Self-consistent Reaction Field Research Articles

The influence of hydrophobic amino acid side groups on the acidity of the aromatic imidazole ring of histidine: A theoretical study

In this study we have calculated the acidity constant (pKa) of imidazole ring in Histidine-Hydrophobic amino acid dipeptides using the quantum chemistry and continuum solvation methods. Density functional theory calculations with the large basis sets are used to determine the Gibbs free energy of deprotonate in the gas and liquid phases. Based on our results ΔGS values are located between −69.38 and −18.82 kcal mol−1 which are related to His+–Gly and His forms, respectively. pKa of the dipeptides in the aqueous phase was obtained from the calculated gas-phase and solvation free energies through a thermodynamic cycle and the solvation model chemistry of Martin Karplus et al. Solvation effects are treated using a self-consistent reaction field formalism involving polarized continuum models. According to our calculations pKa values are between 5.50 and 8.19 that are belong to His+–ILe and His+–Ala forms, respectively. Natural bond orbital analysis of dipeptides reveals that the electron delocalization in imidazole ring is the most effective factor in determination of acidity order for these compounds. Structural analysis confirmed that the orientation of carbonyl group with respect to imidazole ring is an effective factor in imidazole ring stability. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

An experimental and theoretical study of l-tryptophan in an aqueous solution, combining two-layered ONIOM and SCRF calculations

ONIOM and SCRF calculations were performed to study the most stable theoretical structure of l-tryptophan in an aqueous solution phase and to observe the changes occurring in the structural and vibrational properties of l-tryptophan in the aqueous media. l-Tryptophan was characterized by infrared and Raman spectroscopies in the solid state and as an aqueous solution. Optimized geometries and relative stabilities for the l-tryptophan zwitterion were calculated while taking into account solvent effects using the self-consistent reaction field (SCRF) theory. The obtained results in the aqueous solution were compared with those calculated for the zwitterion in the gas phase by means of two-layered ONIOM calculations. For the complete assignments of the IR and Raman spectra of l-tryptophan in the aqueous solution phase, density functional theory (DFT) calculations were combined with Pulay's scaled quantum mechanical force field (SQMFF) methodology in order to fit the theoretical wavenumber values to the experimental values. Additionally, the roles of specific and bulk contributions from solvent effects on the properties of l-tryptophan were analyzed. Furthermore, bands corresponding to the normal modes of vibration were localized and assigned, and they served as the bases for the calculations of the corresponding force constants. Significant effects on the geometrical and vibrational frequencies were found for the studied zwitterion.

Environment-related properties of polyhydroxylated dibenzo-p-dioxins

Polyhydroxylated dibenzo-p-dioxins (PHODDs) are important metabolic and synthetic products of polychlorinated dibenzo-p-dioxins (PCDDs). Two types of hydrogen bonds exist in PHODD molecules: one between a hydroxyl group (HO) and an oxygen atom of the ether bond, and the other between two ortho hydroxyls of a benzene ring. By fully optimized calculation with density functional theory (DFT), their bond energies were ascertained to be approximately 9–14kJ/mol and 15–19kJ/mol respectively by the comparison of standard Gibbs energy of formation (ΔfGθ) between different molecules, which was experimentally verified. The two types of hydrogen bonds affect the hydrophilicity and stability of the molecules. The torsional potential of hydroxyls and the orientation making the congener most stable were obtained. The octanol-water partition coefficients (logKows) were calculated based on the group contribution method, and the standard state entropy (Sθ), standard enthalpy (ΔfHθ) of formation and ΔfGθ were obtained from the combination of DFT calculation and isodesmic reaction for the stable PHODD congeners. The number and position of hydroxyl substitution (NPHOS) were employed as descriptors to establish quantitative structure–property relationship (QSPR) models. Although the hydrophilicity of PHODDs increases with the number of hydroxyl groups, it is impaired by the intramolecular hydrogen bonds. The logKows of PHODDs are much smaller than those of PCDDs, and the variation trend with the number of substituents is different. In addition, the relative stability order of PHODD congeners was theoretically proposed, which is quite different from that of PCDDs. Considering the ionization in water, first-order ionization constants of PHODDs were calculated according to the results of SMD method of Self-Consistent Reaction Field Theory (SCRF), and they were influenced by the hydrogen bonds.

