High-temperature hypersonic Laval nozzle for non-LTE cavity ringdown spectroscopy.

A small dimension Laval nozzle connected to a compact high enthalpy source equipped with cavity ringdown spectroscopy (CRDS) is used to produce vibrationally hot and rotationally cold high-resolution infrared spectra of polyatomic molecules in the 1.67 µm region. The Laval nozzle was machined in isostatic graphite, which is capable of withstanding high stagnation temperatures. It is characterized by a throat diameter of 2 mm and an exit diameter of 24 mm. It was designed to operate with argon heated up to 2000 K and to produce a quasi-unidirectional flow to reduce the Doppler effect responsible for line broadening. The hypersonic flow was characterized using computational fluid dynamics simulations, Pitot measurements, and CRDS. A Mach number evolving from 10 at the nozzle exit up to 18.3 before the occurrence of a first oblique shock wave was measured. Two different gases, carbon monoxide (CO) and methane (CH4), were used as test molecules. Vibrational (Tvib) and rotational (Trot) temperatures were extracted from the recorded infrared spectrum, leading to Tvib = 1346 ± 52 K and Trot = 12 ± 1 K for CO. A rotational temperature of 30 ± 3 K was measured for CH4, while two vibrational temperatures were necessary to reproduce the observed intensities. The population distribution between vibrational polyads was correctly described with Tvib I=894±47 K, while the population distribution within a given polyad (namely, the dyad or the pentad) was modeled correctly by Tvib II=54±4 K, testifying to a more rapid vibrational relaxation between the vibrational energy levels constituting a polyad.

Gas-Phase Infrared Spectroscopy of Substituted Cyanobutadiynes: Roles of the Bromine Atom and Methyl Group as Substituents.

The IR spectra of 5-bromo-2,4-pentadiynenitrile (Br-C≡C-C≡C-CN) and 2,4-hexadiynenitrile (CH3 -C≡C-C≡C-CN), a compound of interstellar interest, have been recorded within the 4000-500 cm(-1) spectral region and calculated by means of high-level ab initio and density functional calculations. Although the calculated structures of both compounds are rather similar, there are very subtle differences, mainly in the strength of the C≡C bond not directly bound to the substituent. These subtle bonding differences are reflected in small, but not negligible, differences in the electron density at the corresponding bond critical points, and, more importantly, are reflected in the IR spectra. Indeed, the IR spectrum for the bromine derivative presents two well-differentiated strong bands around 2250 cm(-1) , whereas for the methyl derivative both absorptions coalesce in a single band. These bands correspond in both cases to the coupling between C≡C and C≡N stretching displacements. A third, very weak, band also associated with C≡C and C≡N coupled stretches is observed for the bromine derivative, but not for the methyl one, owing to its extremely low intensity.

The Electronic Structure of Some Cyanohydrins-A†Spectroscopically Under-Investigated Family of Compounds.

Cyanohydrins are usually formed by addition of hydrogen cyanide to aldehydes or ketones while the elimination of HCN from cyanohydrins is easily observed upon heating. The low thermal stability of these highly boiling compounds leads to difficult studies in the gas phase where partial or complete decomposition is usually observed. Consequently, the reported physicochemical properties of such compounds in the gas phase are still scarce. Keeping with this, four simple cyanohydrins, the glycolonitrile and methyl, vinyl and ethynyl derivatives, have been selected. These are possible candidates for the Interstellar Medium, where the corresponding aldehydes and HCN have been detected, and could have played an important role in prebiotic chemistry, as already proposed for some of them. Three well-suited spectroscopic techniques, namely, UV photoelectron, infrared, and Raman spectroscopies, in tandem with quantum calculations, have been chosen for the structure analysis. Photoelectron spectroscopy, successfully performed with gaseous compounds, provides the first comparative study on cyanohydrins in the gas phase.

Gas-phase infrared spectra of three compounds of astrochemical interest: vinyl, allenyl, and propargyl isocyanides.

