A combined density functional theory and coupled cluster method investigation of the structural properties and stabilities of radical CH(2)CP and its isomers

The doublet potential energy surface of radical system [C(2), H(2), P] is investigated at the UB3LYP/6-311++G(d,p) and UCCSD(T)/6-311++G(2df,2p) (single-point) levels. Eight chainlike and three-membered ring structures are located as energy minima connected by 10 interconversion transition states. At the final UCCSD(T)/6-311++G(2df,2p)//UB3LYP/6-311++G(d,p) level with zero-point vibrational energy correction, species CH(2)CP is found to be thermodynamically the most stable isomer followed by HCCPH, H-cCPC-H, cPCC-H(H), H-cCCP-H, cis-CC(H)PH, trans-CC(H)PH, and CCPH(2) at 11.01, 12.57, 40.07, 43.63, 50.25, 56.82, and 65.36 kcal/mol, respectively. The computed results indicate that the chainlike isomers CH(2)CP and HCCPH and cyclic radical H-cCPC-H possess considerable kinetic stability at extra low pressures and temperatures. Interestingly, radical CCPH(2), whose energy is the highest in all predicted CH(2)CP isomers, can be also regarded as a kinetically stable species with the smallest isomerization barrier of 22.26 kcal/mol at extra low pressures and temperatures. Therefore, considering higher kinetic stability, in addition to the microwave spectroscopy characterized isomer CH(2)CP in previous experiments, the species HCCPH, H-cCPC-H, and CCPH(2) should be considered as excellent candidates for possible experimental observation. Furthermore, the structural nature of stable radical isomers is discussed based on bonding characteristics, single electron spin distribution, and comparison with their analogues.     

Ab initio and DFT energetics of silylenic X–CNSi (X=H, F, Cl, and Br)

Singlet–triplet energy splitting for 24 silylenic reactive intermediates, X–CNSi (where X=H, F, Cl and Br), are compared and contrasted at 11 levels of theory: B1LYP/6-31++G**, B3LYP/6-31++G**, B1LYP/6-311++G**, B3LYP/6-311++G**, MP3/6-31G*, MP3/6-311++G**, MP2/6-31+G**, MP2/6-311++G**, MP4 (SDTQ)/6-311++G**, QCISD(T)/6-311++G** and CCSD(T)/6-311++G**. Each X-substituted silylenic species may either be singlet (s) or triplet (t), with one of the following three structures: 3-X-2-aza-1-silacyclopropenylidene (1_s-X, 1_t-X); [(X-imino)methylene]silylene (2_s-X, 2_t-X); and X-cyanosilylene (3_s-X, 3_t-X). For all X–CNSi species studied, orders of singlet–triplet energy separations (ΔE_s-t,X), appear as a function of electro-negativity (F>Cl>Br>H). For the six H–CNSi isomers (X=H), stability order is: 3_s-H>1_s-H>2_t-H>3_t-H>2_s-H>1_t-H. Likewise, stability order for the six isomers with X=F, is: 3_s-F>3_t-F>1_s-F>1_t-F>2_s-F>2_t-F. For X=Cl, the order of stability is: 3_s-Cl>1_s-Cl>3_t-Cl>2_t-Cl>1_t-Cl>2_t-Cl. Finally, the order of stability for six isomers of Br–CNSi is: 3_s-Br>3_t-Br>1_s-Br>2_s-Br>2_t-Br>1_t-Br. The lowest energy minimum, among all 24 species scrutinized, appears to be the singlet acyclic 3_s-X. Triplet silylene 2_t-H is suggested to be more stable than its corresponding 2_s-H at MP3, MP2 and DFT levels of theory. Comparisons between relative stabilities; multiplicities and geometrical parameters of 1–3 are discussed.     

