Heteroatom effects in XC5H5C Arduengo-type carbenes (X = CH,N, P,and As) in singlet and triplet states

Internal energy difference, ΔE _s-t; enthalpy difference, ΔH _s-t; Gibbs free energy difference, ΔG _s-t, between the singlet (s) and triplet states (t) of XC₅H₅C, 1X (X = CH, N, P, and As) were computed at B3LYP/6-311++G** and MP2/6-311++G**//B3LYP/6-311++G** levels of theory. The ΔG _s-t between the singlet and triplet states of 1 _X were changed in the order: 1 _P > 1 _As > 1 _N .     

The structure of dihydropyrroloindoles: A quantum-chemical estimation of conjugation in cyclohexadiene systems

A non-empirical quantum chemical calculation of isomeric 3,6-divinyl-3,4,5,6-tetrahydropyrrolo[3,2-e] indole 1 and 1,5-divinyl-1,4,5,8-tetrahydro[3,2-f]indole 2 structures carried out by DFT (B3LYP) method with 6-311++G(d, p) and 6-311++G(3df, p) basis sets showed the energy preference of 2 over 1 (1.33 kcal/mol and 1.47 kcal/mol, respectively). The structure of the molecule of 2 is planar while the molecule of 1 is non-planar due to the presence of sp ³-hybridized carbon atoms.     

A potential energy surface survey of OB6: global minima and isomerization stability

In light of the very recent significant discrepancies on the global isomer of the sept-atomic molecule OB₆, we performed a detailed potential energy surface survey of OB₆ covering various isomeric forms. We showed that at the CCSD(T)/6-311+G(2df)//B3LYP/6-311+G(d) level, the planar knife-like isomer 01 with a –BO moiety has the lowest energy, followed by the planar belt-like isomer 02 at 22.6 kcal/mol. Another isomer 05 at 33.1 kcal/mol can be viewed as the direct O-adduct of the pentagonal pyramid B₆. Kinetically, the three isomers 01, 02 and 05 all have considerable barriers (19–29 kcal/mol) (obtained at B3LYP/6-311+G(d) level) against isomerization. However, other isomers either have much higher energy or possess much smaller conversion barriers and are thus of little likeliness for isolation. Moreover, though being isoelectronic to the well-known CB ₆ ^2? molecule, OB₆ does not have any kinetically stabilized wheel-like isomers with O or B centers. The three OB₆ isomers 01, 02 and 05 await future laboratory studies. The detailed results reported in this paper are expected to provide useful information for understanding the growing process of boron oxides, O-doping and oxidation mechanism of boron clusters.     

Ab initio study of dehalohydrogenation reaction of 2-halo-2,3-dihydrophosphinine

Decomposition of 2-fluoro-2,3-dihydrophosphinine (1), 2-chloro-2,3-dihydrophosphinine (3), 2-bromo-2,3-dihydrophosphinine (5) to phosphinine was investigated using Molecular orbital and density functional theory. Study on the B3LYP/6-311+G** level of theory revealed that the required energy for the decomposition of compounds 1, 3, and 5 to phosphinine is 30.56 kcal·mol^?1, 28.23 kcal·mol^?1, and 24.03 kcal·mol^?1, respectively. HF/6-311+G**//B3LYP/6-311+G** calculated barrier height for the decomposition of compound 1, 3, and 5 to phosphinine is 57.56 kcal·mol^?1, 37.26 kcal·mol^?1, and 30.77 kcal·mol^?1, respectively. Also, MP2/6-311+G**//B3LYP/6-311+G** results indicated that the barrier height for the decomposition of compound 1, 3, and 5 to phosphinine is 46.59 kcal·mol^?1, 47.28 kcal·mol^?1, and 42.57 kcal·mol^?1, respectively. Natural bond orbital (NBO) population analysis and nuclear independent chemical shift (NICS) results showed that, reactants are non-aromatic but products of elimination reaction are aromatic, C-H and C-X bonds are broken and H-X bond is appear.     

