Theoretical predictions of the structure,gas‐phase acidity,and aromaticity of tetrathiosquaric acid

Results of ab initio self‐consistent‐field and density functional theory calculations of the gas‐phase structure, acidity (free energy of deprotonation, ΔG⁰), and aromaticity of tetrathiosquaric acid (3,4‐dithiohydroxy‐3‐cyclobutene‐1,2‐dithione, H₂C₄S₄) are reported. The global minimum found on the potential energy surface of tetrathiosquaric acid presents a planar conformation. The ZZ isomer was found to have the lowest energy among the three planar conformers and the ZZ and ZE isomers are very close in energy. The optimized geometric parameters exhibit a bond length equalization relative to reference compounds, cyclobutanedithione, and cyclobutenedithiol. The computed aromatic stabilization energy by homodesmotic reaction is −18.4 (MP2(fu)/6‐311+G**//RHF/6‐311+G**) and −15.1 kcal/mol (B3LYP//6‐311+G**// B3LYP/6‐311+G**). The aromaticity of tetrathiosquaric acid is indicated by the calculated diamagnetic susceptibility exaltation (Λ) −11.77 (CSGT(IGAIM)‐RHF/6‐311+G**// RHF/6‐311+G**) and −18.08 (CSGT(IGAIM)‐B3LYP/6‐311+G**// B3LYP/6‐311+G**). Thus, tetrathiosquaric acid fulfils the geometric, energetic and magnetic criteria of aromaticity. The most reliable theoretical gas‐phase acidities are $\Delta G^{0}_{1(298\mathrm{K})}=303.7$ and $\Delta G^{0}_{2(298\mathrm{K})}=394.1$ kcal/mol. Hence, tetrathiosquaric acid is a stronger acid than squaric acid (3,4‐dihydroxy‐3‐ cyclobutene‐1,2‐dione, H₂C₄O₄). Comparisons of the computed results of tetrathiosquaric acid with squaric acid have also been made. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 78: 443–449, 2000     

3,4-二硫方酸芳香性和气相酸性的理论研究

在RHF/6-311G^**,RHF/6-311+G^**和B3LYP/6-311+G^**水平优化得到3,4-二硫方酸(3,4-二巯基-3-环丁烯-1,2-二酮)三种平面构象异构体的平衡几何构型.用MP2(Full)/6-311G^**//RHF/6-311G^**方法计算单点能量,发现ZZ型异构体是能量最低构象,且ZZ和ZE型能量非常接近.用优化的最稳定构象ZZ型异构体在RHF/6-311G^**//RHF/6-311G^**,RHF/6-311G^**//RHF/6-311G^**,MP2(Full)/6-311G^**//RHF/6-311G^**和B3LYP/6-311G^**//B3LYP/6-311G^**水平计算其气相酸性(ΔG⁰)和同键反应芳香性稳定化能(HASE).用基团加和法(Group Increment Approach)在RHF/6-311G^**//RHF/6-311G^**和B3LYP/6-311G^**//B3LYP/6-311G^**水平计算其磁化率增量(Λ).计算结果表明,标题化合物的同键反应芳香性稳定化能和磁化率增量均为负值,表明它具有芳香性,实现了标题化合物芳香性的几何、能量和磁性的判定.     

Theoretical studies on the reaction of pentafulvenone with water in gas phase and aqueous solvent

In gas phase, the hydrations of pentafulvenone to generate three types of cyclopentadienyl carboxylic acids are studied theoretically at the MP2/6-311+G**//B3LYP/6-311+G** level. A water molecule attacking the C=O double bond of pentafulvenone can yield cyclopentadienyl carboxylic acids via the formation of fulvenediols, and attacking the C=C double bond of pentafulvenone can directly yield cyclopentadienyl carboxylic acid. The barriers of rate-determining transition states are 42.2 and 30.4 kcal mol⁻¹, respectively. The barriers of rate-determining transition states for two water molecules system are 20.2 and 19.6 kcal mol⁻¹, respectively. The products can isomerize to each other. In aqueous solvent, the hydrations of pentafulvenone are investigated using PCM-UAHF model at the MP2 (PCM)/6-311+G**// B3LYP (PCM)/6-311+G** and MP2 (PCM)/6-311+G**// B3LYP/6-311+G** levels. The barriers of all rate-determining transition states are decreased. The added water molecule acts as catalyst in both gas phase and aqueous solvent. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Theoretical study on structures and stability of HC2P isomers

