Liquid Argon Solutions Research Articles

A Combined Experimental/Quantum-Chemical Study of Tetrel, Pnictogen, and Chalcogen Bonds of Linear Triatomic Molecules.

Linear triatomic molecules (CO2, N2O, and OCS) are scrutinized for their propensity to form perpendicular tetrel (CO2 and OCS) or pnictogen (N2O) bonds with Lewis bases (dimethyl ether and trimethyl amine) as compared with their tendency to form end-on chalcogen bonds. Comparison of the IR spectra of the complexes with the corresponding monomers in cryogenic solutions in liquid argon enables to determine the stoichiometry and the nature of the complexes. In the present cases, perpendicular tetrel and pnictogen 1:1 complexes are identified mainly on the basis of the lifting of the degenerate ν 2 bending mode with the appearance of both a blue and a red shift. Van ′t Hoff plots of equilibrium constants as a function of temperature lead to complexation enthalpies that, when converted to complexation energies, form the first series of experimental complexation energies on sp1 tetrel bonds in the literature, directly comparable to quantum-chemically obtained values. Their order of magnitude corresponds with what can be expected on the basis of experimental work on halogen and chalcogen bonds and previous computational work on tetrel bonds. Both the order of magnitude and sequence are in fair agreement with both CCSD(T) and DFA calculations, certainly when taking into account the small differences in complexation energies of the different complexes (often not more than a few kJ mol−1) and the experimental error. It should, however, be noted that the OCS chalcogen complexes are not identified experimentally, most probably owing to entropic effects. For a given Lewis base, the stability sequence of the complexes is first successfully interpreted via a classical electrostatic quadrupole–dipole moment model, highlighting the importance of the magnitude and sign of the quadrupole moment of the Lewis acid. This approach is validated by a subsequent analysis of the molecular electrostatic potential, scrutinizing the σ and π holes, as well as the evolution in preference for chalcogen versus tetrel bonds when passing to “higher” chalcogens in agreement with the evolution of the quadrupole moment. The energy decomposition analysis gives further support to the importance/dominance of electrostatic effects, as it turns out to be the largest attractive term in all cases considered, followed by the orbital interaction and the dispersion term. The natural orbitals for chemical valence highlight the sequence of charge transfer in the orbital interaction term, which is dominated by an electron-donating effect of the N or O lone-pair(s) of the base to the central atom of the triatomics, with its value being lower than in the case of comparable halogen bonding situations. The effect is appreciably larger for TMA, in line with its much higher basicity than DME, explaining the comparable complexation energies for DME and TMA despite the much larger dipole moment for DME.

C–H Infrared Absorption and Solubility of Ethylene, Propyne, 2-methyl-2-butene, and 2-methyl-1, 3-butadiene (Isoprene) in Liquid Argon Solutions

The solubility of ethylene (H2C=CH2), propyne (CH3-C≡C-H), 2-methyl-2-butene (CH3-CH=C(CH3)2), and isoprene or 2-methyl-1, 3-butadiene (H2C=C(CH3)-CH=CH2) in liquid argon has been measured using mid-infrared and near-infrared (NIR) absorption. Spectra were recorded in the C-H infrared (IR) region. Spectra were obtained at increasing solution composition until the magnitude of the integrated absorption band reached a maximum value, indicating a saturated solution. The approximate experimental solubilities are: (600 ± 100) ppm at 92 K for ethylene, (22 ± 9) ppm at 100 K for propyne, (9 ± 5) ppm at 100 K for 2-methyl-2-butene, and (12 ± 2) ppm at 86 K for isoprene. The experimental solubility values at the corresponding temperature were used with solubility parameters of two separate models: the perturbed-chain statistical associating fluid theory (PC-SAFT) and the regular solution theory. Solvent-solute interaction parameters k12 (PC-SAFT) and [Formula: see text] (RST) were obtained for each solute in the presence of argon as the solvent. Data from experimental measurements are important for more realistic simulations of solubility of solids in cryogenic liquids.

