Nuclear Quadrupole Coupling Constants Research Articles

Electric Field Gradient in Chiral and Tetrahedral Molecules within High-Order LRESC Formalism.

In this work, we present the electric field gradient (EFG) given by the linear response elimination of the small component (LRESC) scheme up to the 1/c4 order (c is the speed of light in vacuum) in CHFClX (X = Br, I, At) chiral molecules, together with CHF2Br and CH2FX (X = Br, I, At) tetrahedral systems. The former could be good candidates for further parity violation studies, especially when heavy atoms are surrounding. In this context, the LRESC scheme demonstrates effective applicability to large tetrahedral and chiral molecules that incorporate heavy elements, with relativistic effects playing a crucial role. The LRESC results of EFG exhibit an excellent agreement with those calculated at the four-component level, giving differences of only hundredths order in a.u. (atomic units) for the bromine nucleus and less than 0.1 a.u. for the iodine nucleus. Regarding the other nuclei, for the chiral molecules, there is a heavy atom effect on the light atom (HALA) for chlorine and fluorine atoms as the substituent halogen atom becomes heavier. Furthermore, the electronic part of the EFG for the central carbon and the fluorine nuclei presents an important dependence with the environment in the molecules under study. With accurate calculations of the EFG and tabulated nuclear quadrupole moment, the nuclear quadrupole coupling constant is obtained within the LRESC scheme, including for the first time correlation effects on the spin-dependent corrections with this methodology, providing results close to the experimental ones for Cl, Br, and I atoms. At the Hartree-Fock level, the differences are around 6% for Br and I nuclei, and at the density functional theory level with the LDA and PBE0 functionals, the differences are no more than 2%.

Anti‐Coronavirus Activity of Certain Herbacetin Derivatives, A Theoretical Study

AbstractFlavonoids are natural products commonly found in the human diet with anti‐viral activity. The electrochemical activity of flavonoids is effective in their inhibitory properties. Herbacetin is an anti‐coronavirus drug from the flavonoid family. To understand structure‐electrochemical activity relations, ab initio calculations on some selected herbacetin derivatives were carried out using Gaussian 09 in B3LYP/6‐311G*. The effect of the position and type of the various substituents in the ring A of the flavone backbone are considered. The oxygen charge density of the hydroxyl group on C8 (O8) in the considered herbacetins was compared using nuclear quadrupole coupling constants (NQCC) calculations. Results showed that the presence of the amino group in the ortho position of O8 has the most significant effect on increasing the charge density of the O8 atom (O8‐NQCC=9.23 MHz). Based on the proposed mechanism of these drugs, ortho‐amino herbacetin is expected to show a significant anti‐coronavirus effect. Comparison of calculated O8‐NQCCs of tangeretin and nobiletin with herbacetin shows that with the replacement of OH with OCH3, the charge density of oxygen atoms decreases (about 2 MHz). Therefore tangeretin and nobiletin have a lower drug effect than herbacetin. Natural bond orbital (NBO) results are in agreement with calculated NQCCs.

Accurate Experimental Structure of 1-Chloronaphthalene.

The structure of isolated 1-chloronaphthalene has been investigated in a supersonic expansion by high-resolution chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy in the 2-8 GHz frequency range. Accurate values of the rotational, centrifugal distortion, and nuclear quadrupole coupling constants for the only availabe conformer have been determined. The intensity of the spectrum allowed us to observe all the heavy atoms isotopologues in natural abundance, determining their rotational constants. From the extensive experimental dataset we derived accurate structures for 1-chloronaphthalene using different methodologies and compared with related compounds.

Partial Proton Transfer in the Gas Phase: A Spectroscopic and Computational Analysis of the Trifluoroacetic Acid - Trimethylamine Complex.

