Electronic spectra of linear isoelectronic clusters C2n+1S and C2n+1Cl+ (n = 0-4): an ab initio study

Structures and stabilities of linear carbon chains C2n+1S and C2n+1Cl+ (n=0-4) in their ground states have been investigated by the CCSD and B3LYP approaches. The CASSCF calculations have been used to determine geometries of selected excited states of both isoelectronic series. Linear C2n+1S cluster has a cumulenic carbon framework, whereas its isoelectronic C2n+1Cl+ has a dominant character of acetylenic structure in the vicinity of terminal Cl. The vertical excitation energies of low-lying excited states have been calculated by the CASPT2 method. Calculations show that the excitation energies have nonlinear size dependence. The 2(1)Sigma+<--X1Sigma+ transition energy in C2n+1S has a limit of 1.78 eV, as the chain size is long enough. The predicted vertical excitation energies for relatively strong 1(1)Pi<--X1Sigma+ and 2(1)Sigma+<--X1Sigma+ transitions are in reasonable agreement with available experimental values. The spin-orbit effect on the spin-forbidden transition in both series is generally small, and the enhancement of the spin-forbidden transition by spin-orbit coupling exhibits geometrical and electronic structural dependence.     

Bonding in SCln (n = 1-6): a quantum chemical study

Following a previous study of bonding and isomerism in the SF(n) and singly chloro-substituted SF(n-1)Cl (n = 1-6) series, we describe bonding in the ground and low-lying excited states of the completely substituted series, SCl(n) (n = 1-6). All structures were characterized at least at the RCCSD(T)/aug-cc-pV(Q+d)Z level of theory. Both differences and similarities were observed between SCl(n) and our previous results on SF(n-1)Cl and SF(n). Several minimum structures that exist in SF(n) and SF(n-1)Cl are absent in SCl(n). For example, the optimized structure of SCl(2)((3)A(2)) is a transition state in C(s) symmetry, whereas the analogous states are minima in SF(n) and SF(n-1)Cl. Second, we found a continuation of a trend discovered in the SF(n-1)Cl series, where Cl substitution has a destabilizing effect that weakens bonds with respect to SF(n). This effect is much stronger in the SCl(n) series than it is in the SF(n-1)Cl series, which is why SCl(2) is the most stable observed species in the family and why SCl(4), SCl(5), and SCl(6) are unstable (SCl(n-2) + Cl(2) additions are endothermic for n = 4-6).     

The ground and excited states of polyenyl radicals C2n-1H2n + 1 (n = 2-13): a valence bond study.

The semiempirical valence bond (VB) method, VBDFT(s), is applied to the ground states and the covalent excited states of polyenyl radicals C2n - 1H2n + 1 (n = 2-13). The method uses a single scalable parameter with a value that carries over from the study of the covalent excited states of polyenes (W. Wu, D. Danovich, A. Shurki, S. Shaik, J. Phys. Chem. A, 2000, 104, 8744). Whenever comparison is possible, the VB excitation energies are found to be in good accord with sophisticated molecular orbital (MO)-based methods like CASPT2. The symmetry-adapted Rumer structures are used to discuss the state-symmetry and VB constitution of the ground and excited states, and the expansion to VB determinants is used to gain insight on spin density patterns. The theory helps to understand in a coherent and lucid manner the properties of polyenyl radicals, such as the makeup of the various states, their geometries and energies, and the distribution of the unpaired electrons (the neutral solitons).     

Structure and bonding in third-row main group dicarbides C2X (X=K-Br)

The molecular structures of third-row main group dicarbides C(2)X (X=K-Br) have been studied by theoretical methods. It is found that K, Ca, and Ga favor C(2v)-symmetric (T-shape) ground states, whereas As, Se, and Br have linear or quasilinear ground states. In the case of germanium. a very flat potential energy surface is found and an L-shape structure seems to be the ground state. Dissociation energies into X+C(2) are relatively high. The main features of these compounds, in particular, the preference for linear or cyclic structures, have been rationalized in terms of the most relevant interactions between the third-row atom and dicarbon with the help of an energy decomposition analysis.     

