One- and two-photon absorption spectra of organoboron complexes: vibronic and environmental effects.

We synthesized a series of four parent aza-ß-ketoiminate organoboron complexes and performed spectroscopic studies using both experimental and computational techniques. We studied how benzannulation influences the vibronic structure of the UV/Vis absorption bands with a focus on the bright lowest-energy π → π* electronic excitation. Theoretical simulations, accounting for inhomogeneous broadening effects using different embedding schemes, allowed gaining in-depth insights into the observed differences in band shapes induced by structural modifications. We observed huge variations in the distributions of vibronic transitions depending on the position of benzannulation. By and large, the harmonic approximation combined with the adiabatic hessian model delivers qualitatively correct band shapes for the one-photon absorption spectra, except in one case. We also assessed the importance of non-Condon effects (accounted for by the linear term in Herzberg-Teller expansion of the dipole moment) for S0 → S1 band shapes. It turned out that non-Condon contributions have no effect on the band shape in one-photon absorption spectra. In contrast, these effects significantly change the Franck-Condon band shapes of the two-photon absorption spectra. For one of the studied organoboron complexes we also performed a preliminary exploration of mechanical anharmonicity, resulting in an increase of the intensity of the 0-0 transition, which improves the agreement with the experimental data compared to the harmonic model.

Photoswitchable Molecular Units with Tunable Nonlinear Optical Activity: A Theoretical Investigation.

The first-, second-, and third-order molecular nonlinear optical properties, including two-photon absorption of a series of derivatives, involving two dithienylethene (DTE) groups connected by several molecular linkers (bis(ethylene-1,2-dithiolato)Ni- (NiBDT), naphthalene, quasilinear oligothiophene chains), are investigated by employing density functional theory (DFT). These properties can be efficiently controlled by DTE switches, in connection with light of appropriate frequency. NiBDT, as a linker, is associated with a greater contrast, in comparison to naphthalene, between the first and second hyperpolarizabilities of the "open-open" and the "closed-closed" isomers. This is explained by invoking the low-lying excited states of NiBDT. It is shown that the second hyperpolarizability can be used as an index, which follows the structural changes induced by photochromism. Assuming a Förster type transfer mechanism, the intramolecular excited-state energy transfer (EET) mechanism is studied. Two important parameters related to this are computed: the electronic coupling (VDA) between the donor and acceptor fragments as well as the overlap between the absorption and emission spectra of the donor and acceptor groups. NiBDT as a linker is associated with a low electronic coupling, VDA, value. We found that VDA is affected by molecular geometry. Our results predict that the linker strongly influences the communication between the open-closed DTE groups. The sensitivity of the molecular nonlinear optical properties could assist with identification of molecular isomers.

Decoding the infrared spectra changes upon formation of molecular complexes: the case of halogen bonding in pyridine⋯perfluorohaloarene complexes.

A recently developed computational scheme is employed to interpret changes in the infrared spectra of halogen-bonded systems in terms of intermolecular interaction energy components (electrostatic, exchange, induction, dispersion) taking pyridine⋯perfluorohaloarene complexes as examples. For all complexes, we find a strong linear correlation between the different terms of the interaction-induced changes of the IR band associated with an intermolecular halogen bond stretching mode and the corresponding terms of the interaction energy, which implies that the interaction components play similar roles in both properties. This is not true for other vibrational modes localized in one of the monomers studied here, for which the corresponding interaction-induced changes in IR bands may present a completely different decomposition than the interaction energy.

A new computational tool for interpreting the infrared spectra of molecular complexes.

