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The accuracy of describing the geometry of polyenes in their ground electronic state by various methods is examined. To achieve high accuracy, it is proposed to use the SCS-MP2 method, taking into account the correlation of all electrons, including core ones, and using the cc-pwCVTZ atomic basis. It is shown that, when using this approach, errors in the lengths of C–C bonds do not exceed 0.003 Å. The use of the RIJCOSX and DLPNO approximations speeds up calculations without significantly increasing the errors in bond lengths, but allows the proposed approach to be used to describe such large systems as carotenoids. [ABSTRACT FROM AUTHOR]

This investigation involved an ab initio and Density Functional Theory (DFT) analysis of the hydrolysis mechanism and energetics in a borate network. The focus was on understanding how water molecules interact with and disrupt the borate network, an area where the experimental data are scarce and unreliable. The modeled system consisted of two boron atoms, bridging oxygen atoms, and varying numbers of water molecules. This setup allows for an exploration of hydrolysis under different environmental conditions, including the presence of OH− or H+ ions to simulate basic or acidic environments, respectively. Our investigation utilized both ab initio calculations at the MP2 and CCSD(T) levels and DFT with a range of exchange–correlation functionals. The findings indicate that the borate network is significantly more susceptible to hydrolysis in a basic environment, with respect to an acidic or to a neutral pH setting. The inclusion of explicit water molecules in the calculations can significantly affect the results, depending on the nature of the transition state. In fact, some transition states exhibited closed-ring configurations involving water and the boron–oxygen–boron network; in these cases, there were indeed more water molecules corresponding to lower energy barriers for the reaction, suggesting a crucial role of water in stabilizing the transition states. This study provides valuable insights into the hydrolysis process of borate networks, offering a detailed comparison between different computational approaches. The results demonstrate that the functionals B3LYP, PBE0, and wB97Xd closely approximated the reference MP2 and CCSD(T) calculated reaction pathways, both qualitatively in terms of the mechanism, and quantitatively in terms of the differences in the reaction barriers within the 0.1–0.2 eV interval for the most plausible reaction pathways. In addition, CAM-B3LYP also yielded acceptable results in all cases except for the most complicated pathway. These findings are useful for guiding further computational studies, including those employing machine learning approaches, and experimental investigations requiring accurate reference data for hydrolysis reactions in borate networks. [ABSTRACT FROM AUTHOR]

The spin component scale MP2/molecular mechanics molecular dynamics simulation investigated the hydration shell formation and hydrated Cd2+ dynamics in the water environment. At the first hydration shell, six water molecules with 2.27 Å for the average distance between water and Cd2+. Dynamical properties were analyzed by computing the water molecule's mean residence time (MRT) in its first and second hydration shells. The MRT of each shell was determined to be 31.8 and 1.92 ps, suggesting the strong influence of Cd2+ in the first hydration shell. The second shell was labile, with an average number of water molecules being 18. Despite the strong interaction between Cd2+ and water molecules in the first shell, the influence of ions in the second hydration shell remained weak. [ABSTRACT FROM AUTHOR]

Context: An accurate description of the molecular charge density is crucial for investigating intra- and inter-molecular properties. Among the different ways of describing and analyzing it, the widely used distributed multipole analysis (DMA) is an accurate method for decomposing the molecular charge density into atom-centered electric multipoles (monopole, dipole, quadrupole, and so on) that have a direct chemical interpretation. In this work, DMA was employed to decompose the molecular charge density of six chemically distinct molecules, namely, (2R)-2-amino-3-[(S)-prop-2-enylsulfinyl] propanoic acid (AAP), 4-amine-2-nitro-1,3,5 triazole (ANTA), (RS)-Propan-2-yl methylphosphonofluoridate (SARIN), chloromethane (CLMET), and 2-aminoacetic acid (GLY) into monopole, dipole, and quadrupole values. A hypothetical variation of ANTA built by exchanging all the nitrogen atoms with phosphorus that we named 4-phosphine-2-phosphite-1,3,5-phosphorine (ANTAP) was also studied. These molecules have different chemical structures bearing distinct carbon skeletons, electronegative atoms, and electron-withdrawing/donating groups. We found that although DFT multipole values can depend considerably on the exchange–correlation functional for specific atomic sites, the associated root-mean-square errors (RMSEs) compared to benchmark MP4 mainly were about 10 - 5. The most significant variations were for monopoles and dipoles of sites highly polarized by adjacent atoms, and to a lesser degree, for the quadrupoles. The double hybrid B2PLYP and the hybrid meta M06-2X functionals, as expected in the framework of Jacob's ladder, overall give the most accurate results among the DFT methods. The MP2 DMA multipole values have an RMSE in relation to the MP4 benchmark mainly in the range 10 - 4 - 10 - 6 , thus representing a lower computational cost to obtain results with similar good accuracy without the ambiguity of choosing a DFT functional. The deviations of the HF multipoles from the benchmark in most cases were less than 20%, in agreement with the well-known fact that non-correlated charge densities have a slight dependence on the electronic correlation. We also confirmed that DMA values have a small dependence on the size of the basis set: deviations did not exceed 5% in most cases. However, the dependence of the DMA values on the size of the basis set increases with the rank of the electric multipole. To compute accurate values of DMA multipoles of an atom bonded to very electronegative atoms, especially dipoles and quadrupoles, a large basis set including diffuse functions is necessary. Despite that, for a given polarized basis set, the choice of the basis set to compute accurate DMA multipole values is not critical. Methods: The molecular charge densities were computed using the electronic structure methods Hartree–Fock (HF), MP2, MP4, DFT/PBE, DFT/B3LYP, DFT/B3PW91, DFT/M06-2X, and DFT/B2PLYP implemented in the Gaussian 09 package. MP4 was the benchmark method. The DMA multipoles were obtained with the GDMA program of Stone. The 6-311G + + (d,p) basis set was used for the production calculations, and the augmented correlation-consistent Dunning's hierarchy of basis sets was employed to evaluate the dependence of the DMA multipoles on the basis set size. [ABSTRACT FROM AUTHOR]