Binding properties of Cu(+/2+)-(glycyl)n glycine complexes (n = 1-3)

The structure, relative energies, and binding energies of the complexes formed by the interaction of Cu+ (d10,1S) and Cu2+ (d9,2D) cations with the (glycyl)n glycine (n = 1-3) oligomers have been theoretically determined by means of density functional methods. The most stable structures of the Cu+ systems present linear dicoordination geometries, in agreement with a recent X-ray absorption spectroscopic study of Cu(I) interacting with model dipeptides. This is attributed to an efficient reduction of metal-ligand repulsion through sd sigma hybridization in dicoordinated linear structures. In contrast, for Cu2+ systems the lowest energy structures are tricoordinated (n = 1), tetracoordinated (n = 2), and pentacoordinated (n = 3). For both copper cations, binding energy values show that the interaction energies increase when the peptide chain is elongated. Differences on the coordination properties of the ligands are discussed according to their length as well as to the electronic configuration of the metal cations, which are compared to the Cu+/2+-glycine systems.     

Multireference configuration interaction calculations for the F(2P)+HCl-->HF+Cl(2P) reaction: a correlation scaled ground state (1 2A') potential energy surface

This paper presents a new ground state (1 (2)A(')) electronic potential energy surface for the F((2)P)+HCl-->HF+Cl((2)P) reaction. The ab initio calculations are done at the multireference configuration interaction+Davidson correction (MRCI+Q) level of theory by complete basis set extrapolation of the aug-cc-pVnZ (n=2,3,4) energies. Due to low-lying charge transfer states in the transition state region, the molecular orbitals are obtained by six-state dynamically weighted multichannel self-consistent field methods. Additional perturbative refinement of the energies is achieved by implementing simple one-parameter correlation energy scaling to reproduce the experimental exothermicity (DeltaE=-33.06 kcalmol) for the reaction. Ab initio points are fitted to an analytical function based on sum of two- and three-body contributions, yielding a rms deviation of <0.3 kcalmol for all geometries below 10 kcalmol above the barrier. Of particular relevance to nonadiabatic dynamics, the calculations show significant multireference character in the transition state region, which is located 3.8 kcalmol with respect to F+HCl reactants and features a strongly bent F-H-Cl transition state geometry (theta approximately 123.5 degrees ). Finally, the surface also exhibits two conical intersection seams that are energetically accessible at low collision energies. These seams arise naturally from allowed crossings in the C(infinityv) linear configuration that become avoided in C(s) bent configurations of both the reactant and product, and should be a hallmark of all X-H-Y atom transfer reaction dynamics between ((2)P) halogen atoms.     

(2+1) resonance-enhanced ionization spectroscopy of a state-selected beam of OH radicals

A state-selected beam of hydroxyl radicals is generated using a pulsed discharge source and hexapole field. The OH radicals are characterized by resonance-enhanced multiphoton ionization (REMPI) spectroscopy via the nested D 2Sigma- and 3 2Sigma- Rydberg states. Simplified spectra are observed from the selected |MJ|=3/2 component of the upper Lambda-doublet level of the lowest rotational state (J=32) in ground (v"=0) and excited (v"=1-3) vibrational levels of the OH X 2Pi3/2 state. Two-photon transitions are observed to the D 2Sigma-(v'=0-3) and 3 2Sigma-(v'=0,1) vibronic levels, extending previous studies to higher vibrational levels of the Rydberg states. Spectroscopic constants are derived for the Rydberg states and compared with prior experimental studies. Complementary first-principle theoretical studies of the properties of the D 2Sigma- and 3 2Sigma- Rydberg states [see M. P. J. van der Loo and G. C. Groenenboom, J. Chem. Phys. 123, 074310 (2005), following paper] are used to interpret the experimental findings and examine the utility of the (2+1) REMPI scheme for sensitive detection of OH radicals.     

