Semiclassical calculation of energy transfer in polyatomic molecules. XI. Cross sections and rate constants for Ar + CO2

Data from electron gas calculation on the short-range potential and theoretical van der Waals coefficients C_n (n = 6, 8) have been used to construct a potential surface for the Ar+CO₂ system. The surface has been used to calculate: second virial coefficient, viscosity and diffusion coefficient, rotational relaxation rates, rate constants for vibrational transitions in CO₂ and high-enery/small-angle differential cross sections.     

The vibrational relaxation of ozone by helium atoms

An ab initio, quantum-mechanical method is used to compute rate coefficients for the vibrational relaxation of ozone in three-dimensional collisions with helium atoms. Good agreement is obtained between the calculated and experimental rate coefficients for the process He + O₃(010) ? He + O₃(000).     

Ab initio energies and tunneling lifetimes of the doubly charged AH2+ (A = Mg—Ar) diatomics

Potential energy curves for the ground and low-lying excited states of the AH²⁺ (A = Mg—Ar) dications have been calculated using high-level ab initio methods with large atomic orbital basis sets. Quasi-bound potential energy curves with local minima and deprotonation barriers have been found for most of the dications studied. The energies, tunneling lifetimes, and widths of the quasi-bound states have been calculated by numerical solution of the radial Schrodinger equation using the Numeov method. All these dications except ArH²⁺ have low-lying states which support quasi-bound vibrational states. The ArH²⁺ dication has a ²∏_i potential energy curve with a minimum so shallow that it does not support any quasi-bound vibrational states. Results of our calculations are compared with previous ab initio calculations and available experimental data. © 1995 John Wiley & Sons, Inc.     

Quantum chemical rovibrational analysis of aminoborane and its isotopologues

Aminoborane, H₂NBH₂ and its isotopologues, H₂N¹⁰BH₂, D₂NBD₂, and D₂N¹⁰BD₂, have been studied by high-level ab initio methods. All calculations rely on multidimensional potential energy surfaces and dipole moment surfaces including high-order mode coupling terms, which have been obtained from electronic structure calculations at the level of explicitly correlated coupled-cluster theory, CCSD(T)-F12, or the distinguishable cluster approximation, DCSD. Subsequent vibrational structure calculations based on second-order vibrational perturbation theory, VPT2, and vibrational configuration interaction theory, VCI, were used to determine rotational constants, centrifugal distortion constants, vibrationally averaged geometrical parameters and (an)harmonic vibrational frequencies. The impact of core-correlation effects is discussed in detail. Rovibrational VCI calculations were used to simulate the gas phase spectra of these species and an in-depth analysis of the ν₇ band of aminoborane is provided. Color-coding is used to reveal the identity of the individual progressions of the rovibrational transitions for this particular mode.     

Accurate ab initio determination of the van der Waals interaction in the X 2Σ+ ground state of LiHe

Accurate quantum-chemical ab initio calculations have been performed at the SCF and CEPA (coupled electron pair approximation) levels for the van der Waals interaction in the X ² Σ ⁺ ground state of LiHe. An extended basis set has been used and the counterpoise correction for the basis set superposition error (BSSE) has been applied. The calculated potential energy curve has a very shallow minimum at 11.56 a ₀ with a well depth of only 1.49 cm^?1. This is too small to allow for a bound vibrational level. The analysis of the results shows that the interaction mainly consists of the Pauli repulsion between Li(1s ²2s) and He (1s ²), which is decaying exponentially, and the attractive London dispersion energy. Van der Waals coefficients C₆, C₈, and C₁₀ have been determined by a least squares fit to the long-range part of the calculated potential curve.     

Vibrational relaxation of CO2(v3) by O atoms

The vibrational relaxation time for CO₂(v₃) + O(³P) has been measured by laser fluorescence. The observed value, βCO₂.O = 0.21 ± 0.04 μsec, is an order of magnitude lower than the relaxation time for self-collisions.     