The 1:1 complex formed from trifluoroacetic acid (TFA) and trimethylamine (TMA) has been observed in the gas phase by rotational spectroscopy and further investigated by DFT and MP2 methods. Spectra of both the parent form and the -OD isotopologue have been obtained. The complex is structurally similar to a hydrogen bonded system, with the O-H bond directed toward the nitrogen of the TMA. However, both the spectroscopic and computational results indicate that it is intermediate between a hydrogen bonded complex and a proton-transferred ion pair. Two metrics are used to assess the degree of proton transfer from the acid to the base. The first is based on experimental 14N nuclear quadrupole coupling constants. Specifically, the component of the 14N nuclear quadrupole coupling tensor along the c-inertial axis of the complex, χcc, is 31% of the way between that of free TMA (no proton transfer) and that of TMAH+ (complete proton transfer). A second metric, adapted from that of Kurnig and Scheiner [Int. J. Quantum Chem. Quantum Biol. Symp. 1987, 14, 47-56], is based on calculated O-H and H-N distances and corroborates this description. These results indicate that the degree of proton transfer in TFA-TMA is very similar to that in the TMA complex of HNO3, which has been previously studied and for which the proton affinity of the conjugate anion (NO3-) is almost identical to that of CF3COO-. While the solid salt, TMAH+·CF3COO-, is an ionic plastic above 307 K and exhibits free rotation of the ions, no such motion is observed in the cold 1:1 gas phase adduct.

10-Valence-Electron C≡O and the 14-VE C≡Pt: Two Triple-Bonded Isoelectronic Families Differing by a dδ4 Ring.

10-VE A≡A' Diatomics, such as N≡N, C≡O, etc., have a strong σ2π4 triple bond plus a lone pair at each end. In our studies on 14-VE A≡B systems, such as C≡Pt, we find a similar bonding system plus a (5dδ)4 ring. Here, the A atom belongs to groups 13-17 and the B atom to groups 7-11. Also the BB' combinations, triatomics, such as PtCO or DsCO or uranyl, and longer chains, such as AuCN and [NC-Au-CN]-, are discussed. The δ ring directly contributes to nuclear quadrupole coupling constants, and DFT calculations using the BH and H or mPW1K functionals reproduce the experimental trends of the NQCC.

Noncovalent Interactions in the Molecular Geometries of 4-Methylthiazole···H2O and 5-Methylthiazole···H2O Revealed by Microwave Spectroscopy.

The pure rotational spectra of 4-methylthiazole···H2O and 5-methylthiazole···H2O were recorded by chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy. Each complex was generated within the rotationally cold environment of a gas sample undergoing supersonic expansion in the presence of an argon buffer gas. The spectra of five isotopologues of each complex have been measured and analyzed to determine the rotational constants, A0, B0, and C0; centrifugal distortion constants, DJ, DJK, and d1; nuclear quadrupole coupling constants, χaa(N3) and [χbb(N3) - χcc(N3)]; and parameters describing the internal rotation of the CH3 group, V3 and ∠(i,b). The experimentally deduced parameters were obtained using the XIAM and the BELGI-Cs-hyperfine code. For each complex, parameters in the molecular geometry are fitted to experimentally determined moments of inertia. DFT calculations have been performed at the ωB97X-D/aug-cc-pVQZ level in support of the experiments. Each complex contains two hydrogen bonds; a comparatively strong, primary interaction between the N of thiazole and an O-H of H2O, and a weaker, secondary interaction between O and either the hydrogen atom attached to C2 (in 5-methylthiazole···H2O) or the CH3 group attached to C4 (in 4-methylthiazole···H2O). The barrier to internal rotation of the CH3 group, V3, is slightly lower for 4-methylthiazole···H2O (XIAM result is 340.05(56) cm-1) than that for the 4-methylthiazole monomer (357.6 cm-1). This is likely to be a result of internal charge redistribution within the 4-methylthiazole subunit following its coordination by H2O. At the precision of the experiments, V3 of 5-methylthiazole···H2O (XIAM result is 325.16(38) cm-1) is not significantly different from V3 of the 5-methylthiazole monomer (332.0 cm-1).

The microwave spectra of the conformers of n-butyl nitrate

The microwave spectrum of n-butyl nitrate was recorded in the 5 to 20 GHz frequency range using broadband chirp and narrowband pulse excitation molecular jet Fourier transform microwave spectrometers. A quantum chemistry structural analysis yielded thirteen stable conformers. Among them, the five most energetically stable conformers were observed in the experimental spectra. The most stable conformer features a butyl chain with an anti-gauche-anti conformation (AGA) where the γ-carbon atom is about 64° out of the nitrate plane. For this conformer, spectra of all 13C and 15N minor isotopologues could be measured. The conformer with a straight butyl chain (AAA), and three other conformers (GAA, GGA, and AGG) were also observed. Accurate rotational constants, centrifugal distortion constants, and 14N nuclear quadrupole coupling constants could be deduced and compared to the theoretical values.

