Ab initio molecular orbital calculations on the associated complexes of lithium cyanide with ammonia

Ab initio molecular orbital (MO ) calculations with the 3-21G and 6-31G basis sets are carried out on a series of complexes of NH₃ with Li⁺, C?N^?, LiCN, and its isomer LiNC. The BSSE -corrected interaction energies, geometrical parameters, internal force constants, and harmonic vibrational frequencies are evaluated for 15 species. Complexes with trifurcated (C_3v) structures are calculated to be saddle points on the potential energy surfaces and have one imaginary frequency each. Calculated energies, geometrical parameters, internal force constants, and harmonic vibrational frequencies of the various species considered are discussed in terms of the nature of association of LiCN with ammonia. The vibrational frequencies of the relevant complexed species are compared with the experimental frequencies reported earlier for solutions of lithium cyanide in liquid ammonia. © 1995 John Wiley & Sons, Inc.     

Anion-molecular interaction through CH3 groups: Model ab initio studies

3-21G, 6–31G and 6–31 +G calculations have been performed on Cr…H₃CCl and LiCl…H₃CCl complexes with two different configurations each. Optimized geometries, stabilization energies, CH force constants and harmonic vibrational frequencies for CH₃Cl and its complexes are reported. Comparison of the calculated frequency shifts of CH-stretching bands of CH₃C1, on complexation, with experimental results of related systems indicate that the interaction of Cl^- with CH₃ group takes place in a linear manner with the CH bond. A lower frequency shift for LiCl…H₃CX with reference to the C1^-…H₃CX complexes is explained on the basis of the reduction of the basicity of Cl^- ion in the presence of counter ion in the former complex considered.     

An ab initio molecular orbital study of the structures and energies of neutral and charged bimolecular complexes of NH3 with the hydrides AHn (A = N,O, F,P, S,and Cl)

Hartree-Fock 6-31G(d) structures for the neutral, positive ion, and negative ion bimolecular complexes of NH₃ with the first- and second-row hydrides AH_n (AH_n = NH₃, OH₂, FH, PH₃, SH₂, and ClH) have been determined. All of the stable neutral complexes except (NH₃)₂, the positive ion complexes with NH₃ as the proton acceptor, and the negative ion complexes containing first-row anions exhibit conventional hydrogen bonded structures with essentially linear hydrogen bonds and directed lone pairs of electrons. The positive ion complex NH₄⁺ …? OH₂ has the dipole moment vector of H₂O instead of a lone pair directed along the intermolecular line, while the complexes of NH₄⁺ with SH₂, FH, and ClH have structures intermediate between the lone-pair directed and dipole directed forms. The negative ion complexes containing second-row anions have nonlinear hydrogen bonds. The addition of diffuse functions on nonhydrogen atoms to the valence double-split plus polarization 6-31G(d,p) basis set usually decreases the computed stabilization energies of these complexes. Splitting d polarization functions usually destabilizes these complexes, whereas splitting p polarization functions either has no effect or leads to stabilization. The overall effect of augmenting the 6-31G(d,p) basis set with diffuse functions on nonhydrogen atoms and two sets of polarization functions is to lower computed stabilization energies. Electron correlation stabilizes all of these complexes. The second-order Møller–Plesset correlation term is the largest term and always has a stabilizing effect, whereas the third and fourth-order terms are smaller and often of opposite sign. The recommended level of theory for computing the stabilization energies of these complexes is MP2/6-31+G(2d,2p), although MP2/6-31+G(d,p) is appropriate for the negative ion complexes.     

Geometrical Changes and Their Energies in the Formation of Donor–Acceptor Complexes

We have mapped the energy demands of the geometrical changes in donor–acceptor complexes BH₃NH₃ and AlCl₃NH₃ and in the course of their formation from their monomers. We have varied the individual geometrical parameters systematically and performed ab initio quantum chemical calculations for these structures. We investigated the energy requirements to change bond lengths and bond angles in both the monomers and complexes and the angles of torsion in the complexes. The changes of bond lengths require more energy in the monomers than in the complexes. The energies to change the acceptor bond angles in the monomers are markedly higher than in the complexes. The changes in the geometrical parameters during the complexation process are more moderate in donors than in acceptors, in agreement with prior experimental observations. The geometry versus energy variations related to the process of complexation are in agreement with the notion of relative rigidity of the donor parts and the more compliant nature of the acceptor parts as well as with the notion of competing effects in the structures of the complexes.     

