C. P. Keijzers

Calculation of the Anisotropic Hyperfine Coupling in Cu(II) Bis(dithiocarbamate) and Cu(II) Bis(diselenocarbamate). A Formula for the Anisotropic Hyperfine Coupling Tensor

For a molecule with one unpaired electron a general formula is derived for the elements of the anisotropic hyperfine coupling tensor, taking into account all possible electron excitations. For a check of the formula the hyperfine coupling tensors are calculated for the copper atom in bis(N, N‐diethyl dithiocarbamato) copper(II) and for the copper and selenium atoms in bis(N, N‐diethyldiselenocarbamato) copper(II), using the iterative extended Huckel method. Both the calculated principal values of these tensors and the direction cosines of the principal axes are in good agreement with the experimentally observed ones.

Model calculations of frequency-domain ESEEM spectra of disordered systems

Abstract Model calculations were carried out for electron spin-echo envelope modulation (ESEEM), ENDOR, and pulsed ENDOR spectra of spin systems of one electron coupled with an 1 = 1 2 , I = 1 , or I = 3 2 nucleus. On the basis of these simulations, predictions are made of cases for which resolved powder patterns can be expected. In the I = 1 2 case it is demonstrated that in the ESEEM simulations a degradation of the powder lineshape occurs as compared to the ENDOR simulations. Nevertheless, the shape and the symmetry of the bands may give information about the relative sign of the isotropic and the anisotropic hyperfine interaction and may yield an estimate for the magnitude of the hyperfine interaction. In the I = 1 case, the pure quadrupole frequencies can be observed when the nuclear Zeeman interaction is approximately compensated by the isotropic hyperfine interaction in one Ms manifold. This limitation is much more strict if I = 3 2 . For this nuclear spin the spectra are extremely complicated and virtually no information can be obtained if this “compensation restriction” is not fulfilled.

Exchange interactions in tropylium bis(1,2‐dicyanoethylene dithiolato) nickelate (III)

The compound tropylium bis[1,2‐dicyanoethylenedithiolato] nickelate (III), i.e., (C7H7)+ [Ni(mnt)2]−, has been prepared and its crystal structure determined; the space group is P21/c, Z=2, the unit cell axes are a=6.364, b=7.458, c=17.991 A, and β=91.22 °. The Ni(mnt)2− and C7H7+ ions form segregated stacks in the direction of the b axis. This is in contrast with the structure of the analogous thiete complex (C7H7)+ [NiS2C2 (CF3)2]−, in which crystals alternating stacks of cations and anions are present. The susceptibility appears to be dominated by a strong one‐dimensional, isotropic, antiferromagnetic exchange coupling (J0=−4.4 cm−1). However, the angular dependence of the EPR linewidth at room temperature has to be explained with an additional anisotropic exchange interaction of 0.26 cm−1. Furthermore, Q‐band EPR measurements in the ab plane reveal a very small interstack exchange interaction (0.015 cm−1 at room temperature) which increases by a factor of 4 upon lowering the temperature to 8 K and whic...

Multi-centre contributions to the anisotropic hyperfine interactions in the cu(ii) bis(dithiocarbamate) complex. Proton hyperfine couplings

Abstract One-, two- and three-centre, first-order contributions to the hyperfine coupling tensors of the central metal atom and of the ligand atoms in Cu(II) bis(N,N-diethyldithiocarbamate) are calculated. Conclusions are drawn on the relative magnitude of the multi-centre and the one-centre contributions The multi-centre contributions are always small and are of importance only if the one-centre contributions are negligible themselves The latter holds especially for the anisotropic coupling tensors of the protons the calculated two- and three-centre contributions agree very well with the experimental values.

E.S.R. Study of copper and silver N,N-dialkyldiselenocarbamates

Single-crystal E.S.R. studies of Cu(II) diethyldiselenocarbamate and Cu(II) diethyldithiocarbamate, diluted in the corresponding Zn(II) complexes, are reported. The pure copper and zinc crystals consist of dimeric units. When copper is built into the zinc lattice, dimers are formed with one copper and one zinc atom. The g-tensor, the metal hyperfine and quadrupole coupling tensors and also the hyperfine coupling tensors of the ligand selenium atoms have been obtained. Especially, from the measured selenium hyperfine couplings, it could be concluded that the guest molecules do not accept the structure of the host crystal, but have a structure similar to that of the pure copper compound. This is in contrast to the situation in systems where monomeric host crystals (e.g. nickel (II) complexes) are used and where it is found that the guest molecules do accept the structure of the host. As in the monomeric diselenocarbamate systems, in these dimers the principal axes of the g and metal hyperfine coupling tenso...

