Nuclear spin isotope effect

Regardless of the comments above, we need to speak about nuclear spins, since they are everywhere (at least in the form of protons) and because they also can be connected with quantum effects in bio-systems. Recent experiments (Buchachenko and Kouznetsov 2008 Buchachenko et al. 2005) demonstrate that intramitochondrial nucleotide phosphorylation is a nuclear spin controlled process because the magnetic magnesium isotope Mg(ll) increases the rate of mitochondrial ATP synthesis in comparison with the spinless nonmagnetic Mg(ll), Mg(lI) ions. Such nuclear spin isotope effect is usually interpreted in terms of radical pair theory for separated spins in solvent cage (Buchachenko 1977), but an alternative explanation based on... 

CO was measurably depleted in In part, the elegance of this experiment derives from the fact that it required no synthesis or separation of isotopically enriched starting material. In 1980 R.G. Lawler pointed out that conclusive demonstration of nuclear spin isotope effects demands bracketing of the magnetic isotope between two nonmagnetic ones (e.g. depletion of relative to as well as depletion relative to C) however, few experiments have been reported that fulfil this rigorous requirement. 

The values of S° represent the virtual or thermal entropy of the substance in the standard state at 298.15 K (25°C), omitting contributions from nuclear spins. Isotope mixing effects are also excluded except in the case of the H—system. 

Holt and Pipkin observed the 567.6-nm and 404.7-nm photons emitted in the 9 P,-7 S -6 Po cascade of the zero nuclear spin isotope Hg of mercury. The relevant transitions are shown in Figure 8, from which it can be seen that the final cascade level is not the ground state of the atom. Thus in this experiment no precautions had to be taken to avoid the effects of resonance trapping. 

Isotopes of hydrogen. Three isotopes of hydrogen are known H, 2H (deuterium or D), 3H (tritium or T). Isotope effects are greater for hydrogen than for any other elements (and this may by a justification for the different names), but practically the chemical properties of H, D and T are nearly identical except in matters such as rates and equilibrium constants of reactions (see Tables 5.1a and 5.1b). Molecular H2 and D2 have two forms, ortho and para forms in which the nuclear spins are aligned or opposed, respectively. This results in very slight differences in bulk physical properties the two forms can be separated by gas chromatography. 

The isotope N, with a natural abundance of 99.9%, has nuclear spin 7 = 1 and gives broad signals which are of little use for structural determinations. The N nucleus, with I = 1/2, is therefore preferred. However, the low natural abundance of about 0.4% and the extremely low relative sensitivity (Table 1) make measurements so difficult that N NMR spectroscopy was slow to become an accepted analytical tool. A further peculiarity is the negative magnetogyric ratio since, in proton decoupled spectra, the nuclear Overhauser effect can strongly reduce the signal intensity. DEPT and INEPT pulse techniques are therefore particularly important for N NMR spectroscopy. 

When one considers scattering by more than one atom there is the additional complication that the quantity denoted here by b is different for different isotopes and that nuclear spin is also involved in the process. So far as isotope effects are concerned, we will only be concerned with systems where the different isotopes, hydrogen and deuterium, are in predictable positions and so there is no need to analyse the effects of their random distribution in detail. However, in the case of both hydrogen and deuterium, the nuclear spins of the different atoms are, under normal circumstances, randomly arranged. Now the proton has spin and so does the neutron. Thus the combined system can be in a triplet or in a singlet state and the effective value of b is different for the two cases. 

When SP [T] = SP"[0] (condition 2), AS°[T] can be expressed as v (SP [T] — Sp"[0]) that is, in terms of the observed quantities. We use the difference (Sp [T] — SP"[0]) as the absolute value of the entropy, which is equivalent to assigning the value of zero to SP"[0]. The two effects for which this assignment is valid are (1) the nuclear effects including those of nuclear spin, provided that the isothermal change of state does not involve a nuclear reaction and (2) the isotopic effects, provided there is no change in the isotopic composition of the substances. 

We turn now to the corresponding studies of the isotopic species D2 and HD. The deuterium nucleus has spin Id equal to 1, so that the two equivalent deuterium nuclei in D2 have their spins coupled to give total nuclear spin / equal to 2, 1 or 0. The states with / equal to 2 or 0 correspond to ortho-D2, whilst that with / equal to 1 is known as para-T>2. The molecular beam magnetic resonance studies have been performed on para-D2, in the. 1 = 1 rotational level. Formally, therefore, the effective Hamiltonian is the same as that described above for experimental studies of ortho-H2, also in the J = 1 rotational level. There is one extremely important difference, however, in that the... 

Cu isotopes both with nuclear spin I-3/2. The nucle r g-factors of these two isotopes are sufficiently close that no resolution of the two isotopes is typically seen in zeolite matrices. No Jahn-Teller effects have been observed for Cu2+ in zeolites. The spin-lattice relaxation time of cupric ion is sufficiently long that it can be easily observed by GSR at room temperature and below. Thus cupric ion exchanged zeolites have been extensively studied (5,17-26) by ESR, but ESR alone has not typically given unambiguous information about the water coordination of cupric ion or the specific location of cupric ion in the zeolite lattice. This situation can be substantially improved by using electron spin echo modulation spectrometry. The modulation analysis is carried out as described in the previous sections. The number of coordinated deuterated water molecules is determined from deuterium modulation in three pulse electron spin echo spectra. The location in the zeolite lattice is determined partly from aluminum modulation and more quantitatively from cesium modulation. The symmetry of the various copper species is determined from the water coordination number and the characteristics of the ESR spectra. 

Proton NMR spectra in organic molecules can be interpreted without regard to the structural carbon framework because the predominant 12C has no nuclear spin. However, 13C has a spin of V2, which not only permits its direct observation but also provides features in the H spectrum from the 13C that is present at a natural abundance of 1.1%. As we saw in Chapter 5, J(13C-H) is normally 100—200 Hz, whereas two- and three-bond coupling constants often run 5-10 Hz. Hence a resonance line from a proton attached to a 12C atom is accompanied by weak 13C satellites separated by 1J(13C-H) and placed almost symmetrically about the main line. (The departure from precisely symmetrical disposition arises from the 13C/12C isotope effect on the 1H chemical shift, as described in Section 4.8.) For example, the proton resonance of chloroform in Fig. 6.14a shows 13C satellites. 

Atomic absorption offers a more practical opportunity for determining isotopic composition than atomic emission. Useful reviews of the possibilities of the technique have appeared in two books [233, 234]. Isotopic analysis is in theory possible provided that highly enriched isotope sources are available, the absorption line width available is less than the isotopic displacement and for a given isotope the nuclear spin hyperfine components must be partially resolved from the other isotopic components of the absorption line. In the simplest possible case, for an element with two isotopes, the lamp is prepared from the first isotope and only this isotope in the atom cell will absorb the radiation. The procedure can then be repeated with a lamp prepared with the second isotope. Effectively this is an extension of the impressive selectivity of atomic absorption, because of the classic lock and key effect, treating the different isotopes as different analytes. 

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