Quadrupole Hyperfine Structure Research Articles

C···S Tetrel Bond Favored in the Phenyl Isothiocyanate-CO2 Complex: A Rotational Study.

The rotational spectrum of the phenyl isothiocyanate-CO2 complex was investigated by pulsed-jet Fourier transform microwave spectroscopy complemented by quantum chemical calculations. Only one isomer, with CO2 almost in the plane of phenyl isothiocyanate, has been detected in the pulsed jet, of which the spectrum displays a quadrupole coupling hyperfine structure due to the presence of a 14N nucleus (I = 1). This structure is nearly equal to the lowest energy geometry obtained by B3LYP-D3(BJ)/6-311++G(d,p), which has been dominated by a C···S tetrel bond (n → π* interaction) and one secondary C-H···O hydrogen bond (n → σ* interaction). Molecular electrostatic potential and natural bond orbital analysis were used to characterize the noncovalent interactions of the complex. The results from this study would lay the groundwork for the design and advancement of materials that exhibit high efficiency in capturing CO2.

Variation of the Quadrupole Hyperfine Structure and Nuclear Radius due to an Interaction with Scalar and Axion Dark Matter.

Atomic spectroscopy is used to search for the space-time variation of fundamental constants which may be due to an interaction with scalar and pseudoscalar (axion) dark matter. In this Letter, we study the effects that are produced by the variation of the nuclear radius and electric quadrupole moment. The sensitivity of the electric quadrupole hyperfine structure to both the variation of the quark mass and the effects of dark matter exceeds that of the magnetic hyperfine structure by 1-2 orders of magnitude. Therefore, the measurement of the variation of the ratio of the electric quadrupole and magnetic dipole hyperfine constants is proposed. The sensitivity of the optical clock transitions in the Yb^{+} ion to the variation of the nuclear radius allows us to extract, from experimental data, limits on the variation of the hadron and quark masses, the QCD parameter θ and the interaction with axion dark matter.

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.

Calculation of the hyperfine structure of Dy, Ho, Cf, and Es

A recently developed version of the configuration interaction (CI) method for open shells with a large number of valence electrons has been used to study two heavy atoms, californium (Cf, Z= 98) and einsteinium (Es, Z= 99). Motivated by experimental work to measure the hyperfine structure (HFS) for these atoms, we perform the calculations of the magnetic dipole HFS constants $A$ and electric quadrupole HFS constant $B$ for the sake of interpretation of the measurements in terms of nuclear magnetic moment $\mu$ and electric quadrupole moment $Q$. For verification of our computations, we have also carried out similar calculations for the lighter homologs dysprosium (Dy, Z= 66) and holmium (Ho, Z= 67), whose electronic structures are similar to Cf and Es, respectively. We have conducted a revision of the nuclear moments of some isotopes of Es leading to an improved value of the magnetic moment of $^{253}$Es [$\mu$($^{253}$Es) = 4.20(13)$\mu_N$].

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.

On the planarity of benzyl cyanide

The microwave spectra of benzyl cyanide, C6H5CH2CN, were recorded using a molecular jet Fourier transform microwave spectrometer operating in the frequency range 2–26.5 GHz. Analyses of the 14N quadrupole hyperfine structures were performed simultaneously with the rotational frequencies. Rotational constants, centrifugal distortion constants, and nuclear quadrupole coupling constants χaa and χbb − χcc were all determined with very high accuracy in a fit containing 465 lines with standard deviation within the measurement accuracy. Results from quantum chemical calculations at many levels of theory employing the MP2 method indicate a non-planar heavy atom skeleton structure, where the CH2CN group is tilted up to 50° with respect to the phenyl ring. This conflicts with results from many calculations employing density functional theory methods and also with the conclusion of a previous microwave spectroscopic work (X.-Z. Liu, R.K. Bohn, S.A. Sorenson, N.S. True, J. Mol. Struct.243 (1991) 325–339), stating that the CH2CN group and the phenyl ring are in-plane. By comparing the planar defect with those of several planar molecules reported in the literature, and with extensive support from quantum chemistry, we present for the first time evidence suggesting that the heavy atom structure of benzyl cyanide is probably not planar.

