C3H4: Theoretical study of structures and stabilities of isomers

The various isomers including stable structures, carbenes, and diradicals on the C₃H₄ surface have been investigated. The two carbenes propenylidene and cyclopropylidene have been found to have singlet ground states. Vinylmethylene is predicted to have a triplet ground state with a planar diradical type of structure. The syn and anti forms of this state are degenerate. This is in agreement with the observation of two triplet states in the electron spin resonance (ESR ) spectra. The π electrons are found to be delocalized over the three carbons. The singlet diradical structures are found to be more stable than the carbene structures, which retain the CH₂ (DOUBLE BOND) CH allylic structures. The orbital compositions of the frontier orbitals of all systems have been determined to examine the nature of these orbitals. © 1996 John Wiley & Sons, Inc.     

Theoretical study on structures and stabilities of [H,Ge,C,N

Theoretical investigations are performed for the first time on the simplest hydrogenated germanium cyanide [H,Ge,C,N], whose analogs [H,C(2),N] and [H,Si,C,N] have been detected in space and laboratory, respectively. The detailed potential energy surfaces in both singlet and triplet states are constructed at the CCSD(T)/6-311+G(3df,2p)//B3LYP/6-31G(d)+ZPVE level, including 18 minimum isomers and 26 interconversion transition states. The former three low-lying and kinetically stabilized isomers are HGeCN (1)1 (0.0 kcal/mol), HGeNC (1)2 (5.1 kcal/mol), and cyclic cCHNGe(1)7 (11.1 kcal/mol). In addition, five isomers HCNGe (1)3 (33.8), HNCGe (1)5 (29.8), cNHCGe (1)8 (37.9), HGeCN (3)1 (30.1), and HNCGe (3)5 (26.5) each have considerable barriers, despite their high energies. Future laboratory characterization and astrophysical detection of the eight [H,Ge,C,N] isomers, especially the former three low-lying species (1)1, (1)2, and (1)7, are highly recommended. The accurate spectroscopic data at the QCISD/6-311G(d,p) level are provided. For some species, the CBS-QB3 calculations are also performed. Wherever possible, comparisons with the analogous [H,C(2),N] and [H,Si,C,N] are made on the structural, energetic, and bonding properties.     

Theoretical study on the structures and stabilities of the isomers of N6H6

Forty-eight possible isomers of N₆H₆ have been investigated at the B3LYP/6-311++G^** and G3B3 levels. The results show that heats of formation of all isomers are positive and A2-3 is the lowest and C2 is the highest. In addition, the energies and the heats of formation of the ring-shaped isomers are higher than those of the chain-shaped isomers. Among the ring-shaped isomers, the four-membered ring isomers have the highest heat of formation and the five-membered ring isomers have the lowest. For six-membered ring isomers, only the chair-shaped isomers are stable. Among all those chain-shaped isomers, the ones which have N–N bond and its length is in the range of NN double bond can improve the stability of the compounds.     

Theoretical study on stabilities of multiple hydrogen bonded dimers

A series of self-constituted multiple hydrogen bonded (MHB) complexes has been investigated systematically by density functional theory (PBE1PBE /6-31G**), the Morokuma energy decomposition method (HF/6-31G**) and MP2 (6-31G** and 6-311++G**) calculation. We have discovered that (i) for doubly hydrogen bonded (DHB) complexes, both the interaction energy and stability increase with the charge transfer energy; (ii) for quadruple hydrogen bonded (QHB) complexes, cooperativity is the most important factor determining stability of the complex: stronger cooperative energy correlates well with larger interaction energy and thus more stable complex and vice versa; (iii) correlation energy plays an important role in intermolecular interactions. The correlation energy, mainly consisting of dispersive energy, also exhibits cooperativity in MHB dimers: positive for M-aadd and generally negative for other complexes.     

Ab initio study on structures and stabilities of OLi n hyperlithiated compounds

The geometries of hyperlithiated compounds OLi_n were optimized by means of HF, MP2 and DFT methods with 6-31G basis set. The dissociation energies of those optimized stable geometries of OLi_n were calculated, the results are in good agreement with experimental values; and moreover, the dissociation energy of OLi_6 is predicted. In addition, the fundamental vibrational frequencies were also predicted.     

