Theoretical studies on heats of formation for cubylnitrates using density functional theory B3LYP method and semiempirical MO methods

The heats of formation (HOF) have been calculated for all the 21 cubylnitrate compounds using the semiemprical molecular orbital (MO) methods (MINDO/3, MNDO, AM1, and PM3) and for 8 of 21 cubylnitrates containing 1–4 ? ONO₂ groups using the density functional theory (DFT) method at the B3LYP/6‐31G* level by means of designed isodesmic reactions. The cubane cage skeletons in cubylnitrate molecules have been kept in setting up isodesmic reactions to produce more accurate and reliable results. It is found that there are good linear relationships between the HOFs of the 8 cubylnitrates calculated using B3LYP/6‐31G* and two semiempirical MO (PM3 and AM1) methods, and the linear correlation coefficients of PM3 and AM1 methods are 0.9901 and 0.9826, respectively. Subsequently, the accurate HOFs at B3LYP/6‐31G* level of other 13 cubylnitrates containing 4–8 ? ONO₂ groups are obtained by systematically correcting their PM3‐calculated HOFs. Compared with noncaged nitrates, all the 21 cubylnitrates have high heats of formation implying that they may be very powerful energetic materials and have highly exploitable value. The relationship between the HOFs and the molecular structures of cubylnitrates has been discussed. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Theoretical calculation of bond dissociation energies and heats of formation for nitromethane and polynitromethanes with density functional theory

The C? NO₂ bond dissociation energies (BDEs) and the heats of formation (HOFs) of nitromethane and polynitromethanes (dinitromethane, trinitromethane, and tetranitromethane) system in gas phase at 298.15 K were calculated theoretically. Density functional theory (DFT) B3LYP, B3P86, B3PW91, and PBE0 methods in combination with different basis sets were employed. It was found that the C? NO₂ bond BDEs can be improved from B3LYP to B3PW91 to B3P86 or PBE0 functional. Levels of theory employing B3P86 and PBE0 functionals were found to be sufficiently reliable without the presence of diffusion functions. As the number of NO₂ groups on the same C atom increases, the PBE0 functional performs better than the B3P86 functional. Regarding the calculated HOFs, all four functionals can yield satisfactory results with deviations of <2 kcal mol^?1 from experimental ones for CH₂(NO₂)₂ and CH(NO₂)₃, when the diffusion functions are not augmented. For the C(NO₂)₄ molecule, the large basis sets augmented with polarization functions and diffusion functions are required to yield a good result. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Computational study of structures and proton transfer in hydrogen‐bonded ammonia complexes using semiempirical valence‐bond approach

In this study, the seGVB method was implemented for the N H bonding system, specifically for hydrogen‐bonded ammonia complexes, and the model well reproduces the MP2 geometries and energetics. A comparison between the ammonia dimer and water dimer is given from the viewpoint of valance‐bond structures in terms of the calculated bond energies and pair–pair interactions. The linear hydrogen bond is found to be stronger than the bent bonds in both cases, with the difference in energy between the linear and cyclic structures being comparable in both cases although the NH bonds are generally weaker. The energy decomposition clearly demonstrates that the changes in electronic energy are quite different in the two cases due to the presence of an additional lone pair on the water molecule, and it is this effect which leads to the net stabilization of the cyclic structure for the ammonia dimer. Proton‐transfer profiles for hydrogen‐bonded ammonia complexes [NH₂ H NH₂]⁻ and [NH₃ H NH₃]⁺ were calculated. The barrier for proton transfer in [NH₃ H NH₃]⁺ is larger than that in [NH₂ H NH₂]⁻, but smaller than that in the protonated water dimer. The different bonding structures substantially affect the barrier to proton transfer, even though they are isoelectronic systems. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 73: 357–367, 1999     

Theoretical studies on heats of formation,detonation properties,and bond dissociation energies of monofurazan derivatives