Theoretical study on the mechanism of the gas-phase reaction of diborane(3) anion with carbon disulfide

The complex potential energy surface of the gas-phase reaction of HB(H)BH- with CS2 to give three low-lying products [B2H3S]- + CS, [BH2CS]- + HBS, and [BH3CS] + BS-, involving nine [B2H3CS2]- isomers and 12 transition states, has been investigated at the CCSD(T)/6-311++G(d,p)/B3LYP/6-311++G(d,p) level. Our calculations are in harmony with the recent experimental and theoretical results, and reveal some new bonding and kinetic features of this reaction system. Our theoretical results may help the further identification of the products [BH2CS]- + HBS and [BH3CS] + BS- and may provide useful information on the chemical behaviors of other electron-deficient boron hydride anions.     

Global investigation on the potential energy surface of CH3CN: application of the scaled hypersphere search method

An extensive quantum chemical study of the potential energy surface (PES) for all possible isomerization and dissociation reactions of CH3CN is reported at the DFT (B3LYP/6-311++G(d,p)) and CCSD(T)/ cc-pVTZ//B3LYP/6-311++G(d,p) levels of theory. The pathways around the equilibrium structures can be discovered by the scaled hypersphere search (SHS) method, which enables us to make a global analysis of the potential energy surface for a given chemical composition in combination with a downhill-walk algorithm. Seventeen equilibrium structures and 59 interconversion transition states have been found on the singlet PES. The four lowest lying isomers with thermodynamic stability are also kinetically stable with the lowest conversion barriers of 49.69-101.53 kcal/mol at the CCSD(T)/cc-pVTZ//B3LYP/6-311++G(d,p) level, whereas three-membered-ring isomers c-CH2NCH, c-CH2CNH, and c-CHNHCH can be considered as metastable intermediates which can further convert into the low-lying chain-like isomers and higher lying acyclic isomers with the lowest conversion energies of 21.70-59.99 kcal/mol. Thirteen available dissociation channels depending on the different initial isomers have been identified. A prediction can be made for the possible mechanism explaining the migration of a hydrogen atom in competition with the CC bond dissociation. Several new energetically accessible pathways are found to be responsible for the migration of the hydrogen atom. The present results demonstrate that the SHS method is an efficient and powerful technique for global mapping of reaction pathways on PESs.     

NMR spectra, GIAO and charge density calculations of five-membered aromatic heterocycles

The B3LYP/6-31+G(d) molecular geometry optimized structures of 17 five-membered heterocycles were employed together with the gauge including atomic orbitals (GIAO) density functional theory (DFT) method at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p) and B3LYP/6-311+G(2d,p) levels of theory for the calculation of proton and carbon chemicals shifts and coupling constants. The method of geometry optimization for pyrrole (1), N-methylpyrrole (2) and thiophene (7) using the larger 6-311++G(d,p) basis sets at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p), B3LYP/6-31+G(2d,p) and B3LYP/cc-pVTZ levels of theory gave little difference between calculated and experimental values of coupling constants. In general, the (1)H and 13C chemical shifts for all compounds are in good agreement with theoretical calculations using the smaller 6-31 basis set. The values of nJHH(n=3, 4, 5) and rmnJ(CH)(n=1, 2, 3, 4) were predicted well using the larger 6-31+G(d,p) and 6-311++G(d,p) basis sets and at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p), B3LYP/6-31+G(2d,2p) levels of theory. The computed atomic charges [Mülliken; Natural Bond Orbital Analysis (NBO); Merz-Kollman (MK); CHELP and CHELPG] for the B3LYP/6-311++G(d,p) geometry optimized structures of 1-17 were used to explore correlations with the experimental proton and carbon chemical shifts.     

