Characterizing centrosymmetric two-ring PAHs using jet-cooled high resolution mid-infrared laser spectroscopy and anharmonic quantum chemical calculations.

The presence of Polycyclic Aromatic Hydrocarbon (PAH) molecules in the interstellar medium, recently confirmed by the detection of cyano-naphthalenes, has renewed the interest of extensive spectroscopic and physical-chemistry studies on such large species. The present study reports the jet-cooled rovibrational infrared study of three centrosymmetric two-ring PAH molecules, viz., naphthalene (C10H8), [1,5] naphthyridine (C8H6N2), and biphenyl (C12H10), in the in-plane ring C-H bending (975-1035 cm-1) and C-C ring stretching (1580-1620 cm-1) regions. For the two most rigid PAHs, the accuracy of spectroscopic parameters derived in ground and several excited states (six for naphthalene and six for [1,5] naphthyridine) has significantly improved the literature values. In addition, comparison between experiments and quantum chemical calculations confirms the predictive power of the corrected calculated rotational parameters. The more flexible structure of biphenyl makes the analysis of high resolution jet-cooled spectra of ν19 and ν23 modes recorded at about 1601 and 1013 cm-1, respectively, particularly challenging. The presence of three torsional vibrations below 120 cm-1 together with small values of the rotational constants prevented us from determining the ground and v19 = 1 excited rotational constants independently. In the ν23 band region, the presence of two bands rotationally resolved and separated by only 0.8 cm-1 raises the question of possible splittings due to a large amplitude motion, most probably the torsion of the aliphatic bond between the two phenyl rings.

Tunneling motion and splitting in the CH2OH radical: (Sub-)millimeter wave spectrum analysis.

The (sub-)millimeter wave spectrum of the non-rigid CH2OH radical is investigated both experimentally and theoretically. Ab initio calculations are carried out to quantitatively characterize its potential energy surface as a function of the two large amplitude ∠H1COH and ∠H2COH dihedral angles. It is shown that the radical displays a large amplitude torsional-like motion of its CH2 group with respect to the OH group. The rotation-torsion levels computed with the help of a 4D Hamiltonian accounting for this torsional-like motion and for the overall rotation exhibit a tunneling splitting, in agreement with recent experimental investigations, and a strong rotational dependence of this tunneling splitting on the rotational quantum number Ka due to the rotation-torsion Coriolis coupling. Based on an internal axis method approach, a fitting Hamiltonian accounting for tunneling effects and for the fine and hyperfine structure is built and applied to the fitting of the new (sub)-millimeter wave transitions measured in this work along with previously available high-resolution data. 778 frequencies and wavenumbers are reproduced with a unitless standard deviation of 0.79 using 27 parameters. The N = 0 tunneling splitting, which could not be determined unambiguously in the previous high-resolution investigations, is determined based on its rotational dependence.

Unlocking synchrotron sources for THz spectroscopy at sub-MHz resolution.

Synchrotron radiation (SR) has proven to be an invaluable contributor to the field of molecular spectroscopy, particularly in the terahertz region (1-10 THz) where its bright and broadband properties are currently unmatched by laboratory sources. However, measurements using SR are currently limited to a resolution of around 30 MHz, due to the limits of Fourier-transform infrared spectroscopy. To push the resolution limit further, we have developed a spectrometer based on heterodyne mixing of SR with a newly available THz molecular laser, which can operate at frequencies ranging from 1 to 5.5 THz. This spectrometer can record at a resolution of 80 kHz, with 5 GHz of bandwidth around each molecular laser frequency, making it the first SR-based instrument capable of sub-MHz, Doppler-limited spectroscopy across this wide range. This allows closely spaced spectral features, such as the effects of internal dynamics and fine angular momentum couplings, to be observed. Furthermore, mixing of the molecular laser with a THz comb is demonstrated, which will enable extremely precise determinations of molecular transition frequencies.

Broadband terahertz heterodyne spectrometer exploiting synchrotron radiation at megahertz resolution.

