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This work is a first attempt to understand the mechanism of metformin thermal decomposition under inert conditions. Thermal gravimetric analysis coupled with mass spectrometry was used to probe the thermal degradation reactions. Density functional theory and second-order perturbation (MP2) theoretical calculations were used to construct a reaction mechanism for metformin decomposition. It was evident that the reactions are initiated via formation of methyl radicals, and ammonia via 1.3-H shift, followed by a series of different secondary reaction pathways. The formation of cyanamide, dimethylamine and HCN were among the main secondary products. The proposed mechanism is important for future treatment of wastewater containing metformin and similar drugs formulations, and their possible conversion to useful commodities. [ABSTRACT FROM AUTHOR]

The purpose of this work is to evaluate the efficacy of oversubscription, at the 1n, 2n, and 3n levels for n physical cores, on semi‐direct MP2 methods within NWChem when using two and three Intel nodes. Semi‐direct MP2 energy and gradient calculations were performed on chemical systems ranging from 824 to 1626 basis functions using the cc‐pVDZ basis set. Wall times for semi‐direct MP2 energies were reduced by as much as 36% using two nodes and 44% using three nodes compared to no oversubscription. Total energy consumed by the CPU and DRAM was also reduced by as much as 12% using two nodes and as much as 20% using three nodes when oversubscribing. MP2 gradient wall times improved by as much as 16% using two nodes and 18% using three nodes compared to execution at the 1n level; however, energy savings were insignificant. Intel performance‐counter data show a strong correlation between total wall time saved and less time spent in the idle state, indicating a more efficient use of the processors when oversubscribing. © 2019 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The interaction between a single atom and graphene is an example in which the density functional theory (DFT) presents serious difficulties in giving an appropriate description of the adsorbate–substrate interaction, giving also different predictions according to the chosen approximation. The present calculations sustain that the inclusion of dispersion interactions in the framework of DFT for the Al/graphene system lead to potential energy curves of different nature according to the theoretical approach employed. The adsorption of an Al atom on the graphene surface was studied using both cluster and slab models. Cluster DFT–PBE calculations show the presence of a minimum at hollow site at an Al–graphene distance of about 2.1–2.3 Å corresponding to an exothermic state. Conversely, under B3LYP the same adsorption mode is endothermic. In comparison, our MP2 reference calculations predict the formation of two minima, both of exothermic nature, separated by an important energy barrier (about 0.2–0.4 eV). The incorporation of empirical van der Walls (vdW) corrections to B3LYP changes the original behavior, giving an exothermic adsorption; furthermore, it produces a second, more external minimum. Slab calculations with PBE, and specially using the vdW-DF2 functional, predict also the formation of a minimum of very low depth at about 3.1 Å. The analysis of results obtained with cluster and slab models sustains that the bonding of the inner minima is of ionic character while that of the external ones is of dispersion character. DFT results with the PBE/B3LYP functionals show exothermic/endothermic adsorption on regular graphene, respectively. MP2 reference calculations predict the formation of two minima, both of exothermic nature. The incorporation of empirical vdW corrections to B3LYP changes the original behavior, giving an exothermic adsorption. [ABSTRACT FROM AUTHOR]

A comparative theoretical study of reactivity of methanol, n-butyl alcohol, tert-butyl alcohol, phenol.p-nitrophenol and p-methylphenol acetylation, through the analysis of the corresponding reaction intermediates was carried out. The acetylation of alcohols is one of the most frequently used transformations in organic synthesis as it provides an efficient and inexpensive means for protecting hydroxyl group in a synthetic process. The activation energies and the reaction intermediates energies, were calculated with two levels of theory DFT (B3LYP) and MP2 combined with basis set 3-21G* and 6-31G*.The calculated values were compared with literature data. The methanol presented the higher reactivity in the acetylation reaction and the DFT/6-31G* method reported the lower energy values. [ABSTRACT FROM AUTHOR]

