Theoretical Study on Ionic Liquid Based on 1-Ethyl-3-Methyl- Imidazolium Cation and Hexafluorophosphate or Tetrafluoroborate

The Hartree-Fock and DFT/B3LYP methods have been employed to investigate the electronic structures of 1-ethy1-3-methyl-imidazolium cation(EMIM~ ),BF_4~-,PF_6~-,EMIM~ -BF_4~-,and EMIM~ -PF_6~- using the Gaussian-94 soft-package at 6-31 G(d,p)basis set level for hydrogen,carbon,nitrogen,boron, phosphorus,and fluorine atoms.Comparison of the electronic structures of the lowest energy of EMIM~ - BF_4~- and EMIM~ -PF_6~- pairs,and single EMIM~ ,BF_4~- and PF_6~- showed that the optimized EMIM~ -BF_4~- and EMIM~ -PF_6~- pair conformers were BF_4~- and PF_6~- outside the 5-ring plane between the ethyl group and the methyl group.The cohesion of C—H…F hydrogen bond between cation and anion is reinforced by charge assistance.The interaction energy between EMIM~ and PF_6~- is 328.8 kJ/mol at the B3LYP level and 326.6 kJ/mol at the Hartree-Fock level,whereas that between EMIM~ and BF_4~- is 353.5 kJ/mol at the B3LYP level and 350.5 kJ/mol at the Hartree-Fock level.The low energy interactions caused by bulky asymmetric EMIM~ ,and charge dispersion of cation and anion give rise to the low melting point of ionic liquid EMIM~ -BF_4~- and EMIM~ -PF_6~-.The two hydrogen bonding models of single hydrogen bond formation,and the hydrogen transfer between C_2 in EMIM~ and F in BF_4~- or PF_6~- were principally depicted.     

Ionization of the ammonia dimer: Proton transfer in the ionic state

The adiabatic (I_a) and the vertical (I_v) ionization energy of the ammonia dimer as well as the appearance energies of NH⁺₄ and (NH₃)⁺₂ have been obtained from ab initio potential energy surface calculations. The comparison with the experimental result leads to the following conclusions: (1) the real I_a of (NH₃)₂ is not observed in the experiment due to the unfavorable Franck—Condon factor of the equilibrium dimer cation which has the structure of H₃NH⁺ …NH₂. (2) The value of I_a is estimated to be about 8.6 eV, almost 1 eV lower than the AE of (NH₃)⁺₂. (3) Consequently, the two kinds of dissociation energy of the dimer cation into NH₃ + NH⁺₃ and NH⁺₄ + NH₂, respectively, are about 1 eV larger than the “experimental” estimate from the apparent AE values. The present interpretation solves the question of the large discrepancy in the dissociation energies of the ammonia dimer cation.     

Many-body perturbation theory and polarization propagator studies of the structure, energetics and excitation spectrum of CO3

Geometry optimization has been performed on CO₃ in the SCF approximation and in the second-, third-, and fourth-order MBPT approximations limited to single and double substitutions using a double-zeta plus polarization basis set. The energetics of the formation and decomposition of CO₃ from the reactions CO₂ + O(¹D) → CO₃ and CO₃ + CO → 2CO₂, respectively, have been calculated in several approximations including full fourth-order MBPT. In addition first-order polarization propagator calculations have been performed to identify the low-lying excited states.     

Two-centre model potential energy calculations for the Σ and Π states of Li2, Na2, K2, Rb2 and Cs2

Two-centre model potential calculations have been carried out for the ²Σ⁺_g,u and ²Π_g,u states of Li⁺₂, Na⁺₂, K⁺₂, Rb⁺₂ and Cs⁺₂. Comparison with other model potential calculations suggests that reliable potential curves have been obtained. The results indicate the usefulness of calculating diatomic energies by the method proposed.     