The popularity of infrared (IR) spectroscopy is due to its high interpretive power. This study presents a new computational tool for analyzing the IR spectra of molecular complexes in terms of intermolecular interaction energy components. In particular, the proposed scheme enables associating the changes in the IR spectra occurring upon complex formation with individual types of intermolecular interactions (electrostatic, exchange, induction, and dispersion), thus providing a completely new insight into the relations between the spectral features and the nature of interactions in molecular complexes. To demonstrate its interpretive power, we analyze, for selected vibrational modes, which interaction types rule the IR intensity changes upon the formation of two different types of complexes, namely π⋯π stacked (benzene⋯1,3,5-trifluorobenzene) and hydrogen-bonded (HCN⋯HNC) systems. The exemplary applications of the new scheme to these two molecular complexes revealed that the interplay of interaction energy components governing their stability might be very different from that behind the IR intensity changes. For example, in the case of the dispersion-bound π⋯π-type complex, dispersion contributions to the interaction induced IR intensity of the selected modes are notably smaller than their first-order (electrostatic and exchange) counterparts.

Accurate Nonlinear Optical Properties of Solvated para-Nitroaniline Predicted by an Electrostatic Discrete Local Field Approach.

A general computational protocol for accurate predictions of nonlinear optical (NLO) properties of solvated molecules based on the rigorous local field (RLF) approach taking all relevant effects into account is presented. para-Nitroaniline (pNA) was taken as a model NLO system dissolved in cyclohexane, tetrahydrofuran, and 1,4-dioxane. Molecular dynamics (MD) simulations employing either non-polarizable or polarizable force fields were used to generate representative sets of structures of the solutions. The static NLO properties of the solute were calculated at the MP2/aug-cc-pVDZ level of theory with the multiplicative scaling method used to account for the frequency dispersion of the properties. Focusing on the electric field-induced second harmonic generation (EFISH) and hyper-Rayleigh scattering (HRS), a good agreement between calculated results and experimental measurements was achieved with a polarizable force field. While the solvent effects on the vibrational contributions to the static molecular properties are significant, they remain small for both EFISH and HRS. Our results show that the proposed approach yields reliable predictions of dynamic NLO properties of solvated chromophores, which paves a route to further applications of the RLF approach to study a wide range of NLO phenomena in heterogeneous environments.

Partitioning of interaction-induced nonlinear optical properties of molecular complexes. II. Halogen-bonded systems.

Following our study on hydrogen-bonded (HB) complexes [Phys. Chem. Chem. Phys., 2018, 20, 19841], the physical nature of interaction-induced (non)linear optical properties of another important class of molecular complexes, namely halogen-bonded (XB) systems, was analyzed in this study. The excess electronic and nuclear relaxation (hyper)polarizabilities of nine representative XB complexes covering a wide range of halogen-bond strengths were computed. The partitioning of the excess properties into individual interaction-energy components (electrostatic, exchange, induction, dispersion) was performed by using the variational-perturbational energy decomposition scheme at the MP2/aug-cc-pVTZ level of theory and further supported by calculations with the SCS-MP2 method. In the case of the electronic interaction-induced properties, the physical composition of Δαel and Δγel was found to be very similar for the two types of bonding, despite the different nature of the binding. For Δßel, the XB complexes exhibit a more systematic interplay of interaction-energy contributions compared to the HB systems studied in the previous work. Our analysis revealed that the patterns of interaction-energy contributions to the interaction-induced nuclear-relaxation contributions to the linear polarizability and the first hyperpolarizability are very similar. For both properties the exchange repulsion term is canceled out by the electrostatic and delocalization terms. The physical composition of these contributions is analogous to those observed for the HB complexes.

Partitioning of interaction-induced nonlinear optical properties of molecular complexes. I. Hydrogen-bonded systems.