Isocyanides, isomers of the cyanides detected in the interstellar medium, are also possible components of this medium. The infrared spectra (5000-500 cm(-1) ) of gaseous vinyl isocyanide, allenyl isocyanide, and propargyl isocyanide have been recorded at 0.1 cm(-1) resolution. When prepared on a gram scale to produce a partial pressure of 10 mbar after evaporation in the cell, these three isocyanides, which have previously been reported to be kinetically unstable, do not display any sign of decomposition when recording the spectra. Geometry optimizations and harmonic and anharmonic vibrational frequencies were calculated using the LCCSD(T) method with the cc-pVTZ basis set. Anharmonic frequencies of fundamental, overtone, and combination transitions were calculated using a variational approach implemented in the P Anhar.v2.0 code, to assign the experimental data for each compound. These results improve our knowledge of these under-investigated compounds and pave the way for other physicochemical studies on functionalized isocyanides.

On the structures, lifetimes, and infrared spectra of alkylmercury hydrides.

A series of six small alkylmercury hydrides of the general formula RHgH with R=methyl, ethyl, n-propyl, isopropyl, n-butyl, and 3-butenyl were obtained by reduction in vacuo of the corresponding mercury halide with tributyltin hydride in the presence of a radical inhibitor. These very reactive compounds, which have to be removed from the reaction mixture as they are formed, were characterized by (1)H NMR and (13)C NMR spectroscopy. The IR spectra of n-propyl-, isopropyl-, n-butyl-, and 3-butenylmercury hydride were recorded for the first time. All compounds were then studied by density functional theory calculations on the basis of a recent theoretical assessment for alkylmercury compounds performed by our group. Comparison of the experimental and theoretical results allowed the assignment of the vibrational modes in an unambiguous way, in spite of the low intrinsic stability of some of the derivatives investigated. The experimental procedure implemented for registering the IR spectra of these unstable species in the gas phase allowed us to obtain reasonable estimates of their lifetimes.

Infrared spectra of cyanoacetaldehyde (NCCH2CHO): a potential prebiotic compound of astrochemical interest.

Cyanoacetaldehyde (NC-CH2CH=O) and its isomer, cyanovinylalcohol (NC-CH=CH-OH), as possible components of the interstellar medium, comets, or planetary atmospheres, exist in equilibrium in the gas phase, although the latter compound is very much in the minority (2%). The recording and analysis of the gas-phase infrared spectrum of the former compound within the 4000-500 cm(-1) spectroscopic range and the potential presence of the latter isomer, which could be vital for their detection in these media, are reported. CCSD(T) and G4 high-level ab initio methods, as well as density functional theory calculations, predict the existence of two stable rotamers of cyanoacetaldehyde. The global minimum has a structure with an unusual O-C-C-C dihedral angle (150°) that falls between the antiperiplanar (180°) and anticlinal forms (120°). The second rotamer, which is about 4.0 kJ mol(-1) less stable in terms of free energy, has a planar structure that corresponds to the synperiplanar form (O-C-C-C dihedral angle: 0°). The absorption vibrational bands of the two aldehyde rotamers that are present in the mixture lead to a spectrum with a very complex structure in the region of deformation movements, in which several low-intensity bands overlap. A complete and unambiguous assignment of the experimental spectrum has been achieved by using the calculated harmonic and anharmonic vibrational frequencies.

Infrared spectra of a species of potential prebiotic and astrochemical interest: cyanoethenethiol (NC-CH=CH-SH).

Cyanoethenethiol (NC-CH=CH-SH) was obtained in a 8:1 Z:E ratio by flash vacuum thermolysis of the t-butylsulfide derivative. Density functional theory (DFT) and G3 ab initio calculations predict the existence of Z-and E-isomers, each of which exhibits two rotamers as a function of the relative position of the SH group. All these rotameric forms are planar (C(s) symmetry) and correspond to synperiplanar and antiperiplanar conformations. Calculations indicate that the synperiplanar Z-isomer is the more stable. In pure form, the cyanoethenethiol rapidly decomposes at room temperature, even at low pressure and partially condenses on the wall of the cell. To record its spectrum, a long optical path of 136 m was necessary, and several successive fillings of the cell were required. On the basis of the calculated harmonic and anharmonic vibrational frequencies, a complete and unambiguous assignment of the experimental spectrum has been carried out.

