Amplifying reactivity of bio-inspired [FeFe]-hydrogenase mimics by organic nanotubes.

A bio-inspired FeFe hydrogenase model which catalyses hydrogen evolution reaction (HER) in acidic solutions is immobilized in polyaniline (PANI)-based nanotubes. A combination of analytical techniques reveals that this construct maintains both the molecular signatures of the bio-inspired complex and the material properties of PANI. The amine and imine-rich environment of the PANI chain amplifies the inherent HER activity of the bio-inspired complex, allowing electrocatalytic HER at neutral pH, with lower overpotentials and higher current densities compared to the bio-inspired complex alone. This construct retains the oxygen stability of the bio-inspired complex and remains stable through several hours of aerobic electrolysis, producing only 6.5% H2O2 from the competing oxygen reduction reaction (ORR).

Lysozyme adsorption on carbonaceous nanoparticles probed by second harmonic light scattering.

The first hyperpolarizability (ß) of two different sizes (15 and 35 nm) of carbonaceous nanoparticle (CNP) is reported for the first time using second harmonic light scattering (SHLS). The ß values of the CNPs were found to be larger than those of organic molecules like pNA but lower than those of plasmonic nanoparticles like gold and silver. SHLS was further used to investigate the adsorption of a model protein Lysozyme (Lyz) on these CNPs, which is crucial for the design of safe and effective CNP-based therapeutics. The change in SH intensity from the CNPs on the addition of Lyz was recorded and fitted to the modified Langmuir adsorption model (MLM). The binding constant, free energy changes and surface coverage values show that Lyz is physisorbed on the CNPs forming less than a monolayer. The temperature dependent SH intensity measurements enabled direct determination of enthalpy and entropy changes for Lyz adsorption. The enthalpy and entropy changes reveal that Lyz adsorption is endothermic and entropically driven.

Thermodynamics of multilayer protein adsorption on a gold nanoparticle surface.

We report the thermodynamics of protein adsorption on negatively charged colloidal gold nanoparticles (GNPs) of 16 nm to 69 nm at pH 7.0. Three biologically important proteins of varying size, namely tetrameric alcohol dehydrogenase (ADH) from Saccharomyces cerevisiae, bovine serum albumin (BSA), and insulin from the porcine pancreas were taken and their adsorption on the GNP surface was investigated by dynamic light scattering (DLS), absorption spectroscopy and zeta potential measurements. The hydrodynamic size of the GNPs was found to increase with protein addition. At very low protein concentrations, the adsorption of these proteins was earlier described by Langmuir-type adsorption isotherms. However, when the protein concentration is raised, the adsorption data are found to fit the BET (Brunauer, Emmett, and Teller) adsorption isotherm well, indicating multilayer formation on the GNP surface. The equilibrium binding constants: KS (monolayer) ∼108-109 M-1, KL (subsequent layers) ∼105-106 M-1, and monolayer thickness are obtained from our measurements. KS is found to increase with the size of the GNPs, while KL does not change significantly with either the size of the GNPs or that of the protein. KL, which originates from protein-protein interaction, compares very well with the protein dimerization constant in solution, indicating that the interaction beyond the first layer is not significantly influenced by the nanoparticles. The first layer forms the soft corona on the GNP surface since this layer, although stable, is removable by centrifugation. But the subsequent layers are weakly bound and we call it an ultra-soft corona in order to distinguish it qualitatively from the strongly bound first layer. The modest magnitudes of Gibbs free energy changes of 43-55 kJ mol-1 for the first layer and 31-35 kJ mol-1 for the subsequent layers indicate that ADH, BSA, and insulin are all physiosorbed on the GNP surface. The temperature dependent DLS data point out that the adsorption of these proteins on GNPs is endothermic but is accompanied by a large increase in entropy. The nature of the BET fits to the adsorption data shows that multilayer adsorption starts even before the monolayer formation is complete, and it does not occur in a layer-by-layer sequence.

