Nuclear Quadrupole Coupling Parameters Research Articles

Molecular Rotation Spectrum of Tetracyclic Quinolizidines: Observation of trans-Matrine and the Elusive cis-Matrine.

We characterized the bis-quinolizidine tetracyclic alkaloid (5S, 6S, 7R, 11R)-matrine in a supersonic jet expansion, using chirped-pulsed broadband microwave spectroscopy. Previous crystal diffraction analyses suggested 16 diastereoisomers associated with matrine's four carbon stereocenters but were inconclusive whether the lactamic nitrogen atom would additionally produce separated trans-/cis- diastereoisomers or if both species may interconvert through low potential barriers. Our experiment simultaneously detected trans- and cis-matrine through their rotational spectrum, confirming the possibility of conformational rearrangement in matrine alkaloids. The two matrine conformers mainly differ in the envelope or half-chair lactamic ring, as evidenced by the experimental rotational and nuclear quadrupole coupling parameters. Molecular orbital calculations with ab initio (MP2) and density functional methods (B3LYP-D3(BJ) and MN15) were tested against the experiment, additionally offering an estimation of the cis-/trans- barrier of 24.9-26.9 kJ mol-1. The experiment illustrates the structural potential of chirped-pulsed broadband microwave spectroscopy for high-resolution rotational studies of biomolecules in the range of 20-40 atoms.

Determination of accurate rest frequencies and hyperfine structure parameters of cyanobutadiyne, HC5N

Very accurate transition frequencies of HC5N were determined between 5.3 and 21.4 GHz with a Fourier transform microwave spectrometer. The molecules were generated by passing a mixture of HC3N and C2H2 highly diluted in neon through a discharge valve followed by supersonic expansion into the Fabry-Perot cavity of the spectrometer. The accuracies of the data permitted us to improve the experimental 14N nuclear quadrupole coupling parameter considerably and the first experimental determination of the 14N nuclear spin-rotation parameter. The transition frequencies are also well suited to determine in astronomical observations the local speed of rest velocities in molecular clouds with high fidelity. The same setup was used to study HC7N, albeit with modest improvement of the experimental 14N nuclear quadrupole coupling parameter. Quantum chemical calculations were carried out to determine 14N nuclear quadrupole and spin-rotation coupling parameters of HC5N, HC7N, and related molecules. These calculations included evaluation of vibrational and relativistic corrections to the non-relativistic equilibrium quadrupole coupling parameters; their considerations improved the agreement between calculated and experimental values substantially.

Rotational spectroscopy of the two conformers of 3-methylbutyronitrile (C4H9CN) between 2 and 400 GHz

We present high-resolution rotational spectroscopy of the two conformers of 3-methylbutyronitrile (C4H9CN). Spectra were taken between 2 and 24 GHz by means of Fourier transform microwave spectroscopy. Spectra between 36 and 403 GHz were recorded by means of frequency modulated (FM) absorption spectroscopy. The analysis yields precise rotational constants and higher order distortion constants, as well as a set of 14N nuclear electric quadrupole coupling parameters for each of the two conformers. In addition, quantum chemical calculations were performed in order to assist the assignments. Frequency calculations yield insight into the vibrational energy structure of the two conformers, from which partition functions and vibrational correction factors are determined. These factors are used to determine experimentally and computationally the energy difference between the two conformers, which is revealed to be negligible. Overall, this study provides precise spectroscopic constants for the search of 3-methylbutyronitrile in the interstellar medium. In particular, this molecule is a perfect test case for our knowledge of branched molecule formation in space.

Microwave rotational spectrum and ab initio computations on 4-cyanopyridine: molecular structure and hyperfine interactions

ABSTRACTThe cm-wave rotational spectrum of 4-cyanopyridine was studied on a pulsed supersonic jet-expansion Fourier-transform microwave spectrometer. The ground state rotational parameters for the parent and four 13C and two 15N isotopologues were determined with high accuracy in natural abundance including all quartic centrifugal distortion constants and nuclear quadrupole coupling parameters of the two nitrogen nuclei. With the study of both singly substituted 15N species, the nuclear origins of the hyperfine structures could be differentiated and the respective constants identified unambiguously. Different approaches have been used to determine the structure: empirical (rs), ab initio and semi-experimental () methods. The semi-experimental approach improves the accuracy significantly, whereas the Kraitchman's method fails again.