Gas-phase infrared spectra of vinyl selenol and vinyl tellurol.

The infrared spectra (3500-500 cm(-1)) of gaseous vinyl selenol and vinyl tellurol have been recorded at 0.1 cm(-1) resolution. For the latter the spectra were obtained at room temperature, but for the former a temperature of -40 degrees C was required because of the chemical instability of vinyl selenol at room temperature. To compensate the very weak vapor pressure of vinyl tellurol at room temperature, a long optical path up to 136 m was necessary to record its spectrum. B3LYP density functional theory (DFT) calculations have been performed to assign the different absorption bands. Since an unambiguous assignment of the absorption bands requires a precise knowledge on the relative abundance of the syn and gauche rotamers of these compounds, their relative energies and their anharmonic vibrational frequencies were obtained using a very extended Def2-QZVP basis set. Two rotamers, the syn, which is planar, and a nonplanar gauche, were found to be local minima for both compounds. The gauche rotamer presents two degenerate conformers, which differ by the position of the SeH (TeH) hydrogen atom above or below the molecular plane. Our theoretical results are in good agreement with the main features of the experimental spectra. Fundamental bands and some combination bands of vinyl selenol and vinyl tellurol were assigned and compared with those of vinyl alcohol and vinyl thiol, whose spectra had been reported previously in the literature.

Vinylphosphine-borane: synthesis, gas phase infrared spectroscopy, and quantum chemical vibrational calculations.

Both experimental and theoretical investigations are reported on the infrared spectrum of vinylphosphine-borane (CH(2)=CHPH(2) x BH(3)), a donor-acceptor complex. The gas phase infrared spectra (3500-600 cm(-1)) have been recorded at 0.5 cm(-1) resolution. This first primary alpha,beta-unsaturated phosphine-borane synthesized up to now is kinetically very unstable in the gas phase and decomposes rapidly into two fragments: the free vinylphosphine CH(2)=CHPH(2) and the monoborane BH(3) which dimerizes to form the more stable diborane B(2)H(6). Spectra of free CH(2)=CHPH(2) and B(2)H(6) compounds were also recorded to assign some vibration modes of the complex in very dense spectral regions. The analysis was completed by carrying out quantum mechanical calculations by density functional theory method at the B3LYP/6-31+G(**) level. Anharmonic frequencies and infrared intensities of the two predicted gauche and syn conformers of the vinylphosphine-borane complex were calculated in the 3500-100 cm(-1) region with the use of a variational approach, implemented in the P_ANHAR_V1.2 code. Because of the relatively weak interaction between the vinylphosphine and the monoborane, the vibrations of the complex can easily be subdivided into modes localized in the CH(2)=CHPH(2) and BH(3) moieties and into "intermolecular" modes. Localized modes are unambiguously correlated with the modes of the isolated monomers. Therefore, they are described in terms of the monomer vibrations, and the complexation shifts are defined as Delta nu = nu(complex) - nu(monomer) to make the effect of the complexation precise on each localized mode. In this objective, anharmonic frequencies and infrared intensities of the BH(3) monomer and the stable gauche and syn conformers of the free vinylphosphine were obtained at the same level of theory. In the gas phase, only the syn form of the complex was observed and assigned. All theoretically predicted frequencies and complexation shifts in magnitude and direction are in good agreement with experiment. By infrared spectroscopy assisted by quantum chemical calculations, the consequences of the complexation of an alpha,beta-unsaturated phosphine by borane on the physicochemical properties of the formed 12-atom complex have been efficiently evaluated.

Vibrational spectra of cyclopentadienylphosphine: infrared and theoretical studies from DFT anharmonic potentials.