Effect of Tricarboxylic Acids on the Formation of Hydrogels with Melem or Melamine: Morphological, Structural and Rheological Investigations.

Herein, aggregation behaviors of melem or melamine in the presence of three symmetric carboxylic acids (1,3,5-tris(4-carboxyphenyl)benzene (TPCA), 1,3,5-benzene-tri-carboxylic acid (BTA) and 1,3,5-cyclohexane-tri-carboxylic acid (CHTA)) have been performed to check the influence of acid on the formation of aggregated structures which have been investigated by optical microscopy, FESEM, FTIR, XRD and viscoelastic properties have been explored with rheological studies. Interestingly, melem, that has limited solubility in aqueous medium, forms aggregation that leads to the formation of hydrogels with TPCA. More significantly, hydrogel is formed here by matching the size selectivity. Melem forms hydrogel with only large tricarboxylic acid, whereas melamine produces hydrogel with any kind of its counterpart from small to large tricarboxylic acid derivatives. Present investigations and results provide the strategy of design of organic self-assembled materials having two component systems.

Probing Polymer Chain Folding in Solution Using Second Harmonic Light Scattering.

Periodically grafted amphiphilic copolymers (PGACs) were earlier shown by us to adopt a zigzag folded conformation in the solid state, which enabled the backbone and pendant segments to segregate and occupy alternate layers in a lamellar structure. The conformational transition from a random coil to a zigzag folded chain in solution is an interesting problem, which is largely unexplored. To examine this, an orthogonally clickable parent polyester was sequentially clicked with two types of poly(ethylene glycol) (PEG) segments: one is a simple PEG and the other is a PEG that carries a dipolar chromophore. These two hydrophilic PEG segments, installed in a periodic and alternating fashion along the hydrocarbon-rich (HC) polyester backbone, ensure that the Janus folded chains are formed upon folding and carry chromophoric dipoles oriented along the same direction, thereby generating a large net dipole. The folding-induced alignment of chromophores in solution was followed using second harmonic light scattering (SHLS), wherein the intensity of the frequency-doubled scattered light (I2ω) is measured. Folding was induced by adding a polar solvent, like methanol, to a chloroform solution of the polymer; methanol desolvates the HC backbone but solubilizes the pendant PEG segments, thus inducing folding. The second harmonic intensity (I2ω) increased initially with methanol concentration and then saturated; in contrast, I2ω remained invariant with the solvent composition in the case of an analogous model chromophore. Furthermore, in a model PGAC carrying chromophore-bearing PEG segments on every repeat unit, I2ω decreased with increasing methanol composition, revealing the formation of a centrosymmetric folded chain, wherein the chromophoric dipoles on either side cancel each other. Thus, this study clearly reveals that the zigzag chain folding of PGACs can be induced by a segment-selective solvent, resulting in the rather elusive directional ordering of chromophoric dipoles in solution.

Thermodynamics of adsorption of alcohol dehydrogenase on the gold nanoparticle surface: a model based analysis versus direct measurement.

