Stability and structure of Li(n)H molecules (n=3-6): experimental and density functional study

The molecules Li(3)H and Li(4)H have been identified in mass-spectrometric measurements over solutions of hydrogen in liquid Li, and the gaseous equilibria of the reactions: Li(3)H+Li=Li(2)H+Li(2), Li(3)H+Li(2)=Li(2)H+Li(3), Li(3)H+Li=LiH+Li(3), Li(3)H+LiH=2Li(2)H, and Li(4)H+Li(2)=Li(3)H+Li(3) have been measured. Density functional calculations of Li(n)H molecules (n=3-6) provide structures, vibrational frequencies, ionization energies, and free energy functions of these molecules, and these are used to estimate the enthalpies of these reactions and the atomization energies of Li(3)H (119.4 kcal/mol) and Li(4)H (151.8 kcal/mol).     

高振动激发态吡嗪碰撞传能的QCT计算研究

用准经典轨线(QCT)方法计算了高振动激发态吡嗪(C4N2H4)与N2、O2、NH3、基态吡嗪之间的碰撞传能. C4N2H4通过计算发现, 高振动激发态C4N2H4与N2、O2碰撞发生的主要是V-V传能, 与NH3碰撞发生的主要是V-R传能, 与基态C4N2H4碰撞发生的主要是V-V(R)传能. 通过比较高振动激发态C4N2H4、C6F6、C6H6与其基态分子的碰撞传能， 发现此类碰撞传能中, 若分子的对称性高， 则V-V传能更容易实现.     

Al3+, Ca2+, Mg2+, and Li+ in aqueous solution: calculated first-shell anharmonic OH vibrations at 300 K

The anharmonic OH stretching vibrational frequencies, ν(OH), for the first-shell water molecules around the Li(+), Ca(2+), Mg(2+), and Al(3+) ions in dilute aqueous solutions have been calculated based on classical molecular dynamics (MD) simulations and quantum-mechanical (QM) calculations. For Li(+)(aq), Ca(2+)(aq), Mg(2+)(aq), and Al(3+)(aq), our calculated IR frequency shifts, Δν(OH), with respect to the gas-phase water frequency, are about -300, -350, -450, and -750?cm(-1), compared to -290, -290, -420, and -830?cm(-1) from experimental infrared (IR) studies. The agreement is thus quite good, except for the order between Li(+) and Ca(2+). Given that the polarizing field from the Ca(2+) ion ought to be larger than that from Li(+)(aq), our calculated result seems reasonable. Also the absolute OH frequencies agree well with experiment. The method we used is a sequential four-step procedure: QM(electronic) to make a force field+MD simulation+QM(electronic) for point-charge-embedded M(n+) (H(2)O)(y) (second?shell) (H(2)O)(z) (third?shell) clusters+QM(vibrational) to yield the OH spectrum. The many-body Ca(2+)-water force-field presented in this paper is new. IR intensity-weighting of the density-of-states frequency distributions was carried out by means of the squared dipole moment derivatives.     

Quantitative prediction of gas-phase 19F nuclear magnetic shielding constants

Benchmark calculations of (19)F nuclear magnetic shielding constants are presented for a set of 28 molecules. Near-quantitative accuracy (ca. 2 ppm deviation from experiment) is achieved if (1) electron correlation is adequately treated by employing the coupled-cluster singles and doubles (CCSD) model augmented by a perturbative correction for triple excitations [CCSD(T)], (2) large (uncontracted) basis sets are used, (3) gauge-including atomic orbitals are used to ensure gauge-origin independence, (4) calculations are performed at accurate equilibrium geometries [obtained from CCSD(T)/cc-pVTZ calculations correlating all electrons], and (5) vibrational averaging and temperature corrections via second-order vibrational perturbation theory (VPT2) are included. For the CCSD(T)/13s9p4d3f calculations corrected for vibrational effects, mean and standard deviation from experiment are -1.9 and 1.6 ppm, respectively. Less elaborate theoretical treatments result in larger errors. Consideration of relative shifts can reduce the mean deviation (through an appropriately chosen reference compound), but does not change the standard deviation. Density-functional theory calculations of absolute and relative (19)F nuclear magnetic shielding constants are found to be, at best, as accurate as the corresponding Hartree-Fock self-consistent-field calculations and are not improved by consideration of vibrational effects. Molecular systems containing fluorine-oxygen, fluorine-nitrogen, and fluorine-fluorine bonds are found to be more challenging than the other investigated molecules for the considered theoretical methods.     