Carbon chains of type C2n+1N(-) (n=2-6): A theoretical study of potential interstellar anions

Linear anions of type C(2n+1)N(-) (n=2-6), which are expected to be good candidates for experimental investigation by microwave spectroscopy and radio astronomy, were studied by means of the coupled cluster variant CCSD(T). Making use of corrections taken over from HC(3)NC(3)N(-) and HC(5)N, accurate equilibrium structures ( approximately 0.0005 A accuracy in bond lengths) have been established for all five anions. The electric dipole moments increase strongly with increasing chain length. For C(13)N(-), a very large equilibrium dipole moment of 16.53 D (with respect to center-of-mass coordinate system, negative end of dipole at terminal carbon site) is predicted. The lowest vertical detachment energies, leading to (2)Sigma states of the radicals for C(3)N(-) and C(5)N(-) and to (2)Pi states in the case of the larger anions, are calculated to lie in the range of 4.40-4.63 eV. The ground-state rotational and quartic centrifugal distortion constants of C(5)N(-) are predicted to be 1389.4 MHz and 33.8 Hz, respectively. All anions studied appear to be fairly normal semirigid linear molecules. Throughout, good agreement with available matrix isolation IR spectroscopic data is obtained and many predictions of spectroscopic properties are made.     

Dinuclear Bis(1,2-diaryl-1,2-ethylenedithiolato)iron complexes: [FeIII2(L)4]n (n = 2-, 1-, 0, 1+)

The electronic structures of four members of the electron-transfer series [Fe2(1L)4]n (n = 2-, 1-, 0, 1+) have been elucidated in some detail by electronic absorption, IR, X-band electron paramagnetic resonance (EPR), and M?ssbauer spectroscopies where (1L)(2-) represents the ligand 1,2-bis(4-tert-butylphenyl)-1,2-ethylenedithiolate(2-) and (1L*)- is its pi-radical monoanion. It is conclusively shown that all redox processes are ligand-centered and that high-valent iron(IV) is not accessible. The following complexes have been synthesized: [FeIII2(1L*)2(1L)2]0 (1), [FeIII2(2L*)2(2L)2].2CH2Cl2 (1') where (2L)(2-) is 1,2-bis(p-tolyl)-1,2-ethylenedithiolate(2-) and (2L*)- represents its pi-radical monoanion, [Cp2Co][FeIII2(1L*))(1L)3].4(toluene).0.5Et2O (2), and [Cp2Co]2[FeIII2(1L)4].2(toluene) (3). The crystal structures of 1' and 2 have been determined by single-crystal X-ray crystallography at 100 K. The ground states of complexes have been determined by temperature-dependent magnetic susceptibility measurements and EPR spectroscopy: 1' and 1 are diamagnetic (S(t) = 0); 2 (S(t) = 1/2); 3 (S(t) = 0); the monocation [Fe(III)2(1L*)3(1L)]+ possesses an S(t) = 1/2 ground state (S(t) = total spin ground state of dinuclear species). All species contain pairs of intermediate-spin ferric ions (S(Fe) = 3/2), which are strongly antiferromagnetically coupled (H = -2JS(1).S(2), where S1 = S2 = 3/2 and J = approximately -250 cm(-1)).     

B2Cn+(n=1～9)团簇的结构及其稳定性

采用密度泛函理论(DFT)的B3LYP泛函, 在6-311G*水平上对B2Cn+(n=1～9)团簇的几何构型和电子结构进行了优化和振动频率计算. 结果表明, 在B2Cn+(n=1～9)团簇的基态构型中, B2C2+、B2C3+为具有D∞h对称性的线形结构, B2C7+为具有Cs对称性的立体环状结构, 其余均为平面构型; 其成键顺序为C—C成键优于B—C 成键, B—C成键优于B—B成键. 进一步得到了B2Cn+(n=1～9)团簇的总能量(ET)、零点能(EZ)、摩尔热容(Cp)、标准熵(S0)以及原子化能(ΔEn+). 其结果显示, 随着n的递增, ET、EZ、Cp、S0和ΔEn+数值均呈现增大趋势, 其中EZ数值呈现近似等梯度的增加趋势. 通过对B2Cn+(n=1～9)团簇基态结构的垂直电子亲合势的研究发现, n为奇数的B2Cn+团簇比n为偶数的稳定.     