Theoretical studies on carbon and silicon clusters: comparison of the structures and stabilities of neutral and ionic forms

Optimized molecular geometries and electronic structures are determined for neutral, positively charged, and negatively charged carbon and silicon clusters containing up to ten atoms. The effects of polarization functions and electron correlation are included in these claculations. Carbon clusters have linear or monocyclic ground state geometries whereas silicon clusters containing five or more atoms have three-dimensional ground state structures. Neutral C₄, C₆ and C₈ all have linear and monocyclic isomers of comparable stability whereas the ionic forms appear to be generally more stable as linear geometrical arrangements. In the case of neutral and positively charged carbon clusters, the odd-numbered clusters are significantly more stable than the adjacent even-numbered clusters whereas the opposite order of stability occurs for the negative ions. This is due to the large values of the electron affinities of the linear forms of even-numbered clusters such as C₄ and C₆. The relative stabilities of silicon clusters does not change with the charge state of the clusters.     

LiNiO2及掺杂化合物的结构与稳定性的第一性原理研究

应用第一原理局域密度泛函对LiNiO2及其掺杂化合物的晶体结构、能带结构和态密度进行了研究.结果表明:锂镍氧系正极材料是电子的良导体,在充放电过程中具有良好的稳定性,适合做锂离子电池的正极材料;在LiNiO2晶体中主要是O和Ni之间成键,镍离子和氧离子的相互作用比较强,Li在嵌入层状结构材料后部分失去电子,以离子状态存在.     

YCO分子结构和光谱性质的理论研究

在相对论有效原子实势(RECP)近似下,用密度泛函(B3LYP／SDD) 方法,计算了YCO分子各种几何构型的结构性质．结果表明:YCO分子的基态 构型为Cs构型,基电子态为2A″,基态离解能和零点振动能分别为16．126 7 eV 和14．780 2 kJ／mol;研究得到YCO分子基态谐振频率为v1(A′)=1 968．699 7 cm-1,v2(A′)=215．119 3 cm-1,v3(A′)=287．245 4 cm-1;力常数为f11= 4．475 3×10-16 J／nm2,f12=1．620 3×10-17 J／nm2,f22=2．887 6×10-17 J／nm2,faa=1．459 6×10-19 J／rad2;基态YCO分子的能隙(HLG)较小,化学活 性较强．     

Theoretical study on structures and stabilities of N4X (X = O,S, Se,Te) series

The stable and transition structures of N₄X (X = O, S, Se, Te) series with singlet state are optimized with the ab initio (MP2) and density functional theory (B3LYP) methods using the 6‐311+G(d) basis set. The ring isomers are found to be the global minima for N₄O, N₄S, N₄Se, and the chain isomer is the minimum for N₄Te. The stabilities are studied by evaluating the dissociation barriers with respect to dissociation. The reactants and products connected by transition structures are determined by applying the intrinsic reaction coordinate (IRC) calculations. The C_2v, C_3v and ring isomers decompose into linear NNX and N₂ molecules, however, the chain isomers decompose into cyclic N₂X and N₂ firstly. A new possible isomerization mechanism between the cyclic and linear structures of N₂X series is studied. The cyclic structures of N₂X convert into linear structures easily with the very low barriers. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Theoretical studies of structures and stabilities of a new odd-numbered fullerene dimer: C141

The possible isomers of a newly synthesized C(141) molecule are calculated using MNDO, AM1, PM3, B3LYP/3-21G, and B3LYP/6-31G(d) methods. The geometry optimizations showed that the isomer 8-8 has the lowest total energy in all 64 possible structures of C(141). Unlike those of C(130), C(140), etc., the C(141) 8-8 shows a new structure: two C(70) side cages open [6.6] ring junctions located at the equator (instead of cap) area to create new chemical bonds for the bridge atom. Theoretical measurements of the average length of the long and short axes of C(70) side cages in the C(141) molecule reveal that when two C(70) cages are connected with each other at the equators, their geometric shapes become more spherical compared with the pristine C(70); this leads to a reduction of the molecular polarizability. Analysis of the local and global strain indicates that the global strain of C(70) monomer in the C(141) 8-8 is greatly reduced compared to the pristine C(70). The stable C(70) derivatives that are formed with reacted C-C bonds in the equator area may put new insights into fullerene chemistry, in particular, for C(70) to react with a large molecule. The results are discussed together with the experimental data.     

