Abstract: Publication date: Available online 11 June 2019Source: Computational and Theoretical ChemistryAuthor(s): Vaibhav A. Dixit, Yashita Y. Singh Naphthalene, polyacenes and graphene are considered aromatic. Existing models for polyacenes predict a linearly increasing aromatic stabilization energy (ASE) and give little insights into their high reactivity and decreasing stability. Graphene’s aromaticity has been studied only qualitatively suggesting alternate Clar’s sextet and two-electrons per ring, and ASE estimates are missing. In this paper, various Enthalpy of Hydrogenation (ΔHhydro) and isodesmic schemes have been proposed and compared for the estimation of naphthalene ASE. Results show that ΔHhydro schemes are simple to design, are equivalent to isodesmic schemes, and unconjugated unsaturated reference systems predict ASE values in agreement with literature reports. Partially aromatic reference systems underestimate ASE. ΔHhydro schemes require calculations for a smaller number of structures, and offer scope for experimental validation, and involve enthalpy differences. Polyacene (X-axis extensions of benzene) ASE estimates (using ΔHhydro scheme) correlate well with experimental instability data and offer new chemical physical insights. ASEs extrapolated from quadratic and logarithmic regression models have been used to estimate the largest polyacene with limiting ASE values. ASE values for Pyrene (Y-axis extension of benzene) and higher analogues (here called pyrene-vertacenes) are estimated using ΔHhydro schemes. Further truncated graphene models and graphene are approximated as combinations of polyacene and pyrene-vertacene units. First ever ASE and molecular sizes (22-255 nM) estimates predict nanometer size ranges for flat graphene in agreement with recent experiments and offer new physical insights. These ASE and size estimates for graphene will prove useful in the design of novel energy (hydrogen) storage, electronic and material science applications.Graphical abstract

Abstract: Publication date: Available online 7 June 2019Source: Computational and Theoretical ChemistryAuthor(s): Bruce M. Prince A density functional theory with solvation model density analysis of methane C−H and ammine N−H activations with Group 9 N-pyrrolyl phosphine complexes (Co, Rh, Ir) is presented. Analysis of the reaction, [{(pyr)3P}M(NH2)]q+, (where M signify Co, Rh, Ir; pyr = N−pyrrolyl and q = +1 and +3 for d8 and d6 Group 9) showed that Ir system occurs with a lower free energy barrier. The computed complexes all have reasonable C−H activation barriers, ΔG‡ ∼ 18 to 32 kcal/mol, which tracks with the electron density on the metal center of the transition state complexes. Ammine N−H activation barriers are discreetly low, which emphasize the square planar, [{(pyr)3P}M(Py)(NH3)(OCH3)], complexes are uniquely ready to aid the reaction cycle within a single step. Thus, such ligands are worthy of experimental study due to their ability of strong π-backbonding, which may meet the strong metal-to-ligand coordination demands of an acidic environment of methanol production.Graphical abstract

