NC2S+离子的结构和稳定性的理论研究

在密度泛函和从头算理论水平下计算了单重态的NC2S+离子的结构、能量、光谱以及稳定性. 在B3LYP/6-311G(d)水平下, 得到8个异构体, 它们由15个过渡态相连接. 在CCSD(T)/6-311+G(2df)//QCISD/6-311G(d)+ZPVE水平下, 得到能量最低的异构体是直线型的具有1Σ电子态的NCCS+(1)(0.0 kJ/mol), 其次是直线型的异构体CNCS+(2)(54.8 kJ/mol). 两个低能量的异构体1和2及另外一个高能量的直线型异构体CCNS+(3)(323.8 kJ/mol)都具有相当大的动力学稳定性, 这三个异构体在具备一定条件的实验室和星际条件下是可以进行观测的. 分析了这3个异构体的成键性质.     

高级量子化学从头计算法研究N2和H2O分子间相互作用

在MP2/6-311++G(3d,3p)电子相关校正水平上,对N₂和H₂O分子间可能存在的氢键复合物进行全自由度能量梯度优化,发现了一个接近于直线的弱氢键总能量极小结构(1),进一步在高级电子相关校正的MP4SDTQ和CCSD(T)水平,用6-311++G(3d,3p)基组加上(3s3p2d1f)键函数,用MP4和CCSD(T)计算的结构1的结合能分别为-5.061kJ/mol和-4.715kJ/mol.     

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.     

OH+C_2H_2N←C_2H_3+NO→OH+C_2H_2N反应机理的密度泛函理论研究

应用密度泛函理论研究了反应通道(a)C2H3+NO→CH3+NCO和(b)C2H3+NO→OH+C2H2N的反应机理.在B3LYP/6-31G(d)水平上优化了反应物、中间体、过滤态、产物的几何构型,通过频率分析确定了11个中间体和10个过渡态.所有的反应物、中间体、过渡态、产物都在CCSD/6-311++G(d,p)水平上进行了单点能较正.并讨论了反应的异构化过程.计算结果表明10是能量最低的中间体,比反应物的能量低308 479kJ/mol;过渡态1/3,2/5,3/4,4/8比反应物的能量高,其中3/4是能量最高的过渡态,比反应物的能量高91 894kJ/mol.通道(a)和(b)的理论放热值分别为111 059和96 619kJ/mol.     

HBO2异构体的结构和相对稳定性

用ab initio方法在MP2/6 311＋＋G(d,p)水平下优化得到了HBO2体系的若干异构体和过渡态，并在QCISD(t)/6 311＋＋G(3df,2p)//MP2/6 311＋＋G(d,p)水平下进行了单点能量校正.对计算结果的分析表明，无论是在热力学还是在动力学上，具有链状结构的HOBO异构体(E1)是势能面上最稳定的结构，并对E1的电子结构进行了分析；另一具有C2v对称性的HBO(O)结构的异构体(E2)的能量比E1高381.72 kJ•mol－1，由于E2处于一个较深的势垒中，因此是比较稳定的，可以推断，在适合的实验中应该可以观察到异构体E2.     

B4O分子异构化稳定性的理论研究

采用CCSD(T)/6-311+G(2df)//B3LYP/6-311+G(d)方法, 系统研究了B₄O体系各个异构体的结构和能量, 以及重要异构体的解离和异构化稳定性. 结果表明, 单态平面三角形含-BO单元的异构体cB₃-BO ¹1能量最低, 其次是带状的异构体B₄O ¹2(10.9 kJ/mol), 并且¹1和¹2结构都具有良好的动力学稳定性. 因此对于B₄O体系, ¹1和¹2都是有可能存在的. 而文献报道的三态直线型结构BBBBO(146.0 kJ/mol)的能量比异构体¹1和¹2高得多.     

