Investigations of hyperfine structure and isotope shift predictions in Hf II

The fine structure of the even‐parity low configurations of singly ionized hafnium has been reanalyzed by simultaneous parameterization of the one‐ and two‐body interactions for the model space (5d + 6s)³. Using the calculated eigenfunctions, the magnetic‐dipole A hyperfine constants for the whole 37 existing levels of the model space were predicted and compared partially with those obtained using relativistic configuration‐interaction approach. Moreover, combined with experimental data, previously given in the literature, and pseudo‐relativistic Hartree–Fock estimates, the field shifts (FSs) and specific mass shifts (SMSs) of numerous configurations are deduced: FS(5d¹6s²) = 2265 MHz, FS(5d³) = ?1958 MHz, FS(5d¹6p²) = ?1696 MHz, FS(6s²6p¹) = 3135 MHz, and SMS(5d¹6s²) = ?78 MHz, SMS(5d³) = 64 MHz, SMS(6s²6p¹) = ?228 MHz, referred to 5d²6s for the pair ¹⁷⁸Hf–¹⁸⁰Hf. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

On the hyperfine structure and isotope shift of radium

The hyperfine structure and isotope shift of the heaviest known alkaline earth element radium (Z=88) have been studied in both the atomic Ra I and ionic Ra II spectra. The measurements, carried out by on-line collinear fast-beam laser spectroscopy, yield the hyperfine constantsA andB of the 7s and 7p _1/2 states in Ra II, of all states of the excited 7s 7p configuration and the 7s 7d ³ D ₃ state in Ra I. The data allow a consistent evaluation of the nuclear moments for eight odd-A radium isotopes. In particular, a complete analysis of the hyperfine structure of thesp configuration in the two-electron system provides a stringent test of the validity of the semi-empirical modified Breit-Wills theory. It is shown that the effective operator formalism is equivalent, if relativistic correction functions are used to reduce the number of parameters. The semi-empirical hyperfine fields are evaluated and found to agree generally with those from ab-initio calculations. The isotope shifts in thes?p,s ²?sp,sp?sd transitions are analysed semi-empirically and compared with ab-initio calculations. The consistency of the different analyses proves their validity and classifies the spectrum of Ra I as a model case for a simple and clean two-electron spectrum.     

Difficulties inab initio CI calculations of the hyperfine structure of small radicals

Two of the radicals whose ab initio hyperfine properties exhibited unusually large (30%) deviations from experiment in a recent study of such properties have been reexamined. Inclusion of vibrational averaging effects, MCSCF orbitals and large CI reference spaces resulted in a reduction of the deviation from experiment to 14% for the formaldehyde radical cation. New experimental data on the methoxy radical shows a much larger proton isotropic hyperfine value than previous experimental and theoretical studies. The best current calculations were unable to resolve the discrepancy. The ethyl radical was also included in this study since good experimental beta proton data is available for comparison.     

Finding the needle in a haystack: educing native folds from ambiguous ab initio protein structure predictions

Current ab initio structure‐prediction methods are sometimes able to generate families of folds, one of which is native, but are unable to single out the native one due to imperfections in the folding potentials and an inability to conduct thorough explorations of the conformational space. To address this issue, here we describe a method for the detection of statistically significant folds from a pool of predicted structures. Our approach consists of clustering and averaging the structures into representative fold families. Using a metric derived from the root‐mean‐square distance (RMSD) that is less sensitive to protein size, we determine whether the simulated structures are clustered in relation to a group of random structures. The clustering method searches for cluster centers and iteratively calculates the clusters and their respective centroids. The centroid interresidue distances are adjusted by minimizing a potential constructed from the corresponding average distances of the cluster structures. Application of this method to selected proteins shows that it can detect the best fold family that is closest to native, along with several other misfolded families. We also describe a method to obtain substructures. This is useful when the folding simulation fails to give a total topology prediction but produces common subelements among the structures. We have created a web server that clusters user submitted structures, which can be found at http://bioinformatics.danforthcenter.org/services/scar. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 339–353, 2001     

Long-range interactions in H-He:ab initio potential,hyperfine pressure shift and collision-induced absorption in the infrared