 The structures and isomerization pathways of various HC₂P isomers in both singlet and triplet states are investigated at the B3LYP/6-311G(d,p), QCISD/6-311G(d,p) (for isomers only) and single-point CCSD(T)/6-311G(d,p)//B3LYP/6-311G(d,p) levels. At the CCSD(T)/6-311G(d,p)//B3LYP/6-311G(d,p) level, the lowest-lying isomer is a linear HCCP structure ³ 1 in the ³∑⁻ state. The second low-lying isomer has a CPC ring with exocyclic CH bonding ¹ 5 in a singlet state at 10.5 kcal/mol. The following third and fourth low-lying isomers are a singlet bent HCCP structure ¹ 1 at 20.9 kcal/mol and a bent singlet HPCC structure ¹ 3 at 35.8 kcal/mol, respectively. Investigation of the HC₂P potential-energy surface indicates that in addition to the experimentally known isomer ³ 1, the other isomers ¹ 1, ¹ 3 and ¹ 5 also have considerable kinetic stability and may thus be observable. However, the singlet and triplet bent isomers HCPC ¹ 2 and ³ 2 as well as the triplet bent isomer HPCC ³ 3 are not only high-lying but are also kinetically unstable, in sharp contrast to the situation of the analogous HCNC and HNCC species that are both kinetically stable and that have been observed experimentally. Furthermore, the reactivity of various HC₂P isomers towards oxygen atoms is briefly discussed. The results presented here may be useful for future identification of the completely unknown yet kinetically stable HC₂P isomers ¹ 1, ¹ 3 and ¹ 5 either in the laboratory or in interstellar space. Received: 5 November 2000 / Accepted: 25 November 2001 / Published online: 8 April 2002     

Theoretical study of the potential-energy surface of C2NP

 The B3LYP/6-311G(d) and CCSD(T)/6-311G(2df) (single-point) methods have been used to investigate the singlet potential energy surface of C₂NP, in which seven stationary isomers and seventeen interconversion transition states are involved. At the final CCSD(T)/6-311G(2df)//B3LYP6-311G(d) level with zero-point vibrational energy correction the lowest-lying isomer is a linear NCCP followed by two linear CNCP isomers at 23.9  and CCNP at 65.8 kcal mol⁻¹, respectively. The three isomers are kinetically very stable towards both isomerization and dissociation, and CCNP is even more kinetically stable than CNCP – by 14.3 kcal mol⁻¹ despite its high energy. Further comparative calculations were performed at the QCISD and QCISD(T) levels with the 6-311G(d) and 6-311G(2d) basis sets to obtain more reliable structures and spectroscopy for the three isomers. The calculated bond lengths, rotational constant, and dipole moment for NCCP were in close agreement with the experimentally determined values. Finally, similarities and discrepancies between the potential energy surface of C₂NP and those of the analogous species C₂N₂ and C₂P₂ were compared. The results presented in this paper might be helpful for future identification of the two still unknown yet kinetically very stable isomers CNCP and CCNP, both in the laboratory and in interstellar space. Received: 3 January 2001 / Accepted: 6 June 2001 / Published online: 30 October 2001     

The performance of the rapid estimation of basis set error and correlation energy from partial charges method on new molecules of the G3/99 test set

Experimental enthalpies of formation have been approximated using single-point Hartree–Fock (HF)–self-consistent-field (SCF) total energies plus the rapid estimation of basis set error and correlation energy from partial charges (REBECEP) energy corrections. The energy corrections are calculated from the HF–SCF partial atomic charges and optimized atomic energy parameters. The performance of the method was tested on 51 closed-shell neutral molecules (50 molecules from the G3/99 thermochemistry database plus urea, composed of H, C, N, O, and F atoms). The predictive force of the method is demonstrated, because these larger molecules were not used for the optimization of the atomic parameters. We used the earlier RECEP-3 [HF/6-311+G(2d,p)] and REBECEP [HF/6-31G(d)] atomic parameter sets obtained from the G2/97 thermochemistry database (containing small molecules) together with natural population analysis and Mulliken partial charges. The best results were obtained using the natural population analysis charges, although the Mulliken charges also provide useful results. The root-mean-square deviations from the experimental enthalpies of formation for the selected 51 molecules are 1.15, 3.96, and 2.92 kcal/mol for Gaussian-3, B3LYP/6-11+G(3df,2p), and REBECEP (natural population analysis) enthalpies of formation, respectively (the corresponding average absolute deviations are 0.94, 7.09, and 2.27 kcal/mol, respectively). The REBECEP method performs considerably better for the 51 test molecules with a moderate 6-31G(d) basis set than the B3LYP method with a large 6-311+G(3df,2p) basis set. Received: 10 March 2001 / Accepted: 5 July 2001 / Published online: 11 October 2001     