On the ν1/2 ν2 Resonance in the Complex of Carbon Dioxide with Dimethyl Ether

Infrared and Raman spectra of solutions in liquid argon and krypton containing dimethyl ether or its fully deuterated isotopomer and (12)CO(2) or (13)CO(2) are investigated. The spectra lead to new data on the nu(1) (CO(2) )/2 nu(2) (CO(2) ) resonances appearing in the complex of CO(2) with the ether. The experimental data, and their interpretation, is supported by MP2/6-311++G(2d,2p) calculations of the cubic and quartic force constants and of the first and higher order dipole moment derivatives required for the modelling of the Fermi and Darling-Dennison resonances observed.

CX⋅⋅⋅O Halogen Bonding: Interactions of Trifluoromethyl Halides with Dimethyl Ether

The formation of weakly bound molecular complexes between dimethyl ether (DME) and the trifluoromethyl halides CF(3)Cl, CF(3)Br and CF(3)I dissolved in liquid argon and in liquid krypton is investigated, using Raman and FTIR spectroscopy. For all halides evidence is found for the formation of C-X...O halogen-bonded 1:1 complexes. At higher concentrations of CF(3)Br, a weak absorption due to a 1:2 complex is also observed. Using spectra recorded at temperatures between 87 and 125 K, the complexation enthalpies for the complexes are determined to be -6.8(3) kJ mol(-1) (DME x CF(3)Cl), -10.2(1) kJ mol(-1) (DME x CF(3)Br), -15.5(1) kJ mol(-1) (DME x CF(3)I), and -17.8(5) kJ mol(-1) [DME(x CF(3)Br)(2)]. Structural and spectral information on the complexes is obtained from ab initio calculations at the MP2/6-311++G(d,p) and MP2/6-311++G(d,p)+LanL2DZ* levels. By applying Monte Carlo free energy perturbation calculations to account for the solvent influences, and statistical thermodynamics to estimate the zero-point vibrational and thermal influences, the ab initio complexation energies are converted into complexation enthalpies for the solutions in liquid argon. The resulting values are compared with the experimental data deduced from the cryosolutions.

Thermal Lens Spectroscopy in Cryogenic Solutions: Analysis and Comparison of Intensities in CH4−N2 and CH4−Ar Liquid Solutions

The C-H (Delta upsilon = 6) absorption spectrum of methane has been obtained in liquid nitrogen and argon solutions using thermal lens spectroscopy. Mathematical models for continuous and periodic excitation were used to describe the concentration dependence of thermal lens intensities. Better fitting of the experimental results was accomplished by taking into account the periodic nature of the excitation. Comparison of thermal lens intensities in nitrogen and argon allowed calculation of relative enhancement factors. In dilute solutions, the intensity in liquid nitrogen was 1.39 +/- 0.08 times higher than that in argon. The literature estimation of this ratio is 1.49 +/- 0.61. In contrast to the estimated value, our result confidently shows actual variation in enhancement factors. This article not only shows the applicability of thermal lensing to cryosamples but also demonstrates that accurate measurements at low temperature are possible.

Thermal Lens Spectroscopy in Liquid Argon Solutions: (Δv = 6) C−H Vibrational Overtone Absorption of Methane

A dual-beam thermal lens technique has been used to obtain the absorption spectrum of the (Deltav = 6) C-H stretch of liquid methane and methane in liquid argon solutions. The lowest concentration detected was 1 x 10(-3) (mole fraction) of CH(4) in liquid Ar with a continuous wave laser power of 20 mW. The thermal lens signal is linear with the mole fraction of methane up to 1 x 10(-2) but not for higher concentrations. Considering the system CH(4)-Ar as an ideal solution, the factors that contribute to the thermal lens signal were calculated as a function of the concentration of methane. A mechanism of energy transfer based on the gas-phase results could explain qualitatively the dependence of the magnitude of the signal on the mole fraction of methane.

The vibrational spectrum of NF3 and the manifestation of resonant dipole-dipole interaction in NF3 solutions in liquid argon.

