Nuclear Quadrupole Coupling Constants Research Articles

Rovibrational spectroscopic constants and anharmonic force fields of CH3AsH2 and CH2AsH3: An ab inito study

The spectroscopic constants and anharmonic force fields of Methylarsine and its isomer are systematically studied by B3LYP, CAM-B3LYP, M06-2X methods utilizing the cc-PVTZ and cc-PVQZ basis sets. The calculated equilibrium geometries, fundamental frequencies, and nuclear quadrupole coupling constants, et al of CH3AsH2 are supported by experimental or theoretical data. The harmonic frequencies, anharmonic constants, et al of CH3AsH2 are predicted for the first time. The anharmonic force fields and spectroscopic constants of CH2AsH3 are also investigated with the same theoretical level. The IR spectra of CH3AsH2 and CH2AsH3 are predicted theoretically, and the red-shifts are found in their peaks.

Laboratory Detection of Cyanoacetic Acid: A Jet-Cooled Rotational Study.

Herein we present a laboratory rotational study of cyanoacetic acid (CH2(CN)C(O)OH), an organic acid as well as a -CN bearing molecule, that is a candidate molecular system to be detected in the interstellar medium (ISM). Our investigation aims to provide direct experimental frequencies of cyanoacetic acid to guide its eventual astronomical search in low-frequency surveys. Using different jet-cooled rotational spectroscopic techniques in the time domain, we have determined a precise set of the relevant rotational spectroscopic constants, including the 14N nuclear quadrupole coupling constants for the two distinct structures, cis- and gauche- cyanoacetic acid. We believe this work will potentially allow the detection of cyanoacetic acid in the interstellar medium, whose rotational features have remained unknown until now.

Proton Transfer in a Bare Superacid-Amine Complex: A Microwave and Computational Study of Trimethylammonium Triflate.

The complex formed from trimethylamine ((CH3)3N) and trifluoromethanesulfonic acid (triflic acid, CF3SO3H) has been observed by Fourier transform microwave spectroscopy in a supersonic jet. Spectroscopic data, most notably 14N nuclear quadrupole coupling constants, are combined with computational results at several levels of theory to unambiguously demonstrate complete or near-complete proton transfer from the triflic acid to the trimethylamine upon complexation. Thus, the system is best regarded as a trimethylammonium triflate ion pair in the gas phase. The formation of an isolated ion pair in a 1:1 complex of a Brønsted acid and base is unusual and likely arises due to the strong acidity of triflic acid. Simple energetic arguments based on proton affinities and the Coulomb interaction energy can be used to rationalize this result.

Facilitating Interstellar Searches for Simple Amino Alcohols with Accurate Rest Frequencies into the Millimeter-wave Regime: Alaninol, Valinol, and Leucinol

Abstract The detection of larger complex organic molecules, such as molecules consisting of several functional groups or those which show conformational flexibility, in the interstellar medium could lead to insights into the availability of biomolecules in space. We present the rotational spectroscopic study of three amino alcohols: alaninol, valinol, and leucinol. The spectra were recorded over the 2–110 GHz region, which included the utilization of a newly developed instrument operating between 18–26 GHz. We report accurately determined line lists, rotational constants, centrifugal distortion constants, and nuclear quadrupole coupling constants for two conformers of alaninol, four conformers of valinol, and three conformers of leucinol, as well as for several singly substituted heavy-atom isotopologues, which also provide structural insights. Further, a number of spectra of vibrationally excited states were assigned, and the associated motions were revealed with anharmonic frequency calculations. Accurate predictions of rotational transitions into the millimeter-wave regime for all species were made, facilitating searches for these complex molecules by observational facilities such as ALMA. Their detection would establish a new family of interstellar molecules.

Distance measurements to quadrupolar nuclei: Evolution of the rotational echo double resonance technique.