Rotational Spectroscopy of the Lowest Energy Conformer of 2-Cyanobutane.

Isopropyl cyanide was recently detected in space as the first branched alkyl compound. Its abundance with respect to n-propyl cyanide in the Galactic center source Sagittarius B2(N2) is about 0.4. Astrochemical model calculations suggest that for the heavier homologue butyl cyanide the branched isomers dominate over the unbranched n-butyl cyanide and that 2-cyanobutane is the most abundant isomer. We have studied the rotational spectrum of 2-cyanobutane between 2 and 24 GHz using Fourier transform microwave spectroscopy and between 36 and 402 GHz employing (sub)millimeter absorption spectroscopy. Transitions of the lowest energy conformer were identified easily. Its rotational spectrum is very rich, and the quantum numbers J and Ka reach values of 111 and 73, respectively. This wealth of data yielded rotational and centrifugal distortion parameters up to tenth order, diagonal and one off-diagonal 14N nuclear quadrupole coupling parameters, and one nuclear spin-rotation coupling parameter. We have also carried out quantum chemical calculations in part to facilitate the assignments. The molecule 2-cyanobutane was not found in the present ALMA data of Sagittarius B2(N2), but it may be found in the more sensitive data that have been completed very recently in the ALMA Cycle 4.

55Mn pulsed ENDOR spectroscopy of Mn2+ ions in ZnO thin films and single crystal

55Mn pulsed electron nuclear double resonance (ENDOR) experiments were performed at X-band on high spin S=5/2 Mn2+ ions incorporated at zinc lattice sites in heteroepitaxial ZnO thin films. The films have been prepared by pulsed laser deposition and the manganese ions were doped during the growth process. We examine how the c/a lattice axes ratio of the ZnO films influences the 55Mn hyperfine (hf) and nuclear quadrupole (nq) coupling parameters of the Mn2+ probe ions. The results are compared with those obtained for Mn2+ ions present as impurities in ZnO single crystals and revealed that the 55Mn nq coupling monitors sensitively the structural distortions in the bonding environment of the Mn2+ ions. The experiments provided the full axially symmetric 55Mn hf and nq interaction tensors. The latter is found to be very sensitive to small axial distortions of the MnO4 tetrahedrons. In particular, the 55Mn pulsed ENDOR spectra of the ZnO:Mn thin films are strongly subjected to strain effects in the nq coupling parameter indicating a variation of the local structural parameters for the heteroepitaxial films. In the analysis of the 55Mn pulsed ENDOR spectra the axial and cubic zero field splitting of the Mn2+ ions was taken into account and intensity effects in the ENDOR spectra due to the zero field splitting effects were discussed.

THE CM-, MM-, AND SUB-MM-WAVE SPECTRUM OF ALLYL ISOCYANIDE AND RADIOASTRONOMICAL OBSERVATIONS IN ORION KL AND THE SgrB2 LINE SURVEYS

Organic isocyanides have an interesting astrochemistry and some of these molecules have been detected in the interstellar medium (ISM). However, rotational spectral data for this class of compounds are still scarce. We provide laboratory spectra of the four-carbon allyl isocyanide covering the full microwave region, thus allowing a potential astrophysical identification in the ISM. We assigned the rotational spectrum of the two cis (synperiplanar) and gauche (anticlinal) conformations of allyl isocyanide in the centimeter-wave region (4-18 GHz), resolved its 14N nuclear quadrupole coupling (NQC) hyperfine structure, and extended the measurements into the millimeter and submillimeter-wave (150-900 GHz) ranges for the title compound. Rotational constants for all the monosubstituted 13C and 15N isotopologues are additionally provided. Laboratory observations are supplemented with initial radioastronomical observations. Following analysis of an extensive dataset (>11000 rotational transitions), accurate ground-state molecular parameters are reported for the cis and gauche conformations of the molecule, including rotational constants, NQC parameters, and centrifugal distortion terms up to octic contributions. Molecular parameters have also been obtained for the two first excited states of the cis conformation, with a dataset of more than 3300 lines. The isotopic data allowed determining substitution and effective structures for the title compound. We did not detect allyl isocyanide either in the IRAM 30 m line survey of Orion KL or in the PRIMOS survey toward SgrB2. Nevertheless, we provided an upper limit to its column density in Orion KL.

