An ab initio calculation of symmetric bending and stretching vibrational states of the H3O+ and D3O+ ions

Calculations are reported for the symmetric bending and stretching vibrational states of H₃O⁺ and D₃O⁺ including coupling between these two modes. The calculations were carried out by using a potential surface calculated by the SCF CI method and expressed in terms of symmetric internal coordinates. The transition energy of the ν₂ (1^? ← 0⁺) inversion mode is found to be 985 cm^?1, which is comparable to the experimental value of 954.417 cm^?1 observed by Haese and Oka. The calculated inversion doubling of the lowest state is 51 cm^?1.     

A kinetic study of proton transfer reactions of NH+4, CH3NH+3, and PH+4 with calcium atoms

Cross section measurements for the proton transfer reactions of NH⁺₄, CH₃NH⁺₃, and PH⁺₄ with Ca(g) have been obtained over a range of low ion kinetic energies. For all reactions studied the cross sections drop sharply with increase in ion kinetic energy, indicating exothermic behavior. The results show that Ca(g) is an unusually strong base with a proton affinity in excess of 9.2 eV. Cross sections for the PH⁺₄Ca reaction are an order to magnitude higher than those for the NH⁺₄Ca reaction for ion energies between one and three eV. This effect is not explained by simple theories of ion-induced dipole interactions. It is suggested that the enhanced rate of the PH⁺₄Ca reaction may be due to d-orbital participation.     

Excited states of gaseous ions. V. The predissociation of the h2o+ ion

The B?² state of H₂O⁺ is predissociated twice. First, by the ã⁴B₁ state, giving OH⁺ + H fragments via spinorbit coupling interaction. Secondly, by a²A state, giving H ⁺ OH fragments via spin-orbit coupling and Coriolis interactions. A vibrational analysis of the photoelectron band of the B? state of H₂O⁺ and D₂O⁺ is carried out. This provides the vibrational frequencies of the H₂O⁺, D₂O⁺ and HDO⁺ ions, as well as a vibrational assignment of the peaks. The H₂O⁺ ion in its B?²B₂ state is found to have a OH bond length of 1.12 A and a valence angie of 78°.In order to describe the unimolecular fragmentation process, a distinction is introduced between the totally symmetric, optically active vibrational modes, and the antisymmetric ones which are coupled to the continuum. The former are supplied with photon or electron impact energy, but only the latter are chemically efficient. The dynamics of the dissociation process depends therefore on the couplings among normal modes. This is studied in the framework of two models. In Model 1, it is assumed that, as a result of the anharmonicity of the potential energy surface, only even overtones of the antisymmetric vibration are excited by Fermi resonance. In Model II, excitation of the odd overtones is provided by vibronic coupling. Model II is in better agreement with experiment than Model I. Calculated and experimental results have been compared on the following points: isotopic shift on the appearance potential of OH⁺ and OD⁺ ions, shapes of the photoionization curves, fragmentation pattern with 21 eV photons, presence of a unimolecular metastable transition, production of O⁺ ions. All the vibrational levels situated above the dissociation asymptote are totally predissociated. Autoionization is shown in this case to contribute only to the formation of molecular H₂O⁺ ions, and not to that of the OH⁺ fragments. For 21 eV electrons, the contribution due to direct ionization is calculated to represent about 25% of the total cross section, the rest being due to autoionization.     

Radiative lifetimes of the B1Π11 states of Li2

Model potential calculations of the B¹Π_uX¹Σ⁺_g transition dipole moment are used in conjunction with the empirical potential energy curves of Vidal and Hessel to calculate the radiative lifetimes of the vibrational levels of the B¹Π_u state. The lifetimes are nearly independent of the vibrational quantum number with a value between 8 and 9 ns.     

Triatom-triatom collisions: Fixed angle close-coupling study of vibrational excitation in the 12CO2+13CO2 system

A close-coupling approach to the calculation of quantal vibrational transition probabilities for the fixed angle scattering of a linear triatomic molecule with another linear triatomic molecule is described. The method is applied to the ¹²CO₂+¹³C0₂ collisional system. For a calculated inelastic transition probability to have an appreciable magnitude, it is found that the amount of energy transferred in a transition must be very small and just one quantum of energy must be exchanged between either the symmetric stretch or the asymmetric stretch vibrational modes of ¹²C0₂ and ¹³CO₂. For collisional energies away from threshold, the probabilities for transitions involving the symmetric stretch ¹²CO₂ and ¹³CO₂ modes are insensitive to long range multipole terms in the potential energy surface, while the probabilities for energy exchange between the asymmetric stretch modes are considerably diminished when the long range terms are removed from the potential energy surface. A brief discussion is presented on the possibilities of extending the technique to the calculation of vibrational excitation cross sections for three-dimensional triato—triatom collisions.     