Ab initio investigation of tautomeric stability,molecular structure,and internal rotation of methylphosphonic dicyanide,methoxydicyanophosphine, and their isocyano analogs

The equilibrium geometric parameters and structures of the transition states of internal rotation for MeP(O)(CN)₂, McOP(CN)₂, and their isocyano analogs, MeP(O)(NC)₂ and MeOP(NC)₂, have been calculated by theab initio SCF method and with inclusion of electron correlation effects according to the second-order Muuller-Plesset perturbation theory (MP2). At both levels the 6-31G* basis set has been used. The estimation of relative stability of these tautomeric forms depends largely on the calculation level. The total energies of the cyanides calculated by the MP2 method are 25–30 kcal mol^–1 lower than those of the corresponding isocyanides. The oxo-tautomeric forms containing four-coordinate phosphorus are 15–25 kcal mol^–1 more stable than the three-coordinate phosphorus aci-derivatives. The internal rotation potential curves of the aci-forms are characterized by a deep minimum for thetrans-arrangement of the methoxy group and phosphorus lone electron pair. Two additional less clearly pronounced minima are located symmetrically on both sides of the weak maximum, which corresponds to thecis-arrangement. The equilibrium oxo-form structures have a staggered configuration of the methyl group with respect to the phosphorus atom bonds.Translated from izvestiyaAkademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1104–1115, May, 1996.     

A molecular orbital study of the electronic structure and stereochemistry of dithiocyanate diamine and dicyanate diamine cupric complexes

CNDO/2—UHF molecular orbital calculations have been performed with the aim of studying the electronic structure and stereochemistry of the complexes Cu(NH₃)₂(NCX)₂ (X = O, S). Orbital energies, atomic charges, Wiberg indices, spin densities, total molecular energies and their partitioning into metal—ligand and ligand—ligand interactions have been calculated for three model structures of each complex. The role of axial interactions in stabilizing the planar arrangement of theCuN₄ chromophore is discussed. The harmonic force constants of some deformation vibrations of coordinated NCX groups have been estimated for the trans isomers and these estimates are compared with the values for the free ligands.     

Coupling characteristics and proton transfer mechanisms of guanine–Na monohydrate

The coupling characteristics and the proton transfer mechanisms of guanine–Na⁺ monohydrate are determined in this investigation after the implementation of the geometry optimization and the harmonic vibrational frequency calculations. There are two elementary coupling modes: the interaction of monohydrated sodium ion with two heteroatoms which form a ringed coupling, and hydrogen-bond involved coupling mode. Two potential reaction pathways, coupling mode and hydration have been taken into account, and the accurate values of binding energy are corrected for basis set superposition error (BSSE) and zero-point vibrational energy (ZPVE). Relative energies of the hydrated guanine–sodium ion complexes indicate that the ringed-coupling complexes are predominant geometries with much lower energies. Monohydrated sodium ion coupling with O6 and N7 generates the most stable geometry with a five-member cycle. Sodium ion plays an important role in the tautomerization for guanine–sodium ion complexes. This investigation indicates that the stable cation-π complexes cannot be optimized for guanine–sodium ion monohydrate. Amino-involved coupling often gives rise to a twisted four-membered cycle with unrealistic distribution of positive charge and higher energies. The rotation of amino group is likely to lead to the redistribution of the base pair hydration bonding. Effective distribution of the positive charge is an important factor in the stabilization of biological systems and binding energies for the monohydrated guanine–sodium ion complexes. The enolic coupling complex has the higher energy than the keto type due to the hindrance for the positive charge.     

