Ab initio and DFT studies on nitrosoguanidine tautomers

Isolated nitrosoguanidine tautomers have been subjected to 6-31G(d,p), 6-31G(d,p) /(MP2), B3LYP/6-311G(d,p) and B3LYP/6-311++G(d,p) type quantum chemical analyses in the gas phase. The geometrical features and energetics of some conformers of the tautomers are reported. The nitrosimine form has the highest stability than the others and the diazoic acid form is the least stable one. The nitrosoimine form has the highest HOMO and LUMO energies. Whereas, the nitrosamine form possesses the lowest HOMO and the diazoic acid form has the lowest LUMO energies.     

Quantum chemical treatment of nivalenol and its tautomers

Nivalenol, a highly poisonous mycotoxin, and its possible tautomers have been considered theoretically by RHF/6-31G/d,p) and B3LYP/6-31G(d,p) calculations together with a semi-empirical PM3 method. The calculations revealed that some of the tautomers are more stable and exothermic than nivalenol. The calculated IR spectra as well as some geometrical and physicochemical properties of the structures considered have been presented.     

Ab initio and DFT study on 1,4-dinitroglycoluril configurational isomers: cis-DINGU and trans-DINGU

cis-1,4-Dinitroglycoluril (1,4-dinitrotetrahydroimidazo[4,5-d]imidazole-2,5-(1H,3H)-dione, C(4)H(4)N(6)O(6), known as DINGU in the literature) is an important explosive regarded as one of the potential ingredients for LOVA (low vulnerability ammunition) applications. However, there is no study concerning trans-DINGU, to the best of our knowledge. Thus, in the present study the structural and electronic properties of the configurational isomers of DINGU (cis- and trans-isomers) have been investigated by performing density functional theory (DFT) calculations at B3LYP/6-31G(d,p), UB3LYP/6-31G(d,p) levels and also ab initio calculations at RHF/6-31G(d,p) and UHF/6-31G(d,p) levels. The optimized geometries, vibrational frequencies, electronic structures, and some thermodynamical values for the presently considered DINGU isomers have been obtained in their ground states. Comparing the calculated energy data (total electronic energy, heat of formation values and homolytic bond dissociation values of N-NO(2)) and the calculated bond lengths, cis-DINGU was found to be a more stable molecule than the trans-isomer.     

DFT study of the interaction between 3-nitro-1,2,4-triazole-5-one and hydrogen fluoride

Three fully optimized geometries of 3-nitro-1,2,4-triazol-5-one-hydrogen fluoride (NTO-HF) complexes have been obtained with density functional theory (DFT) method at the B3LYP/6-311++G** level. The intermolecular interaction energy is calculated with zero point energy (ZPE) correction and basis set superposition error (BSSE) correction. The greatest corrected intermolecular interaction of the NTO-HF complexes is -34.155kJ/mol. Electrons in complex systems transfer from NTO to HF. Natural bond orbital (NBO) analysis is performed to reveal the origin of the interaction. The strong hydrogen bonds contribute to the interaction energies dominantly. Frequency calculations are carried out on each optimized structure, and their IR spectra are discussed. Vibrational analysis show that there are large red-shifts for H-X (X=N and F) stretching vibrational frequencies in the NTO and hydrogen fluoride complexes. The changes of thermodynamic properties from the monomer to complexes with the temperature ranging from 200K to 1500K have been obtained using the statistical thermodynamic method. It is found that two of three NTO-HF complexes can be produced spontaneously from NTO and HF at room temperature.     

Ab initio molecular orbital calculations of the NMR chemical shieldings for mannose and mannobiose

The ¹³C and ¹H NMR shielding constants for -methylmannose and -methylmannobiose have been calculated using the ab initio gauge-including atomic orbital (GIAO) method to study the conformational dependencies of the NMR chemical shifts of the sugars. The molecular structures were fully optimized using B3LYP/6-31G^* and the NMR shielding constants were calculated at both Hartree–Fock (HF) and density functional (DF) levels of theory with various kinds of basis sets. The values determined using the B3LYP hybrid functional were a little closer to those obtained experimentally than those determined at the HF level. Both, HF and B3LYP with the 6-311+G(2d,p) basis were found to give a very good correlation between the experimental and calculated shielding constants, especially for ¹³C.     