Boronic acids and their conjugate esters have been employed in a variety of biomedical applications. This work reports structural, bonding, and thermochemical calculations for the boronic acids, RB(OH)2 (R = H, methyl, phenyl, and ortho-methyl-phenyl) and the corresponding ethylene glycol diesters, RB(−OCH2CH2O−), in the presence of explicit NH3 and/or H2O molecules. Calculations were performed in vacuo and in polarizable continuum model (PCM) aqueous media using density functional theory (DFT) and second-order Moller-Plesset perturbation theory (MP2) with the Dunning-Woon aug-cc-pVTZ basis set. These results quantify the relative stability of N→B dative-bonded, water-inserted Owater→B (Ow→B) dative-bonded, Zwitterionic, and hydrogen-bonded conformers on the model RB(OH)2⋅NH3⋅H2O and RB(−OCH2CH2O−)⋅NH3⋅H2O potential energy surfaces (PESs). In the PCM aqueous media model, conformers containing an H3N→B dative bond and a water molecule bridging the NH3 and acid/ester moieties were found consistently to be lower in energy than arrangements with either a H3N⋯H(H)Ow→B linkage or Zwitterionic forms, [H4N]+[HOw−BR(OH)2]− or [H4N]+[HOw−BR(−OCH2CH2O−)]−. For the acids, however, conformers with a H3N⋯HOwH moiety hydrogen bonded to the boronate hydroxyl group(s) proved to be lowest in energy. [ABSTRACT FROM AUTHOR]

This study has centered on potentially important enzyme‐ligand interaction currently not incorporated: stacked cation–π interaction between the guanidinium group of arginine 590 residues in the HMG‐CoA reductase active site and the 4‐fluorophenyl group of type II statins. The geometry of interaction between these planar groups has been considered based on the X‐ray crystallographic structures already available in the protein data bank (PDB) archive. Electronic interaction energies between this residue and statins have been acquired by M06 and MP2 methods. In addition, stacking interaction and hydrogen bonding as two important investigated interactions are verified through prominent analyses: (1) quantum theory of atoms in molecules (QTAIM) analysis; (2) plotting and quantitative molecular surface analyses such as electrostatic potential (ESP) analysis, Hirshfeld surface (HS) and Becke surface (BS) analyses; (3) visual study methods such as noncovalent interaction (NCI), independent gradient model (IGM) analyses; and (4) localized orbital locator integrated pi over plane (LOLIPOP) index. The results indicate the absolute binding energy for type II statins is overall more than for type I statins. [ABSTRACT FROM AUTHOR]

The binding energy and structures of the Be cations with H2 molecules are studied theoretically at the MP2/aug-cc-pVTZ method level. The structures of the Be + · H 2 1 - 3 complexes are determined. A tee and a vee-shaped structure are obtained for mono- and di-ligated complexes. Trigonal prism geometry is discovered in the tri-ligated complex. The sequential bond energies of the Be + · H 2 1 - 3 complexes are also examined. The variation in the trend in sequential bond dissociation energies that have been observed may be due to variations in the electrostatic interaction energies, electronic charge transfer, and repulsive forces between the H2 ligands in these complexes. The figure represents the potential energy surface (PES) of the interaction between the Be+ ion and hydrogen molecule, H2, at different polar angle, θ.. Our results show that the Be+,H2 complex exhibits a T configuration in which the cation is vertically directed a long the mid bond length of H2 molecule. The PES diagram has a potential well depth at around - 6.0 kcal/mol. [ABSTRACT FROM AUTHOR]

Theoretical studies have been carried out using DFT, ab initio MP2 and Monte Carlo (MC) simulations of corrosion inhibitors from lawsone derivatives against carbon steel. The research focuses on studying the effect of substituent groups in the lawsone structure on the efficiency of corrosion inhibition in mild steel. Quantum chemical parameters of lawstone inhibitors in neutral and protonated conditions have been calculated. Fukui's function analysis predicts that the active side of the inhibitor will be adsorbed on the mild steel surface. MC simulation is used to understand the adsorption patterns of lawsone compounds on metal surfaces. The organic inhibitor L-NH2 has better performance as a corrosion inhibitor for mild steel in neutral or protonated conditions. [ABSTRACT FROM AUTHOR]

This investigation involved an ab initio and Density Functional Theory (DFT) analysis of the hydrolysis mechanism and energetics in a borate network. The focus was on understanding how water molecules interact with and disrupt the borate network, an area where the experimental data are scarce and unreliable. The modeled system consisted of two boron atoms, bridging oxygen atoms, and varying numbers of water molecules. This setup allows for an exploration of hydrolysis under different environmental conditions, including the presence of OH− or H+ ions to simulate basic or acidic environments, respectively. Our investigation utilized both ab initio calculations at the MP2 and CCSD(T) levels and DFT with a range of exchange–correlation functionals. The findings indicate that the borate network is significantly more susceptible to hydrolysis in a basic environment, with respect to an acidic or to a neutral pH setting. The inclusion of explicit water molecules in the calculations can significantly affect the results, depending on the nature of the transition state. In fact, some transition states exhibited closed-ring configurations involving water and the boron–oxygen–boron network; in these cases, there were indeed more water molecules corresponding to lower energy barriers for the reaction, suggesting a crucial role of water in stabilizing the transition states. This study provides valuable insights into the hydrolysis process of borate networks, offering a detailed comparison between different computational approaches. The results demonstrate that the functionals B3LYP, PBE0, and wB97Xd closely approximated the reference MP2 and CCSD(T) calculated reaction pathways, both qualitatively in terms of the mechanism, and quantitatively in terms of the differences in the reaction barriers within the 0.1–0.2 eV interval for the most plausible reaction pathways. In addition, CAM-B3LYP also yielded acceptable results in all cases except for the most complicated pathway. These findings are useful for guiding further computational studies, including those employing machine learning approaches, and experimental investigations requiring accurate reference data for hydrolysis reactions in borate networks.

The fragment molecular orbital (FMO) method is a fast quantum‐mechanical method that divides systems into pieces of fragments and performs ab initio calculations. The system truncation enables further speed improvement. In this article, we systematically study the effects of system truncations on binding affinity calculations obtained with FMO in combination with either the polarizable continuum model (FMO/PCM) or in combination with the Møller–Plesset method (FMO‐MP2). We have used five protein complexes with ligands of several charged states. The calculated binding energies of the size variants of the truncated system, including only a restricted number of atoms around the ligand, are compared to the energy obtained from a full system. The result shows that the systems could be truncated to a radius of 8 Å from neutral ligands within an error of 0.7 kcal/mol, and 12 Å from charged ligands within an error of 1.1 kcal/mol for calculating the binding energy in solution. [ABSTRACT FROM AUTHOR]

The relativistic effects of the values of the shielding constants of 1H, 13C, 15N, 57Fe, 59Co, 61Ni, 105Pd, and 195Pt nuclei were studied at the four-component relativistic level and were compared to the results of non-relativistic calculations perfomed on a series of biologically important Fe(II), Co(III), Ni, Pd, and Pt glycinates. The accuracy factors affecting the calculation of the chemical shifts of the title heavy nuclei were analyzed. First of all, the advantages and limitations of the different levels of theory used to take into account the electron correlation effects (namely HF, DFT, MP2, and CCSD) at the geometry optimization stage were thoroughly scrutinized. Among the employed DFT functionals, the behavior of 11 dedicated functionals of different types and hierarchies were analyzed. The contribution of the exact-exchange admixture was established both in the geometrical search and during the calculation of the shielding constants, which was exemplified with the PBE family of functionals. The main result of the performed study was that relativistic effects were of major importance to the theoretical calculations of the shielding constants and chemical shifts of the chelate complexes of the transition metals of the 8–10 groups. Thus, the relativistic effects of the values of the shielding constants of those metals, as well as those of the light nuclei located in the α-position to the latter, were found to reach as much as 35 ppm for nitrogen and up to an enormous 4300 ppm for platinum. [ABSTRACT FROM AUTHOR]