Many-body quantum muon effects and quadrupolar coupling in solids

Strong quantum zero-point motion (ZPM) of light nuclei and other particles is a crucial aspect of many state-of-the-art quantum materials. However, it has only recently begun to be explored from an ab initio perspective, through several competing approximations. Here we develop a unified description of muon and light nucleus ZPM and establish the regimes of anharmonicity and positional quantum entanglement where different approximation schemes apply. Via density functional theory and path-integral molecular dynamics simulations we demonstrate that in solid nitrogen, α–N2, muon ZPM is both strongly anharmonic and many-body in character, with the muon forming an extended electric-dipole polaron around a central, quantum-entangled [N2–μ–N2]+ complex. By combining this quantitative description of quantum muon ZPM with precision muon quadrupolar level-crossing resonance experiments, we independently determine the static 14N nuclear quadrupolar coupling constant of pristine α–N2 to be –5.36(2) MHz, a significant improvement in accuracy over the previously-accepted value of –5.39(5) MHz, and a validation of our unified description of light-particle ZPM.

The chirped pulse, Fourier transform microwave spectrum of 1-chloromethyl-1-fluorosilacyclopentane

The microwave spectrum of 1-chloromethyl-1-fluorosilacyclopentane has been recorded for the first time using the chirped pulse, Fourier transform microwave technique. Quantum chemical calculations show the two lowest energy conformers as being a twist-trans and a gauche form with the gauche form previously being shown as having two separate conformations, a gauche+ (lower in energy) and gauche- (higher in energy) form. Analysis of the spectrum provided the observation of the twist-trans conformer only, with 253 and 85 transitions being assigned to the 35Cl and 37Cl isotopologues, respectively. R-branch, a- and b-type transitions were observed. The spectrum was fit to a Watson S-reduced Hamiltonian and consisted of rotational constants, quartic centrifugal distortion constants, and nuclear quadrupole coupling constants, including the determination of the off-diagonal nuclear quadrupole coupling constant, χab. Interpretation of the structure was provided using second moments and is found to have a similar ring structure to other known silacyclopentanes. Analysis of the χzz has been carried out and compared to other similar molecules. An investigation of the known quantum chemical energies of the gauche conformer reveals that the reported B3LYP energies do not align with the observed microwave results.

The Microwave Rotational Electric Resonance (RER) Spectrum of Benzothiazole.

The microwave spectra of benzothiazole were measured in the frequency range 2-26.5 GHz using a pulsed molecular jet Fourier transform microwave spectrometer. Hyperfine splittings arising from the quadrupole coupling of the 14N nucleus were fully resolved and analyzed simultaneously with the rotational frequencies. In total, 194 and 92 hyperfine components of the main species and the 34S isotopologue, respectively, were measured and fitted to measurement accuracy using a semi-rigid rotor model supplemented by a Hamiltonian accounting for the 14N nuclear quadrupole coupling effect. Highly accurate rotational constants, centrifugal distortion constants, and 14N nuclear quadrupole coupling constants were deduced. A large number of method and basis set combinations were used to optimize the molecular geometry of benzothiazole, and the calculated rotational constants were compared with the experimentally determined constants in the course of a benchmarking effort. The similar value of the χcc quadrupole coupling constant when compared to other thiazole derivatives indicates only very small changes of the electronic environment at the nitrogen nucleus in these compounds. The small negative inertial defect of -0.056 uÅ2 hints that low-frequency out-of-plane vibrations are present in benzothiazole, similar to the observation for some other planar aromatic molecules.

Nuclear Quadrupole Splittings in Rotational Spectra for the Possible Detection of Fine Structure Constant Variations: The Diatomic Gold Halides and Gold Hydride as Case Studies

The dependence of nuclear quadrupole coupling constants CNQC(α) on the fine-structure constant α for various diatomic gold molecules AuX (X = H, F, Cl, Br, and I) at the density functional level of theory is investigated. While the electric field gradient at gold is very sensitive to the density functional applied, the derivative with respect to α is less sensitive. From this, we can estimate the upper limit for the α variation in time, ∂CNQC/∂t, for the 197Au nuclear quadrupole coupling constant, which is on the order of 10-9 Hz/year. This is currently beyond the limit of high-precision spectroscopy. I demonstrate that α∂CNQC∂α can be estimated from relativistic effects in CNQC, which will be useful in further investigations.

