The role of electron- and proton-transfer processes in the photophysics of hydrogen-bonded molecular systems has been investigated with ab initio electronic-structure calculations. Adopting indole, pyridine, and ammonia as molecular building blocks, we discuss generic mechanisms of the photophysics of isolated aromatic chromophores (indole), complexes of π systems with solvent molecules (indole−ammonia, pyridine−ammonia), hydrogen-bonded aromatic pairs (indole−pyridine), and intramolecularly hydrogen-bonded π systems (7-(2‘-pyridyl)indole). The reaction mechanisms are discussed in terms of excited-state minimum-energy paths, conical intersections, and the properties of frontier orbitals. A common feature of the photochemistry of the various systems is the electron-driven proton-transfer (EDPT) mechanism:  highly polar charge-transfer states of 1ππ*, 1nπ*, or 1πσ* character drive the proton transfer, which leads, in most cases, to a conical intersection of the S1 and S0 surfaces and thus ultrafast internal...

Computational Studies of the Photophysics of Hydrogen-Bonded Molecular Systems

Using density functional theory, we have studied the interactions between pyridine (Py) and coinage metals (silver, copper, and gold) as well as the transition metal platinum. We present here a detailed analysis of the influence of chemical enhancement effects on the SERS signals. We analyze the differential Raman scattering cross sections of the a1 vibrational modes related to Py. The results show that the relative Raman intensities of SERS spectra depend strongly on the binding interaction between Py and the SERS active centers, the electronic property of metal materials, and the incident wavelengths. When the bonding between Py and a SERS site is very weak, analogous to physical adsorption, the Raman spectra of the adsorbed Py are similar to that of free Py. For Py interacting strongly with copper, gold, and platinum clusters, we find that the Raman intensities of the v1, v6a, v9a, and v8a modes of Py are enhanced, whereas the intensity of the v12 mode decreases. To check the enhancement effect of the ...

Chemical enhancement effects in SERS spectra: A quantum chemical study of pyridine interacting with copper, silver, gold and platinum metals

Raman and infrared (IR) spectroscopies have been used extensively to study the adsorption of pyridine on various surfaces for more than 40 years. However, no satisfactory assignment on all the vibrational modes has been achieved. In the present study, density functional theory at the level of B3LYP/6-311+G** (for C, N, H)/LANL2DZ (for metals) has been used for the normal coordinate calculations of the neutral and cationic species of the pyridine−metal atom (ion) complexes. Based on the present calculations, new assignments of the fundamental frequencies for the bands attributed to v17a, v3, v5, and v18b modes of the free pyridine in the reported IR and Raman spectra have been suggested. The calculated frequency shifts indicate that the coupling among v1, v12, and v18a modes and the coupling between these modes and the N−metal (Cu, Ag, Au, or Pt) bond depend on the strength of the N−metal bond and its bonding properties. The fundamental frequency of the v6a mode has a nearly linear relationship with the ch...

Density Functional Study and Normal-Mode Analysis of the Bindings and Vibrational Frequency Shifts of the Pyridine-M (M = Cu, Ag, Au, Cu+, Ag+, Au+, and Pt) Complexes

Density functional theory (DFT) at the B3LYP/6-31++G(d,p) level was employed to obtain the optimized geometries and vibrational spectra of several pyridine(Py)−water(W) complexes with stoichiometric ratios ranging from 2:1 (Py2W) to 1:3 (PyW3). The harmonic vibrational wavenumbers of pyridine ring modes and the fundamental modes of water were calculated in order to examine the influence of hydrogen bonding on the normal modes of both pyridine and water upon complexation. The Raman spectra in the wavenumber region 960−1060 cm-1 covering the ring modes ν1 and ν12 of pyridine (in Wilson's notation) as a function of pyridine mole fraction were recorded. The integrated Raman intensities in the isotropic components of the spectra were used to determine the relative concentration of “free” pyridine molecules in close neighborhood with other Py−W complexes. The combination of both experimental wavenumbers yielding the overall shift induced by the entirety of hydrogen-bonded complexes in the mixture and the DFT-de...

Hydrogen-Bonded Pyridine-Water Complexes Studied by Density Functional Theory and Raman Spectroscopy

The synergy of self- and directed-assembly processes and lithography provides intriguing avenues to fabricate translationally ordered nanoparticle arrangements, but currently lacks the robustness necessary to deliver complex spatial organization. Here, we demonstrate that interparticle spacing and local orientation of gold nanorods (AuNR) can be tuned by controlling the Debye length of AuNR in solution and the dimensions of a chemical contrast pattern. Electrostatic and hydrophobic selectivity for AuNR to absorb to patterned regions of poly(2-vinylpyridine) (P2VP) and polystyrene brushes and mats was demonstrated for AuNR functionalized with mercaptopropane sulfonate (MS) and poly(ethylene glycol), respectively. For P2VP patterns of stripes with widths comparable to the length of the AuNR, single- and double-column arrangements of AuNR oriented parallel and perpendicular to the P2VP line were obtained for MS-AuNR. Furthermore, the spacing of the assembled AuNR was uniform along the stripe and related to t...

Control over position, orientation, and spacing of arrays of gold nanorods using chemically nanopatterned surfaces and tailored particle-particle-surface interactions.

Raman study of vibrational dephasing in hydrogen-bonded binary and ternary complexes of C6H5Cl and methanol

Isotopic dilution study of self association in (CH3CN+CD3CN) mixture by scanning multichannel Raman difference technique and ab-initio calculations

Raman spectra of terephthalidine-bis-heptylaniline (TB7A) were recorded in the region 1125–1225 cm−1 from room temperature to 10 K at 18 different temperatures The splitting of the ∼1165 cm−1 band, and the variation of the ratio of integrated intensities of the original band at ∼1165 cm−1 to the new component at 1170/1171 cm−1 reveals a solid modification The analysis of the temperature dependence of the linewidth was made using Rakov's relation A logarithmic plot as a function of 1/T gives two straight lines with an abrupt discontinuity, which clearly exhibits a phase transition at about 210 K The temperature dependence of the linewidth of the ∼1165 cm−1 band was used to characterize this transition A relatively large variation (∼3 cm−1) of the linewidth on going from 250 to 10 K in TB7A having an odd (n = 7) number of carbon atoms in the alkyl tail compared with those observed (∼1 cm−1) in other homologues having an even (n = 2 or 10) number of carbon atoms in the alkyl tail is a clear demonstration of the even–odd effect Copyright © 2002 John Wiley & Sons, Ltd

B.P. Asthana

Papers

Hydrogen-Bonded Pyridine-Water Complexes Studied by Density Functional Theory and Raman Spectroscopy

Raman study of vibrational dephasing in hydrogen-bonded binary and ternary complexes of C6H5Cl and methanol

Isotopic dilution study of self association in (CH3CN+CD3CN) mixture by scanning multichannel Raman difference technique and ab-initio calculations

Raman investigation of the low-temperature phase transition in a liquid crystalline system: terephthalidine-bis-heptylaniline (TB7A)

Study of the dynamics of phase transition in NH4SCN by a precise Raman linewidth and frequency shift measurement