Electronic absorption spectra of pyridine and nicotine in aqueous solution with a combined molecular dynamics and polarizable QM/MM approach

The electronic absorption spectra of pyridine and nicotine in aqueous solution have been computed using a multistep approach. The computational protocol consists in studying the solute solvation with accurate molecular dynamics simulations, characterizing the hydrogen bond interactions, and calculating electronic transitions for a series of configurations extracted from the molecular dynamics trajectories with a polarizable QM/MM scheme based on the fluctuating charge model. Molecular dynamics simulations and electronic transition calculations have been performed on both pyridine and nicotine. Furthermore, the contributions of solute vibrational effect on electronic absorption spectra have been taken into account in the so called vertical gradient approximation. © 2016 The Authors. Journal of Computational Chemistry Published by Wiley Periodicals, Inc.     

Exploring concerted effects of base pairing and stacking on the excited‐state nature of DNA oligonucleotides by DFT and TD‐DFT studies

We have taken (dA)₅, (dT)₅, and (dA)₅·(dT)₅ as model systems to study concerted effects of base pairing and stacking on excited‐state nature of DNA oligonucleotides using density functional theory (DFT) and time dependent DFT methods. The spectroscopic states are determined to be of a partial A → A charge‐transfer nature in the A·T oligonucleotides. The T → T charge‐transfer transitions produce dark states, which are hidden in the energy region of the steady‐state absorption spectra. This is different from the previous assignment that the T → T charge‐transfer transition is responsible for a shoulder at the red side of the first strong absorption band. The A → T charge‐transfer states were predicted to have relatively high energies in the A·T oligonucleotides. The present calculations predict that the T → A charge‐transfer states are not involved in the spectra and excited‐state dynamics of the A·T oligonucleotides. In addition, the influence of base pairing and stacking on the nature of the ¹nπ* and ¹ππ* states are discussed in detail. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

The MC‐DFT approach including the SCS‐MP2 energies to the new minnesota‐type functionals

We have applied the multicoefficient density functional theory (MC‐DFT) to four recent Minnesota functionals, including M06‐2X, M08‐HX, M11, and MN12‐SX on the performance of thermochemical kinetics. The results indicated that the accuracy can be improved significantly using more than one basis set. We further included the SCS‐MP2 energies into MC‐DFT, and the resulting mean unsigned errors (MUEs) decreased by approximately 0.3 kcal/mol for the most accurate basis set combinations. The M06‐2X functional with the simple [6–311+G(d,p)/6–311+G(2d,2p)] combination gave the best performance/cost ratios for the MC‐DFT and MC‐SCS‐MP2|MC‐DFT methods with MUE of 1.58 and 1.22 kcal/mol, respectively. © 2014 Wiley Periodicals, Inc.     

Analytic energy gradients for orbital‐optimized MP3 and MP2.5 with the density‐fitting approximation: An efficient implementation

Efficient implementations of analytic gradients for the orbital‐optimized MP3 and MP2.5 and their standard versions with the density‐fitting approximation, which are denoted as DF‐MP3, DF‐MP2.5, DF‐OMP3, and DF‐OMP2.5, are presented. The DF‐MP3, DF‐MP2.5, DF‐OMP3, and DF‐OMP2.5 methods are applied to a set of alkanes and noncovalent interaction complexes to compare the computational cost with the conventional MP3, MP2.5, OMP3, and OMP2.5. Our results demonstrate that density‐fitted perturbation theory (DF‐MP) methods considered substantially reduce the computational cost compared to conventional MP methods. The efficiency of our DF‐MP methods arise from the reduced input/output (I/O) time and the acceleration of gradient related terms, such as computations of particle density and generalized Fock matrices (PDMs and GFM), solution of the Z‐vector equation, back‐transformations of PDMs and GFM, and evaluation of analytic gradients in the atomic orbital basis. Further, application results show that errors introduced by the DF approach are negligible. Mean absolute errors for bond lengths of a molecular set, with the cc‐pCVQZ basis set, is 0.0001–0.0002 Å. © 2017 Wiley Periodicals, Inc.     