Molecular structure determination is the basis for understanding chemical processes and the property of materials. The direct dependence of the magnetic dipolar interaction on the distance makes solid-state nuclear magnetic resonance (NMR) an excellent tool to study molecular structure when X-ray crystallography fails to provide atomic-resolution data. Although techniques to measure distances between pairs of isolated nuclear spin-1/2 pairs are routine and easy to implement using the rotational echo double resonance (REDOR) experiment (Gullion & Schaefer, 1989), the existence of a nucleus with a spin > 1/2, appearing in approximately 75% of the elements in the periodic table, poses a challenge due to difficulties stemming from the large nuclear quadrupolar coupling constant (QCC). This mini-review presents the existing solid-state magic-angle spinning NMR techniques aimed toward the efficient and accurate determination of internuclear distances between a spin-1/2 and a "quadrupolar" nucleus having a spin larger than one half. Analytical expressions are provided for the various recoupling curves stemming from different techniques, and a coherent nomenclature for these various techniques is suggested. Treatment of some special cases such as multiple spin effects and spins with close Larmor frequencies is also discussed. The most advanced methods can recouple spins with quadrupolar frequencies up to tens of megahertz and beyond, expanding the distance measurement capabilities of solid-state NMR to an increasingly growing number of applications and nuclear spin systems.

Microwave spectrum and iodine nuclear quadrupole coupling constants of 1,1-diiodoethane

The high resolution rotational spectroscopic observation of 1,1-diiodoethane is investigated using a pulsed jet, cavity Fourier transform microwave (FTMW) spectrometer over the frequency range 11.5–18 GHz for the first time. The rotational constants, the centrifugal distortion constants, the nuclear spin-rotation coupling constants, and the complete tensor components of the nuclear quadrupole coupling for both iodine nuclei have been determined and reported. The fitted rotational constants are A = 4548.320446(47), B = 625.629141(55), C = 558.798939(43) MHz and the nuclear quadrupole coupling constants are χaa = −1089.8125(7), χbb − χcc = −542.3162(13), |χab| = 1215.7505(10), χbc = 340.8983(14), and |χac| = 562.4206(19) MHz. No A-E splittings due to the methyl group internal rotation were observed. Many dipole-forbidden/electric quadrupole coupling allowed transitions were observed in the spectrum due to the large iodine quadrupole coupling effect. Quantum chemical calculations were performed at the CCSD(T)/aug-cc-pVTZ-pp level of theory. The calculated rotational constants, centrifugal distortion constants, and hyperfine constants were used to guide the data analysis.

σ-Hole activation and structural changes upon perfluorination of aryl halides: direct evidence from gas phase rotational spectroscopy.

Enhancement of the σ-hole on the halogen atom of aryl halides due to perfluorination of the ring is demonstrated by use of the Extended Townes-Dailey (ETD) model coupled to a Natural Atomic Orbital Bond analysis on two perfluorinated aryl halides (C6F5Cl and C6F5Br) and their hydrogenated counterparts. The ETD analysis, which quantifies the halogen p-orbitals populations, relies on the nuclear quadrupole coupling constants which in this work are accurately determined experimentally from the rotational spectra. The rotational spectra investigated by Fourier-transform microwave spectroscopy performed in supersonic expansion are reported for the parent species of C6F5Cl and C6F5Br and their 13C, 37Cl or 81Br substituted isotopologues observed in natural abundance. The experimentally determined rotational constants combined with theoretical data at the MP2/aug-cc-pVTZ level provide precise structural information from which an elongation of the ring along its symmetry axis due to perfluorination is proved.

Internal rotation and chlorine nuclear quadrupole coupling in 2-chloro-4-fluorotoluene explored by microwave spectroscopy and quantum chemistry.

2-Chloro-4-fluorotoluene was investigated using a combination of molecular jet Fourier transform microwave spectroscopy in the frequency range from 5 to 21GHz and quantum chemistry. The molecule experiences an internal rotation of the methyl group, which causes fine splittings of all rotational transitions into doublets with separation on the order of a few tens of kHz. In addition, hyperfine effects originating from the chlorine nuclear quadrupole moment coupling its nuclear spin to the end-over-end rotation of the molecule are observed. The torsional barrier was derived using both the rho and the combined-axis-method, giving a value of 462.5(41) cm-1. Accurate rotational constants and quadrupole coupling constants were determined for the 35Cl and 37Cl isotopologues and compared with Bailey's semi-experimental quantum chemical predictions. The gas phase molecular structure was deduced from the experimental rotational constants supplemented with those calculated by quantum chemistry at various levels of theory. The values of the methyl torsional barrier and chlorine nuclear quadrupole coupling constants were compared with the theoretical predictions and with those of other chlorotoluene derivatives.