Internal energy distributions of tungsten hexacarbonyl ions after neutralization—Reionization

The internal energy distributions P(ε) transferred to W(CO)^+√₆ during the kiloelectronvolt collisions that occur upon neutralization-reionization (NR) have been estimated based on the relative abundances of the W(CO)^+√₀₋₆ products present in NR spectra (thermochemical method). The average internal energy of the incipient W(CO)^+√₆ ^* ions arising after near thermoneutral neutralization with trimethylamine followed by reionization with O₂ is −9 eV for 8-keV precursor ions and is mainly deposited during reionization. For comparison, the mean internal energy of W(CO)^+√₆* after electron ionization (EI) or collisionally activated dissociation (CAD) is −6 eV. Making the neutralization step endothermic slightly increases the overall excitation gained; however, a large increase in endothermicity ( > 16 eV) causes only a modest rise of the average internal energy (< 2 eV). The P(ε) curve for NR increases exponentially up to 6 eV and levels off at higher energies, showing that the probability of imparting large internal energies (6–17 eV) is high. In sharp contrast, the most probable excitation on CAD is ≤ 6 eV, and the probability of deposition of larger energies declines exponentially. The mean internal energies after CAD and NR decrease steadily when the kinetic energy is lowered. The structure (minima-maxima) observed in the P(ε) distribution for EI, which most likely originates from Franck-Condon factors, is not reproduced in the distributions for NR or high energy CAD, despite the fact that all three methods involve electronic excitation. Because of the large internal energies transferred upon NR, NR mass spectrometry could be particularly useful in the differentiation of ionic isomers with high dissociation but low isomerization thresholds. (J Am Soc Mass Spectrom 1994, 5, 1093-1101)     

Autoionization spectra of Li2 and the XΣg ground state of Li2: Experimental and theoretical investigations

Autoionizing Rydberg levels of Li₂ molecules in a supersonic molecular beam are populated by stepwise excitation with two tunable pulsed dye lasers. The observed autoionization spectra show severe perturbations. Based on calculations of quantum defects and a perturbation treatment of l-uncoupling a tentative assignment of Rydberg series up to n = 32 is proposed. The convergence limits of these series yield a value of IP = 41475 cm⁻¹ for the adiabatic ionization potential and a vibrational constant ω_e = 263 cm⁻¹ for the X²Σ⁺_g ground state of Li⁺₂. The experimental results are compared with ab initio calculations combined with a core polarization potential, which yield the potential curve. the dissociation energy, the quadrupole moment and the vibrational frequency for the X²Σ⁺_g ground state of Li⁺₂, in the excellent agreement with experimental findings.     

Thermochemical study of the complex formation of ag(I) with some sulphur-containing aminopyridines

The enthalpies of formation of the complexes between the silver(I) ion and some sulphur-containing aminopyridines of general formula N(CH₂)_n−1-S-(CH₂)_m-NH₂ where n = 1, and m = 1,2; 1,3; 2,2; 2,3 have been determined by direct calorimetric titration at 25°C in 0.5 M (K)NO₃ solution. The corresponding entropy terms, ΔS, have been calculated using the obtained enthalpy values and the previously reported ΔG values.

In acid medium (pH < 3) coordination occurs through the thioether group and the protonated species AgLH³⁺₂ and AgL₂H⁵⁺₄ are enthalpy-stabilized and entropy-destabilized. At higher pH values (pH &>; 3) additional chelation through the pyridine nitrogen is obvious by a marked increase in the enthalpy of formation of the complexes AgL₂H⁴⁺₃, AgL₂H³⁺₂ and AgLH²⁺. The last complex dimerizes into a cyclic dimer Ag₂L, H⁴⁺₂. At still higher pH values (pH &>; 6) the participation of the amino group in the dimeric chelates Ag₂L₂H³⁺ and Ag₂L²⁺₂ is revealed again by a marked increase in the heat of complexation.     

The enthalpy of mixing of liquid NaF and NaMgF3 from drop calorimetry

The enthalpy of mixing of three liquid mixtures of NaF and NaMgF₃ has been measured by drop calorimetry and was found to be negative. This energy release is attributed to a change in the equilibrium

Mg_1/4[MgF²⁻₄]_3/4+f⁻ å MgF²⁻₄ to the formation of complex MgF²⁻₄-ions. A ΔH^M diagram for the system NaF-MgF₂ up to 50 mol % MgF₂ has been constructed.     