Rotational spectroscopic studies of para-nitrobenzoic acid, para-aminobenzoic acid, para-chlorobenzoic acid, and para-hydroxybenzoic acid

The rotational spectra of four substituted benzoic acids, i.e., para-aminobenzoic acid, para-nitrobenzoic acid, para-chlorobenzoic acid, and para-hydroxybenzoic acid were recorded with a chirp-pulse Fourier transform microwave spectrometer and analysed in terms of rotational constants and, where applicable, nuclear quadrupole coupling constants and tunnelling motions. In all instances, the experimentally identified conformer contains a carboxylic acid group in the cis-arrangement. In the case of para-hydroxybenzoic acid, spectra of two conformers were observed which differ in the orientation of the para-OH group. Quantum chemical calculations at the B3LYP-D3BJ/def2-TZVP and B2PLYP-D3BJ/cc-pCVTZ levels show that the trans-conformers are about 23 kJ mol−1 higher in energy than the global minimum cis-conformers. In general, the calculated rotational constants are in good agreement with the experimental derived ones. Agreement between calculated and experimental 14N and 35/37Cl nuclear quadrupole coupling constants is somewhat worse and necessitated the use of the B2PLYP functional in the case of para-nitrobenzoic acid to reduce the discrepancy. Conversion barriers between possible conformers were calculated to explain the absence of tunnelling splittings and/or higher energy conformers in the experimental spectra. An analysis of the electron density distribution was used to rationalize the calculated out-of-plane excursions of the carboxylic acid group in the trans-conformers.

The microwave spectra and molecular structures of (Z)-1-chloro-3,3,3-trifluoropropene and its gas-phase heterodimer with the argon atom

The microwave spectra of (Z)-1-chloro-3,3,3-trifluoropropene and its gas-phase heterodimer with the argon atom in the 5.6 to 18.1 GHz frequency range are first observed and assigned using broadband, chirped pulse, Fourier transform microwave (FTMW) spectroscopy. Subsequent analysis of higher-resolution spectra obtained between 5 and 21 GHz with a narrowband, Balle-Flygare FTMW instrument provides spectroscopic constants, including nuclear quadrupole coupling constants, for five isotopologues of the propene molecule and two isotopologues of the complex with argon. Structural parameters obtained from these spectra show the existence of an intramolecular hydrogen bond between one of the fluorine atoms of the trifluoromethyl group and the hydrogen atom on the adjacent carbon atom. No evidence is seen for internal rotation of the trifluoromethyl group. The location of the argon atom in the heterodimer is consistent with the expectation that it will be positioned so to interact with the greatest number of heavy atoms, and in particular, the polarizable chlorine atom.

Cooperative hydrogen bonding in thiazole⋯(H2O)2 revealed by microwave spectroscopy.

Two isomers of a complex formed between thiazole and two water molecules, thi⋯(H2O)2, have been identified through Fourier transform microwave spectroscopy between 7.0 and 18.5GHz. The complex was generated by the co-expansion of a gas sample containing trace amounts of thiazole and water in an inert buffer gas. For each isomer, rotational constants, A0, B0, and C0; centrifugal distortion constants, DJ, DJK, d1, and d2; and nuclear quadrupole coupling constants, χaa(N) and [χbb(N) - χcc(N)], have been determined through fitting of a rotational Hamiltonian to the frequencies of observed transitions. The molecular geometry, energy, and components of the dipole moment of each isomer have been calculated using Density Functional Theory (DFT). The experimental results for four isotopologues of isomer I allow for accurate determinations of atomic coordinates of oxygen atoms by r0 and rs methods. Isomer II has been assigned as the carrier of an observed spectrum on the basis of very good agreement between DFT-calculated results and a set of spectroscopic parameters (including A0, B0, and C0 rotational constants) determined by fitting to measured transition frequencies. Non-covalent interaction and natural bond orbital analyses reveal that two strong hydrogen bonding interactions are present within each of the identified isomers of thi⋯(H2O)2. The first of these binds H2O to the nitrogen of thiazole (OH⋯N), and the second binds the two water molecules (OH⋯O). A third, weaker interaction binds the H2O sub-unit to the hydrogen atom that is attached to C2 (for isomer I) or C4 (for isomer II) of the thiazole ring (CH⋯O).