Dithio biuret conformers were optimised and frequency calculations were performed at MP2/6-311++G(d,p) level of theory. Classification of conformers was done to five different tautomers and the most stable conformer of each tautomer was selected. Tautomer 3 having trans conformation of amino and thiocarbonyl functional group was the most stable tautomer in the gas phase. Optimisation and frequency calculations were also done in four different solvents as follows: benzene, acetonitrile, DMSO and water. Stability of tautomers were compared in these solvents to explore effect of polarity on stabilisation of tautomers. Inter-conversion of selected tautomers was investigated both in the gas phase and in four different media. Results showed that when solvent is used as an implicit media, it has negligible effect on proton transfer. Water-explicit effect was also investigated and the results exhibited high dependency of proton transfer mechanism on water molecules when they participate in breaking and formation of bonds. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • First hyperpolarizability of cis and trans forms of azochromophores are calculated. • Frequency and solvent effects on first hyperpolarizability are studied. • M06-2X provides first hyperpolarizability values most close to the MP2 ones. • First hyperpolarizability is controlled by the strength of the acceptor moiety. Abstract The effect of acceptor on static and dynamic first hyperpolarizability, β, of trans- and cis-isomers of azochromophores AAB-DCV and AAB-TCV is studied. Calculations were performed by MP2 and DFT with three density functionals: M06-2X, CAM-B3LYP and ωB97X-D in gas phase and chloroform. Dynamic first hyperpolarizabilities were calculated by TD-DFT at 1.165 and 0.65 eV. The first hyperpolarizability values for trans-isomers are approximately three times greater than those for cis-isomer; solvent effect is essential comprising 2.7 and 3.2 times growth for static values for AAB-DCV and AAB-TCV , respectively, the corresponding dynamic estimations for β tot (2ω) being slightly smaller (2.2 and 2.6 times). [ABSTRACT FROM AUTHOR]

The alanine dipeptide is a standard system to model dihedral angles in proteins. It is shown that obtaining the Ramachandran plot accurately is a hard problem because of many local minima; depending on the details of geometry optimizations, different Ramachandran plots can be obtained. To locate all energy minima, starting from geometries from MD simulations, 250,000 geometry optimizations were performed at the level of RHF/6‐31G*, followed by re‐optimizations of the located 827 minima at the level of MP2/6–311++G**, yielding 30 unique minima, most of which were not previously reported in literature. Both in vacuo and solvated structures are discussed. The minima are systematically categorized based on four backbone dihedral angles. The Gibbs energies are evaluated and the structural factors determining the relative stabilities of conformers are discussed. © 2018 Wiley Periodicals, Inc. Using molecular dynamics to obtain starting geometries for geometry optimizations, minima on the energy surface of alanine dipeptide as a function of two dihedral angles are systematically located, and their energetics are analyzed both in vacuo and in aqueous solution. It is found that two other dihedral angles are useful to label the minima. The proposed approach can be used to locate stationary points in other systems. [ABSTRACT FROM AUTHOR]

Historic debates on the true structure of C4 cluster based on the experimental results of EPR, infrared spectroscopy and Coulomb explosion, as well as theoretical calculations have prompted us to comprehensively survey the geometries of C4 clusters. A total of 37 possible geometries for neutral C4, 24 C4 anions (C4−) and 27 C4 cations (C4+) have been optimized with both MP2 and density functional theory (B3LYP) calculations and verified with subsequent frequency calculations. The results align very well with all experimental observations, including the detection of rhombic C4 structure in Coulomb explosion experiment and the cis‐bent structure, in addition to the most stable linear geometries for C4 and C4−, and the electron affinity and ionization potential of C4. Both MP2 and B3LYP geometry optimization of C4 cluster (including neutral C4, C4+ and C4−) at various shapes and spin states indicate that the most stable structures for C4 and C4− are in linear shape, whereas the most stable C4+ is the rhombic structure. Potential energy surface search verifies the linearity of the most stable C4 and C4−. [ABSTRACT FROM AUTHOR]

Abstract: Dimethyldichlorosilane is the most versatile of organosilicon monomers and is produced in large quantites. In this paper, MP2/6-311++G (3df, 2pd) basis set has been used to calculate the reaction mechanism of disproportionation for producing dimethyldichlorosilane by catalyst AlCl3/(AlCl2)-γ-Al2O3 (Z = 0-2). The activation energy (Z = 0-2) of the rate-determining step of main reaction is: 73.00, 60.26, and 66.39 kJ mol-1. By comparing the activation energy and reaction heat, it is found that the catalytic effect of AlCl3/(AlCl2)-γ-Al2O3 (Z = 1) is the best. [ABSTRACT FROM AUTHOR]