The nature of the CO2–nH2O interaction where n = 1, 2 and 3 is explored to evaluate the energetics of the CO2 molecule and small water clusters using ab initio MP2 and CCSD(T). We are interested in the absorbance of CO2 onto the water cluster and not those interactions that significantly alter the original water cluster. Thus, we characterize the interaction energy in terms of CO2 binding to an intact water cluster. We provide 3 lowest energy isomers of the CO2–water trimer not previously reported. We include a description of the bond lengths between the CO2 and the water and include AIM critical points to identify the hydrogen bond interactions. [ABSTRACT FROM AUTHOR]

Understanding the solvation process requires an understanding of how the solvent affects the molecular cluster configurations. The current study examined the impact of solvents on the structures of the hydrated Cu 2 + cation, as well as the temperature dependence, and the energetics of the hydrated Cu 2 + cation clusters from sizes n = 1 from n = 10 (Cu 2 + (H 2 O) n = 1 − 10). The potential energy surfaces (PESs) in the solvent phase (represented by a dielectric continuum medium) at the MP2/6-31++G(d,p) level of computation have been comprehensively investigated. We employed the integral equation formalism polarized continuum model (IEF-PCM) to describe the dielectric continuum medium. It is pointed out that changes in PESs of hydrated copper dication isomers and structural morphologies occur in the continuum medium. Thus, the relative energies are smallest at T=0 K and for the cluster populations of Cu 2 + (H 2 O) n = 1 − 10 are modified. Thus, in the solvent phase, the competition between the conformers is more severe than in the gas phase. Furthermore, highly coordinated conformers are more favorable, and favored conformers in the gas phase are disfavored in the solvent phase. The structural investigations are validated by the analysis of Wiberg Bond Index (WBI) at 300 K. In the solvent phase, the WBIs of the most stable isomers are the smallest and the WBIs of axial bonds are lower than those of equatorial bonds. Concerning the energy analysis, the binding energies were predicted for enormous sizes (at saturation) using a fit function. Thus, the new values of the binding energies obtained are -5467.8 and ▪ in the gas and solvent phases respectively. All these observed differences are due to the effects of the implicit solvent. • The Cu2+ ion is hexa- and penta-hydrated in Cu2+(H 2 O) n=1-10 clusters in solvent phase. • The dative bonds are shorter in solvent phase than in gas phase. • The structural investigations are validated by the analysis of WBI at 300 K. • New values of the binding energies per water molecule at n = ∞ are predicted by fitting the data with a fit function. • The binding energies were predicted for enormous sizes (at saturation) using a fit function. [ABSTRACT FROM AUTHOR]

Quantum tunneling of the ammonia inversion motion and energy level splittings in He and Ar clusters were investigated. It was found that the double well potential (DWP) in He clusters is symmetrical and that the first layer of He atoms is able to model the system. The calculated tunneling splitting was in good agreement with the experimental, 36.4 and 24.6 cm - 1 respectively. For NH 3 in Ar clusters, the DWP becomes slightly asymmetric, which is enough to decrease the resonance and make the symmetric DWP unable to model the system. An asymmetric potential was used and the result was in excellent agreement with the experimental splitting, of 9.0 and 10.6 cm - 1 respectively. Non-covalent interactions revealed that the asymmetry is caused by dissimilar interactions in each minimum of the double well potential. The effects of different methodologies were analyzed via a design of experiments approach. For the gas-phase NH 3 molecule, only diffuse functions were statistically significant while for the NH 3 embedded in He cluster both the MP2 method and polarization functions were significant. This tendency suggests higher order polarization functions may be essential to generate accurate barrier heights. [ABSTRACT FROM AUTHOR]

The domain of application of the G3(MP2)//B3-SBK theory was expanded, and its efficiency was evaluated to determinate enthalpies of formation of forty-one iodine compounds. The results were compared to those obtained with the G2 theory for the same set of molecules. The G3(MP2)//B3-SBK theory showed a mean deviation and deviation standard equal to 3.7 kcal mol−1 and 6.0 kcal mol−1, respectively. The G2 theory (mean deviation = 3.1 kcal mol−1 and standard deviation = 4.9 kcal mol−1) presented a lower error and standard deviation, but at a significantly higher computational cost. For a more complete evaluation, as a secondary part of the work, it also used different functionals B3LYP, M06-2X, WB97XD, and MP2 method with four different basis sets 6-311G(d,p), LANL2DZ, jorge-ADZP, and CEP-31G(d). The best density functional/basis set combination was obtained with M06-2X/CEP-31G(d) among the three mentioned functionals. However, the produced mean deviation is significant and equal to 17.3 kcal mol−1, with a standard deviation equal to 23.0 kcal mol−1. The 6-311G(d,p) basis achieved the best performance with the MP2 method, generating an equally significant mean deviation of 12.8 kcal mol−1 with a standard deviation equal to 18.7 kcal mol−1. [ABSTRACT FROM AUTHOR]

Geometrical parameters and vibrational spectra of the dimer Ba2X4 molecules were calculated; different isomeric structures were revealed. For the Ba2F4, Ba2Cl4, and Ba2Br4, three isomers of C3v, C2v, and C2h symmetries were confirmed to exist while for Ba2I4 only two isomers, C3v and C2h, were confirmed. For all halides, the bipyramidal C3v isomers possessed the lowest energy. The structural parameters and vibrational frequencies were employed in calculation of thermodynamic functions and enthalpies of dissociation reactions of the species. The enthalpies of formation ∆fH(0) of the C3v isomers were obtained (in kJ mol−1): − 1874 ± 2 (Ba2F4); − 1236 ± 7 (Ba2Cl4); − 1023 ± 8 (Ba2Br4); − 787 ± 11 (Ba2I4). The relative abundance of the isomeric forms was evaluated for a broad temperature range, between 200 and 2000 K. It was shown that the concentrations of different isomers appeared to be comparable in saturated vapors at temperatures close to experimental conditions. [ABSTRACT FROM AUTHOR]