Revealing Internal Rotation and 14N Nuclear Quadrupole Coupling in the Atmospheric Pollutant 4-Methyl-2-nitrophenol: Interplay of Microwave Spectroscopy and Quantum Chemical Calculations.

The structure and interactions of oxygenated aromatic molecules are of atmospheric interest due to their toxicity and as precursors of aerosols. Here, we present the analysis of 4-methyl-2-nitrophenol (4MNP) using chirped pulse and Fabry-Pérot Fourier transform microwave spectroscopy in combination with quantum chemical calculations. The rotational, centrifugal distortion, and 14N nuclear quadrupole coupling constants of the lowest-energy conformer of 4MNP were determined as well as the barrier to methyl internal rotation. The latter has a value of 106.4456(8) cm-1, significantly larger than those from related molecules with only one hydroxyl or nitro substituent in the same para or meta positions, respectively, as 4MNP. Our results serve as a basis to understand the interactions of 4MNP with atmospheric molecules and the influence of the electronic environment on methyl internal rotation barrier heights.

Millimeter-Wave and High-Resolution Infrared Spectroscopy of 2-Furonitrile─A Highly Polar Substituted Furan

The rotational spectrum of 2-furonitrile (2-cyanofuran) has been obtained from 140 to 750 GHz, capturing its most intense rotational transitions at ambient temperature. 2-Furonitrile is one of two isomeric cyano-substituted furan derivatives, both of which possess a substantial dipole moment due to the cyano group. The large dipole of 2-furonitrile allowed over 10 000 rotational transitions of its ground vibrational state to be observed and least-squares fit to partial octic, A- and S-reduced Hamiltonians with low statistical uncertainty (σfit = 40 kHz). The high-resolution infrared spectrum, obtained at the Canadian Light Source, allowed for accurate and precise determination of the band origins of its three lowest-energy fundamental modes (ν24, ν17, and ν23). Similar to other cyanoarenes, the first two fundamental modes (ν24, A″, and ν17, A', for 2-furonitrile) form an a- and b-axis Coriolis-coupled dyad. More than 7000 transitions from each of these fundamental states were fit to an octic A-reduced Hamiltonian (σfit = 48 kHz), and the combined spectroscopic analysis determines fundamental energies of 160.1645522 (26) cm-1 and 171.9436561 (25) cm-1 for ν24 and ν17, respectively. The least-squares fitting of this Coriolis-coupled dyad required 11 coupling terms, Ga, GaJ, GaK, GaJJ, GaKK, Fbc, FbcJ, FbcK, Gb, GbJ, and FacK. Using both the rotational and high-resolution infrared spectra, a preliminary least-squares fit was obtained for ν23, providing its band origin of 456.7912716 (57) cm-1. The transition frequencies and spectroscopic constants provided in this work, when combined with theoretical or experimental nuclear quadrupole coupling constants, will provide the foundation for future radioastronomical searches for 2-furonitrile across the frequency range of currently available radiotelescopes.

The conformational behavior of N‑ethylformamide as observed by rotational spectroscopy and quantum chemistry

A peptide linkage (CO)NH containing molecule, N-ethylformamide, was investigated by rotational spectroscopy in order to determine the molecular constants of its highest-energy conformer, cis-ac. Its rotational spectrum was observed in two different frequency ranges, in the 4–26 GHz frequency region using a Fourier transform microwave spectrometer and at millimeter wave frequencies between 75 and 116 GHz, employing a broadband high-resolution rotational spectrometer. The measurements at low frequencies allowed to resolve the hyperfine structure components due to nitrogen nuclear quadrupole coupling while the data at higher frequencies provided spectroscopic information about high order centrifugal effects. From a merged fit using all the observational data we have determined a total of thirteen molecular constants that provide a more accurate spectral modelling of the cis-ac conformer and serves a basis for their astronomical search. We have also observed spectra of five singly substituted isotopologues for the cis-ac conformer, three 13C and one for each of 15N and the deuterated species on the N–D position, from which we derived a partial r0 structure, in fair agreement with an ab initio result. In addition, the rotational transitions of the deuterated species of the most stable trans-sc conformer were observed and assigned and three rotational, five centrifugal distortion constants and nuclear quadrupole coupling constants of the nitrogen and deuterium nuclei were determined.

