Nuclear Quadrupole Hyperfine Structure Research Articles

Rotational spectrum and interstellar detection of the first torsionally excited state of methylamine

Context. Methylamine (CH3NH2) was first detected in the interstellar medium (ISM) toward Sgr B2 almost 50 years ago by observation of rotational transitions in its torsional ground state. Methylamine exhibits two large-amplitude motions (LAMs), the methyl torsion and amine wagging, which complicate the spectral analysis, especially in excited vibration states. The lack of an accurate model of the two coupled LAMs has also hampered the identification in the ISM of rotational transitions in excited vibrational states. Aims. The aim of this work is to study the terahertz and microwave rotational spectra of methylamine experimentally and theoretically in order to provide a reliable basis for the detection of its rotational transitions in the first torsionally excited state, υt = 1, in the ISM. Methods. The terahertz spectrum of methylamine was measured from 150 to 1520 GHz with the Lille fast scan spectrometer. Using a new “hybrid” Hamiltonian model, we were able to analyze the nuclear quadrupole hyperfine structure and to accurately fit the rotational spectrum of the υt = 1 state of methylamine. We used the imaging spectral line survey ReMoCA performed with the Atacama Large Millimeter/submillimeter Array (ALMA) to search for rotational transitions of methylamine in its first torsionally excited state toward the high-mass star forming region Sgr B2(N). The observed spectra are modeled under the assumption of local thermodynamic equilibrium (LTE). Results. Accurate spectral predictions were obtained for the ground and first excited states of CH3NH2. We report the first interstellar detection of methylamine in the υt = 1 state toward the offset position Sgr B2(N1S) in the hot molecular core Sgr B2(N1). The LTE parameters derived previously from the rotational emission of methylamine in its torsional ground state toward Sgr B2(N1S) yield synthetic spectra of methylamine in the υt = 1 state that are fully consistent with the ALMA spectra and allow us to identify five rotational lines of this state.

Electric Dipole Moments from Stark Effect in Supersonic Expansion: n-Propanol, n-Butanol, and n-Butyl Cyanide.

The orientation and magnitude of the molecular electric dipole moment are key properties relevant to topics ranging from the nature of intermolecular interactions to the quantitative analysis of complex gas-phase mixtures, such as chemistry in astrophysical environments. Stark effect measurements on rotational spectra have been the method of choice for isolated molecules but have become less common with the practical disappearance of Stark modulation spectrometers. Their role has been taken over by supersonic expansion measurements within a Fabry-Perot resonator cavity, which introduces specific technical problems that need to be overcome. Several of the adopted solutions are described and compared. Presently, we report precise electric dipole moment determinations for the two most stable conformers of the selected molecules of confirmed or potential astrophysical relevance: n-propanol, n-butanol, and n-butyl cyanide. All dipole moment components have been precisely determined at supersonic expansion conditions by employing specially designed Stark electrodes and a computer program for fitting the measured Stark shifts, inclusive of cases with resolved nuclear quadrupole hyperfine structure. The experimental values are compared with suitable quantum chemistry computations. It is found that, among the tested levels of computation, vibrationally averaged dipole moments are the closest to the observation and the molecular values are, as in the lighter molecules in the series, largely determined by the hydroxyl or nitrile groups.

Laboratory detection and astronomical study of interstellar acetohydroxamic acid, a glycine isomer