Investigation of drug delivery on anticancer drug by SWCNT with theoretical studies

The interaction of anticancer drug Sn(CH3)2(N-acetyl-L-cysteinate), that is called (N-acetyl-L-cysteinato-O,S) dimethyl tin(IV), with single walled carbon nanotube (SWCNT) is investigated by Quantum chemical ab initio calculations at HF/(LanL2DZ+STO-3G) and HF/(LanL2DZ+6-31G) levels in gas phase and solution. The solvent effect is taken into account via the self-consistent reaction field (SCRF) method. Carbon nanotubes can act as a suitable drug delivery vehicle for internalization, transportation and translocation of Sn(CH3)2(N-acetyl-L-cysteinate) within biological systems. Thermodynamical analysis indicates that the relative energies (ΔE), enthalpies (ΔH) and free Gibbs energies (ΔG) are negative for Sn(CH3)2(N-acetyl-L-cysteinate)-CNT system, but the calculated entropies (ΔS) are positive, suggesting thermodynamic favorability for covalent attachment of Sn(CH3)2(N-acetyl-L-cysteinate) into carbon nanotube. Also, the results show that with increasing dielectric constant of solvent, the stability of Sn(CH3)2(N-acetyl-L-cysteinate)-CNT complex decreases. Furthermore, anisotropic chemical shift tensor (Δσ), total atomic charge and asymmetry parameter (η) have been calculated using the gauge-including atomic orbital (GIAO) method, results being compared with CGST data. From the nuclear magnetic resonance (NMR) calculations, it can be seen that the NMR (Δσ, η) parameters at the sites of nitrogen and oxygen, as well as C-2 and C-3 nuclei are significantly influenced by intermolecular hydrogen-bonding interactions, but the quantity at the site of S-27 is influenced by nonspecific solute-solvent interaction, such as polarisability/polarity. Key words: Sn(CH3)2(N-acetyl-L-cysteinate), nuclear magnetic resonance (NMR) parameters, single walled nanotube (SWNT), solvent.

Solvents Level Dipole Moments

The dipole moments of highly polar molecules measured in solution are usually smaller than the molecular dipole moments that are calculated with reaction field methods, whereas vacuum values are routinely calculated in good agreement with available vapor phase data. Whether from Onsager's theory (or variations thereof) or from quantum mechanical methods, the calculated molecular dipoles in solution are found to be larger than those measured. The reason, of course, is that experiments measure the net dipole moment of solute together with the polarized (perturbed) solvent "cloud" surrounding it. Here we show that the reaction field charges that are generated in the quantum mechanical self-consistent reaction field (SCRF) method give a good estimate of the net dipole moment of the solute molecule together with the moment arising from the reaction field charges. This net dipole is a better description of experimental data than the vacuum dipole moment and certainly better than the bare dipole moment of the polarized solute molecule.

Influence of Hydrophobic amino acid side groups on the acidity of aromatic imidazole ring of Histidine: A theoretical study

In this study we have calculated the acidity constant (pKa) of imidazole ring in Histidine-Hydrophobic amino acid dipeptides using the quantum chemistry and continuum solvation methods. Density functional theory calculations with the large basis sets are used to determine the Gibbs free energy of deprotonate in the gas and liquid phases. Based on our results DGS values are located between � 69.38 and � 18.82 kcal mol � 1 which are related to His þ –Gly and His forms, respectively. pKa of the dipeptides in the aqueous phase was obtained from the calculated gas-phase and solvation free energies through a thermodynamic cycle and the solvation model chemistry of Martin Karplus et al. Solvation effects are treated using a self-consistent reaction field formalism involving polarized continuum models. According to our calculations pKa values are between 5.50 and 8.19 that are belong to His þ –ILe and His þ –Ala forms, respectively. Natural bond orbital analysis of dipeptides reveals that the electron delocalization in imidazole ring is the most effective factor in determination of acidity order for these compounds. Structural analysis confirmed that the orientation of carbonyl group with respect to imidazole ring is an effective factor in imidazole ring stability. V C 2011 Wiley Periodicals, Inc.