Characterizing the n→π* interaction of pyridine with small ketones: a rotational study of pyridine⋯acetone and pyridine⋯2-butanone.

Complexes formed by pyridine and small ketones such as acetone and 2-butanone have been generated in a supersonic jet and characterized by broadband Fourier transform microwave spectroscopy combined with high-level theoretical computations. The spectra of the complexes show a quadrupole coupling hyperfine structure due to the presence of a nitrogen atom and the splittings owing to the low barriers of the internal rotation of the methyl groups bonded to the carbonyl group. The corresponding barriers have been determined from the analysis of the spectra. We show in both complexes that pyridine closes a cycle with a ketone carbonyl group through an N⋯CO n→π* tetrel interaction and a C-H⋯O contact. The n→π* tetrel bond involves the pyridine N atom lone pair and the ketone carbonyl group with a geometry approaching the Bürgi-Dunitz trajectory for the nucleophilic attack to a carbonyl group.

Precision Calculation of Hyperfine Constants for Extracting Nuclear Moments of ^{229}Th.

Determination of nuclear moments for many nuclei relies on the computation of hyperfine constants, with theoretical uncertainties directly affecting the resulting uncertainties of the nuclear moments. In this work, we improve the precision of such a method by including for the first time an iterative solution of equations for the core triple cluster amplitudes into the relativistic coupled-cluster method, with large-scale complete basis sets. We carried out calculations of the energies and magnetic dipole and electric quadrupole hyperfine structure constants for the low-lying states of ^{229}Th^{3+} in the framework of such a relativistic coupled-cluster single double triple method. We present a detailed study of various corrections to all calculated properties. Using the theory results and experimental data, we found the nuclear magnetic dipole and electric quadrupole moments to be μ=0.366(6)μ_{N} and Q=3.11(2) eb, respectively, and reduce the uncertainty of the quadrupole moment by a factor of 3. The Bohr-Weisskopf effect of the finite nuclear magnetization is investigated, with bounds placed on the deviation of the magnetization distribution from the uniform one.

Theoretical study of the electronic structure of hafnium (Hf,Z=72) and rutherfordium (Rf,Z=104) atoms and their ions: Energy levels and hyperfine-structure constants

Energy levels, magnetic dipole, and electric quadrupole hyperfine structure of the superheavy element rutherfordium (Rf, $Z$=104) and its first three ions are calculated using a combination of the configuration interaction, coupled-cluster single-doubles, and many-body perturbation theory techniques. The results are to be used in future interpretations of the measurements in terms of nuclear magnetic dipole and electric quadrupole momenta. To have a guide on the accuracy of the study, we perform similar calculations for hafnium (Hf, $Z$=72) and its ions. Hf is a lighter analog of Rf with a similar electronic structure. Good agreement with the experiment for Hf and with available previous calculations of the energy levels of Rf is demonstrated.

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.

Semi-empirical determination of the nuclear quadrupole moment of ^{109}Sn

The nuclear quadrupole moment (Q) of ^{109}Sn was determined by means of hyperfine structure (hfs) many-body parametrization method. The hyperfine structure splittings for isotopes ^{117-131}Sn recently measured by Yordanov et al. (Commun Phys 3:1, 2020) were used in multiconfiguration semi-empirical calculations. The contributions from the second-order perturbation theory to the magnetic dipole hyperfine structure, concerning electrostatically correlated hyperfine interactions, were taken into consideration for even and odd configurations simultaneously. Contributions from the second-order perturbation theory to the electric quadrupole hyperfine structure, concerning spin–orbit correlated hyperfine interactions, were included for the first time.

Rotational spectrum and quantum chemical calculations of methyl cyanoacetate: A compound of potential astrochemical interest

The rotational spectrum of methyl cyanoacetate (H3CO(CO)CH2CN) was investigated for the first time using Fourier transform microwave spectroscopy in the 6–19 GHz range. The observed spectral pattern is that of a single, dominant conformer and reveals both 14N quadrupole hyperfine structure and characteristic A/E splittings due to the methyl internal rotor. The rotational constants determined from analysis of the complex spectral pattern confirm that the observed spectrum is that of the lowest energy conformer of methyl cyanoacetate predicted at the B3LYP-D3(BJ) and MP2 levels of theory using the aug-cc-pVTZ basis set. This global minimum corresponds to a geometry which orients the COCO and CCCO dihedral angles in syn and near anti arrangements, respectively and is governed by a balance of stabilizing orbital interactions and destabilizing steric effects identified using non-covalent interaction and natural bond orbital analyses.