Energetics and structure of the hydrated gaseous halide anions

Energetics and geometries for the hydrated gaseous halide anions have been computed from a simple model in which the molecular dipole of water was composed of two parts, one due to a lone pair on oxygen (60%) and the rest to formal charges on the nuclei. The calculations were made for both the symmetric and nonsymmetric structures. A variety of structures were used to compute potential energies and distances with up to six water molecules. The total energy consisted of a sum of electrostatic, polarization, dispersion, and repulsion terms. Various sets of repulsive potential parameters, ranging from those determined from molecular beam experiments to those determined using experimental ion–water distances or energies, have been employed to compute repulsive interaction energies. It was found that the range parameters play a significant role in deciding the magnitudes of the distances and energies, as the latter are most sensitive to them. It was also shown that with a simple correlation scheme the consistency of the experimental energies and distances can be tested separately without using repulsive potential parameters from other sources. It also suggests that a range of parameters can be used to compute repulsion energies. Despite the fact that the model is greatly simplified, the agreement of both the predicted ion-oxygen distances and energies with both experiment and other calculations is excellent. A detailed analysis of our calculation suggests that the negative ion clusters with one to three water molecules contain symmetric orientation of water molecules, while those with more than three may contain asymmetric orientations of water molecules or a mixture of both. From the log–log plots of hydration energies versus (R + radius of water molecule), we have proposed empirical expressions of the type ΔE_n?1,n = 10·0^x (R + 1.38)^?y with both Pauling's and Ladd's radii for univalent ions with which stepwise hydration energies of the latter can be predicted if we know thier radii. The values predicted for the alkali cations are in excellent agreement with the experimental and theoretical values, indicating the consistency of the simple model.     

Geometrical and electronic structures of the dication and ion pair in the geminal dicationic ionic liquid 1,3-bis[3-methylimidazolium-yl]propane bromide

Geminal dicationic ionic liquids (ILs), a new category of IL family, have been developed recently and found to possess unique properties compared to conventional monocationic ILs. To establish a basis for understanding their novel properties, we studied the geometrical and electronic structures of the dication ([(mim)C₃(mim)]²⁺) and the ion pair ([(mim)C₃(mim)]²⁺-2Br⁻) in the geminal dicationic IL 1,3-bis[3-methylimidazolium-yl]propane bromide by performing density functional theory calculations. The geometrical structures and relative stabilities for the dication and the ion pair are discussed, and their electronic properties are analyzed in detail. The intrinsic interaction between the dication and Br anions in the most stable conformer was investigated by performing the natural bond orbital analyses. Results for the dication and the ion pair are compared with those of the corresponding monocation ([C₄mim]⁺) and ion pair ([C₄mim]⁺-Br⁻). ¹H NMR spectroscopy for the most stable ion pair has been calculated and the general trend is found to be in fairly agreement with the experimental data.     

Ab initio studies of RO− … HOR′ complexes. Solvent effects on the relative acidities of water and methanol

Ab initio molecular orbital calculations are reported for complexes of hydroxide and methoxide anions with water and methanol. The basis set dependence of the results is carefully considered for HO^? ? H₂O. 4-31G and 6-31G* calculations yield similar geometrical predictions; however, the 6-31G* basis set is superior for computing dissociation energies. Further extension to the 6-31G** level provides little change. The dissociation energies for the complexes range from 25 to 37 kcal/mole with hydroxide ion and methanol acting as the strongest base and acid. The difference in gas phase acidities of water and methanol is halved by the introduction of one solvent molecule.     

The monomolecular mechanism of the gas‐phase thermal decomposition of primary N‐nitramines

Using nonempirical methods and DFT‐methods the geometrical parameters formation enthalpies of molecules and radicals, energies dissociation of N? NO₂ bonds of primary and secondary N‐nitramines have been investigated. The basic tendencies in the changes of the geometrical and electronic structures, formation enthalpies, and dissociation energies have been analyzed in basic homologous series of nitramines. Various alternative mechanisms of the gas‐phase monomolecular thermal decomposition have been studied by of the example of N‐methylnitramine. The process of the aci‐form formation and its further multistage destruction is the most advantageous way of decomposition of the primary N‐nitramines. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Measuring internal energy deposition in collisional activation using hydrated ion nanocalorimetry to obtain peptide dissociation energies and entropies

The internal energy deposited in both on- and off-resonance collisional activation in Fourier transform ion cyclotron resonance mass spectrometry is measured with ion nanocalorimetry and is used to obtain information about the dissociation energy and entropy of a protonated peptide. Activation of Na⁺(H₂O)₃₀ results in sequential loss of water molecules, and the internal energy of the activated ion can be obtained from the abundances of the product ions. Information about internal energy deposition in on-resonance collisional activation of protonated peptides is inferred from dissociation data obtained under identical conditions for hydrated ions that have similar m/z and degrees-of-freedom. From experimental internal energy deposition curves and Rice-Ramsperger-Kassel-Marcus (RRKM) theory, dissociation data as a function of collision energy for protonated leucine enkephalin, which has a comparable m/z and degrees-of-freedom as Na⁺(H₂O)₃₀, are modeled. The threshold dissociation energies and entropies are correlated for data acquired at a single time point, resulting in a relatively wide range of threshold dissociation energies (1.1 to 1.7 eV) that can fit these data. However, this range of values could be significantly reduced by fitting data acquired at different dissociation times. By measuring the internal energy of an activated ion, the number of fitting parameters necessary to obtain information about the dissociation parameters by modeling these data is reduced and could result in improved accuracy for such methods.     