Abstract: Publication date: Available online 3 June 2019Source: Computational and Theoretical ChemistryAuthor(s): Muhammad Ans, Khurshid Ayub, Shabbir Muhammad, Javed Iqbal Continuous strides are being made to explore non-fullerene acceptors for organic solar cells. Here, optoelectronic properties of four new acceptor-donor-acceptor (A-D-A) type non-fullerene acceptor molecules are evaluated for their potential use in organic solar cells. The designed molecules contain indacenodithiophene (IDT) donor core connected with various acceptor groups through benzothiadiazole (BT) bridge unit. The designed molecules differ from each other in end-capped acceptor groups. The end-capped acceptors are 2-methylene-3-oxo-2,3-dihydroinden-1-ylidene) malononitrile (M1), 2,5,6-difluoro-2-methylene-3-oxo-2,3-dihydroinden-1-ylidene) malononitrile (M2), 2-methylene-2-H-indene-1,3-dione (M3), and 2,5-methylene-6-oxo-5,6-dihydrocyclopenta-c-thiphen-4-ylidene-malononitrile (M4). The optoelectronic properties are evaluated in comparison with the recently reported R which is structurally similar to the designed molecules. The reference compound R contains indacenodithiophene (IDT) donor core unit with 3-ethyl-d-ethylidene-2-thioxothiazolidin-4-one end capped acceptor group. The A-D-A acceptor molecules evaluated here exhibit proper frontier molecular orbital diagram to facilitate the charge mobility. M2, containing 2-(5,6-difluoro-2-methylene-3-oxo-2,3-dihydroinden-1-ylidene) malononitrile end-capped acceptor group, exhibits maximum absorption at 805 nm which is attributed to extended conjugation with the BT unit. Results of transition density matrix (TDM) show clear effect of end caped acceptor group on electro-hole exciton. Small coupling of electron and hole in M4 illustrates easy dissociation of exciton as compared with other molecules. Among all, M4 shows the highest hole transfer rate as revealed from low hole reorganization energy (λh). The open circuit voltage (Voc) of reference R is 1.96 V with donor polymer PTB7-Th while the designed molecule M3 exhibits the Voc of 2.02 V. Hence, the designed fullerene free acceptor molecules are suitable for transparent organic solar cells.Graphical abstract

Abstract: Publication date: Available online 3 June 2019Source: Computational and Theoretical ChemistryAuthor(s): Fumiyuki Ito The supermolecule approach has been used to model molecules embedded in a solid argon matrix. The interaction between the guest and the host atoms in the first solvation shell (FSS) are evaluated using density functional calculations while that beyond the FSS is incorporated using ONIOM calculations to extend the results of our previous study [F. Ito, J. Chem. Phys. 133 (2010) 214502]. The vibrational spectra of a formic acid dimer (FAD) in an Ar matrix was simulated using (FAD)-Arn clusters (n = 94–1095). Structural relaxation in the first solvation shell is crucial to quantitatively reproduce the matrix shifts of fundamental bands, according to calculations at the B971/6-31++G(3df,3pd) level. Moreover, the influence of the outer Ar atoms is negligible.Graphical abstractStructural relaxation of Ar atoms in the first solvation shell.

Abstract: Publication date: Available online 3 June 2019Source: Computational and Theoretical ChemistryAuthor(s): Claude Pouchan We present here a quantum molecular dynamic approach from DFT molecular dynamics (DFT/MD) trajectories around 150K, 300K, 450K and 600K to compute the IR spectra of the propynenitrile (or cyanoacetylene) molecule HC3N, the simplest linear cyanopolyyne detected in interstellar clouds and circumstellar envelopes of stars. The DFT/MD results in fair agreement with the experimental data for the isolated molecule are close to those obtained, for all the valence modes, from an effective second order treatment (VPT2) at the CCSD(T) level of theory. This DFT/MD simulation is used to predict the IR spectra of the1:1 complexes with water and analyse the shifts expected for the micro-hydrated molecule.Graphical abstract

Abstract: Publication date: Available online 1 June 2019Source: Computational and Theoretical ChemistryAuthor(s): J. Tabet, Z. Adem, F. Taher The theoretical studies of the electronic structure of ZrO molecule to predict the molecular characteristics and the spectroscopic constants are performed with Ab-initio methods, the (MRCI-SD)/CASSCF. The configuration interaction (CI) method which employed consistent basis sets with effective core potentials for Zr atom, determined 13 singlet and 11 triplet low-lying electronic states below 38600 cm-1. 7 new singlet and 6 new triplet states not yet observed, are calculated for the first time. In addition to the electronic energy Te referred to the ground state energy at equilibrium, for the states presented as2S+1Λ±, we computed the spectroscopic constants including ωe, Re, the permanent electric dipole moments and the transition dipole moments (TDM). We are also reporting 47 electronic components in the representation Ω(±) below 38600 cm-1, when calculating the spin orbit effects. These calculated energies and spectroscopic constants are in excellent agreement with the previous experimental and theoretical results.Graphical abstract