Theoretical study of the thermal interconversion mechanism between the norbornadiene and quadricyclane radical cations

Geometry optimizations at the UHF/6-31G^* and UMP2/6-31G^* levels of theory were performed to find the transition state in the interconversion between norbornadiene (N) and quadricyclane (Q) radical cations. Two transition structures, TS^+·₁ and TS^+·₂, were obtained which have C₁ and C₂ symmetry, respectively. Vibrational analysis at the UHF and UMP2 levels of theory and IRC calculation showed that TS^+·₁ is the true transition state connecting N^+· and Q^+·, while TS^+·₂ is a second order saddle point.     

Reaction dynamics of the Ca(D2, PJ) + CH3I → CaI + CH3 system: chemiluminescence, energy disposal and product polarization

The Ca(¹D₂, ³P_J) + CH₃ → CaI(A,B) + CH₃ reactions system has been studied by measuring its chemiluminescence under beam-gas conditions. Absolute values of the state-to-state reaction cross-sections were determined at low collision energy . In addition, the electronic branching ratio and product energy disposal have been determined for each metastable reaction. The major changed observed in the chemiluminescence when comparing the Ca(¹D₂) reaction versus that of Ca(³P_J) is the total yield associated with the former reaction. To the best of our spectral resolution neither the electronic branching ratio e.g. CaI(A)/CaI(B) nor the internal CaI energy disposal change significantly as the metastable Ca(¹D₂)/Ca(³P_J) ratio is varied. In spite of the fact that the Ca(³P_J) reaction is less exoergic, the CaI product appears with a higher fraction of internal energy than that of Ca(¹D₂) reaction. Thus, the fraction of the total energy appearing in CaI internal energy amounts to 57.5% in the Ca(³P_J) reaction while it is 19.3% only for the Ca(¹D₂) reaction. This difference is discussed in the light of a distinct mechanism associated with the attack of the excited Ca atom into the C---I bond. No significant chemiluminescence yield was found for the energetically open CaCH^*₃ channels.

The product chemiluminescence polarization was also measured as a function of the metastable concentration. A significant degree of polarization was found depending upon the specific electronic excitation. The analysis of the polarization emission associated to the parallel CaI(X ²Σ⁺ ← B ²Σ⁺) emission led into a strong polarization of the product rotational angular momentum. The comparison of the product rotational alignment for the kinematically identical Ca(¹D₂, ³P_J, ¹P₁) + CH₃ → CaI^* (B₂Σ⁺) + CH₃ reaction system showed that the CaI rotational polarization diminishes in the ³P_J → ¹D₂ → ¹P₁ sequence, e.g. as the reaction exothermicity increases. In addition the degree of polarization associated with other emission bands as for example CaI(X ²Σ⁺ ← A ²Π_1/2) indicates the presence of a parallel transition which was been interpreted as mixing of Hund's case (a) and (c) appropriate for this heavy CaI diatom produced with a high rotational excitation.     

Ab initio molecular orbital calculations of nuclear spin-spin coupling constants in PH2, PH3, PH4 and P2H4

Ab initio molecular orbital calculations of nuclear spin-spin coupling constants in PH⁻₂, PH₃, PH⁺₄ and P₂H₄ have been carried out employing SCF perturbation theory. Basis set dependence of all the four contributing terms has been studied in order to find the criterion for the selection of basis sets to be employed for computing this property. The dependence of the coupling constants of PH⁻₂ on its geometry has also been found. This study also discusses the requirement for satisfactory computation of couplings in cases where none of the coupling nuclei is a proton. It is found that bond-centred functions along with at least double zeta basis sets reproduce coupling constants quite satisfactorily. In all the cases studied, uncontracted core basis functions yield couplings which are in better agreement with experimental couplings than those obtained with contracted core functions.     