A transition state view on reactive scattering: initial state-selected reaction probabilities for the H + CH4 → H2 + CH3 reaction studied in full dimensionality

Initial state-selected reaction probabilities for the H+CH(4)→H(2)+CH(3) reaction are computed for vanishing total angular momentum by full-dimensional calculations employing the multiconfigurational time-dependent Hartree approach. An ensemble of wave packets completely describing reactivity for total energies up to 0.58 eV is constructed in the transition state region by diagonalization of the thermal flux operator. These wave packets are then propagated into the reactant asymptotic region to obtain the initial state-selected reaction probabilities. Reaction probabilities for reactants in all rotational states of the vibrational 1A(1), 1F(2), and 1E levels of methane are presented. Vibrational excitation is found to decrease reactivity when reaction probabilities at equivalent total energies are compared but to increase reaction probabilities when the comparison is done at the basis of equivalent collision energies. Only a fraction of the initial vibrational energy can be utilized to promote the reaction. The effect of rotational excitation on the reactivity differs depending on the initial vibrational state of methane. For the 1A(1) and 1F(2) vibrational states of methane, rotational excitation decreases the reaction probability even when comparing reaction probabilities at equivalent collision energies. In contrast, rotational energy is even more efficient than translational energy in increasing the reaction probability when the reaction starts from the 1E vibrational state of methane. All findings can be explained employing a transition state based interpretation of the reaction process.     

Theoretical search for alternative nine-electron ligands suitable for superhalogen anions

The calculations performed at the OVGF/6-311++G(3df,3pd)//MP2/6-311++G(d,p) level for the representative NaX(2)(-) and AlX(4)(-) anions matching the MX(k+1)(-) superhalogen formula and utilizing 9-electron systems (i.e., consisting of various possible combinations of atoms containing nine electrons when brought together) revealed that the OH, Li(2)H(3), and NH(2) groups might be considered as alternative ligands X due to their thermodynamic stability and large values of electron binding energy (approaching or even exceeding 6 eV in some cases). All aluminum-containing AlX(4)(-) anions (excluding Al(HBLi)(4)(-)) were predicted to be thermodynamically stable, whereas the NaX(2)(-) anions for X = CH(3), HBLi, CLi, BeB, and H(2)BeLi were found to be susceptible to the fragmentations leading to Na(-) loss. Among the MX(k+1)(-) (M = Na, Al; X = Li(2)H(3), OH, H(2)BeLi, BeB, NH(2), HBLi, CH(3), Be(2)H, CLi) anions utilizing systems containing 9 electrons (and thus isoelectronic with the F atom) the largest vertical electron detachment energy of 6.38 eV was obtained for Al(OH)(4)(-).     

A first principles molecular dynamics study of excess electron and lithium atom solvation in water-ammonia mixed clusters: structural, spectral, and dynamical behaviors of [(H2O)5NH3]- and Li(H2O)5NH3 at finite temperature

First principles molecular dynamics simulations are carried out to investigate the solvation of an excess electron and a lithium atom in mixed water-ammonia cluster (H(2)O)(5)NH(3) at a finite temperature of 150 K. Both [(H(2)O)(5)NH(3)](-) and Li(H(2)O)(5)NH(3) clusters are seen to display substantial hydrogen bond dynamics due to thermal motion leading to many different isomeric structures. Also, the structures of these two clusters are found to be very different from each other and also very different from the corresponding neutral cluster without any excess electron or the metal atom. Spontaneous ionization of Li atom occurs in the case of Li(H(2)O)(5)NH(3). The spatial distribution of the singly occupied molecular orbital shows where and how the excess (or free) electron is primarily localized in these clusters. The populations of single acceptor (A), double acceptor (AA), and free (NIL) type water and ammonia molecules are found to be significantly high. The dangling hydrogens of these type of water or ammonia molecules are found to primarily capture the free electron. It is also found that the free electron binding motifs evolve with time due to thermal fluctuations and the vertical detachment energy of [(H(2)O)(5)NH(3)](-) and vertical ionization energy of Li(H(2)O)(5)NH(3) also change with time along the simulation trajectories. Assignments of the observed peaks in the vibrational power spectra are done and we found a one to one correlation between the time-averaged populations of water and ammonia molecules at different H-bonding sites with the various peaks of power spectra. The frequency-time correlation functions of OH stretch vibrational frequencies of these clusters are also calculated and their decay profiles are analyzed in terms of the dynamics of hydrogen bonded and dangling OH modes. It is found that the hydrogen bond lifetimes in these clusters are almost five to six times longer than that of pure liquid water at room temperature.     