HPS2异构体结构与相对稳定性

在MP2/6-311++G(d,p)和QCISD(t)/6-311++G(3df,2p)(单点)水平下计算得到了包括9个异构体和15个过渡态的HPS2势能面.其中,异构体trans-HSPS(E1)的能量最低, 其次是cis-HSPS(E2)和HPS(S)(C2_V,E3),能量分别比trans-HSPS(E1)高3.43和14.17 kJ/mol.计算结果表明,异构体E1,E2,E3和立体的三元环结构的异构体HP(S)S(C_S,E4), 及与E4共存的trans-HPSS(E5)和cis-HPSS(E6)具有一定的稳定性.在QCISD(t)/6-311++G(3df,2p)//MP2/6-311++G(d,p)并包含零点能水平下,PH和S₂反应生成的E6和E5分别越过65.75和71.73 kJ/mol的势垒就可以异构化为具有较高动力学稳定性的产物E4, 计算结果对实验认定的产物是cis-HPSS(E6)的结论进行了修正.     

(H_2O)_n(n=0～2)催化HS和HOCl反应机理的理论研究

在CCSD(T)/6-311+G(3df,2p)//M06-2X/6-311+G(3df,2p)水平上研究了(H_2O)n(n=0~2)催化HS和HOCl的反应机理.结果表明,HS与HOCl反应中HS夺取HOCl上的H原子形成产物H_2S和ClO.在无水催化时,该反应存在2种不同的路径(分别经过过渡态TS1和TS2,二者互为顺反结构),对应的能垒分别为100.28和100.91kJ/mol,到达产物(H_2S+ClO)需吸收18.99kJ/mol能量,反应不易发生;在单个水分子参与时,水分子可通过形成弱相互作用或者作为H原子转移桥梁影响反应机理,获得了4种水催化路径,能垒(间于53.97~92.39kJ/mol之间)均低于无水催化过程.同时发现,在反应到达产物前,水分子可以与产物形成中间体IM,IM相对能仅为0.46kJ/mol,有利于产物形成;有2个水分子参与反应时,找到了3条催化路径,最优反应路径过渡态TS7的能垒为45.05kJ/mol,低于无水催化过程,相比单个水分子最优路径能垒(53.97kJ/mol)并无显著降低.     

OH+ C2H2N←C2H3 + NO→CH3 + NCO反应机理的密度泛函理论研究

应用密度泛函理论研究了反应通道(a)C2H3 NO→CH3 NCO和(b)C2H3 NO→OH C2H2N的反应机理．在B3LYP／6-31G(d)水平上优化了反应物、中间体、过滤态、产物的几何构型，通过频率分析确定了11个中间体和10个过渡态．所有的反应物、中间体、过渡态、产物都在CCSD／6-311 G(d，P)水平上进行了单点能较正．并讨论了反应的异构化过程．计算结果表明10是能量最低的中间体，比反应物的能量低308．479kJ／mol；过渡态1／3，2／5，3／4，4／8比反应物的能量高，其中3／4是能量最高的过渡态，比反应物的能量高91．894kJ／mol．通道(a)和(b)的理论放热值分别为111．059和96．619kJ／mol．     

SiNP分子结构及稳定性的理论研究

采用B3LYP/6-311G(d),QCISD和CCSD(T)方法,对单重态和三重态SiNP体系的异构化进行了研究.在QCISD/6-311G(d)水平下,得到了7个稳定的异构体,它们由4个过渡态所连接,其中三重态线型异构体SiNP(31,3Σ1),单重态环状异构体SiNP(14,1A')和单重态线型异构体SiNP(11,1Σ)都具有较大的热力学及动力学稳定性.     

Theoretical study on stability and properties of NC2O isomers

The structures, energetics, spectroscopies, and stabilities of the doublet NC(2)O radical are explored at density functional theory and ab initio levels. Nine minimum isomers are located connected by 22 interconversion transition states. At the CCSD(T)/6-311+G(2df)//QCISD/6-311G(d)+ZPVE level, the lowest-lying isomer is bent NCCO 1 (0.0 kcal/mol) with (2)A' state followed by bent isomer CNCO 2 (16.7). Two isomers (1 and 2) and another high-lying species CCNO 4 (99.4) with bent structure are considerably stabilized by a barrier of at least 20 kcal/mol. All of the three isomers should be experimentally or astrophysically observable. This result is consistent with their indication of neutralization-reionization mass spectrometry experiments. Also, the calculated spectroscopic properties and bond distances of known NCCO 1 are consistent with recent experimental observations and theoretical studies. The bonding natures of the isomers 1, 2, and 4 are analyzed. Their molecular properties including the heats of formation, adiabatic ionization potentials, and adiabatic electronic affinities are calculated at the higher levels G3//B3LYP, G3(MP2)//B3LYP, QCISD, and CCSD(T) (single-point). Possible formation strategies of the isomers 1, 2, and 4 in laboratory and space are also discussed in detail.     