Ligands of low electronegativity in the vsepr model: electron-rich hydrides MH3E2, MH4E,MH5E,and MH6E

Hydrides FH₃, ClH₃, and OH^?₃ of type MH₃E₂ are calculated to adopt D_3h structures: NH₃^2?, PH₃^2?, and SH₃^? each have two energy minima, one at D_3h and the other at a T-shaped geometry, of which the D_3his the more stable for SH₃^? but the less stable for NH₃^2? and PH₃^2?. Hydrides NH₄^2?, OH₄, and ClH₄⁺ of type MH₄E have a single energy minimum at T_d: CH₄^2?, SiH₄^2?, PH₄^?, and SH₄ each have two minima, one at T_d (more stable for SH₄ only) and one at an SF₄-like C_2v geometry, which is the more stable for CH₄^2?, SiH₄^2? and PH₄^?. D_3h and C_4V structures are very close in energy for all hydrides of type MH₅E, with no activation barrier between the two configurations: D_3h is the more stable configuration for OH₅^?, FH₅, SH₅, and ClH₅, but C_4V is the more stable for NH₅^2?, SiH₅^3?, and PH₅^?. The T_1u bending force constant in hydrides MH₆E becomes negative, for C_3V distortion, in PH₆^3? and SiH₆^4?. Both the equilibrium geometries and the force constants strongly support an interpretation, in terms of the second-order Jahn-Teller effect, of the observed stereochemical inactivity of non-bonding electrons in the presence of ligands of low electronegativity. Molecular energies, equilibrium geometries, orbital energies and electron populations are reported for all species considered in this study. Three molecular states of ClH₄^?, of type MH₄E₂, were also briefly investigated.     

Internal-energy-selected measurements on the relative cross section of the ion-molecule reaction NH3+(NH3, NH2)NH4+

A photoelectron-photoion coincidence technique is used to measure the internal-energy dependence of the ion-molecule reaction NH₃⁺(E_int+NH₃ → NH₄⁺ + NH₂ at thermal collision energy. The range in which the internal energy is varied, is enlarged by including in the experiment the electronically excited state of the NH₃⁺ ion. Special attention is paid to the possible influence of the product's kinetic energy on the measurements. The experimental results are analysed using a modified statistical model and compared with previous data.     

Vibrational energy transfer from N2 to CO on electron excitation of N2-CO solids at 4.2 K

Spectra emitted from 0.1% CO-N₂ solids excited with high energy electrons at 4 K show evidence for resonant transfer of vibrational energy from highly excited vibrational levels of N₂ to CO in the process N₂(X¹Σ_g⁺, ν) + CO(ν = 0) → N₂(X¹Σ_g⁺, ν - 1) + CO(ν = 1) + phonons. Energy transfer from levels with ν ? 9 has been observed.     

Effect of rotation on vibrational excitation of H2 by Li impact

Coupled channel calculations of integral cross sections for rotational and vibrational excitation of H₂(X¹Σ⁺_g by collision with Li⁺ are reported for 1.2 eV in the c.m. system employing an ab initio potential energy surface and numerical vibration—rotation functions of the Koo?s—Wolniewicz potential function including adiabatic correction. Pure rotational excitation is found to strongly dominate the inelastic scattering occurring at this energy. Preparation of H₂ in various allowed non-zero rotational states is seen to enhance the 0 → 1 vibrational cross section by approximately an order of magnitude.     