The heats of formation (HOFs) for a series of monofurazan derivatives were calculated by using density functional theory. It is found that the ? CN or ? N₃ group plays a very important role in increasing the HOF values of the furazan derivatives. The detonation velocities and detonation pressures of the furazan derivatives are evaluated at two different levels. The results show that the ? NF₂ group is very helpful for enhancing the detonation performance for the furazan derivatives, but the case is quite the contrary for the ? CH₃ group. An analysis of the bond dissociation energies and bond orders for the weakest bonds indicate that the substitutions of ? CN group are favorable and enhances the thermal stability of the furazan derivatives, but the ? NO₂ groups produce opposite effects. These results provide basic information for the molecular design of novel high‐energy density materials. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Accurate prediction of the enthalpies of formation for xanthophylls

This study investigates the applications of computational approaches in the prediction of enthalpies of formation (ΔH(f)) for C-, H-, and O-containing compounds. Molecular mechanics (MM4) molecular mechanics method, density functional theory (DFT) combined with the atomic equivalent (AE) and group equivalent (GE) schemes, and DFT-based correlation corrected atomization (CCAZ) were used. We emphasized on the application to xanthophylls, C-, H-, and O-containing carotenoids which consist of ～ 100 atoms and extended π-delocaization systems. Within the training set, MM4 predictions are more accurate than those obtained using AE and GE; however a systematic underestimation was observed in the extended systems. ΔH(f) for the training set molecules predicted by CCAZ combined with DFT are in very good agreement with the G3 results. The average absolute deviations (AADs) of CCAZ combined with B3LYP and MPWB1K are 0.38 and 0.53 kcal/mol compared with the G3 data, and are 0.74 and 0.69 kcal/mol compared with the available experimental data, respectively. Consistency of the CCAZ approach for the selected xanthophylls is revealed by the AAD of 2.68 kcal/mol between B3LYP-CCAZ and MPWB1K-CCAZ.     

C-C bond formation through olefin-thiocarbyne coupling in diiron complexes

The bridging diiron thiocarbyne complex [Fe₂{μ-CS(Me)}(μ-CO)(CO)₂(Cp)₂][SO₃CF₃] (1) reacts with activated olefins (methyl acrylate, acrylonitrile, styrene, diethyl maleate), in the presence of Me₃NO and NaH, to give the corresponding μ-allylidene complexes [Fe₂{μ-η¹:η³-C_α(SMe)C_β(R′)C_γ(H)(R″)} (μ-CO)(CO)(Cp)₂] (R″ = CO₂Me, R′ = H, 3a; R″ = CN, R′ = H, 3b; R″ = C₆H₅, R′ = H, 3c; R″ = R′ = CO₂Et, 3d). The coupling reaction of olefin with thiocarbyne is regio- and stereospecific, leading to the formation of only one isomer. C-C bond formation occurs between the less substituted alkene carbon and the thiocarbyne. Moreover, olefinic hydrogens of the bridging ligands are mutually trans.The reactions of 3a-b with MeSO₃CF₃ result, selectively, in the formation of the cationic μ-sulphonium allylidene complexes [Fe₂{μ-η¹:η³-C_α(SMe₂)C_β (H)C_γ(H)(R)}(μ-CO)(CO)(Cp)₂][SO₃CF₃] (R = CO₂Me, 4a; R = CN, 4b). Compound 4a undergoes displacement of the SMe₂ group by nucleophiles such as NaBH₄, NBu₄CN and NaOMe, affording the complexes [Fe₂{μ-η¹:η³-C_α(R)C_β (H)C_γ(H)(CO₂Me)}(μ-CO)(CO)(Cp)₂] (R = H, 5a; R = CN, 5b; R = OMe, 5c), respectively. The molecular structures of 3a and 5a have been determined by X-ray diffraction studies.     