An ab initio molecular orbital study on the gas-phase rapid reaction of the silicon cation Si with ammonia

The gas-phase rapid ion-molecule reaction Si⁺ (²P) + NH₃→ SiNH₂⁺ + H is theoretically investigated by the ab initio molecular orbital methods. Several possible pathways (A, B, C) on its potential energy surface have been examined, discussed and compared. Theoretical calculations indicate that pathway A is favourable in energy and that the reaction begins by forming a collision complex of the ion-dipole molecule Si-NH⁺₃, which forms with no barrier into the first energy well of the reaction coordinate. Migration of an H atom from an N atom to a Si atom forms the intermediate HSi-NH⁺₂, which corresponds to the second energy well and can fragment to the observed product SiNH⁺₂ by losing an H atom from the Si atom. The barriers for migration and fragmentation are 52.5 and 38.6 kcal mol⁻¹ respectively. Pathway A has a negative activation energy of −42.1 kcal mol⁻¹.     

The vapour pressure of di-arsenic pentoxide (As2O5)

The arsenic oxide pressure of As₂O₅ has been studied using mass spectrometry and a transportation method. Mass spectrometry revealed the presence of the species As₄O⁺₆, As₄O⁺₇, and As₄O⁺₈ in the vapour. The existence of volatile species up to As₄O₁₀(g) as a result of the reaction As₄O₁₀(g) As₄O_(10−y) (g) +1/2yO₂(g) has been assumed.

The oxygen pressure of this equilibrium builds up very slowly. The equilibrium pressure can be expressed by log(p_O2/atm) (880−952 K) = −(13940±930)/T + (14.53 ± 1.01)

A stationary arsenic oxide pressure has been measured using the transportation method. Since the oxygen pressure in the transportation gas did not influence the arsenic oxide pressure, it is assumed that only the As₄O₁₀(g) pressure has been measured. The results can be expressed by the linear function log(p_As4O10/atm) (865−1009 K) = −(15741 ± 410)/T + (13.87 ± 0.42).     

MRSDCI STUDIES OF LOW-LYING ELECTRONIC STATES OF THE N_2F~+ MOLECULE

Ab initio electronic structure calculations are reported for low-lying electronic states,X ~1Σ~+andA ~1Π of the N_2F~+ molecule.Geometric parameters for the ground state X ~1Σ~+ are predicted by means of mul-tireference single and double excitation configuration interaction(MRSDCI)calculations with a double zeta pluspolarization(DZ+P)basis set.Vertical excitation energy for these two electronic states is determined usingMRSDCI/DZ+P calculations at the ground state equilibrium geometry.The oscillator strength for the X~1Σ+→A ~1Π transition and the radiative lifetime for the A~1Π state are calculated based on the MRSDCI wavefunc-tions.     

Structure and stability of ammonium-sulfate and guanidium-sulfate complex

Ab initio (HF/6-31G^** and B3LYP/6-31 + + G^**) methods have been used to study the stability and structure of complexes between CH₃SO⁻₃ and CH₃NH⁺₃ or C(NH₂)⁺₃. Results show that no hydrogen jump is involved in the complex formations, which is different from previous work studying complexes between CH₃COO⁻ and CH₃NH⁺₃. In addition, we have studied complexes between CH₃SO⁻₃ and HC(NH₂)⁺₃ or ⁺H₃NC(NH₂)₃, all of which have a cage structure.     

Gas-phase stability of cluster ions SF m + (SF6) n with m = 0–5 and n = 1–3

The gas-phase stabilities of cluster ions SF⁺_m (SF₆)_n with m = 0−5 were determined by using a high pressure mass spectrometer. The bond energies of SF⁺_m (SF₆)₁ were found to be less than 10 kcal/mol and to decrease with m = 0 → 5. There appear to be rather large gaps in the bond energies between n = 1 and 2 for the clusters SF⁺_m (SF₆)_n with m = 0−4. The structures of SF⁺₅, SF⁺ (SF₆)₁, SF⁺₃ (SF₆)₁, and SF⁺₅ (SF₆)₁ were investigated by ab initio molecular orbital calculations. For SF⁺₅, the D_3h geometry is found to be most stable andC_4v is a transition state of the Berry pseudorotation. For the ion-molecule complexes, the “on-top hat” models were found to be the most stable structures.     