Excited state properties,fluorescence energies,and lifetimes of a poly(fluorene‐phenylene), based on TD‐DFT investigation

The structural and electronic properties of fluorene‐phenylene copolymer (FP)_n, n = 1–4 were studied by means of quantum chemical calculations based on density functional theory (DFT) and time dependent density functional theory (TD‐DFT) using B3LYP functional. Geometry optimizations of these oligomers were performed for the ground state and the lowest singlet excited state. It was found that (FP)_n is nonplanar in its ground state while the electronic excitations lead to planarity in its S₁ state. Absorption and fluorescence energies were calculated using TD‐B3LYP/SVP and TD‐B3LYP/SVP+ methods. Vertical excitation energies and fluorescence energies were obtained by extrapolating these values to infinite chain length, resulting in extrapolated values for vertical excitation energy of 2.89 and 2.87 eV, respectively. The S₁ ← S₀ electronic excitation is characterized as a highest occupied molecular orbital to lowest unoccupied molecular orbital transition and is distinguishing in terms of oscillator strength. Fluorescence energies of (FP)_n calculated from TD‐B3LYP/SVP and TD‐B3LYP/SVP+ methods are 2.27 and 2.26 eV, respectively. Radiative lifetimes are predicted to be 0.55 and 0.51 ns for TD‐B3LYP/SVP and TD‐B3LYP/SVP+ calculations, respectively. These fundamental information are valuable data in designing and making of promising materials for LED materials. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

DFT/TD‐DFT investigation on Ir(III) complexes with N‐heterocyclic carbene ligands: Geometries,electronic structures,absorption, and phosphorescence properties

Iridium(III) complexes with N‐heterocyclic (NHC) ligands including fac‐Ir(pmb)₃ (1), mer‐Ir(pmb)₃ (2), (pmb)₂Ir(acac) (3), mer‐Ir(pypi)₃ (4), and fac‐Ir(pypi)₃ (5) [pmb = 1‐phenyl‐3H‐benzimidazolin‐2‐ylidene, acac = acetoylacetonate, pypi = 1‐phenyl‐5H‐benzimidazolin‐2‐ylidene; fac = facial, mer = meridional] were investigated theoretically. The geometry structures of 1–5 in the ground and excited state were optimized with restricted and unrestricted DFT (density functional theory) methods, respectively (LANL2DZ for Ir atom and 6‐31G for other atoms). The HOMOs (highest occupied molecular orbitals) of 1–3 are composed of d(Ir) and π(phenyl), while those of 4 and 5 are contributed by d(Ir) and π(carbene). The LUMOs (lowest unoccupied molecular orbitals) of 1, 2, 4, and 5 are localized on carbene, but that of 3 is localized on acac. The calculated lowest‐lying absorptions with TD‐DFT method based on Perdew‐Burke‐Erzenrhof (PBE) functional of 1 (310 nm), 2 (332 nm), and 3 (347 nm) have ML_carbeneCT/IL_{phenyl→carbene}CT (MLCT = metal‐to‐ligand charge transfer; ILCT = intraligand charge transfer) transition characters, whereas those of 4 (385 nm) and 5 (389 nm) are assigned to ML_carbeneCT/IL_{carbene→carbene}CT transitions. The phosphorescences calculated by TD‐DFT method with PBE0 functional of 1 (386 nm) and 2 (388 nm) originate from ³ML_carbeneCT/³IL_{phenyl→carbene}CT excited states, but those of 4 (575 nm) and 5 (578 nm) come from ³ML_carbeneCT/³IL_{carbene→carbene}CT excited states. The calculated results showed that the carbene and phenyl groups act as two independent chromophores in transition processes. Compared with 1 and 2, the absorptions of 4 and 5 are red‐shifted by increasing the effective π‐conjugation groups near the C_carbene atom. We predicated that (pmb)₂Ir(acac) is nonemissive, because the LUMO of 3 is contributed by the nonemissive acac ligand. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Photo‐physical Properties of N‐methyl‐3,4‐fulleropyrrolidine and Its Derivatives: A DFT and TD‐DFT Investigation