[Display omitted] • First microwave spectroscopic study of 3,4,5-trifluorobenzoic acid. • Precise determination of rotational constants for nine isotopologues. • Molecular structure determined for 3,4,5-trifluorobenzoic acid. • Good agreement between experimental and calculated spectroscopic parameters. Calculations on the structure of 3,4,5-trifluorobenzoic acid were made using the Gaussian 16 program. The potential energy surfaces were scanned along C C C O and O C O H dihedral angles at the B3LYP/6-311G level to analyze its conformational landscape. Two conformations were identified and reoptimized at the B3LYP/aug-cc-pVTZ level. The result indicates that 3,4,5-trifluorobenzoic acid prefers a planar structure in its global minimum conformation. The pure rotational spectra of 3,4,5-trifluorobenzoic acid were measured in the frequency range of 6 – 12.5 GHz using a chirped pulse Fourier transform microwave (CP-FTMW) spectrometer. The spectra of the parent, seven 13C, and one deuterium singly substituted isotopologues were analyzed and fitted to measurement accuracy for a semi-rigid asymmetric top molecule. The rotational constants and centrifugal distortion constants were accurately determined. The rotational constants for the parent isotopologue are A = 1535.31408(32) MHz, B = 650.31751(16) MHz, and C = 456.98499(12) MHz. The effective structure of its ground vibrational state was determined from the spectra of the mono-substituted isotopologues. The agreement between the calculated and experimental spectroscopic constants is excellent. [ABSTRACT FROM AUTHOR]

Quantum chemical calculations have been performed for the substitution reactions of the aluminum chlorogermylenoid H2GeClAlCl2 with HF, H2O, NH3, HCl, H2S, and PH3 to get more insights into the reactivity of H2GeClAlCl2. The theoretical calculated results indicated that the substitution reactions of H2GeClAlCl2 with HF, H2O, NH3, HCl, H2S, and PH3 proceeded in a concerted manner. There were one transition state and one intermediate which connected the reactants and the products along the potential energy surface. The six substitution reactions of H2GeClAlCl2 with HF, H2O, NH3, HCl, H2S, and PH3 are compared with the addition reaction s of H2Ge with these hydrides. And based on the calculated results we concluded that the substitution reactions of H2GeClAlCl2 with these hydrides involve two steps, one is dissociation onto H2Ge with AlCl3, and the other is the addition reactions of H2Ge with HF, H2O, NH3, HCl, H2S, and PH3.

A TrF2 group (Tr = B, Al, Ga, In, Tl) is placed on one of the α positions of naphthalene, and its ability to engage in a triel bond (TrB) with a weak (NCH) and strong (NC−) nucleophile is assessed by ab initio calculations. As a competitor, an NH2 group is placed on the neighboring Cα, from which point it forms an intramolecular TrB with the TrF2 group. The latter internal TrB reduces the intensity of the π-hole on the Tr atom, decreasing its ability to engage in a second external TrB. The intermolecular TrB is weakened by a factor of about two for the smaller Tr atoms but is less severe for the larger Tl. The external TrB can be quite strong nonetheless; it varies from a minimum of 8 kcal/mol for the weak NCH base, up to as much as 70 kcal/mol for CN−. Likewise, the appearance of an external TrB to a strong base like CN− lessens the ability of the Tr to engage in an internal TrB, to the point where such an intramolecular TrB becomes questionable.

Losartan inhibits the renin-angiotensin-aldosterone system by blocking the angiotensin II receptor. It is commonly used in cardiovascular diseases, such as hypertension. Several publications applied the ab initio and density functional theory methods to investigate the molecule of losartan. Only in one of them were the nuclear magnetic resonance spectra calculations carried out, and their results were correlated with the experimental values. The authors focused their attention on calculations of the anion form of losartan, taking into consideration both its synperiplanar and antiperiplanar configurations. Coefficients of determination and mean absolute deviation parameters were calculated for the experimental and calculated chemical shifts for every used basis set. They showed a noticeably stronger correlation for the anti-isomers than for the syn-isomers. Moreover, the solvation model increased the value of this parameter. The results of calculations confirmed that an anti-conformation of the analyte seems to be the preferred one, and such an orientation might be most potent within the receptor cavity, which is in agreement with the results of previous studies.