Heteroatom effects on allenic seven-membered ring X2C4H4C (X = CH,N, P,and As)

Stable configurations of seven-membered rings X₂C₄H₄C (1 _X , X = CH, N, P, and As) in the singlet (s) and triplet (t) states are found at B3LYB/6-311++G** level of theory. Thermal energy gaps, ΔE _s-t; enthalpy gaps, ΔH _s-t; Gibbs free energy gaps, ΔG _s-t, between the singlet and triplet states of 1 _X were estimated at the same level of theory. The ΔG _s-t gap between the singlet and triplet states of 1 _X changes in the order: 1 _P > 1 _As > 1 _N , respectively.     

Radical reaction HCNO + 3NH: a mechanistic study

The complex triplet potential energy surface for the reaction of HCNO with NH is investigated at the G3B3 level using the B3LYP/6-311++G(d,p), and QCISD/6-311++G(d,p) geometries. Various possible isomerization and dissociation pathways are probed. The initial association between HCNO and NH is found to be carbon to nitrogen attack leading to HNCHNO 2a, which can convert to 2b, 2c, and 2d. Subsequently, 1,4-H-shift of 2a to form NCHNOH 3a followed by dissociation to P ₂ (¹HCN + ³HON) is the most feasible pathway. Much less competitively, 2d undergoes successive 1,3-H-shift and C-N cleavage to form HNCNOH 8b, and then to product P ₃ (¹HNC + ³HON), the second feasible pathway. 8b can alternatively isomerize to 8c followed by N–O bond rupture to generate P ₆ (²OH + ²HNCN), the lesser followed feasible pathway. In addition, 2b takes continuously 1,3- and 1,2-H-shift to form NC(H)NHO 6a, then to ONHCNH 7a which can convert to 7b. Eventually, 7b may take C-N bond fission to produce P ₅ (¹HNC + ³HNO), the least feasible pathway. The present paper may be helpful for future experimental identification of the product distributions for the title reaction, and may be helpful to deeply understand the mechanism of the title reaction.     

NMR chemical shielding and spin–spin coupling constants across hydrogen bonds in uracil–α-hydroxy-N-nitrosamine complexes

One- and two bond spin–spin coupling constants, ¹ J, ^1h J , and ^2h J across X–H?O hydrogen bonds and shielding constants of bridging hydrogens have been computed for complexes formed from interaction between the α-hydroxy-N-nitrosamine (NP) and four preferential binding sites of the uracil (U) at B3LYP/6-311++G(2d,2p)//MP2/6-311++G(2d,2p) level of theory. All complexes are stabilized by two H_U?O_NP and H_NP?O_U hydrogen bonds. Very good correlations were found between NMR spin–spin coupling constant as well as isotropic shielding constant and the binding energy, H-bond distance, red-shift of vibration frequency, charge transfer energy, and electron density at H-bond critical point.     

Aromatic stability energy studies on five-membered heterocyclic C4H4M (M = O, S, Se, Te, NH, PH, AsH and SbH): DFT calculations

Energetic, geometric and magnetic criteria were applied to examine the stability and/or aromatic character for the cyclic molecules C ₄ H ₄ M (M = O, S, Se, Te, NH, PH, AsH and SbH) at B3LYP/6-311++G^** and MP2/6-311++G^** levels of theory. The isodesmic reactions and nuclear independent chemical shifts (NICS) calculations were utilized to examine the molecules for energetic and magnetic criteria, respectively. The isodesmic reaction energies reveal that thiophene (C ₄ H ₄ S, ?23.269 kcal/mol) and pyrrole (C ₄ H ₄ NH, ?20.804 kcal/mol) have the greatest aromatic stabilization energies and tellurophene (C ₄ H ₄ Te, ?15.114 kcal/mol) and stibole (C ₄ H ₄ SbH, ?1.169 kcal/mol) have the lowest aromatic stabilization energies in their corresponding groups at MP2/6-311++G^**. The NICS calculations confirmed the results obtained through isodesmic reaction energies.     