Influence of diffuse and polarization functions on the second-order Møller–Plesset optimized dihedral angle of biphenyl

 Using 6-31G and 6-311G basis sets to which diffuse and polarization functions were added in a stepwise fashion (a total of 16 basis sets), Hartree–Fock (HF), MP2 and B3LYP geometry optimizations were performed on biphenyl. With the MP2 method, diffuse functions raise the dihedral angle φ, for example, from 46.3° for 6-31G to 54.1° for 6-311++G, while polarization functions lower it, for example, from 54.1° for 6-311++G to 42.1° for 6-311++G(2d,2p). For a single set of polarization functions, φ(MP2) lies close to or above φ(HF) (44–47°), but for a double set it is below φ(HF) and is close to B3LYP values (38–42°) which show little basis set dependence. The most reliable value for φ, 42.1° [MP2/6-311++G(2d,2p)], is expected to increase slightly by adding more diffuse functions. The corresponding best calculated energy barrier at 0° (coplanar conformation) is 2.83 kcal/mol, much higher than the experimental estimate (1.4 ± 0.5 kcal/mol). The barrier at 90° is 1.82 kcal/mol, in line with the experimental estimate (1.6 ± 0.5 kcal/mol) and with previous theoretical results. Received: 9 September 2002 / Accepted: 15 November 2002 / Published online: 1 April 2003 Correspondence to: Friedrich Grein e-mail: fritz@unb.ca Acknowledgement. The author would like to thank NSERC (Canada) for financial support.     

3,4-二硒方酸芳香性和气相酸性的理论研究

在ＲＨＦ／６－３１１Ｇ水平优化得到３,４－二硒方酸（３,４－二氢硒基－３－环丁烯－１,２－二酮）３ 种平面构象异构体的平衡几何构型。进一步用ＭＰ２（ｆｕｌｌ）／６－３１１Ｇ／／ＲＨＦ／／６－３１１Ｇ方法计算单点能量,发现ＺＺ型异构体是能量最低构象,且ＺＺ和ＺＥ型能量非常接近。用优化的最稳定构象ＺＺ型异构体在ＲＨＦ／６－３１１Ｇ／／ＲＨＦ／６－３１１Ｇ、ＲＨＦ／６－３１１＋ Ｇ／／ＲＨＦ／６－３１１＋ Ｇ、ＭＰ２（ｆｕｌｌ）／６－３１１＋ Ｇ／／ＲＨＦ／６－３１１＋ Ｇ 和Ｂ３ＬＹＰ／６－３１１＋ Ｇ／／Ｂ３ＬＹＰ／６－３１１＋ Ｇ水平计算其气相酸性（ΔＧ°）和同键反应芳香性稳定化能（ＨＡＳＥ）。用基团加和法（ｇｒｏｕｐ ｉｎｃｒｅｍ ｅｎｔ ａｐ－ｐｒｏａｃｈ ） 在 ＲＨＦ／６－３１１ ＋ Ｇ／／ＲＨＦ／６－３１１ ＋ Ｇ 和 Ｂ３ＬＹＰ／６－３１１ ＋ Ｇ／／Ｂ３ＬＹＰ／６－３１１＋ Ｇ水平计算其磁化率增量（Λ）。计算结果指出标题化合物的同键反应芳香性稳定化能和磁化率增量均为负值,表明它具有芳香性,实现了标题化合物芳香性的几何、能量和磁性的判定。     

Action of HCl on 3-hydroxypyrazolo(iso)quinolines to give 1-chloropyrazoles: evidence for an addition–elimination mechanism by ab initio calculations in gas phase and water