Theoretical Study of Different Solvents and Temperatures Effects on Single-Walled Carbon Nanotube and Temozolomide Drug: A QM/MM Study

Recently, the majority of the investigation into computational chemistry has been focused on the examination of molecules submerged in various solvents and the interaction of anticancer drug with carbon nanotubes (CNTs). The most likely tendency is toward the interaction between anticancer drug and single-walled carbon nanotubes (SWCNTs). In this investigation, the interaction of Temozolomide, a common form of anticancer drug with open-end of SWCNT, are examined, with MM+, Amber and OPLS force field in molecular mechanic (MM) method. We investigate effects of water, methanol and a mixture of them on interaction of Temozolomide with SWCNT, utilizing these force fields. The interaction of Temozolomide with open-end of SWCNT in gas phase, water and methanol have been processed applying the abinitio calculations. The calculations have been done with the GAUSSIAN 98 program according to Hartree-Fock (HF) theory at the HF/STO-3G level. Gibbs free energy, enthalpy, entropy and dipole moment values are compared in gas phase, water and methanol in this research. Thus, by utilizing a Hartree-Fock method, we studied the effects of different solvents on interaction of Temozolomide with open-end SWCNT within the onsager self-consistent reaction field (SCRF) model, as well as the temperature effects on the stability of the interaction between Temozolomide and single-walled carbon in various solvents.

Structure and vibrational assignment of tautomerism of 4-hydroxyquinazoline in gaseous and aqueous phases

The tautomerizm and conformation of the 4-hydroxyquinazoline molecule were investigated, and its three tautomers (4-HQ1, 4-HQ2, 4-HQ3) and conformer of 4-HQ2 (4-HQ2A) were considered. The tautomeric and conformational equilibrium geometry of 4-hydroxyquinazoline have been studied using density functional theory (DFT) and Møller–Plesset second-order (MP2) methods in the gas phase and in water. The tautomerization processes in water ( ε = 78.39) were studied by using self-consistent reaction field theory with the polarized continuum model (PCM). Energetics and relative stabilities of the tautomers were compared and analyzed in both the gaseous and water. In agreement with experimental results, our calculations show that the 4-HQ1 was the most stable form in the gas phase and in water. Solvent effect induced slight changes in geometry of rings and the order of stability changed. The harmonic wavenumbers of 4-HQ1 were calculated in the gas phase and in water by the B3LYP/6-311++G ∗∗ DFT method and the wavenumbers were given. The infrared spectra of 4-hydroxyquinazoline were recorded in the regions 400–4000 cm −1. The variation of dipole moments and charges on atoms were studied in the gas phase and in water.

Ab Initio Study of the Anomalous Solvatochromic Behavior of Large Betaines

The structure and spectroscopic properties of a diluted compound can be deeply affected by its interaction with the neighboring molecules of the solvent, and the associated solvatochromism is an effect that becomes more noticeable with the increase in both the dipole moment of the solute and the polarity of the medium. The correct description of the complex set of interactions that prevail in the solvation process remains a challenge for theoreticians not only when interpreting an observed behavior but also when considering the possible existence of novel properties in untested solute-solvent systems. On the basis of an ab initio study, we examine here how the presence of solvents of different polarities should affect the electronic properties of a family of molecules, formally related to Betaine-30 (aka Reichardt's dye), whose donor (D) and acceptor (A) groups are terminally connected to conjugated chains of different sizes. Because these molecules exhibit elevated ground-state dipole moment that should strongly interact with molecules of a polar solvent, a large hypsochromic shift is predicted for them. However, in a recent gas-phase study of these molecules, we have established the existence of an "inversion" in the spatial localization of their frontier orbitals when the size of the conjugated bridge connecting the D and A groups is progressively increased. This fact has led us to suggest that the increase in size of dissolved betaines should be accompanied by a large variation in their solvatochromic properties. In this work, we first use the self-consistent reaction field approach at the configuration interaction level to estimate the expected bathochromic shift in the absorption spectra (positive solvatochromism) in the largest members of the investigated betaine family when dissolved in different low polarity solvents and then discuss the conformational changes as a consequence of the solute--solvent interactions. We then use these results to interpret the observed solvatochromic properties of push--pull molecules of varying size and discuss the corresponding implications on their photochemical properties.