Correlation between the bond dissociation energies and impact sensitivities in nitramine and polynitro benzoate molecules with polynitro alkyl groupings

The geometries of nine CHNO nitro-contained explosive molecules were fully optimized employing the B3LYP method of density functional theory with the 6-31G* basis set. The bond dissociation energy (BDE) for removal of the NO(2) group in nitroamine molecules with nitro alkyl, and benzoate with nitro alkyl were calculated at the same level. The calculational results of B3LYP/6-31G* and B3P86/6-311G* levels showed that the strength of C-NO(2) is weaker than that of N-NO(2) bond in nitroamine molecules with nitro alkyl. The weakest bond is the C-NO(2) in these computed molecules. The relationship between the impact sensitivities and the weakest C-NO(2) bond dissociation energy values was examined. The results indicate a nearly linear correlation between the impact sensitivity and the ratio of the BDE value to the molecular total energy.     

Combined Computational and Experimental Study on the Adsorption and Inhibition Effects of N2O2 Schiff Base on the Corrosion of API 5L Grade B Steel in 1 mol/L HCI

The electrochemical behavior of low carbon steel （API 5L grade B） in 1 mol/L HCI solution with different concentrations of N,Nr-bis（4-formylphenol）-trimethylenediamine Schiff base was studied by electrochemical techniques and density functional theory analysis. The inhibition efficiency was found to increase with increasing inhibitor concentration and decreased with increasing temperature. The high inhibition efficiency was attributed to the blocking of active sites by adsorption of inhibitor molecules on the steel surface. Thermodynamic parameters for the adsorption and activation processes were determined. Galvanostatic polarization data indicated that Schiff base act as a mixed-type inhibitor and the adsorption isotherm obeyed the Langmuir adsorption isotherm. Results obtained from quantum chemical studies show excellent correlations between the quantum chemical parameters and the experimental inhibition efficiencies using density functional theory at the B3LYP/6-31G（d,p） and B3LYP/3-21G basis set levels and ab initio calculations using HF/6-31G（d,p） and HF/3-21G methods.     

Quantum chemical study on nitroimidazole, polynitroimidazole and their methyl derivatives

The insensitive explosive candidates, nitroimidazoles, polynitroimidazoles and their methyl derivatives, are investigated using density functional theory (DFT). The homolytic bond dissociation energies (BDEs) corresponding to -NO2 group removal from carbon or nitrogen site on imidazole ring were calculated at B3P86/6-311G** level, and the weakest bond has been determined. Further, a correlation is developed between impact sensitivity h50 and the ratio (BDE/E) of the weakest bond BDE to the total energy E, and we extrapolate this relationship to predict the impact sensitivities for compounds where experiments are not available. It is found that most of the title compounds are insensitive towards impact stimuli with their h50 larger than 60.0cm. Heats of formation (HOFs) for the 21 title compounds at 298K in gas are also determined both at B3LYP/6-311G** and B3P86/6-311G** levels using isodesmic work reactions. The calculated BDEs and HOFs consistently indicate that C-nitro-substituted imdazole is more stable than the corresponding N-substituted one, and the introduction of methyl on C increases the stability whereas the methyl attached to N atom decreases the stability.     

A theoretical study on vomitoxin and its tautomers

In the present work, the structural and electronic properties of vomitoxin (deoxynivalenol, a mycotoxin) and all of its possible tautomers have been investigated by the application of B3LYP/6-31G(d,p) type quantum chemical calculations. According to the results of the calculations, tautomer V(4) has been found to be the most stable one among all the structures both in the gas and aqueous phases. The calculations also indicated that, vomitoxin and V(2) possess the deepest and the highest lying HOMO levels, respectively. Hence, V(2) is to be more susceptible to oxidations than the others. On the other hand, V(5)(S) and vomitoxin have the lowest and the next lowest LUMO energies, respectively. Whereas, V(1) and V(2) possess quite highly lying (within the group) LUMO energy levels which result in comparatively unfavorable reduction potentials. Some important geometrical and physicochemical properties and the calculated IR spectra of the systems have also been reported in the study.     

