Prashant Chandra Singh

Structure of the phenylacetylene-water complex as revealed by infrared-ultraviolet double resonance spectroscopy.

The structure of the phenylacetylene-water complex has been elucidated based on spectral shifts in electronic and vibrational transitions. Phenylacetylene forms a cyclic complex with water incorporating C-H...O and O-H...pi hydrogen bonds, which is different from both the benzene-water and acetylene-water complexes, even though phenylacetylene combines the features of both benzene and acetylene. Formation of such a complex can be rationalized on the basis of cooperativity between the two sets of hydrogen bonds.

IR-UV Double Resonance Spectroscopic Investigation of Phenylacetylene-Alcohol Complexes. Alkyl Group Induced Hydrogen Bond Switching

The electronic transitions of phenylacetylene complexes with water and trifluoroethanol are shifted to the blue, while the corresponding transitions for methanol and ethanol complexes are shifted to the red relative to the phenylacetylene monomer. Fluorescence dip infrared (FDIR) spectra in the O-H stretching region indicate that, in all the cases, phenylacetylene is acting as a hydrogen bond acceptor to the alcohols. The FDIR spectrum in the acetylenic C-H stretching region shows Fermi resonance bands for the bare phenylacetylene, which act as a sensitive tool to probe the intermolecular structures. The FDIR spectra reveal that water and trifluoroethanol interact with the pi electron density of the acetylene C-C triple bond, while methanol and ethanol interact with the pi electron density of the benzene ring. It can be inferred that the hydrogen bonding acceptor site on phenylacetylene switches from the acetylene pi to the benzene pi with lowering in the partial charge on the hydrogen atom of the OH group. The most significant finding is that the intermolecular structures of water and methanol complexes are notably distinct, which, to the best of our knowledge, this is first such observation in the case of complexes of substituted benzenes.

Electronic and Vibrational Spectroscopic Investigation of Phenylacetylene-Amine Complexes. Evidence for the Diversity in the Intermolecular Structures

Shifts in the electronic transitions for the complexes of phenylacetylene with ammonia, methylamine, and triethylamine clearly indicate the variation in the intermolecular structures of the three complexes. The infrared spectrum of phenylacetylene in the acetylenic C-H stretching region shows Fermi resonance bands, which act as a sensitive tool to probe the intermolecular structures. The IR-UV double resonance spectra of the three complexes are disparate and signify the formation of distinct structures. The formation of C-H...N hydrogen-bonded complex with ammonia and two distinct types of pi complexes with methylamine and triethylamine can be inferred from the analysis of electronic and vibrational spectra in combination with ab initio calculations. These complexes clearly point out the fact that marginal changes in the interacting partner can significantly alter the intermolecular structure.

PHOTODISSOCIATION DYNAMICS OF METHYLAMINE CATION AND ITS RELEVANCE TO TITAN'S IONOSPHERE

Photodissociation of CH3NH2 + has been studied using the DC sliced ion imaging technique and ab initio calculations in order to understand the formation of HCNH+, an important molecule in Titans ionosphere. Our experimental and theoretical observations show that hydrogen loss from CH3NH2 + has two channels: one giving rise to the triplet species CH3NH+, while the other product is CH2NH2 +. The latter then decomposes further to form HCNH+. H2 loss from CH3NH2 + has only one channel, yielding CH2NH+. This species further loses H to form HCNH+. The branching ratio of the H, H2, and H+H2 loss channels is found to be 4.2:1:2.5. This is ascribed to the fact that, at these energies, the H loss has one stable triplet product channel, while most of the H2 loss product further decomposes to HCNH+.

Infrared-Optical Double-Resonance Measurements on O-H···H-Ge Dihydrogen-Bonded Phenol-Triethylgermanium Hydride Complex in the Gas Phase

Spectroscopic investigation of a dihydrogen-bonded complex between phenol and triethylgermanium hydride is reported here. Laser-induced fluorescence excitation, fluorescence-detected infrared, and IR-UV hole-burning spectroscopic studies were carried out in supersonic jet to investigate the complex formation between phenol and triethylgermanium hydride. The lowering of the O-H stretching frequency of the phenol moiety in the complex with triethylgermanium hydride clearly establishes the role of phenol as hydrogen bond donor. The experimental results together with the ab-initio calculations unambiguously confirm formation of an O-H...H-Ge dihydrogen-bonded complex between phenol and triethylgermanium hydride.

