure, which is also in agreement with the experiment.