A new spectrometer allowing both high resolution and broadband coverage in the terahertz (THz) domain is proposed. This instrument exploits the heterodyne technique between broadband synchrotron radiation and a quantum-cascade-laser-based molecular THz laser that acts as the local oscillator. Proof of principle for exploitation for spectroscopy is provided by the recording of molecular absorptions of hydrogen sulfide (H2S) and methanol (CH3OH) around 1.073 THz. Ultimately, the spectrometer will enable to cover the 1-4 THz region in 5 GHz windows at Doppler resolution.

Synthesis, High-Resolution Infrared Spectroscopy, and Vibrational Structure of Cubane, C8H8.

Carbon-cage molecules have generated a considerable interest from both experimental and theoretical points of view. We recently performed a high-resolution study of adamantane (C10H16), the smallest hydrocarbon cage belonging to the diamandoid family ( Pirali , O. ; et al. J. Chem. Phys. 2012 , 136 , 024310 ). There exist another family of hydrocarbon cages with additional interesting chemical properties: the so-called platonic hydrocarbons that comprise dodecahedrane (C20H20) and cubane (C8H8). Both possess C-C bond angles that deviate from the tetrahedral angle (109.8°) of the sp(3) hybridized form of carbon. This generates a considerable strain in the molecule. We report a new wide-range high-resolution study of the infrared spectrum of cubane. The sample was synthesized in Bari upon decarboxylation of 1,4-cubanedicarboxylic acid thanks to the improved synthesis of literature. Several spectra have been recorded at the AILES beamline of the SOLEIL synchrotron facility. They cover the 600-3200 cm(-1) region. Besides the three infrared-active fundamentals (ν10, ν11, and ν12), we could record many combination bands, all of them displaying a well-resolved octahedral rotational structure. We present here a preliminary analysis of some of the recorded bands, performed thanks the SPVIEW and XTDS software, based on the tensorial formalism developed in the Dijon group. A comparison with ab initio calculations, allowing to identify some combination bands, is also presented.

High resolution spectroscopy of six SOCl2 isotopologues from the microwave to the far-infrared.

Despite its potential role as an atmospheric pollutant, thionyl chloride, SOCl2, remains poorly characterized in the gas phase. In this study, the pure rotational and ro-vibrational spectra of six isotopologues of this molecule, all detected in natural abundance, have been extensively studied from the cm-wave band to the far-infrared region by means of three complementary techniques: chirped-pulse Fourier transform microwave spectroscopy, sub-millimeter-wave spectroscopy using frequency multiplier chain, and synchrotron-based far-infrared spectroscopy. Owing to the complex line pattern which results from two nuclei with non-zero spins, new, high-level quantum-chemical calculations of the hyperfine structure played a crucial role in the spectroscopic analysis. From the combined experimental and theoretical work, an accurate semi-experimental equilibrium structure (r(e)(SE)) of SOCl2 has been derived. With the present data, spectroscopy-based methods can now be applied with confidence to detect and monitor this species, either by remote sensing or in situ.

High-Resolution Far-Infrared Spectroscopy of N-Substituted Two-Ring Polycyclic Aromatic Hydrocarbons: An Extended Study.

Polycyclic aromatic hydrocarbons (PAHs) and their N-substituted derivatives are among the largest species for which gas-phase high-resolution spectroscopy can be performed nowadays. In this paper we report the observation and analysis of spectra from several N-substituted two-ring PAHs, all taken in the "fingerprint" far-infrared region (<850 cm(-1)). Together with accurate measurements of their pure rotational transitions in the millimeter and submillimeter ranges, these synchrotron-based Fourier transform infrared (FTIR) measurements provide an accurate description of the rotational energy levels in the ground and low-energy excited vibrational states of these species. To complement the experimental data, anharmonic DFT calculations were performed to obtain relatively accurate rotational and vibrational parameters. The calculated results strongly support the rotational analysis and provide a good estimate of the equilibrium structures for each species. Extended measurements, analysis, and calculations are presented here for the far-IR bands of quinoline (C9H7N), isoquinoline (C9H7N), quinoxaline (C8H6N2), quinazoline (C8H6N2), [1,5]-naphthyridine (C8H6N2), [1,6]-naphthyridine (C8H6N2), and indole (C8H7N) molecules.