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.

Hyperfine structures of 2-cyanothiophene and 3-cyanothiophene: A quantum chemical study

A sufficient amount of spectroscopic data for S-substituted heterocyclic molecules are not available in the literature to adequately examine interstellar sulfur depletion. A quantum chemical investigation of quadrupole hyperfine structures of 2CNT, 3CNT, and some of their isotopologues is undertaken here in order to supplement the literature and supply new data that are valuable for the detection of new species in the ISM. The nuclear hyperfine quadrupole structures of 2-cyanothiophene (2CNT), 3-cyanothiophene (3CNT), and their 33S, 2H-isotopologues were analyzed at temperatures prevailing in regions where these compounds are expected to be found (T∼5K to T∼150). The geometrical parameters were calculated using density functional theory (M06-2X/6-31G(d,p)) with empirical corrections for systematic errors. The 14N, 33S, and 2H quadrupole hyperfine constants, and dipole moments for 2CNT, 3CNT, 33S-2C15NT, 33S-3C15NT, 2C15NT-3D, and 2C15NT-2D were calculated, and their corresponding hyperfine structures were analyzed. The strongest transition lines and optimal regions of detectability were also discussed.

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.

Coupled large amplitude motions: The effects of two methyl internal rotations and 14N quadrupole coupling in 4,5-dimethylthiazole investigated by microwave spectroscopy

The molecular jet Fourier-transform microwave spectrum of 4,5-dimethylthiazole has been recorded between 2.0 and 26.5 GHz, revealing torsional splittings arising from two inequivalent methyl internal rotations with relatively low hindering barriers and nitrogen quadrupole hyperfine structures. Two global fits with 1009 hyperfine components of 97 rotational transitions involving 315 torsional lines were performed using the program XIAM and BELGI-Cs-2Tops-hyperfine, an extended version of the BELGI-Cs-2Tops code which includes the effect of the 14N quadrupole coupling, giving a root-mean-square deviation of 399.8 kHz and 4.2 kHz, respectively. Compared to the monomethyl substituted thiazole derivatives, the barriers to internal rotation are drastically lower. This is also in contrast to chemical intuition which suggests high barriers due to steric hindrance. Because of the strong interaction between the methyl groups, strong top-top couplings in both the potential energy and kinetic parts of the Hamiltonian were observed.

Theoretical investigation of protonated thiophene and two of its nitrile substituted derivatives (2-cyanothiophene and 3-cyanothiophene).

Theoretical and experimental spectroscopic data for protonated cyano-thiophenes (R-CNH+ with R = C4H3S), which are needed for their interstellar search and/or detection, are still lacking in the literature. Considering the high abundance and reactivity of H3+ in the interstellar medium (ISM), a quantum chemical investigation on protonated thiophene and two of its nitrile-substituted derivatives (2-cyanothiophene and 3-cyanothiophene) is undertaken for their characterization. The geometrical structures for the title species are calculated at the M06-2X/6-31G(d,p) level of theory, followed by an empirical correction for systematic errors. At the same level of theory, IR and Raman spectra are explored and the rotational parameters are calculated. The proton affinity (PA) of R-CN and the enthalpy, entropy and Gibbs free energy changes (ΔrH, ΔrS and ΔrG) of the reactions producing R-CNH+ are computed at the G2(MP2) and G3B3 levels of theory and at different temperatures. The PA calculations show that the protonation favors the nitrogen atom, while ΔrH, ΔrS, and ΔrG reveal the spontaneous reactions producing R-CNH+ and their neutral forms. In addition, quadrupole hyperfine structures are predicted, while the region where the brightest lines fall at different temperatures is discussed. These results are expected to assist astrophysicists and astrochemists in the search for new species in the ISM.