Aims. In this work, we aim to achieve the first laboratory detection of acetohydroxamic acid (CH3CONHOH), a relevant glycine isomer, to enable its eventual identification in the ISM. Methods. We employed a battery of state-of-the-art rotational spectroscopic techniques in the time domain to measure the microwave spectrum of acetohydroxamic acid. We then used the spectral GOTHAM survey performed with the Green Bank Telescope (GBT) to search for the lowest-energy Z-conformer toward the cold and quiescent molecular cloud TMC-1. We also employed a sensitive spectral survey of the chemically rich Galactic Center molecular cloud G+0.693-0.027, based on IRAM 30 m and Yebes 40 m observations. Results. We report direct experimental frequencies of the ground state of acetohydroxamic acid (up to 40 GHz). The 14N nuclear quadrupole hyperfine structure and the A-E splittings due to the internal rotation were observed and analyzed. Hence, a precise set of the rotational spectroscopic parameters were determined for the two distinct conformers, Z- and E-acetohydroxamic acid, which is the initial and prerequisite step of their radio astronomical search in the ISM using low-frequency surveys. We report the nondetection of acetohydroxamic acid toward both astronomical sources. We derive an upper limit to the column density of this molecule very similar to that obtained for glycine. Its corresponding molecular abundance with respect to molecular hydrogen is found to be ≤1 × 10−9 and 2 × 10−10 in TMC-1 and G+0.693-0.027, respectively, which further constrain the abundance of this glycine isomer in the ISM.

From Molecular to Cluster Properties: Rotational Spectroscopy of 2-Aminopyridine and of Its Biomimetic Cluster with Water

We report the observation and analysis of the rotational spectrum of a 1:1 cluster between 2-aminopyridine and water (AMW) carried out with supersonic expansion Fourier transform microwave spectroscopy at 4.7–16.5 GHz. Measurements of the 2-aminopyridine monomer (AMP) were also extended up to 333 GHz for the room-temperature rotational spectrum and to resolved hyperfine splitting resulting from the presence of two 14N quadrupolar nuclei. Supersonic expansion measurements for both AMP and AMW were also carried out for two synthesized isotopic species with single deuteration on the phenyl ring. Nuclear quadrupole hyperfine structure has also been resolved for AMW and the derived splitting constants were used as an aid in structural analysis. The structure of the AMW cluster was determined from the three sets of available rotational constants and the hydrogen bonding configuration is compared with those for clusters with water of similarly sized single-ring molecules. Experimental results aided by quantum chemistry computations allow the conclusion that the water molecule is unusually strongly bound by two hydrogen bonds, OH...N and O...HN, to the NCNH atomic chain of AMP with the potential to replace hydrogen bonds to the identical structural segment in cytosine and adenine in CT and AT nucleic acid base pairs.

The 2,2,4,4-tetrafluoro-1,3-dithietane⋯NH3 complex: A rotational study reveals a N⋯σ-hole interaction

Six isotopologues of the 2,2,4,4-tetrafluoro-1,3-dithietane⋯ammonia complex were characterized by rotational spectroscopy. Accurate molecular structures of the observed dimer were obtained combining quantum chemical calculations and experimental results employing Kraitchman’s equations. To further investigate the non-covalent interactions between the two subunits, the nuclear quadrupole hyperfine structure was analyzed and interpreted. Additionally, natural bond orbital and non-covalent interactions analyses were applied. A N⋯S σ-hole interaction characterizes the structure of the observed complex.

Laser spectroscopy of indium Rydberg atom bunches by electric field ionization

This work reports on the application of a novel electric field-ionization setup for high-resolution laser spectroscopy measurements on bunched fast atomic beams in a collinear geometry. In combination with multi-step resonant excitation to Rydberg states using pulsed lasers, the field ionization technique demonstrates increased sensitivity for isotope separation and measurement of atomic parameters over previous non-resonant laser ionization methods. The setup was tested at the Collinear Resonance Ionization Spectroscopy experiment at ISOLDE-CERN to perform high-resolution measurements of transitions in the indium atom from the text {5s}^2text {5d},^2text {D}_{5/2} and text {5s}^2text {5d},^2text {D}_{3/2} states to text {5s}^2np ^2P and text {5s}^2ntext {f},^2F Rydberg states, up to a principal quantum number of n=72. The extracted Rydberg level energies were used to re-evaluate the ionization potential of the indium atom to be 46,670.107(4),hbox {cm}^{-1}. The nuclear magnetic dipole and nuclear electric quadrupole hyperfine structure constants and level isotope shifts of the text {5s}^2text {5d},^2text {D}_{5/2} and text {5s}^2text {5d},^2text {D}_{3/2} states were determined for ^{113,115}In. The results are compared to calculations using relativistic coupled-cluster theory. A good agreement is found with the ionization potential and isotope shifts, while disagreement of hyperfine structure constants indicates an increased importance of electron correlations in these excited atomic states. With the aim of further increasing the detection sensitivity for measurements on exotic isotopes, a systematic study of the field-ionization arrangement implemented in the work was performed at the same time and an improved design was simulated and is presented. The improved design offers increased background suppression independent of the distance from field ionization to ion detection.