Synthesis,spectroscopy, computational study and prospective biological activity of two novel 1,3,2-diazaphospholidine-2,4,5-triones

The preparation of two new 1,3,2-diazaphospholidine-2,4,5-triones is reported. Thus, 2-chloro-1,3,2-diazaphospholidine-2,4,5-trione [ClP(O)(NHC(O)C(O)NH) (I)] and 2-benzylamino-1,3,2-diazaphospholidine-2,4,5-trione [C₆H₅CH₂NHP(O)(NHC(O)C(O)NH) (II)] have been synthesized by the reaction of POCl₃ with the corresponding carboxylic diamide salts. The characterization of the compound I was performed by multinuclear (¹H, ¹³C, ³¹P) NMR and FTIR spectroscopies, elemental analysis and also mass spectrometry. Both compounds show two signals at room temperature in the low field region of the ¹H NMR spectrum, which collapsed to a single peak when the temperature is increased. Dynamic NMR (¹H DNMR) and quantum chemical studies were performed to gain insight from this conversion process. The free activation energies, calculated at the coalescence temperatures are 18.51 and 17.45 kcal/mol for compounds (I) and (II), respectively, which are associated with a tautomeric interconversion process, most likely between the lactam and lactim forms. The relative energy, molecular geometry and vibrational properties of several plausible tautomers were analyzed by using quantum chemical calculations at the HF/6-311G** and B3LYP/6-311++G** levels of the theory. The nuclear magnetic shielding tensors have been calculated for both tautomeric forms using the gauge independent atomic orbital (GIAO) method at the B3LYP/6-311++G(3df,2p) level of approximation. A biological activity prediction using the PASS software shows that compound (I) can be characterized by a superb anti-HIV activity whereas compound (II) is a very good antineoplastic.     

Theoretical study on the singlet and triplet potential energy surfaces of NH (X3Σ−) + HCNO reaction

Both the singlet and triplet potential energy surfaces (PESs) of the NH (X³Σ^?) + HCNO reaction have been investigated at the BMC-CCSD level based on the UB3LYP/6-311++G(d, p) structures. The results show that the title reaction is more favorable through the singlet potential energy surface than the triplet one. For the singlet potential energy surface of the NH (X³Σ^?) + HCNO reaction, the most feasible association of NH (X³Σ^?) with HCNO is found to be a non-barrier nitrogen-to-carbon attack forming the adduct a (trans-HNCHNO), which can isomerize to the adduct b (cis-HNCHNO). The most feasible channel is that the 1, 3-H shift with N2–H2 and C–N1 bonds cleavage associated with the N1–H2 bond formation of adduct a leads to the product P ₁ (HCN + HNO). Moreover, P ₂ (HNC + HNO) should be the competitive product. The other products, including P ₃ (NH₂ + NCO) and P ₄ (N₂H₂ + CO), are minor products. The product P ₁ can be obtained through two competitive channels Path 1: R → a → P ₁ and Path 3: R → b → d → P ₁ , whereas the product P ₂ can be formed through Path 2: R → b → d → P ₂ . At high temperatures, the nitrogen-to-nitrogen approach may become feasible. For the triplet potential energy surface of the NH (X³Σ^?) + HCNO reaction, the Path 10: R → ³ a → ³ a ₁ → P ₁ should be the most feasible pathway due to the less reaction steps and lower barriers. These conclusions will have impacts on further experimental investigations.     