Quantum chemical computational studies on bis-thiourea zinc acetate

In this study, quantum chemical calculations of vibrational spectra, Raman spectra, electronic properties (total energy, dipole moment, electronegativity, chemical hardness and softness), Mulliken atomic charges and thermodynamic parameters of bis-thiourea zinc acetate (BTZA) have been performed using Gaussian 09 program. Additionally, nonlinear optical (NLO), conformational, natural bond orbital (NBO) analyses of BTZA have been carried out using the same program. The structural and spectroscopic data of the molecule in the ground state have been calculated using Hartree-Fock (HF) and density functional method (DFT/B3LYP) with the 6-311++G(d,p) basis set. In addition, the molecular frontier orbital energies (HOMO, HOMO-1, LUMO and LUMO+1) of the title compound have been calculated at the HF and B3LYP levels. The calculated HOMO and LUMO energies show that charge transfer occurs within the molecule. Finally, the calculated results were applied to simulate infrared and Raman spectra of the title compound which showed good agreement with the experimental ones.     

B3LYP/6-31G*//SAM1 Calculations of C36 Fullerene and quasi-Fullerene Cages

The recently isolated fullerene C₃₆ is computed by the SAM1 method with energetics refined at the B3LYP/6-31G* level. Twelve low-energy cages are considered in detail, exhibiting both fullerene (pentagons and hexagons) and quasi-fullerene (also squares and heptagons) pattern. In contrast to other fullerene systems, the SAM1 and B3LYP/6-31G* methods do produce somewhat different energetics. Consequently, the equilibrium isomeric composition at high temperatures is different, too. In the best affordable B3LYP/6-31G* approach the fullerene cage with the topological D_6d symmetry represents the most populated species at high temperatures. Hence, an agreement with the solid-state NMR experiment is achieved at the B3LYP/6-31G* theoretical level.     

Eu@C86 isomers: Calculated relative populations

Abstract

Relative populations of four energy-lowest IPR (isolated-pentagon-rule) isomers of Eu@C₈₆ are computed using the Gibbs energy based on characteristics from density functional theory calculations (M06-2X/3-21G?～?SDD entropy term, M06-2X/6-31G*～SDD or B2PLYP(D)/6-31G*～SDD energetics). The calculations confirm that the recently isolated Eu@C₁(7)-C₈₆ species is a major isomer in a relevant temperature region. Relationship to the empty C₈₆ cages is discussed, too.     

Crystal density predictions for nitramines based on quantum chemistry

An efficient and convenient method for predicting the crystalline densities of energetic materials was established based on the quantum chemical computations. Density functional theory (DFT) with four different basis sets (6-31G(**), 6-311G(**), 6-31+G(**), and 6-311++G(**)) and various semiempirical molecular orbital (MO) methods have been employed to predict the molecular volumes and densities of a series of energetic nitramines including acyclic, monocyclic, and polycyclic/cage molecules. The relationships between the calculated values and experimental data were discussed in detail, and linear correlations were suggested and compared at different levels. The calculation shows that if the selected basis set is larger, it will expend more CPU (central processing unit) time, larger molecular volume and smaller density will be obtained. And the densities predicted by the semiempirical MO methods are all systematically larger than the experimental data. In comparison with other methods, B3LYP/6-31G(**) is most accurate and economical to predict the solid-state densities of energetic nitramines. This may be instructive to the molecular designing and screening novel HEDMs.     

Ideal Gas Thermodynamic Properties of Propyl tert-Butyl Ethers from Density Functional Theory Results Combined with Experimental Data

Ideal gas thermodynamic properties, S°(T), C _p°(, T), H°(T)–H°(0), _f H°(T), and _f G°(T), are obtained on the basis of density functional B3LYP/6-31G(d,p) and B3LYP/6-311 + G(3df,2p) calculations for two propyl tert-butyl ethers. All torsional motions about C–C and C–O bonds were treated as hindered internal rotations using the independent-rotor model. An empirical approximation was assumed to account for the effect of the coupling of rotor potentials. The correction for rotor–rotor coupling was found by fitting to entropy values determined from calorimetric measurements. Enthalpies of formation were calculated using isodesmic reactions.     