Hydration of l-tyrosine in aqueous medium. An experimental and theoretical study by mixed quantum mechanical/molecular mechanics methods

Quantum mechanical/molecular mechanics (QM/MM) calculations were carried out in order to study the theoretical structures of l-tyrosine in both gas phase and in aqueous solution and observe the changes that occur on the structural and vibrational properties in two phases. Therefore, the molecule was characterized by infrared and Raman spectroscopy in solid phase and aqueous solution. Optimized geometries and relative stabilities for the zwitterion l-tyrosine derivatives have been calculated taking into account the solvent effects by using the self-consistent reaction field (SCRF) theory. For a complete assignment of the IR and Raman spectra of l-tyrosine in solid and aqueous solution phases, density functional theory (DFT) calculations were combined with Pulay's scaled quantum mechanical force field (SQMFF) methodology in order to fit the theoretical wavenumber values to the experimental ones. A good agreement between theoretical and available experimental results is found.

The role of the heteroatom (X = SiIV, PV, and SVI) on the reactivity of {γ-[(H2O)RuIII(μ-OH)2RuIII(H2O)][X n+W10O36]}(8−n)− with the O2 molecule

The mechanism of reaction of the di-Ru-substituted polyoxometalate, {γ-[(H2O)RuIII(μ-OH)2RuIII(H2O)][X n+W10O36]}(8−n)−, I_X, with O2, i.e. I_X + O2 → {γ-[(·O)RuIV(μ-OH)2RuIV(O·)][X n+W10O36]}(8−n)− + 2H2O, (1), was studied at the B3LYP density functional and self-consistent reaction field IEF-PCM (in aqueous solution) levels of theory. The effect of the nature of heteroatom X (where X = Si, P and, S) on the calculated energies and mechanism of the reaction (1) was elucidated. It was shown that the nature of X only slightly affects the reactivity of I_X with O2, which is a 4-electron oxidation process. The overall reaction (1): (a) proceeds with moderate energy barriers for all studied X’s [the calculated rate-determining barriers are X = Si (18.7 kcal/mol) P [21.4 (9.8) kcal/mol] > S [12.3 (5.0) kcal/mol]. The resulting $$ \left\{ {\gamma - \left[ {\left( {^{ \cdot } {\text{O}}} \right) {\text{Ru}}^{\text{IV}} \left( {\mu - {\text{OH}}} \right)_{2} {\text{Ru}}^{\text{IV}} \left( {{\text{O}}^{ \cdot } } \right)} \right]\left[ {{\text{X}}^{{{\text{n}} + }} {\text{W}}_{10} {\text{O}}_{36} } \right]} \right\}^{{\left( {8 - {\text{n}}} \right) - }} $$ , VI_X, complex was found to have two RuIV = O· units, rather than RuV = O units. The “reverse” reaction, i.e., water oxidation by VI_X is an endothermic process and unlikely to occur for X = Si and P, while it could occur for X = S under specific conditions. The lack of reactivity of VI_X biradical toward the water molecule leads to the formation of the stable [{Ru 4 IV O4(OH)2(H2O)4}[(γ-XW10O36]2}m− dimer. This conclusion is consistent with our experimental findings; previously we prepared the $$ \left[ {\left\{ {{\text{Ru}}_{4}^{\text{IV}} {\text{O}}_{4} ({\text{OH}})_{2} \left( {{\text{H}}_{ 2} {\text{O}}} \right)_{4} } \right\}} \right[\left( {\gamma - {\text{XW}}_{10} {\text{O}}_{36} } \right]_{2} \}^{{{\text{m}} - }} $$ dimers for X = Si (m = 10) [Geletii et al. in Angew Chem Int Ed 47:3896–3899, 2008 and J Am Chem Soc 131:17360–17370, 2009] and P (m = 8) [Besson et al. in Chem Comm 46:2784–2786, 2010] and showed them to be very stable and efficient catalysts for the oxidation of water to O2.