One-pot microwave-assisted solvent-free synthesis,theoretical and experimental studies on barrier rotation of C–N bond of N-alkenyl-1,2,3-triazoles

The reactions on benzotriazoles continue to happen to reach interesting varieties of their derivatives. This study reports a fast one-pot microwave-assisted solvent-free synthesis of N-alkenyl-1,2,3-benzotriazole (3, 5, and 7) and 1-(2-Alkyloxycarbonyl-vinyl)-1H-[1–3] triazole-4-carboxylic acid methyl ester (8 and 9) derivatives by nucleophilic addition reactions of 1,2,3-benzotriazole (C₆H₅N₃) (1) and 1H-[1–3] triazole-4-carboxylic acid methyl ester (C₄H₄N₃O₂) (1′) with R-propiolates (R = Me, Et; 2 & 4) and phenylacetylene 6 in good yields. The values of activation energy for rotation around C–N bond in the synthesized N-alkenyl-1,2,3-triazole compounds were studied by DFT-B3LYP/6-31G* method.     

Gas-phase basicity: Parameter determining the efficiency of laser desorption/ionization from silicon surfaces

The efficiency of laser desorption/ionization of twenty compounds from the surface of amorphous silicon is studied as a function of proton affinity (PA) and gas-phase basicity (GB). The values of GB and PA are obtained from quantum-chemical calculations using the density functional theory in the B3LYP model with the 6–311++G(3df,3pd) basis set. The values of GB lie in the range from 845 to 977 kJ/mol. The efficiency of laser desorption/ionization exponentially depends on the GB and PA values and for the studied compounds varies from 7 × 10^?6 to 1.4 × 10^?2.     

The theoretical research of the medium effect on the vibrational spectrum and the energy of intramolecular coordination O → Si in (benzoyloxymethyl)trifluorosilane molecule

The characteristics of (benzoyloxymethyl)trifluorosilane C₆H₅C(O)OCH₂SiF₃ containing a five-membered heterocycle closed by intramolecular coordination O → Si bond (Ia) and its most stable acyclic isomer (Ib) have been calculated by HF, MP2(Full) non-empirical methods, and DFT(B3LYP) using 6-311G(d) and 6-311 + G(2d,p) basis sets. The (C₈H₁₈, C₆H₆, (C₄H₉)₂O, CHCl₃, (CH₂)₄O, CH₂Cl₂, CH₃CN) medium effect on the energy and structural characteristics, dipole moments, and vibrational spectra of Ia and Ib isomers was calculated by the DFT(B3LYP)/6-311 + G(2d,p) method in the Onsager SCRF model approximation. The DFT(B3LYP)/6-311 + G(2d,p) calculation reasonably reproduces the medium effect on coordination energy, geometry, dipole moments, and band frequencies in the vibrational spectrum of Ia.     

Structural and electronic properties of heptachlor

In this work, the molecular geometry of heptachlor is investigated using ab initio HF, DFT, LDA, and GGA methods. The natural bond orbital (NBO) analysis is performed at the B3LYP/6-311++G(d,p) level of theory. The first order hyperpolarizability β_total, the mean polarizability Δα, the anisotropy of the polarizability Δα, and the dipole moment μ, are calculated by B3LYP/6-311++G(d,p) and HF/6- 311++G(d,p) methods. The first order hyperpolarizability (β_total) is calculated based on the finite field approach. UV spectral parameters along with HOMO, LUMO energies for heptachlor are determined in vacuum and the solvent phase using HF, DFT, and TD-DFT/B3LYP methods implemented with the 6-311++G(d,p) basis set. Atomic charges and electron density of heptachlor in vacuum and ethanol are calculated using DFT/B3LYP and TD-DFT/B3LYP methods and the 6-311++G(d,p) basis set. In addition, after the frontier molecular orbitals (FMOs), the molecular electrostatic potential (MEP), the electrostatic potential (ESP), the electron density (ED), and the solvent accessible surface of heptachlor are visualized as a results of the B3LYP/6-311++G(d,p) calculation. Densities of states (DOS), the external electric field (EF) effect on the HOMO-LUMO gap, and the dipole moment are investigated by LDA and GGA methods.     