We have calculated global and local DFT reactivity descriptors for aminocyclopyrachlor herbicide at the MP2/6-311++G (2d,2p) level of theory in the aqueous phase. Global reactivity descriptors such as ionization energy, molecular hardness, electrophilicity, and total energies were calculated to evaluate the aminocyclopyrachlor reactivity. Local reactivity was evaluated through the Fukui function. Our results suggest that the cationic and dipolar forms of aminocyclopyrachlor exhibit similar global reactivity and they are susceptible to deamination and decarboxylation. Also, the opening of the ring might become factible through free radical attacks to the neutral form, while a similar process is caused by nucleophilic attacks on the anionic form.

Especially for larger molecules relevant to life sciences, vibrational self-consistent field (VSCF) calculations can become unmanageably demanding even when only first and second order potential coupling terms are considered. This paper investigates to what extent the grid density of the VSCF’s underlying potential energy surface can be reduced without sacrificing accuracy of the resulting wavenumbers. Including single-mode and pair contributions, a reduction to eight points per mode did not introduce a significant deviation but improved the computational efficiency by a factor of four. A mean unsigned deviation of 1.3% from the experiment could be maintained for the fifteen molecules under investigation and the approach was found to be applicable to rigid, semi-rigid and soft vibrational problems likewise. Deprotonated phosphoserine, stabilized by two intramolecular hydrogen bonds, was investigated as an exemplary application.

The molecular structure of the 2, 4, 6-Tri-Phenyl- λ 3 -Phosphinine dianion was investigated. The geometric parameters are calculated by B3LYP density functional model using the 6-31G**, 6-31+G**, 6-311G** and 6311+G** basis set, resulting in a stable structure with minimal energy. The molecular structure of titled compound admits C2 symmetry, which coincides with the bisector of the heterocycle ring. The NBO analysis was calculated using both variation principle (B3LYP) and perturbation theory (MP2). The results from natural bond orbital (NBO) analysis describe the type of hybridization for each NBO and indicate different transfers of intra-molecular charge (ICT), which stabilizes this system. Also, the NBO calculations show that σ (P-C) bonds are formed from an sp2,26, sp2.22 and sp2.09 hybrid on carbon, respectively, at B3LYP/6-31G (d, p), B3LYP/6-31+G (d, p) and MP2/6-31G levels of theory. These calculations determined that strongest electron donation occurs from a lone pair orbital on the carbon atom LP (C34) to the anti-bonding acceptors π *(C32-C33) and π *(C35-C36); the corresponding E2 measures 67.42 k.cal/mol and 115.42 k.cal/mol, respectively, at B3LYP and MP2 theoretical levels. The MP2 method overestimates the energy E2 compared to B3LYP calculation. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The [Ag(CN) 2 −] 3 trimer was found in [Cu(Dach) 2 -Ag(CN) 2 -Cu(Dach) 2 ][Ag(CN) 2 ] 3 complex. • This entity is stabilized by unsupported argentophilic interaction. • MP2 interaction energy for this trimer is equal to −6.95 kcal/mol. • Energy decomposition revealed the prominent role of electrostatics and dispersion. • Normal vibrations of the Ag⋯Ag⋯Ag interaction confirmed its weak character. Abstract Crystal structure study of a copper(II) complex of cis -1,2-diaminocyclohexane (Dach), [Cu(Dach) 2 -Ag(CN) 2 -Cu(Dach) 2 ][Ag(CN) 2 ] 3 (1) reveals the presence of unsupported Ag⋯Ag⋯Ag interactions with the interatomic Ag⋯Ag distance of 3.230 Å. Detailed HF, MP2 and BLYP-D3 theoretical investigations of [Ag(CN) 2 −] 3 trimer, together with AIM, NCI and EDA analyses verified these interactions as noncovalent Ag Ag bonds with the magnitude of a weak hydrogen bond, mainly electrostatically and dispersively driven. [ABSTRACT FROM AUTHOR]

