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The dopamine-TiO2 system shows a specific spectroscopic response, surface enhanced Raman scattering (SERS), whose mechanism is not fully understood. In this study, the goal is to reveal the key role of the molecule–nanoparticle interface in the electronic structure by means of ab initio modeling. The dopamine adsorption energy on anatase surfaces is computed and related to changes in the electronic structure. Two features are observed: the appearance of a state in the material band gap, and charge transfer between molecule and surface upon electronic excitation. The analysis of the energetics of the systems would point to a selective adsorption of dopamine on the (001) and (100) terminations, with much less affinity for the (101) plane.

IR-UV double resonance spectroscopy and ab initio calculations were employed to investigate the structures and vibrations of the aromatic amino acid, L-phenylalanine–(H2O)n clusters formed in a supersonic free jet. Our results indicate that up to three water molecules are preferentially bound to both the carbonyl oxygen and the carboxyl hydrogen of L-phenylalanine (L-Phe) in a bridged hydrogen-bonded conformation. As the number of water molecules is increased, the bridge becomes longer. Two isomers are found for L-Phe–(H2O)1, and both of them form a cyclic hydrogen-bond between the carboxyl group and the water molecule. In L-Phe–(H2O)2, only one isomer was identified, in which two water molecules form extended cyclic hydrogen bonds with the carboxyl group. In the calculated structure of L-Phe–(H2O)3 the bridge of water molecules becomes larger and exhibits an extended hydrogen-bond to the π-system. Finally, in isolated L-Phe, the D conformer was found to be the most stable conformer by the experiment and by the ab initio calculation., The authors would like to acknowledge the support from the Grant-in-Aids for Scientific Research (18205003) by the Ministry of Education, Science, Sports, and Culture, Japan and from the French-German CNRS-DFG binational project. T. E. gratefully acknowledges financial support from Satake foundation.

In earlier studies, the interactions of isolated ionic species with various solvents were investigated using ab initio calculations. The ionic species investigated included cations (proton, hydronium, ammonium, and metal cations) and anions (single electron, hydroxide, and halide anions). However in the present study, we investigate the interactions of these ionic species with the solvent in the presence of other competing ionic species. We also elaborate on how the information obtained from these extensive studies have been employed in designing and synthesizing various kinds of novel ionophores and receptors.

The nature of the anion−π interaction has been investigated by carrying out high level ab initio calculations of the complexes of halide (F-, Cl-, and Br-), linear organic (CN-, NC-), and trigonal planar organic (NO3- and CO32-) anions with different kinds of π systems, viz. olefinic (tetrafluoroethene), aromatic (hexafluorobenzene), and heteroaromatic (1,3,5-triazine). In an effort to comprehend the underlying basis of this interaction, we have also carried out a rigorous decomposition of the interaction energies using the symmetry adapted perturbational theory (SAPT) method. Contrary to our expectations, the results indicate that the magnitudes of total interation energies of anion−π and cation−π interactions are similar. In contrast to cation−π interactions, anion−π interactions are, however, marked by substantial contributions from dispersion energies. As in the case of cation−π interactions, the role of anions also have a marked influence on the nature and magnitude of the anion−π interaction with in...

The structures of pure gold and silver clusters (Auk, Agk, k = 1−13) and neutral and anionic gold−silver binary clusters (AumAgn, 2 ≤ k = m + n ≤ 7) have been investigated by using density functional theory (DFT) with generalized gradient approximation (GGA) and high level ab initio calculations including coupled cluster theory with relativistic ab initio pseudopotentials. Pure Auk clusters favor 2-D planar configurations, while pure Agk clusters favor 3-D structures. In the case of Au, the valence orbital energies of 5d are close to that of 6s. This allows the hybridization of 6s and 5d orbitals in favor of planar structures of Auk clusters. Even 1-D linear structures show reasonable stability as local minima (or as global minima in a few small anionic clusters). This explains the ductility of gold. On the other hand, the Ag-4d orbital has a much lower energy than the 5s. This prevents hybridization, and so the coordination number (Nc) of Ag in Agk tends to be large in s-like spherical 3-D coordination i...