 Addition–elimination reactions involving a nucleophile and a remote leaving group [S^H _N(AE)^tele] are well-known under basic conditions, especially amongst electron-poor six-membered heterocycles, but are less commonly encountered for five-membered heterocycles and are rare under acidic conditions. Concentrated HCl converts 1-hydroxy-1H-pyrazolo[3,4-c] isoquinoline and 1-hydroxy-1H-pyrazolo[3,4-c]quinoline into 3-chloro-1H-pyrazolo[3,4-c]isoquinoline and 3-chloro-1H-pyrazolo[3,4-c]quinoline, respectively. However, apparently neither the isomeric 1-hydroxy-1H-pyrazolo[4,3-c](iso)-quinolines nor the parent 1-hydroxypyrazole undergo this reaction. Additionally, all these systems are refractory under basic conditions. We present a plausible mechanism for the reaction, involving the 3-addition of Cl^- to the diprotonated heterocycle, followed by the elimination of water. Calculations of the initial transition states and intermediates, using optimisation at B3LYP/6-311+G(d,p), including thermochemistry [HF/6-31+G(d)], and single-point Poisson–Boltzmann self-consistent reaction field determination of the free energy of solvation (Jaguar Poisson–Boltzmann self-consistent reaction field), support this mechanism and reproduce the observed order of reactivity, the addition step being 2–4 kcal less favourable for the isomeric 1-hydroxy-1H-pyrazolo[4,3-c](iso)quinolines and provide a rationalisation for the role of strong acid. Received: 27 June 2002 / Accepted: 6 September 2002 / Published online: 14 February 2003     

Theoretical study of the structural character of weakly bonding silicon carbonyl complexes

The structures, properties and the bonding character for sub-carbonyl Si, SiCO and Si(CO)₂, in singlet and triplet states have been investigated using complete-active-space self-consistent field (CASSCF), density functional theory and second-order M?ller–Plesset methods with a 6-311+G* basis set. The results indicate that the SiCO species possesses a ³∑⁻ ground state, and the singlet ¹Δ excited state is higher in energy than the ³∑⁻ state by 17.3 kcalmol⁻¹ at the CASSCF–MP2/6-311+G* level and by 16.4 kcalmol⁻¹ at the CCSD(T)/6-311+G* level. The SiCO ground state may be classified as silene (carbonylsilene), and its CO^δ− moiety possesses CO⁻ property. The formation of SiCO causes the weakening of CO bonds. The Si–C bond consists of a weak σ bond and two weak π bonds. Although the Si–C bond length is similar to that of typical Si–C bonds, the bond strength is weaker than the Si–C bonds in Si-containing alkanes; the calculated dissociation energy is 26.2 kcalmol⁻¹ at the CCSD(T)/6-311+G* level. The corresponding bending potential-energy surface is flat; therefore, the SiCO molecule is facile. For the bicarbonyl Si systems, Si(CO)₂, there exist two V-type structures for both states. The stablest state is the singlet state (¹A₁), and may be referred to the ground state. The triplet state (³B₁) is energetically higher in energy than the ¹A₁ state by about 40 kcalmol⁻¹ at the CCSD(T)/6-311 + G* level. The bond lengths in the ¹A₁ state are very close to those of the SiCO species, but the SiCO moieties are bent by about 10°, and the CSiC angles are only about 78°. The corresponding ³B₁ state has a CSiC angle of about 54° and a SiCO angle of about 165°, but its Si–C and C–O bonds are longer than those in the ¹A₁ state by about 0.07 and 0.03 ?, respectively. This Si(CO)₂ (¹A₁) has essentially silene character and should be referred to as a bicarbonyl silene. Comparison of the CO dissociation energies of SiCO and Si(CO)₂ in their ground states indicates that the first CO dissociation energy of Si(CO)₂ is smaller by about 7 kcalmol⁻¹ than that of SiCO; the average one over both CO groups is also smaller than that of SiCO. A detailed bonding analysis shows that the possibility is small for the existence of polycarbonyl Si with more than three CO. This prediction may also be true for similar carbonyl complexes containing other nonmetal and non-transition-metal atoms or clusters. Received: 17 April 2002 / Accepted: 11 August 2002 / Published online: 4 November 2002 Acknowledgements. This work was supported by the National Natural Science Foundation of China (29973022) and the Foundation for Key Teachers in University of the State Ministry of Education of China. Correspondence to: Y. Bu e-mail: byx@sdu.edu.ch     