High density terahertz frequency comb produced by coherent synchrotron radiation.

Frequency combs have enabled significant progress in frequency metrology and high-resolution spectroscopy extending the achievable resolution while increasing the signal-to-noise ratio. In its coherent mode, synchrotron radiation is accepted to provide an intense terahertz continuum covering a wide spectral range from about 0.1 to 1 THz. Using a dedicated heterodyne receiver, we reveal the purely discrete nature of this emission. A phase relationship between the light pulses leads to a powerful frequency comb spanning over one decade in frequency. The comb has a mode spacing of 846 kHz, a linewidth of about 200 Hz, a fractional precision of about 2 × 10(-10) and no frequency offset. The unprecedented potential of the comb for high-resolution spectroscopy is demonstrated by the accurate determination of pure rotation transitions of acetonitrile.

Rotation-vibration interactions in the spectra of polycyclic aromatic hydrocarbons: Quinoline as a test-case species.

Polycyclic aromatic hydrocarbons (PAHs) are highly relevant for astrophysics as possible, though controversial, carriers of the unidentified infrared emission bands that are observed in a number of different astronomical objects. In support of radio-astronomical observations, high resolution laboratory spectroscopy has already provided the rotational spectra in the vibrational ground state of several molecules of this type, although the rotational study of their dense infrared (IR) bands has only recently become possible using a limited number of experimental set-ups. To date, all of the rotationally resolved data have concerned unperturbed spectra. We presently report the results of a high resolution study of the three lowest vibrational states of quinoline C9H7N, an N-bearing naphthalene derivative. While the pure rotational ground state spectrum of quinoline is unperturbed, severe complications appear in the spectra of the ν45 and ν44 vibrational modes (located at about 168 cm(-1) and 178 cm(-1), respectively). In order to study these effects in detail, we employed three different and complementary experimental techniques: Fourier-transform microwave spectroscopy, millimeter-wave spectroscopy, and Fourier-transform far-infrared spectroscopy with a synchrotron radiation source. Due to the high density of states in the IR spectra of molecules as large as PAHs, perturbations in the rotational spectra of excited states should be ubiquitous. Our study identifies for the first time this effect and provides some insights into an appropriate treatment of such perturbations.

Low-energy vibrational spectra of flexible diphenyl molecules: biphenyl, diphenylmethane, bibenzyl and 2-, 3- and 4-phenyltoluene.

Gas phase absorption far-infrared (FIR) spectra of six flexible hydrocarbon molecules containing two phenyl groups -biphenyl, diphenylmethane, bibenzyl and 2-, 3-, 4-phenyltoluene- are reported for the first time, allowing an accurate determination of most of their active low-frequency vibrational modes. DFT calculations have been carried out at the harmonic and perturbative anharmonic levels to predict the vibrational spectra of these molecules and unambiguously assign observed vibrational modes.

High resolution measurements supported by electronic structure calculations of two naphthalene derivatives: [1,5]- and [1,6]-naphthyridine--estimation of the zero point inertial defect for planar polycyclic aromatic compounds.

