Millimeter-wave emission spectrometer based on direct digital synthesis.

We present a millimeter-wave Fourier transform emission spectrometer whose design is based on the application of a direct digital synthesizer (DDS) up-converted into the Ku-band with subsequent frequency multiplication. The spectrometer covers the frequency range from 50 GHz to 110 GHz and from 150 GHz to 330 GHz. Owing to the fast frequency switching ability of the DDS in the spectrometer, the same radiation source is used both as a generator of short polarizing pulses and as a local oscillator for the heterodyne receiving system. Such a design provides intrinsically coherent reception that allows very long-term data averaging in the time domain, which improves considerably the maximum sensitivity of the spectrometer. The performances of the spectrometer including the data acquisition rate, the sensitivity, and the accuracy of line frequency measurements were tested on the rotational spectra of OCS, NH2CHO, and CH3CH2CN. We show that in the frequency range of 150-300 GHz, the maximum sensitivity of the spectrometer for a 10 min integration time is around 10-9 cm-1 (the minimal value of the absorption coefficient of detectable rotational transition) in the case of narrowband single frequency pulse excitation, and around 10-8 cm-1 in the case of broadband chirped-pulse excitation.

Internal Rotation of OH Group in 4-Hydroxy-2-butynenitrile Studied by Millimeter-Wave Spectroscopy.

Cyanoacetylene, HCC-CN is a ubiquitous molecule in the Universe. However, its interstellar chemistry is not well understood and its understanding requires laboratory data including rotational spectroscopy of possible products coming from a reaction with another compounds. In this study we present the first spectroscopic characterization of gauche conformation of 4-hydroxy-2-butynenitrile (HOCH2CCCN), a formal adduct of cyanoacetylene on formaldehyde, in the frequency range up to 500 GHz. The analysis of the rotational spectrum was complicated by internal rotation of the OH group that connects two equivalent gauche configurations. The spectral assignment was aided by high-level quantum chemical calculations that were particularly useful in the interpretation of torsional-rotational part of the problem. The applied reduced-axis-system (RAS) formalism allowed fitting within experimental accuracy the lines with K a < 18. We also present the method of search for initial global solution of torsional-rotational problem within RAS formalism. Accurate spectroscopic parameters obtained in this study provide a reliable basis for the detection of 4-hydroxy-2-butynenitrile in the interstellar medium.

Laboratory spectroscopy of methoxymethanol in the millimeter-wave range.

Methoxymethanol, CH3OCH2OH is a very interesting candidate for detection in the interstellar medium since it can be formed in the recombination reaction between two radicals considered as intermediates in methanol formation: CH3O (already detected in the ISM) and CH2OH. It could also be formed by the addition of CH3O to formaldehyde (another abundant compound in the ISM) followed by abstraction of a hydrogen radical. In this study, we present the first spectroscopic characterization of methoxymethanol in the millimeter-wave range augmented by high level quantum chemical calculations. The analysis revealed three stable conformations all exhibiting different large amplitude motions (LAMs). For the analysis of the most stable conformation (I) we applied a model that accounts for hindered internal rotation of the methyl top. The analysis of conformation III was performed taking the interaction between the overall rotation and OH torsional motion into account. Conformation II was only tentatively assigned, as it exhibits several LAMs that significantly complicate the theoretical description. Accurate spectroscopic parameters obtained in this study provide a reliable basis for the detection of methoxymethanol in the ISM.

Conformational Landscape and Torsion-Rotation-Vibration Effects in the Two Conformers of Methyl Vinyl Ketone, a Major Oxidation Product of Isoprene.

Methyl vinyl ketone is the second major oxidation product of isoprene, and as such an important volatile organic compound present in the troposphere. In the present study, quantum chemical calculations coupled to high-resolution millimeter-wave spectroscopy have been performed to characterize the ground and first excited vibrational states of the two stable conformers. Equilibrium structures, internal rotation barriers, and relative energies have been calculated at the MP2 and M062X levels of theory. Experimental molecular parameters have been obtained that model the rotational and torsional structures, including splitting patterns due to the internal rotation of the methyl group. For the most stable antiperiplanar (s-trans) conformer, the set of parameters obtained for the ground state should be useful to further model IR spectra up to room temperature. By combining theoretical and experimental data, we obtained a relative energy value of 164 ± 30 cm-1 in the gas phase between the more stable antiperiplanar and the less stable synperiplanar conformers. Moreover, we compared our system with related molecules for the variation in the barriers of methyl rotors in different molecular environments. In addition, the inverse sequence of A and E tunneling substates for the rotational lines of the first excited skeletal torsional state and Coriolis-type coupling with methyl torsion have been observed. For the less stable synperiplanar (cis) conformer, molecular parameters for the ground and first excited torsional states as well as of the first excited skeletal torsional state are presented.

High-resolution millimeter wave spectroscopy and ab initio calculations of aminomalononitrile.

The HCN trimer aminomalononitrile (H2NCH(CN)2, AMN) is considered as a key compound in prebiotic chemistry and a potential candidate for detection in the interstellar medium. In this view, we studied the rotational spectrum of AMN in the 120-245 GHz frequency range. The spectroscopic work was augmented by high-level ab initio calculations. The calculations showed that between two existing rotamers, symmetric and asymmetric, the most stable is the asymmetric conformation, and it is the only conformation observed in the recorded spectra. The symmetric conformation is 6.7 kJ/mol higher in energy and thus has a very low Boltzmann factor. The analysis of the rotational spectra of the A conformation has shown that the observed lines exhibit a doublet or quartet structure owing to two large-amplitude motions, C-N torsion and amino group inversion. To study the large-amplitude motions in detail, we calculated a two-dimensional potential energy surface and determined the barrier heights for the torsion and inversion, Vt = 12.5 kJ/mol and Vi = 19.1 kJ/mol. About 2500 assigned rotational transitions in the ground vibrational state were fitted within experimental accuracy using the reduced axes system Hamiltonian. The set of obtained spectroscopic parameters allows accurate calculation of transition frequencies and intensities for an astrophysical search of AMN.