A series of N‐methyl‐3,4‐fulleropyrrolidine (NMFP) derivatives were designed by selecting different π‐conjugated linkers and electron‐donating groups as D‐π‐A and D‐A systems. The optimised structures and photo‐physical properties of NMFP and its derivatives have been determined using density functional theory (DFT) and time‐dependent density functional theory (TD‐DFT) methods with the B3LYP functional and the 6‐31G basis set. According to the computation analysis, both the π‐conjugated linkers and the electron‐donating groups can influence the electronic and photo‐physical properties of the NMFP derivatives. Our calculated results demonstrated that the electron‐donating groups, with significant electron‐donating ability, had the tendency to increase the highest occupied molecular orbital (HOMO) energy. The π‐conjugated linkers with lower resonance energy decreased the lowest occupied molecular orbital (LUMO) energy and caused a significant decrease in the energy gap (E_g) between the E_HOMO and E_LUMO. A Natural Bond Orbital (NBO) analysis examines the effect of the electron‐donating group, π conjugated linker, and electron‐withdrawing group for these NMFP derivatives. For the NMFP derivatives, a projected density of state (PDOS) analysis demonstrated that the electron density of HOMO and LUMO are concentrated on the electron‐donating group and the π‐conjugated linker, respectively. A TD‐DFT/B3LYP calculation was performed to calculate the electronic absorption spectra of these NMFP derivatives. Both the electron‐donating group and the π‐conjugated linker contribute to the major absorption peaks, which are assigned as HOMO to LUMO transitions and are red‐shifted relative to those of non‐substituted NMFP.     

Revealing the physical nature and the strength of charge‐inverted hydrogen bonds by SAPT(DFT), MP2, SCS‐MP2, MP2C,and CCSD(T) methods

The physical nature of charge‐inverted hydrogen bonds in H₃XH YH₃ (X = Si, Ge; Y = Al, Ga) dimer systems is studied by means of the SAPT(DFT)‐based decomposition of interaction energies and supermolecular interaction energies based on MP2, SCS‐MP2, MP2C, and CCSD(T) methods utilizing dimer‐centered aug‐cc‐pCVnZ (n = D, T, Q) basis sets as well as an extrapolation to the complete basis set limit. It is revealed that charge‐inverted hydrogen bonds are inductive in nature, although dispersion is also important. Computed interaction energies form the following relation: . It is confirmed that the aug‐cc‐pCVDZ basis set performs poorly and that very accurate values of interaction and dispersion energies require basis sets of at least quadrupole‐ζ quality. Considerably large binding energies suggest potential usefulness of charge‐inverted hydrogen bonds as an important structural motif in molecular binding. Terminology applying to σ‐ and π‐hole interactions as well as to triel and tetrel bonds is discussed. According to this new terminology the charge‐inverted hydrogen bond would become the first described case of a hydride‐triel bond. © 2017 Wiley Periodicals, Inc.     

DFT and TD‐DFT study of iridium complexes with low‐color‐temperature and low‐efficiency roll‐off properties

A series of heteroleptic cyclometalated Ir (III) complexes with low‐color‐temperature and low‐efficiency roll‐off properties, which cause a fast reduction in efficiency when the drive current increases, for organic light‐emitting devices are investigated theoretically to explore their electronic structures and spectroscopic properties. The geometries, electronic structures, lowest‐lying singlet absorptions and triplet emissions of (ptpy)₂Ir(acac), and the theoretically designed models (ptpy)₂Ir(tpip), (F‐ptpy)₂Ir(acac), (F‐ptpy)₂Ir(tpip), (F₂‐ptpy)₂Ir(acac) and (F₂‐ptpy)₂Ir(tpip), are investigated with density functional theory approaches, where ptpy denotes 4‐phenylthieno [3,2‐c] pyridine, acac denotes acetylacetonate, tpip denotes tetraphenylimido‐diphosphinate, F‐ptpy denotes 4‐(3‐fluorophenyl) thieno [3,2‐c] pyridine, and F₂‐ptpy denotes 4‐(2,4‐difluorophenyl) thieno [3,2‐c] pyridine.     

Intramolecular interaction energies in model alanine and glycine tetrapeptides. Evaluation of anisotropy, polarization, and correlation effects. A parallel ab initio HF/MP2, DFT, and polarizable molecular mechanics study

An extension of the SIBFA polarizable molecular mechanics procedure to flexible oligopeptides is reported. The procedure is evaluated by computing the relative conformational energies, deltaE(conf), of the alanine tetrapeptide in 10 representative conformations, which were originally derived by Beachy et al. (J Am Chem Soc 1997, 119, 5908) to benchmark molecular mechanics procedures with respect to ab initio computations. In the present study, a particular emphasis is on the separable nature of the components of the energy and the particular impact of the polarization energy component on deltaE(conf). We perform comparisons with respect to single-point HF, DFT, LMP2, and MP2 computations done at the SIBFA-derived energy minima. Such comparisons are made first for the 10 conformers derived from phi/psi torsional angle energy-minimization (the rigid rotor approach), and, in a second step, after allowing additional relaxation of the C(alpha) centered valence angles. In both series of energy-minimization, the SIBFA deltaE(conf) compared best with the LMP2 results using the 6-311G** basis set, the rms being 1.3 kcal/mol. In the absence of the polarization component, the rms is 3.5 kcal/mol. In both series of minimizations, the magnitudes of deltaE(conf), computed as differences with respect to the most stable conformer taken as energy zero, decrease along the series: HF > DFT > LMP2 > SIBFA > MP2, indicative of increasing stabilization of the most highly folded conformers.     