Theoretical Study on the Microhydration of Atmospherically Important Carbonyl Sulfide in Its Neutral and Anionic Forms: Bridging the Gap between the Bulk and Finite Size Microhydrated Cluster

Carbonyl sulfide (OCS) is the most abundant and stable sulfur-containing triatomic gas present in the atmosphere that plays an important role in aerosol formation. Structure, energetics, and photoelectron spectral properties of the microhydrated OCS in its neutral and anionic forms have been studied by using the BP86, B3LYP, and MP2 methods. OCS is linear in the neutral state but bent in the anionic state. Water binds with the OCS through a single hydrogen bond (O-H···O) in the OCS-(H2O)n [n = 1-6], whereas binding of OCS(-) with water takes place through single as well as double hydrogen bonds (O-H···S and O-H···O). Energy decomposition analysis shows that electrostatic and exchange energies are the main contributors to the stabilization energy of the microhydrated OCS and OCS(-) clusters. Detachment as well as solvation energies are calculated with different levels of theory and compared with the existing experimental values. Finally, an analytical expression has been used to obtain the bulk value of the detachment and solvation energies from the existing information on the finite size clusters. The present study reveals that hydration increases the detachment energy of the OCS(-) by 3.2 eV. In the absence of experimental bulk values of the detachment and solvation energies for this system, the values obtained by the solvent-number-dependent theoretical expression will definitely reduce this gap and may be used for the modeling of the OCS in the atmosphere.

Water Complexes of Styrene and 4-Fluorostyrene: A Combined Electronic, Vibrational Spectroscopic and Ab-Initio Investigation

The binary complexes of water with styrene and fluorostyrene were investigated using LIF and FDIR spectroscopic techniques. The difference in the shifts of S 1 <-- S 0 electronic transitions clearly points out the disparity in the intermolecular structures of these two binary complexes. The FDIR spectra in the O-H stretching region indicate that water is a hydrogen bond donor in both complexes. The formation of a single O-H...pi hydrogen-bonded complex with styrene and an in-plane complex with fluorostyrene was inferred based on the analysis of the FDIR spectra in combination with ab initio calculations. The in-plane complex with fluorostyrene is characterized by the presence of O-H...F and C-H...O hydrogen bonds, leading to formation of a stable six-membered ring. The synergistic effect of O-H...F and C-H...O hydrogen bonds overwhelms the O-H...pi interaction in fluorostyrene-water complexes.

Combined Molecular Dynamics, Atoms in Molecules, and IR Studies of the Bulk Monofluoroethanol and Bulk Ethanol To Understand the Role of Organic Fluorine in the Hydrogen Bond Network

The presence of the fluorocarbon group in fluorinated alcohols makes them an important class of molecules that have diverse applications in the field of separation techniques, synthetic chemistry, polymer industry, and biology. In this paper, we have performed the density function theory calculation along with atom in molecule analysis, molecular dynamics simulation, and IR measurements of bulk monofluoroethanol (MFE) and compared them with the data for bulk ethanol (ETH) to understand the effect of the fluorocarbon group in the structure and the hydrogen bond network of bulk MFE. It has been found that the intramolecular O-H···F hydrogen bond is almost absent in bulk MFE. Molecular dynamics simulation and density function theory calculation along with atom in molecule analysis clearly depict that in the case of bulk MFE, a significant amount of intermolecular O-H···F and C-H···F hydrogen bonds are present along with the intermolecular O-H···O hydrogen bond. The presence of intermolecular O-H···F and C-H···F hydrogen bonds causes the difference in the IR spectrum of bulk MFE as compared to bulk ETH. This study clearly depicts that the organic fluorine (fluorocarbon) of MFE acts as a hydrogen bond acceptor and plays a significant role in the structure and hydrogen bond network of bulk MFE through the formation of weak O-H···F as well C-H···F hydrogen bonds, which may be one of the important reasons behind the unique behavior of the fluoroethanols.

Ion and electron imaging study of isobutanal photoionization dynamics

We present a detailed photoion and photoelectron imaging study of isobutanal cation dynamics. The 2 + 1 REMPI spectrum via the (n,3s) Rydberg transition was recorded and analyzed under both cold and warm beam conditions in an effort to identify the predicted trans conformer. Photoelectron imaging was used to establish the ion energetics accurately and to aid in the assignment of the REMPI spectra. On the basis of the photoelectron spectra and ab initio calculations, a peak at 54336.8 cm(-1) is assigned to the trans conformer. We have also assigned the photoelectron spectra and identified some hot band transitions not reported in previous work. We determined the adiabatic ionization energy to be 9.738 eV. We also studied the photodissociation dynamics of isobutanal cations leading to several product channels, but no evidence of vibrational mode or conformational isomer dependence on either the product branching or dynamics was seen for this system.

Production of O2 Herzberg states in the deep UV photodissociation of ozone

High-resolution imaging experiments combined with new electronic structure and dynamics calculations strongly indicate that the O((3)P)+O(2) products with very low kinetic energy release (E(tr)<0.2 eV) formed in the deep UV (226 nm) photodissociation of ozone reflect excitation of the Herzberg states of O(2): A()(3)Delta(u)(v=0,1,2) and A (3)Sigma(u)(+)(v=0,1). This interpretation contradicts the earlier assignment to very high (v> or =26) vibrational states of O(2)((3)Sigma(g)(-)).