Characterization of the nanoparticle protein corona has gained tremendous importance lately. The parameters which quantitatively establish a specific nanoparticle-protein interaction need to be measured accurately since good quality data are necessary for the elucidation of the underlying mechanism and accurate molecular dynamics simulation. Here, we have employed surface sensitive second harmonic light scattering (SHLS) for investigating the adsorption of a tetrameric protein, alcohol dehydrogenase (ADH, Saccharomyces cerevisiae 147 kDa), on 16 nm, 27 nm, 41 nm, and 69 nm citrate capped gold nanoparticles (GNPs) in aqueous phosphate buffer at pH 7. We have extracted the binding constant, number of ADH bound per GNP, Gibbs free energy (ΔG°) from the decay of the second harmonic scattered signal as a function of protein concentration using a modified version of the Langmuir adsorption isotherm. The data obtained were checked with another technique, dynamic light scattering, using the same modified Langmuir model (MLM). While the binding constants measured by the two methods are in agreement, the number of ADH bound to each GNP obtained by the two methods varies a lot. In order to further probe this binding independent of a model fitting, we used an orthogonal fluorescence assay which measures the number of ADH bound to a GNP directly, and no model-fitting is necessary. We then used temperature dependent SHLS to measure the heat of adsorption (ΔH°) and entropy (ΔS°) for ADH-GNP corona formation. We found that the equilibrium binding constant (Kb) obtained from SHLS is of the order of 109 M-1 and the formation of the GNP-ADH corona is spontaneous with ΔG° ∼ -55 kJ mol-1. However, the adsorption is modestly endothermic, accompanied by a large increase in entropy. Stated differently, GNP-ADH corona formation is entropically driven. This is perhaps due to the tremendous disruption of the water structure at the negatively charged interface upon the arrival of the protein within the bonding distance to it. We believe that the SHLS technique is highly sensitive and reliable, at very low concentrations of both nanoparticles and proteins, for the quantitative estimation of the thermodynamic parameters of nanoparticle-protein corona formation, where many other techniques may fall short.

Thermodynamics of adsorption of lysozyme on gold nanoparticles from second harmonic light scattering.

Gold nanoparticle (GNP) interaction with hen egg white lysozyme (Lyz) has been investigated by many groups in order to understand protein mediated aggregation of GNPs and the underlying mechanism of aggregation. In this article, we have studied the interaction of citrate-capped GNPs of 16, 28, 41, and 69 nm sizes with Lyz by the non-destructive label-free second harmonic light scattering (SHLS) technique at physiological pH in phosphate buffer. The surface sensitivity of the nonlinear optical SHLS technique is very high and we have looked at the GNP-Lyz interaction at nanomolar concentrations. We have followed the increase in the SHLS intensity of GNPs as a function of the added concentration of Lyz in small aliquots. The SH intensity profile exhibits saturation behaviour and was fitted with a modified Langmuir adsorption model which yielded the binding constant (Kb), the binding stoichiometry (nsat) at saturation and the free energy change (ΔG) in the adsorption process. The free energy change was further decomposed into changes in the enthalpy (ΔH) and entropy (ΔS) of adsorption by carrying out temperature dependent SHLS measurements in a specially designed cell. The thermodynamic quantities extracted from the measurements show that the binding is exothermic (ΔH < 0) as well as spontaneous (ΔS > 0). We find that the first step in the adsorption of Lyz on the GNP surface is nanoparticle protein corona (NP-PC) formation driven predominantly by electrostatic attraction. In the second step of adsorption, the adsorbed lysozymes on the surface form a bridge between two or more GNPs leading to the latter's aggregation, which is the main reason for the enhancement of the SH scattering signal. Although the interaction between the GNPs and Lyz is driven by strong electrostatic attraction, the thermodynamic quantities reported here indicate that the protein is physisorbed on the nanoparticle surface. We have also demonstrated that SHLS provides a new tool for full thermodynamic characterization of protein adsorption on metal nanoparticles at ultralow concentrations.

Blue-Shifted Hydrogen Bonding in the Gas Phase CH/D3CN···HCCl3 Complexes.