An ab initio potential energy surface and spectroscopic constants for the XΣg state of NO2

A three-dimensional potential energy function has been calculated for the X¹Σ⁺_g state of NO⁺₂ from ab initio MRD-CI data. With this PE function, converged vibrational calculations have also been performed for ten vibrational states, with the aid of a computer program developed in the present work for this purpose. The calculated harmonic frequencies, vibrational term values and rotational constants are in good agreement with experimental data.     

Ab initio prediction of the vibrational spectra of the hydrogen‐bonded complexes between CO and HONO2

The vibrational characteristics (vibrational frequencies and infrared intensities) for free and complexed CO and HONO₂ have been predicted using ab initio calculations at SCF and MP2 levels with different basis sets and B3LYP/6?31G(d,p) calculations. The ab initio calculations show that the complexation between HONO₂ and CO leads to two stable structures: CO … HONO₂ (1A) and OC … HONO₂ (1B). The changes in the vibrational characteristics from free monomers to complexes have been estimated. It was established that the most sensitive to the complexation is the stretching O? H vibration. In agreement with the experiment, its vibrational frequency in the complexes is shifted to lower frequency (Δν = ?123 cm^?1). The magnitude of the wave number shift is indicative of relatively strong hydrogen‐bonded interaction. The ab initio calculations at different levels predict an increase of the infrared intensity of the stretching O? H vibration for structure 1A more than five times and for structure 1B more than nine times. The most consistent agreement between the computed values of the frequency shifts for structure 1B and those experimentally observed suggests that this structure is preferred. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003     

Experimental and theoretical determination of rate constants for vibrational relaxation of CO2 and CH3F by He

Rate constants have been measured for the vibrational relaxation of CO₂ and CH₃F in the temperature range 300-150 K by the collision partners ⁴He and ³He. These results are compared with those calculated with a vibrational close-coupling, rotational infinite-order sudden approximation.     

Semiclassical calculation of cross sections and rate constants for vibrational deactivation of HD (v = 1) colliding with 4He

A semiclassical collision model has been used to calculate the rate constant for vibrational relaxation in HD (v = 1, j = 0) colliding with ⁴He. The He + HD potential surface was obtained from an analytical He + H₂ surface previously used for similar calculations on He + H₂ and He + D₂. The theoretically calculated rate constant is about 50% below that experimentally determined in the temperature range 80–300 K.     

A New Glimpse into the CO2‐Philicity of Carbonyl Compounds

We report a theoretical study on non‐conventional structures of 1:1 complexes between carbon dioxide and carbonyl compounds. These structures have never been reported before but are relevant for understanding the solubility of carbonyl compounds in supercritical CO₂. The work is based on the results of ab initio calculations at the MP2 and CCSD(T) levels using aug‐cc‐pVDZ and aug‐cc‐pVTZ basis sets. Investigated systems include aldehydes, ketones and esters, together with some fluorinated derivatives. The results are interpreted in terms of natural bond orbital analyses. Harmonic vibrational frequency calculations have also been done in order to compare them with available experimental data. We show for the first time that complexes where CO₂ behaves globally as a Lewis base are stable in the case of ketones and esters, but not in the case of aldehydes, and their stability is similar to that of traditional complexes in which CO₂ behaves as a Lewis acid. This finding considerably modifies the concept of CO₂‐philicity and may have important ramifications in the development of green reactions in supercritical CO₂.     

Proton-CO2(X1Σ) orientational anisotropy from an ab initio SCF potential energy surface

Ab initio calculations have been carried out, over an extended range of relative distances and geometries, to obtain the SCF potential energy surface that describes the ground-state interaction between H⁺ and the CO₂ molecular target treated as a rigid rotor. Various aspects of the forces are discussed with special attention to their general dependence on internal angle, affecting rotational excitation of CO₂ during colisional encounters. The corresponding multipolar coefficients are calculated and the asymptotic behaviour of their lowest terms compared with results from perturbation theory, thus providing some qualitative indication on the expected energy-transfer probability in molecular-beam experiments or in ion diffusion processes.     