Summary The collisional complex H-He, with both atoms in their electronic ground-states, is treated as a molecule in self-consistent field (SCF) and multi-reference configuration interaction (MR-CI) calculations to determine interaction energy, dipole moment and spin density as function of internuclear separation. A basis set tailored for long-range interactions was used and the basis set super-position errors were controlled. The resulting functions are analyzed and presented in analytical form, in terms of exchange and damped dispersion contributions. For all three properties there is full agreement with the accurately known long-range coefficients, but the dipole moment function shows rather large overlap effects even at large distances which obscure higher-order dispersion coefficients. The well depth of 22.56 µE_h is significantly deeper than most recentab initio calculations and model potentials have suggested, but our number corroborates existing semi-empirical values. Likewise, the calculated spin density variations are more pronounced than recent work has suggested. The resulting hyperfine pressure shift of H atoms in a helium buffer gas is in very good agreement with experiment, except for temperatures of the order of 1 K. Infrared absorption continua associated with the induced dipole moment are evaluated for their astrophysical interest.Dedicated to Prof. W. Kutzelnigg on the occasion of his 60th birthday     

The influence of hyperfine structure and isotope shift on the detection of Rb by 2f-wavelength modulation diode laser absorption spectrometry-experimental verification of simulations

This work presents an experimental verification of a previously developed methodology for simulation of the 2f-wavelength modulation diode laser absorption spectrometry technique (2f-WM-DLAS) when the influence of hyperfine structure, isotope shift and collisional broadening and shift of an atomic transition is taken into account [J. Gustafsson, D. Rojas and O. Axner, Spectrochim. Acta, 52B, 1937–1953 (1997)]. The pilot element in the simulations was Rb, detected at the 780 nm 5s ²S_1/2−5p ²P_3/2 transition, in low-pressure cells and atmospheric-pressure reservoirs (e.g. graphite furnaces). This experimental investigation verifies that the simulations are able to predict, with good accuracy, experimental 2f-WM signals from Rb atoms under both low-pressure, room-temperature conditions and atmospheric-pressure, high-temperature conditions. This implies that the previously published simulation methodology can be used for predicting and optimizing 2f-WM signal strengths and shapes from Rb atoms (and thereby presumably also from other atoms) under a variety of pressure and temperature conditions.     

ChemInform Abstract: Comparison of the Bulk and Surface Properties of Ceria and Zirconia by ab initio Investigations.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.     

Combined application of 2D NMR correlation methods and ab initio chemical shift calculations to the structure determination of new heterocyclic compounds

The combined use of 2D NMR correlation methods and ab initio chemical shift calculations is efficient and, in some cases, virtually the only way to determine the structures of new organic compounds. This approach enabled us to establish the structure of the major unusual product of the three-component reaction of imidazo[1,5-a]quinoxalin-4-one, bis(2-chloroethyl)amine hydrochloride, and potassium carbonate in DMF. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2172–2179, December, 2006. Dedicated to Professor A. V. Il’yasov on the occasion of his 70th birthday.     

An ab initio study of the hyperfine structure in the X2 Pi electronic state of CCCH

The results of an ab initio study of the magnetic hyperfine structure in the X (2)Pi electronic state of CCCH are reported. The potential surfaces for two components of the X (2)Pi electronic state were computed by means of an extensive configuration interaction approach. The electronically averaged hyperfine coupling constants of H and (13)C for (12)C (12)C (12)CH, (13)C (12)C (12)CH, (12)C (13)C (12)CH, and (12)C (12)C (13)CH are obtained as functions of two bending vibrational modes by the density functional theory method. The vibronic wave functions are calculated with the help of a variational approach which takes into account the Renner-Teller effect and spin-orbit coupling. The model Hamiltonian is expressed in terms of the normal bending coordinates. It is found that, due to the generally strong geometry dependence of the hyperfine coupling constants, it is necessary to carry out the vibronic averaging of the corresponding functions in order to obtain the values which can be compared to the results of the measurements. The results of the present study help to reliably interpret the experimental data previously published. They also predict the yet unobserved hyperfine structure in excited vibronic states.     

