Zero-field splitting of quintet conjugated dinitrenes: 2,6-biphenylenedinitrene

2,6-Diazidobiphenylene was synthesized and photolyzed in frozen matrix. The resulting electron spin resonance spectrum showed the formation of 2,6-biphenylenedinitrene as a quintet species with zero field splitting (zfs) parameters ∣D/hc∣=0.260±0.002 cm⁻¹, ∣E/hc∣≤0.0005 cm⁻¹. The zfs parameters are in excellent accord with dipolar models for a quintet state produced by interaction between triplet state nitrene sites.     

Zero-field splitting in phosphorescent triplet states of aromatic hydrocarbons IV. Phenyl naphthalenes

The zero-field splitting parameters D, E, and D* are calculated theoretically for four phenyl naphthalenes. Esr measurements show that the theoretical calculations are in good agreement with experiment. Comparison of the theoretical and experimental results gives some insight into the geometry of these molecules in their lowest triplet state.     

Zero-field splitting and the low-temperature magnetic susceptibility of tris (pyrrolidine-I-carbodithioato) iron (III)

It is demonstrated that the existing discrepancy in the interpretation of low-temperature magnetic-susceptibility data on high-spin tris (pyrrolidine-I-carbodithioato) iron (III) disappears when high-order magnetic-field effects and fourth-order ligand-field terms are considered.     

First-principles calculations of zero-field splitting parameters

In this work, an implementation of an approach to calculate the zero-field splitting (ZFS) constants in the framework of ab initio methods such as complete active space self-consistent field, multireference configuration interaction, or spectroscopy oriented configuration interaction is reported. The spin-orbit coupling (SOC) contribution to ZFSs is computed using an accurate multicenter mean-field approximation for the Breit-Pauli Hamiltonian. The SOC parts of ZFS constants are obtained directly after diagonalization of the SOC operator in the basis of a preselected number of roots of the spin-free Hamiltonian. This corresponds to an infinite order treatment of the SOC in terms of perturbation theory. The spin-spin (SS) part is presently estimated in a mean-field fashion and appears to yield results close to the more complete treatments available in the literature. Test calculations for the first- and second-row atoms as well as first-row transition metal atoms and a set of diatomic molecules show accurate results for the SOC part of ZFSs. SS contributions have been found to be relatively small but not negligible (exceeding 1 cm(-1) for oxygen molecule). At least for the systems studied in this work, it is demonstrated that the presented method provides much more accurate estimations for the SOC part of ZFS constants than the emerging density functional theory approaches.     

Zero-field splitting in molecules with closely spaced triplet states : A theoretical investigation of three isomeric dimethylbenzaldehydes

It is shown by quantum-chemical calculations that the unusually large zero-field splitting observed in ³ππ^* states of 2,4-, 2,5- and 3,4-dimethylbenzaldehyde are mainly due to second-order spin—orbit coupling with ³nπ^* states modulated by Franck—Condon factors.     

Zero-field NMR and NQR line shapes

A general density operator formalism is given for the NMR and NQR experiments where the evolution period is initiated by a sudden switching to zero field and is terminated by a sudden restoration of the field.     

Zero-field splitting of Mn2+ ions in tetragonal and rhombic symmetry. The effect of excited state contributions

Zero-field splitting parameters D and E are derived by a complete crystal field and spin-orbit interaction calculation for the d⁵ configuration in fields of tetragonal and rhombic symmetry. The differences as compared to the results of approximate calculations are due to the combined effect of excited states on the splitting of the ground state.     

Zero-field splittings in aromatic triplet states

The zero-field splitting parametersD andE have been calculated for naphthalene using T.B.M. and I. R. M. wave functions, and accurate values of all significant two, three and four centre dipolar interaction integrals. Agreement with experiment is improved in the case ofE, but is less satisfactory forD.     

Ab initio studies on the zero-field splitting parameters of manganese porphyrin complexes

The magnetic property of a one-dimensional magnetic chain, 5,10,15,20-tetrakis(4-bromophenyl)porphyrinatomanganese(III) tetracyanoethenide ([MnTBrPP]⁺[TCNE]⁻), is investigated by using model complexes and ab initio calculations. In these models, MnTBrPP is reduced to a manganese porphyrin complex (MnP). The spin-polarized density functional theory (UDFT) and the hybrid UDFT were used to calculate the complexes, and Pederson’s scheme was used to calculate their zero-field splitting (ZFS) parameters. We found from the model calculations that the TCNE coordination hardly affects the magnetic anisotropy of MnP.     

