Nuclear isomers

One should remember in this connection that many cases of spontaneous fission of excited (isomer) nuclear states with lifetimes about 10 2 sec (quite close to the time of addition of a new link to the growing polyformaldehyde chain near absolute zero of temperature) were observed and successfully studied after the pioneering works of Dubna scientists. 

A useful method for selective O-acetylation of 5,7-dihydroxyisoflavones has been described. Acetic anhydride and pyridine gives the 7-acetoxy-compound whereas acetic anhydride and perchloric acid yields the 5-acetoxy-isomer nuclear chlorination of the latter occurred at C-8 when it was treated with ethyl AW-dichlorocarbamate. Interaction of isoflavones (222) with Grignard reagents and copper(l) chloride gave the deoxybenzoin (223), which was used in the synthesis of stilbene-like compounds. ... 

The P61ya method is widely used for enumeration of various classes of isomers benzenoid hydrocarbons, substituted benzenoid hydrocarbons, alkanes, cycloalkanes, isotopically labeled compounds, porphyrins, adamantane and its valence isomers, valence isomers of annulenes, dodeca-hedrane, fullerene isomers, nuclear-spin statistics, and ESR hyperfine patterns. 

Both vapor-phase chromatography and high performance Hquid chromatography, along with nuclear magnetic resonance spectroscopy, have been used for isomer and composition analysis. 

Nuclear Overhauser enhancement (NOE) spectroscopy has been used to measure the through-space interaction between protons at and the protons associated with the substituents at N (20). The method is also useful for distinguishing between isomers with different groups at and C. Reference 21 contains the chemical shifts and coupling constants of a considerable number of pyrazoles with substituents at N and C. NOE difference spectroscopy ( H) has been employed to differentiate between the two regioisomers [153076 5-0] (14) and [153076 6-1] (15) (22). N-nmr spectroscopy also has some utility in the field of pyrazoles and derivatives. 

Technetium-99m coordination compounds are used very widely as noniavasive imaging tools (35) (see Imaging technology Radioactive tracers). Different coordination species concentrate ia different organs. Several of the [Tc O(chelate)2] types have been used. In fact, the large majority of nuclear medicine scans ia the United States are of technetium-99m complexes. Moreover, chiral transition-metal complexes have been used to probe nucleic acid stmcture (see Nucleic acids). For example, the two chiral isomers of tris(1,10-phenanthroline)mthenium (IT) [24162-09-2] (14) iateract differentiy with DNA. These compounds are enantioselective and provide an addition tool for DNA stmctural iaterpretation (36). 

In decoupling the methyl protons, the NOE difference spectrum shows a nuclear Overhauser enhancement on the cyclopropane proton at = 1.60 and on the terminal vinyl proton with trans coupling at <5// = 5.05 and, because of the geminal coupling, a negative NOE on the other terminal proton at Sh= 4.87. This confirms the trans configuration G. In the cis isomer H no NOE would be expected for the cyclopropane proton, but one would be expected for the alkenyl-// in the a-position indicated by arrows in H. 

Despite its very simple electronic configuration (Is ) hydrogen can, paradoxically, exist in over 50 different forms most of which have been well characterized. This multiplicity of forms arises firstly from the existence of atomic, molecular and ionized species in the gas phase H, H2, H+, H , H2" ", H3+. .., H11 + secondly, from the existence of three isotopes, jH, jH(D) and jH(T), and correspondingly of D, D2, HD, DT, etc. and, finally, from the existence of nuclear spin isomers for the homonuclear diatomic species. 

Kem-isomer, n. nuclear isomer, ring isomer, -isomerie, /. nucleus (or nuclear) isomerism, -kbrper, m., -korperchen, n. nucleolus, -la-dung,/. nuclear charge main charge, -la-dungszahl, /. nuclear-charge number, atomic number, -leder, n. butt or bend leather. 

Several 3-azido[l,2,4]triazines 1013 were prepared (76JOC2860 77JHC1221) by treating the corresponding 3-hydrazino derivatives with nitrous acid. Azidotriazines 1013 were spontaneously cyclized to tet-razolo[l,5-/j][l,2,4]triazines 1014 rather than their isomers 1015. These transformations were studied using nuclear magnetic resonance and infra-... 

Nuclear y-ray resonance spectra of solid solutions of Fe and Co in /3-rh boron give inconclusive results, although the large isomer shifts as compared to Fe metal indicate that the accommodation of Fe atoms in the boron structure is associated with changes in the electronic state. The magnitudes of the shifts are... 

Strohschein, S., Pursch, M., and Albert, K., Hyphenation of high performance liquid chromatography with nuclear magnetic resonance spectroscopy for the characterization of (3-carotene isomers employing a Cjq stationary phase, J. Pharm. Biom. Anal., 21, 669, 1999. 