Low-lying structures of water cationic clusters and the compounds with the OH radical have become a hot topic in recent years. We here investigate the cluster H2O10+ and calculate its ideal structures by the quantum chemical calculation together with the particle swarm optimization method. We analyzed the properties of the obtained lower-energy isomers of H2O10+. Their energies are further re-optimized and demonstrated at three different methods with two basis sets. Based on our numerical calculations, a new cage-like structure of H2O10+ with the lowest energy is obtained at MP2/aug-cc-pVDZ level. Our results showed the comparison of energy order at different conditions and demonstrated the influence of temperature on the relative Gibbs energy and IR spectra. Moreover, we also contained the molecule orbitals to discuss the stability of these representative isomers. [ABSTRACT FROM AUTHOR]

Both static and dynamic molecular quadratic nonlinear-optical characteristics of trans - and cis -isomers of azochromophore Disperse Orange, DO3, are studied by correlated MP2 technique and TD-DFT with two density functionals: meta -GGA M06-2X and range-separated ωB97X ones. The effect of solvent and radiation frequency on the first hyperpolarizability values is considered. All the tendencies obtained at the MP2 level are found to be valid at DFT level, the first hyperpolarizability values estimated with M06-2X being quite close to those calculated at MP2 level. The solvent (chloroform) effect results in 3 times growth of the static first hyperpolarizability values for trans -isomer, and ca 2.5 times – for cis -DO3. The effect of radiation frequency on the first hyperpolarizability values reveals the systematic decrease of the β tot values with the decrease of frequency. The comparison of the first hyperpolarizability values for trans - and cis -isomers demonstrates the dominating value for trans -DO3, MP2 providing systematically larger ratio β tot [ t r a n s ] / β tot [ c i s ] than DFT; the estimations obtained by TD-DFT being in a very good agreement with the values presented in literature. [ABSTRACT FROM AUTHOR]

Abstract: Density functional theory calculation on BHb←:X interaction (X= N, O, P, S, F, Cl, Br) is performed. HOMO energy predicts the feasibility of such complexation. Steric and electronic effects play significant role on geometry of the complexes. Interaction energy suggests that the interaction is moderately strong (< 5.00 kcal mol−1) in nature and the complexes are stable in both gas and solvent phase. Electron donating group on: X facilitates the interaction whereas electron withdrawing group impedes the same. MP2 and CCSD(T) calculations further confirm the suitability of ωB97X‐D and M06‐2X functional for studying such interactions. Dispersive interaction is the primary mode of interaction in stabilizing the complexes. 1H NMR and IR study are also performed. Thermochemical analysis advocates exothermic nature of complexation. [ABSTRACT FROM AUTHOR]

1-(Dimethylamino)-1-phenyl-1-silacyclohexane 1 , was synthesized, and its molecular structure and conformational properties studied by gas-phase electron diffraction (GED), low temperature 13 C NMR spectroscopy and quantum-chemical calculations. The predominance of the 1 -Ph ax conformer ( 1 -Ph eq : 1 -Ph ax ratio of 20:80%, Δ G °(317 K) = −0.87 kcal/mol) in the gas phase is close to the theoretically estimated conformational equilibrium. In solution, low temperature NMR spectroscopy showed analyzable decoalescence of C ipso and C(1,5) carbon signals in 13 C NMR spectra at 103 K. Opposite to the gas state in the freon solution employed (CD 2 Cl 2 /CHFCl 2 /CHFCl 2  = 1:1:3), which is still liquid at 100 K, the 1 -Ph eq conformer was found to be the preferred one [( 1 -Ph eq : 1 -Ph ax  = 77%: 23%, K  = 77/23 = 2.8; −Δ G ° = −R T ln K (at 103 K) = 0.44 ± 0.1 kcal/mol]. When comparing 1 with 1-phenyl-1-(X)silacylohexanes (X = H, Me, OMe, F, Cl), studied so far, the trend of predominance of the Ph ax conformer in the gas phase and of the Ph eq conformer in solution is confirmed. [ABSTRACT FROM AUTHOR]

Glycolic acid (GA) dimers were studied in low temperature argon matrices by means of FTIR spectroscopy. Experimentally, the dimers were produced when monomeric glycolic acid molecules were thermally mobilized upon annealing of argon matrices at 25–35 K. The experimental spectra observed upon annealing indicate the presence of three different dimer structures. Computationally, MP2 and DFT calculations were used to study the potential dimer species in order to scrutinize the possible dimer structures, their energetics and their spectral features. Altogether 27 local minima were found for dimer structures for the three lowest conformers of glycolic acid considered based on previous studies on glycolic monomer in argon matrices. Comparing the computational and the experimental spectra especially in the O H and C O stretching regions it was possible to assign the experimental observations to the three most stable dimer species. [ABSTRACT FROM AUTHOR]