Vibrational spectra of mono-, di- and trichloroacetamide and mono- and trifluoroacetamide

The Raman and infrared spectra of five α-halogenoacetamides; mono-, di- and trichloroacetamide (MCA, DCA and TCA) and mono- and trifluoroacetamide (MFA and TFA), in solid phase (KBr pellets and Nujol dispersion) and in solution in several solvents (ClCH₃, Cl₂CH₂, Br₂CH₂, CH₃CN and benzene), have been recorded in order to assign the fundamental bands. The vibrational assignments, the intermolecular hydrogen bonds and the equilibrium mixture of cis and trans isomers have been discussed.     

Zero-field NMR enhanced by parahydrogen in reversible exchange

We have recently demonstrated that sensitive and chemically specific NMR spectra can be recorded in the absence of a magnetic field using hydrogenative parahydrogen induced polarization (PHIP) (1-3) and detection with an optical atomic magnetometer. Here, we show that non-hydrogenative parahydrogen-induced polarization (4-6) (NH-PHIP) can also dramatically enhance the sensitivity of zero-field NMR. We demonstrate the detection of pyridine, at concentrations as low as 6 mM in a sample volume of 250 μL, with sufficient sensitivity to resolve all identifying spectral features, as supported by numerical simulations. Because the NH-PHIP mechanism is nonreactive, operates in situ, and eliminates the need for a prepolarizing magnet, its combination with optical atomic magnetometry will greatly broaden the analytical capabilities of zero-field and low-field NMR.     

Synthesis of mono- and diarylacetylenes

Arylacetylenes have been synthesized by CC coupling of lithium acetylide with aryl bromide in the presence of a palladium(0) catalyst.     

Synthesis of cyclic mono- and bis-disulfides and their selective conversion to mono- and bis-thiosulfinates

Twelve-membered ring pseudopeptidic cyclic disulfides have been prepared by iodine oxidation of the parent dithiols. However, oxidation of N,N′-(1,2-phenylene)bis(2-mercapto-2-methylpropanamide) afforded a 25/75 mixture of cyclic mono- and bis-disulfides that were separated by selective precipitation in CHCl₃. The cyclic bis-disulfide was selectively prepared by iodine oxidation of the Ni complex of this dithiol and crystallized. Its crystal structure was solved by X-ray diffraction. All these cyclic mono- or bis-disulfides were selectively converted to cyclic mono- and bis-thiosulfinates upon stoichiometric oxidation with dimethyldioxirane at low temperature. ¹H NMR of the cyclic bis-thiosulfinate revealed the presence of four isomers, two couples of stereoisomers, as expected from the insertion of two oxygen atoms in this compound, one on each disulfide bond. The two couples of cis/trans isomers were separated by preparative TLC and identified after alkaline cleavage of the two S(O)-S bonds and metalation with Ni(II). As HO⁻ attack is selective for the sulfinyl sulfur, the nature of the Ni complexes obtained is a signature of each couple of stereoisomers.     

Calculation of the critical conversion and topological parameters for hyperbranched polyisocyanurate polymers prepared via cocyclotrimerization of mono- and diisocyanates

A new approach to the design of hyperbranched polymers, the cocyclotrimerization of mono-and diisocyanates is proposed. The kinetic scheme for the synthesis of hyperbranched polyisocyanurates was considered and calculated in the generating-function formalism. New theoretical relationships were obtained that make it possible to predict the critical conversion and a change in the structural and molecular-mass characteristics of hyperbranched polyisocyanurates in the course of the reaction, depending on the initial ratio of monoand diisocyanates and on the relative reactivity of isocyanate groups.     

Amidoalkylation of mono- and dimercapto-pyrimidines

Reaction of 2- and 4-mercapto- and 2,4-dimercaptopyrimidines with N-(1-chloroalkyl)amides of carboxylic acids and N-acylimines forms only the S-substituted products.Institute of Bioorganic Chemistry and Petroleum Chemistry, Ukraine Academy of Sciences, Kiev 253660. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1234–1238, September, 1994. Original article submitted August 4, 1994.     

Zero-field NMR and NQR line-shapes in the presence of brownian rotational reorientations

The stochastic Liouville method is applied to analyze the general problem of zero-field (ZF) NMR and NQR line shapes for molecules undergoing brownian reorientations in powders and liquids. System isotropy suffices to transform the integral kinetic equations to ordinary differential ones. Then a compact expression for the spectrum is obtained which is valied for the complete tumbling regime.     

Zero-field optically detected magnetic resonance of model compounds for pheophytins

ZFS parameters and kinetic constants of the lowest triplet state of chlorin and tetraphenylchlorin free base in n-octane have been determined by fluorescence-detected ODMR at 4.2 K. These compounds can be considered as model compounds for pheophytin, a compound of biological interest. For both compounds the middle spin-level is the most active one in the populating and depopulating pathway. In the lowest triplet state the NHNH axis in both chlorins is probably fixed to one orientation not involving the reduced ring, and no evidence was found for the occurrence of two tautomeric forms as in the corresponding porphyrins.