Synthesis, high-resolution millimeter-wave spectroscopy, and ab initio calculations of ethylmercury hydride.

The millimeter-wave rotational spectrum of an organomercury compound, ethylmercury hydride, has been recorded and assigned for the first time. The spectroscopic study is complemented by quantum chemical calculations taking into account relativistic effects on the mercury atom. The very good agreement between theoretical and experimental molecular parameters validates the chosen ab initio method, in particular its capability to predict accurate quartic centrifugal distortion constants related to this type of compound. Estimations of the nuclear quadrupole coupling constants have less predictive power than those of the structural parameters, but are good enough to satisfy the spectroscopic needs. In addition, the orientation of the axis of the H-Hg-C bonds deduced from the experimental nuclear quadrupole coupling constants compares well with the corresponding ab initio value. From the good agreement between experimental and theoretical results, together with the observation of the six most abundant isotopes of mercury, ethylmercury hydride is unambiguously identified as the product of the chemical reaction described here, and its calculated equilibrium geometry is confirmed.

Rotational spectrum and conformational composition of cyanoacetaldehyde, a compound of potential prebiotic and astrochemical interest.

The rotational spectrum of cyanoacetaldehyde (NCCH(2)CHO) has been investigated in the 19.5-80.5 and 150-500 GHz spectral regions. It is found that cyanoacetaldehyde is strongly preferred over its tautomer cyanovinylalcohol (NCCH═CHOH) in the gas phase. The spectra of two rotameric forms of cyanoacetaldehyde produced by rotation about the central C-C bond have been assigned. The C-C-C-O dihedral angle has an unusual value of 151(3)° from the synperiplanar (0°) position in one of the conformers denoted I, while this dihedral angle is exactly synperiplanar in the second rotamer called II, which therefore has C(s) symmetry. Conformer I is found to be preferred over II by 2.9(8) kJ/mol from relative intensity measurements. A double minimum potential for rotation about the central C-C bond with a small barrier maximum at the exact antiperiplanar (180°) position leads to Coriolis perturbations in the rotational spectrum of conformer I. Selected transitions of I were fitted to a Hamiltonian allowing for this sort of interaction, and the separation between the two lowest vibrational states was determined to be 58794(14) MHz [1.96112(5) cm(-1)]. Attempts to include additional transitions in the fits using this Hamiltonian failed, and it is concluded that it lacks interaction terms to account satisfactorily for all the observed transitions. The situation was different for II. More than 2000 transitions were assigned and fitted to the usual Watson Hamiltonian, which allowed very accurate values to be determined not only for the rotational constants, but for many centrifugal distortion constants as well. Two vibrationally excited states were also assigned for this form. Theoretical calculations were performed at the B3LYP, MP2, and CCSD levels of theory using large basis sets to augment the experimental work. The predictions of these calculations turned out to be in good agreement with most experimental results.

High resolution millimeter-wave spectroscopy of cyclopropylphosphine-borane.

The microwave spectrum of cyclopropylphosphine-borane, C(3)H(5)PH(2)-BH(3), has been investigated in the frequency range 150-195 GHz. The spectral assignment was supported by high level ab initio calculations. Two stable conformations have been predicted: the most stable antiperiplanar form and synclinal form that is higher in energy by 7.3 kJ/mol. In the observed spectra, only the most stable antiperiplanar (ap) form has been assigned. The analysis of the rotational spectra in the lowest excited vibrational states of the ap conformer has enabled determination of the potential function for the C-P torsional mode in the vicinity of equilibrium position. The barrier to internal rotation of the BH(3) top has been determined to be 9.616(15) kJ/mol and agrees well with quantum chemical calculations.

High resolution millimeter-wave spectroscopy of vinyltellurol.

The millimeter-wave rotational spectrum of vinyltellurol has been recorded and assigned for the first time. To support the spectrum assignment, high level ab initio calculations have been carried out. Geometries, total electronic energies, and harmonic vibrational frequencies have been determined at the MP2 level. A small-core relativistic pseudopotential basis set (cc-pVTZ-PP) was employed to describe the tellurium atom. Two stable conformers, synperiplanar (sp) and anticlinal (ac), have been identified. The sp conformer is planar with a small negative inertia defect of -0.025 u Å(2). The ac conformer was found to be nonplanar with a C-C-Te-H dihedral angle of about 140° from sp. This conformer exhibits a large amplitude motion associated with the torsion about the C-Te bond. The barrier to internal rotation is about 1 kJ/mol, according to the theoretical calculations. For the ac conformation, a torsional potential function consisting of quartic and quadratic terms of the torsional angle has been partially determined from the observed rotational constants.

First high resolution spectroscopic studies and ab initio calculations of ethanetellurol.

The millimeter-wave rotational spectrum of ethanetellurol has been recorded and assigned for the first time. The spectroscopic study has been complemented by high level ab initio calculations. Geometries, total electronic energies, and harmonic vibrational frequencies have been determined at the MP2 level. A small-core relativistic pseudopotential basis set (cc-pVTZ-PP) was employed to describe the tellurium atom. Two stable conformers, synclinal and antiperiplanar, have been identified. Both theory and experiment have shown the synclinal form to be more stable by 2 kJ/mol. The doublet structure observed in the rotational spectrum of synclinal conformer is attributed to tunneling motion of tellurol functional group. The energy difference between 0(+) and 0(-) substates split by tunneling has been determined from the observed spectra.