Structures and stability of N7+ and N7− clusters

Ab initio molecular orbital theory and density functional theory have been used to study nine isomers of N₇ ionic clusters with low spin at the HF/6-31G*, MP2/6-31G*, B3LYP/6-31G*, and B3LYP/6-311(+)G* levels of theory. All stationary points are examined with harmonic vibrational frequency analyses. Four N₇ ⁺ isomers and five N₇ ⁻ isomers are determined to be local minima or very close to the minima on their potential-energy hypersurfaces, respectively. For N₇ ⁺ and N₇ ⁻, the energetically low lying isomers are open-chain structures (C _{2 v} and C _{2 v} or C₂). The results are very similar to those of other known odd-number nitrogen ions, such as N₅ ⁺, N₉ ⁺, and N₉ ⁻, for which the open-chain structures are also the global minima. This research suggests that the N₇ ionic clusters are likely to be stable and to be potential high-energy-density materials if they could be synthesized. Received: 16 July 2001 / Accepted: 8 October 2001 / Published online: 21 January 2002     

Ab initio and density functional theory studies of the structure,gas-phase acidity and aromaticity of tetraselenosquaric acid

Results of ab initio self-consistent-field (SCF) and density functional theory (DFT) calculations of the gas-phase structure,acidity (free energy of deprotonation,G0) and aro-maticity of tetraselenosquaric acid (3,4-diselenyl-3-cy-dobutene-1,2-diselenone,H2C4Se4) are reported.The global minimum found on the potential energy surface of tetraselenosquaric acid presents a planar conformation.The ZZ iso-mer was found to have the lowest energy among the three planar conformers and the ZZ and ZE isomers are very dose in energy.The optimized geometric parameters exhibit a bond length equalization relative to reference compounds,cyclobu-tanediselenone,and cydobutenediselenol.The computed aromatic stabilization energy (ASE) by homodesmotic reaction is -77.4 (MP2(fu)/6 - 311 G //RHF/6 - 311 G) and - 54.8 kJ/mol (B3LYP/6 - 311 G //B3LYP/6 -311 G).The aromaticity of tetraselenosquaric add is indicated by the calculated diamagnetic susceptibility exaltation (A) - 19.13 (CSGT(IGAEM) - RHF/6 - 311 G// RHF/6-     

1,2-二硫方酸的气相酸性和芳香性

在ＲＨＦ／６－３１１Ｇ、ＲＨＦ／６－３１１＋Ｇ　和Ｂ３ＬＹＰ／６－３１１＋Ｇ　水平,优化得到１,２－二硫方酸（３,４．二羟基－３－环丁烯－１,２－二硫酮）三种平面构象异构体的平衡几何构型．进一步用ＭＰＺ（ｆｕｌｌ）／６－３１１＋Ｇ　、／／ＲＨＦ／６－３１１＋Ｇ　方法计算三种异构体的单点能量,发现ＺＺ型异构体是能量最低构象,且ＺＺ和ＺＥ型能量非常接近．用优化的最稳定构象ＺＺ型异构体,在ＲＨＦ／６－３１１Ｇ　／／ＲＨＦ／６－３１１Ｇ　、ＲＨＦ／６．３１１＋Ｇ　／／ＲＨＦ／６－３１１＋Ｇ　、ＭＰＺ（ｆｕｌｌ）／６．３１１＋Ｇ　／／ＲＨＦ／６－３１１＋Ｇ　和Ｂ３ＬＹＰ／６－３１１＋Ｇ　斤Ｂ３ＬＹＰ／６－３１１＋Ｇ　水平,计算其气相酸性（△Ｇ＿２９８）,并用同键反应方法在同样水平计算其芳香性稳定化能．用基团加和法（ｇｒｏｕｐｉｎｃｒｅｍｅｎｔａｐｐｒｏａｃｈ）在ＲＨＦ／６．３１１＋Ｇ　／／ＲＨＦ／６－３１１＋Ｇ　和Ｂ３ＬＹＰ／６－３１１＋Ｇ　／／Ｂ３ＬＴｈ／６－３１１＋Ｇ　水平计算其磁化率增量（　）．计算结果指出,标题化合物的键长发生了平均化,芳香性稳定化能和磁化率增量均为负值,表明它具有芳香性,实现了标题化合物芳香     