Vibrational spectra of trifluoramine, NF3, dissolved in liquid Ar were studied at 90 K in the concentration range between 2 x 10(-5) and 0.1 mole fraction, using Fourier transform spectroscopy. The concentration dependence of the band shapes in the region of the combination transitions nu1+nu3, nu2+nu3, and 2nu3 involving the strong nu3 mode was studied and the absorption associated with NF3 dimers was isolated. This absorption is compared with spectra of NF3 dimers calculated on the basis of resonant dipole-dipole interaction between two doubly degenerate oscillators. Spectra of pure liquid NF3 were recorded for comparison. Using the nu1+nu3 absorption band of the NF3 dimer the distance R between two NF3 molecules was determined to be R=4.5(1) A in solution in liquid Ar. This distance is compared with the separation between two NF3 molecules in liquid NF3 and with the value calculated from the pair distribution function obtained from Monte Carlo simulations.

Van der Waals Complex of Dimethyl Ether with Carbon Dioxide

The 1:1 complex between dimethyl ether and carbon dioxide was studied experimentally in solutions in liquid argon, using infrared spectroscopy, and theoretically, using ab initio calculations at the MP2/6-311++G(d,p) level. The complex is formed through the interaction of the dimethyl ether oxygen atom with the CO2 carbon atom, with the CO2 perpendicular to the 2-fold axis of dimethyl ether, in the plane of the heavy atoms of the latter. Observed vibrational bands are in good agreement with the predicted vibrational frequencies. The standard complexation enthalpy of the complex in liquid argon was determined from a temperature dependent study of the spectra and was found to be −8.0(3) kJ mol-1. Some aspects of the Fermi resonance between the symmetric stretch and the first overtones of the two bending modes of the CO2 moiety of the complex are discussed.

The nature of improper, blue-shifting hydrogen bonding verified experimentally.

In the infrared spectra of solutions in liquid argon of dimethyl ether ((CH(3))(2)O) and fluoroform (HCF(3)), bands due to a 1:1 complex between these monomers have been observed. The C-H stretch of the HCF(3) moiety in the complex appears 17.7 cm(-1) above that in the monomer, and its intensity decreases by a factor of 11(2). These characteristics situate the interaction between the monomers in the realm of improper, blue-shifting hydrogen bonding. The complexation shifts the C-F stretches downward by some 9 cm(-1), while the C-H stretches in (CH(3))(2)O are shifted upward by 9-15 cm(-1), and the C-O stretches are shifted downward by 5 cm(-1). These shifts are in very good agreement with those calculated by means of correlated ab initio methods, and this validates a two-step mechanism for improper, blue-shifting hydrogen bonding. In the first step, the electron density is transferred from the oxygen lone electron pairs of the proton acceptor ((CH(3))(2)O) to fluorine lone electron pairs of the proton donor (CHF(3)) which yields elongation of all CF bonds. Elongation of CF bonds is followed (in the second step) by structural reorganization of the CHF(3) moiety, which leads to the contraction of the CH bond. It is thus clearly demonstrated that not only the spectral manifestation of H-bonding and improper H-bonding but also their nature differ.

Ab Initio and Cryospectroscopic Investigation of the Van der Waals Complexes of Methylcyclopropane with Hydrogen Chloride and Boron Trifluoride

The mid infrared spectra of methylcyclopropane−HCl and methylcyclopropane−BF3 mixtures, dissolved in liquid argon (LAr) or liquid nitrogen (LN2), have been examined. In LAr solutions, evidence was found for the formation of two different isomers of the 1:1 van der Waals complexesan asymmetric complex, in which the electron acceptor interacts with the C−C bond adjacent to the methyl group, and a symmetric complex in which the interaction occurs with the opposite C−C bond. At higher concentrations, indications were found for the formation of a chain-type 1:2 complex. In LN2 solutions, only the two 1:1 isomers of the HCl complex have been observed. Using spectra recorded at different temperatures, the complexation enthalpy ΔH° in LAr was determined to be −9.5(4) and −8.3(3) kJ mol-1 for the symmetric and asymmetric MCP·HCl complexes, and −9.1(2) and −6.7(5) kJ mol-1 for the symmetric and asymmetric MCP·BF3 complexes, respectively. In addition, for all investigated species, full MP2/6-31+G(d,p) geometry optim...