The rotational spectrum of Methylarsine

The rotational spectrum of an organoarsenic compound, methylarsine, CH3AsH2, was studied in the frequency range 10–80 GHz using the Stark-modulated spectrometer in Oslo, and in the frequency range 150–660 GHz using the Lille spectrometer. The experimental work was augmented by high-level ab initio calculations. The analysis of large amplitude torsional motion in CH3AsH2 was performed using the rho axis method and the RAM36 code modified to take into account the nuclear quadrupole hyperfine structure. The final fit used 48 parameters (including 3 parameters of quadrupole coupling) to give an overall unitless root-mean-square deviation of 0.92 for the dataset consisting of 2753, 991, and 414 transitions belonging, respectively, to the ground, first, and second excited torsional states. Compact grouping of rotational transitions due to the small asymmetry (κ=-0.9986), torsional splittings due to internal rotation of the CH3 group, and relatively large hyperfine splittings due to a rather large value of the arsenic nuclear quadrupole moment (χaa=35.4 MHz, χcc=−117.2 MHz, χac=115.7 MHz) represent an interesting case of interplay of different splittings in a molecule with a rather strong coupling between internal and overall rotations (ρ=0.413).

Rotational spectra and molecular structures of ethylanilines

The molecular structures of three ethylanilines, ortho-, meta- and para-ethylaniline, have been obtained by means of Fourier-transform microwave spectroscopy. Rotational spectra of all three molecules display the nuclear quadrupole hyperfine structures arising from the 14N nucleus. Comparison of the determined structures allows a direct study of the influence of the position of the ethyl substituent on the structure of the amino group communicated through the phenyl ring.

A Comprehensive Spectral Rotational Analysis of the Interstellar Methyl Isocyanate CH3NCO

Abstract Methyl isocyanate (CH3NCO) is a recently identified interstellar molecule giving rise to many detected lines. Interestingly, its delayed identification was due not to weak lines but to a very complex rotational spectrum. To date, the only published laboratory transitions for this molecule are those between rotational energy levels with K ≤ 3. In the present work, Stark-modulation spectroscopy was used to record the room-temperature rotational spectrum of CH3NCO in the spectral region from 32 to 90 GHz. Observation of characteristic Stark effects, measured at specific low-voltage modulation conditions, and 14N nuclear quadrupole hyperfine structure allowed unambiguous assignment of rotational transitions up to K = 10. These newly assigned transitions were subsequently followed up to 364 GHz with the aid of Loomis–Wood-type displays. Since there are no reports on astrophysical detection of 13C isotopic species, first laboratory measurements between 50 and 300 GHz have also been performed for CH3N13CO and 13CH3NCO isotopologues. A comprehensive spectral analysis undertaken in this work made it possible to extend the knowledge of the rotational spectrum of CH3NCO to more than 2500 new transitions. Furthermore, more than 1200 lines were identified and analyzed for each of the isotopologues. The extensive line lists and sets of molecular parameters reported in this work provide the basis for further astrophysical searches of CH3NCO.

A highly flexible molecule: The peculiar case of ethynyl isothiocyanate HCCNCS.

The rotational spectrum of the parent isotopic species of HCCNCS, along with those of three 13C singly substituted variants and one 34S minor isotopologue, has been observed with high resolution using Fourier transform microwave spectroscopy from 4 to 26 GHz. Based on the observed spectral pattern and assignment of the 14N nuclear quadrupole hyperfine structure, the geometry of HCCNCS is effectively linear, which is further supported by the observation of -type doubling for rotational transitions in an excited bending state of the parent HCCNCS. The experimental observations are supported by high level quantum chemical calculations using the coupled-cluster singles and doubles model augmented by a perturbative correction for triple excitations, CCSD(T), level of theory that reveal an extremely shallow potential energy well along the CNC bending angle with a surprising basis set dependence.