羰基硼化合物(BCO)n(n==1～12)的理论研究

运用HF/3-21G方法和密度泛函理论(DFT)的B3LYP/6-31G*方法, 对羰基硼化合物(BCO)n(n=1~12)的各种可能结构进行了优化, 对在B3LYP/6-31G*水平上得到的几何构型、电子态、结合能、振动频率、核独立化学位移(NICS)、能量二次差分和热力学性质进行了理论研究, 得到了(BCO)n(n=1~12)结构的稳定性信息. 十二种基态结构都是端配位(μ1-CO), (BCO)n(n=1~3, 5, 6) 的基态是线型或平面结构, (BCO)n(n=4, 7~12)的基态是笼状结构; B—C平均键能呈现奇偶交替现象, 偶数的结构比奇数稳定; 能量二次差分得到同样的结论；羰基的振动频率与实验值非常吻合; 热力学性质的研究对实验具有重要的指导意义.     

Near-infrared electronic spectrum of CH2+

Four vibronic bands of the A 2B1(Piu)<--X 2A1 electronic transition of CH2+ have been observed in the near infrared from approximately 11,000-13,000 cm(-1) using a highly sensitive spectrometer based on a Ti:sapphire laser and a positive column plasma. The two states are derived from the lowest 2Piu state of the linear conformation as a result of a strong Renner-Teller effect. As expected, the rovibronic structure of the CH2+ spectrum is very similar to that of the isoelectronic BH2 radical reported by Herzberg and Johns. Results of a preliminary analysis are given in which the rotational structures of the two states are treated independently, the ground state as a near-prolate asymmetric rotor and the excited state as a linear molecule.     

Density functional and Ab initio study of Cr(CO)n (n = 1-6) complexes

Cr(CO)n (n = 1-6) systems were studied for all possible spin states using density functional and high-level ab initio methods to provide a more complete theoretical understanding of the structure of species that may form during ligand dissociation of Cr(CO)6. We carried out geometry optimizations for each system and obtained vibrational frequencies, sequential bond dissociation energies (BDE), and total CO binding energies. We also compared the performance of various DFT functionals. Generally, the ground states of Cr(CO)6, Cr(CO)5, and Cr(CO)4, whose spin multiplicity is a singlet, are in good agreement with both previous theoretical results and currently available experimental data. Calculations on Cr(CO)3, Cr(CO)2, and CrCO provide new findings that the ground state of Cr(CO)3 might be a quintet with C2v symmetry instead of a singlet with C3v symmetry, and the ground state of Cr(CO)2 is not a linear quintet, as suggested by previous DFT calculations, but rather a linear septet. We also found that nonet states of Cr(CO)2 and CrCO display partial C-O bond breakage.     

乙醇-水分子团簇C2H5OH(H2O)n (n=1-9)稳定结构的量子化学研究

采用密度泛函理论B3LYP方法, 在B3LYP/6-311++G(2d,2p)//B3LYP/6-311++G(d,p)基组水平上对乙醇-水分子团簇(C₂H₅OH(H₂O)_n (n=1-9))的各种性质进行研究, 如: 优化的几何构型、结构参数、氢键、结合能、平均氢键强度、自然键轨道(NBO)电荷分布、团簇的生长规律等. 结果表明, 从二维(2-D)环状结构到三维(3-D)笼状结构的过渡出现在n=5的乙醇-水分子团簇中. 此外, 利用团簇结合能的二阶差分、形成能、能隙等性质, 发现在n=6时乙醇-水分子团簇的最低能量结构稳定性较好, 可能为幻数结构. 最后, 为了进一步探讨氢键本质, 将C₂H₅OH(H₂O)_n (n=2-9)最低能量结构的各种性质与纯水分子团簇(H₂O)_n (n=3-10)比较, 结果表明前者与后者中的水分子之间氢键相似.     

