B. Nogaj

Electron density redistribution on complexation in non-transition element complexes

Abstract Complexes of tin, antimony, silicon, germanium, iodine, boron and titanium halogenides with organic ligands were investigated using nuclear quadrupole resonance (NQR) and X-ray fluorescence spectroscopies, and Mossbauer effect and quantum-chemical calculations. A discussion of results which we obtained previously as well as literature data is included. The correlations between 35 Cl NQR and ClKα line shifts in SnCl 4 L 2 , SbCl 5 L and TiCl 4 L 2 complexes were obtained. The geometry of the acceptor was found to influence the value of effective charge on the chlorine atom ( q Cl ) in SnCl 4 L 2 complexes, but not to influence the chlorine atom in TiCl 4 L 2 complexes. Values of q Sn and q Sb were calculated from Mossbauer isomer shifts in the SnCl 4 L 2 and SbCl 5 L complexes. Electron-density transfer from the ligand to the vacant d orbital of the central atom on complexation was observed.

Electron density distribution in 2‐nitro‐5‐methylimidazole derivatives studied by NMR–NQR double resonance

An NMR–NQR spectroscopic study of three 2‐nitroimidazole derivatives was performed, prompted by the prospective medical applications of these compounds. A comparison of the NMR–NQR spectra of the derivatives taken at 193 and 296 K was made. Moreover, the electronic structure of each derivative was referred to that of pure imidazole, which was possible on the basis of analysis of the bond populations. The effect of substitution of the imidazole ring at the 1 H position was analysed. Copyright

Low-temperature single-crystal X-Ray diffraction, 1H and 13C solid-state NMR and 14N NQR studies of 1,8-bis(dimethyl-amino)naphthalene

Low-temperature 13C and 1H MAS NMR and 14N NQR spectra of solid 1,8-bis(dimethylamino)naphthalene (DMAN) have been measured. X-Ray diffraction shows that the molecule is more symmetric than at room temperature. The low-temperature geometry of DMAN is very similar to the ab initio optimized geometry. Solid-state NMR and nuclear quadrupole resonance (NQR) confirm the symmetry of the molecule at low temperature despite the asymmetry of intermolecular interactions in the crystal. Experimental values of the principal components of electric field gradients at 14N, which characterize the properties of the lone pair electrons of the nitrogen atoms, are in good agreement with the ab initio calculated values.

Hydrogen bonds in 1:2 complexes of substituted pyridine N-oxides with pentachlorophenol

Abstract Pyridine N-oxides form two types of crystalline complexes with phentachlorophenol, with 1:1 and 1:2 base-to-acid ratios. The 1:2 complex of 2,6-dimethylpyridine N-oxide with pentachlorophenol crystallizes in space group P1 with a = 7.335(1) A , b = 11.324(2) A , c = 15.824(2) A , α = 100.38(1)°, β = 94.63(1)°, γ = 106.60(1)°, V = 1226.7(6) A 3 and Z = 2. The structure has been refined to R = 0.046 for 3408 observed Mo Kα reflections. The oxygen atom of the N-oxide group accepts hydrogen bonds from two molecules of pentachlorophenol, with Otctdot;O distances of 2.639(5) and 2.642(5) A and OHO angles of 141.2° and 157.6°, respectively. Both NOtctdot;HO bridges are formed in, or near, the directions of the electron lone-pairs of the N-oxygen atom. The two pentachlorophenol rings (A and B) are nearly parallel to each other and they are almost perpendicular to the pyridine ring. FTIR spectra of eleven 1:2 complexes in the solid state are similar and independent of the proton acceptor properties of the N-oxides. Five lines in the 35 Cl NQR spectra of the 1:2 complexes provide evidence that both molecules of pentachlorophenol are equivalent. In CHCl 3 solution, all the 1:2 complexes exist as a mixture of the 1:1 complex and pentachlorophenol.

14N nuclear quadrupole resonance studies of 1,8-bis(dimethylamino)naphthalene.

The presence of four resonance peaks from two nitrogen atoms in the 14N nuclear quadrupole resonance (NQR) spectrum of 1,8-bis(dimethylamino)naphthalene (DMAN) confirms the asymmetry of this molecule in the crystal. Spectroscopic parameters such as transition frequencies as well as the asymmetry parameters of the electric field gradient (EFG) tensors and differences in sigma bond occupation numbers have been assigned to particular nitrogen atoms in the molecule.