DFT-GIAO calculations of 19F NMR chemical shifts for perfluoro compounds

The (19)F NMR shieldings for 53 kinds of perfluoro compounds were calculated by the B3LYP-GIAO method using the 6-31G(d), 6-31+G(d), 6-31G(d,p), 6-31++G(d,p), 6-311G(d,p), 6-311++G(d,p), 6-311G(2d,2p), 6-311++G(2d,2p), 6-311++G(2df,2p), 6-311++G(3d,2p), and 6-311++G(3df,2p) basis sets. The diffuse functions markedly reduce the difference between the calculated and experimental chemical shifts. The calculations using the 6-31++G(d,p) basis set give the chemical shifts within 10 ppm deviations from experimental values except for the fluorine nuclei attached to an oxygen atom, a four- and a six-coordinated sulfur atom, and FC(CF(3))(2) attached to a sulfur atom.     

Structures and stabilities of HPO_2 isomers

The potential energy surface of HPO2 system including eight isomers and twelve transition states is predicated at MP2/6-311++G(d, p) and QCISD(t)/6-311++G(3df,2p)(single-point) levels of theory. On the potential energy surface, cis-HOPO(E1) is found to be thermodynamically and kinetically most stable isomer followed by trans-HOPO(E2) and HPO(O)(C2v, E3) at 10.99 and 48.36 kJ/mol higher, respectively. Based on the potential energy surface, only E1 and E3 are thermodynamically stable isomers, and should be experimentally observable. The products cis-HPOO(E5) and frans-HPOO(E6) in the first-step reaction of HP with O2 can isomerize into isomer E1 that has higher stability. The reaction of OH with PO will directly lead to the formation of isomer E1. The computed results are well consistent with the previous experimental studies.     

Structures and stabilities of HPS_2 isomers

The potential energy surface of HPS2 system containing nine isomers and fifteen transition states is obtained at MP2/6-311++G(d, p) and QCISD(t)/6-311++G(3df, 2p)(single-point) levels. On the potential energy surface, the lowest-lying frans-HSPS(EI) is found to be thermodynami-cally the most stable isomer followed by cis-HSPS(E2) and HP(S)S(C2v, E3) at 3.43 and 14.17 kJ/mol higher, respectively. The computed results show that species E1, E2, E3, stereo HP(S)S(Cs, E4) with PSS three-membered ring, isomers trans-HPSS(E5) and cis-HPSS(E6) which coexist with E4 are kinetically stable isomers. The products E6 and E5 in the reaction of HP with S2 can be isomerized into higher kinetic stable isomer E4 with 65.75 and 71.73 kJ/mol reaction barrier height, respectively. The predicated results may correct the possible inaccurate conclusion in that the product was experimentally assigned as isomer cis-HPSS(E6).     

Insight into global reaction mechanism of [C2, H4, O] system from ab initio calculations by the scaled hypersphere search method

A detailed computational study is performed on the singlet potential energy surface (PES) for possible isomerization and dissociation reactions of CH(3)CHO at the DFT (B3LYP/6-311++G(d,p)) and CCSD(T)/cc-pVTZ//B3LYP/6-311++G(d,p) levels. The pathways around the equilibrium structures can be discovered by the scaled hypersphere search (SHS) method, which enables us to make a global analysis of the PES for a given chemical composition. Fourteen isomers inclusive of 11 single-molecules and three "non-stabilized" oxygen-based ylides, 5 energetically favored complexes, and 79 interconversion transition states have been found on the singlet PES. Four lowest lying isomers with thermodynamic stability are also kinetically stable with respect to metastable intermediates. It was revealed that vinyl alcohols, which could be generated by the tautomerization of acetaldehyde, could undergo dissociation to form acetylene and water. In addition, recombination channels between some fragments, such as H(2)CO + (1)CH(2) and (1)CHOH + (1)CH(2), are energetically accessible via collision complex or oxygen-based ylides. Most of available unimolecular decompositions are found to be responsible for favorable hydrogen abstraction processes.     