Laboratory spectroscopy of allylimine and tentative detection towards the G+0.693-0.027 molecular cloud

Context. Substituted methanimines and ethylenes have been identified in the interstellar medium. Therefore, allylimine (CH2 =CH–CH=NH) represents a promising candidate for a new interstellar detection. Aims. The goal of the present work is to perform a comprehensive laboratory investigation of the rotational spectrum of allylimine in its ground vibrational state in order to obtain a highly precise set of rest frequencies to assist its search for astronomical sources. Methods. The rotational spectra of the two most stable trans–anti and trans–syn geometrical isomers of allylimine were recorded in the laboratory in the 84–300 GHz frequency interval. Measurements were performed using a source-modulation millimetre-wave spectrometer equipped with a pyrolysis system for the production of unstable species. High-level ab initio calculations were performed to assist the analysis and to obtain reliable estimates for an extended set of spectroscopic parameters. Guided by new laboratory data, allylimine was searched for in space using a sensitive spectral survey of the G+0.693-0.027 molecular cloud, located at the Galactic centre. Results. Almost 1000 rotational transitions have been recorded for trans–anti and trans–syn allylimine. These new data have enabled the determination of a very accurate set of spectroscopic parameters including rotational, quartic and sextic centrifugal distortion con-stants, as well as nuclear quadrupole coupling constants. The improved spectral data allowed us to report a tentative detection for both allylimine isomers in the G+0.693-0.027 molecular cloud, located at the Galactic centre.

Structure and Spectroscopic Signatures of Interstellar Sodium Isocyanate Isomers

The investigation of metal-containing interstellar molecules stands as a prolific field for current astrochemical research. However, the search for many of these systems in the interstellar medium has remained inaccessible to date due to the lack of preliminary spectroscopic data. In this context, pioneering theoretical studies have inspired quantum chemists to study new appealing candidates to enable their subsequent search in space. The aim of this study is to provide high-level theoretical spectroscopic signatures of the tetratomic system [Na, N, C, O]. We have performed a thorough exploration of its potential energy surface employing different state-of-the-art quantum chemical methods and nine different species have been characterized. Moreover, we have evaluated the stability of the most stable isomers against dissociation and explored their main isomerization processes. We therefore suggest sodium isocyanate (NaNCO,1Σ) and sodium cyanate, (NaOCN, 1Σ) as the most relevant candidates for laboratory and interstellar detection. To aid in their eventual spectral search by means of rotational spectroscopy, we report a complete set of the required spectroscopic parameters including the nuclear quadrupole coupling constants, which are needed to interpret their complex hyperfine structure. NaNCO and NaOCN present exceptionally high values of the electric dipole moment (11.4 and 13.6 Debyes, respectively at the CCSD(T,rw)/aug-cc-pVTZ level), which strongly support to perform an eventual radio astronomical search. Furthermore, both isomers exhibit rather small vibrational frequencies, which indicates that these species are certainly floppy molecules.

How Water Interacts with the NOH Group: The Rotational Spectrum of the 1:1 N,N-diethylhydroxylamine·Water Complex

The rotational spectrum of the 1:1 N,N-diethylhydroxylamine-water complex has been investigated using pulsed jet Fourier transform microwave spectroscopy in the 6.5-18.5 GHz frequency region. The most stable conformer has been detected as well as the 13C monosubstituted isotopologues in natural abundance and the 18O enriched water species, allowing to determine the nitrogen nuclear quadrupole coupling constants and the molecular structure in the vibrational ground state. The molecule has a Cs symmetry and the water lies in the bc symmetry plane forming two hydrogen bonds with the NOH frame with length: dHOH·NOH = 1.974 Å and dH2O·HON = 2.096 Å. From symmetry-adapted perturbation theory calculations coupled to atoms in molecule approach, the corresponding interaction energy values are estimated to be 24 and 13 kJ·mol-1, respectively. The great strength of the intermolecular interaction involving the nitrogen atom is in agreement with the high reactivity of hydroxylamine compounds at the nitrogen site.