Ab initio study of rate constants of the reaction: HCN + OH → CN + H2O

A method applying ab initio direct dynamics has been utilized in studying the hydrogen abstraction reaction HCN + OH → CN + H₂O. The geometries of the reactants, products, and the transition state have been optimized at the QCISD/6-311G(d, p) level. Single-point energies were further evaluated at the QCISD(T)/6-311+G(2df, 2p)//QCISD/6-311G(d, p) level. The barrier heights for the forward and reverse reactions were predicted to be 15.95 and 7.51 kcal mol⁻¹ at the QCISD(T)/6-311 + G(2df, 2p)//QCISD/6-311G(d, p) level, respectively. The reaction rate constants were calculated in the temperature range from 298 to 4,000 K using the canonical variational transition-state theory with a small-curvature tunneling correction. The results of the calculation show that the theoretical rate constants are in good agreement with experimental data over the measured temperature range of 400–2,600 K. Received: 18 August 2002 / Accepted: 30 August 2002 / Published online: 20 November 2002 Acknowledgements. Our thanks are due to D.G. Truhlar for providing the POLYRATE 8.2 program. This work was supported by the National Science Foundation of China. We also thank D.C. Fang and Y. M. Xie for their valuable help, and P.R. Yan for reading our paper. Correspondence to: Q. S. Li e-mail: qsli@mh.bit.edu.cn     

The influence of protecting the hydroxyl group of β-oxy-α-diazo carbonyl compounds in the competition between Wolff rearrangement and [1,2]-hydrogen shift. Density functional theory study and topological analysis of the charge density

The influence of protecting the hydroxyl group of a β-oxy-α-diazo carbonyl compound on the competition between the Wolff rearrangement (WR) and the [1,2]-hydrogen shift (HS) was investigated theoretically. Stationary points on the potential-energy surface were located with the B3LYP density functional and the 6-31G** basis set. For the basic system geometry optimisations at B3LYP/6-311+G** were performed to validate the reliability of the B3LYP/6-31G** calculations. Single-point energy calculations were carried out at the B3LYP/6-311+G** level on the B3LYP/6-31G**-optimised geometries. Further insight into the processes was achieved with the aid of the theory of “atoms in molecules” of Bader. The calculated energy barriers qualitatively predicted the yields of HS and WR obtained experimentally. In order to rationalise the calculated energy barriers, it was necessary to take into account not only the electronegativity of the protective groups but also the alignment of the migrating groups with the depletion sites at the carbene centre. Further, when the hydroxyl group was not protected the existence of an intramolecular hydrogen bond played an important role in both HS and WR. Received: 30 December 1998 / Accepted: 7 May 1999 / Published online: 4 October 1999     

Ab initio and density functional theory studies on the mechanism of nucleophilic vinylic substitution of 4<Emphasis Type="Italic">H</Emphasis>-pyran-4-one and 2-methyl-4<Emphasis Type="Italic">H</Emphasis>-pyran-4-one with ammonia

 Nucleophilic vinylic substitutions of 4H-pyran-4-one and 2-methyl-4H-pyran-4-one with ammonia were calculated by the B3LYP method using the 6-31G(d,p) basis set. Bulk solvent effects of aqueous solution were estimated by the polarized continuum and Poisson–Boltzmann self-consistent reaction field models using the 6-311+G(d,p) basis set. In the gas phase different mechanisms were found for the two reaction systems calculated. The reaction of 4H-pyran-4-one proceeds through enol, whereas a feasible path for the less reactive 2-methyl-4H-pyran-4-one is the mechanism through a keto intermediate. Addition of ammonia in concert with proton transfer is the rate-determining step ofthe reaction. The mechanism proceeding either by a bimolecular nucleophilic substitution (S_N2) or by one involving a tetrahedral zwitterionic intermediate is shown to be unlikely in the gas phase or nonpolar solution. The effects of bulk solvent not only consist in a reduction of the various activation barriers by about 25–40 kJ mol⁻¹ but also in a change in the reaction mechanism. Received 26 May 2002 / Accepted 26 July 2002 / Published online: 14 February 2003     

Effects of fused benzene rings on tautomerizations and inversions of benzo, azabenzo, and oxabenzocycloheptatrienes at theoretical levels