Molecular orbital theory of the hydrogen bond. 27. Substituent effects in water: 4-R-pyrimidine complexes

Ab initio SCF calculations with the STO -3G basis set have been performed to investigate substituent effects on the structures and stabilization energies of water:4-R-pyrimidine complexes, with R including CH₃, NH₂, OH, F, C₂H₃, CHO, and CN. Except for the cyclic water:4-aminopyrimidine complex hydrogen bonded at N₃, these complexes have open structures stabilized by a nearly linear hydrogen bond formed through a nitrogen lone pair of electrons. When hydrogen bonding occurs at N₃, the complexes may have planar or perpendicular conformations depending on the substituent, but when hydrogen bonding occurs at N₁, the perpendicular is generally slightly preferred, and there is essentially free rotation of the 4-R-pyrimidine. Primary substituent effects alter the electronic environment at the nitrogens, and tend to make N₃ a poorer site for hydrogen bonding than N₁, primarily because of a stronger π electron-withdrawing effect at N₃. However, the relative stabilities of complexes hydrogen bonded at N₁ and N₃ are also influenced by secondary substituent effects, which may be significant in stabilizing complexes bonded at N₃. Substitutent effects on the structures and stabilization energies of the water:4-R-pyrimidine complexes are similar to substitutent effects in water:2-R-pyridine and water:4-R-pyrimidine complexes are similar to substitutent effects in water:2-R-pyridine and water:4-R-pyridine complexes. Configuration interaction calculations indicate that although absorption of energy by the pyrimidine ring destabilizes the water:4-R-pyrimidine complexes, these may still remain bound in the excited n → π* state. This is in contrast to the fate of open water:2-R-pyridine and water:4-R-pyridine complexes, which dissociate in this state.     

Structures and bonding patterns of nanoannular carbon clusters (C4–C20) through AIM analyses

Structures, binding energies, harmonic frequencies, dipole moments, HOMO–LUMO energy gaps and particularly atoms in molecules (AIM) analyses of some nanoannular carbon clusters (C₄–C₂₀) are investigated at B3LYP/6-31+G(d) level of theory. No correlation is found by plotting the calculated binding energies as a functional number of carbon atoms of carbon clusters. The calculated binding energies sharply increase from C₄ to C₁₀ while slowly from C₁₀ to C₂₀. The binding energies of C_4n+2 clusters including C₆, C₁₀, C₁₄, and C₁₈ have a clear increase when compared with others indicating their aromatic characters which is confirmed by results of HOMO–LUMO energy gaps and geometrical parameters. AIM analyses show that most of our carbon clusters are topologically normal (non-conflict) with stable structures. Nevertheless, the topological networks of small antiaromatic rings, C₄ and C₈, at their equilibrium geometries may change via molecular vibrations. The existence of straight bond paths in 3D molecular graphs of carbon clusters with n > 10 implies that ring strains are decreased as the ring sizes grow. Except for C₄ and C₈, the ellipticity values for the remaining carbon clusters are small indicating that the C–C bond is conserved in these clusters. Dipole moments of even-numbered structures are negligible, whereas odd-numbered ones have μ values of 0.09−0.73 D.     

Theoretical studies of complexes between Hg(II) ions and l‐cysteinate amino acids

In this study, ab initio and density functional theory methods have been used to understand the structures and thermodynamic stabilities of complexes formed between l ‐cysteine and mercury (II) ions in neutral aqueous solution. To better understand the interaction between sulfur and mercury (II) ion, the MP2, B3LYP, M06‐2X, and TPSS methods have been used to optimize [HgSH_x]^2?x, x = 1–4, complexes and compared to benchmark QCISD(T) structures. Furthermore, energies from these same methods are compared to CCSD(T)/CBS(2,3) energies. From these benchmark calculations, the M06‐2X method was selected to optimize l ‐cysteinate‐Hg(II) complexes and the MP2 method for estimating complex energies. l ‐cysteinate‐mercury (II) ion complexes are formed primarily by forming a bond between cysteinate sulfur and the mercury ion. Stable complexes of l ‐cysteinate and mercury can be formed in 1:1, 2:1, 3:1, and 4:1 ratios. Each complex is stabilized further by interaction between carboxylate oxygen and mercury as well as hydrogen bonding among complex cysteinate ligands. The results indicate that at high cysteinate to Hg(II) ratios high‐coordinate complexes can be present but at lower ratios the 2:1 complex should be dominant. © 2013 Wiley Periodicals, Inc.     