A barrier-free molecular radical-molecule reaction: {^{3}C_{2} (a^{3}\Pi) {+} O_{2} (X^{3} \Sigma)}

The reaction of ³C₂ (a³Π) radical with O₂ (X³Σ) molecule has been studied theoretically using ab initio Quantum Chemistry method. Both singlet and triplet potential energy surfaces (PES) are calculated at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-311+G(d) + ZPE and G3B3 levels of theory. On the singlet PES of the title reaction, it is shown that the most feasible pathway should be the O-atom of O₂ attacking the C-atom of the  ³C₂ molecule first to form the adduct 1 CCOO, followed by the O-shift to give intermediate 2 CC(OO), and then to the major products P1 (2CO). Alternatively, 1 can be directly dissociated to P1 via transition state TS1-P1. The other reaction pathways are less competitive due to thermodynamical or kinetic factors. On the other hand, the pathways on the triplet PES are less competitive than those on the singlet PES in low temperature range, whereas it is not the case in high temperature ranges. On the basis of the analysis of the kinetics of all pathways through which the reactions proceed, we expect that the competitive power of reaction pathways may vary with experimental conditions for the title reaction. The reaction heats of formation calculated are in good agreement with that obtained experimentally.     

Thermochemical and structural properties of anthraquinones

We have investigated the energetic, structural, and other physical–chemical properties (aromaticity, intrinsic strain, hydrogen bond interaction) of 1,4-anthraquinone (1), its better known isomer 9,10-anthraquinone (2) and the derivatives 9-hydroxy-1,4-anthraquinone (3) and 9-methoxy-1,4-anthraquinone (4). In particular, the standard enthalpy of formation in the gas phase at 298.15 K of 1,4-anthraquinone was determined [ $\Updelta_{\text{f}}^{{}} H_{\text{m}}^{\text{o}} \left( {{\text{g}},{\mathbf{1}}} \right) \, = \, - 4 4. 9 { } \pm { 5}. 7\;{\text{kJ}}\;{\text{mol}}^{ - 1} ]$ . Using isodesmic/homodesmotic reaction schemes, we have experimentally estimated: (i) the stabilization energy of 1 (162.2 ± 7.2 kJ mol^?1) and 2 (193.2 ± 5.2 kJ mol^?1), (ii) strength of intramolecular hydrogen bonding in 3 (HB = 79.8 ± 10.8 kJ mol^?1), and (iii) additional strain energy due to peri-oxygen interaction in 4 (?34.2 ± 7.6 kJ mol^?1). A computational study of these species, at the B3LYP/6-311++G(3df,2p) level, sheds light on structural, aromatic, intrinsic strain, or hydrogen bond effects and further confirmed the consistency of the experimental results.     

Insight into the acidic behavior of oxazolidin-2-one,its thione and selone analogs through computational techniques

Deprotonation thermochemistry of Oxazolidin-2-one (OXA), Oxazolidine-2-thione (OXA-S), and Oxazolidine-2-selone (OXA-Se) has been studied in order to find the most acidic site and relative acidities of these heterocyclics at various sites. The deprotonation enthalpies at MP2/6-311++G**//MP2/6-31+G* and B3LYP/6-31+G* levels, while the free energies for deprotonation process and pKa values at B3LYP/6-31+G* level both in gas and aqueous phase (using PCM continuum model) of the anions of the three heterocyclics have been computed at 298 K. Calculated aqueous phase pKa values of OXA vary by ~6–7 units from the experimental aqueous phase pKa values of OXA and its derivatives. The deprotonation at the nitrogen is favored in OXA over the carbon atoms in contrast to the OXA-S and OXA-Se where in the deprotonation at the carbon attached to the nitrogen is most preferred. Deprotonation at this carbon induces an important C–O bond rupture in OXA-S and OXA-Se promoting an energetically favored ring-opening process. The finding offers a rare case when C–H acidity is able to dominate over the N–H acidity. In order to explain the relative stabilities, relative acidities and deprotonation enthalpies various characteristics of these molecules as well as their anions such as molecular electrostatic potential surface (MEP), frontier molecular orbital (FMO) features, chemical hardness, softness have been governed. The three dimensional MEP maps and HOMO–LUMO orbitals encompassing these molecules yield a reliable relative stability and reactivity (in terms of acidity) map displaying the most probable regions for deprotonation. The differential distribution of the electrostatic potential over the neutral and anionic species of OXA, OXA-S, and OXA-Se molecules is authentically reflected by HOMO–LUMO orbitals and NBO charge distribution analysis. The lone pair occupancies, second order delocalization energies for orbital interactions and the distribution of atomic charges over the entire molecular framework as obtained from natural bond orbital (NBO) analysis are found to faithfully replicate the predictions from the MEP maps and HOMO–LUMO band gaps in respect of explaining the relative stabilities and acidities in most of the cases. Good linear correlations have been obtained between HOMO–LUMO gap and pKa values in the aqueous phase for OXA and OXA-S molecules.     