1,2-Eliminations from (CH3)2NH+CH2CH3 and (CH3)2NH 2+ : Guided dissociations

1,2-Eliminations are a varied and extensive set of dissociations of ions in the gas phase. To understand better such dissociations, elimination of CH₂=CH₂ and CH₃CH₃ from (CH₃)₂NH⁺CH₂CH₃ (1) and of CH₄ from (CH₃)₂NH₂⁺ are characterized by quantum chemical calculations. Stretching of the CN bond to ethyl is followed by shift of an H from methyl to the bridging position in ethyl and then to N to reach (CH₃)₂NH₂⁺ + CH₂=CH₂ from 1. CH₃CH₃ elimination by H-transfer to C₂H₅⁺ to form CH₃NH⁺=CH₂ + CH₃CH₃ also takes place. (CH₃)₂NH₂⁺ eliminates methane by CN bond extension followed by β-H-transfer to give CH₂=NH⁺ + CH₄. Low-energy reactions resembling complex-mediated 1,2-eliminations occur and constitute a hitherto largely unrecognized type of reaction. As in many complex-mediated reactions, these reactions transfer H between incipient fragments. They are distinguished from complex-mediated processes by the fragments not being able to rotate freely relative to each other near the transition state for reaction, as they do in complexes. Most 1,2-eliminations are ion-neutral complex-mediated, occur by the just described lower energy reactions, have 1,1-like transition states, or utilize highly asynchronous 1,2 transition states. All of these avoid synchronized 1,2-transition states that would violate conservation of orbital symmetry.     

IR and Raman spectra of two layered aluminium phosphates Co(en)3Al3P4O16 · 3H2O and [NH4]3[Co(NH3)6]3[Al2(PO4)4]2 · 2H2O

The FT IR and FT Raman spectra of Co(en)₃Al₃P₄O₁₆ · 3H₂O (compound I) and [NH₄]₃[Co(NH₃)₆]₃[Al₂(PO₄)₄]₂ · 2H₂O (compound II) are recorded and analysed based on the vibrations of Co(en)₃³⁺, Co(NH₃)₆³⁺, NH₄⁺, Al---O---P, PO₃, PO₂ and H₂O. The observed splitting of bands indicate that the site symmetry and correlation field effects are appreciable in both the compounds. In compound I, the overtone of CH₂ deformation Fermi resonates with its symmetric stretching vibration. The NH₄ ion in compound II is not free to rotate in the crystalline lattice. Hydrogen bonding of different groups is also discussed.     

Hydrogen bonded complexes and the HF vibrational energy distributions from the reaction of F atoms with NH2 and NH3

The gas phase reactions of fluorine atoms with amino radicals and ammonia molecules: F(²P)+NH₂(²B₁) → HF(¹Σ⁺)+NH(³Σ⁻) and F(²P)+NH₃(¹A₁) → HF(¹Σ⁺)+NH₂(²B₁) produce hydrogen fluoride with very different primary vibrational energy distributions as determined by low-pressure chemiluminescence studies. The reaction with NH₂ yields HF with an inverted primary vibrational energy distribution, P(v′=1:2:3:4)=0.23:0.68:0.08:0.01. The HF from the reaction with ammonia is cold (non-inverted), P(v′=1:2)=0.60:0.40. Recent experimental work on these reactions is critically assessed and some discrepancies between low-pressure chemiluminescence results and fast-flowing afterglow studies are resolved. The results of high-level ab initio calculations (up to 6–311G^** CISD) on reactants, products, and the hydrogen bonded complexes FH … NH and FH … NH₂ in the exit channels are reported. The most reliable of the computations predict that FH … NH₂ is significantly more bound than FH … NH (8.1 versus 4.1 kcal mol⁻¹ in comparison with products at the 6-311G^** MP2 level).Also, the calculated vibrational frequencies for the two hydrogen bonded complexes indicate that the FH stretch and NH₂ asymmetric stretch are much closer in frequency in FH … NH₂ than are the FH and NH stretches in FH … NH. The strong interaction and the close match of vibrational frequencies in the FH … NH₂ case both will lead to fast internal vibrational relaxation (IVR) of the reaction exoergicity from the FHN bonds, where it is released, to the NH₂ fragment in the F/NH₃ reaction. Thus, the HF produced in this reaction is expected to have less vibrational excitation than that created in the F/NH₂ reaction, for which these IVR mechanisms are not as important, and simple direct abstraction dynamics are expected.     

C2 and CN formation by optical pumping of CO/Ar and Co/N2/Ar mixtures at room temperature

A continuous wave carbon monoxide laser is used to excite the vibrational mode of CO in CO/Ar and CO/N₂/Ar mixtures flowing through a gas absorption cell. High steady-state excitation of the CO vibrational mode (0.3 eV/molecule) is achieved, while a translational—rotational temperature near 300 K is maintained by the steady flow of cold gas into the cell. These non-equilibrium conditions result in extreme vibration—vibration pumping, population high-lying vibrational quantum levels (to V = 42) of CO. N₂ can also be pumped by vibrational energy transfer from CO. Under these conditions, C₂ and CN molecules are formed, and are observed to fluoresce on various electronic band transitions, notably C₂ Swan (A ³Π_g—X ³Π_u) and CN violet (B ²Σ⁺—X²Σ⁺).     