On the flexibility of beta-peptides

The full conformational space was explored for an achiral and two chiral beta-peptide models: namely For-beta-Ala-NH2, For-beta-Abu-NH2, and For-beta-Aib-NH2. Stability and conformational properties of all three model systems were computed at different levels of theory: RHF/3-21G, B3LYP/6-311++G(d,p)//RHF/3-21G, B3LYP/6-311++G(d,p), MP2//B3LYP/6-311++G(d,p), CCSD//B3LYP/6-311++G(d,p), and CCSD(T)//B3LYP/6-311++G(d,p). In addition, ab initio E = E(phi, micro, psi) potential energy hypersurfaces of all three models were determined, and their topologies were analyzed to determine the inherent flexibility properties of these beta-peptide models. Fewer points were found and assigned than expected on the basis of Multidimensional Conformational Analysis (MDCA). Furthermore, it has been demonstrated, that the four-dimensional surface, E = E(phi, mu, psi), can be reduced into a three-dimensional one: E = E[phi, f(phi), psi]. This reduction of dimensionality of freedom of motion suggests that beta-peptides are less flexible than one would have thought. This agrees with experimental data published on the conformational properties of peptides composed of beta-amino acid residues.     

HPOS体系异构体结构与稳定性

在MP2／6－311＋＋G(d,p)和QCISD（T）／6－311＋＋G（3df,2p)（单点）水 平下计算得到了HPOS体系势能面上18个异构体和25个过渡态及解离碎片等驻点，并 分析了这些异构体的结构及异构化过程，讨论了可能的解离方式。在得到的异构体 中，有8个异构体是动力学较稳定的，它们是dis-HOPS,trans-HOPS,trans-HSPO, cis-HSPO,HP(O)S(Cs),trans-HPSO,cis-HPSO和HP（O）S（C1）。这些异构体在实 验中应该可以观测到。理论研究表明，P与S原子较强的超价能力在降低异构体能量 ，提高异构体动力学稳定性方面起到了关键的作用。得到的计算结果与HPO2， HPS2，HNOS等价电子相同的体系进行了比较。     

Theoretical investigation on the protonation reactions and products of the stable [N,C,C,S] isomers

A theoretical study on the protonation system of [N,C,C,S], [H,N,C,C,S]+, was performed at the B3LYP/6-311++G(d,p) and CCSD(T)/6-311++G(2df,2p) (single point) levels of theory. On the doublet [H,N,C,C,S]+ surface, 24 species were located as energy minima and 10 of them were considered as kinetically stable species. The species HNCCS+ with 2A' state and a shallow W-shaped skeleton was predicted to be the global minimum and kinetically the most stable species, being in good agreement with previous experimental findings. Furthermore, the protonation reactions of the stable [N,C,C,S] isomers were investigated in detail. The calculation results indicated that the [N,C,C,S] isomers may be significantly stabilized upon protonation. Finally, the possible covalent structures of the [H,N,C,C,S]+ isomers with considerable stability were briefly discussed.     

CAl4X (X=Si,Ge): molecules with simultaneous planar tetracoordinate carbon, aluminum, and silicon/germanium

The authors report the first theoretical study on the hexa-atomic molecules CAl(4)X (X=Si,Ge) at the B3LYP/6-311++G(2d), MP2/6-311++G(2d), and CCSD(T)/6-311++G(3df) (single point) levels. Three low-lying isomers (within 2.0 kcal/mol) can be formally viewed as constructed by one Al+ interacting with the planar CAl3X- at the side Al-X bond (X-1), side Al-Al bond (X-2), and central C atom (X-3). The isomers X-1 and X-2 both have planar structures that include the planar tetracoordinate carbon, aluminum, and silicon/germanium, while the three-dimensional isomer X-3 has the pentacoordinate carbon. The planarity of X-1 and X-2 is ascribed to the ligand five-center two-electron bonding molecular orbital, similar to the orbital responsible for the planarity of CAl3X- (X=Si,Ge). Kinetically, the two planar structures X-1 and X-2 can be easily interconverted to each other via the intermediate X-3, indicative of their coexistence. Of particular interest, isomer X-1 represents the first example that simultaneously contains three types of planar centers in a single molecule, to the best of our knowledge. The three low-lying and structurally interesting isomers X-1, X-2, and X-3 await future experimental verification. The present results could enrich the planar chemistry.     