Prediction of the concentration polarization in the nanofiltration/reverse osmosis of dilute multi-ionic solutions

The aim of this work was to develop a simple and accurate model for predicting the concentration polarization index in the nanofiltration (NF)/reverse osmosis (RO) of dilute multi-ionic solutions. On the grounds of this model, the total flux of the ion i at the feed-solution/membrane interface consists of the sum of the diffusion, convection and migration fluxes, the former of which is determined by conventional mass-transfer correlations duly corrected to take into account the permeation through the membrane (suction effect). The coupling of the ionic fluxes is enforced by the electroneutrality requirement at the feed-solution/membrane interface. The model developed dispenses with the arbitrary assumption of the thickness of a film layer in the vicinity of the membrane surface.

Assessing the accuracy/validity of this model with multi-ionic solutions would be rather harsh, thus the model accuracy and ranges of validity were ascertained for a benchmark case: NF/RO of single salt solutions. The model predicts approximate concentration polarization indexes of the salts A⁺B⁻, A⁺₂B²⁻ and A⁺₃B³⁻ (or A²B⁻₂ and A³⁺B⁻₃) with positive deviations lower than 10% with respect to the benchmark concentration polarization index, for ions diffusivities ratios, D₁/D₂ (or D₂/D₁), in the range 0.16–5.5 and ≡J_v/k_c<3, where J_v is the permeation flux and k_c is the mass-transfer coefficient of the salt for vanishing mass-transfer rates at impermeable walls. The main assumption of the model – the individual mass-transfer coefficients of the ions are independent of each other – appears to hold in a broad range of conditions, for single salt solutions.

The model developed was expeditely applied to predict the concentration polarization in the nanofiltration of solutions containing Na⁺, Cl⁻ and a dye³⁻ (experimental data of Bowen and Mohammad [AIChE J. 44 (8) (1998) 1799–1812]), and its predictions are in fair agreement with the predictions of the extended Nernst–Planck equations in the film layer of the “slowest” ion.     

Magnetic circular dichroism assignment of the longest wavelength band of a series of tetrahedral d OXY- and Thioanions

Room temperature magnetic circular dichroism spectra have been obtained for MoS²⁻₄, MoO²⁻₄, WS²⁻₄, ReS⁻₄, VS³⁻₄, VO³⁻₄ and OsO₄ through their first and second charge-transfer bands. These measurements demonstrate that the band at longest wavelength (ν₁ band) must be assigned to a t₁ → 2e transition for all the compounds investigated.     

Rainbow scattering for Ar + Ar and Xe + Xe

Elastic differential scattering measurements have been performed on Ar⁺ + Ar and Xe⁺ + Xe. The rainbow scattering angle is found at τ = Eθ ≈ 115 eV deg for Ar⁺₂ and τ ≈ 93 eV deg for Xe⁺₂. These data are consistent with a potential well depth of 1.25 eV for Ar⁺₂ and 0.97 eV for Xe⁺₂.     

Spectroscopic investigation of ground state pyrrole (C4H5N): the NH stretch

We have recorded the infrared absorption spectrum of pyrrole at 0.005 cm⁻¹ spectral resolution using a Fourier transform interferometer. The rotational analysis of the fundamental N---H stretch (1¹₀) at 3530.811343(82) cm⁻¹ was performed. A set of 13 upper state rovibrational parameters was determined, allowing the 2715 assigned rovibrational lines to be reproduced with a standard deviation of 1.3 10⁻³ cm⁻¹. An attempt to record the fundamental band under slit-jet conditions is reported. The role of hot bands accompanying the series of the N---H stretch excitation is investigated. Effective vibrational parameters — ω⁰₁, X⁰₁₁, Y₁₁₁, X_1,24 — are obtained. The lower level in the hot band series is unambiguously identified as the V₂₄ = 1 level, by retrieving X_1,24 independently, from other spectral data. The observation of the complex band pattern accompanying the N---H series in the higher overtone range is discussed with the help of new data, recorded around the 1⁵₀ band at different temperatures using intracavity laser optoacoustic spectroscopy.