The hydration-free energy of the Cu 2 + cation stands as a crucial parameter for comprehending its reactivity with various molecules. However, the existing literature lacks a definitive consensus on this value. To solve this problem, we use the cluster continuum model to evaluate this parameter accurately, requiring the determination of the energy potential of Cu 2 + (H 2 O) n clusters at different temperatures. Our project initiates with an exploration of structures, relative stabilities, infrared spectroscopy, binding energies, and clustering energies of hydrated Copper dication clusters, utilizing the MP2 ab initio method. We propose a unique fit function capable of precisely deriving binding and clustering energies per solvent molecule for the solvated Cu 2 + cation in water. Furthermore, we extend our findings to encompass ammonia as a solvent. The binding and clustering energies exhibit minimal changes showcasing remarkable consistency between water and ammonia as solvents. This saturation phenomenon allows us to understand the behavior of solvated ions. [Display omitted] • The potential energy surfaces are investigated for the Cu2+(H 2 O) n=1-10 clusters and are predicted as a function of temperature. • IR spectra confirms that the hydrated Cu2+ ion is tetra- and penta-coordinated as a function of temperature. • New values of the binding energies per water molecule at n = ∞ are predicted. • This function gives satisfactory results with Cu2+, Fe2+, Mg2+ and Mg+ions. [ABSTRACT FROM AUTHOR]

[Display omitted] • DFT-D will remain the work horse of computational heterogeneous catalysis and relevant work will use it acknowledging the limitations of a given level of DFT-D for a given problem. • MLP trained on DFT-D will take over in computational heterogeneous catalysis enabling simulations over longer times, larger configuration spaces and for larger simulation boxes. This way, more complex models will become accessible to simulations with DFT-D quality. • Progress in computer technology and algorithm development will allow climbing up the Jacobs ladder to higher rungs (hybrid functionals, RPA) and to wavefunction methods. • MD or MC simulations beyond the DFT-D level will remain a computational and methodological challenge with an increasing number of studies meeting this challenge. Besides Δ -learning, most promising is the combination of high-level treatments on cluster models with low level treatments on periodic models including the transfer of MLP that are trained on cluster models to periodic models. The future of computational heterogeneous catalysis is shaped by machine learning in two different but equally important areas: (i) development of atomistic potentials that closely approximate DFT and wavefunction based ab initio methods (MP2, CCSD(T)), but are computationally more efficient, and (ii) finding structure and reactivity descriptors for predicting catalytic materials and reactions. Machine Learning Potentials will enable improved sampling of the potential energy surface (PES) for reaction conditions, but they will not do better than the data on which they are trained. Therefore, this perspective focusses on ways of improving the quality of the PES beyond the generalized gradient approximation (GGA) climbing the Jacob's ladder of functionals in DFT up to RPA and using wavefunction methods such as MP2 and CCSD(T). It is problem solving in close collaboration with experiment that has made computational methodology relevant and this remains an important aspect of data science methods in heterogeneous catalysis. [ABSTRACT FROM AUTHOR]

The essential biological role of seleno amino acids has been approved in many theoretical and experimental studies. These seleno amino acids may deliver selenium to cells to prevent cancer or resist infections. In this research, d-threonine and its oxygen, sulfur, and selenium analogues were analysed using the second order Møller–Plesset theory (MP2) and density functional theory (DFT). The neutral and zwitterion forms were investigated to predict the most stable forms in the gas and aqueous phases. The polarizable continuum models within the self-consistent reaction field method (SCRF) were applied to simulate the solvent effect. Calculations confirmed that the zwitterion form of all analogues is the most stable form of threonine in water. In the gas and solution phases, the selenium analogue had the lowest energy, followed by the sulfur analogue, and then by the oxygen analogue with the highest energy. Thermodynamic parameters and molecular orbital analysis are listed and discussed in this study. [ABSTRACT FROM AUTHOR]

The tautomeric equilibrium of sildenafil molecule was theoretically studied using B3LYP and M06-2X density functional theory (DFT) methods in connection with aug-cc-pVDZ correlation consistent basis set. Calculations were performed for gas phase and water solution conditions modelled by polarizable continuum model (PCM). Three tautomeric forms are possible. Two keto forms: A — where the tautomeric proton in more distant from carbonyl group and B — where it is closer, and one enol form denoted, C. Both DFT methods qualitatively give similar tautomer stability order: B > A > C. The B tautomer is dominant in gas phase and water environment, whereas the C tautomer is too high in energy to be present in the tautomeric mixture. Regarding the A tautomer, it is not present in the gas phase but is present in small amounts in water solution. According to B3LYP/aug-cc-pVDZ, the relative Gibbs-free energies for A and C relative to B are 10.05 kcal/mol and 11.91 kcal/mol for gas phase and 5.49 kcal/mol and 12.49 kcal/mol for water solution. According to M06-2X/aug-cc-pVDZ, the relative Gibbs-free energies for A and C are 9.12 kcal/mol and 10.60 kcal/mol for gas phase and 4.27 kcal/mol and 10.23 kcal/mol for water solution. Therefore, for in vivo conditions, we expect that the B tautomer is dominant, and there may exist small amounts of the A tautomer. The C enol tautomer is not present at all. This picture is very different from the parent tautomeric system: 4-hydroxypyrimidine/4-pyrimidinone where the C enol tautomer is less stable than keto B only by about 1 kcal/mol in the gas phase and the A keto tautomer is the least stable and not present in the tautomeric mixture. In order to understand these differences, we performed additional calculations for a series of parent molecules starting from 4-hydroxypyrimidine/4-pyrimidinone, going through two in-between model molecules and ending at Sildenafil molecule. We found that the most important reasons of C form destabilization are dearomatization of the 6-membered ring caused by the fusion with pyrazole ring, lack of strong intramolecular hydrogen bond in C form of sildenafil and presence of destabilizing steric interaction of oxygen and nitrogen atoms of two 6-memberd rings in this tautomer. [ABSTRACT FROM AUTHOR]

The B3LYP, MP2, and CBS-QB3 quantum chemical methods are used to study the relative energy and isomerization reactions of C5H5+ cations. Ease of generation of 14 C5H5+ isomers by ionic dissociation of halide precursors does not correlate well with carbocation stability. The reaction profiles of concerted isomerization of various C5H5+ cations to six select cations are established along with the respective transition states. The rate coefficients of these processes are estimated by using transition state theory and activation energies computed. The transition states for these six reactions are characterized with regard to position along the isomerization pathway as per Hammond's postulate. The 6 isomerization reactions are combined to yield multi-step conversions of various C5H5+ species to the lowest energy vinylcyclopropenyl cation 1. Finally, three different routes for obtaining the select cations from C5H5Br precursors are profiled and the most favored pathways predicted. [ABSTRACT FROM AUTHOR]

Unlike pyranoside cycles which are generally characterized by strictly defined conformational preferences, furanosides are flexible and may adopt a wide range of available conformations. During our previous studies, conformational changes of galactofuranoside cycles upon total sulfation were described computationally, using a simple Hartree–Fock (HF) method, and principal conformers of the 5-membered galactose ring were revealed. However, in the case of more complex disaccharide structures, it was found that this method and the widely applied DFT-B3LYP produced results that deviated from experimental evidence. In this study, other DFT functionals (PBE0 and double hybrid B2PLYP) along with RI-MP2 are employed to study the conformational behavior of the galactofuranoside ring. Reinvestigation of galactofuranosides with a lactic acid substituent at O-3 revealed that changes in the orientation of lactic acid residue at O-3 might induce conformational changes of the furanoside cycle. Such findings are important for further modeling of carbohydrate–protein interaction.