Polycyclic aromatic hydrocarbons (PAHs) molecules are suspected to be present in the interstellar medium and to participate to the broad and unresolved emissions features, the so-called unidentified infrared bands. In the laboratory, very few studies report the rotationally resolved structure of such important class of molecules. In the present work, both experimental and theoretical approaches provide the first accurate determination of the rotational energy levels of two diazanaphthalene: [1,5]- and [1,6]-naphthyridine. [1,6]-naphthyridine has been studied at high resolution, in the microwave (MW) region using a Fourier transform microwave spectrometer and in the far-infrared (FIR) region using synchrotron-based Fourier transform spectroscopy. The very accurate set of ground state (GS) constants deduced from the analysis of the MW spectrum allowed the analysis of the most intense modes in the FIR (ν38-GS centered at about 483 cm(-1) and ν34-GS centered at about 842 cm(-1)). In contrast with [1,6]-naphthyridine, pure rotation spectroscopy of [1,5]-naphthyridine cannot be performed for symmetry reasons so the combined study of the two intense FIR modes (ν22-GS centered at about 166 cm(-1) and ν18-GS centered at about 818 cm(-1)) provided the GS and the excited states constants. Although the analysis of the very dense rotational patterns for such large molecules remains very challenging, relatively accurate anharmonic density functional theory calculations appeared as a highly relevant supporting tool to the analysis for both molecules. In addition, the good agreement between the experimental and calculated infrared spectrum shows that the present theoretical approach should provide useful data for the astrophysical models. Moreover, inertial defects calculated in the GS (ΔGS) of both molecules exhibit slightly negative values as previously observed for planar species of this molecular family. We adjusted the semi-empirical relations to estimate the zero-point inertial defect (Δ0) of polycyclic aromatic molecules and confirmed the contribution of low frequency out-of-plane vibrational modes to the GS inertial defects of PAHs, which is indeed a key parameter to validate the analysis of such large molecules.

Synchrotron based FT-FIR pure rotational spectroscopy of the NH2 radical in its two lowest vibrational states.

Six Fourier-transform FIR spectra of the NH2 radical have been recorded at high resolution (0.001 cm(-1)) using synchrotron radiation on the AILES beamline at SOLEIL Synchrotron. Three different experimental discharge setups have been used to observe, in absorption, 1009 pure rotational transitions of NH2 in the vibrational ground state (000) and 170 pure rotational transitions within the first excited vibrational state (010). These results constitute a significant extension of the observed quantum numbers for these two states. The spectra permitted several couplings to be resolved (asymmetric coupling, spin-rotation coupling, hyperfine structure) for relatively highly excited energy levels. An effective fit has been realized using both standard Watson-S and -A reductions despite an abnormal centrifugal distortion effect for this light hydride.

The cyclic ground state structure of the HF trimer revealed by far infrared jet-cooled Fourier transform spectroscopy.

The rovibrationally resolved Fourier transform (FT) far infrared (FIR) spectra of two intermolecular librations of (HF)3, namely the in-plane ν6 and out-of-plane ν4 bending fundamentals centered, respectively, at about 494 cm(-1) and 602 cm(-1), have been recorded for the first time under jet-cooled conditions using the supersonic jet of the Jet-AILES apparatus. The simultaneous rotational analysis of 245 infrared transitions belonging to both bands enabled us to determine the ground state (GS), ν6 and ν4 rotational and centrifugal distortion constants. These results provided definite experimental answers to the structure of such a weakly bound trimer: firstly the vibrationally averaged planarity of cyclic (HF)3, also supported by the very small value of the inertia defect obtained in the GS, secondly the slight weakening of the hydrogen bond in the intermolecular excited states evidenced from the center of mass separations of the HF constituents determined in the ground, ν6 = 1 and ν4 = 1 states of (HF)3 as well as the decrease of the fitted rotational constants upon excitation. Finally, lower bounds of about 2 ns on ν6 and ν4 state lifetimes could be derived from the deconvolution of experimental linewidths. Such long lifetimes highlight the interest in probing low frequency intermolecular motions of molecular complexes to get rid of constraints related to the vibrational dynamics of coupled anharmonic vibrations at higher energy, resulting in loss of rotational information.

Coupled large amplitude motions: a case study of the dimethylbenzaldehyde isomers.

The microwave spectra of the 3,4- (syn and anti), 2,5- (syn), and 3,5-dimethylbenzaldehyde (DMBA) molecules have been recorded for the first time in the 2-26.5 GHz frequency range, using the high resolution COBRA-FTMW spectrometer in Hannover. The experimental assignments and fits are supplemented by ab initio quantum chemical calculations of the conformational energy landscape and dipole moment components. The analysis of the spectra of the four observed isomers, including spectroscopic constants and large amplitude motion parameters, are presented in this paper. The DMBA isomers belong to a series of similar molecules obtained formally by adding one or more methyl group(s) at the aromatic ring. These molecules serve as prototype systems for the development of the theoretical model of asymmetric top molecules having C(s) symmetry while containing in addition two nonequivalent methyl tops (C(3v)), exhibiting different barrier heights and coupling terms. Thus, the DMBA isomers represent good species for testing the recently written two-top internal rotors BELGI program.