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.     

Structures and stability of medium silicon clusters. II. Ab initio molecular orbital calculations of Si12-Si20

Ab initio all-electron molecular-orbital calculations are carried out to study the structures and relative stability of low-energy silicon clusters (Si(n),n = 12-20). Selected geometric isomers include those predicted by Ho et al. [Nature (London) 392, 582 (1998)] based on an unbiased search with tight-binding/genetic algorithm, as well as those found by Rata et al. [Phys. Rev. Lett. 85, 546 (2000)] based on density-functional tight-binding/single-parent evolution algorithm. These geometric isomers are optimized at the M?ller-Plesset (MP2) MP2/6-31G(d) level. The single-point energy at the coupled-cluster single and double substitutions (including triple excitations) [CCSD(T)] CCSD(T)/6-31G(d) level for several low-lying isomers are further computed. Harmonic vibrational frequency analysis at the MP2/6-31G(d) level of theory is also undertaken to assure that the optimized geometries are stable. For Si12-Si17 and Si19 the isomer with the lowest-energy at the CCSD(T)/6-31G(d) level is the same as that predicted by Ho et al., whereas for Si18 and Si20, the same as predicted by Rata et al. However, for Si14 and Si15, the vibrational frequency analysis indicates that the isomer with the lowest CCSD(T)/6-31G(d) single-point energy gives rise to imaginary frequencies. Small structural perturbation onto the Si14 and Si15 isomers can remove the imaginary frequencies and results in new isomers with slightly lower MP2/6-31G(d) energy; however the new isomers have a higher single-point energy at the CCSD(T)/6-31G(d) level. For most Si(n) (n = 12-18,20) the low-lying isomers are prolate in shape, whereas for Si19 a spherical-like isomer is slightly lower in energy at the CCSD(T)/6-31G(d) level than low-lying prolate isomers.     

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).     

Theoretical Studies on Structures and Stabilities of C4H2+ Isomers

The structures, energies, stabilities and spectroscopies of doublet C₄H₂⁺ cations were explored at the DFT/B3LYP/6-311G(d,p), CCSD(T)/6-311+G(2df,2pd)(single-point), and G3B3 levels. Ten minimum isomers including the chainlike, three-member-ring, and four-member-ring structures are interconverted by means of 15 interconversion transition states. The potential energy surface was investigated. At the CCSD(T)/6-311+G(2df,2pd) and G3B3 levels, the global minimum isomer was found to be a linear HCCCCH. The structures of the stable isomer and its relevant transition state are further optimized at the QCISD/6-311G(d,p) level. The bonding nature and structure of isomer HCCCCH were analyzed.     

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.     

HAsO~2异构体结构、相对稳定性与体系势能面

在MP2/6-311++G(d,p)和QCISD(T)/6-311++G(3df,2p)(单点)水平下计算得到了包括9个异构体和10个过滤态的HAsO~2体系势能面。在势能面上，异构体cis-HOAsO(E1)的能量是最低的，其次是trans-HOAsO(E2)和HAsO(O)(C~2~V,E3)，能量分别比cis-HOAsO高13.15和192.74kJ/mol。根据体系的势能面，异构体E1，E3及cis-HOOAs(E6),trans-HOOAs(E5)具有一定的动力学稳定性，在实验中应该可以观测到。AsH和O~2反应的第一步产物将会异构化为具有较高动力学稳定性的异构体E3；而OH和AsO反应可直接生成E1。计算结果与HPO~2,HPS~2,HNO~2,HNS~2等价电子相同的分子的势能面进行了比较。