This work duly investigates the recovery of Pd (II) chlorocomplexes from industrial wastewater. Chemical structures and thermodynamic stabilities of the complex formed are evaluated via density functional theory (DFT). By applying synergistic solutions of thiourea mixed with hydrochloric acid (HCl), the stripping reaction of Pd (II) in the loaded Aliquat 336 occurs and Pd (II) chlorocomplexes coordinated thiourea ligands are formed, thus 80.19% of Pd (II) chlorocomplexes can be recovered. The aim of this study is to gain a better understanding of the stripping mechanisms and the structure of the complexes formed via the synergistic system. Such an understanding is still limited since little research has been conducted in this field. Owing to their molecular geometry, ligand coordination and donor groups play a vital role in the reactivity of palladium (II) complex. Quantum models have been developed to evaluate the chemical structure and thermodynamics stability of ((NH2)2CS·PdCl2) namely: (i) DFT with B3LYP/6-31g(d,p) and MP2/6-31g(d,p) basis set, (ii) MP2 with cc-pVTZ basis set and (iii) CCSD(T)/cc-pVTZ. Results demonstrate that the highest geometric stability exhibited is the structure of Pd-S bonding with 180° Cl-Pd-Cl. The distance (r) and angle (a) of the selected geometrical parameters for (NH2)2CS·PdCl2 complex are reported. Additionally, FTIR and UV–vis spectroscopies have been conducted to analyze the sampling solutions. Further, the calculated vibrational frequencies and experimental spectroscopic results show good agreement with the optimized geometry.

Ligand protected metal nanoclusters (NCs) have gained much attention due to their unique physicochemical properties and potential applications in material science. Noble metal NCs protected with thiolate ligands have been of interest because of their long-term stability. The detailed structures of most of the ligandstabilized metal NCs remain unknown due to the absence of crystal structure data for them. Theoretical calculations using quantum chemistry techniques appear as one of the most promising tools for determining the structure and electronic properties of NCs. That is why finding a cost-effective strategy for calculations is such an important and challenging task. In this work, we compare the performance of different theoretical methods of geometry optimization and absorption spectra calculation for silver-thiolate complexes. We show that second order Moller-Plesset perturbation theory reproduces nicely the geometries obtained at a higher level of theory, in particular, with RI-CC2 method. We compare the absorption spectra of silver-thiolate complexes simulated with different methods: EOM-CCSD, RI-CC2, ADC(2) and TDDFT. We show that the absorption spectra calculated with the ADC(2) method are consistent with the spectra obtained with the EOM-CCSD and RI-CC2 methods. CAM-B3LYP functional fails to reproduce the absorption spectra of the silver-thiolate complexes. However, M062X global hybrid meta-GGA functional seems to be a nice compromise regarding its low computational costs. In our previous study, we have already demonstrated that M062X functional shows good accuracy as compared to ADC(2) ab initio method predicting the excitation spectra of silver nanocluster complexes with nucleobases.

We investigate the minimum-energy path for the rotation of formal C=N double bonds in molecules with guanidine-like substructures as present in the chemical class of neonicotinoids. The transitions between the E- and Z-isomers of several neonicotinoids using scans of the torsional potential energy hypersurfaces are quantified at the DFT-level of theory. The validity of using this ansatz is checked by single-point CCSD(T) calculations for model systems like nitroguanidine. A combined approach of theory and experiment permits to unambiguously identify the relevant isomers present at ambient conditions. As an example, MP2-GIAO predictions of the NMR spectra of E- and Z-Clothianidin are experimentally confirmed by low-temperature NMR-experiments identifying for the first time the hitherto unknown Z-Isomer of Clothianidin. [ABSTRACT FROM AUTHOR]

We have reached at new acyclic plumbylenes which show high acidity, via B3LYP, Def2‐TZVP, MP2, and wB97XD functionals. Our scrutinized plumbylenes include dicyclopropaplumbylene (1), dicyclopentaplumbylene (2), dicycloheptaplumbylene (3), and their unsaturated analogs (1'‐3', respectively). All scrutinized plumbylenes and their corresponding protonated forms, 1H(1'H)‐3H(3'H), appear as minima on their energy surfaces. The conductor‐like polarizable continuum model (CPCM) and integral equation formalism polarized continuum model (IEFPCM) are used to predict the pka values for electrophilic plumbylenes, 1(1')‐3(3'), in chloroform and water. In most cases, plumbylenes turn out as strong acids for showing relatively high negative pka values. The acidity trend of plumbylenes (pka) appears consistent with their proton affinity (PA), electrophilicity (ω), nucleophilicity (N), Mulliken electronegativity (X), absolute chemical hardness (ηabs), singlet‐triplet energy gap (ΔEs‐t), and highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gap (ΔEH‐L). Structure 1, with two cyclopropyl groups, turns out as the most acidic specie in chloroform and water. [ABSTRACT FROM AUTHOR]

This work presents a systematic investigation of the spectroscopic properties at anharmonic force fields of ground electronic state ( X ˜ 1 A 1 ) of LiNH2, which are calculated using second-order Møller-Plesset perturbation theory (MP2) and density functional theory (DFT) with hybrid GGA and meta-hybrid GGA (M06-2X) exchange-correlation functional. Two high angular momentum basis sets of 6-311+g (2d, p) and 6-311++g (3df, 2pd) are used. The equilibrium geometries, ground-state rotational constants, harmonic frequencies, and quartic and sextic centrifugal distortion constants of LiNH2 are calculated and compared with corresponding experimental or theoretical data. The predicted accuracy of the calculated constants has been confirmed by analyzing the deviations with respect to experiment. In addition, the anharmonic constants, vibration-rotation interaction constants, force constants, and Coriolis coupling constants of LiNH2 are firstly obtained. The infrared spectrum is predicted and together with the first prediction on the higher-order anharmonic constants contributes to a better understanding of the vibrational and rotational characteristics of LiNH2, thus revealing its internal structure. [ABSTRACT FROM AUTHOR]

We present an implementation of the canonical and Laplace-transformed formulation of the second-order Møller–Plesset perturbation theory under periodic boundary conditions using numerical atomic orbitals. To validate our approach, we show that our results of the Laplace-transformed MP2 correlation correction for the total energy and the band gap are in excellent agreement with the results of the canonical MP2 formulation. We have calculated the binding energy curve for the stacked trans-polyacetylene at the Hartree–Fock + MP2 level as a preliminary application.