Blue-shifting H-bonded complexes between CHCl3 and CH/D3CN have been identified by Fourier transform infrared spectroscopy in the gas phase at room temperature. The change in FTIR peak intensity of the mixture of the two components as a function of temperature and composition provides the basis for identification of the H-bonded band in the infrared spectrum. On complex formation with CH3CN and CD3CN, the C-H stretching frequency of CHCl3 shifts to the blue by +8.7 and +8.6 cm-1, respectively. The molecular electrostatic potential calculation at the MP2/6-311++G** level has been used to arrive at the geometry of the complex. It has been reported in the literature that CHCl3 and CH/D3CN form red-shifting H-bonded complex in Ar matrix. The red shifting has been verified by doing ab initio calculations in the presence of Ar atoms, which has been attributed to the matrix effect at low temperature. The interaction of Ar with CH3CN makes the CH3CN more basic and as a result it becomes better hydrogen bond acceptor and causes red shift. The potential energy scans and NBO analysis of the Cl3CH···NCCH3 complex have been compared with those of F3CH···NCCH3 and Cl3CH···NH3 complexes. The change in electron density of the CHCl3 as a function of C-H···N distance shows that the approach of CH3CN to CHCl3 induces a shift in electron density from the H atom to the Cl atoms of CHCl3 which leads to C-H bond contraction and blue shifting of C-H stretching frequency. However, in the complex Cl3CH···NH3, where frequency shift to the red is reported, charge transfer and electrostatic interaction dominate.

Blue- and Red-Shifting Hydrogen Bonding: A Gas Phase FTIR and Ab Initio Study of RR'CO···DCCl3 and RR'S···DCCl3 Complexes.

Blue-shifting H-bonded (C-D···O) complexes between CDCl3 and CH3HCO, (CH3)2CO, and C2H5(CH3)CO, and red-shifting H-bonded (C-D···S) complexes between CDCl3 with (CH3)2S and (C2H5)2S have been identified by Fourier transform infrared spectroscopy in the gas phase at room temperature. With increasing partial pressure of the components, a new band appears in the C-D stretching region of the vibrational spectra. The intensity of this band decreases with an increase in temperature at constant pressure, which provides the basis for identification of the H-bonded bands in the spectrum. The C-D stretching frequency of CDCl3 is blue-shifted by +7.1, +4, and +3.2 cm-1 upon complexation with CH3HCO, (CH3)2CO, and C2H5(CH3)CO, respectively, and red-shifted by -14 and -19.2 cm-1 upon complexation with (CH3)2S and (C2H5)2S, respectively. By using quantum chemical calculations at the MP2/6-311++G** level, we predict the geometry, electronic structural parameters, binding energy, and spectral shift of H-bonded complexes between CDCl3 and two series of compounds named RCOR' (H2CO, CH3HCO, (CH3)2CO, and C2H5(CH3)CO) and RSR' (H2S, CH3HS, (CH3)2S, and (C2H5)2S) series. The calculated and observed spectral shifts follow the same trends. With an increase in basicity of the H-bond acceptor, the C-D bond length increases, force constant decreases, and the frequency shifts to the red from the blue. The potential energy scans of the above complexes are done, which show that electrostatic attraction between electropositive D and electron-rich O/S causes bond elongation and red shift, and the electronic and nuclear repulsions lead to bond contraction and blue shifts. The dominance of the two opposing forces at the equilibrium geometry of the complex determines the nature of the shift, which changes both in magnitude and in direction with the basicity of the hydrogen-bond acceptor.

Linear and Nonlinear Optical Properties of Tricyanopropylidene-Based Merocyanine Dyes: Synergistic Experimental and Theoretical Investigations.

New merocyanines dyes with tricyanopropylidene-based acceptor units connected to dihexylaminophenyl or dihexylaminothiophenyl donor moieties through polyenic bridges of different lengths have been designed. All derivatives exhibited a strong dipolar character and showed a typical intramolecular charge transfer (ICT) transition. NMR spectroscopy experiments combined with DFT calculations demonstrated that both the nature of the donor-acceptor pair and the length of the conjugated linker strongly impact the electronic structure of the dyes and induce alteration in the bond-length alternation (BLA) and marked shifts in the ICT absorption bands. Hyper-Rayleigh scattering experiments revealed an exponential increase in the second-harmonic generation response as the polyenic chain length was increased. Strikingly, the largest chromophores with the strongest donor-acceptor pair exhibited a very high first hyperpolarizabilty together with a cyanine-like electronic structure, which apparently contradicts the paradigm of optimal BLA predicted by the two-state model. Although it decreased as the polyenic chain length increased, all dyes also exhibited high thermal stability, which demonstrates their potential for applications in nonlinear optical devices.