Thermal-energy charge transfer of Ar+ with H2O: Internal and kinetic energy of the product H2O+

The reaction of Ar⁺ with H₂O has been investigated at near-thermal energy. The product ions H₂O⁺ and ArH⁺ account for 90 and 10% of the total reaction rate, respectively. Kinetic energy measurements and emission spectroscopy of the H₂O⁺ product ions are reported. It is concluded that at least 60% of H₂O⁺ ions are in the X? state with ≈2.4 eV vibrational energy while up to 40% are in the à state with a mean vibrational energy of 1.4 eV; the à state vibrational distribution has been determined. It is shown that both H₂O⁺ states are populated via an energetically “non-resonant” charge transfer process.     

Energy disposal in thermal-energy charge-transfer reactions: Ar+,Kr+ and Xe+ with NH3

Thermal-energy charge-transfer reactions from the ²P_3/2 state of Ar⁺, Kr⁺ and Xe⁺ with NH₃ are shown to be non-energy resonant: the kinetic energy released in each case has been measured, and the internal energy of the NH⁺₃ product ions deduced. Possible quenching of ²P_1/2 state of rare-gas ions in ICR cells is discussed.     

Diatomics-in-molecules correlation diagrams for (BH2)+

The diatomics-in-molecules method is applied to calculate potential energy surfaces of the system B⁺(¹S, ³P) + H₂ (X¹ Σ_g⁺. Results are presented as correlation diagrams following the approximate minimum energy paths for C_∞v and C_2v geometries of the reactants. Two possible non-adiabatic mechanisms of complex formation are discussed.     

Substituent effects in second row molecules: Molecular orbital studies of phosphorus(III) compounds

Substituent effects in directly bonded P(III) compounds are investigated by ab initio MO calculations of relative energies and the results compared with those for the corresponding nitrogen species. The investigation covers substitution by X = BH₂, CH₃, NH₂, OH, F in PHX^-, PH₂X, and PH₃X⁺ series molecules with some attention also to PX₃ and PX₃H⁺ species. Except for compounds containing the π-acceptor substituent BH₂, σ-interactions dominate substitution behaviour but the second row species tolerate electron withdrawal better than their first row analogues, the severe destabilization of NH₂X and NH₃X⁺ by σ-electron withdrawal being absent from PH₂X and PH₃X⁺. In contrast to the σ-withdrawing NH₂ group, the PH₂ group is characterized as a mild σ-donor. PH^- is a σ-donor and PH⁺₃ a σ-acceptor. π-Bonding to the second row atom is an important means of maintaining electroneutrality in the PH₃X⁺ series, where dπ functions have a bigger role than pπ functions.     

Millimeter-wave spectrum of isocyanamide,H2N-NC

The a-type rotational spectrum of isocyanamide was observed in the frequency range 147–300 GHz. Ether extraction of the hydrolysis products of diazomethyllithium was employed to isolate the isocyanamide. 53 rotational lines were assigned to molecules in the ground vibrational state (NH₂-inversion state 0⁺) and in the lowest excited vibrational state (NH₂-inversion state 0^?.     

Effect of internal and translational energy on the NH3+(ν) + D2 ion-molecule reaction

Using 2+1 multiphoton ionization, NH₃⁺ is prepared in selected levels of the ν₂ bending mode (ν = 0−7) and the NH₃⁺(ν) + D₂ reaction is studied as a function of the center-of-mass collision energy (1–10 eV). The exchange channel (NH₂D⁺ + HD or H + D) is enhanced by ion vibrational excitation whereas the addition channel (NH₃D⁺ + D) is almost unaffected.     

Time-dependent close coupling for vibrational excitation in three dimensions: Application to H+ - H2

Impact parameter calculations for the non-reactive H⁺ + H₂ (n_i = 0) → H⁺ + H₂ (n_f) collision are reported for energies 10 eV ? E_cm ? 200 eV describing the rotational motion of the molecule in the sudden limit. The time-dependent Schrödinger equation for the vibrational motion has been solved by close coupling techniques expanding the vibrational wavefunction into both harmonic and numerically exact H₂ bound states. The convergence in vibrational basis sets, where up to six vibrational levels are considered, becomes worse with decreasing energy and increasing inelasticity. Furthermore, the harmonic wavefunctions are not suitable over a large range of energies to calculate proper cross sections. The various integral and differential cross sections have been compared with the classical results of Giese and Gentry.