Implementation of nuclear gradients of range‐separated hybrid density functionals and benchmarking on rotational constants for organic molecules

We have implemented the nuclear gradient for several range‐separated hybrid density functionals in the general quantum chemistry code ORCA. To benchmark the performance, we have used a recently published set of back‐corrected gas phase rotational constants, which we extended by three molecules. In our evaluation, CAM‐B3LYP‐D3 and ωB97X‐D3 show great accuracy, and are surpassed by B2PLYP‐D3 only. Lower‐cost alternatives to quadruple‐ζ basis set‐based calculations, among them a smaller basis set and the use of resolution‐of‐the‐identity approaches, are assessed and shown to yield acceptable deviations. In addition, the Hartree‐Fock‐based back‐correction method is compared to a density functional theory alternative, which largely shows consistency between the two. A new, well‐performing, spin‐component scaled MP2 variant is designed and discussed, as well. © 2014 Wiley Periodicals, Inc.     

A new phosphorescent heteroleptic cuprous complex with a neutral 2‐methylquinolin‐8‐ol ligand: synthesis,structure characterization,properties and TD–DFT calculations

Luminescent Cu^I complexes have emerged as promising substitutes for phosphorescent emitters based on Ir, Pt and Os due to their abundance and low cost. The title heteroleptic cuprous complex, [9,9‐dimethyl‐4,5‐bis(diphenylphosphanyl)‐9H‐xanthene‐κ²P ,P ](2‐methylquinolin‐8‐ol‐κ²N ,O )copper(I) hexafluorophosphate, [Cu(C₁₀H₉NO)(C₃₉H₃₂OP₂)]PF₆, conventionally abbreviated as [Cu(Xantphos)(8‐HOXQ)]PF₆, where Xantphos is the chelating diphosphine ligand 9,9‐dimethyl‐4,5‐bis(diphenylphosphanyl)‐9H‐xanthene and 8‐HOXQ is the N ,O‐chelating ligand 2‐methylquinolin‐8‐ol that remains protonated at the hydroxy O atom, is described. In this complex, the asymmetric unit consists of a hexafluorophosphate anion and a whole mononuclear cation, where the Cu^I atom is coordinated by two P atoms from the Xantphos ligand and by the N and O atoms from the 8‐HOXQ ligand, giving rise to a tetrahedral CuP₂NO coordination geometry. The electronic absorption and photoluminescence properties of this complex have been studied on as‐synthesized samples, whose purity had been determined by powder X‐ray diffraction. In the detailed TD–DFT (time‐dependent density functional theory) studies, the yellow emission appears to be derived from the inter‐ligand charge transfer and metal‐to‐ligand charge transfer (M +L ′)→LCT excited state (LCT is ligand charge transfer).     

Quantum chemical comparison of vertical,adiabatic, and 0‐0 excitation energies: The PYP and GFP chromophores

A number of benchmark studies investigating the performance of quantum chemical methods for calculating vertical excitation energies are today available in the literature. However, less established is the variation between methods in their estimates of the differences between vertical, adiabatic, and 0‐0 excitation energies. To this end, such excitation energies are here calculated for the bright S₁ states of the anionic chromophores of the photoactive yellow protein (PYP) and the green fluorescent protein (GFP) in the gas phase using configuration interaction singles, complete active space self‐consistent field, coupled‐cluster singles and doubles, and time‐dependent density functional theory methods. Although the estimates of the excitation energies vary by more than 1 eV between the methods, the differences between the different types of excitation energies are found to be relatively method‐insensitive, varying by ～0.1 eV only for these particular chromophores. Specifically, the adiabatic energies are uniformly 0.10–0.17 (PYP) and 0.06–0.17 eV (GFP) lower than the vertical energies, and the 0‐0 energies are similarly 0.09–0.14 (PYP) and 0.07–0.17 eV (GFP) lower than the adiabatic energies. © 2012 Wiley Periodicals, Inc.     