BaO(X 1Σ+) vibrational relaxation

Vibrationally excited BaO(X ¹Σ⁺) was produced by reacting Ba atoms with O₂ under “jet flow” conditions in which the convective flow velocity was large compared with the diffusion velocities so that the relaxation could be spatially resolved. The vibrational level populations were determined by laser-induced fluorescence measurements. By using modeling calculations to fit the spatial variation of the apparent vibrational temperature, we obtained a vibrational relaxation rate for BaO(X ¹Σ⁺), ν = 1 → ν = 0, by Ar, of 9 × 10^?13 cm³/molecule s.     

Discrimination of two low-energy conformers of octahydroanthracene in supersonic jet

The S₀ and S₁ states’ vibronic features of two low-energy conformers of octahydroanthracene have been investigated by laser induced fluorescence excitation, dispersed emission and UV–UV hole burning spectroscopy in a cold seeded jet. UV–UV hole burning spectra have been used to distinguish the two conformers and their S₁ state vibrational modes. In addition, semi-empirical and ab initio calculations have been performed to correlate the calculated S₀ frequencies of vibrational modes with the experimental results. It is found that the observed spectra correspond well with sym-twist (a) (C_2h) and antisym-twist (b) (D₂) forms.     

Infrared Spectra of Krypton Solutions of Methylamine

Variable temperature (–105 to –145°C) studies of the infrared spectra (3500–400 cm^–1) of methylamine, CH₃NH₂, dissolved in liquid krypton have been recorded. From these data, the hydrogen bonding enthalpy has been determined to be 530 ± 29 cm^–1 (6.34 ± 0.35 kJ/mol). The elusive ₁₃ and ₁₄ fundamentals, which are strongly mixed CH₃ rock and NH₂ twist, have been observed at 1244 and 876 cm^–1, respectively. These assignments are supported by frequency predictions from ab initio MP2/6-31G(d) calculations where the predicted infrared intensities for these two vibrations are 0.054 and 0.002 km/mol. The ab initio predicted infrared spectrum compares very favorably with that observed in the krypton solution. Normal coordinate calculations have also been carried out for four other isotopomers of methylamine, CH₃NHD, CH₃ND₂, CD₃NH₂, and CD₃ND₂ and vibrational assignments given from previously reported infrared spectra of matrix isolated samples. The Raman spectrum of these latter three isotopes, along with the normal species, have been predicted from MP2/6-31G(d) calculations and the results compared to the experimental spectra. The equilibrium structural parameters have been obtained from ab initio calculations utilizing several different basis sets with full electron correlation by the perturbation method to second order. These predicted values are compared to the previously reported experimental structural parameters.     

Ab initio and density functional theory study of the enthalpies of formation of F2SOx and FClSOx (x=1, 2)

Enthalpies of formation of F₂SO, F₂SO₂, FClSO and FClSO₂ molecules have been determined using ab initio molecular orbital theory and density functional theory (DFT) calculations. Different DFT approaches and levels of the Gaussian-3 and the complete basis set (CBS) ab initio model chemistries have been employed to calculate enthalpies of formation from both total atomization energies and isodesmic reaction schemes. The best values at 298 K for F₂SO, F₂SO₂, FClSO and FClSO₂ as derived from an average of G3, G3B3, CBS-Q and CBS-QB3 isodesmic energies are −140.6, −181.1, −92.6 and −132.3 kcal mol⁻¹, respectively. The results obtained suggest that the accumulated small component errors found in the DFT-based methods are significantly reduced at the ab initio levels employed. Structural properties, harmonic vibrational frequencies, mode assignations and infrared intensities derived from B3LYP and mPW1PW91 functional with the 6-311+G(3df) basis set are presented.     