Systematic errors in DFT calculations of haloalkane heats of formation

The B3LYP and B3PW91 density functionals were employed with a large [BS1 = 6-311+G(3df,2p)] and small [BS2 = 6-311G(d,p)] basis set to compute enthalpies of formation (at optimized MP2/6-31G(d) geometries and with scaled HF/6-31G(d) frequencies) in the following series of haloalkanes: (1) the 15 fluoro-, chloro-, and chlorofluoromethanes, (2) the 18 fluorinated and chlorinated ethanes. Similar to earlier higher level calculations on haloalkanes, the computed enthalpies exhibited very large, systematic deviations from experiment. It was found that these errors could be largely eliminated using a very simple Bond Additivity Correction (BAC) formula, Delta(f)H degrees (BAC) = Delta(f)H degrees (calc) - n(CX). Delta(CX) [X = F, Cl], in which the BAC parameters, Delta(CF) and Delta(CCl) were determined by fitting the equation to experimental data on the four fluoroethanes and chloroethanes, respectively. The resultant BAC corrected enthalpies of formation are in excellent agreement with experiment, with RMS deviations in the same range as quoted RMS errors in measured enthalpies. Therefore, this simple BAC procedure may be utilized to provide reliable semiquantitative estimates of enthalpies of formation in larger haloalkanes, for which higher level ab initio calculations are not feasible.     

Theoretical prediction on heats of formation for polyisocyanocubanes

The heats of formation (HOF) for all the 21 polyisocyanocubanes are calculated systematically with density functional theory (DFT) B3LYP and semiempirical MO(MINDO/3, MNDO, AM1 and PM3) methods. First, the accurate HOFs for the 8 title compounds are obtained by means of designed isodesmic reactions at DFT-B3LYP/6-31G* level, and the cubane cage skeleton has not been broken (i.e. choosing cubane as a reference compound) to produce more accurate and reliable results. It is found that there are good linear relationships between the HOFs calculated using the B3LYP/6-31G* and four semiempirical MO methods, respectively, and all of the linear correlation coefficients are more than 0.9971. The HOFs obtained from PM3 calculation are the best among the four semiempirical MO methods. Then, the accurate HOFs at B3LYP/6-31G* level of other 13 polyisocyanocubanes are obtained by systematically correcting their PM3-calculated HOFs. Polyisocyanocubanes have very high HOFs, and the HOFs increase linearly with the increasing of the number of isocyano groups in a molecule. The results show that polyisocyanocubanes are the new generation explosives with highly potential and exploitable value.     

C-N bond formation followed by N-Cl bond breaking. One more and unexpected case of the formation of a hydroxamic group via heterolytic bond cleavage

An unexpected and previously unknown reaction sequence in the interactions of the acyl halides with nitrosobenzenes, which involves carbon-nitrogen bond formation followed by heterolytic nitrogen-chlorine bond cleavage giving the corresponding unsubstituted N-phenylalkylhydroxamic acids (or N-phenylarylhydroxamic acids) and chlorine as the products has been observed. The kinetic and other evidence obtained suggest that the carbon-nitrogen bond formation is the consequence of a nucleophilic interaction of an N-phenylchlorohydroxylamine intermediate, formed in the first reaction step, with the acyl halide in the second step of the complex sequence, which leads to an N-acyl-N-chlorophenylhydroxylamine cation intermediate. The key reaction step involves the interaction of an N-acyl-N-chlorophenylhydroxylamine cation intermediate with chloride ion, which leads to the N-Cl heterolytic bond cleavage and the final formation of the hydroxamic group and a molecule of chlorine.     