Rotational Spectra of Bicyclic Decanes: The Trans Conformation of (−)-Lupinine

The conformational and structural properties of the bicyclic quinolizidine alkaloid (-)-lupinine have been investigated in a supersonic jet expansion using microwave spectroscopy. The rotational spectrum is consistent with a single dominant trans conformation within a double-chair skeleton, which is stabilized by more than 10.4 kJ mol(-1) with respect to other conformations. In the isolated conditions of the jet, the hydroxy methyl side chain of the molecule locks in to form an intramolecular O-H···N hydrogen bond to the electron lone pair at the nitrogen atom. Accurate rotational constants, centrifugal distortion corrections, and (14)N nuclear quadrupole coupling parameters are reported and compared to ab initio (MP2) and DFT (M06-2X) calculations. The stability of lupinine is further compared computationally with epilupinine and decaline in order to gauge the influence of intramolecular hydrogen bonding, absent in these molecules.

The Conformational Landscape of Nicotinoids: Solving the Conformational Disparity of Anabasine

The conformational landscape of the alkaloid anabasine (neonicotine) has been investigated by using rotational spectroscopy and ab initio calculations. The results allow a detailed comparison of the structural properties of the prototype piperidinic and pyrrolidinic nicotinoids (anabasine vs. nicotine). Anabasine adopts two most stable conformations in isolation conditions, for which we determined accurate rotational and nuclear quadrupole coupling parameters. The preferred conformations are characterized by an equatorial pyridine moiety and additional N-H equatorial stereochemistry at the piperidine ring (eq-eq; eq=equatorial). The two rings of anabasine are close to a bisecting arrangement, with the observed conformations differing by an approximately 180 degrees rotation of the pyridine subunit, denoted either syn or anti. The preference of anabasine for the eq-eq-syn conformation has been established by relative intensity measurements (syn/anti approximately 5(2)). The conformational preferences of free anabasine are directed by a weak N...H-C hydrogen bond interaction between the nitrogen lone pair at piperidine and the closest C-H bond in pyridine, with N...H distances ranging from 2.686 (syn) to 2.667 A (anti). Supporting ab initio calculations by using MP2 and the recent M05-2X density functional are provided, evaluating the predictive performance of both methods.

Density Functional Calculations of 55Mn, 14N and 13C Electron Paramagnetic Resonance Parameters Support an Energetically Feasible Model System for the S2 State of the Oxygen‐Evolving Complex of Photosystem II

Metal and ligand hyperfine couplings of a previously suggested, energetically feasible Mn(4)Ca model cluster (SG2009(-1)) for the S(2) state of the oxygen-evolving complex (OEC) of photosystem II (PSII) have been studied by broken-symmetry density functional methods and compared with other suggested structural and spectroscopic models. This was carried out explicitly for different spin-coupling patterns of the S=1/2 ground state of the Mn(III)(Mn(IV))(3) cluster. By applying spin-projection techniques and a scaling of the manganese hyperfine couplings, computation of the hyperfine and nuclear quadrupole coupling parameters allows a direct evaluation of the proposed models in comparison with data obtained from the simulation of EPR, ENDOR, and ESEEM spectra. The computation of (55)Mn hyperfine couplings (HFCs) for SG2009(-1) gives excellent agreement with experiment. However, at the current level of spin projection, the (55)Mn HFCs do not appear sufficiently accurate to distinguish between different structural models. Yet, of all the models studied, SG2009(-1) is the only one with the Mn(III) site at the Mn(C) center, which is coordinated by histidine (D1-His332). The computed histidine (14)N HFC anisotropy for SG2009(-1) gives much better agreement with ESEEM data than the other models, in which Mn(C) is an Mn(IV) site, thus supporting the validity of the model. The (13)C HFCs of various carboxylates have been compared with (13)C ENDOR data for PSII preparations with (13)C-labelled alanine.