Understanding the effects of different fundamental intermolecular interactions on nonlinear optical properties is crucial for proposing efficient strategies to obtain new materials with tailored properties. In this study, we computed the electronic and vibrational (hyper)polarizabilities of ten hydrogen-bonded molecular complexes employing the MP2, CCSD and CCSD(T) methods combined with the aug-cc-pVTZ basis set. The vibrational contributions to hyperpolarizabilities included nuclear-relaxation anharmonic corrections. The effect of intermolecular interactions was analyzed in terms of excess properties, which are defined as the difference between a property of the complex and the net properties of the noninteracting subsystems. Considering systems covering a wide range of hydrogen bond strengths, the electronic and vibrational excess (hyper)polarizabilities were decomposed into different interaction energy contributions (electrostatic, exchange, induction and dispersion). This systematic study, the very first of this kind, revealed that the physical origins of the electronic and vibrational excess properties are completely different. In the case of vibrational contributions, the decomposition pattern is very similar for the polarizability and first and second hyperpolarizabilities. The exchange contributions to excess vibrational properties are the largest and they have an opposite sign to the electrostatic, induction and dispersion terms. On the other hand, no general patterns can be established for the electronic excess properties.

Dipole and quadrupole polarizabilities of the water molecule as a function of geometry.

Dipolar, dipole-quadrupole and quadrupole-quadrupole static polarizabilities of the water molecule have been determined by ab initio calculations at coupled cluster level of theory with single, double and perturbative triple excitations CCSD(T) with an aug-cc-pVTZ basis set using a finite field and field-gradient method. The geometry dependence of polarizability tensor components has been explored and modeled by power series expansion in bond length and angle variations up to sum of powers equal to 4. The results provide a very detailed description of the static polarizability of water up to quadrupolar rank which can be used for the test and development of novel accurate polarizable interaction potentials for modeling aqueous solutions. © 2016 Wiley Periodicals, Inc.

Numerical and exact kinetic energy operator using Eckart conditions with one or several reference geometries: Application to HONO.

For the computation of rovibrational levels and their spectroscopic intensities, the Eckart conditions are essential to achieve the optimal separation between rotation and vibration. Dymarsky and Kudin [J. Chem. Phys. 122, 124103 (2005)] proposed a procedure for a simplified calculation of the Eckart rotation matrix. In the present work, we have adapted their approach to obtain a kinetic energy operator in curvilinear coordinates using a numerical but exact procedure without resorting to finite differences. Furthermore, we have modified this approach for the study of molecular systems with several minima, for which several Eckart reference geometries are required. The HONO molecular system has been used to show the efficiency of our implementation. Using the Eckart conditions with multi-reference geometries allows for a calculation of the rotational levels as well as frequencies and intensities of the infrared spectra of both HONO isomers with a single calculation.

A Comprehensive Computational Study of the Interaction between Human Serum Albumin and Fullerenes.

Human serum albumin (HSA) is the most abundant blood plasma protein, which transports fatty acids, hormones, and drugs. We consider nanoparticle-HSA interactions by investigating the binding of HSA with three fullerene analogs. Long MD simulations, quantum mechanical (fragment molecular orbital, energy decomposition analysis, atoms-in-molecules), and free energy methods elucidated the binding mechanism in these complexes. Such a systematic study is valuable due to the lack of comprehensive theoretical approaches to date. The main elements of the mechanism include the following: binding to IIA site results in allosteric modulation of the IIIA and heme binding sites with an increase in α-helical structure of IIIA. Fullerenes displayed high binding affinities for HSA; therefore, HSA can be used as a fullerene carrier, facilitating any toxic function the fullerene may exert. Complex formation is driven by hydrogen bonding, van der Waals, nonpolar, charge transfer, and dispersion energy contributions. Proper functionalization of C60 has enhanced its binding to HSA by more than an order of magnitude. This feature may be important for biological applications (e.g., photodynamic therapy of cancer). Satisfactory agreement with relevant experimental and theoretical data has been obtained.

Elucidation of the binding mechanism of renin using a wide array of computational techniques and biological assays.