Absorption spectrum of the (CF4) dimer in liquid argon solution

The concentration dependence of the shape of absorption bands in the spectrum of CF4 in liquid argon is studied in the concentration range (0.01–17)×10−3 molar fractions at 93 K. In all spectral regions related to ν3, the shape of the spectral function is determined, along with the Fermi resonance 〈νi,ν3+1,ν4|≈〈νi,ν3,ν 4+2|, by the resonance dipole-dipole interaction. In the spectral region of the Fermi doublet ν 1+ν3≈ ν1+2ν4, the spectrum of the contact (CF4)2 dimer is identified. Agreement between this spectrum and the calculated spectrum is achieved by simultaneously taking intramolecular and intermolecular resonances into account. The distance R C-C in the dimer is 4.85(15) A. The calculations of the spectra of (12CF4)2 and (13CF4−12CF4) dimers with this value of R C-C in the region ν 3≈2ν4 agree with the experiment.

Conformational Characteristics of Methyl Nitrite: A Cryospectroscopic Study

Mid-infrared spectra of solutions in liquid argon, krypton, and xenon of methyl nitrite, which occurs as a mixture of cis and trans conformers, have been investigated. In liquid argon and in liquid...

Characterization of the CH 3F·BF 3 van der Waals complex in the vapor phase

Using variable-temperature infrared spectroscopy, the vapor phase complexation enthalpy for the van der Waals complex CH 3F·BF 3 has been determined to be −16.5(7) kJ mol −1. This value is compared with literature values obtained for solutions in liquid argon and in liquid krypton. The complexation enthalpy was transformed by statistical thermodynamics into a complexation enthalpy of −19.8(14) kJ mol −1, and this value is compared with ab initio results in which the geometry of the complex is optimized at different levels between MP2/6-31++G(d,p) and MP2/aug-cc-PVQZ, using BSSE-corrected potential energy surfaces.

Vibrational energy transfer between the isotopomers of carbon monoxide at low temperatures

Rate constants have been measured for vibration to vibration energy transfer from C12O16(v=1) to the first vibrationally excited state of both C13O16 (Δν̄=47 cm−1) and C13O18 (Δν̄=100 cm−1) using the infrared laser fluorescence technique over the temperature range 50 to 270 K in the gas phase and in liquid neon, deuterium and argon solutions. Fluorescence from C12O16(v=1) was filtered out, and the time-dependence of the fluorescence intensity from the other isotopomer used to determine the rate constants for energy transfer. The results for the two isotopomeric systems are markedly different. For that with the smaller energy mismatch, C12O16–C13O16, the rate constants increase with decrease in temperature from 270 to 80 K. This is shown to be consistent with near-resonant energy transfer mediated by transition dipole-transition dipole couplings. Below 80 K, the temperature dependence of the rate constants flattens. For the C12O16–C13O18 system, the rate constants decrease with decrease in temperature from 270 to 100 K, below which they increase with further decrease in temperature. The temperature dependence in the range 270 to 100 K is shown to be consistent with energy transfer mediated by a combination of by long range and short range forces. Below 100 K, the temperature dependence of the rate constants is governed by attractive forces which influence the collision pair during their encounter. The rate constants are compared with previous studies of near-resonant vibrational energy transfer in other systems at low temperatures.

Cis- and trans-3-hexene: infrared spectrum in liquid argon solution, ab initio calculations of equilibrium geometry, normal coordinate analysis, and vibrational assignments

The infrared spectra of cis-3-hexene and trans-3-hexene dissolved in liquid argon have been obtained at temperatures from 93 to 120 K. The absorptions were observed with a low-temperature cell and a Fourier transform infrared spectrophotometer. Ab initio molecular orbital calculations were performed to obtain the equilibrium geometry, vibrational frequencies, force fields, and infrared intensities. The calculations were done at the Hartree-Fock level using 6-31G basis set. The Cartesian force fields from ab initio calculations have been converted to the force field in symmetry coordinates. The scale factors of ab initio calculated force fields were determined. Normal coordinate calculations were performed using a scaled quantum mechanical (SQM) force field. Vibrational normal modes calculated for the lowest energy rotamers of cis- and trans-3-hexene have been assigned to infrared absorption bands observed in liquid argon solution. The assignments were based on calculated frequencies and potential energy distributions. The equilibrium geometries of the two lowest energy rotamers (symmetry C 2 and C s ) of cis-3-hexene and of the three lowest energy rotamers (symmetry C i , C 2, and C 1) of trans-3-hexene were calculated. Variable temperature studies of the infrared spectrum of cis- and trans-3-hexenes dissolved in liquid argon were done to obtain the ΔH of conversion between the rotamers C 2 and C s of cis-3-hexene and between the rotamers C i , C 2, and C 1 of trans-3-hexene.