秋水仙碱立体异构体电子结构及圆二色谱性质的理论研究

采用量子化学密度泛函理论(DFT)在B3LYP/6-311++G二水平上对秋水仙碱四个立体异构体分子几何构型进行了优化,在优化的基础上进行了振动圆二色谱(VCD),紫外-可见光谱(UV-Vis)和电子圆二色谱(ECD)研究.为模拟真实条件,以水为溶剂,计算其对分子电子结构和光谱性质的影响.研究结果表明:秋水仙碱四个立体...     

层间化合物hNB—Cu稳定性的理论研究

以分子簇为模型，用从头算方法，在６－３１Ｇ＾＊基组水平上对六方氮化硼（ｈＢＮ）进行几何全优化，再用优化构型对Ｃｕ的六方氮化硼层间化合物（ｈＮＢ－Ｃｕ）做单点计算，根据计算结果，从层间距、Ｍｕｌｌｉｋｅｎ布居、轨道相互作用、原子净电荷及前线轨道等角度分析了ｈＢＮ－Ｃｕ的电子结构及其稳定性，通过劝分析计算确认了这一层间化合物只是一个介稳结构，阐明了Ｃｕ使六方氮化硼陶瓷致密化的作用机理以及ｈＮＢ－Ｃｕ的弱抗氧化性。     

槲皮素及其异构体分子的结构性质研究

槲皮素是一类有着重要药理活性的黄酮类化合物。本文在B3LYP/6-31G**水平上对槲皮素及其14种异构体进行了几何优化,得到各种异构体的结构与性质。由电荷布居数分析出B环是此类多羟基化合物的活性部分,3位-OH的存在抑制了A、C环的活性。同时,表明此类化合物中必有分子内氢键形成。并给出了最稳构型、槲皮素和标准结构三类分子的振动光谱。探讨了它们的构效关系。     

ANTA的结构、性质及其互变异构的理论研究

对3-硝基-5-氨基-1,2,4-三唑(ANTA)的三种异构体,1H-ANTA(Ⅰ),2－ANTA(Ⅱ)和4H-ANTA(Ⅲ)在,bainitio-HF/3-21G和DFT-B3LYP/3-21G势能面计算的基础上,进行6-311G^**几何参数全优化,MP2总能量和SCRF溶剂(四氢呋喃)效应计算。以振动分析和统计热力学为基础,作标题物热力学性质以及Ⅰ和Ⅱ之间的互变异构反应计算,求得分子几何,电子结构和300～1000K范围的焓、熵和热容以及Ⅰ和Ⅱ互变异构平衡常数和速率常数。发现在三种异构体中在通常温度下以Ⅱ在气相下最稳定,Ⅰ在溶液中最稳定。低温下难以发生异构化反应,温度可提高Ⅰ与Ⅱ之间的互变速率,在800K时两种异构体在气相中等量共存;大于800K时Ⅰ更为稳定。     