A study of the conformational isomerism of n-propyl nitrate by microwave spectroscopy

The rotational spectrum of n-propyl nitrate was measured in the frequency range between 6 and 18 GHz using a Balle-Flygare Fourier transform microwave jet/cavity spectrometer. Parent, 13C and 15N isotopologue transitions for the lower-energy anti-gauche (AG) conformer were found using this instrument. The search for spectra from other conformers was performed using a broadband chirped-pulse jet spectrometer. Transitions from the anti-anti (AA) conformer were observed in this manner. Parent, 13C and 15N isotopologue transitions for the AA conformer were rescanned using the cavity instrument to improve resolution and the signal-to-noise ratio. Rotational, centrifugal distortion, and nuclear electric quadrupole coupling constants for all conformers/isotopologues were fit using Pickett’s SPFIT program. The structure of n-propyl nitrate is discussed considering these results.

Quasi-relativistic study of nuclear electric quadrupole coupling constants in chiral molecules containing heavy elements

A quasi-relativistic (two-component) zeroth-order regular approximation model potential approach is employed to calculate nuclear electric quadrupole coupling tensors of the chiral polyhalomethane derivatives CHBrClF and CHClFI. Each of these two chiral compounds contains two nuclei with electric quadrupole moments, which poses special challenges for an analysis of high-resolution rovibrational spectra and, thus, renders predictions of electric field gradients and resulting nuclear electric quadrupole coupling tensors in such molecules particularly valuable. The theoretical approach is studied with respect to the influence of relativistic effects, by comparison to scalar-relativistic and non-relativistic calculations, and electron correlation effects, which are included within different flavours of density functional theory. The quality of the calculations is assessed by comparison to results from experiments reported previously by Bauder et al. and Soulard et al., respectively. It is shown that the local density approximation exchange-correlation functional gives results that agree with experiment within 12 % for all nuclear quadrupole coupling tensor components of 79Br and 127I. This benchmark test of the quasi-relativistic approach indicates its applicability to support upcoming microwave experiments with chiral heavy-element containing main group compounds.

Microwave spectrum and structure of the SO3−SO2 weakly bound complex

The microwave spectrum and structure of the weakly bound complex SO3–SO2 are reported. The parent species, as well as the isotopologues with 34S and 33S single substitution on each sulfur atom have been observed. Together with the measured 33S nuclear quadrupole coupling constants, the structure of the complex has been determined. MP2/6-311++G(3df) calculations identify two nearly isoenergetic conformations, one of which corresponds to the observed structure. In the observed complex, an oxygen atom of the SO2 approaches the sulfur of the SO3 with an intermolecular S⋯O distance of 2.635(28) Å. The experimental S⋯S distance is 3.7517(83) Å, and the SO2 is positioned so as to direct an oxygen lone electron pair roughly along the C3 axis of the SO3. This structure reflects the strong acidity of SO3 insofar as it is suggestive of incipient electron pair donation from the SO2 oxygen to the acidic site of the SO3.

Microwave Spectra and Theoretical Calculations for Two Structural Isomers of Methylmanganese Pentacarbonyl.

The first microwave rotational spectra for two structural isomers of methylmanganese pentacarbonyl were measured in the 4-9 GHz range using a pulsed-beam Fourier transform microwave spectrometer. The spectra for the two isomers, a symmetric-top structure and an asymmetric-top acyl isomeric structure, were fit to obtain rotational and centrifugal distortion constants and 55Mn quadrupole coupling parameters. The rotational constants, the manganese (55Mn) nuclear quadrupole coupling constant, the centrifugal distortion constants, and the spin-rotation constant were determined for the symmetric CH3Mn(CO)5 and have the following values: A = B = 793.153(3) MHz, DJ = 0.00040(4) MHz, DJK = 0.0018(2) MHz, Ccc = 0.183(6) MHz, and eQqcc= -87.4(3) MHz. Rotational constants and 55Mn quadruple coupling constants were determined for the isomeric acyl-CH3C(O)Mn(CO)4 and have the following values: A = 839.96(4) MHz, B = 774.20(7) MHz, C = 625.63(1) MHz, and 1.5 eQqaa= 44.9(47) MHz and 0.25(eQqbb - eQqcc) = 11.9(12) MHz. The measured rotational constants from the isomeric acyl-CH3C(O)Mn(CO)4 were compared with various theoretical computations. The calculated rotational constants for the dihapto-acyl and the agostic-acyl structures are reasonably close to the experimental values. We note that the calculated dihapto-acyl structure most closely matches the experimental data, as the calculation for the dihapto structure using the B3LYP functional with the aug-cc-pVDZ basis set closely reproduced the experimental values for A, B, and C.

Rotational spectroscopy and astronomical search for glutaronitrile★.