Reversed Lewis Acidity of Mixed Boron Halides: An Infrared Study of the Van der Waals Complexes of BFxCly with CH3F in Cryosolution

Complexes of mixed boron halides BFxCl3-x (x = 1, 2) with CH3F have been detected in solutions in liquid argon (86−115 K), using infrared spectroscopy. The spectra of the complexes have been interp...

Vibrational overtone spectroscopy of methane in liquid argon solutions

The spectra of the fundamental, and first and second overtones (Δ ν = 1,2,3) around the C-H stretch of methane in liquid argon solutions have been measured at approximately 95 K. Combination bands involving one quantum of a bending mode in the region 3500–5000 cm -1 and three quanta of bending modes in the region 6500–7600 cm -1 were also measured. The near-infrared (IR) spectra of pure liquid methane was recorded between 6500 and 12 000 cm -1 at 94 K. Measurements were made using a cryostat, a low-temperature cell, and a Fourier transform IR and near-IR spectrometer. Visible spectra of the fourth (Δ ν = 5) and fifth (Δ ν = 6) overtones of liquid methane and methane in liquid argon solutions were measured at 95 K. Visible spectra were recorded with a photoacoustic spectrometer employing resonant continuous-wave excitation and piezoelectric detection. Spectra in solutions are greatly simplified compared with the gas phase. This simplification is attributed to a narrowing of the rotational distribution at low temperatures and partial hindering of the rotational motion of sample molecules. Peak positions are systematically red shifted with respect to the gas phase, and the magnitude of the shift increases with the vibrational quantum number. The harmonic frequency and anharmonicity were obtained from a Birge–Sponer fit of the C–H vibrations. Previously reported assignments in normal-mode notation and local-mode calculations of energy levels of methane were used to assign the vibrational transitions observed in our experiments. The local mode notation is based on the harmonically coupled anharmonic oscillator model. Linewidths of the pure overtones increase proportionally as a function of the vibrational quantum number ν. There is indication that, for transitions with Δ ν = 5 and 6, the bandwidths are due to a dephasing mechanism in the slow modulation limit.

Solvent effects on non-resonant vibrational deactivation: N 2( ν = 1) deactivated by CH 4 in liquid Ar/liquid N 2 mixtures

New measurements have been made of rate constants for the vibrational deactivation of N 2( ν = 1) by CH 4 in liquid Ar/liquid N 2 mixtures. The ratio of the liquid phase rate constants, k L,M for the liquid mixture over k L,Ar for liquid argon solution, varies non-linearly with composition. The results imply a saturation effect which occurs when one solvent N 2 molecule is present in the first solvation shell of the excited molecule. It is proposed that this is due to the formation of a N 2( ν = 1) … N 2 collision complex.

Vibrational overtone spectroscopy of CH2D2 in liquid argon solutions

Abstract The spectra of the fundamental and overtones of the CH ( ν = 1–5) and CD ( Δν = 1, 2, 3) stretches of CH 2 D 2 have been measured in liquid argon solutions at temperatures between 94 and 101 K. Absorption in the visible was obtained with a low temperature cell and a resonant continuous wave laser technique with acoustic detection. Absorptions in the IR and near-IR were observed with a Fourier transform spectrophotometer. To interpret the experimental results, overtone transitions are described in terms of the local mode model. The harmonically coupled anharmonic oscillator (HCAO) model was used to determine the overtone energy levels and assign the absorption bands to vibrational transitions. The linewidths of the fundamental and overtone transitions are discussed in terms of rotational relaxation for the fundamental bands and vibrational relaxation in the form of dephasing for the overtones.

Near-Infrared and Visible Spectroscopy of CH3D in Liquid Argon Solutions

The near-infrared and visible spectra of CH3D in liquid argon solutions have been measured around 95 K. The fundamental and overtones, Δv = 1, 2, 3, 4, and 5 around the C−H stretch and Δv = 1, 2, a...