HAsS2异构体结构与稳定性

在MP2/6-311++G(d,p)和QCISD(t)/6-311++G(3df,2p)(单点)水平下计算得到9个异构体和10个过渡态的HAsS₂体系势能面.异构体cis-HSAsS(E1)的能量最低,其次是trans-HSAsS(E2)、具有AsSS三元环的立体HAs(S)S(C_{s,E3)和HAs(S)S(C2v,E4)结构的异构体,能量分别比cis-HSAsS高1.46,60.78和93.63kJ/mol.根据体系的势能面,异构体E1,E2,E3和E4具有一定的动力学稳定性.AsH和S2第一步反应产物将会异构化为具有较高动力学稳定性的异构体E3,而SH和AsS第一步反应产物将会异构化为E1.计算结果与HNO2,HNS2,HPO2,HPS2和HAsO2等价电子相同的分子的势能面进行了比较.}     

Isomerizations of CH3SO2: Quantum Chemistry and Topological Study

The structures, stabilities and the isomerization reactions of CH₃SO₂ isomers in a doublet electronic state have been studied at B3LYP/6‐311+ +G (d,p), MP2/6‐311++G (d,p) and CCSD(T)/6‐311++G (d,p) levels. The three different levels of calculation give the similar results: thirteen minimum isomers were located and they were connected by eleven transition states. Among the thirteen isomers, cis‐CH₃OSO, trans‐CH₃OSO and CH₃SO₂ are the most stable species, and they should be detected easily in experiment. This is well consistent with the experimental result. These isomers could isomerize to each other by chemical bond vibration, chemical bond rotation and atom migration. The non‐planar ring structure transition state (STS), which was found in this paper, extended the concept of ring STS to the non‐planar systems.     

Theoretical studies on dynamics and thermochemistry of the reactions CF(3)CHCl(2)+Cl-->CF(3)CCl(2)+HCl and CF(3)CHFCl+Cl-->CF(3)CFCl+HCl

A dual-level direct dynamics study has been carried out for the two hydrogen abstraction reactions CF(3)CHCl(2)+Cl and CF(3)CHFCl+Cl. The geometries and frequencies of the stationary points are optimized at the BHLYP/6-311G(d,p), B3LYP/6-311G(d,p), and MP2/6-31G(d) levels, respectively, with single-point calculations for energy at the BHLYP/6-311++G(3df,2p), G3(MP2), and QCISD(T)/6-311G(d,p) levels. The enthalpies of formation for the species CF(3)CHCl(2), CF(3)CHFCl, CF(3)CCl(2), and CF(3)CFCl are evaluated at higher levels. With the information of the potential energy surface at BHLYP/6-311++G(3df,2p)//6-311G(d,p) level, we employ canonical variational transition-state theory with small-curvature tunneling correction to calculate the rate constants. The agreement between theoretical and experimental rate constants is good in the measured temperature range 276-382 K. The effect of fluorine substitution on reactivity of the C-H bond is discussed.     

Theoretical Study on Reaction Mechanism of Aluminum-Water System

A theoretical study on the reaction of aluminum with water in the gas phase was performed using the hybrid density functional B3LYP and QCISD(T) methods with the 6-311+G(d,p) and the 6-311++G(d,p) basis sets. The results show that there are three possible reaction pathways that involve four isomers, seven transition structures, and two possible products for the reaction of aluminum with water. The two most favorable reaction pathways were found, whose intermediates and products agreed quite well with experimental results. The enthalpy and Gibbs free energy change of the reaction between Al and H2O at 298 and 2000 K were calculated. Some results are also in good agreement with the previous calculations or experimental results.     