We investigate the binding mechanism in renin complexes, involving three drugs (remikiren, zankiren and enalkiren) and one lead compound, which was selected after screening the ZINC database. For this purpose, we used ab initio methods (the effective fragment potential, the variational perturbation theory, the energy decomposition analysis, the atoms-in-molecules), docking, molecular dynamics, and the MM-PBSA method. A biological assay for the lead compound has been performed to validate the theoretical findings. Importantly, binding free energy calculations for the three drug complexes are within 3 kcal/mol of the experimental values, thus further justifying our computational protocol, which has been validated through previous studies on 11 drug-protein systems. The main elements of the discovered mechanism are: (i) minor changes are induced to renin upon drug binding, (ii) the three drugs form an extensive network of hydrogen bonds with renin, whilst the lead compound presented diminished interactions, (iii) ligand binding in all complexes is driven by favorable van der Waals interactions and the nonpolar contribution to solvation, while the lead compound is associated with diminished van der Waals interactions compared to the drug-bound forms of renin, and (iv) the environment (H2O/Na(+)) has a small effect on the renin-remikiren interaction.

Theoretical modelling of photoswitching of hyperpolarisabilities in ruthenium complexes.

Static excited-state polarisabilities and hyperpolarisabilities of three Ru(II) ammine complexes are computed at the density functional theory (DFT) and several correlated ab initio levels. Most accurate modelling of the low energy electronic absorption spectrum is obtained with the hybrid functionals B3LYP, B3P86 or M06 for the complex [Ru(II)(NH3)5(MeQ(+))](3+) (MeQ(+)=N-methyl-4,4'-bipyridinium, 3) in acetonitrile. The match with experimental data is less good for [Ru(II)(NH3)5L](3+) (L=N-methylpyrazinium, 2; N-methyl-4-{E,E-4-(4-pyridyl)buta-1,3-dienyl}pyridinium, 4). These calculations confirm that the first dipole- allowed excited state (FDAES) has metal-to-ligand charge-transfer (MLCT) character. Both the solution and gas-phase results obtained for 3 by using B3LYP, B3P86 or M06 are very similar to those from restricted active-space SCF second-order perturbation theory (RASPT2) with a very large basis set and large active space. However, the time-dependent DFT λ(max) predictions from the long-range corrected functionals CAM-B3LYP, LC-ωPBE and wB97XB and also the fully ab initio resolution of identity approximate coupled-cluster method (gas-phase only) are less accurate for all three complexes. The ground state (GS) two-state approximation first hyperpolarisability ß(2SA) for 3 from RASPT2 is very close to that derived experimentally via hyper-Rayleigh scattering, whereas the corresponding DFT-based values are considerably larger. The ß responses calculated by using B3LYP, B3P86 or M06 increase markedly as the π-conjugation extends on moving along the series 2→4, for both the GS and FDAES species. All three functionals predict substantial FDAES ß enhancements for each complex, increasing with the π-conjugation, up to about sevenfold for 4. Also, the computed second hyperpolarisabilities γ generally increase in the FDAES, but the results vary between the different functionals.

Performance of density functional theory in computing nonresonant vibrational (hyper)polarizabilities.

A set of exchange-correlation functionals, including BLYP, PBE0, B3LYP, BHandHLYP, CAM-B3LYP, LC-BLYP, and HSE, has been used to determine static and dynamic nonresonant (nuclear relaxation) vibrational (hyper)polarizabilities for a series of all-trans polymethineimine (PMI) oligomers containing up to eight monomer units. These functionals are assessed against reference values obtained using the Møller-Plesset second-order perturbation theory (MP2) and CCSD methods. For the smallest oligomer, CCSD(T) calculations confirm the choice of MP2 and CCSD as appropriate for assessing the density functionals. By and large, CAM-B3LYP is the most successful, because it is best for the nuclear relaxation contribution to the static linear polarizability, intensity-dependent refractive index second hyperpolarizability, static second hyperpolarizability, and is close to the best for the electro-optical Pockels effect first hyperpolarizability. However, none of the functionals perform satisfactorily for all the vibrational (hyper)polarizabilities studied. In fact, in the case of electric field-induced second harmonic generation all of them, as well as the Hartree-Fock approximation, yield the wrong sign. We have also found that the Pople 6-31+G(d) basis set is unreliable for computing nuclear relaxation (hyper)polarizabilities of PMI oligomers due to the spurious prediction of a nonplanar equilibrium geometry.