Infrared spectra of the Li+-(H2)n (n=1-3) cation complexes

The Li+-(H2)n n=1-3 complexes are investigated through infrared spectra recorded in the H-H stretch region (3980-4120 cm-1) and through ab initio calculations at the MP2/aug-cc-pVQZ level. The rotationally resolved H-H stretch band of Li+-H2 is centered at 4053.4 cm-1 [a -108 cm-1 shift from the Q1(0) transition of H2]. The spectrum exhibits rotational substructure consistent with the complex possessing a T-shaped equilibrium geometry, with the Li+ ion attached to a slightly perturbed H2 molecule. Around 100 rovibrational transitions belonging to parallel Ka=0-0, 1-1, 2-2, and 3-3 subbands are observed. The Ka=0-0 and 1-1 transitions are fitted by a Watson A-reduced Hamiltonian yielding effective molecular parameters. The vibrationally averaged intermolecular separation in the ground vibrational state is estimated as 2.056 A increasing by 0.004 A when the H2 subunit is vibrationally excited. The spectroscopic data are compared to results from rovibrational calculations using recent three dimensional Li+-H2 potential energy surfaces [Martinazzo et al., J. Chem. Phys. 119, 11241 (2003); Kraemer and Spirko, Chem. Phys. 330, 190 (2006)]. The H-H stretch band of Li+-(H2)2, which is centered at 4055.5 cm-1 also exhibits resolved rovibrational structure. The spectroscopic data along with ab initio calculations support a H2-Li+-H2 geometry, in which the two H2 molecules are disposed on opposite sides of the central Li+ ion. The two equivalent Li+...H2 bonds have approximately the same length as the intermolecular bond in Li+-H2. The Li+-(H2)3 cluster is predicted to possess a trigonal structure in which a central Li+ ion is surrounded by three equivalent H2 molecules. Its infrared spectrum features a broad unresolved band centered at 4060 cm-1.     

Isotope effects in hydrogen-bonded complexes. Calculation of geometrical and vibrational characteristics of asymmetric isotopologues of [F(HF)2]-

The geometrical and vibrational characteristics of isolated H-bonded anionic complexes [FHFDF](-), [FHFTF](-), and [FDFTF](-) are calculated quantum-mechanically. The four-dimensional anharmonic vibrational problems are solved by the variational method using the potential energy and dipole moment surfaces calculated in the MP2/6-311++G(3df,3pd) approximation with the basis set superposition error taken into account. Changes in the bond lengths of molecular fragments LF (L = H, D, T) and in the distances between the F(-) anion and the centers of mass of LF are used as the vibrational coordinates. For each isotopologue, the vibrational energy levels, the transition frequencies and absolute intensities for the H-bond and L-F stretching vibrations are determined. To study the isotope effects on the geometrical parameters, the values of internuclear separations and the asymmetry parameter of the F(-)···L-F bridges, averaged over the ground state and several excited vibrational states, are calculated, as well as their standard deviations. The calculations revealed an extremely strong influence of anharmonic coupling between different vibrations on the absorption intensities and a significant mass-dependence of spectroscopic and structural parameters. The geometry and harmonic frequencies of KH(2)F(3), KD(2)F(3), and KHDF(3) are also calculated at a lower ab initio level. The results obtained for [FHFDF](-), [FHFTF](-), and [FDFTF](-) are compared with the available experimental data and the results of earlier calculations of the symmetric complexes [F(HF)(2)](-), [F(DF)(2)](-), and [F(TF)(2)](-) and complexes containing a positive K-meson.     

Arachno, nido, and closo aromatic isomers of the Li6B6H6 molecule

We analyzed chemical bonding in low-lying isomers of the recently computationally predicted B(6)H(6)Li(6) molecule. According to our calculations the benzene-like B(6)H(6)Li(6) (D(2h), (1)A(1g)) arachno structure with the planar aromatic B(6)H(6)(6-) anion is the most stable one. A nido isomer with two aromatic B(6)H(6)(4-) (pentagonal pyramid) and Li(3)(+) (triangular) moieties, which can be considered as derived from the global minimum structure through a two-electron intramolecular transfer from B(6)H(6)(6-) to three Li(+) cations, was found to be 10.7 kcal/mol higher in energy. A closo isomer with three aromatic moieties (octahedral B(6)H(6)(2-) and two Li(3)(+)) was found to be 31.3 kcal/mol higher in energy than the global minimum. Another isomer with three aromatic moieties (two B(3)H(3)(2-) and Li(3)(+)) was found to be substantially higher in energy (74.4 kcal/mol). Thus, the intramolecular electron transfers from the highly charged B(6)H(6)(6-) anion to cations are not favorable for the B(6)H(6)Li(6) molecule, even when a formation of three-dimensional aromatic B(6)H(6)(2-) anion and two sigma-aromatic Li(3)(+) cations occurs in the closo isomer.     