Investigation of Solvent Effects on Armchair Single-Walled Carbon Nanotubes: A QM/MD Study

In this research, we studied solvent effects on the relative energies, dipole moment values, and structural properties of water, methanol, ethanol, and mixtures of them surrounding single-walled carbon nanotubes (SWCNT), by using molecular dynamics simulation. Calculations were carried out with the Hyper Chem program. We used different force fields to determine energy and other types of geometrical parameters on the particular SWCNT. Because of the differences among force fields, the energy of a molecule calculated using two different force fields will not be the same. In this study, differences in force fields was illustrated by comparing the energy calculated by using force fields MM+, Amber and OPLS. The quantum Mechanics (QM) calculations were carried out with the GAUSSIAN 98 program based on density functional theory (DFT) at the B1LYP/3-21G level. In this study, we have compared gas phase and solvent calculations that considered solvents such as water, methanol and ethanol and a mixture of them. Therefore, we investigate polar solvents effects on SWCNT within the Onsager self-consistent reaction field (SCRF) model using a Hartree-Fock method and the temperature effect on the stability of SWCNT in various solvents.

Theoretical Study of the Gauche and Trans Conformers of SiH2X–CH2X, SiH2F–CH2Y and SiH2Y–CH2F (X = F, Cl, Br, I and Y = Cl, Br, I) in the Gas and Solution Phases

The gauche and trans rotamers of halogeno(halogenomethyl)silane (XSiH2CH2X; X = F, Cl, Br, I), fluoro(halogenomethyl)silane and halogeno(fluoromethyl)silane (SiH2F–CH2Y and SiH2Y–CH2F; Y = Cl, Br, I) have been studied in the gas phase using theoretical methods. The transition state arising from gauche-trans isomerization has also been modeled. The methods used are density functional theory (DFT) and second-order Moller–Plesset theory (MP2). B3LYP is the functional used for the DFT method. The basis set used is 6-311++G(d,p) for all atoms except that 6-311G(d,p) is used for the iodine atom only. The results indicate that the trans conformers are preferred in the gas phase and both energy difference and rotational barrier height increase as the size of the halogen increases. This study has been extended to include the solvent effect with the dielectric constant of the solvents varying from 2 to 80. The solvent effect was explored using Self-Consistent Reaction Field and the conformers have been fully optimized at the DFT/B3LYP level of theory. The net effect of a solvent is that energy difference decreases but the rotational barrier is not much affected. The findings from this work are explained in terms of different interactions and these are supported by a Natural Bond Orbital analysis.

Hydrogen bond interactions in hydrated acetylsalicylic acid

Post Hartree–Fock and density functional theory (DFT) methods have been employed to study the molecular properties of the conformers of acetylsalicylic acid and their hydrated forms. The structures of monomer and hydrated acetylsalicylic acid are theoretically investigated at MP2 and B3LYP level of theories, in gaseous and aqueous phase environment implementing the 6-311G (2d, 2p) atomic basis set. Their interaction energy and the strength of the hydrogen bonds have been analyzed. The complex with strong hydrogen bond is found to be the most stable among the monohydrated acetylsalicylic acid complexes. The self-consistent reaction field theory (SCRF) has been employed to optimize acetylsalicylic acid complexes in aqueous phase ( ε = 78.5) environment and the solvent effect has been studied. The atoms in molecules theory (AIM) is employed to investigate H-bonding patterns in the complexes. From the above topological analysis, an excellent linear correlation is shown between electron density [ ρ( r)] and its Laplacian [∇ 2 ρ( r)] at bond critical points. The charge transfer from the proton acceptor to the anti-bonding orbital of proton donor has been analyzed through natural bond orbital (NBO) analysis. NMR chemical shift has been calculated to analyze the molecular environment as well as the delocalization of electron density.