NMR,MP2, and DFT study of thiophenoxyketenimines (o‐thio‐Schiff bases): Determination of the preferred form

Five new thiophenoxyketinimines have been synthesized. ¹H and ¹³C NMR spectra as well as deuterium isotope effects on ¹³C chemical shifts are determined, and spectra are assigned. DFT and MP2 calculations of both structures, chemical shifts, and isotope effects on chemical shifts are done. The combined analysis reveals that the compounds are primarily on a zwitterionic form with an NH⁺ and a S⁻ group and with a little of the neutral form mixed in. Very strong intramolecular hydrogen bonding is found and very high NH chemical shifts are observed. The theoretical calculations show that calculations at the MP2 level are best to obtain correct “C═S” chemical shifts.     

Three zinc iodide complexes based on phosphane ligands: syntheses,structures, optical properties and TD–DFT calculations

Three zinc iodide complexes based on phosphane ligands, namely diiodidobis(triphenylphosphane‐κP)zinc(II), [ZnI₂(C₁₈H₁₅P₂)₂], ( 1 ), diiodidobis[tris(4‐methylphenyl)phosphane‐κP]zinc(II), [ZnI₂(C₂₁H₂₁P₂)₂], ( 2 ), and [bis(diphenylphosphoryl)methane‐κ²O,O′]zinc(II) tetraiodidozinc(II), [Zn(C₂₅H₂₂O₂P₂)₃][ZnI₄], ( 3 ), have been synthesized and characterized. Single‐crystal X‐ray diffraction revealed that the structures of ( 1 ) and ( 2 ) are both mononuclear four‐coordinated ZnI₂ complexes containing two monodentate phosphane ligands, respectively. Surprisingly, ( 2 ) spontaneously forms an acentric structure, suggesting it might be a potential second‐order NLO material. The crystal structure of complex ( 3 ) is composed of two parts, namely a [Zn(dppmO₂)₃]²⁺ cation [dppmO₂ is bis(diphenylphosphoryl)methane] and a [ZnI₄]²⁻ anion. The UV–Vis absorption spectra, thermal stabilities and photoluminescence spectra of the title complexes have also been studied. Time‐dependent density functional theory (TD–DFT) calculations reveal that the low‐energy UV absorption and the corresponding light emission both result from halide‐ligand charge‐transfer (XLCT) excited states.     

A DFT and TD‐DFT Approach to the Understanding of Statistical Kinetics in Substitution Reactions of M3Q4 (M=Mo,W; Q=S,Se) Cuboidal Clusters

For many years it has been known that the nine water molecules in [M₃Q₄(H₂O)₉]⁴⁺ cuboidal clusters (M=Mo, W; Q=S, Se) can be replaced by entering ligands, such as chloride or thiocyanate, and kinetic studies carried out mainly on the substitution of the first water molecule at each metal centre reveal that the reaction at the three metal centres occurs with statistical kinetics; that is, a single exponential with a rate constant corresponding to the reaction at the third centre is observed instead of the expected three‐exponential kinetic trace. Such simplification of the kinetic equations requires the simultaneous fulfilment of two conditions: first that the three consecutive rate constants are in statistical ratio, and second that the metal centres behave as independent chromophores. The validity of those simplifications has been checked for the case of the reaction of [Mo₃S₄(H₂O)₉]⁴⁺ with Cl^? by using DFT and TD‐DFT theoretical calculations. The results of those calculations are in agreement with the available experimental information, which indicates that the H₂O ligands trans to the μ‐S undergo substitution much faster than those trans to the μ₃‐S. Moreover, the energy barriers for the substitution of the first water molecule at the three metal centres are close to each other, the differences being compatible with the small changes in the numerical values of the rate constants required for observation of statistical kinetics. TD‐DFT calculations lead to calculated electronic spectra, which are in reasonable agreement with those experimentally measured, but the calculations do not indicate that the three metal centres behave as independent chromophores, although the mathematical conditions required for simplification of the kinetic traces to a single exponential are reasonably well fulfilled at certain wavelengths. A re‐examination of the kinetics of the reaction by using global fitting procedures yields results, which are compatible with statistical kinetics, although an alternative interpretation in which substitution only occurs at a single metal centre under reversible conditions is also possible.