Conformational effects in the vibrational and electronic spectra of propionaldehyde: Experimental and theoretical studies.

We report here investigations on conformational effects in the vibrational and electronic spectra of the propionaldehyde (propanal) molecule using FTIR (600-3200 cm-1) and vacuum ultraviolet (VUV) synchrotron radiation photoabsorption (52 500-85 000 cm-1) spectroscopy respectively. Detailed theoretical calculations (using DFT and TDDFT methodologies) on ground and excited states of the cis and gauche conformers of propanal are performed; a comprehensive spectral analysis of the IR and VUV spectra is presented. A reinvestigation of the IR spectrum reveals several new bands assigned to the gauche conformer based on theoretical calculations. The VUV spectrum exhibits rich Rydberg series structure assigned to ns, np and nd series converging to the first ionization potentials of the two conformers. Earlier assignments of the 3s cis and gauche origins are revised in addition to extending Rydberg series analysis to several higher members. Vibronic bands accompanying the 3s, 4s and 4p Rydberg states are assigned using estimated vibrational frequencies of cis and gauche conformers in the cationic ground state. Simulated potential energy curves of the first few excited states (singlets and triplets) of cis and gauche conformers of propanal help in gaining insights into photodissociation mechanisms and possible conformational effects therein.

VUV photoabsorption of thermally processed carbon disulfide and ammonia ice mixtures - Implications for icy objects in the solar system.

Many icy bodies in the solar system have been found to contain a rich mixture of simple molecules on their surfaces. Similarly, comets are now known to be a reservoir of molecules ranging from water to amides. The processing of planetary/cometary ices leads to the synthesis of more complex molecules some of which may be the harbingers of life. Carbon disulphide (CS2) and ammonia (NH3) are known to be present on many icy satellites and comets. Reactions involving CS2 and NH3 may lead to the formation of larger molecules that are stable under space conditions. In this paper we present temperature dependent VUV spectra of pure CS2 in the ice phase, and of CS2 and NH3 ices deposited as (i) layered, and (ii) mixed ices at 10 K and warmed to higher temperatures until their sublimation. Pure CS2 ice is found to have a broad absorption in the VUV region, which is unique for a small molecule in the ice phase. In layered and mixed ices, the molecules tend to affect the phase change and sublimation temperature of each other and also leave behind a form of CS2-NH3 complex after thermal annealing. This study of CS2-NH3 ice systems in layered and mixed configurations would support the detection of these species/complexes in mixed molecular ices analogous to that on planetary and cometary surfaces.

Vacuum ultraviolet photoabsorption spectra of icy isoprene and its oligomers.

Isoprene and its oligomers, terpenes, are expected to be present, along with other complex organic molecules in the diverse environments of the ISM and in our solar system. Due to insufficient spectral information of these molecules at low temperature, detection and understanding the importance of these molecules has been rather incomplete. For this purpose, we have carried out the vacuum ultraviolet (VUV) photoabsorption measurements on pure molecular ices of isoprene and a few simple terpenes: limonene, α-pinene and ß-pinene by forming icy mantles on cold dust analogs. From these experiments, we report the first low temperature (10 K) VUV spectra of isoprene and its oligomers limonene, α-pinene and ß-pinene. VUV photoabsorption spectra of all the molecules reported here reveal similarities in the ice and gas phase as expected, with an exception of isoprene where a prominent red shift is observed in the ice phase absorption. This unqiue property of isoprene along with distinctive absorption at longer wavelengths supports its candidature for detection on icy bodies.

Structural, vibrational and electronic spectroscopic study of 6-hydroxycoumarin using experimental and theoretical methods.

Understanding the photochemical behavior of structural isomers of hydroxycoumarin (HC) having different properties of consequence in biological activities demand spectroscopic information of this class of compounds. Barring 6-hydroxycoumarin (6-HC), other isomers of HC's are well studied spectroscopically. To understand and compare the photochemical activity of 6-HC with other isomers, a detailed study of this molecule has been taken up. For this purpose, electronic, vibrational and structural properties of 6-HC have been studied using ultraviolet absorption and Infrared spectroscopy techniques. Quantum chemical calculations have been performed at DFT/B3LYP level of theory to get the optimized geometry and vibrational frequencies of normal modes to support and analyze experimental data. The detailed vibrational assignments were made on the basis of potential energy distributions. Chemical activity, molecular orbital energies, band gap and hyper-polarizability information have been computed from quantum chemical simulations. NBO analysis helped in understanding the stability of the molecule arising from hyper-conjugative interaction and charge delocalization. UV-Visible spectrum of the compound was recorded in the region 300-600 nm helped in obtaining band gap data of the compound. Molecular Electrostatic Potentials (MESP) were plotted and the respective centers of electrophilic and nucleophilic attacks were predicted with the help of Fukui functions calculations. Further, it was observed that the negative electrostatic potential regions are mainly localized over the oxygen atoms and the positive regions are localized over the benzene ring. Details of the results and analysis of experimental and theoretical spectroscopy studies are presented in this paper.