E.S.R. study of copper dithio- and diselenophosphate

Single-crystal E.S.R. studies of Cu(II)diethyldithiophosphate and Cu(II)-diethyldiselenophosphate, diluted in the corresponding Ni(II) complexes, are reported. Enrichment of the magnetic isotope of sulphur (33S) has been used in order to detect the sulphur hyperfine coupling. The g tensor, the metal hyperfine and quadrupole coupling tensors, the phosphorus and selenium hyperfine coupling tensors, and the sulphur hyperfine and quadrupole coupling tensors have been obtained. An almost isotropic coupling of the phosphorus atom has been found. The 33S hyperfine tensor largely differs from previously reported results. As in the case of other Se-containing complexes the principal axes of g and metal hyperfine coupling tensors of Cu(II)diethyldiselenophosphate do not coincide. Different kinds of low intensity ‘extra lines’ have been detected also. They have been interpreted as due to copper-copper interactions and to the dipolar coupling of neighbouring protons with the copper 3d electron. The results of extende...

Proton ENDOR study of copper(II) bis(dithiocarbamate)

The proton hyperfine coupling tensors of the methylene protons in methyl-deuterated copper(II) bis(N,N-diethyldithiocarbamate) in a diamagnetic host crystal of the corresponding nickel complex have been measured by ENDOR spectroscopy. Two intermolecular and all four intramolecular proton coupling tensors could be determined. With the aid of spin densities, obtained from extended Huckel molecular orbital calculations, the anisotropic part of the tensors can be reproduced quantitatively, taking into account all two- and three-centre contributions. Comparison of the transition frequencies which are computed from the theoretical tensors with the experimental transitions enables the tracing of another five tensors which cannot be completely determined experimentally.

Comparison of ESEEM, ESE-ENDOR, and CW-ENDOR on 14N in a powder

Abstract Three different techniques were applied for the determination of the 14 N couplings in powdered 0.3% copper-doped nickel(II)bis( NN -diethyl dithiocarbamate): electron spinecho envelope modulation (ESEEM), electron spin-echo ENDOR, and continuous-wave ENDOR. Comparison of the experimental data obtained with these methods shows their complementary nature and suggests that ESEEM and one of the ENDOR techniques should be applied together to obtain maximum information. The various powder spectra were simulated using tensors which are known from single-crystal studies. Intensities and transition probabilities were computed by complete diagonalization of the spin-Hamiltonian matrix. The correspondence with the experimental spectra is quite promising.

The nature of the Jahn-Teller effect in the copper-doped hexaimidazole zinc dichloride tetrahydrate complex

Abstract The CU(IM)62+ (Im = imidazole) ion doped into the triclinic zinc host crystal Zn(Im)6Cl2 · 4H2O was reexamined with X- and Q-band single-crystal EPR spectroscopy over the temperature range 4 to 300 K. Contrary to the earlier study, which characterized this system as an example of Jahn-Teller tunneling between three tetragonally elongated configurations, it was found from our more extensive measurements, that the Cu(Im)62+ ion represents an example of strong Jahn-Teller coupling. Below the transition temperature from dynamic to static behavior, two wells in the “Mexican hat” potential energy surface are preferentially and almost equally occupied. The g and hyperfine tensors for these two sites are consistent with the tetragonally elongated geometry typically found for the static Jahn-Teller effect in CuL62+ systems. At higher temperatures the third site 1758 becomes populated leading to a near isotropic spectrum at room temperature. Measurement of the variation of g and hyperfine coupling with temperature leads to an approximate value of 100 to 150 cm−1 for the height of the barrier separating this third site from the two lower energy sites. Examination of the structure of the host crystal allows an understanding of the observed EPR behavior.

Single-crystal EPR, 35,37Cl and 14,15N endor and eseem studies on (Ph4As)[TcVINCl4/TcVOCl4]. II, Determination of the 35,37Cl and 14,15N hyperfine and quadrupole tensors by endor and eseem

Abstract Single-crystal ENDOR and ESEEM studies on 35,37Cl and 14,15N nuclei are reported for the 4d1 (S= 1 2 ) complex (Ph4As) [TcNCl4], a spin system consisting of four 35Cl and four 37Cl nuclei (I= 3 2 ), and one 14N (15N) nucleus with I = 1 (I = 1 2 ), respectively (I(99Tc) = 9 2 ). The 35,37Cl spectra are characterized by different ratios of the hyperfine, nuclear Zeeman and the nuclear quadrupole interaction depending on the B0 field orientation: HQ