On the vibrational linear and nonlinear optical properties of compounds involving noble gas atoms: HXeOXeH, HXeOXeF, and FXeOXeF.

The vibrational (hyper)polarizabilities of some selected Xe derivatives are studied in the context of Bishop-Kirtman perturbation theory (BKPT) and numerical finite field methodology. It was found that for this set of rare gas compounds, the static vibrational properties are quite large, in comparison to the corresponding electronic ones, especially those of the second hyperpolarizability. This also holds for the dc-Pockels ß(-ω;ω,0), Kerr γ(-ω;ω,0,0) and electric field second harmonic generation γ (-2ω;ω,ω,0) effects, although the computed nuclear relaxation (nr) vibrational contributions are smaller in magnitude than the static ones. HXeOXeH was used to study the effects of electron correlation, basis set, and geometry. Geometry effects were found to lead to noticeable changes of the vibrational and electronic second hyperpolarizability. A limited study of the effect of Xe insertion to the nr vibrational properties is also reported. Assessment of the results revealed that Xe insertion has a remarkable effect on the nr (hyper)polarizabilities. In terms of the BKPT, this is associated with a remarkable increase of the electrical and mechanical anharmonicity terms. The latter is consistent with the anharmonic character of several vibrational modes reported for rare gas compounds.

A Full Dimensionality Approach to Evaluate the Nonlinear Optical Properties of Molecules with Large Amplitude Anharmonic Tunneling Motions.

Previously, a reduced dimensionality approach was used to determine the vibrational contribution to nonlinear optical properties for molecules with large amplitude anharmonic modes that takes into account tunneling between potential wells (Luis, J. M.; Reis, H.; Papadopoulos, M. G.; Kirtman, B. J. Chem. Phys.2009, 131, 034116). Here, the treatment is extended, again using ammonia as an example, to include the remaining modes at several approximate levels. It is shown that this extension is essential to obtaining the correct results. Our new approach fully accounts for tunneling and avoids possible convergence problems associated with the normal coordinate expansion of the potential energy surface in a single-well treatment. For accurate numerical values, a good treatment of electron correlation is required along with a flexible basis set including diffuse functions.

Computational studies of darunavir into HIV-1 protease and DMPC bilayer: necessary conditions for effective binding and the role of the flaps.

Human immunodeficiency virus type 1 protease (HIV-1 PR) is one of the main targets toward AIDS therapy. We have selected the potent drug darunavir and a weak inhibitor (fullerene analog) as HIV-1 PR substrates to compare protease's conformational features upon binding. Molecular dynamics (MD), molecular mechanics Poisson-Boltzmann surface area (MM-PBSA), and quantum-mechanical (QM) calculations indicated the importance of the stability of HIV-1 PR flaps toward effective binding: a weak inhibitor may induce flexibility to the flaps, which convert between closed and semiopen states. A water molecule in the darunavir-HIV-1 PR complex bridged the two flap tips of the protease through hydrogen bonding (HB) interactions in a stable structure, a feature that was not observed for the fullerene-HIV-1 PR complex. Additionally, despite that van der Waals interactions and nonpolar contribution to solvation favored permanent fullerene entrapment into the cavity, these interactions alone were not sufficient for effective binding; enhanced electrostatic interactions as observed in the darunavir-complex were the crucial component of the binding energy. An alternative pathway to the usual way of a ligand to access the cavity was also observed for both compounds. Each ligand entered the binding cavity through an opening between the one flap of the protease and a neighboring loop. This suggested that access to the cavity is not necessarily regulated by flap opening. Darunavir exerts its biological action inside the cell, after crossing the membrane barrier. Thus, we also initiated a study on the interactions between darunavir and the DMPC bilayer to reveal that the drug was accommodated inside the bilayer in conformations that resembled its structure into HIV-1 PR, being stabilized via HBs with the lipids and water molecules.