Abstract: Publication date: Available online 1 June 2019Source: Computational and Theoretical ChemistryAuthor(s): Zemin Zhang, Rongxing He Three hole-transporting molecules S-O, S-S and S-Se are designed through introducing heterocyclic spacers into the silafluorene core of S101. The effects of heterocyclic spacer on the geometry, electronic property and hole mobility of these materials are investigated systematically through using the DFT, Marcus formula and Einstein relation. The HOMO energy levels of S-O, S-S and S-Se are lower than that of S101 from calculated results, indicating that introduction of heterocyclic spacer can improve the open circuit voltage of device. Further, all of the designed molecules with heterocyclic spacer have larger electronic coupling relative to S101 because of more efficient intermolecular π-π stacking and interactions, which makes their hole mobility be higher than that of S101. The hole mobility will decrease gradually with the increase of heteroatom number. That is, the way introducing heterocyclic spacer into the silafluorene core of hole-transporting materials can enhance the hole transfer ability. The present theoretical investigation provides a meaningful way for improving the hole transport property of silafluorene-based materials.Graphical abstract

Abstract: Publication date: Available online 31 May 2019Source: Computational and Theoretical ChemistryAuthor(s): A.E. Sitnitsky A theoretical scheme for the analysis of experimental data on IR spectroscopy for a quantum particle in a double well potential (DWP) is suggested. The analysis is based on the trigonometric DWP for which the exact analytic solution of the Schrödinger equation is available. The corresponding energy levels along with their wave functions are expressed via special functions implemented in Mathematica (spheroidal function and its spectrum of eigenvalues). As a result trigonometric DWP makes the calculation of the energy levels an extremely easy procedure. It contains three parameters allowing one to model the most important characteristics of DWP (barrier height and the distance between the minima of the potential) along with the required asymmetry. Our approach provides an accurate calculation of the energy spectrum for hydrogen bonds in chromous acid (CrOOH) and potassium dihydrogen phosphate (KH2PO4) along with their polarizability in agreement with available experimental data.Graphical abstract

Abstract: Publication date: Available online 31 May 2019Source: Computational and Theoretical ChemistryAuthor(s): Xiao Wang, Rui-xue Chen, Andrew C.-H. Sue, Han Zuilhof, Adelia J.A. Aquino, Hans Lischka Pillar[5]arenes, a type of novel macrocycles containing di-substituted hydroquinone units linked by methylene bridges in para-positions, have attracted extensive attention in supramolecular chemistry as interesting candidates to be used in the preparation of host-guest complexes. Functionalization by means of rim substitution and sustaining an ordered substituent arrangement on both sides of the rim is important for the development of new pillararene-based materials. In order to achieve this, the rim inversion process of rotating the hydroquinone units through the pillar[5]arenes has to be controlled. In this context we have studied the effect of different types of hydroquinone substituents on the rotational energy profile using density functional theory combined with the hybrid M06-2X functional. The influence of polar (-CH2F, -CH2Cl, -CH2OH, -CH2SH, -CH2NH2) and nonpolar alkyl (-CH3, –CH2CH3, –CH2CH2CH3, -CH(CH3)2, and –CH2CH2CH2CH3) substituents on the on the energy barriers of the rotation mechanism, and different local minima was investigated. The stabilization of the intermediate structures by non-covalent van der Waals and interactions and also by hydrogen bonds constitute a major factor affecting barrier heights. In case of polar substituents, the largest barriers were found for -CH2OH and -CH3 substitutions and the lowest ones for -CH2SH and -CH2NH2. For the alkyl series, the barrier decreased significantly up to propyl due to increasing stabilizing dispersion interactions while it increased again for n-butyl since the chain did not fit in well the cavity to rotate through.Graphical abstract