We have investigated a new type of interaction between silane (SiH4) and borane (BH3) using high level ab initio calculations. The SiH4−BH3 complex is found to be extremely stable with the formation of a bridged hydrogen bond between SiH4 and BH3. Hence, it might have a hitherto unknown role in the mechanism of chemical vapor deposition (CVD), which is employed in the fabrication of boron doped silicon semiconductor materials. In an attempt to unravel this role and the underlying reasons responsible for the stability of this complex, we have carried out a detailed analysis based on the structure and molecular orbitals. The results indicate that the binding strength and electronic character of the bridged hydrogen bond in the SiH4−BH3 complex is between those observed in double hydrogen bridged B2H6 and mono-hydrogen bridged anion B2H7-. In contrast to B2H6 and B2H7-, it should be noted that the complex is stabilized by direct strong electrostatic interaction between the positively charged Si atom and the ...

The optimal structures, binding energies, and harmonic vibrational frequencies of clusters containing a substituted benzene molecule microsolvated by four water molecules, termed as π-(water tetramer) clusters (π: p-difluorobenzene, fluorobenzene, benzene, toluene) have been evaluated at the second order perturbation level of theory (MP2) using both the 6-31+G* and aug-cc-pVDZ basis sets. In sharp contrast to the complexes of smaller water clusters with these π systems, wherein the water subcluster is most strongly bound to toluene, the water tetramer is most strongly bound to fluorobenzene. This exceptionally high binding energy results from both a π⋅⋅⋅OH H-bond and a competing σ F⋅⋅⋅OH bond between the water tetramer moiety and the aromatic molecule. The magnitudes of the many-body energy terms and their contribution to the binding energies of these π-(water tetramer) systems indicates that the contributions of three- and higher-order terms are much smaller when compared to the neutral water clusters. The two-body terms associated with the π- and σ-type of interaction indicates that in both the fluorobenzene and p-difluorobenzene complexes, the increase in the size of the water cluster enhances the π-H-bonding interaction and weakens the σ F⋅⋅⋅H interaction. This observation is in consonance with the calculated and experimentally observed redshifts of the OH vibrational frequencies. Thus, with an increase in the size of a water cluster bound to the fluorinated π system, there is a lowering of the redshift induced by the σ F⋅⋅⋅H interaction and an increase in the redshift due to the π-H interaction. The calculated redshift of the π H-bonded OH mode is very much dependent on the basis set, with larger basis sets yielding shifts which are in better agreement with the experimentally determined shifts.

The results of a combined experimental and theoretical investigation of the molecular structure of p-cyclohexylaniline (pCHA) in the electronic ground and the first electronically excited state are reported. The experimental investigations are performed for the crystalline phase by X-ray diffraction for the first time and related to former gas phase results obtained by time-resolved rotational laser spectroscopy. The theoretical results, from new ab initio calculations at the MP2/6-31+G(d) and CIS/6-31+G(d) level of theory for the electronic ground and excited state, respectively, give an adequate description of the rotational constants as obtained by the gas phase experiments. Thus, a detailed comparison of the ab initio structure for the ground state with the X-ray structure is performed in order to ascertain differences in the molecular geometry between the gas and crystalline phase. In particular, the size of the aromatic and cyclohexyl ring, their mutual orientation, and the conformation of the NH2 g...

Laser spectroscopy, i.e., resonant two-photon ionization (R2PI), IR/R2PI ion depletion and hole burning spectroscopy have been applied in an experimental study of heterogenous clusters consisting of fluorobenzene (FB) or p-difluorobenzene (pDFB), respectively, microsolvated by up to three methanol molecules. Their infrared ion depletion spectra were taken in the region of the OH and CH stretches of methanol. In these complexes the methanol molecules form subclusters, which are weakly hydrogen bonded to either the aromatic π-system (πOH) or to the fluorine substituent (σF) and a CH group (σCH). In FB⋅(MeOH)1, pDFB⋅(MeOH)1, pDFB⋅(MeOH)2 and one isomer of FB⋅(MeOH)2 the methanol subunits exclusively exhibit σF and σCH H-bonds. A further isomer of FB⋅(MeOH)2 exhibits a πOH type interaction. For FB⋅(MeOH)3 and pDFB⋅(MeOH)3 (1:3) complexes the methanol subcluster may take on either a chainlike or a ringlike conformation. In the chainlike isomer of FB⋅(MeOH)3 the methanol trimer interacts with the chromophore vi...