Spectroscopy and structures of two complete families, one mononuclear, the other binuclear, of 1 : 2 CuX : dptu stoichiometry (X = Cl, Br, I; ‘dptu’ = N,N′-diphenylthiourea)

Crystallization of the copper(i) halides (CuX; X = Cl, Br, I) with N,N'-diphenylthiourea ('dptu' = (PhNH)(2)CS) in 1 : 2 ratio from acetonitrile solution in ambient conditions yields a mononuclear isomorphous series of complexes of the form [XCu(dptu)(2)].H(2)O, an unusually complete array, with the central copper atoms in quasi-trigonal planar environments, as shown by single crystal X-ray studies, in turn providing a platform for comparative spectroscopic studies. Under anhydrous conditions, anhydrous compounds may be obtained, a full series again being accessible, but now of the binuclear form [(dptu)XCu(mu-S-dptu)(2)CuX(dptu)], the chloride being a bis(acetonitrile) solvate, the isomorphous bromide and iodide pair being unsolvated. A (solvated) sulfate salt is found to be of the form [Cu(dptu)(2)](2)(SO(4)), providing a novel example of a copper(i) atom being (quasi-)linearly coordinated by the pair of unidentate (dptu) sulfur ligands, Cu-S 2.1770(5), 2.1811(5) A, S-Cu-S 162.18(2) degrees . The nu(CuX) vibrational frequencies are assigned in the far-IR spectra of [XCu(dptu)(2)].H(2)O (X = Cl, Br, I) at 274, 207, and 190 cm(-1) respectively. The broadline static (65)Cu NMR spectra of [Cl(2)Cu(2)(dptu)(4)].2CH(3)CN and [X(2)Cu(2)(dptu)(4)] (X = Br, I) have been recorded at 9.4 and 7.05 T, and the spectra have been analysed to yield the (65)Cu nuclear quadrupole coupling and chemical shift parameters. The (63,65)Cu nuclear quadrupole resonance frequencies of [XCu(dptu)(2)].H(2)O (X = Cl, Br, I) have also been measured, and the resulting (65)Cu quadrupole coupling constants are ca. 4x those of the dimeric [X(2)Cu(2)(dptu)(4)] compounds.

The hyperfine interaction in the AΠ1∕22 and XΣ+2 states of ytterbium monofluoride

The fine and hyperfine interaction parameters in the A (2)Pi(1/2)(v=0) and X (2)Sigma(+)(v=0) states of the odd metal nuclear spin isotopologues of ytterbium monofluoride, (171)YbF and (173)YbF, have been determined from an analysis of high-resolution laser induced fluorescence spectra of the A (2)Pi(12)<--X (2)Sigma(+)(0,0) band. The observed ground X (2)Sigma(+)(v=0) state (171)Yb(I=1/2) Fermi contact parameter is significantly smaller than that determined from the matrix isolation electron spin resonance measurement [Van Zee et al., J. Phys. Chem. 82, 1192 (1978)]. An interpretation of the (173,171)Yb magnetic hyperfine and nuclear electric quadrupole coupling parameters is given.

Conformations of α‐Aminobutyric Acid in the Gas Phase

A laser-ablation molecular-beam Fourier transform microwave (LA-MB-FTMW) spectrometer has been successfully applied to the structural study of alpha-aminobutyric acid. Three neutral conformers have been identified in the gas phase by comparing their experimental rotational and 14N nuclear quadrupole coupling parameters with those predicted by ab initio calculations at the MP2/6-311++G(d,p) level. The most stable conformer is stabilized by a bifurcated amine-to-carbonyl hydrogen bond (N--HO=C) and a cis-COOH group, and the side-chain adopts a configuration with a torsion angle tau(C(gamma)-C(beta)-C(alpha)-C') of about 180 degrees. The second most stable conformer exhibits the same configuration for the amino acid skeleton but adopts a different orientation for the side chain with tau(C(gamma)-C(beta)-C(alpha)-C') approximately -60 degrees. In the third conformer an intramolecular hydrogen bond is established between the hydroxyl group and the nitrogen atom (NH--O), with a side-chain orientation similar to that of the most stable conformer.