Vibration—vibration energy exchange between carbon monoxide and carbon dioxide

Measurements have been made on the vibration—vibration (V—V) energy exchange rate between carbon monoxide and carbon dioxide in the temperature range 180 to 345 K. A steady-state vibrational fluorecence quenching technique was used in conjunction with an open flow gas system. Vibrational excitation of the carbon monoxide was accomplished by absorption of infrared radiation from prospane—oxygen flames. The measured rate constant for the process CO* (υ = 1) + CO₂ → CO + CO^*₂(001) increased linearly with temperature, and after correction for the V—V exchange rate fo the back reaction, the rate constant has a value of (2.2 ± 0.3) × 10³ torr^?1 s^?1 at 296 K. The data are compared to results at highest temperatures and to available theoretical calculations.     

Spectra and Structure of Silicon-Containing Compounds. XXVIII1 Infrared and Raman Spectra,Vibrational Assignment,and Ab Initio Calculations of Vibrational Spectrum and Structural Parameters of Vinyltrichlorosilane

The infrared spectra (3200-50 cm^–1) of gaseous and solid vinyltrichlorosilane, CH₂=CH-SiCl₃, have been recorded. In addition, the Raman spectrum (3200-10 cm^–1) of the liquid has been recorded and quantitative depolarization values obtained. The infrared spectrum of the sample dissolved in liquid xenon (–80°C) has also been recorded. Using the experimental data and normal coordinate calculations with scaled ab initio force constants, the complete vibrational assignment is proposed. The torsional mode was observed in the infrared spectrum of the gas at 69 cm^–1 and the threefold barrier of internal rotation was calculated to be 500 cm^–1 (5.98 kJ/mol). Ab initio calculations have been carried out at the restricted Hartree–Fock level of the theory as well as with full electron correlation by the perturbation method to second order with different basis sets up to 6-311+G(d,p) to obtain the optimized geometries, harmonic force constants, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies. The ab initio predicted structural parameters are compared with those obtained from a previous electron diffraction study.     

Vibrational kinetics in HeCO reacting discharges

Combined measurements of vibrational distributions (N_υ) of CO and CO₂ yields (β) in HeCO discharges have been performed at different residence times in radiofrequency discharges. The experimental results on N_υ have been obtained by IR emission spectroscopy and on β by gas-chromatographic and mass-spectrometric techniques. A theoretical model including the most important relaxation channels of the vibrational energy has been set up and coupled to the plasma chemistry describing the rate of formation of species such as CO₂, C, and O. Theoretical and experimental results are in good agreement, emphasizing the role of a vibrational mechanism in dissociating CO in HeCO mixtures.     

The vibrational and NMR spectra,conformations and ab initio calculations of aminomethylene,propanedinitrile and itsN-methyl derivatives

The IR and Raman spectra of aminomethylene propanedinitrile (AM) [H₂N-CH=C(CN)₂], (methylamino)methylene propanedinitrile (MAM) [CH₃NH-CH=C(CN)₂] and (dimethylamino)methylene propanedinitrile (DMAM) [(CH₃)₂N-CH=C(CN)₂] as solids and solutes in various solvents have been recorded in the region 4000-50 cm^–1. AM and DMAM can exist only as one conformer. From the vibrational and NMR spectra of MAM in solutions, the existence of two conformers with the methyl group orientedanti andsyn toward the double C=C bond were confirmed. The enthalpy difference H ⁰ between the conformers was measured to be 3.7±1.4 kJ mol^–1 from the IR spectra in acetonitrile solution and 3.4±1.1 kJ mol^–1 from the NMR spectra in DMSO solution. Semiempirical (AM1, PM3, MNDO, MINDO3) and ab initio SCF calculations using a DZP basis set were carried out for all three compounds. The calculations support the existence of two conformersanti andsyn for MAM, withanti being 7.8 kJ mol^–1 more stable thansyn from ab initio and 8.6, 13.4, 11.6, and 10.8 kJ mor^–1 from AM1, PM3, MNDO, and MINDO3 calculations, respectively. Finally, complete assignments of the vibrational spectra for all three compounds were made with the aid of normal coordinate calculations employing scaled ab initio force constants. The same scale factors were optimized on the experimental frequencies of all three compounds, and a very good agreement between calculated and experimental frequencies was achieved.