Recently, Brutschy and co-workers have reported the spectra of (substituted benzene)⋯(H2O)n systems. To investigate the possibility of these systems exhibiting a π–H kind of bonding interaction as observed in benzene⋯(H2O)n systems, we have carried out extensive ab initio calculations on different conformations of the fluorobenzene⋯(H2O) and p-difluorobenzene⋯(H2O) systems using various basis sets. Our results indicate that unlike the π interaction observed in benzene⋯(H2O)n, the O–H of the water molecule is involved in the formation of a six-membered ring system with the F–C–C–H of the aromatic ring. This six-membered ring which results from the formation of two H-bonds (water hydrogen and fluorine, water oxygen and benzene hydrogen), is extensively stabilized by electrostatic interactions. The strength of this σ-bonding interaction of water to fluorobenzene in C6H5F⋯H2O is nearly equal to the corresponding π-bonding interaction of water to benzene in C6H6⋯H2O. However the σ interaction of water to diflu...

Quantum mechanical ab initio calculations at the MP2/6-311++G** level of theory have been used to predict the binding energies and geometries of benzene−BX3 and ethene−BX3 (X = H, F, Cl) complexes. Single point calculations at a much higher level of correlation (MP4) and larger basis sets (6-311++G(2df,p) + diffuse(d,p)) have also been carried out. The calculations reveal interesting trends in their binding energies and geometries. The binding energies indicate that all of them are weakly bound van der Waals complexes with the exception of the C2H4−BH3 complex. While complexes involving BF3 are the weakest (binding energies) in cases of both ethene and benzene, there is a reversal in the relative order of the binding energies as one moves from ethene to benzene. Thus C6H6−BCl3 is more tightly bound than C6H6−BH3. The geometry exhibited by the lowest energy conformer in cases of complexes involving benzene is different from those involving ethene. In contrast to most weak van der Waals interactions involvi...

The geometries and vibrational frequencies of two conformers of pyruvic acid have been obtained at the ab initio second order Moller-Plesset level of theory using the 6–311++G∗∗ basis set. While the calculated geometries have been compared to the experimental microwave data, the vibrational frequencies have been assigned, using the experimental gas phase IR spectra of 13 isotopes of pyruvic acid by a recently developed scaling procedure (IRPROG). An attempt has been made to explain the stability of the eclipsed conformation over the staggered conformation of pyruvic acid by taking account of the molecular orbitals.

Ab initio calculations of proline and hydroxyproline zwitterions have been performed at the Hartree-Fock level using the 4-21++G basis set. The evaluated geometries have been compared to the corresponding X-ray parameters. The probable assignments for the experimental solid state vibrational spectra of proline and hydroxyproline based on the calculated ab initio frequencies and intensities were made.

The conformations of the higher dicarboxylic acids HOOC(CH 2 ) n COOH, n = 1–4 , the hydroxymalonic acid have been studied at the HF/4-21G level of theory. The conformational characteristics of these acids have been elucidated through potential energy scans at this level. The vibrational frequencies of all the dicarboxylic acids have also been evaluated at this level. Calculations at the HF/6-31G ** level of theory were also carried out on malonic, hydroxymalonic and succinic acids. They have been assigned to the solid state Raman and IR spectra of the dicarboxylic acids. The potential energy distributions, Raman activities and IR intensities calculated at the levels indicated have been used as aids in making the assignments. The calculated frequencies have also been compared with the frequencies evaluated at the MP2/4-31G * level for oxalic acid.

The vibrational frequencies of cysteine and serine zwitterions are evaluated using ab initio methods at the 4–31G∗ level. The calculated frequencies along with the corresponding potential energy distributions are used to assign the IR and Raman spectra of the zwitterions of l -cysteine and d,l -serine. The calculated Raman activities, IR intensities along with the experimental Raman optical activities of these amino acids are used as aids in the assignments. The similarities and differences between the spectra of these amino acids are highlighted.