Theoretical study on the structures, isomerization and stability of SiC3H isomers

The calculations of the geometry optimizations, energies, dipole moments, vibrational spectra, rotational constants, and isomerization of doublet SiC₃H species were performed using density functional theory and ab initio methods. Four types of isomers, a total of 18 minima, connected by 16 interconversion transition states, were located on the potential energy surface (PES) at the B3LYP/6-311G (d, p) level. More accurate energies were obtained at the CCSD(T)/6-311G(2df, 2p), and G3(MP2) levels. With the highest isomerization barrier, the lowest lying structure, linear A1 possesses the largest kinetic stability. Besides, the isomerization barriers of A2, A4, C2, F1, F4 and F5 are over 10 kcal/mol, and these isomers are also considered to be higher kinetically stable. Other isomers cannot be kinetically stabilized with considerably low isomerization barriers. Investigation on the bonding properties and the computations of vibrational spectra, dipole moments, and rotational constants for SiC₃H isomers are helpful for understanding their structures and also valuable for their detections in the interstellar space and laboratory.     

C-S barrier and vibrational analyses of (halocarbonyl)sulfenyl halides XCO-SX (X = F, Cl, and Br)

The structural stability of (halocarbonyl)sulfenyl halides XCO-SX (X is F, Cl, and Br) was investigated by DFT-B3LYP and ab initio MP2 calculations using 6-311 + G(**) basis set. From the calculations the molecules were found to exist predominantly in the trans conformation (two halogen atoms are trans to each other). Full energy optimizations were carried out for the minima and the transition states (TS) at the two levels, from which the rotational barriers about C-S bond in the three molecules were calculated to be about 12-13 kcal mol(-1). The vibrational frequencies of (fluorocarbonyl)sulfenyl fluoride (FCO-SF), (chlorocarbonyl)-sulfenyl chloride (ClCO-SCl), and (bromocarbonyl)-sulfenyl bromide (BrCO-SBr) were computed at the DFT-B3LYP level and the vibrational assignments for the normal modes of the stable forms of the compounds were made on the basis of normal coordinate calculations and experimental data of the chloride.     

Quasiclassical trajectory calculations for Li(2(2)P(J)) + H2 → LiH(X(1)Σ+) + H: influence by vibrational excitation and translational energy

Ab initio potential energy surfaces and the corresponding analytical energy functions of the ground 1A' and excited 2A' states for the Li(2(2)P) plus H(2) reaction are constructed. Quasiclassical trajectory calculations on the fitted energy functions are performed to characterize the reactions of Li(2(2)P) with H(2)(v = 0, j = 1) and H(2)(v = 1, j = 1) as well as the reaction when the vibrational energy is replaced by collision energy. For simplicity, the transition probability is assumed to be unity when the trajectories go through the crossing seam region and change to the lower surface. The calculated rotational distributions of LiH(v = 0) for both H(2)(v = 0, j = 1) and H(2)(v = 1, j = 1) reactions are single-peaked with the maximum population at j' = 7, consistent with the previous observation. The vibrational excitation of H(2)(v = 1) may enhance the reaction cross section of LiH(v' = 0) by about 200 times, as compared to a result of 93-107 reported in the experimental measurements. In contrast, the enhancement is 3.1, if the same amount of energy is deposited in the translational states. This endothermic reaction can be considered as an analog of late barrier. According to the trajectory analysis, the vibrational excitation enlarges the H-H distance in the entrance channel to facilitate the reaction, but the excess energy may not open up additional reaction configuration.     

Crystal structure of Li2B12H12: a possible intermediate species in the decomposition of LiBH4

The crystal structure of solvent-free Li2B12H12 has been determined by powder X-ray diffraction and confirmed by a combination of neutron vibrational spectroscopy and first-principles calculations. This compound is a possible intermediate in the dehydrogenation of LiBH4, and its structural characterization is crucial for understanding the decomposition and regeneration of LiBH4. Our results reveal that the structure of Li2B12H12 differs from other known alkali-metal (K, Rb, and Cs) derivatives.     

不饱和类硅烯H2C＝SiMBr (M＝Li, Na)的DFT研究

采用密度泛函理论方法, 在B3LYP/6-311G (d,p)水平上研究了不饱和类硅烯H₂C＝SiMBr (M＝Li, Na)的结构. 结果表明, 不饱和类硅烯H₂C＝SiLiBr与H₂C＝SiNaBr各有三种平衡构型, 其中非平面的p-配合物型构型能量最低, 是这两种不饱和类硅烯存在的主要构型. 对平衡构型间异构化反应的过渡态进行了计算, 求得了转化势垒. 计算预言了最稳定构型的振动频率和红外强度.     