5-Methylfurfural (5MF) was studied by vibrational (IR and Raman) spectroscopy and computational methods (DFT/B3LYP&MP2). FT-IR and FT-Raman spectra in KBr (at room temperature) were collected. The Gaussian 09 and Spartan 08 programs were used for conformational analysis and calculations of molecular structure, torsional barrier, and vibrational spectral data for the 5MF molecule. The obtained results were used in the analysis of experimental vibrational spectra of 5MF molecule. [ABSTRACT FROM AUTHOR]

Quantum chemical techniques reasonable in practical simulation of carbon-centered radical reactions were evaluated scaling up from addition and cyclization of small radicals to growing chains in homo- , cross- and cyclo- polymerization. The methods preselected via “cost – deviation from experiment” criteria were explored relative to growing molecular systems of radical cyclocopolymerization between divinyl ether and maleic anhydride: alternating cross-propagation with cyclization and side homopolymerization reactions. The following methods were found to be optimal for estimation of activation barriers and thermodynamics parameters: DFT M06-2X with 6-311+G(d) and 6-311+G (2df,p) basis sets or ONIOM M06-2X/6-311+G(d):B3LYP/6-31G(d) – for more complex systems. Spin-restricted open shell calculations by ROMP2/6-311+G (3df, 2p) have shown unfeasible increase in computational costs retaining similar accuracy. The optimal techniques facilitated new mechanistic insights into regulating the polymerization including kinetic-thermodynamic control factors for regulation of furan-/pyran-related isomerism within the chain that is essential for tailoring functionality of the derivatives. The approach builds methodology basis for research of similar macromolecular systems including alike polymeric platforms for preparations of broad biomedical applications. [ABSTRACT FROM AUTHOR]

The 129 local minima of the alanine tetrapeptide with relative energy < 10 kcal/mol were identified at the ω B97X-D/6-311++G(d,p) level of theory from initial structures generated by combining nine local minima of each residue. The CCSD(T), MP2, and dispersion-corrected DFT levels of theory with various basis sets were assessed for relative energies of the 24 representative conformations. The best performance was obtained at the double-hybrid DSD-PBEP86-D3BJ/def2-QZVP level of theory with RMSD = 0.12 kcal/mol against the CCSD(T)/CBS-limit energies. The ω B97X-D/def2-QZVP and CAM-B3LYP-D3BJ/def2-QZVP levels of theory can be an alternative level of theory with marginal deviations for conformational study of peptides. [ABSTRACT FROM AUTHOR]

B3LYP and MP2 calculations with the commonly-used 6-31+G(d) basis set predict qualitatively different structures for the Tyr-Gly conformer book1, which is the most stable conformer identified in a previous study. The structures differ mainly in the ψtyr Ramachandran angle (138° in the B3LYP structure and 120° in the MP2 structure). The causes for the discrepant structures are attributed to missing dispersion in the B3LYP calculations and large intramolecular BSSE in the MP2 calculations. The correct ψtyr value is estimated to be 130°. The MP2/6-31+G(d) profile identified an additional conformer, not present on the B3LYP surface, with a ψtyr value of 96° and a more folded structure. This minimum is, however, likely an artefact of large intramolecular BSSE values. We recommend the use of basis sets of at least quadruple-zeta quality in density functional theory (DFT), DFTaugmented with an empirical dispersion term (DFT-D) and second-order Møller-Plesset perturbation theory (MP2 ) calculations in cases where intramolecular BSSE is expected to be large. [ABSTRACT FROM AUTHOR]