The far infrared spectrum of naphthalene characterized by high resolution synchrotron FTIR spectroscopy and anharmonic DFT calculations.

Using synchrotron radiation, we performed the rotationally resolved Fourier transform infrared absorption spectroscopy of three bands of naphthalene C10H8, namely ν(46)-0 (centered at 782 cm(-1), 12.7 µm), ν(47)-0 (centered at 474 cm(-1), 21 µm), and ν(48)-0 (centered at 167 cm(-1), 60 µm). The intense CH bending out of plane ν(46)-0 band was recorded under supersonic jet-cooled conditions using a molecular beam (the Jet-AILES apparatus) and the low frequency ν(47)-0 and ν(48)-0 bands were measured at room temperature in a long absorption path cell. The simultaneous rotational analysis of these bands permitted us to refine the ground state (GS) and ν(46) rotational spectroscopic constants and to provide the first sets of constants for the ν(47) and ν(48) modes. The experimental rotational constants were then used as reference data to calibrate theoretical models in order to provide new insights into the accuracy of anharmonic calculations. The B97-1 functional associated with the cc-pVTZ and ANO-RCC basis sets gave a consistent set of results, for rotational constants and fundamental frequencies. The data presented here pave the way for the search of naphthalene through its far-infrared spectrum in different objects of the interstellar medium.

Synchrotron FTIR spectroscopy of weak torsional bands: a case study of cis-methyl formate.

The far infrared spectrum of cis-methyl formate has been recorded on the AILES beamline of the synchrotron SOLEIL using a Fourier transform infrared spectrometer coupled to a long path cell. The very weak fundamental band associated with the methyl-top torsion mode (ν(18)) was observed. The frequency analysis was performed using the "rho axis method", and the microwave and millimeter-wave data from the literature. A precise determination of the band origins (ν(18)(A) = 132.4303 cm(-1) and ν(18)(E) = 131.8445 cm(-1)) and of the barrier height [V(3) = 370.7398 (58) cm(-1)] have been obtained. The intensity of the ν(18) fundamental band was determined to be 3.4 × 10(-21) cm(-1)∕(molecule cm(-2)) at 297 K, equally shared among A-A and E-E transitions, thus leading to a dipole moment component µ(c)(3) equal to 0.0483 D. The results were compared with the ab initio calcula-tions of Senent et al. [Astrophys. J. 627, 567 (2005)].

Rotationally resolved infrared spectroscopy of adamantane.

We present the first rotationally resolved spectra of adamantane (C(10)H(16)) applying gas-phase Fourier transform infrared (IR) absorption spectroscopy. High-resolution IR spectra are recorded in the 33-4500 cm(-1)range using as source of IR radiation both synchrotron radiation (at the AILES beamline of the SOLEIL synchrotron) as well as a classical globar. Adamantane is a spherical top molecule with tetrahedral symmetry (T(d) point group) and has no permanent dipole moment in its vibronic ground state. Of the 72 fundamental vibrational modes in adamantane, only 11 are IR active. Here we present rotationally resolved spectra for seven of them: ν(30), ν(28), ν(27), ν(26), ν(25), ν(24), and ν(23). The typical rotational structure of spherical tops is observed and analyzed using the STDS software developed in the Dijon group, which provides the first accurate energy levels and rotational constants for seven fundamental modes. Rotational levels with quantum numbers as high as J = 107 have been identified and included in the fit leading to a typical standard deviation of about 10(-3) cm(-1).

High resolution far-infrared Fourier transform spectroscopy of radicals at the AILES beamline of SOLEIL synchrotron facility.