Theoretical calculations of hypersurfaces of the C chemical shift anisotropy in the COHN hydrogen bond and the benefit for the ab initio structure determination

We investigated the influence of structural changes on the anisotropic part of the carbonyl ¹³C chemical shift tensor in a model complex containing hydrogen bonded cyanuric acid and pyrrole. The model was chosen for its chemical resemblance to cyameluric acid. In the solid state this compound comprises three different hydrogen bonds which are well distinguishable based on the anisotropy parameters δ_aniso and η of the carbonyl ¹³C atoms. The variation of six relevant structural variables in the model system produced hypersurfaces for the isotropic shift, δ_aniso and η. Our goal was to investigate whether such surfaces can be used for the ab initio structure determination of hydrogen bonds. With a medium size basis set it could be shown that although the absolute values differ DFT describes the relative change in δ_aniso and η close to the quality of MP2 calculations. Due to the high dimensionality of the hypersurface we had to reduce the number of variables in our study. We systematically created subsurfaces each described by three of the six variables and investigated their isolated influence on the NMR observables. We identified the most important structure parameters and on this base built a minimal model. For a fixed NO distance the hydrogen bond arrangement was altered by two angular variations and one dihedral distortion. In this model evidence was found that the η surfaces for different NO distances exhibit a uniform shape and can be transformed into one another by a simple shift and multiplication by a mean factor. Furthermore, the experimental parameters δ_aniso and η of cyameluric acid were taken as a base for the extraction of structures from the hypersurfaces. δ_aniso and η unequivocally selected ensembles of similar structure and the COHN arrangement in two of the three cyameluric hydrogen bonds could be predicted with good quality from the theoretical model. Our results show that it is possible to predict the distance and at least qualitatively the orientation in a hydrogen bond environment from an analysis of the anisotropic part of the ¹³C chemical shift tensor.     

Changes in electronic,magnetic and bonding properties from Zr2FeH5 to Zr3FeH7 addressed from ab initio

Potential hydrogen storage ternaries Zr₃FeH₇ and Zr₂FeH₅, are studied from ab initio with the purpose of identifying changes in electronic structures and bonding properties. Cohesive energy trends: E_coh. (ZrH₂) > E_coh. (Zr₂FeH₅) > E_coh. (Zr₃FeH₇) > E_coh. (hypothetic-FeH) indicate a progressive destabilization of the binary hydride ZrH₂ through adjoined Fe so that Zr₃FeH₇ is found less cohesive than Zr₂FeH₅. From the energy volume equations of states EOS the volume increase upon hydriding the intermetallics leads to higher bulk moduli B₀ explained by the Zr/Fe–H bonding. Fe–H bond in Zr₂FeH₅ leads to annihilate magnetic polarization on Fe whereas Fe magnetic moment develops in Zr₃FeH₇ identified as ferromagnetic in the ground state. These differences in magnetic behaviors are due to the weakly ferromagnetic Fe largely affected by lattice environment, as opposed to strongly ferromagnetic Co. Hydrogenation of the binary intermetallics weakens the inter-metal bonding and favors the metal–hydrogen bonds leading to more cohesive hydrides as with respect to the pristine binaries. Charge analyses point to covalent like Fe versus ionic Zr and hydrogen charges ranging from covalent H^−0.27 to more ionic H^−0.5.     

Electronic structure of NH+: An ab initio study

We report ab initio self‐consistent field MRSD‐CI electronic structure calculations of the NH⁺ cation. A basis set of DZ + POL quality augmented with Rydberg and bond functions was employed together with an extensive treatment of electron correlation. More than 50 electronic states of NH⁺ are reported, including doublets, quartets, and sextets. Leading configurations, vertical ionization energies of NH, vertical excitation energies of NH⁺, and potential energy curves are reported. Spectroscopic properties calculated for the known bound electronic states of NH⁺ are found in good agreement with experiment. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Excited-state structure and vibrations of p-diaminobenzene studied by ab initio calculations

The structures and vibrations of p-diaminobenzene (PDAB) in the S₀ and S₁ states have been studied by ab initio quantum-chemical calculations. Results from geometry optimization show that the two stable cis and trans conformers of PDAB are non-planar in the S₀ state. Upon electronic excitation to the S₁ state, enhanced interaction between the ring and the amino substituent causes the molecule to become planar and contract along the long in-plane axis. A detailed analysis of the normal vibrations of PDAB in both states has been done on the basis of the motions of individual atoms as well as reduced masses, force constants and frequencies. The computed frequencies are in reasonably good agreement with the available experimental data.     