New calculations of the functions for YF3 and Y2F6 in the gas phase using quantum-chemical calculations by MP2 and CCSD(T) methods are performed in connection with the ongoing work on obtaining reliable thermodynamic data of yttrium halides. The obtained values are entered in the database of the IVTANTERMO software complex. Equations approximating the temperature dependence of the reduced Gibbs energy in the Т = 298.15-6000 K range of temperatures are presented. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The equilibrium bond lengths of 41 small molecules are calculated by Gaussian09 and ADF2013 programs. We use five different basis sets: 6-31G*, cc-pVDZ, 6-311G+(2d,p), cc-pVTZ, and cc-pVQZ, for six different methods: Hartree-Fock, MP2, MP3, CCSD, CCSD(T), and B3LYP. The reliability of each level of theory on 89 bond lengths compared with CCSD(T)/cc-pVQZ is examined in terms of the mean absolute deviation. In particular, basis set dependence of the relative reliability of the two popular methods MP2 versus B3LYP is important to computational chemists. In addition, the efficient even-tempered basis set of Slater-type orbital called et-pVQZ, available in the ADF2013 program, is tested with the popular density functional B3LYP. [ABSTRACT FROM AUTHOR]

Four new organotin(IV) ferrocenyl dithiocarbamate complexes viz. [(FcCH 2 EtOHdtc) 2 SnR 2 ] (R = Me ( 1 ), n -Bu ( 2 ) and Ph ( 3 )) and [(FcCH 2 EtOHdtc)SnPh 3 ] ( 4 ) have been synthesized and characterized by electronic absorption, IR, 1 H, 13 C and 119 Sn NMR spectroscopy. The molecular structures of 1 and 2 have been confirmed by single crystal X-ray diffraction technique. The X-ray analyses for 1 and 2 reveal a skew trapezoidal bipyramid geometry around Sn(IV) which is being satisfied by the two sulfur atoms of the two dithiocarbamate ligands in anisobidentate fashion. In the crystal structure of 1 , an interesting one dimensional chain held by O H⋯O intermolecular interactions and also display weak C H⋯π and C H⋯S interactions. The crystal structure of 2 exhibit the formation of one dimensional chain held by weak O H⋯S, (Cp)C H⋯O and π⋯π interactions. Additionally 2 also display the formation of one dimensional chain because of the (Cp)C H⋯S and (Cp)C H⋯π interactions. These interactions have been addressed by ab initio and atoms-in-molecules analyses. [ABSTRACT FROM AUTHOR]

Graphene can be viewed as sheet of benzene rings fused together forming a variety of structures including the trioxotriangulenes (TOTs) which is a class of organic molecules with electro-active properties. In order to clarify such properties, structures and electronic properties of the graphene fragments phenalenyl, triangulene, 6-oxophenalenoxyl, and X 3 TOT (X = H, F, Cl) are computed. Validation of the methodologies are carried out using the density functionals B3LYP, M06-2X, B2PLYP-D, and the MP2 theory, giving equilibrium geometries of benzene, naphthalene, and anthracene with mean unsigned error (MUE) of only 0.003, 0.007, 0.004, and 0.007 Å, respectively in relation to experiment. [ABSTRACT FROM AUTHOR]

In this work a detailed quantum-chemical comparison of the relative stability of six tautomers of 1,2,5-oxadiazol-4,3-diamine studied in the gas phase and solution. Theoretical calculations are carried out by the density functional theory (DFT/B3LYP) and MP2 methods using the standard 311++G(d,p) basis set. The results indicate that A is the most stable form in the gas phase and also is the predominant tautomer in solution at the DFT and MP2 methods. The transition states of proton transfer reaction are calculated. The variation of dipole moments and charges on atoms are studied in various solvent. Specific solvent effects with addition of one water molecule near the electrophilic centers of tautomer investigated. Also the transition state of proton transfer assisted by a water molecule was investigated. [ABSTRACT FROM AUTHOR]

Relativistic Atomic Natural Orbitals (ANO-RCC) are extrapolated to the complete basis set limit. ANO-RCC-VXZP (X = D,T,Q) basis sets were extrapolated using standard extrapolation techniques. Five noncovalent complexes, characterized by hydrogen, dispersion and halogen interactions, were chosen. Accurate description of the studied complexes is allowed only after the inclusion of electron correlation and large basis sets which have to include polarization and diffuse functions. Results are in quantitative agreement with the benchmark data obtained by standard aug-cc-pVXZ-DK (X = D, T, Q) basis sets considering chemical accuracy of ±1 kcal/mol. [ABSTRACT FROM AUTHOR]