One of the main tasks of quantum chemistry is to predict total energies for a given system within an almost nonapproximate method for a complete basis set. Unfortunately, although computational power is growing, approximate methods such as MP2 or CCSD(T) still have to be used as higher‐level methods such as full CI are computationally too demanding. Nevertheless, when using these approximations, at least an almost‐complete basis set should be used in order to avoid errors from basis set superposition or incompleteness. The aim of this work is to introduce a new method to analyze the local basis set superposition error (LBSSE) using local counterpoise corrections in order to converge a basis set for a given compound. Using this approach, we are able to define a basis set composition for a CO molecule adsorbed on rutile(110), which can be regarded as complete. In the presented work, CO adsorbed on rutile(110) and CO2 adsorbed on a model Ti3O9 cluster will be used as case examples. Therefore, in addition to a new LBSSE analysis method, adsorption energies of CO on rutile(110) at the MP2 and CCSD levels of theory are presented. [ABSTRACT FROM AUTHOR]

Alloy clusters of NaxLiy (4 ≤ x + y ≤ 10) are studied by exploring the potential energy surface in the ab initio MP2 level with the support of a quantum genetic algorithm (QGA). In some cases, the structures have been also refined with DFT and coupled-cluster methods. The general trends of sodium-lithium structures are in line with previous studies. The ionization potentials and polarizabilities to all structures were calculated with MP2 method and the average error between these two properties compared with experimental data was 6% and 13%, respectively. The topological analysis based on quantum theory of atoms in molecules (QTAIM) showed that by increasing the cluster size of the diatomic system there was a decrease of atomic interaction energies. The degree of degeneracy from D3BIA aromaticity index and the analysis of the atomic charges showed the influence (by charge transfer) of the chemical element in lower quantity in the cluster with respect to the other atoms. Our achievements of comparing our theoretical results with available experimental data have demonstrated that our approach can also predict satisfactorily quantum atomic and alloy clusters properties, at least, for low nuclearities. [ABSTRACT FROM AUTHOR]

The conformational preferences of the cationic nylon-3 βNM [(3R,4)-diaminobutanoic acid, dAba] dipeptide in water were explored as the first step to understand the mode of action of polymers of βNM against phylogenetically diverse and intrinsically drug-resistant pathogenic fungi. The CCSD(T), MP2, M06-2X, ωB97X-D, B2PLYP-D3BJ, and DSD-PBEP86-D3BJ levels of theory with various basis sets were assessed for relative energies of the 45 local minima of the cationic Ac-dAba-NHMe located at the SMD M06-2X/6-31+G(d) level of theory in water against the benchmark CCSD(T)/CBS-limit energies in water. The best performance was obtained at the double-hybrid DSD-PBEP86-D3BJ/def2-QZVP level of theory with RMSD = 0.12 kcal/mol in water. The M06-2X/def2-QZVP level of theory predicted reasonably the conformational preference with RMSD = 0.38 kcal/mol in water and may be an alternative level of theory with marginal deviations for the calculation of conformational energies of relatively longer cationic peptides in water. In particular, the H14–helical structures appeared to be the most feasible conformations for the cationic Ac-dAba-NHMe populated at 48–64% by relative free energies in water. The hexamer built from the H14–structure of the cationic Ac-dAba-NHMe adopted a left-handed 314-helix, which has a slightly narrower radius and a longer rise than the regular 314-helix of β-peptides. Hence, the 314-helices of oligomers or polymers of the cationic dAba residues are expected to be the active conformation to exhibit the ability to bridge between charged lipid head groups that might cause a local depression or invagination of the membrane of fungi.

Broadcasters need to decide on bitrates of the services in the multiplex transmitted via Digital Audio Broadcasting Plus system. The bitrate should be set as low as possible for maximal number of services, but with high quality, not lower than in conventional analog systems. In this paper, the objective method Perceptual Evaluation of Audio Quality is used to analyze the perceived audio quality for appropriate codecs --- MP2 and AAC offering three profiles. The main aim is to determine dependencies on the type of signal --- music and speech, the number of channels --- stereo and mono, and the bitrate. Results indicate that only MP2 codec and AAC Low Complexity profile reach imperceptible quality loss. The MP2 codec needs higher bitrate than AAC Low Complexity profile for the same quality. For the both versions of AAC High-Efficiency profiles, the limit bitrates are determined above which less complex profiles outperform the more complex ones and higher bitrates above these limits are not worth using. It is shown that stereo music has worse quality than stereo speech generally, whereas for mono, the dependencies vary upon the codec/profile. Furthermore, numbers of services satisfying various quality criteria are presented.

In this study, we theoretically investigated Methyl hydrazinecarbodithioate by quantum chemical calculations for geometry optimization, vibration frequencies, and electronic structure parameters. The geometry optimization by DFT, ab initio MP2 method and the frequency calculation by DFT method was performed at the highest available Pople style 6-311G++(3df,3pd) basis set level. The semi-empirical calculations were performed by the latest PM7 method. The theoretically obtained results were compared with the experimental data. Conformational behavior, frontier molecular orbitals, molecular electrostatic potential, electron localization function, and non-covalent interaction plots were also analyzed. The study explained the geometry, conformational flexibility and relative stability of different conformers.

Hydrogen adsorption properties of the CN3Be3+ cluster have been studied using density functional theory and MP2 method with a 6–31++G** basis set. Five hydrogen molecules get adsorbed on the CN3Be3+ cluster with a hydrogen storage capacity of 10.98 wt%. Adsorption of three H2 molecules on one of the three Be atoms in a cluster is reported for the first time. It is due to the more positive charge on this Be atom than the remaining two. The average value for H2 adsorption energy in CN3Be3+ (5H2) complexes is 0.41 (0.43) eV/H2 at MP2 (wB97XD) level, which fits well within the ideal range. Adsorption energy from electronic structure calculations plays an important role in retaining the number of H2 molecules on a cluster during atom-centered density matrix propagation (ADMP) molecular dynamics (MD) simulations. According to ADMP-MD simulations, out of five H2 adsorbed molecules on CN3Be3+, four and two H2 molecules remain absorbed on CN3Be3+ cluster at 275 K and 350 K, respectively, during the simulation. [ABSTRACT FROM AUTHOR]