Nitriles constitute almost 15% of the molecules observed in the interstellar medium (ISM), surprisingly only two dinitriles have been detected in the ISM so far. The lack of astronomical detections for dinitriles may be partly explained by the absence of laboratory rotational spectroscopic data. Our goal is to investigate the rotational spectrum of glutaronitrile, N≡C-CH2-CH2-CH2-C≡N, in order to allow its possible detection in the ISM. The rotational spectrum of glutaronitrile was measured using two different experimental setups. A Fourier transform microwave spectrometer was employed to observe the supersonic jet rotational spectrum of glutaronitrile between 6 and 20 GHz. In addition, the mmW spectrum was observed in the frequency range 72-116.5 GHz using a broadband millimetre-wave spectrometer based on radio astronomy receivers with fast Fourier transform backends. The spectral searches were supported by high-level ab initio calculations. A total of 111 rotational transitions with maximum values of J and Ka quantum numbers 54 and 18, respectively, were measured for the gg conformer of glutaronitrile. The analysis allowed us to accurately determine the rotational, nuclear quadrupole coupling, quartic and sextic centrifugal distortion constants. These rotational parameters were employed to search for glutaronitrile in the cold and warm molecular clouds Orion KL, Sgr B2(N), B1-b and TMC-1, using the spectral surveys captured by IRAM 30m at 3mm. Glutaronitrile was not detected, and the upper limits' column densities were derived. Those are a factor of 1.5 and 5 lower than those obtained for the total column densities of the analogous succinonitrile in Orion KL and Sgr B2, respectively.

Bifunctional Hydrogen Bonding of Imidazole with Water Explored by Rotational Spectroscopy and DFT Calculations

Laser vaporization of imidazole in the presence of an argon buffer gas has allowed the generation and isolation of two isomers of an imidazole monohydrate complex, denoted herein as imid···H2O and H2O···imid, within a gas sample undergoing supersonic expansion. Imidazole and water are respectively proton-accepting and proton-donating in imid···H2O, but these roles are reversed in the H2O···imid complex. Both isomers have been characterized by chirped-pulse Fourier transform microwave spectroscopy between 7.0 and 18.5 GHz. The ground-state rotational spectra of four isotopologues of imid···H2O and three isotopologues of H2O···imid have been measured. All spectra have been assigned and fitted to determine rotational (A0, B0, C0), centrifugal distortion (DJ, DJK), and nuclear quadrupole coupling constants (χaa(N1), [χbb(N1) - χcc(N1)], χaa(N3), and [χbb(N3) - χcc(N3)]). Structural parameters (r0 and rs) have been accurately determined from measured rotational constants for each isomer. The imid···H2O complex contains a nonlinear hydrogen bond (∠(O-Hb···N3) = 172.1(26)° in the experimentally determined, r0 geometry) between the pyridinic nitrogen of imidazole and a hydrogen atom of H2O. The DFT calculations find that the H2O···imid complex also contains a nonlinear hydrogen bond between the oxygen atom of water and the hydrogen attached to the pyrrolic nitrogen of imidazole (∠(O···H1-N1) = 174.7°). Two states observed in the spectrum of H2O···imid, assigned as 0- and 0+ states, confirm that large amplitude motions occur on the time scale of the molecular rotation. Density functional theory has been performed to characterize these large amplitude motions.

Gas-Phase Conformational Map of the Amino Acid Isovaline.

Four conformers of the non-proteinogenic α-amino acid isovaline, vaporized by laser ablation, are characterized by Fourier-transform microwave techniques in a supersonic expansion. The comparison between the experimental rotational and 14 N nuclear quadrupole coupling constants and the ab initio calculated ones provides conclusive evidence for the identification of the conformers. The most stable species is stabilized by an N-H⋅⋅⋅O =C intramolecular hydrogen bond and a cis-COOH interaction, whereas the higher-energy conformers exhibit an N⋅⋅⋅H-O intramolecular hydrogen bond and trans-COOH, as in other aliphatic amino acids. The spectroscopic data herein reported can be used for the astrophysical purpose in a possible detection of isovaline in space.