Solid‐state NMR investigation of sodium nucleotide complexes

Solid-state NMR has been used to analyze the chemical environments of sodium sites in powdered crystalline samples of sodium nucleotide complexes. Three of the studied complexes have been previously characterized structurally by crystallography (disodium deoxycytidine-5'-monophosphate heptahydrate, disodium deoxyuridine-5'-monophosphate pentahydrate and disodium adensoine-5'-triphosphate trihydrate). For these salts, the nuclear quadrupole coupling parameters measured by (23)Na multiple-quantum magic-angle-spinning NMR could be readily correlated with sodium ion coordination environments. Furthermore, two complexes that had not been previously characterized structurally, disodium uridine-3'-monophosphate and a disodium uridine-3'-monophosphate/disodium uridine-2'-monophosphate mix, were identified by solid-state NMR. A spectroscopic assignment of the four sites of an additional salt, disodium adensoine-5'-triphosphate trihydrate, is also presented and discussed within the context of creating a general approach for the spectroscopic assignment of multiple sites in sodium nucleotide complexes.

Pulsed ENDOR Study of Cu(I)−NO Adsorption Complexes in Cu−L Zeolite

The local environments of Cu(I)-NO adsorption complexes formed in zeolites Cu-L and Cu-ZSM-5 were studied by electron spin resonance (ESR), pulsed electron nuclear double resonance (ENDOR), and hyperfine sublevel correlation spectroscopy (HYSCORE). Cu(I)-NO complexes have attracted special interest because they are important intermediates in the catalytic decomposition of nitric oxide over copper exchanged zeolites. Recently, detailed structures of the complexes in Cu-ZSM-5 zeolites, O2-Al-O2-Cu(I)-NO, have been proposed on the basis of quantum chemical calculations (Pietrzyk, et al. J. Phys. Chem. B 2003, 107, 6105. Dedecek, et al. Phys. Chem. Chem. Phys. 2002, 4, 5406). 27Al pulsed ENDOR and HYSCORE experiments allowed the hyperfine coupling parameters of an aluminum nuclei found in the vicinity of the Cu(I)-NO complex formed in zeolite Cu-L to be estimated. The data indicate that the aluminum atom is located in the third coordination sphere of the adsorbed NO molecule in agreement with the suggested geometry of the adsorption sites. Broad distributions of aluminum nuclear quadrupole and hyperfine coupling parameters and short electron spin relaxation times of the Cu(I)-NO species prevented the determination of the 27Al hyperfine couplings for zeolite Cu-ZSM-5.

Electronic Structure of the Aqueous Vanadyl Ion Probed by 9 and 94 GHz EPR and Pulsed ENDOR Spectroscopies and Density Functional Theory Calculations.

The aqueous vanadyl ion ([VO(H(2)O)(5)](2+)) has been investigated by X-band EPR, 94 GHz W-band EPR, and ESE-ENDOR. These experiments reveal information about the hyperfine (|A(xx)| = 208.5 MHz, |A(yy)| = 208.5 MHz, |A(zz)| = 547.0 MHz), and nuclear quadrupole coupling (|e(2)qQ| = 5.6 MHz) of the (51)V nucleus. The measured nuclear quadrupole coupling parameters are compared to values determined by density functional theory calculations (|e(2)qQ| = 5.2 MHz). These theoretical calculations illustrate that axial ligands and molecular distortions can alter the magnitude of the nuclear quadrupole interaction.

Atomic charges in RBa2Cu4O8 lattices derived from a comparison of experimental and calculated nuclear-quadrupole coupling parameters

The nuclear-quadrupole coupling parameters at the rare-earth metal and copper sites in the RBa2Cu4O8 lattices (R=Sm, Y, Er) have been determined by 67Cu(67Zn) and 67Ga(67Zn) Mossbauer emission spectroscopy. The crystal electric-field-gradient tensor has been calculated for all sites of the above lattices in the point-charge approximation. A comparison of the experimental with calculated parameters shows that holes in the RBa2Cu4O8 lattices are localized primarily at chain-oxygen sites.

Universality of NMR Results in LISICON Systems and Other Solid Lithium Conductors