The impact of solvent flexibility and electron correlation on the simulation results of Cu2+ in liquid ammonia has been investigated via an ab initio quantum mechanical charge field molecular dynamics (QMCF MD) simulation approach. To achieve this, three different simulation systems were considered in this study, namely Cu2+ in rigid and flexible ammonia at Hartree‐Fock (HF) level of theory, as well as resolution of identity second order Møller‐Plesset (MP2) perturbation theory in the rigid body case. In all cases, a stable octahedral [Cu(NH3)6]2+ complex subject to dynamic Jahn‐Teller distortions without the occurrence of ligand exchange was observed. The Cu2+ − NH3 distance in the first shell agrees well with the experimental and other theoretical data. In all three cases, the structural data shows that the rigid‐body ammonia model in conjunction with the HF level of theory provides accurate data for the first solvation shell, while at the same time, the computational demand and thus the achievable simulation time are much more beneficial. The vibrational analysis of the Cu2+ − NH3 interaction yields similar force constants in the three investigated systems indicating that there is no distinct difference on the dynamical properties of the first solvation shell. In addition to the QMCF MD simulations, a number of natural bond orbital (NBO) analyses were carried out, confirming the strong electrostatic character of the Cu2+ − NH3 interaction. [ABSTRACT FROM AUTHOR]

We describe an efficient implementation of the partition of the second‐order Møller–Plesset (MP2) correlation energy within the interacting quantum atoms (IQA) energy decomposition. We simplify the IQA integration bottleneck by considering only the occupied to virtual elements of the second order reduced density matrix, a procedure that reduces substantially the size of the two‐electron matrix, which has to be addressed. The algorithmic improvements described herein allow to perform the decomposition of the MP2 correlation energy for medium size molecular systems using moderate computational resources. We expect that the methods developed in this investigation will prove useful to understand electron correlation effects through a real space perspective. [ABSTRACT FROM AUTHOR]

A theoretical study on the intermolecular dihydrogen bonding (DHB) of ground state B3N3H6···HM (Li, Na, and K) complexes were investigated by B3LYP and MP2 methods with 6-311++G** basis set. Thermodynamic parameters (entropy, enthalpy, heat capacity and Gibbs free energy) of the complexes were calculated at different temperatures in the gas phase. The vibrational analysis of N–H···H–M DHB bond formation reveals that the calculated N–H and M–H stretching frequencies undergo red and blue shifts, respectively. The calculated interaction energies correlate well with the geometrical parameters wherein the shortest H···H intermolecular distance is obtained for B3N3H6···HK complex. The chemical shift of 1H, 11B and 15N NMR predict large variation for B3N3H6···HK complex which has large protonic hydrogen than the hydridic hydrogen. Furthermore, natural bond orbital and quantum theory of atoms in molecule analyses were carried out to explore the non-covalent interaction along with the molecular electrostatic potential to predict the reactive sites of electrophilic and nucleophilic attack. [ABSTRACT FROM AUTHOR]

We seek correlation‐consistent diffuse‐augmented double‐zeta and triple‐zeta basis sets that perform optimally in extrapolating the correlation energy to the one‐electron complete basis set limit, denoted oAVXZ and oAV(X + d)Z. The novel basis sets are method‐dependent in that they are trained to perform optimally for the correlation energy at each specific level of theory. They are shown to yield accurate results in calculating both the energy and tensorial properties such as polarizabilities while not significantly altering the Hartree‐Fock energy. Quantitatively, complete basis set limit (CBS)‐/(oAVdZ,oAVtZ)‐extrapolated correlation energies typically outperform, by 3‐ to 5‐fold, the ones calculated with traditional ansatzes of similar flexibility. Attaining energies of CBS/(AVtZ,AVqZ) type or better accuracy, they frequently outperform expensive raw explicitly correlated ones. Promisingly, a limited test on CBS‐extrapolated energies based on conventional basis sets has shown that they compare well even with extrapolated explicitly correlated ones. Calculated atomization and dissociation energies, molecular geometries, ionization potentials, and electron affinities also tend to outperform the ones obtained with traditional Dunning's ansatzes from which the new basis sets have been determined. The method for basis set generation is simple, and there is no reason of principle why the approach could not be adapted for handling other bases in the literature. [ABSTRACT FROM AUTHOR]

The reaction pathway of the isomerization in the process of butyl rubber bromination was proposed based on density functional theory (DFT) and MP2 calculations with the 6-31+G(d) basis set. The microstructural composition of the brominated butyl rubber was determined via proton nuclear magnetic resonance spectroscopy (1H-NMR).The transition state of isomerization reaction was identified. The geometries of the reactant, transition state, and product structures were optimized. The effect of solvation on model compounds was simulated using the polarizable continuum model (PCM). The energy barriers for isomerization reactions were calculated with different solvents (vacuum, n-hexane, and 245/105 mL n-hexane/dichloromethane (DCM) mixture). The increase in solvent polarity decreased the activation energy and facilitated the isomerization reaction. This finding was consistent with the experimental result. [ABSTRACT FROM AUTHOR]

Pyridazine-derivative compounds have attracted considerable attention in different fields for many years due to their various biological activities such as antimicrobial and antitumoral. Simultaneously, antioxidative activity is one of the most important properties of them. With this perspective, antioxidative properties of eight different pyridazine derivatives have been investigated in accordance with three main mechanisms (HAT, SET, and SPLET). The investigation has been carried out with HF/6-31+G(d,p), B3LYP/6-31G(d,p), B3LYP/6-31+G(d,p), and MP2/6-31+G(d,p) level of theories in both gas and liquid phases. Furthermore, several molecular descriptors, bond and proton dissociation enthalpies, ionization potentials, proton affinities, and electron transfer enthalpies of investigated molecules have been calculated and antioxidative properties of the molecules have been sorted in accordance with the calculated values. Besides, effects of methods, basis sets, and solvent have been discussed. This computational study has determined the antioxidative capacity of 1A2D as the highest in each studied method and phases. [ABSTRACT FROM AUTHOR]

• 1-methyl-germacyclohexane stable conformers that can be observed experimentally are 1C 4 axial (global minimum) and 1C 4 equatorial structures. • The conformational rearrangement path for the 1-methyl-1-germacyclohexane is: chair → half-chair → skew-boat → boat → skew-boat. • The heteroatom as well as substituent type influence the conformational stability of the six-member saturated cyclic molecules. The conformational analysis of newly synthesized compound, 1-methyl-1-germacyclohexane, is performed by means of a combined vibrational spectroscopy and theoretical methods. The liquid 1-methyl-1-germacyclohexane at room temperature is investigated using conventional ATR-FTIR, FT-IR (gas phase) and Raman methods. FT-IR spectra of the isolated in low-temperature neon and nitrogen matrices are registered. The further detailed vibrational conformational analysis is performed utilizing density functional theory and perturbation theory based methods. All possible 12 canonical ring conformations considering axial/equatorial CH 3 group position are analyzed by utilizing DFT/B3LYP, DFT/M06-2X and MP2 at aug-cc-pVTZ theory level. The most stable conformer with the global energy minimum structure in the chair axial conformation is investigated in detail. Analysis of the potential energy surface reveals transition states (TS) and energy barriers. The conformational path is found to be as follows: chair (1C 4)→ envelope/half-chair (TS)→ skew-boat (1S 3 - C 1 symmetry)→ boat (TS)→ skew-boat (1S 5 - C 2 symmetry). The energy barrier for 1C 4 (chair) to 1S 3 (skew-boat) conversion is 4.8 kcal/mol while for the reverse process equals to 0.5 kcal/mol. A similar conformational analysis is performed for different configurations of germacyclohexane, silacyclohexane and cyclohexane with mono- and disubstituents: CH 2 Cl, CH 3 , Cl and F. Such approach enables investigation of influence of the heteroatoms and substituents of various size and type on the structural parameters and conformational diversity. [ABSTRACT FROM AUTHOR]