Electric field polarization in conventional density functional theory: from quasilinear to two-dimensional and three-dimensional extended systems.

The large overshoot in (hyper)polarizabilities of quasilinear (1D) chains calculated by applying density functional theory with conventional functionals is investigated for several 2D and 3D extended systems. These systems include arrays of molecular hydrogen chains, as well as 2D coronene-type structures and LiF in 1D, 2D, and 3D. Contrary to a recently proposed model it is found that the overshoot persists in all of these cases. A simple explanation is provided by an analysis of the field-induced charges for molecular hydrogen, which shows an excessive buildup at the chain ends regardless of where the chain is located within the 2D and 3D array.

Electronic structure, bonding, spectra, and linear and nonlinear electric properties of Ti@C28.

The potential energy surface (PES) of Ti@C(28) has been revisited, and the stationary points have been carefully characterized. In particular, the C(2v) symmetry structure considered previously turns out to be a transition state lying 2.3 kcal/mol above the ground state of C(3v) symmetry at the MP2/6-31G(d) level. A large binding energy of 181.3 kcal/mol is found at the ROMP2/6-31G(d) level. Topological analysis of the generalized Ti@C(28) density reveals four bond paths between Ti and carbon atoms of the host. The character of all four contacts corresponds to a partially covalent closed shell interaction. UV-vis, IR, and Raman spectra are calculated and compared with C(28)H(4). The dipole moment and the static electronic and double harmonic vibrational (hyper)polarizabilities have been obtained. Distortion of the fullerene cage due to encapsulation leads to nonzero diagonal components of the electronic first hyperpolarizability ß, and to an increase in the diagonal components of the electronic polarizability α and second hyperpolarizability γ. However, introduction of the Ti atom causes a comparable or larger reduction in most cases due to localized bonding interactions. At the double harmonic level, the average vibrational ß is much larger than its electronic counterpart, but the opposite is true for α and for the contribution to γ that has been calculated. There is also a very large anharmonic (nuclear relaxation) contribution to ß which results from a shallow PES with four minima separated by very low barriers. Thus, the vibrational γ (and α) may, likewise, become much larger when anharmonicity is taken into account.

Comparative biophysical studies of sartan class drug molecules losartan and candesartan (CV-11974) with membrane bilayers.

The interactions of the antihypertensive AT(1) antagonists candesartan and losartan with membrane bilayers were studied through the application of DSC, Raman, and solid state (31)P NMR spectroscopies. (1)H and (13)C NMR resonances of candesartan were assigned on the basis of 1D and 2D NMR spectroscopy. A (31)P CP NMR broadline fitting methodology in combination with ab initio computations was implemented and, in conjunction with DSC and Raman results, provided valuable information regarding the perturbation, localization, orientation, and dynamic properties of the drugs in membrane models. In particular, results indicate that losartan anchors in the mesophase region of the lipid bilayers with the tetrazole group oriented toward the polar headgroup, whereas candesartan has less definite localization spanning from water interface toward the mesophase and upper segment of the hydrophobic region. Both sartan molecules decrease the mobilization of the phospholipids alkyl chains. Losartan exerts stronger interactions compared with candesartan, as depicted by the more prominent thermal, structural, and dipolar (1)H-(31)P changes that are caused in the lipid bilayers. At higher concentrations, candesartan strengthens the polar interactions and induces increased order at the bilayer surface. At the highest concentration used (20 mol %), only losartan induces formation of microdomains attributed to the flexibility of its alkyl chain. These results in correlation to reported data with other AT(1) antagonists strengthen the hypothesis that this class of molecules may approach the active site of the receptor by insertion in the lipid core, followed by lateral diffusion toward the binding site. Further, the similarities and differences of these drugs in their interactions with lipid bilayers establish, at least in part, their pharmacological properties.