Protein-Protein Interaction Probed by Label-free Second Harmonic Light Scattering: Hemoglobin Adsorption on Spectrin Surface as a Case Study.

In this article, we have studied the binding of different naturally occurring hemoglobin (Hb) variants on erythrocyte skeletal protein, spectrin surface using the label free nondestructive second harmonic light scattering (SHLS) technique in aqueous buffer. Hemoglobin variants like sickle hemoglobin (HbS) and hemoglobin E (HbE) were chosen as they associate with sickle cell disease and HbEß-thalassemia, respectively, and their interaction with spectrin is compared with normal adult hemoglobin (HbA). The concentration dependent change in the second harmonic light intensity from nanomolar spectrin solution has been measured after addition of small aliquots of hemoglobins. From the second harmonic titration data, the binding constant is calculated using a modified Langmuir adsorption model of hemoglobin binding to the spectrin surface. Interestingly, it is found that the binding constant for HbE (13.8 × 108 M-1) is 1 order of magnitude higher than that of HbS (1.6 × 108 M-1) or HbA (2.1 × 108 M-1) which indicates higher affinity of HbE for spectrin compared to HbA and HbS. The number of the Hb molecules bound to the spectrin surface was estimated to be of the order of hundred's which is determined for the first time.

Enzyme Adsorption on Nanoparticle Surface Probed by Highly Sensitive Second Harmonic Light Scattering.

Recent developments in second harmonic light scattering technique and the associated theoretical models have provided a deeper insight of molecular interactions on micro- and nanoparticle surfaces. This technique is extended to probe the thermodynamics of protein adsorption on nanoparticle surface which is crucial for understanding the fate of nanoparticle-based formulations in biomedical applications. A modified Langmuir adsorption model has been applied to extract the thermodynamic parameters from the experimental data. The general applicability of the technique is established by extracting free energy change, association constant, and binding stoichiometry of adsorption of a moderate size protein, alcohol dehydrogenase, and a small size protein, insulin, on gold nanoparticles. The free energy change for the adsorption is found to be of the order of -55kJ/mol, which indicates that the interaction of proteins with the nanoparticle surface involves weak forces. On the other hand, the low value of the free energy change makes the detachment of the protein from the particle surface easier and guarantees reversibility of the binding process. In addition, one gets the binding stoichiometry of the proteins with the nanoparticle surface which opens up the possibility of controlling the payload of the protein- or peptide-based therapeutics in future biomedical applications.

Anharmonicity in the Vibrational Spectra of Naphthalene and Naphthalene-d8: Experiment and Theory.

In this paper, we report the gas phase infrared (IR) spectra of naphthalene and naphthalene-d8 recorded in the mid-infrared region (3200-500 cm-1) using a heated multipass long path gas cell. Several combination bands appear as shoulders and satellite peaks in the 3200-2600 cm-1region along with the C-H stretch fundamental bands. Experimental IR spectra of these molecules were systematically analyzed with vibrational self-consistent field (VSCF) theory, vibrational second order perturbation theory (VPT2) and vibrational couple cluster method (VCCM) with two different potential energy surfaces obtained using B3LYP and MP2 methods. A comparative study between these two PESs was made to match the observed spectra. Final assignment of the IR spectra of naphthalene and naphthalene-d8 was done using the VCCM with MP2 potential, which provided the best match.

Combination of Cyanine Behaviour and Giant Hyperpolarisability in Novel Merocyanine Dyes: Beyond the Bond Length Alternation (BLA) Paradigm.