Probing the structural and electronic properties of Ag(n)H(-) (n = 1-3) using photoelectron imaging and theoretical calculations

Structural and electronic properties of silver hydride cluster anions (Ag(n)H(-); n = 1-3) have been explored by combining the negative ion photoelectron imaging spectroscopy and theoretical calculations. The photoelectron spectrum of AgH(-) exhibits transitions from AgH(- 2)Σ(+) to AgH (1)Σ(+) and AgH (3)Σ(+), with the electron affinity (EA) 0.57(3) eV. For Ag(2)H(-), the only observed transition is from Ag(2)H(-) (C(∞v)) (1)Σ(+) to Ag(2)H (C(2v)) (2)A(') and the electron affinity is 2.56(5) eV. Two obvious electron bands are observed in photoelectron imaging of Ag(3)H(-), which are assigned to the transitions from Ag(3)H(-) (C(2v)-T, which means C(2v) geometry with top site hydrogen) (2)B(2) to Ag(3)H (C(2v)-T) (1)A(1) and Ag(3)H (C(2v)-T) (3)B(2). The electron affinity is determined to be 1.61(9) eV. The Ag-H stretching modes in the ground states of AgH and Ag(2)H are experimentally resolved and their frequencies are measured to be 1710(80) and 1650(100) cm(-1), respectively. Aside from the above EAs and the vibrational frequencies, the vertical detachment energies to all ground states and some excited states of Ag(n)H (n = 1-3) are also obtained. Theoretical calculations reproduce the experimental energies quite well, and the results are used to assign the geometries and electronic states for all related species.     

Optical properties of (GaAs)n clusters (n = 2-16)

The electronic and geometrical structures of the lowest triplet states of (GaAs) n clusters ( n = 2-16) are studied using density functional theory with generalized gradient approximation (DFT-GGA). It is found that the triplet-state geometries are different from the corresponding singlet-state geometries; for n = 2-8, 10, and 11, the triplets and singlets have different topologies, while the (GaAs) 9, (GaAs) 12, (GaAs) 15, and (GaAs) 16 triplets possess a reduced symmetry, due to Jahn-Teller distortions. Except for GaAs, the singlet states are the ground states. Excitation energies and oscillator strengths are computed for excitations from the ground state to ten singlet states of all (GaAs) n clusters using time-dependent density functional theory. The adiabatic singlet-triplet gap is compared to the vertical gap, and the difference in the eigenvalues of the highest-occupied and lowest-unoccupied molecular orbitals (the HOMO-LUMO gap). While these three values show large oscillations for small n, they approach each other as the cluster size grows. Thus, the HOMO-LUMO gap computed using the DFT-GGA approach presents a rather reliable estimate of the adiabatic singlet-triplet gap.     

Electronic absorption spectra of C3Cl, C4Cl, and their ions in neon matrices

Electronic absorption spectra of C3Cl, C3Cl+, C3Cl-, C4Cl, and C4Cl+ have been recorded in 6 K neon matrices following mass selection. Ab initio calculations were performed (CCSD(T) and CASSCF) to identify the ground and accessible excited states of each molecule. The estimated excitation energies and transition moments aid the assignment. The absorptions observed for C3Cl are the 5(2)A' <-- X(2)A' and 3(2)A' <-- X(2)A' transitions of the bent isomer and the (2)A1 <-- X(2)B2 transition of the cyclic form in the UV (336.1 nm), visible (428.7 nm), and near-IR (1047 nm) regions, respectively. The band systems for bent C3Cl- (435.2 nm) and linear C3Cl+ (413.2 nm) are both in the visible region and correspond to 2(1)A' <-- X(1)A' and (1)pi <-- X(1)sigma+ type transitions. The C4Cl and C4Cl+ chains are linear, and the band origins of the 2(2)pi <-- X(2)pi and 2(3)pi <-- X(3)pi electronic transitions are at 427.0 and 405.7 nm. The spectral assignments are supported by analysis of the vibrational structure associated with each electronic transition.     