Ab inito study of the bonding and nuclear quadrupole coupling in the Py–ICl complex

Abstract As a result of the ICl complexation with Py, the 14N- and 35Cl-NQR frequencies are shifted towards lower values and 127I-NQR frequency towards higher values. In order to explain this phenomenon, ICl, Py and Py–ICl complex have been subjected to Gaussian-94 analysis on the 6-31G** level of the theory. The ab initio calculations indicate a good correlation between the calculated and the experimental e2Qqzz as well as the values of the asymmetry parameter and their shifts upon complexation. As follows from the Townes–Dailey approximation, increasing ionicity of the I–Cl bond implies a decrease in 35Cl- and 14N-NQR frequencies and an increase in the 127I-NQR frequency, which has been confirmed by both experimental and calculated results. The increase in the ionic character of the I–Cl bond can be observed as an increase in the negative charge on the chlorine atom and a decrease in the positive charge on the iodine atom. A decrease in the e2Qqzz 14N can be explained by an increase in the population of the π orbital while the population of the σ orbital remains unchanged. Stability of ICl complexes is determined by the contribution of the unpopulated orbital of the acceptor and coupling between the populated orbitals of the acceptor and the donor. The frequency shifts have been explained by the redistribution of electron density taking place on complexation.

Investigation of the electronic structure of SnCl4L2, TiCl4L2 and SbCl5L complexes by X-ray fluorescence spectroscopy

Abstract The electronic structure of SbCl 5 L complexes has been analyzed on the basis of the fluorescence spectra of the sulfur and chlorine atoms as well as quantum chemical XαDV and PM3 calculations. The results have been compared with the corresponding data for SnCl 4 L 2 and TiCl 4 L 2 complexes. An increase in electron density on the chlorine atoms accompanied by a decrease on the central metal atom has been observed for the complexes arranged according to their stability. The arrangement of Sn, Sb, and Ti complexes with dimethyl sulfide according to their increasing stability is in disagreement with the order of the complexes with respect to electron density on the sulfur atom.

Non-cyclic geometric phase of nuclear quadrupole resonance signals of powdered samples

The non-cyclic geometric phase of ¹⁴N and ³⁵Cl NQR signals induced by the character of trajectory of nuclear magnetization motion upon pulse r.f. excitation of powdered samples is studied. Analytical expressions for the geometric phases of NQR signals of the nuclei of spins I=1 and 3/2 upon nuclear magnetization rotation induced by means of r.f. pulses with frequency detuned from the resonance and for any impulse duration for a separate crystallite are obtained. It is shown that the geometric phase recorded for the signal from a powdered sample at Δω=0 can be different from zero and can oscillate upon changes in duration of the r.f. excitation pulse. An alternative variant of the nutation experiment aimed at obtaining the asymmetry parameter η from locations of frequency singularities in the nutation phase spectrum for nuclei of spin I=3/2 in powder substances is proposed.

NQR study of chalcogenide glasses Ge-As-Se.

A three‐component Ge‐As‐Se system is studied by the nuclear quadrupole resonance (NQR) method on 75As nuclei and by the nutation NQR spectroscopy. The NQR 75As spectra of the glasses Ge0.021As0.375Se0.604, Ge0.043As0.348Se0.609 and Ge0.068As0.318Se0.614reveal broad lines with two peaks assigned to the main structural unit of As2Se3. With increasing average coordination number (r̄), the spectrum signals are shifted towards higher frequencies. At r̄ > 2.54, the spectrum becomes complex, which is a consequence of formation of more complex molecular structures in the glasses of high content of germanium. At fixed frequencies the asymmetry parameter η of the samples studied is determined. Copyright

Study of As-Sb-Se chalcogenide glasses by NQR and EPR spectroscopy methods

In this paper experimental results obtained by both 75As NQR and EPR spectroscopy are presented for the three-component system As-Sb-Se. The 75As NQR spectra of glasses of structures (As2Se3)0.78 (Sb2Se3)0.22, (As2Se3)0.75 (Sb2Se3)0.25, (As2Se3)0.5 (Sb2Se3)0.5 have broad lines with two Sb-NQR lines (corresponding to the Sb2Se3 units) and two 75As-NQR lines (corresponding to the As2Se3 units). Differences in the EPR spectra of the different glasses arise because of the different amounts of arsenic and antimony in their structure.