Isotope shift and hyperfine structure measurements in titanium I

High accuracy measurements of hyperfine structure due to⁴⁷Ti and⁴⁹Ti in the 3d ² 4s ² a ³ F ₂?3d ² 4s4p z ⁵ D ₁ absorption line at σ=18482.772 cm^?1 have been performed by use of a Doppler-free experiment, where a beam of titanium atoms is crossed by a CW single mode tunable dye laser. They have allowed for the determination of isotope shifts between⁴⁶Ti,⁴⁷Ti,⁴⁸Ti,⁴⁹Ti and⁵⁰Ti. By use of accurate values of mean square nuclear charge radii for the even isotopes, it has been possible to separate mass shifts from field shifts and to determine accurate values for the mean square nuclear charge radii of⁴⁷Ti and⁴⁹Ti. The field shift presents a marked odd-even staggering.     

Hyperfine structure of some near-infrared Xe I and Xe II lines

This work reports on the experimental determination of the hyperfine splitting of the Xe I lines at 828.01 nm and 834.68 nm and the Xe II line at 834.72 nm. Measurements were performed by means of Doppler-free saturation spectroscopy in a low-pressure radio-frequency discharge. The absolute wavelength of all hyperfine components is obtained by way of a high-precision wavemeter backed-up with the absorption spectrum of the NO₂ molecule. We provide an accurate estimate of hyperfine constants for the lower level of the Xe II transition at 834.72 nm. The two Xe I transition outcomes of our experimental study are compared with data available in the literature.     

An ab initio analysis of the electronic structure and harmonic frequencies of nickel porphyrin

Ab initio calculations are performed to understand the geometry, electronic structure, and vibrational frequencies of nickel porphyrin (NiP). Hartree-Fock (HF) and second-order perturbation (MP2) theories are applied with polarized basis sets. The calculated geometrical parameters are in very good agreement with the crystal structure determination. The electronic structure and bonding are analyzed in terms of complexation and correlation effects. Not unexpectedly, the HF depiction of the metal-porphyrin interaction is rather ionic while ligand σ donation is dominant at the MP2 level. Scaled HF frequencies of NiP and its isotopomers are in very good agreement with observed infrared and resonance Raman data. Received: 7 January 1997 / Accepted: 6 May 1997     

Experimental and ab initio calculational studies on nitrato[2,4-dichloro-6-((pyridin-2-ylmethylimino)methyl)phenolato]copper(II)

The mononuclear Schiff-base copper(II) compound, [Cu(C₁₃H₉C_l2N₂O)(NO₃)], has been synthesized and characterized by elemental analysis and X-ray single crystal determination. The compound crystallizes in the orthorhombic, space group Cmc2₁ with unit cell dimensions a?=?6.713(1), b?=?22.147(3), c?=?9.976(1)?Å?, M _r?=?405.67, V?=?1483.2(3)?ų, Z?=?4, R ₁?=?0.0568 and wR ₂?=?0.1331. X-ray structure determination revealed that the compound possesses crystallographic mirror symmetry. The Cu^II ion in the compound is five-coordinate in a distorted trigonal bipyramid with one O and two N atoms of the Schiff-base and by two O atoms of the nitrate anion. In the crystal structure, the molecules are linked via intermoleclular C?H?···?O non-classical hydrogen bonds, forming a two-dimensional network. Density functional theory (DFT) and Hartree-Fock (HF) calculations of the structure, atomic charge distribution and natural bond orbital analyses have been performed. The calculated results show that the Cu^II ion mainly adopts spd² hybridization and forms five bonds with the NNO donor set of the Schiff-base ligand and with two O atoms of nitrate from four orientations. The coordinate stabilization energies show that the Schiff-base copper(II) compound is very stable.