Both experimental and theoretical infrared investigations of cyclopentadienylphosphine (CpP) are reported. The infrared spectra (3500-500 cm(-1)) in the gas phase have been recorded at 0.5 cm(-1) resolution. Infrared absorptions bands of the two lowest stable conformers were observed and assigned. Average integrated intensities of isolated and overlapping vibrational bands were also determined experimentally. The vibrational frequencies of the CpP system and its P-dideuterated isotopologue have been calculated by means of density functional theory. The Becke exchange functional and Lee-Yang-Parr correlation functional method with a combination of the two basis sets, namely 6-31+G(d,p) and the correlation-consistent triple-zeta cc-pVTZ set of Dunning, were used. Hybrid B3LYP/B3LYP//cc-pVTZ/6-31+G(d,p) anharmonic frequencies of the fundamental, overtone, and combination transitions were calculated in the 3500-200 cm(-1) area with the use of a variational approach, implemented in the P_Anhar_v1.1 code, to assign the experimental data for each conformer.

Structural and conformational properties of 2-propenylgermane (allylgermane) studied by microwave and infrared spectroscopy and quantum chemical calculations.

The structural and conformational properties of allylgermane have been investigated using Stark and Fourier transform microwave spectroscopies, infrared spectroscopy, and high-level quantum chemical calculations. The parent species H2C=CHCH2GeH3 was investigated by microwave spectroscopy and infrared spectroscopy, while three deuterated species, namely, H2C=CDCH2GeH3, H2C=CHCHDGeH3, and H2C=CHCH2GeD3, were studied only by infrared spectroscopy. The microwave spectra of the ground vibrational state as well as of the first excited state of the torsion vibration around the sp2-sp3 carbon-carbon bond were assigned for the 70Ge, 72Ge, and 74Ge isotopomers of one conformer. This rotamer has an anticlinal arrangement for the C=C-C-Ge chain of atoms. The infrared spectrum of the gas in the 500-4000 cm(-1) range has been assigned. No evidence of additional rotameric forms other than anticlinal was seen in the microwave and infrared spectra. Several different high-level ab initio and density functional theory calculations have been performed. These calculations indicate that a less stable form, having a synperiplanar conformation of the C=C-C-Ge link of atoms, may coexist with the anticlinal form. The energy differences between the synperiplanar and anticlinal forms were calculated to be 5.6-9.2 kJ/mol depending on the computational procedure. The best approximation of the equilibrium structure of the anticlinal rotamer was found in the MP2/aug-cc-pVTZ calculations. The barrier to internal rotation of the germyl group was found to be 6.561(17) kJ/mol, from measurements of the splitting of microwave transitions caused by tunneling of the germyl group through its threefold barrier.

Infrared spectra of a species of astrochemical interest: aminoacrylonitrile (3-amino-2-propenenitrile).

Ammonia easily reacts on cyanoacetylene in the gas phase or in a solvent to form the Z- and E-isomers of aminoacrylonitrile (3-amino-2-propenenitrile, 2). This kinetically stable enamine presents interest for its possible presence in the interstellar medium, the comets, the atmospheres of Planets including the Primitive Earth, and from a theoretical point of view. B3LYP/6-311+G(3df,2p) and G2 calculations indicate that the imine isomer is significantly less stable than the enamine 2. DFT and G2 calculations indicate that the Z-isomer of compound 2 lies ca. 8.0 kJ mol(-1) lower in energy than the E-isomer. The infrared spectra of the aminoacrylonitrile, in both the gas and condensed phases were recorded in the range 500-4000 cm(-1). Consistent with the theoretical calculations, the imine and the E-isomer of the enamine have never been detected in the infrared spectrum of a gaseous sample and only the Z-isomer has been observed. With a neat sample in the condensed phase, IR spectra of a 1:1 and 20:1/Z:E mixtures were recorded. The comparison of these data with the spectrum of the Z-isomer in the gas phase allowed us to deduce the IR spectrum of the E-isomer. The E-Z isomerization takes place through a torsion around the C=C bond. A possible mechanism involving a previous enamine-imine tautomerism must be discarded because it implies a much larger barrier than the direct isomerization process. Consistently, the presence of a deuterium atom has not been observed on the sp2 carbon of the products of distillation of a 1:1/E:Z mixture of the NCCH=CHND2.