Substituent effects on heats of formation, group interactions, and detonation properties of polyazidocubanes

We have calculated the heats of formation (HOFs) for a series of polyazidocubanes by using the density functional theory (DFT), Hartree-Fock, and MP2 methods with 6-31G* basis set as well as semiempirical methods. The cubane skeleton was chosen for a reference compound, that is, the cubane skeleton was not broken in the process of designing isodesmic reactions. There exists group additivity for the HOF with respect to the azido group. The semiempirical AM1 method also produced reliable results for the HOFs of the title compounds, but the semiempirical MINDO3 did not. The relationship between HOFs and molecular structures was discussed. It was found that the HOF increases 330-360 kJ/mol for each additional number of the azido group being added to the cubane skeleton. The distance between azido groups slightly influences the values of HOFs. The interacting energies of neighbor azido groups in polyazidocubanes are in the range of 2.3 approximately 6.6 kJ/mol, which are so small and less related to the substituent numbers. The average interaction energy between nearest neighbor --N3 groups in the most stable conformer of octaazidocubane is 2.29 kJ/mol at the B3LYP/6-31G* level. The relative stability related to the number of azido groups of the title compounds was assessed based on the calculated HOFs, the energy gaps between the frontier orbitals, and the bond orders of the C--N3 and C--C bonds. The predicted detonation velocity of hepta- and octa-derivatives is over 9 km/s, and the detonation pressure of them is ca. 40 GPa or over.     

Surface energies of hematite faces and heats of oxygen adsorption: calculations by modified semiempirical interacting bonds method

Modified semiempirical Interacting Bonds Method in the slab approximation with due regard for relaxation after free surface formation was used to calculate surface energies and heats of oxygen adsorption for various faces of hematite. The results obtained explained faceting of some faces after high-temperature annealing and formed bases for analysis of the structure sensitivity of CO oxidation reaction catalyzed by oxides with corundum-type structures.     

Extension of the PDDG/PM3 and PDDG/MNDO semiempirical molecular orbital methods to the halogens

The new semiempirical methods, PDDG/PM3 and PDDG/MNDO, have been parameterized for halogens. For comparison, the original MNDO and PM3 were also reoptimized for the halogens using the same training set; these modified methods are referred to as MNDO' and PM3'. For 442 halogen-containing molecules, the smallest mean absolute error (MAE) in heats of formation is obtained with PDDG/PM3 (5.6 kcal/mol), followed by PM3' (6.1 kcal/mol), PDDG/MNDO (6.6 kcal/mol), PM3 (8.1 kcal/mol), MNDO' (8.5 kcal/mol), AM1 (11.1 kcal/mol), and MNDO (14.0 kcal/mol). For normal-valent halogen-containing molecules, the PDDG methods also provide improved heats of formation over MNDO/d. Hypervalent compounds were not included in the training set and improvements over the standard NDDO methods with sp basis sets were not obtained. For small haloalkanes, the PDDG methods yield more accurate heats of formation than are obtained from density functional theory (DFT) with the B3LYP and B3PW91 functionals using large basis sets. PDDG/PM3 and PM3' also give improved binding energies over the standard NDDO methods for complexes involving halide anions, and they are competitive with B3LYP/6-311++G(d,p) results including thermal corrections. Among the semiempirical methods studied, PDDG/PM3 also generates the best agreement with high-level ab initio G2 and CCSD(T) intrinsic activation energies for S(N)2 reactions involving methyl halides and halide anions. Finally, the MAEs in ionization potentials, dipole moments, and molecular geometries show that the parameter sets for the PDDG and reoptimized NDDO methods reduce the MAEs in heats of formation without compromising the other important QM observables.     

Description of halogen bonding in semiempirical quantum-mechanical and self-consistent charge density-functional tight-binding methods

This article analyzes the ability of semiempirical quantum-mechanical methods (PM6 and PM7) and self-consistent charge density-functional tight-binding (SCC-DFTB) method DFTB3 to describe halogen bonds. Calculations of the electrostatic potential on the surface of molecules containing halogens show that the σ-hole could be described well in modified neglect of diatomic overlap-based methods. The situation is more complex in the case of DFTB3 where a simpler model is used for the electrostatics, but short-ranged effects are covered in the Hamiltonian. All these methods can thus capture the effects that, for example, define the geometry of halogen bonds. The interaction energies are, however, affected by generally underestimated repulsion, which has been addressed earlier by standalone empirical corrections. Another approach to correcting this issue in DFTB3 is presented here—a modification of the energies of d-orbitals on halogens yields better results than the empirical correction in DFTB3-D3X, although it remains difficult to describe halogen and hydrogen bonds simultaneously. © 2019 Wiley Periodicals, Inc.     