Merocyanine dyes that exhibit antithetic cyanine-like behaviour and giant first-order hyperpolarisability (ß) values have been designed. These cyanine-type dyes open up an intriguing route towards molecular-based electro-optic materials as well as new second-harmonic generation dyes for imaging.

Conformational stability and intramolecular hydrogen bonding in 1,2-ethanediol and 1,4-butanediol.

The gas-phase infrared spectra of 1,2-ED and 1,4-BD have been recorded at three different temperatures using a multipass gas cell of 6 m optical path length. DFT calculation has also been carried out using 6-311++G** and aug-cc-pVDZ basis sets to look for the existence of intramolecular hydrogen bonding in them from the red shift and infrared absorption intensity enhancement of the bonded O-H band compared to that of the free O-H band. Equilibrium population analysis with 10 conformers of 1,2-ED and 1,4-BD at experimental temperatures were carried out for the reconstruction of the observed vibrational spectra at that temperature using standard statistical relationships. The most abundant conformer at experimental temperatures was identified. In 1,2-ED a red shift of 45 cm(-1) in the intramolecularly interacting O-H stretching vibrational band position and no significant intensity enhancement compared to that of the free O-H have been observed. On the contrary, in one of the hydrogen-bonded conformers of 1,4-BD, a 124 cm(-1) red shift in the O-H stretching frequency and a 8.5 times intensity enhancement for the "bonded" O-H compared to that of the "free" O-H is seen. On the basis of this comparative study, we have concluded that strong intramolecular hydrogen bonding exists in 1,4-BD. But there appears to be weak intramolecular hydrogen bonding in 1,2-ED at temperatures of 303, 313, and 323 K in the gas phase. We have found that most stable hydrogen-bonded conformers of 1,4-BD are less populated than some of the non-hydrogen-bonded conformers. Even for the 1,4-BD, the relative population of the g'GG'Gt conformer, which has a strong intramolecular hydrogen bond, is less than what is predicted. Perhaps the intramolecular hydrogen bond plays a less significant role in the relative stability of the various conformers than what has been predicted from calculations and prevails in the literature.

Combined transparency and optical nonlinearity enhancement in flexible covalent multimers by operating through-space interactions between dipolar chromophores.

Subtle manipulation of mutual repulsion and polarisation effects between polar and polarisable chromophores forced in closed proximity allows achieving major (100%) enhancement of the first hyperpolarisability together with increased transparency, breaking the well-known nonlinearity-transparency trade-off paradigm.

Base triggered enhancement of first hyperpolarizability of a keto-enol tautomer.

This work describes the base triggered enhancement of first hyperpolarizability of a tautomeric organic molecule, namely, benzoylacetanilide (BA). We have used the hyper-Rayleigh scattering technique to measure the first hyperpolarizability (ß) of BA which exists in the pure keto form in water and as a keto-enol tautomer in ethanol. Its anion exists in equilibrium with the keto and enol forms at pH 11 in aqueous solution. The ß value of the anion form is 709 × 10(-30) esu, whereas that of the enol is 232 × 10(-30) esu and of the keto is 88 × 10(-30) esu. There is an enhancement of ß by ~8 times for the anion and ~3 times for the enol compared to the keto form. All these are achieved by altering the equilibrium between the three forms of BA by simple means. MP2 calculations reproduce the experimental trend, but the computed ß values are much lower than the measured values. DFT calculations with the standard B3LYP functional could not predict the right order in the ß values. The difference between experimental and calculated values is, perhaps, due to the fact that electron correlation effects are important in computing optical nonlinearities of large organic molecules and MP2 and B3LYP calculations done here for different forms of BA could not account for such effects adequately.

Influence of anion on the quadratic nonlinearity and depolarization ratios of scattered second harmonic light from cation-π complexes.