Experimental far-infrared (FIR) spectroscopy of transient species (unstable molecules, free radicals, and ions) has been limited so far in both emission and absorption (mainly by the low probability of spontaneous emission in that spectral range and the low brightness of continuum sources used for absorption measurements, respectively). Nevertheless, the FIR spectral range recently became of high astrophysical relevance thanks to several new observational platforms (HERSCHEL, ALMA...) dedicated to the study of this region suitable for the detection of the emission from cold objects of the interstellar medium. In order to complete the experimental dataset concerning transient species, three discharge experiments dedicated to the recording of high resolution FIR spectra of radicals have been developed at the Advanced Infrared Line Exploited for Spectroscopy (AILES) which extracts the bright FIR synchrotron continuum of the synchrotron facility SOLEIL. These experiments make use of a high resolution (R = 0.001 cm(-1)) Bruker IFS125 Fourier transform (FT) spectrometer. An emission setup (allowing to record spectra of radicals excited at high rotational and vibrational temperatures) and two absorption setups (exploiting the bright synchrotron source at the highest resolution available on the FT) are alternatively connected to the FT. The advantages and limitations of these techniques are discussed on the basis of the recent results obtained on OH and CH radicals. These results constitute the first FIR spectra of radicals using synchrotron radiation, and the first FIR spectrum of a C-bearing radical using FT-spectroscopy.

The (CH2)2O-H2O hydrogen bonded complex. Ab Initio calculations and Fourier transform infrared spectroscopy from neon matrix and a new supersonic jet experiment coupled to the infrared AILES beamline of synchrotron SOLEIL.

A series of hydrogen bonded complexes involving oxirane and water molecules have been studied. In this paper we report on the vibrational study of the oxirane-water complex (CH(2))(2)O-H(2)O. Neon matrix experiments and ab initio anharmonic vibrational calculations have been performed, providing a consistent set of vibrational frequencies and anharmonic coupling constants. The implementation of a new large flow supersonic jet coupled to the Bruker IFS 125 HR spectrometer at the infrared AILES beamline of the French synchrotron SOLEIL (Jet-AILES) enabled us to record first jet-cooled Fourier transform infrared spectra of oxirane-water complexes at different resolutions down to 0.2 cm(-1). Rovibrational parameters and a lower bound of the predissociation lifetime of 25 ps for the v(OH)(b) = 1 state have been derived from the rovibrational analysis of the ν(OH)(b) band contour recorded at respective rotational temperatures of 12 K (Jet-AILES) and 35 K (LADIR jet).

Gas-phase synchrotron FTIR spectroscopy of weakly volatile alkyl phosphonate and alkyl phosphate compounds: vibrational and conformational analysis in the terahertz/far-IR spectral domain.

The high brilliance of the AILES beamline at the SOLEIL synchrotron facility has been exploited for the study of the gas-phase vibrational spectra of weakly volatile organophosphorous compounds. The propagation of the synchrotron radiation in long path length gas cells allowed improvements in the sensitivity limits and spectral coverage compared with a previous study, performed by our group with conventional thermal sources. A ppm level detection in the entire IR domain up to terahertz (THz) frequencies has been realized for dimethyl methylphosphonate (DMMP), trimethyl phosphate (TMP), triethyl phosphate (TEP), and diethyl (2-methylallyl)phosphonate (DEMaP). In the present study, the assignment of the gas-phase vibrational and the conformational analysis of the two most stable conformers of DMMP and TMP have been extended to the torsional THz spectra in the 20-120 cm(-1) range. The improvement of the S/N ratio below 600 cm(-1) has permitted for the first time a gas-phase conformational analysis of the two weakly volatile and highly flexible TEP and DEMaP compounds. The experimental far-infrared (FIR)/THz spectra have been studied taking into account four low-energy conformers determined by means of high level of theory quantum chemistry calculations. Finally, due to its particularly low vapor pressure, the detection of gas-phase tributyl phosphate (TBP) in the FIR domain was unsuccessful. Nevertheless, the mid-IR/near-IR spectra of TBP recorded in a multipass cell heated to 355 K have been assigned with the harmonic vibrational predictions of the most stable conformer.