Using quantum-chemical methods, it has been found that the structure of fluorite coincides with the symmetry and coordination number of central calcium atom in the ( O )-CaF cluster. The Ca-F and F-F interatomic distances in the cluster are 3-4% shorter than in the crystal. The symmetry of the ( O )-(ScCaF) cluster mimicking the cationic defect after the β-decay of Ca does not correspond to the energy minimum. The increase in the cation charge from 1.79 to 2.80 a. u. reduces the radius of the first coordination sphere by 0.14 Å. For ytterbium dihalides, the bond lengths Yb-F 2.344, Yb-Cl 2.897 Å and the cation charges 1.81, 1.64 a. u., respectively, have been found. [ABSTRACT FROM AUTHOR]

As theoretical approaches and technical methods improve over time, the field of computer simulations for water has greatly progressed. Water potential models become much more complex when additional interactions and advanced theories are considered. Macroscopic properties of water predicted by computer simulations using water potential models are expected to be consistent with experimental outcomes. As such, discrepancies between computer simulations and experiments could be a criterion to comment on the performances of various water potential models. Notably, water can occur not only as liquid phases but also as solid and vapor phases. Therefore, the melting temperature related to the solid and liquid phase equilibrium is an effective parameter to judge the performances of different water potential models. As a mini review, our purpose is to introduce some water models developed in recent years and the melting temperatures obtained through simulations with such models. Moreover, some explanations referred to in the literature are described for the additional evaluation of the water potential models.

MP2, DFT and CCSD methods with 6-311++G** and aug-cc-pvdz basis sets have been used to probe the structural changes and relative energies of E-prop-2-ynylideneamine (I), Z-prop-2-ynylideneamine (II), prop-1,2-diene-1-imine (III) and vinyl cyanide (IV). The energy near-equivalence and provenance of preference of isomers and tautomers were investigated by NBO calculations using HF and B3LYP methods with 6-311++G** and aug-cc-pvdz basis sets. All substrates have Cs symmetry. The optimized geometries were found to be mainly theoretical method dependent. All elected levels of theory have computed I/II total energy of isomerization (ΔE) of 1.707 to 3.707 kJ/mol in favour of II at 298.15 K. MP2 and CCSD methods have indicated clearly the preference of II over III; while the B3LYP functional predicted nearly similar total energies. All tested levels of theory yielded a global II/IV tautomerization total energy (ΔE) of 137.3–148.4 kJ/mol in support of IV at 298.15 K. The negative values of ΔS indicated that IV is favoured at low temperature. At high temperature, a reverse tautomerization becomes spontaneous and II is preferred. The existence of II in space was debated through the interpretation and analysis of the thermodynamic and kinetic studies of this tautomerization reaction and the presence of similar compounds in the Interstellar Medium (ISM).

When an N-base approaches the tetrel atom of TF4 (T = Si, Ge, Sn) the latter molecule deforms from a tetrahedral structure in the monomer to a trigonal bipyramid. The base can situate itself at either an axial or equatorial position, leading to two different equilibrium geometries. The interaction energies are considerably larger for the equatorial structures, up around 50 kcal/mol, which also have a shorter R(T··N) separation. On the other hand, the energy needed to deform the tetrahedral monomer into the equatorial structure is much higher than the equivalent deformation energy in the axial dimer. When these two opposite trends are combined, it is the axial geometry which is somewhat more stable than the equatorial, yielding binding energies in the 8⁻34 kcal/mol range. There is a clear trend of increasing interaction energy as the tetrel atom grows larger: Si < Ge < Sn, a pattern which is accentuated for the binding energies.

A series of silyl and germanium complexes containing halogen atoms (fluorine and chlorine atoms) and exhibiting tetrel bonds with Lewis bases were analyzed by means of Møller-Plesset computational theory. Binding energies of germanium derivatives were more negative than silicon ones. Amongst the different Lewis bases utilized, ammonia produced the strongest tetrel bonded complexes in both Ge and Si cases, and substitution of the F atom by Cl led to stronger complexes with an ethylene backbone. However, with phenyl backbones, the fluorosilyl complexes were shown to be less stable than the chlorosilyl ones, but the opposite occurred for halogermanium complexes. In all the cases studied, the presence of a hydroxyl group enhanced the tetrel bond. That effect becomes more remarkable when an intramolecular hydrogen bond between the halogen and the hydrogen atom of the hydroxyl group takes places.