Abstract: Publication date: Available online 30 May 2019Source: Computational and Theoretical ChemistryAuthor(s): T.J. Penfold, M. Pápai, K.B. Møller, G.A. Worth The Variational Multi-Configurational Gaussian (vMCG) approach offers a framework to perform exact trajectory-based quantum dynamics. Herein we use two model vibronic coupling Hamiltonians of pyrazine to explore, for the first time, the influence of the coupling between the external field and the Gaussian basis functions (GBFs) in vMCG on the dynamics. We show that when the excitation pulse is short compared to the nuclear dynamics, vertical projection without a field and explicit description of the external field converge. For longer pulses, a sizeable change is observed. We demonstrate that comparatively few GBFs are sufficient to provide qualitative agreement to MCTDH dynamics and a quantitative agreement can be achieved using ∼100 GBFs. Longer pulses require more GBFs due to the prolonged coupling between the ground and excited states. Throughout this work the single set formalism offers the fastest convergence.

Abstract: Publication date: Available online 25 May 2019Source: Computational and Theoretical ChemistryAuthor(s): Chun Gao, Shu-Xian Hu, Huixian Han, Guina Guo, Bingbing Suo, Wenli Zou As a group 12 element, mercury locating at the sixth row with a valent electronic configuration of 5d106s2 has been treated as a main group element featuring +I and +II oxidation states for a long time. C. K. Jørgensen conjectured the existences of HgF4 and HgF6 molecules in early 1960s, where HgF4 was first synthesized in 2007, but HgF6 as the Hg(+VI) compound was less known. In this paper we explored the electronic structure and decomposition paths of HgF6 by scalar (X1C) and 2-component relativistic (X2C) density functional calculations and more accurate multi-reference NEVPT2 calculations. It is found that the HgF6 molecule, where the mercury atom presents an extraordinary high oxidation state of +VI, is protected by a considerable barrier height of about 22 kcal/mol for the decomposition into HgF4 and two isolated fluorine atoms. The results propose that HgF6 is kinetically stable and may exist under exorbitant conditions.Graphical abstract

Abstract: Publication date: Available online 22 May 2019Source: Computational and Theoretical ChemistryAuthor(s): Alfred Karpfen The potential energy surfaces of the complexes of propynal, (propiolaldehyde, propargyl aldehyde, HCCCOH), with a series of small molecules (HNO, HF, HCl, H2O, CH3OH, and NH3) have been investigated theoretically at the MP2 and CCSD(T) levels using several extended basis sets. Thirty low-lying minima have been detected, eight for HCCCOH-HNO, four for each of the dimers HCCCOH-HF, HCCCOH-HCl, and HCCCOH-H2O, seven for the HCCCOH-CH3OH complex, and three for HCCCOH-NH3. The most stable HCCCOH-HNO complex has a non-planar, orthogonal structure with a tetrel bond and a hydrogen bond. In most cases the calculated intramolecular bond length changes as induced by complex formation are allegeable on the basis of the intramolecular harmonic force fields of propynal and the partner molecules. The vibrational frequency shifts, typical for formyl groups engaged in hydrogen bonding, red shifts of C=O stretches and blue shifts of C-H stretches, occur in most of the dimers, whereas the acetylenic C-H and C≡C stretches are red-shifted when this C-H bond is involved in hydrogen bonding. In almost all HCCCOH-HNO complexes, the N–H stretching frequency is also blue-shifted. However, in case of the most stable HCCCOH-HNO complex a red-shift of the harmonic N-H stretch is calculated. Inclusion of anharmonic contributions reduces this red shift substantially and even converts it to a blue shift. Conventional red shifts of X-H (X=F, Cl, O) stretching frequencies and blue shifts for formyl C-H stretches are calculated for most of the complexes with HF, HCl, H2O, CH3OH, and NH3.Graphical abstract