Ab initio force fields for the vibrations of citric acid and citrate trianion in their ground electronic states have been determined at the Hartree—Fock level using the 4–21G basis set. The calculated geometries have been compared to the corresponding X-ray structures of citric acid and magnesium citrate, respectively. The calculated vibrational frequencies have been used to assign the experimental IR and Raman spectra of citric acid and sodium citrate. The calculated potential energy distribution, Raman activities and IR intensities have been used as aids in the assignment of the vibrational spectra of citric acid and sodium citrate.

The design of cesium-selective ionophores must include the nature of cesium-water interactions. The authors have carried out extensive ab initio and density functional theory calculations of hydrated cesium cations to obtain reasonably accurate energetics, thermodynamic quantities, and IR spectra. An extensive search was made to find the most stable structures. Since water...water interactions are important in the aqua-Cs+ clusters, the authors investigated the vibrational frequency shifts as a function of the number of water molecules and the frequency characteristics with and without the presence of outer-shell water molecules. The predicted vibrational frequencies were then compared with the infrared photodissociation spectra of argon-tagged hydrated cesium cluster ions. This comparison allowed the identification of specific hydrogen-bonding structures present in the experimental spectra.

Electrophilic aromatic substitution reactions, which are highly exemplified by the widely used Friedel-Craft's reaction, have been extensively studied using theoretical and experimental techniques. A number of elegant mechanisms have been proposed for the Friedel-Craft's reaction till date. In all the proposed mechanisms, the role of the Lewis acid has been limited to the generation of the electrophile, which subsequently attacks the aromatic system to form either a π or a σ complex. A recent experimental report on the reaction of CO with benzene in zeolite catalysts intrigued us because the presence or absence of AlCl3 was found to govern the reaction product. These clearly indicated that AlCl3 has an additional role in the reaction. We probed this role theoretically by performing high-level ab initio calculations on two model systems C6H6−BCl3 and C6H6−AlCl3. Our results clearly indicate that one of the benzene carbon tends to become highly nucleophilic, thereby facilitating an attack by an incipient el...

This chapter discusses theoretical approaches to the design of functional nanomaterials. A theoretical description of nanomaterials that belongs to the mesoscopic phase is fraught with a number of problems because neither quantum–chemical methods used to investigate the microscopic phase nor solid-state physics methods used to investigate the macroscopic phase can adequately describe all the desired properties. A judicious combination of both these methods apart from providing a detailed insight of the properties of these nanomaterials also aids the de novo design of novel molecular systems and functional materials. Another area of interest is molecular recognition, wherein quantum chemical calculations aid the design of microscopic chemical/biochemical sensors/monitors such as DNA chips. This chapter examines the ways in which an effective combination of diverse theoretical methods ranging from traditional ab initio , density functional, tight binding, Monte Carlo to molecular dynamics simulations, have helped elucidate the nature of intermolecular interactions in a large number of widely disparate molecular complexes (aqueous and metal clusters, biomolecules), and have also aided the design of novel nanomaterials (ionophores, receptors, endo/exohedral fullerenes, fullerides, nanotori, nanotubes, nanowires, and molecular devices).

The vibrational frequencies of two conformations of proline and hydroxyproline in the non-ionized form have been evaluated using the ab initio method at the HF/4-21G level. The barrier heights of the rotation of the carboxy and hydroxy hydrogen atoms have been evaluated at the HF/4-21G level. The calculated frequencies are assigned to the experimental vibrational spectra of proline and hydroxyproline.

The vibrational frequencies of three conformations of cysteine and serine in the non-ionized form have been evaluated using ab initio methods at the 4-31G ∗ level. The calculated frequencies along with the corresponding potential energy distributions were used to assign the IR spectra of the methyl ester, hydrochloride and zwitterion of cysteine and serine. It was found that the v (C β X) frequency is dependent on the value of X 1 [CC α C β X] torsional angle. The conformational dependence of some of the frequencies have been highlighted.