内含式化合物X@Al12P12的结构与稳定性研究

采用B3LYP／6—31G*方法,对内含式化合物X@Al12P12(X=Li^0/ ,Na^0/ ,K^0/2 ,Be^0/2 ,Mg^0/2 ,Ca^0/2 ,H和He)的不同对称性构型进行计算,讨论其最稳定构型的几何参数、布居分析、偶极矩、电离势、包含能、频率、HOMO—LUMO能隙和自旋密度．发现X@Al12P12化合物中,客体X=Na^0/ ,K^0/ ,Mg和He几乎处在笼的中心,Be和Ca^0/2 处在中心附近0．033nm的半径内,Li^0/ ,Be^2 ,Mg^2 和H很大程度上偏离笼的中心位置．大部分金属内含式化合物的C3对称性构型稳定．Li^0/ 。,Be^0/2 ,Mg^2 ,Ca^2 和H与其它离子相比更易嵌入笼内形成稳定的内含式化合物．     

Triplet electronic states in d(2) and d(8) complexes probed by absorption spectroscopy: a CASSCF/CASPT2 analysis of [V(H2O)6]3+ and [Ni(H2O)6]2+

Octahedral complexes of transition metal ions with d(2) and d(8) electron configurations have triplet electronic states with identical T(2g), A(2g), T(1g)((3)F), and T(1g)((3)P) symmetry labels. CASSCF and CASPT2 calculations indicate the predominant electronic configurations for each triplet state. The two (3)T(1g) states show strong configuration mixing in the d(8) complex [Ni(H(2)O)(6)](2+), but much weaker mixing occurs between these states in the d(2) compound [V(H(2)O)(6)](3+). Calculated vibrational frequencies and equilibrium geometries for the triplet states are used to obtain theoretical absorption spectra that are in agreement with the experimental data.     

Vibrational corrections to magneto-optical rotation: a computational study

Vibrational corrections to the Verdet constants of nine molecules (H2, N2, CO, H2O, CH4, benzene, toluene, p-xylene, and o-xylene) were calculated with pure density functional theory (DFT), hybrid DFT, and an approximate coupled-cluster theory. Comparisons are made for the accuracy of the vibrational averages among different methods and with respect to experimental data where available. It is found that vibrational corrections to magneto-optical rotation can be as large as 10% of the equilibrium value. Hybrid DFT with the B3LYP hybrid functional offers reasonable accuracy at a relatively inexpensive computational cost for accurate calculations of vibrationally averaged Verdet constants.     

Heavy atom tunneling in chemical reactions: study of H + LiF collisions

The H+LiF(X (1)sigma(+),upsilon=0-2,j=0)-->HF(X (1)sigma(+),upsilon',j')+Li(2S) bimolecular process is investigated by means of quantum scattering calculations on the chemically accurate X 2A' LiHF potential energy surface of Aguado et al. [A. Aguado, M. Paniagua, C. Sanz, and J. Roncero, J. Chem. Phys. 119, 10088 (2003)]. Calculations have been performed for zero total angular momentum for translational energies from 10(-7) to 10(-1) eV. Initial-state selected reaction probabilities and cross sections are characterized by resonances originating from the decay of metastable states of the H...F-Li and Li...F-H van der Waals complexes. Extensive assignment of the resonances has been carried out by performing quasibound states calculations in the entrance and exit channel wells. Chemical reactivity is found to be significantly enhanced by vibrational excitation at low temperatures, although reactivity appears much less favorable than nonreactive processes due to the inefficient tunneling of the relatively heavy fluorine atom strongly bound in van der Waals complexes.     

Theoretical studies on vibrational spectra of some halides of group IVB elements

The vibrational spectra of group IVB elements halides MX4 (M=Ti(IV), Zr(IV), Hf(II); X=F, Cl, Br and I), have been investigated by ab initio RHF, MP2 and density functional theory B3LYP method with LanL2DZ basis sets. The optimized geometries, calculated vibrational frequencies and Far-IR intensities of MX4 are evaluated via comparison with experimental data. The vibrational frequencies, calculated by these methods, are compared to each other. The results indicate that B3LYP method is more reliable than RHF and MP2 methods for the frequencies calculations for these compounds. With this method, some vibrational frequencies of M2X6(2+)(M=Ti(IV), Zr(IV) and Hf(II); X=F, Cl, Br and I) are also predicted.