Residue from vacuum ultraviolet irradiation of benzene ices: Insights into the physical structure of astrophysical dust.

We have irradiated benzene ices deposited at 4 K on a cold, interstellar dust analog with vacuum ultraviolet (9 eV) irradiation for periods lasting from several hours to nearly a day, after which the irradiated ice was warmed to room temperature. Vacuum ultraviolet photoabsorption spectra of the aromatic residue left at room temperature were recorded and showed the synthesis of benzene derivatives. The residue was also imaged using an electron microscope and revealed crystals of various sizes and shapes. The result of our experiments suggests such geometrically shaped dust particles may be a key component of interstellar dust.

Excited states of aniline by photoabsorption spectroscopy in the 30,000-90,000 cm(-1) region using synchrotron radiation.

The photoabsorption spectrum of aniline (C6H5NH2) in gas phase in the 30,000-90,000 cm(-1) (3.7-11.2 eV) region is recorded at resolution limit of 0.008 eV using synchrotron radiation source for the first time to comprehend the nature of the excited valence and Rydberg states. The first half of the energy interval constitutes the richly structured valence transitions from the ground to excited states up to the first ionization potential (IP) at 8.02 eV. The spectrum in the second half consists of vibrational features up to second IP (9.12 eV) and structureless broad continuum up to the third IP (10.78 eV). The electronic states are assigned mainly to the singlets belonging to π → π* transitions. A few weak initial members of Rydberg states arising from π → 4s, np or nd transitions are also identified. Observed vibrational features are assigned to transitions from the ground state A' to the excited states 1A", 3A', 5A", 6A', and 10A" in C(s) symmetry. Time dependent density functional theory (TDDFT) calculations at B3LYP level of theory are employed to obtain the vertical excitation energies and the symmetries of the excited states in equilibrium configuration. The computed values of the transition energies agree fairly well with the experimental data. Further the calculated oscillator strengths are used to substantiate the assignments of the bands. The work provides a comprehensive picture of the vacuum ultraviolet photoabsorption spectrum of aniline up to its third ionization limit.

Photoionization of CH(3)I mediated by the C state in the visible and ultraviolet regions.

Three/two-photon resonant multiphoton ionization (MPI) of the CH3I monomer has been studied in the gas phase at 532 and 355 nm using time-of-flight mass spectrometry. Under low laser intensity (approximately 10(9) W/cm2) the mass spectra showed peaks at m/z 15, 127 and 142, corresponding to [CH3]+, [I]+ and [CH3I]+ species, at both these wavelengths. The laser power dependence for [CH3I]+, [I]+ and [CH3]+ ions showed a three-photon dependence at 532 nm. For the same three ions, photoionization studies at 355 nm gave a power dependence of 2. Both these results suggest that a vibronic energy level at approximately 7 eV, lying in the Rydberg C state, acts as a resonant intermediate level in ionization of CH3I. In the case of 355 nm, with increasing intensity additional peaks at m/z 139 and 141 were observed which could be assigned to [CI]+ and [CH2I]+ fragments. In contrast, for high intensity radiation at 532 nm ( approximately 2 x 10(10) W/cm2), only the [CI]+ fragment was observed. At these wavelengths, fragment ions observed in mass spectra mainly arise from photodissociation of the parent ion. Experiments at another wavelength in the visible region (564.2 nm) confirmed the results obtained at 532 nm. In order to assess the role of the A state in these MPI experiments, additional experiments were performed at 266 and 282.1 nm, which access the A state directly via a one-photon transition, and showed absence of a surviving precursor ion. Reaction energies for various possible dissociation channels of CH3I/[CH3I]+/[CH2I]+ were calculated theoretically at the MP2 level using the GAMESS electronic structure program.