Intramolecular hydrogen bonds in the sulfonamide derivatives of oxamide, dithiooxamide, and biuret. FT-IR and DFT study, AIM and NBO analysis

The hydrogen bonding in [(1-arylsulfonylamino-2,2,2-trichloro)ethyl]biuret 1, [(1-arylsulfonylamino-2,2,2-trichloro)ethyl]oxamide 2, and [(1-arylsulfonylamino-2,2,2-trichloro)ethyl]dithiooxamide 3, the sulfonamide derivatives of biuret 4, oxamide 5, and dithiooxamide 6, has been studied by molecular spectroscopy and DFT theoretical calculations including frequency calculations, at the B3LYP/6-311+G(d,p) level of theory. The analysis of the CO?HN and CS?HN intramolecular hydrogen bonds closing the five- and six-membered rings employing the atoms-in-molecules (AIM) method using the MP2(full)/6-311++G(d,p) wave functions has shown that their stability is increased in comparison to the original molecules and is much higher in the thiocarbonyl compounds. The results of the AIM and the NBO analysis of donor-acceptor interactions are in good agreement with each other and with the experimental FT-IR spectroscopy data.     

Theoretical study on the mechanism of C2Cl3 + NO2 reaction

The radical-molecule reaction of C₂Cl₃ with NO₂ is explored at the B3LYP/6-311G(d,p) and CCSD(T)/6-311+G(d,p) (single-point) levels. On the singlet potential energy surface (PES), the association between C₂Cl₃ and NO₂ is found to be carbon-to-nitrogen attack forming the adduct C₂Cl₃NO₂ (1) without any encounter barrier, followed by isomerization to C₂Cl₃ONO (2). Starting from 2, the most feasible pathway is the N–O1 bond cleavage which lead to P ₁ (C₂Cl₃O + NO). Much less competitively, 2 transforms to the three-membered ring isomer c-OCCl₂C–ClNO (4 ^a ) which can easily interconvert to c-OCCl₂C–ClNO 4 ^b . Then 4 (4 ^a , 4 ^b ) takes direct C1–C2 and C2–O1 bonds cleavage to give P ₂ (COCl₂ + ClCNO). The lesser competitive channel is the 4 ^a isomerizes to the four-membered ring intermediate O-c-CNClOCCl₂ (5) followed by dissociation to P₃ (CO + ClNOCCl₂). The concerted 1,2-Cl shift along with C1–O1 bond rupture of 4 ^b to form ONC(O)CCl₃ (6) followed by dissociation to P ₄ (ClNO + OCCCl₂) is even much less feasible. Moreover, some of P ₃ and P ₄ can further dissociate to P ₅ (ClNO + CO + CCl₂). Compared with the singlet pathways, the triplet pathways may have less contribution to the title reaction. Our results are in marked difference from previous theoretical studies which showed that two initial adducts C₂Cl₃–NO₂ and C₂Cl₃–ONO are obtained. Moreover, in the present paper we focus our main attentions on the cyclic isomers in view of only the chain-like isomers are considered by previous studies. The present study may be helpful for understanding the halogenated vinyl chemistry.