Structures and stabilities of HPS2 isomers

The potential energy surface of HPS² system containing nine isomers and fifteen transition states is obtained at MP2/6-311++G(d, p) and QCISD(t)/6-311++G(3df, 2p)(single-point) levels. On the potential energy surface, the lowest-lying trans-HSPS(E1) is found to be thermodynamically the most stable isomer followed by cis-HSPS(E2) and HP(S)S(C_2v, E3) at 3.43 and 14.17 kJ/mol higher, respectively. The computed results show that species E1, E2, E3, stereo HP(S)S(C_s, E4) with PSS three-membered ring, isomers trans-HPSS(E5) and cis-HPSS(E6) which coexist with E4 are kinetically stable isomers. The products E6 and E5 in the reaction of HP with S² can be isomerized into higher kinetic stable isomer E4 with 65.75 and 71.73 kJ/mol reaction barrier height, respectively. The predicated results may correct the possible inaccurate conclusion in that the product was experimentally assigned as isomer cis-HPSS(E6).     

An ab initio correlated study of the potential energy surface for the HOBr.H2O complex

The potential energy surface (PES) for the HOBr.H(2)O complex has been investigated using second- and fourth-order M?ller-Plesset perturbation theory (MP2, MP4) and coupled cluster theory with single and doubles excitations (CCSD), and a perturbative approximation of triple excitations (CCSD-T), correlated ab initio levels of theory employing basis sets of triple zeta quality with polarization and diffuse functions up to the 6-311++G(3dp,3df ) standard Pople's basis set. Six stationary points being three minima, two first-order transition state (TS) structures and one second-order TS were located on the PES. The global minimum syn and the anti equilibrium structure are virtually degenerated [DeltaE(ele-nuc) approximately 0.3 kcal mol(-1), CCSD-T/6-311++G(3df,3pd) value], with the third minima being approximately 4 kcal mol(-1) away. IRC analysis was performed to confirm the correct connectivity of the two first-order TS structures. The CCSD-T/6-311++G(3df,3pd)//MP2/6-311G(d,p) barrier for the syn<-->anti interconversion is 0.3 kcal mol(-1), indicating that a mixture of the syn and anti forms of the HOBr.H(2)O complex is likely to exist.     

Computational study of hydrogen-bonded complexes of HOCO with acids: HOCO⋯HCOOH, HOCO⋯H(2)SO(4), and HOCO⋯H(2)CO(3)

Quantum chemistry calculations at the density functional theory (DFT) (B3LYP), MP2, QCISD, QCISD(T), and CCSD(T) levels in conjunction with 6-311++G(2d,2p) and 6-311++G(2df,2p) basis sets have been performed to explore the binding energies of open-shell hydrogen bonded complexes formed between the HOCO radical (both cis-HOCO and trans-HOCO) and trans-HCOOH (formic acid), H(2)SO(4) (sulfuric acid), and cis-cis-H(2)CO(3) (carbonic acid). Calculations at the CCSD(T)∕6-311++G(2df,2p) level predict that these open-shell complexes have relatively large binding energies ranging between 9.4 to 13.5 kcal∕mol and that cis-HOCO (cH) binds more strongly compared to trans-HOCO in these complexes. The zero-point-energy-corrected binding strengths of the cH?Acid complexes are comparable to that of the formic acid homodimer complex (～13-14 kcal∕mol). Infrared fundamental frequencies and intensities of the complexes are computed within the harmonic approximation. Infrared spectroscopy is suggested as a potential useful tool for detection of these HOCO?Acid complexes in the laboratory as well as in various planetary atmospheres since complex formation is found to induce large frequency shifts and intensity enhancement of the H-bonded OH stretching fundamental relative to that of the corresponding parent monomers. Finally, the ability of an acid molecule such as formic acid to catalyze the inter-conversion between the cis- and trans-HOCO isomers in the gas phase is also discussed.