Synthesis, structure, catalytic and calculated non-linear optical properties of cis- and trans-, mer-chlorobis(triphenyl phosphine/triphenyl arsine)-dipicolinato ruthenium III complexes

The new mononuclear Ru III complexes cis-, mer-[Ru( k 3-dipic)(EPh 3) 2Cl]·2H 2O (E = P ( 1), As ( 2)), (L = 2,6-pyridyldicarboxylate; dipic) have been synthesized and characterized. The X-ray crystal structure of 1 reveals that the coordination geometry around the Ru III center is distorted octahedral in which three sites are occupied by tridentate dipic ligand and remaining three sites are occupied by two mutually cis triphenyl phosphine ligands and one chloride ions. In chloroform solvent, cis-, mer-[Ru( k 3-dipic)(EPh 3) 2Cl]·2H 2O (E = P ( 1), As ( 2)) complexes undergoes thermal rearrangement to more stable trans-, mer-[Ru( k 3-dipic)(EPh 3) 2Cl] (E = P( 3), As( 4)) complexes. Complexes 1 and 2 show rhombic EPR spectral features, while complex 3 and 4 show tetragonal distortion. The new Ru III complexes 1 and 2 display both Ru III–Ru II reduction and Ru III–Ru IV oxidation processes. These geometric isomers exhibit textbook differences both in spectroscopic as well as structural properties in the solid state. The new complexes 1 and 2 were found to catalyze the oxidation of alcohols to carbonyl derivatives using N-methyl morpholine- N-oxide as co-oxidant. The first static hyperpolarizability ( β) for all of the complexes have been investigated by density functional theory (DFT) which suggests β value increases from trans to cis form. Also, the solvent-induced effects on the non-linear optical properties (NLO) were studied by using self-consistent reaction field (SCRF) method. As the solvent polarity increases, the β value increases monotonically. The electronic absorption bands of 1 have been assigned by time dependent density functional theory (TD-DFT).

Solvation analysis of halogenated 3-phenyl 5-amino pyrazoles

Quantum mechanical solvation analysis accounts for electric and electro static components of solvation by microscopic surface tension. Solvation analysis is performed for two halo substituted phenyl 1H pyrazole-5 amines in different solvents. The free energy and its components such as electrostatic interaction, dispersion energy, and repulsion energy are computed. The systematic comparison of the electrostatic contribution of the free energy of solvation is carried out from the self-consistent reaction field. The effects of cavities and the dipole moments are examined. The investigated set consists of ten solvents with wide range of dielectric constant.

Solvent Dependence of Absorption Intensities and Wavenumbers of the Fundamental and First Overtone of NH Stretching Vibration of Pyrrole Studied by Near-Infrared/Infrared Spectroscopy and DFT Calculations

Near-infrared (NIR) and IR spectra were measured for pyrrole in CCl(4), CHCl(3), and CH(2)Cl(2) to study solvent dependence of absorption intensities and wavenumbers of the fundamental and first overtone of NH stretching vibration. It was found that the wavenumbers of the NH fundamental and its first overtone decrease in the order of CCl(4), CHCl(3), and CH(2)Cl(2), which is the increasing order for of the dielectric constant of the solvents. Their absorption intensities increase in the same order, and the intensity increase is more significant for the fundamental than the overtone. These results for the solvent dependence of the wavenumbers and absorption intensities of NH stretching bands of pyrrole are quite different from those due to the formation of hydrogen bonds. Quantum chemical calculations of the wavenumbers and absorption intensities of NH stretching bands by using the 1D Schrödinger equation based on the self-consistent reaction field (SCRF)/isodensity surface polarized continuum model (IPCM) suggest that the decreases in the wavenumbers of both the fundamental and the overtone of the NH stretching mode with the increase in the dielectric constant of the solvents arise from the anharmonicity of vibrational potential and their intensity increases come from the gradual increase in the slope of the dipole moment function.