• The intramolecular aromatic π-π interactions were investigated by high-level quantum mechanical methods in the transition states of Ir(I)-catalyzed alkane C H activation. • The truncated phenyl radical model was applied for the computation of intramolecular π-π interactions. • The intramolecular π-π interaction forces were found to be slightly repulsive. • Comparison of calculation results were made between restricted open shell and unrestricted open methods. • The DFT M06HF functional provided a reasonable interaction energy compared with that of CCSD(T) method. Major π-π interactions within Ir(I) complex Cp*Ir(I)PPh3 were quantum mechanically characterized using the major hybrid DFT, MP2 and CCSD(T) methods. Almost all the neighborhood π-π interactions are found to be slightly repulsive, and MP2 interaction energies range from -0.49 to 19.16 kcal/mol. The representative A-B π-π pair (See Figure 2) is further addressed using DFT methods. Interaction energies of the A-B π-π pair calculated from 6 hybrid DFT methods ranges from 0.52 kcal/mol by the SVWN method to 8.23 kcal/mol by the O3LYP method. The interaction energies calculated with unrestricted open shell MP2 theory can be up to 25% higher than those computed using restricted open shell MP2 method. Comparison between the DFT results and the benchmark CCSD(T) results shows that the DFT calculations of π-π interaction are reasonable but still need to be interpreted with caution. This study helps to pinpoint intramolecular aromatic π-π interactions of transition state for alkyl C H bond activation of saturated hydrocarbons and rational molecular design. [ABSTRACT FROM AUTHOR]

Our long-term investigations have been devoted the characterization of intramolecular hydrogen bonds in cyclic compounds. Our previous work covers naphthazarin, the parent compound of two systems discussed in the current work: 2,3-dimethylnaphthazarin (1) and 2,3-dimethoxy-6-methylnaphthazarin (2). Intramolecular hydrogen bonds and substituent effects in these compounds were analyzed on the basis of Density Functional Theory (DFT), Møller–Plesset second-order perturbation theory (MP2), Coupled Clusters with Singles and Doubles (CCSD) and Car-Parrinello Molecular Dynamics (CPMD). The simulations were carried out in the gas and crystalline phases. The nuclear quantum effects were incorporated a posteriori using the snapshots taken from ab initio trajectories. Further, they were used to solve a vibrational Schrödinger equation. The proton reaction path was studied using B3LYP, ωB97XD and PBE functionals with a 6-311++G(2d,2p) basis set. Two energy minima (deep and shallow) were found, indicating that the proton transfer phenomena could occur in the electronic ground state. Next, the electronic structure and topology were examined in the molecular and proton transferred (PT) forms. The Atoms In Molecules (AIM) theory was employed for this purpose. It was found that the hydrogen bond is stronger in the proton transferred (PT) forms. In order to estimate the dimers’ stabilization and forces responsible for it, the Symmetry-Adapted Perturbation Theory (SAPT) was applied. The energy decomposition revealed that dispersion is the primary factor stabilizing the dimeric forms and crystal structure of both compounds. The CPMD results showed that the proton transfer phenomena occurred in both studied compounds, as well as in both phases. In the case of compound 2, the proton transfer events are more frequent in the solid state, indicating an influence of the environmental effects on the bridged proton dynamics. Finally, the vibrational signatures were computed for both compounds using the CPMD trajectories. The Fourier transformation of the autocorrelation function of atomic velocity was applied to obtain the power spectra. The IR spectra show very broad absorption regions between 700 cm−1–1700 cm−1 and 2300 cm−1–3400 cm−1 in the gas phase and 600 cm−1–1800 cm−1 and 2200 cm−1–3400 cm−1 in the solid state for compound 1. The absorption regions for compound 2 were found as follows: 700 cm−1–1700 cm−1 and 2300 cm−1–3300 cm−1 for the gas phase and one broad absorption region in the solid state between 700 cm−1 and 3100 cm−1. The obtained spectroscopic features confirmed a strong mobility of the bridged protons. The inclusion of nuclear quantum effects showed a stronger delocalization of the bridged protons.

Intra- and intermolecular forces competition was investigated in the 9,10-anthraquinone (1) and its derivatives both in vacuo and in the crystalline phase. The 1,8-dihydroxy-9,10-anthraquinone (2) and 1,8-dinitro-4,5-dihydroxy-anthraquinone (3) contain Resonance-Assisted Hydrogen Bonds (RAHBs). The intramolecular hydrogen bonds properties were studied in the electronic ground and excited states employing Møller-Plesset second-order perturbation theory (MP2), Density Functional Theory (DFT) method in its classical formulation as well as its time-dependent extension (TD-DFT). The proton potential functions were obtained via scanning the OH distance and the dihedral angle related to the OH group rotation. The topological analysis was carried out on the basis of theories of Atoms in Molecules (AIM—molecular topology, properties of critical points, AIM charges) and Electron Localization Function (ELF—2D maps showing bonding patterns, calculation of electron populations in the hydrogen bonds). The Symmetry-Adapted Perturbation Theory (SAPT) was applied for the energy decomposition in the dimers. Finally, Car–Parrinello molecular dynamics (CPMD) simulations were performed to shed light onto bridge protons dynamics upon environmental influence. The vibrational features of the OH stretching were revealed using Fourier transformation of the autocorrelation function of atomic velocity. It was found that the presence of OH and NO2 substituents influenced the geometric and electronic structure of the anthraquinone moiety. The AIM and ELF analyses showed that the quantitative differences between hydrogen bonds properties could be neglected. The bridged protons are localized on the donor side in the electronic ground state, but the Excited-State Intramolecular Proton Transfer (ESIPT) was noticed as a result of the TD-DFT calculations. The hierarchy of interactions determined by SAPT method indicated that weak hydrogen bonds play modifying role in the organization of these crystal structures, but primary ordering factor is dispersion. The CPMD crystalline phase results indicated bridged proton-sharing in the compound 2.