Second rank direction cosine spherical tensor operators and the nuclear electric quadrupole hyperfine structure Hamiltonian of rotating molecules

Transformations of vector or tensor properties from a space-fixed to a molecule-fixed axis system are often required in the study of rotating molecules. Spherical components λμ,ν of a first rank irreducible tensor can be obtained from the direction cosines between the two axis systems, and a second rank tensor with spherical components λμ,ν(2) can be built from the direct product λ × λ.It is shown that the treatment of the interaction between molecular rotation and the electric quadrupole of a nucleus is greatly simplified, if the coefficients in the axis-system transformation of the gradient of the electric field of the outer charges at the coupled nucleus are arranged as spherical components λμ,ν(2). Then the reduced matrix elements of the field gradient operators in a symmetric top eigenfunction basis, including their dependence on the molecule-fixed z-angular momentum component k, can be determined from the knowledge of those of λ(2). The hyperfine structure Hamiltonian Hq is expressed as the sum of terms characterized each by a value of the molecule-fixed index ν, whose matrix elements obey the rule Δk = ν. Some of these terms may vanish because of molecular symmetry, and the specific cases of linear and symmetric top molecules, orthorhombic molecules, and molecules with symmetry lower than orthorhombic are considered.Each ν-term consists of a contraction of the rotational tensor λ(2) and the nuclear quadrupole tensor in the space-fixed frame, and its matrix elements in the rotation-nuclear spin coupled representation can be determined by the standard spherical tensor methods.

The Structure of the Elusive Simplest Dipeptide Gly-Gly.

Among the hundreds of peptide compounds for which conformations have been determined by using different spectroscopic techniques, the structure of the simplest dipeptide glycylglycine (Gly-Gly) is conspicuously absent. Herein, for the first time, solid samples of Gly-Gly have been vaporized by laser ablation and three different structures have been revealed in a supersonic expansion by Fourier transform microwave spectroscopy. The intramolecular hydrogen bonding interactions that stabilize the observed forms have been established based on the 14 N nuclear quadrupole hyperfine structure. We have illustrated how conformer interconversion distorts the equilibrium conformational distribution, giving rise to missing conformers in the conformational landscape.

The Structure of the Elusive Simplest Dipeptide Gly‐Gly

AbstractAmong the hundreds of peptide compounds for which conformations have been determined by using different spectroscopic techniques, the structure of the simplest dipeptide glycylglycine (Gly‐Gly) is conspicuously absent. Herein, for the first time, solid samples of Gly‐Gly have been vaporized by laser ablation and three different structures have been revealed in a supersonic expansion by Fourier transform microwave spectroscopy. The intramolecular hydrogen bonding interactions that stabilize the observed forms have been established based on the 14N nuclear quadrupole hyperfine structure. We have illustrated how conformer interconversion distorts the equilibrium conformational distribution, giving rise to missing conformers in the conformational landscape.

Rotational spectra of two six-membered heterocyclic N-methyl-piperidinol compounds: Conformations by OH rotation, N-methyl inversion, and ring puckering.

Rotational spectra of two nitrogen containing six-membered heterocycles which are commonly used in syntheses of pharmaceuticals, namely, N-methyl-3-piperidinol (NMP3) and N-methyl-4-piperidinol (NMP4), were measured using a broadband chirped pulse and a cavity based Fourier transform microwave spectrometer. The possible conformers due to the OH rotation, N-methyl inversion, and ring puckering were investigated theoretically for these two heterocycles. The substituent position of the hydroxyl group with respective to the N atom in the heterocyclic ring has a strong influence on the preferred conformations. While one dominant conformer, favoring the OH⋯N close contact, was predicted for NMP3, several close energy conformers with OH pointing at different directions were predicted for NMP4. In contrast, only one conformer was identified for each compound experimentally. The 14N nuclear quadrupole hyperfine structures were observed for all rotational transitions and analyzed. In addition, rotational spectra of all 13C and 15N isotopologues of NMP4 were studied in their natural abundance, leading to a definite identification of the NMP4 conformer observed. The differences in the conformational landscapes and the OH motions in the two compounds are presented and also discussed in the context of the 1,3-diaxial interaction rule commonly used in organic chemistry.