Biologically important bicyclic species, including 6H-, 6H-6-aza-, and 6-oxabenzocycloheptatrienes (in which the benzene moiety is fused meta with respect to the tetrahedral constituents: –CH₂–, –NH–, and –O–, respectively), show strong shifts of tautomerizations in favor of the corresponding tricyclic benzonorcaradienes (with ΔH values of −11.49, −14.55, and −19.20 kcal mol⁻¹, respectively), at B3LYP/6-311++G**//B3LYP/6-31G*, and MP2/6-311++G**//MP2/6-31G* levels, and at 298 K. In contrast, such shifts are strongly disfavored by the isomeric bicyclic species in which the benzene moieties are fused ortho or para with respect to –CH₂–, –NH–, and –O–, respectively. Hence for species with ortho benzene rings including 5H-, 5H-5-aza- and 5-oxabenzocycloheptatrienes, tautomerization ΔH values are 30.76, 31.89, and 25.27 kcal mol⁻¹, respectively, while for species with para fused benzene moieties including 7H-, 7H-7-aza-, and 7-oxabenzocycloheptatrienes, tautomerization ΔH values are 24.12, 26.00, and 19.55 kcal mol⁻¹, respectively. NICS calculations are successfully used to rationalize these results. The calculated energy barriers for inversion of the seven-membered rings of bicyclic species predict a dynamic nature for all the structures except for the virtually planar 6H-6-aza- and 6-oxabenzocycloheptatrienes. Finally, our theoretical data are compared to the experimental results where available. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Molecular structure, barriers to internal rotation, and the enthalpy of formation of cumene hydroperoxide PhCMe2OOH: a DFT study

Conformational analysis of cumene hydroperoxide PhCMe₂OOH (1) has been carried out using the density functional methods B3LYP/6-31G(d,p) and B3LYP/6-311+G(3df,2p). Ignoring rotation of methyl groups, molecule 1 has seven conformers differing in orientation of the — CMe₂OOH fragment relative to the benzene ring and in mutual position of atoms in this fragment. The molecular structures, relative energies, and statistical distribution of the conformers were determined, and intramolecular rotational barriers were estimated. The enthalpies of formation of all conformers of molecule 1 were calculated using two approximations with inclusion of zero-point vibrational energy and temperature correction. Calculations using the isodesmic reaction (IDR) scheme made it possible to reduce the systematic error of the determination of the enthalpy of reactions. The total enthalpy of formation of compound 1 calculated with inclusion of statistical distribution of rotamers equals −19.7±3.6 kcal mol⁻¹. The combination of the B3LYP/6-31G(d,p) approximation and the IDR scheme gives fairly accurate results (relative error is ±0.4 kcal mol⁻¹) as compared to those obtained with the extended basis set 6-311+G(3df,2p). Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1157–1164, June, 2008.     

Boron rings containing planar octacoordinate iron and cobalt

The boron rings containing planar octacoordinate transition metals, D _8h FeB₈ ²⁻, CoB₈ ⁻ and CoB₈ ³⁺, C _2v FeB₈, D _2h CoB₈ ⁺ and CoB₈, are optimized with all real vibrational frequencies at the B3LYP/6–311+G* level of the theory. The D _8h FeB₈ ²⁻ and CoB₈ ⁻ isomers are global minima, while D _8h CoB₈ ³⁺ is only local minimum. The electronic structure character of these systems is revealed by natural bond orbital (NBO) analysis, showing that the boron rings containing planar octacoordinate transition metals have stability and aromaticity with six π electrons. The aromaticity is confirmed by nucleus independent chemical shifts (NICS) calculations. Supported by the specialized research fund for the doctoral program of higher education (20060007030)     

Structures and stability of N9, N9 − and N9 + clusters

 Ab initio molecular orbital calculations for N₉, N⁻ ₉ and N⁺ ₉ isomers were carried out at the HF/ 6-31G*, B3PW91/6-31G*, B3LYP/6-31G* and MP2/ 6-31G* levels of theory. Stable equilibrium geometric structures were determined by harmonic vibrational frequency analyses at the HF/6-31G*, B3PW91/6-31G* and B3LYP/6-31G* levels of theory. The most stable free-radical N₉ cluster is structure 1 with C _{2 v} symmetry and that of anion N⁻ ₉ is structure 3 with C _s symmetry. Only one stable structure of the N⁺ ₉ cation with C _{2 v} symmetry was predicted. Their potential application as high-energy-density materials has been examined. Received: 15 June 1999 / Accepted: 11 October 1999 / Published online: 14 March 2000