Theoretical study on structures and stability of Si2CP isomers

The structures, energetics, spectroscopies, and isomerization of various doublet Si2CP species are explored theoretically. In contrast to the previously studied SiC2N and SiC2P radicals that have linear SiCCN and SiCCP ground states, the title Si2CP radical has a four-membered-ring form cSiSiPC 1 (0.0 kcal/mol) with Si-C cross-bonding as the ground-state isomer at the CCSD(T)/6-311G(2df)//B3LYP/6-311G(d)+ZPVE level, similar to the Si2CN radical. The second low-lying isomer 2 at 11.6 kcal/mol has a SiCSiP four-membered ring with C-P cross-bonding, yet it is kinetically quite unstable toward conversion to 1 with a barrier of 3.5 kcal/mol. In addition, three cyclic species with divalent carbene character, i.e., cSiSiCP 7, 7' with C-P cross-bonding and cSiCSiP 8 with Si-Si cross-bonding, are found to possess considerable kinetic stability, although they are energetically high lying at 44.4, 46.5, and 41.4 kcal/mol, respectively. Moreover, a linear isomer SiCSiP 5 at 44.3 kcal/mol also has considerable kinetic stability and predominantly features the interesting cumulenic /Si=C=Si=P/* form with a slight contribution from the silicon-phosphorus triply bonded form /Si=C*-Si[triple bond]P/. The silicon-carbon triply bonded form *Si[triple bond]C-Si[triple bond]P/ has negligible contribution. All five isomers are expected to be observable in low-temperature environments. Their bonding nature and possible formation strategies are discussed. For relevant species, the QCISD/6-311G(d) and CCSD(T)/6-311+G(2df) (single-point) calculations are performed to provide more reliable results. The calculated results are compared to those of the analogous C3N, C3P, SiC2N, and Si2CN radicals with 17 valence electrons. Implications in interstellar space and P-doped SiC vaporization processes are also discussed.     

Theoretical study on structures and stability of C4P isomers

The structures, energetics, spectroscopies, and stabilities of doublet C(4)P isomeric species are explored at the DFT/B3LYP, QCISD, and CCSD(T) (singlet-point) levels. A total of 12 minimum isomers and 27 interconversion transition states are located. At the CCSD(T)/6-311G(2df)//QCISD/6-311G(d)+ZPVE level, the lowest-lying isomer is a floppy CCCCP 1 (0.0 kcal/mol) mainly featuring a cumulenic structure |C=C=C=C=P*|, which differs much from the analogous C4N radical (|*C-C[triple bond]C-C[triple bond]N|). The quasi-linearity and the low bending mode of 1 are in contrast to the previous prediction. The second energetically followed isomer PC-cCCC 3 (14.9 kcal/mol) possesses a CCC ring-bonded to CP. The two low-lying isomers are separated by a high-energy ring-closure/open transition state (26.5 kcal/mol) and thus are very promising candidates for future laboratory and astrophysical detection. Furthermore, four high-energy isomers, that is, two bent isomers CCPCC 2 (68.4 kcal/mol) and CCPCC 2' (68.5 kcal/mol) and two cagelike species 10 (56.0 kcal/mol) and 11 (67.9 kcal/mol), are also stabilized by considerable barriers. The present work is the first detailed potential energy survey of CnP clusters and can provide useful information for the investigation of larger CnP radicals and for understanding the isomerism of P-doped C vaporization processes.     

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     

三重态SiCP2异构体的结构和稳定性的理论研究

采用DFT,QCISD和CCSD(T)等理论计算方法对三重态SiCP2异构体的结构和稳定性进行了理论研究.在B3LYP/6-311G(d)水平下,共计算得到由17个过渡态相连接的15个异构体.在CCSD(T)/6-311 +G(2df)//QCISD/6-311G(d)水平下,考虑重点振动能相对能量最低的三元环状异构体P-cCSiP 8(0.0 kJ/mol)及四元环状结构的cPCSiP 4具有相当大的动力学稳定性,在一定的实验室和星际条件下可能被检测到.另外,对它们的成键性质也进行了分析.     

Theoretical study on the structures, isomerization and stability of SiC4 isomers

The structures, energetics, dipole moments, vibrational spectra, rotational constants, and isomerization of singlet SiC₄ isomers were explored using ab initio methods. Five types of isomers, a total of 11 minima, connected by 11 interconversion transition states, were located on the potential energy surface at the MP2/6-311G(d, p) level. More accurate energies were obtained at the G3(MP2) level. With the highest isomerization barrier, a C2v tetra-angular cone possesses the largest kinetic stability. The lowest-lying structure, linear SiCCCC is also highly kinetically stabilized. Besides, D2d bicyclic c-Si(CC)₂, C2v five-membered ring c-SiCCCC, another C2v tetra-angular cone isomer and C3v trigonal bipyramid isomer are also considered to be kinetically stable, because their isomerization barriers are all over 10 kcal/mol. Other isomers cannot be kinetically stabilized with considerably low isomerization barriers. Investigation on the vibrational spectra, dipole moments, and rotational constants for SiC₄ isomers are valuable for their detections in the interstellar space and laboratory.