Hentschel, P. et ah. Structure elucidation of deoxylutein 11 isomers by on-line capillary high performance liquid chromatography- H nuclear magnetic resonance spectroscopy, J. Chromatogr. A, 1112, 285, 2006. 

Figure 2A. Schematic diagram of Mossbauer parameters isomer shift (6), quadrupole splitting (AEq) and magnetic dipole splitting of the nuclear energy states of 57pe leading to various hyperfine splitting in Mossbauer spectra.

One is purely formal, it concerns the departure from symmetry of an approximate solution of the Schrodinger equation for the electrons (ie within the Bom-Oppenheimer approximation). The most famous case is the symmetry-breaking of the solutions of the Hartree-Fock equations [1-4]. The other symmetry-breaking concerns the appearance of non symmetrical conformations of minimum potential energy. This phenomenon of deviation of the molecular structure from symmetry is so familiar, confirmed by a huge amount of physical evidences, of which chirality (i.e. the existence of optical isomers) was the oldest one, that it is well accepted. However, there are many problems where the Hartree-Fock symmetry breaking of the wave function for a symmetrical nuclear conformation and the deformation of the nuclear skeleton are internally related, obeying the same laws. And it is one purpose of the present review to stress on that internal link. 

The description of states participating in a spin-state transition as electronic isomers with discrete nuclear configurations, in particular different metal-ligand distances, requires that separate electronic and vibrational spectra of the two spin states exist. Indeed, a superposition of the individual vibrational spectra of the two states is in general observed, the relative contribution of the states being a function of temperature [41, 139, 140, 141, 142]. This observation sets a lower limit for the spin-state lifetime longer than the nuclear vibrational period, i.e.,... 

A unique situation is encountered if Fe-M6ssbauer spectroscopy is applied for the study of spin-state transitions in iron complexes. The half-life of the excited state of the Fe nucleus involved in the Mossbauer experiment is tj/2 = 0.977 X 10 s which is related to the decay constant k by tj/2 = ln2/fe. The lifetime t = l//c is therefore = 1.410 x 10 s which value is just at the centre of the range estimated for the spin-state lifetime Tl = I/Zclh- Thus both the situations discussed above are expected to appear under suitable conditions in the Mossbauer spectra. The quantity of importance is here the nuclear Larmor precession frequency co . If the spin-state lifetime Tl = 1/feLH is long relative to the nuclear precession time l/co , i.e. Tl > l/o) , individual and sharp resonance lines for the two spin states are observed. On the other hand, if the spin-state lifetime is short and thus < l/o) , averaged spectra with intermediate values of quadrupole splitting A q and isomer shift 5 are found. For the intermediate case where Tl 1/cl , broadened and asymmetric resonance lines are obtained. These may be the subject of a lineshape analysis that will eventually produce values of rate constants for the dynamic spin-state inter-conversion process. The rate constants extracted from the spectra will be necessarily of the order of 10 -10 s"F... 

Fig. 3.4 Calibration spectrum of metallic iron and magnetic hyperfine splitting of the nuclear levels. The values of the hyperfine splitting in a-iron are = 1.677 mm >2 = 6.167mms >3 = 10.657 mm s. The center of the calibration spectrum is defined as velocity zero left). The isomer shift of a specific sample with respect to metallic iron is indicated as 5 (right)...

The term causes a uniform shift of the nuclear energy states which, however, is different for the ground and excited state because the nuclear volume and, therefore, also the entity R ) are different for ground and excited states. This gives rise to the isomer shift 6 of the Mbssbauer spectrum. The notation 6E = E is introduced to emphasize the very small change in energy ( 10 eV), which is only a fraction (about 10 ) of the transition energy. The isomer shift will be discussed in detail in Sect. 4.2. 

The electric monopole interaction between a nucleus (with mean square radius k) and its environment is a product of the nuclear charge distribution ZeR and the electronic charge density e il/ 0) at the nucleus, SE = const (4.11). However, nuclei of the same mass and charge but different nuclear states isomers) have different charge distributions ZeR eR ), because the nuclear volume and the mean square radius depend on the state of nuclear excitation R R ). Therefore, the energies of a Mossbauer nucleus in the ground state (g) and in the excited state (e) are shifted by different amounts (5 )e and (5 )g relative to those of a bare nucleus. It was recognized very early that this effect, which is schematically shown in Fig. 4.1, is responsible for the occurrence of the Mossbauer isomer shift [7]. 

Fig. 4.1 The electric monopole interaction between the nuclear charge and the electron density at the nucleus shifts the energy of the nuclear states and gives the Mossbauer isomer shift...

The entity ot is the so-called isomer shift calibration constant, c is the speed of light, Co is the electric constant, and Eq is the nuclear transition energy. (The Coulomb constant k = l/(47rco), which was dropped in (4.1), is re-inserted here.) A comprehensive derivation of this expression is found in [8, 9]. 

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