Systematic search for energetically favored isomers of large fullerenes C122–C130 and C162–C180

Based on the methodology by Cioslowski et al. [J. Cioslowski, N. Rao, D. Moncrieff, J. Am. Chem. Soc. 122 (2000) 8265–8270], two empirical fit equations to predict the standard enthalpy of formation are obtained over large number of calculation results at B3LYP/6-31G* theory level for fullerene isomers, which can be used as a preliminary and second-level screening tool, respectively, for large fullerenes. By applying these equations in screening the whole isolated pentagon rule (IPR) isomers, the energetically favored isomers of large fullerenes C₁₂₂–C₁₃₀ and C₁₆₂–C₁₈₀ were predicted at the B3LYP/6-31G* density functional theory level for the first time. Our results show that the lowest energy isomers of C₁₇₄ (2473259: C_3v) and C₁₈₀ (4071832: I_h) possess much lower relative energy and larger HOMO–LUMO gaps. Moreover, the ionization energy and electron affinity of the lowest energy isomers were also investigated.     

Theoretical Studies on Endofullerenes X@Cn (X=He, Ne, Ar; n=20, 24, 30, 32, 40, 50, 60)

The equilibrium geometries, electronic structures, and binding energies between the noble gases X (X = He, Ne, Ar) and C_n (n = 20, 24, 30, 32, 40, 50, 60) in X@C_n were calculated at B3LYP level of theory using 6-31G* and 6-311G* basis sets. We expect that the introduction of He, Ne, and Ar into C_n creates molecules with a high energy content. The relative stabilities of C_n fullerenes and corresponding endohedrals X@C_n are discussed.     

B3LYP/6-31G* Computations of C60F36(g) Isomers

Three isomers of C₆₀F₃₆ have been known from the experiment. Previous computations, however, suggested five low-energy isomers though different methods do not always agree on the separation energetics. The present B3LYP/6-31G* computations support the observation of just three species. However, the computed energetics must still be adjusted in order to reproduce the observed concentration ratios.     

Vibrational and surface-enhanced Raman spectra of 1,6-diphenyl-1,3,5-hexatriene

The vibrational spectra and surface-enhanced Raman scattering (SERS) of 1,6-diphenyl-1,3,5-hexatriene (DPH) are discussed. The fundamental vibrational frequencies, overtones, and combinations observed in the infrared and Raman spectra of DPH are reported. The interpretation of the observed vibrational spectra was supported by a complete geometry optimization, followed by vibrational frequency and intensity computations for the cis- and trans- isomers of the DPH using density functional theory at the B3LYP/6-31G(d,p) level of theory. Because the molecule is photo-chemically active on Ag metal surfaces, the best SERS results for silver islands were obtained at low temperature and low energy density of the exciting laser line. DPH SERS on Au films was obtained at room temperature.     

B3LYP/6-31G∗//SAM1 Calculations of C36 Fullerene and quasi-Fullerene Cages

Abstract

The recently isolated fullerene C₃₆ is computed by the SAM1 method with energetics refined at the B3LYP/6-31G? level. Twelve low-energy cages are considered in detail, exhibiting both fullerene (pentagons and hexagons) and quasi-fullerene (also squares and heptagons) pattern. In contrast to other fullerene systems, the SAM1 and B3LYP/6-31G? methods do produce somewhat different energetics. Consequently, the equilibrium isomeric composition at high temperatures is different, too. In the best affordable B3LYP/6-31G? approach the fullerene cage with the topological D _6d symmetry represents the most populated species at high temperatures. Hence, an agreement with the solid-state NMR experiment is achieved at the B3LYP/6-31G? theoretical level.     

Cyclic and Linear Structures of C13: A Computational Study

Recently, a linear structure of C₁₃ was observed and its infrared spectrum recorded. Computations of the linear and cyclic structures of C₁₃ are thus presented. The computations are performed at ab initio level using the standard 6-31G* basis set. Electron correlation is treated by density functional theory (Becke3LYP) and perturbation treatment (MP2=FC). With the density-functional approach the standard 6-311G* and 6-311G(2d,p) basis sets are applied, too. In all the treatments the cyclic structure (C_2v symmetry) is located about 90 kJ/mol below the linear form (D_∞h symmetry), while a tricyclic structure is considerably higher in energy (about 280 kJ/mol above the monocyclic species). Even at high temperatures the linear form represents less than 10% of the equilibrium isomeric mixture. Computed IR spectra are also reported. The stability evaluation corresponds to the findings of ion chromatography.