Molecular Structure of ortho-Fluoronitrobenzene Studied by Gas Electron Diffraction and Ab Initio MO Calculations

The molecular structure of ortho-fluoronitrobenzene (o-FNB) has been investigated by gas-phase electron diffraction and ab initio MO calculations. The geometrical parameters and force fields of o-FNB were calculated by ab initio and DFT methods. The obtained force fields were used to calculate vibrational amplitudes required as input parameters in an electron diffraction analysis. Within the experimental error limits, the geometrical parameters obtained from the gas-phase electron diffraction analysis are mostly in agreement with the results obtained from the ab initio calculations. The main results are: the molecular geometry of o-FNB is nonplanar with a dihedral angle about C–N of 38(3)°. The r _g (C–F) bond is shortened to 1.307(13) Å in comparison with r _g (C–F) = 1.356(4) Å in C₆H₅F.     

Hydrogen-bonded and van der Waals complexes studied by a Gaussian density functional method. The case of (HF)2, ArHCl and Ar2HCl systems

Summary Linear combination of Gaussian-type orbitals local spin density calculations (LCGTO-LSD) have been performed to further test the applicability to the method of hydrogen-bonded and van der Waals systems. The calculated minimum energy structures and binding energies for the (HF)₂, ArHCl and Ar₂HCl complexes are presented. In addition, the harmonic vibrational frequencies are reported for (HF)₂. The results show that by using nonlocal corrections and increasing the number of radial points in the grid, the calculated parameters are close to experimental ones and provide some encouraging evidence for the reliable use of density functional theory for these complex systems.     

α-取代甲苯定向硝化的理论研究

运用密度泛函理论(DFT), 研究了吸电子氟基和供电子羟基在取代甲苯的α-H以后, 其邻、间、对各位次进行硝化反应的速控步骤, 在B3LYP/6-311G**水平上, 计算了该速控步骤基元反应各反应驻点(反应物、过渡态和中间体)的优化几何、电子结构和能量性质, 并首次给出了目标硝化反应速控步骤的IR谱学的动态特征及解析, 从微观层面上验证了反应坐标C—N的形成和C—H的断裂是非协同的, 从而无一级动力学同位素效应的实验事实. 通过对目标硝化反应速控步骤的微观动态计算, 验证了氟基对甲基定位的影响. 氟基的电负性大, 吸电子能力强, 取代甲苯的α-H以后对硝酰阳离子的进攻有抑制作用, 活化能较取代前高, 但比较苄基氟各位次硝化活化能的相对大小得知, －CH₂F仍为邻、对位定向基团. 而供电子羟基取代甲苯的α-H以后, 则对硝酰阳离子的进攻有促进作用, 因而各反应驻点络合物的稳定化能较α-H取代前甲苯的有所增大, 且邻、对位硝化的活化能较间位低, 故－CH₂OH为邻、对位定位基. 但对位因硝化活化能低, 反应放热多, 空间位阻小, 为亲电试剂NO₂⁺最有利的进攻位; 而邻位则因羟基取代甲苯α-H后多了一个氧原子, 增大了邻位进攻的空间位阻, 使得其络合物的能量比相应对位的高.     

Cation-pi interactions of bare and coordinatively saturated metal ions: contrasting structural and energetic characteristics

In the present work, we address an apparent disparity in the structural parameters of the X-ray structures and theoretical models of cation-pi complexes in biological and chemical recognition. Hydrated metal ion (Li+, Na+, K+, Mg2+, Ca2+) complexes with benzene (cation-pi) are considered as model systems to perform quantum mechanical calculations in evaluating the geometrical parameters and interaction energies of these complexes. The computations disclose that there is a variation in the structural parameters as well as in the interaction energies of these complexes with the multiple additions of water molecules. The distance between the cation and the pi-system increases with the addition of water molecules, delineating the influence of solvent or the neighborhood atoms on the structural parameters of cation-pi systems present in crystal structures.