Abstract The temperature evolution of the 7Li NMR spectra and relaxation rates in many investigated solid lithium conductors shows more or less the same behavior. These compounds are characterized by a disordered lithium sublattice with a surplus of cation sites in cavities and channels of the anionic network. At low temperature, the spectra consist of a central line and a distributed satellite base structure. Upon increasing temperature, both components narrow until a reduced constant width with a well resolved quadrupole structure is reached. The mean nuclear quadrupole coupling parameters reduce by either about 5 or by a factor of 15 in all the compounds. The spin-lattice relaxation rates 1/T1 are asymmetric as a function of reciprocal temperature and of quadrupolar origin. The activation energy of the main process of ionic motion may best be obtained from the temperature dependence of the dipolar spin-spin-relaxation rate 1/T2 . The spectral densities of the relaxation dependences correspond to those for inhomogeneous motions; they may be described by modification of the BPP equation, a Cole-Davidson distribution or a Kohlrausch-Williams-Watts function. Within this study three LISICON systems, Li4-3x Gax GeO4 , and two phosphates Li3M2 (PO4)3 (M = Sc, In) were investigated or re-investigated which fit well into this scheme. Activation energies of 39-43 kJ/mol (Li4-3x Gax GeO4 with x = 0.06,0.14, 0.24), 53 kJ/mol (Li 3 Sc 2 (PO4)3) and 75 kJ/mol (Li3 In2 (PO4)3) were obtained.

14N electron spin-echo envelope modulation of the S = 3/2 spin system of the Azotobacter vinelandii nitrogenase iron-molybdenum cofactor.

Wild-type nitrogenase MoFe protein shows a deep 14N electron spin-echo envelope modulation (ESEEM) arising from a nitrogen nucleus (N1) coupled to the S = 3/2 spin system of the FeMo-cofactor of the MoFe protein. A previous ESEEM study on altered MoFe proteins generated by substitutions at the alpha-195-histidine position suggested that alpha-195-histidine provides a hydrogen bond to the FeMo-cofactor but is not the source of the 14N1 modulation [DeRose et al. (1995) Biochemistry 34, 2809-2814]. This study also raised the possibility of a correlation between ESEEM spectroscopic properties and the nitrogenase phenotype. We now report ESEEM studies on altered MoFe proteins with substitutions at residues alpha-96-arginine, alpha-359-arginine, and alpha-381-phenylalanine to (i) assign the first-shell hydrogen bonding as revealed by the 14N modulation; (ii) explore the mechanistic relevance of the ESEEM signatures to nitrogenase activity; and (iii) study microscopic changes within the polypeptide environment of the FeMo-cofactor. Present ESEEM data reveals that two kinds of 14N modulations are present in wild-type MoFe protein. A new 2-dimensional procedure for high-precision analysis of the ESEEM data of the MoFe proteins shows that the deep wild-type ESEEM modulation (denoted N1) has a hyperfine-coupling constant of Aiso = 1.05 MHz and nuclear quadrupole coupling parameters of e2qQ = 2.17 MHz, eta = 0.59; the other (denoted N2) has a smaller hyperfine coupling of Aiso = approximately 0.5 MHz and e2qQ = approximately 3.5 MHz, eta = approximately 0.4. The N2 ESEEM pattern is more obvious when unmasked by substitutions that result in the loss of the deep N1 modulation. Correlations of the ESEEM properties and catalytic activities of the altered MoFe proteins suggest that (i) the side chain of the alpha-359-arginine is the source of the deep ESEEM N1 modulation; (ii) one or both of the amide nitrogens of alpha-356-glycine/alpha-357-glycine are responsible for the weak N2 modulation; (iii) substitution of the nonpolar alpha-381-phenylalanine residue, as well as substitution of either the alpha-195-histidine or alpha-359-arginine residues, can eliminate the N1 interaction with FeMo-cofactor; and (iv) ESEEM can be used to detect slight reorientations of FeMo-cofactor within its polypeptide pocket, although the mechanistic relevance of the loss or perturbation of the hydrogen-bonding interactions between FeMo-cofactor and polypeptide environment has not yet been established.

139La Nuclear magnetic resonance characterisation of La 2O 3 and La 1− xSr xMO 3 where M = Cr, Mn or Co

139La Nuclear magnetic resonance (NMR) spectra have been used to obtain nuclear quadrupole coupling parameters for La 2O 3 and a series of perovskites La 1− x Sr x MO 3 (where M = Cr, Mn or Co). Depending on the doping level of SrO 2 these materials are either paramagnetic or ferromagnetic at room temperature. Magnetic transferred hyperfine effects are strongly in evidence in the Mn compound. A 59Co NMR spectrum was observed in LaCoO 3. A precision measurement of the nuclear quadrupole coupling constant in La 2O 3 was made by nuclear quadrupole resonance (NQR) spectroscopy.