The influence of hydrogen bond formation on the dual luminescence of n-phenyl-2,3-naphthalimide derivatives

Summary The influence of hydrogen bond formation on the spectroscopic properties of dual luminescent naphthalimides was examined in n-hexane using fluorinated alcohols as hydrogen bond donors. Complex formation causes considerable red shift of the dominant intramolecular charge transfer (ICT) fluorescence maxima.     

Radical-mediated intramolecular C-C bond formation and the deoxygenation of alcohols under solvent-free conditions with tributyl methyl ammonium hypophosphite

A green, solvent-free protocol was developed for the radical-mediated intramolecular cyclization of haloacetals and the deoxygenation of S-methyl dithiocarbonates and cyclic thionocarbonate. This process uses tributyl methyl ammonium hypophosphite as a H-donor in the presence of triethylborane or t-butyl peroxide. This methodology provides eco-friendly reaction conditions.     

Theoretical prediction on heats of formation for polyisocyanocubanes——Looking for typical high energetic density material(HEDM)

The heats of formation (HOP) for all the 21 polyisocyanocubanes are calculated systematically with density functional theory (DFT) B3LYP and semiempirical MO(MINDO/3, MNDO, AM1 and PM3) methods. First, the accurate HOFs for the 8 title compounds are obtained by means of designed isodesmic reactions at DFT-B3LYP/6-31G* level, and the cubane cage skeleton has not been broken (i.e. choosing cubane as a reference compound) to produce more accurate and reliable results. It is found that there are good linear relationships between the HOFs calculated using the B3LYP/6-31G* and four semiempirical MO methods, respectively, and all of the linear correlation coefficients are more than 0.9971. The HOFs obtained from PM3 calculation are the best among the four semiempirical MO methods. Then, the accurate HOFs at B3LYP/6-31G* level of other 13 polyisocyanocubanes are obtained by systematically correcting their PM3-calculated HOFs. Polyisocyanocubanes have very high HOFs, and the HOFs increase linearly with the increa     

Uncatalyzed peptide bond formation between two double amino acid molecules in the gas phase

The gas phase mechanism for peptide bond formation between two double amino acid (DAA) molecules ((NH₂)₂C(COOH)₂) is investigated in the absence of any catalysts. Two different paths, concerted and stepwise, each leading to both cis and trans DAA‐DAA dipeptide products (four mechanisms total) are examined on the basis of theoretical calculations carried out at the CCSD(T)/aug‐cc‐pVDZ//MP2/aug‐cc‐pVDZ level. The investigation indicates that the concerted mechanism leading to the trans configuration of the peptide bond in the DAA‐DAA dipeptide product is thermodynamically favored by about 5 kcal mol^?1 and requires slightly less energy than the remaining pathways considered. Moreover, the peptide bond formation process between two DAA molecules in the gas phase resembles the analogous reactions between two natural amino acids.     

Enantioselective C--C bond formation to sulfonylimines through use of the 2-pyridinesulfonyl group as a novel stereocontroller

Enantioselective C--C bond formation to 2-pyridinesulfonylimines afforded products with good enantioselectivity. Dynamic induction of chirality on the sulfur by coordination of a chiral Lewis acid to the pyridine nitrogen and one of the prochiral sulfonyl oxygens induces enantioselectivity. Since the 2-pyridinesulfonyl group can easily be removed after the reaction, it acts not only as an activating group but also as an efficient stereocontroller.