Characterization of the [18.42]0+ – X1Σ+ (0, 0) band of tantalum nitride, TaN

High resolution laser-induced fluorescence (LIF) spectra of the [18.42]0+ − X1Σ+ (0,0) band of tantalum nitride, TaN, near 543nm, have been recorded and analyzed field-free and in the presence of a static electric field. The nuclear electric quadrupole hyperfine structure due to 181Ta(I=7/2) was resolved and analysis of the field-free spectrum yielded nuclear electric quadrupole coupling constants, eQq0, of −0.24025(75)cm−1 and −0.05755(76)cm−1 for the X1Σ+ and [18.42]0+ states, respectively. The Stark tuning of the R(0) and P(1) lines was analyzed to determine the magnitude of the molecular frame electric dipole moments, |μ→el|, of 3.33(4)D and 4.48(3)D for the X1Σ+ and [18.42]0+ states, respectively. The |μ→el| and eQq0 values are discussed in terms of predicted molecular orbitals for the X1Σ+ and [18.42]0+ states.

Effect of Chlorine Substitution in Modulating the Relative Importance of Two Intermolecular Interactions: The Microwave Spectrum and Molecular Structure of (E)-1-Chloro-2-fluoroethylene-HCl.

Fourier transform microwave rotational spectroscopy is used to determine the structure of the gas-phase bimolecular complex formed between (E)-1-chloro-2-fluoroethylene and hydrogen chloride. Extensively split by nuclear quadrupole hyperfine structure and isotopic dilution, the spectrum is first identified via weak features observed using a broadband chirped pulse spectrometer in the 5.6-18.1 GHz range and studied in detail with greater sensitivity and resolution over 6.0-20.0 GHz with a Balle-Flygare, narrowband instrument. The complex has a geometry similar to that of vinyl fluoride-HCl, with HCl binding across the C═C double bond, forming a hydrogen bond to the fluorine atom of the haloethylene and bending to allow a secondary interaction to develop with the hydrogen atom in the cis position. Further consideration of structural details among the complexes of hydrogen fluoride and hydrogen chloride with (E)-1-chloro-2-fluoroethylene and vinyl fluoride suggests that the addition of a trans Cl atom in vinyl fluoride enhances the significance of the secondary interaction while deemphasizing that of the hydrogen bond.

Probing the Electronic Environment of Methylindoles using Internal Rotation and (14)N Nuclear Quadrupole Coupling.

High-resolution rotational spectra were recorded in the 10.5-21.0 GHz frequency range for seven singly methylated indoles. (14)N nuclear quadrupole hyperfine structure and spectral splittings arising from tunneling along the internal rotation of the methyl group were resolved for all indole species. The nuclear quadrupole coupling constants were used to characterize the electronic environment of the nitrogen atom, and the program XIAM was used to fit the barrier to internal rotation to the measured transition frequencies. The best fit barriers were found to be 277.1(2), 374.32(4), 414.(5), 331.6(2), 126.8675(15), 121.413(4), and 426(3) cm(-1) for 1-methylindole through 7-methylindole, respectively. The fitted barriers were found to be in good agreement with barriers calculated at the ωB97XD/6-311++G(d,p) level. The complete set of experimental barriers is compared to theoretical investigations of the origins of methyl torsional barriers and confirms that the magnitude of these barriers is an overall effect of individual hyperconjugative and structural interactions of many bonding/antibonding orbitals.

The Trimethylamine-Formic Acid Complex: Microwave Characterization of a Prototype for Potential Precursors to Atmospheric Aerosol.