Microwave spectrum and structure of 2-(trifluoromethyl)pyridine

The high resolution rotational spectrum of 2-(trifluoromethyl)pyridine in 2 20 GHz was recorded and analyzed. Spectroscopic parameters including rotational constants, nuclear quadrupole coupling constants of 14N as well as the centrifugal distortion constants were determined. The rotational spectra of five mono-substituted 13C and one 15N isotopologues were also measured and assigned in natural abundance. Experimental results complemented by ab initio calculations lead to an accurate determination of the skeleton structure. The values of the planar moment inertia Pcc were determined to be 44.46 uÅ2 for all the measured isotopologues, indicating a Cs symmetry of this molecule. The molecular electrostatic surface potential was calculated to illustrate the trifluoromethyl substitution effects on the electron distribution.

Relativistic Effects on NMR Parameters of Halogen-Bonded Complexes.

Relativistic effects are found to be important for the estimation of NMR parameters in halogen-bonded complexes, mainly when they involve the heavier elements, iodine and astatine. A detailed study of 60 binary complexes formed between dihalogen molecules (XY with X, Y = F, Cl, Br, I and At) and four Lewis bases (NH3, H2O, PH3 and SH2) was carried out at the MP2/aug-cc-pVTZ/aug-cc-pVTZ-PP computational level to show the extent of these effects. The NMR parameters (shielding and nuclear quadrupolar coupling constants) were computed using the relativistic Hamiltonian ZORA and compared to the values obtained with a non-relativistic Hamiltonian. The results show a mixture of the importance of the relativistic corrections as both the size of the halogen atom and the proximity of this atom to the basic site of the Lewis base increase.

Accurate nuclear quadrupole moment of ruthenium from the molecular method.

The nuclear electric quadrupole moment (NQM) of ruthenium was reevaluated by means of the molecular method. Therefore, four-component relativistic electronic structure calculations of the electric field gradient at the Ru nucleus in ruthenium monocarbide, which were done with the coupled cluster methodology and carefully augmented large basis sets, were combined with the respective nuclear quadrupole coupling constant from experiments. This provided a recommended NQM value of (425 ± 13) mbarn for 101Ru.

Barriers to internal rotation in methylimidazole isomers determined by rotational spectroscopy.

The rotational spectra of N-, 2-, 4-, and 5-methylimidazole are reported and analyzed. Liquid N-methylimidazole was vaporized from a reservoir, and each of 2-, 4-, and 5-methylimidazole was laser-vaporized from a solid target prior to mixing with argon buffer gas and undergoing supersonic expansion from a pulsed nozzle. The spectra were recorded by chirped-pulse Fourier transform microwave spectroscopy in the 7.0-18.5 GHz frequency range. Rotational constants, A0, B0, and C0, centrifugal distortion constants, DJ, DJK, DK, d1, and d2, and nuclear quadrupole coupling constants of nitrogen atoms, χaa(N1), χbb(N1) - χcc(N1), χaa(N3), and χbb(N3) - χcc(N3), are determined from experimentally measured transition frequencies. Data recorded for isotopologues containing 13C or 15N are used to determine the rs coordinates of all heavy atoms in N-, 2-, and 4-methylimidazole. The results allow fitting of parameters in the Hamiltonian that describes internal rotation of the CH3 group about its local C3 axis. The V3 terms in the periodic potential energy functions that describe the internal rotation in N-, 2-, 4-, and 5-methylimidazole are determined to be 185.104(11), 122.7529(38), 317.20(14), and 386.001(19) cm-1, respectively, by the internal axis method. The experiments are supported by density functional theory calculations. Observed variations in barrier height are explained with reference to the symmetry of overlap between a π-like orbital on the CH3 group and π-orbitals on the aromatic ring.

14N Nuclear quadrupole coupling and methyl internal rotation in the microwave spectrum of 2-methylpyrrole

Using two molecular jet Fourier transform microwave spectrometers, the rotational spectrum of 2-methylpyrrole was recorded in the frequency range from 2 to 40 GHz. From the torsional splittings due to the internal rotation of the methyl group a barrier height of 279.7183(26) cm−1 was deduced. Because of the 14N nucleus, all lines show a quadrupole hyperfine structure. The microwave spectra were analysed using the XIAM and BELGI-Cs-hyperfine codes. The XIAM code enabled us to reproduce the whole data set with a root-mean-square deviation of 5.6 kHz while the BELGI-Cs-hyperfine code could provide a better root-mean-square almost by a factor of 2 compared to that of XIAM. The experimental results were complemented by quantum chemical calculations. The values of the methyl torsional barrier and the 14N nuclear quadrupole coupling constants are discussed and compared with other methyl substituted pyrroles as well as other aromatic five-membered rings.