Characterization of cyclic and linear C3H- and C3H via anion photoelectron spectroscopy

Anion photoelectron spectroscopy of C3H- and C3D- is performed using both field-free time-of-flight and slow electron velocity-map imaging. We observe and assign transitions originating from linear/bent (l-C3H) and cyclic (c-C3H) anionic isomers to the corresponding neutral ground states and low-lying excited states. Transitions within the cyclic and linear manifolds are distinguished by their photoelectron angular distributions and their intensity dependence on the neutral precursor. Using calculated values for the energetics of the neutral isomers [Ochsenfeld et al., J. Chem. Phys. 106, 4141 (1997)], which predict c-C3H to lie 74 meV lower than l-C3H, the experimental results establish c-C3H- as the anionic ground state and place it 229 meV below l-C3H-. Electron affinities of 1.999+/-0.003 and 1.997+/-0.005 eV are determined for C3H and C3D from the X 2B2<--X 1A1 photodetachment transition of c-C3H. Term energies for several low-lying states of c-C3H and l-C3H are also determined. Franck-Condon simulations are used to make vibrational assignments for the bands involving c-C3H. Simulations of the l-C3H bands were more complicated owing to large amplitude bending motion and, in the case of the neutral A 2Pi state, strong Renner-Teller coupling.     

Interatomic potential parameters of CdHe van der Waals complex derived from excitation spectrum of the C1 1(51P1)<--X10+(51S0) vibrational transition

The first time observed excitation spectrum of the C(1)1(5(1)P(1))<--X(1)0+(5(1)S(0)) transition in CdHe van der Waals molecules is reported. Vibrational spectrum in the UV region (2286.0-2296 A) was excited in a continuous molecular-jet-expansion beam of CdHe seeded in helium using an in-house-built nitrogen-dye laser system. The excitation spectrum exhibits two vibrational components (v'<--v'=0) highly broadened by means of unresolved rotational structure and some additional contributions of "hot-bands" components (v'<--v'=1). The last effect is due to an extremely small separation of the vibrational levels in the ground X(1)0+ state of the CdHe molecule, where v'=0 vibrational level is separated from v'=0 by merely 6.0 cm(-1). It follows therefore that even in an extremely cold environment (T(v) approximately 10K) of a jet-expansion beam the population of v'=1 level is feasible, due to some residual collisions, and hence the v'<--v'=1 transitions are highly probable. The assignment of vibrational bands and numerical analysis of the spectrum was based and obtained with the aid of a rigorous computer simulation of the C(1)1<--X(1)0+ transition including the impact of rotational structure and hot-bands contributions. As a result we obtained optical potential parameters of the C(1)1(5(1)P(1)) state of CdHe molecule which are further discussed in terms of our recent (and only existing) experimental results regarding the X(1)0+, B1(5(3)P(1)) and A0+(5(3)P(1)) states of CdHe as well as in terms of ab initio calculations results.     

Electronic states of sodium dimer in ammonia clusters: theoretical study of photoelectron spectra for Na2-(NH3)n (n = 0-6)

The geometries, energetics, and vertical detachment energies of Na2-(NH3)n (n = 0-6) were examined by ab initio molecular orbital methods in connection with their photoelectron spectra. One of the Na atoms is selectively solvated in the most stable structures for each n. The solvated Na is spontaneously ionized and the formation of a solvated electron occurs with increasing n, giving rise to the Na-Na+(NH3)n(e-)-type state. The ground and two lowest-lying excited states derived from the 11Sigma g+, 13Sigma u+, and 13Pi u states of Na2, respectively, are of ion-pair character though the 13Sigma u+-type state has an intermediate nature slowly changing to the radical-pair state with increasing n. On the other hand, the higher states stemming from the 11Sigma u+, 13Sigma g+, and 11Pi u states of Na2 show a developing radical-pair nature as n increases. The size dependences of the photoelectron spectra such as the near parallel shifts of the first and second bands, as well as the rapid red shifts of the higher bands, are studied on the basis of the electronic change of the neutrals by solvation.