The reactions of amines and carboxylic acids have recently received attention for their possible role in the formation of atmospheric aerosol. Here, we report a microwave study of the trimethylamine-formic acid hydrogen-bonded complex, a simple prototype in which to study amine-carboxylic acid interactions. Spectra of three isotopologues of the system have been observed using a tandem cavity and chirped-pulse Fourier transform microwave spectrometer. The complex has a plane of symmetry, with the acidic proton of the formic acid directed toward the lone pair of the nitrogen. The zero-point-averaged hydrogen bond length is 1.702 Å, and the O-H···N angle is 177°. (14)N nuclear quadrupole hyperfine structure has been used to assess the degree of proton transfer from the formic acid to the trimethylamine. Experimental results are supplemented with density functional theory calculations. M06-2X/6-311++G(3df,3pd) calculations indicate a binding energy of 16.8 kcal/mol with counterpoise correction (17.4 kcal/mol without counterpoise correction).

A new millimeter-wave observation of the weakly bound CO–N2 complex

New millimeter-wave transitions of the CO–N2 van der Waals complex have been observed using the intracavity OROTRON jet spectrometer in the frequency range of 103–159GHz. For the less abundant form, CO–paraN2, a total of 37 rotational transitions were assigned to three K=0–0, 0–1, 2–1 subbands connecting the (jCO, jN2)=(1,1) and (jCO, jN2)=(0,1) internal rotor states. The upper K=0 and K=2 “stacks” of rotational levels were probed for the first time here by millimeter-wave spectroscopy following a recent infrared study by Rezaei et al. (2013). The observation of new subbands fixes with higher precision not only these upper K=0 and K=2 but also lower K=1(f) levels, not linked with other stacks in earlier rotational studies. For the more abundant form, CO–orthoN2, five new P-branch rotational transitions of the K=0–0 “CO bending” subband are reported, thus extending previous measurements. Nuclear quadrupole hyperfine structure due to the presence of two equivalent 14N nuclei was partly resolved and analyzed to give additional information about the angular orientation of the N2 molecule in the complex.

The formic acid-nitric acid complex: microwave spectrum, structure, and proton transfer.

Rotational spectra are reported for seven isotopologues of the complex HCOOH-HNO3 in a supersonic jet. The system is planar and bound by a pair of hydrogen bonds, much like the more widely studied carboxylic acid dimers. Double proton exchange interconverts the system between a pair of equivalent structures, as revealed by a splitting of the a-type spectrum that disappears when one of the hydrogen bonding protons is replaced by deuterium. The observation of relative intensities that are consistent with nuclear spin statistics in a symmetric and antisymmetric pair of tunneling states provides additional evidence for such a motion. The observed splittings in the pure rotational spectrum are 1-2 orders of magnitude smaller than those recently reported in the pure rotational spectra of several related carboxylic acid dimers. This is a curious difference, although we note that because the observed spectra do not cross the tunneling doublet, the splittings are a measure of the difference in effective rotational constants for the two states, not the tunneling frequency itself. The observed rotational constants have been used to determine an accurate vibrationally averaged structure for the complex. The two hydrogen bond lengths, 1.686(17) Å and 1.813(10) Å for the hydrogen bonds involving the HNO3 and HCOOH protons, respectively, differ by 0.127(27) Å. Likewise, the associated oxygen-oxygen distances determined for the parent species, 2.631 and 2.794 Å, differ by 0.163 Å. These results suggest that the double proton transfer is necessarily accompanied by substantial motion of the heavy atom frame, and thus this system, in principle, provides an excellent prototype for multidimensional tunneling processes. Ab initio calculations of the binding energy and the barrier height are presented. Excellent agreement between the calculated equilibrium structure and the experimental, vibrationally averaged structure suggests that the vibrational wave function is not highly delocalized in the region between the equivalent potential wells. (14)N nuclear quadrupole hyperfine structure is interpreted in terms of the degree to which the HNO3 releases its proton in either of the equivalent potential energy minima.