We have investigated quadratic nonlinearity (ß(HRS)) and linear and circular depolarization ratios (D and D('), respectively) of a series of 1:1 complexes of tropyliumtetrafluoroborate as a cation and methyl-substituted benzenes as π-donors by making polarization resolved hyper-Rayleigh scattering measurements in solution. The measured D and D(') values are much lower than the values expected from a typical sandwich or a T-shaped geometry of a complex. In the cation-π complexes studied here, the D value varies from 1.36 to 1.46 and D(') from 1.62 to 1.72 depending on the number of methyl substitutions on the benzene ring. In order to probe it further, ß, D and D(') were computed using the Zerner intermediate neglect of differential overlap-correction vector self-consistent reaction field technique including single and double configuration interactions in the absence and presence of BF(4) (-) anion. In the absence of the anion, the calculated value of D varies from 4.20 to 4.60 and that of D(') from 2.45 to 2.72 which disagree with experimental values. However, by arranging three cation-π BF(4)(-) complexes in a trigonal symmetry, the computed values are brought to agreement with experiments. When such an arrangement was not considered, the calculated ß values were lower than the experimental values by more than a factor of two. This unprecedented influence of the otherwise "unimportant" anion in solution on the ß value and depolarization ratios of these cation-π complexes is highlighted and emphasized in this paper.

Infrared spectra of dimethylphenanthrenes in the gas phase.

Infrared spectra of atmospherically and astronomically important dimethylphenanthrenes (DMPs), namely 1,9-DMP, 2,4-DMP, and 3,9-DMP, were recorded in the gas phase from 400 to 4000 cm(-1) with a resolution of 0.5 cm(-1) at 110 °C using a 7.2 m gas cell. DFT calculations at the B3LYP/6-311G** level were carried out to get the harmonic and anharmonic frequencies and their corresponding intensities for the assignment of the observed bands. However, spectral assignments could not be made unambiguously using anharmonic or selectively scaled harmonic frequencies. Therefore, the scaled quantum mechanical (SQM) force field analysis method was adopted to achieve more accurate assignments. In this method force fields instead of frequencies were scaled. The cartesian force field matrix obtained from the gaussian calculations was converted to a nonredundant local coordinate force field matrix and then the force fields were scaled to match experimental frequencies in a consistent manner using a modified version of the UMAT program of the QCPE package. Potential energy distributions (PEDs) of the normal modes in terms of nonredundant local coordinates obtained from these calculations helped us derive the nature of the vibration at each frequency. The intensity of observed bands in the experimental spectra was calculated using estimated vapor pressures of the DMPs. An error analysis of the mean deviation between experimental and calculated intensities reveal that the observed methyl C-H stretching intensity deviates more compared to the aromatic C-H and non C-H stretching bands.

Structure of the pyridine-chloranil complex in solution: a surprise from depolarized hyper-Rayleigh scattering measurements.

In this article, we report the structure of a 1:1 charge transfer complex between pyridine (PYR) and chloranil (CHL) in solution (CHCl(3)) from the measurement of hyperpolarizability (ß(HRS)) and linear and circular depolarization ratios, D and D', respectively, by the hyper-Rayleigh scattering technique and state-of-the-art quantum chemical calculations. Using linearly (electric field vector along X) and circularly polarized incident light, respectively, we have measured two macroscopic depolarization ratios D = I(X,X)(2ω)/I(X,Z)(2ω) and D' = I(X,C)(2ω)/I(Z,C)(2ω) in the laboratory fixed XYZ frame by detecting the second harmonic (SH) scattered light in a polarization resolved fashion. The stabilization energy and the optical gap calculated through the MP2/cc-pVDZ method using Gaussian09 were not significantly different to distinguish between the cofacial and T-shape structures. Only when the experimentally obtained ß(HRS) and the depolarization ratios, D and D', were matched with the theoretically computed values from single and double configuration interaction (SDCI) calculations performed using the ZINDO-SCRF technique, we concluded that the room temperature equilibrium structure of the complex is cofacial. This is in sharp contrast to an earlier theoretical prediction of the T-shape structure of the complex.