Biophysical chemistry of macrocycles for drug delivery: a theoretical study

Ab initio (RHF/STO-3G) quantum chemical calculations using the self-consistent reaction field (SCRF) model were carried out to analyze the effect of solvent polarity on the relative Gibbs free energies, the dipole moments, and the structural stability of peptide macrocycles based on unsubstituted cyclo[Gly6] and its trisubstituted derivatives containing Me, NH2, or OH groups at the Cα atom. The macrocycles studied are stable in water at both room temperature and at body fever temperature, which is important for the design of a stable nanovehicle for drug delivery in water.

Geometry and quadratic nonlinearity of charge transfer complexes in solution: A theoretical study

In this paper, we have computed the quadratic nonlinear optical (NLO) properties of a class of weak charge transfer (CT) complexes. These weak complexes are formed when the methyl substituted benzenes (donors) are added to strong acceptors like chloranil (CHL) or di-chloro-di-cyano benzoquinone (DDQ) in chloroform or in dichloromethane. The formation of such complexes is manifested by the presence of a broad absorption maximum in the visible range of the spectrum where neither the donor nor the acceptor absorbs. The appearance of this visible band is due to CT interactions, which result in strong NLO responses. We have employed the semiempirical intermediate neglect of differential overlap (INDO∕S) Hamiltonian to calculate the energy levels of these CT complexes using single and double configuration interaction (SDCI). The solvent effects are taken into account by using the self-consistent reaction field (SCRF) scheme. The geometry of the complex is obtained by exploring different relative molecular geometries by rotating the acceptor with respect to the fixed donor about three different axes. The theoretical geometry that best fits the experimental energy gaps, β(HRS) and macroscopic depolarization ratios is taken to be the most probable geometry of the complex. Our studies show that the most probable geometry of these complexes in solution is the parallel displaced structure with a significant twist in some cases.

Geometry and quadratic nonlinearity of charge transfer complexes in solution using depolarized hyper-Rayleigh scattering

We report large quadratic nonlinearity in a series of 1:1 molecular complexes between methyl substituted benzene donors and quinone acceptors in solution. The first hyperpolarizability, β(HRS), which is very small for the individual components, becomes large by intermolecular charge transfer (CT) interaction between the donor and the acceptor in the complex. In addition, we have investigated the geometry of these CT complexes in solution using polarization resolved hyper-Rayleigh scattering (HRS). Using linearly (electric field vector along X direction) and circularly polarized incident light, respectively, we have measured two macroscopic depolarization ratios D=I(2ω,X,X)/I(2ω,Z,X) and D(')=I(2ω,X,C)/I(2ω,Z,C) in the laboratory fixed XYZ frame by detecting the second harmonic scattered light in a polarization resolved fashion. The experimentally obtained first hyperpolarizability, β(HRS), and the value of macroscopic depolarization ratios, D and D('), are then matched with the theoretically deduced values from single and double configuration interaction calculations performed using the Zerner's intermediate neglect of differential overlap self-consistent reaction field technique. In solution, since several geometries are possible, we have carried out calculations by rotating the acceptor moiety around three different axes keeping the donor molecule fixed at an optimized geometry. These rotations give us the theoretical β(HRS), D and D(') values as a function of the geometry of the complex. The calculated β(HRS), D, and D(') values that closely match with the experimental values, give the dominant equilibrium geometry in solution. All the CT complexes between methyl benzenes and chloranil or 1,2-dichloro-4,5-dicyano-p-benzoquinone investigated here are found to have a slipped parallel stacking of the donors and the acceptors. Furthermore, the geometries are staggered and in some pairs, a twist angle as high as 30° is observed. Thus, we have demonstrated in this paper that the polarization resolved HRS technique along with theoretical calculations can unravel the geometry of CT complexes in solution.