Shift . chemical

Chemical shift relates the Larmor frequency of a nuclear spin to its chemical environment l 3. The Larmor frequency is the precession frequency v0 of a nuclear spin in a static magnetic field (Fig. 1.1). This frequency is proportional to the flux density Bo of the magnetic field (v0 B0 = const.) 3. It is convenient to reference the chemical shift to a standard such as tetramethylsilane [TMS, (C//j)4Si] rather than to the proton ft. Thus, a frequency difference (Hz) is measured for a proton or a carbon-13 nucleus of a sample from the H or 13C resonance of TMS. This value is divided by the absolute value of the Larmor frequency of the standard (e.g. 400 MHz for the protons and 100 MHz for the carbon-13 nuclei of TMS when using a 400 MHz spectrometer), which itself is proportional to the strength B0 of the magnetic field. The chemical shift is therefore given in parts per million (ppm, 5 scale, SH for protons, 5C for carbon-13 nuclei), because a frequency difference in Hz is divided by a frequency in MHz, these values being in a proportion of 1 106. 

Chemical shift relates the Larmor frequency of a nuelear spin to its ehemieal environment The Larmor frequency is the preeession frequency Vg of a nuclear spin in a static magnetic field (Fig. 1.1). This frequency is proportional to the flux density Bg of the magnetic field vglBg = const.) 

In such expression, Hcs is the chemical-shift interaction of the nucleus with the orbital motion of the surrounding electrons Hd is the direct (through space) dipolar interaction between nuclei 7 / is the electron-mediated interaction between nuclei and Hq is the quadrupolar interaction between a nucleus with spin 1/2 and the electric field gradient at the nuclear position. Each of these terms are briefly described below and simplified forms for the Hamiltonians are given. 

The quantity a is known as the chemical shielding tensor associated with that particular nuclear site. The tensorial character of a implies that B/oc is in general in a direction different from that of Bo, which reflects the anisotropy of the molecular environment of the considered nucleus. As this is a purely magnetic interaction, analogous to the Zeeman one, the Hamiltonian Hcs is given by  

The last step in the equation above is known as secular approximation and it is a generally appropriate simplification valid as consequence of the much larger magnitude of the Zeeman interaction with the external magnetic field as compared to the chemical shift one [5]. 

It is important to stress that the component Ozz depends on the relative orientation of the electron cloud in the molecule with respect to the external magnetic field. In a monocrystalline solid there is only one value of the parameto Ozz for each orientation of the specimen. For an isotropic liquid substance, the average of all possible molecular orientations leads to an average value for the chemical shift known as the isotropical chemical shift (fiso) [4]  

The parameter aiso is related to the trace of the tensor a, which is usually written in a molecular reference frame where this tensor is diagonal, known as the principal axis system (PAS) of the tensor a  

Of course, these relationships do not take into account the variations in SO4 chemical shift due to the effects of counterions, pH, concentration and temperature. It has not been possible to adopt the IUPAC chemical shift scale exactly, because that would require measuring the chemical shifts of all secondary references reported in the literature with respect to the standard suggested by IUPAC. 

Sulphur can assume three formal oxidation states and forms as many as six bonds S(II), in which it forms two covalent bonds and has two lone pairs (i.e. H2S and (CH3)2S) or forms three covalent bonds and has one electron lone pair ((CH3)3S+), and S(IV) and S(VI), in which it is hypervalent and can use its lone 

As for all nuclei, the 33S nuclear screening constant a can be approximated as the sum of two independent terms of opposite sign, the diamagnetic and the paramagnetic contribution 32 

The most significant contribution to the overall screening constant is due to the diamagnetic term, which arises from the electrons in the filled shells surrounding the nucleus. It depends on the electronic ground state of the molecule and has a shielding effect. The variations in chemical shift can be ascribed mainly to variations in the paramagnetic term. This has been demonstrated experimentally for 33S in 3- and 4-substituted benzenesulpho-nates.33 

There are many analogies in the structural influences on S, P and Se NMR chemical shifts. Wasylishen et al.36 have compared the relative 33S and 77Se chemical shift scales and found that the correspondence is noticeable. For example, the resonances of the hydrides H2S and PH3 lie at the extreme upheld end of the total spectral range. Substitution of a hydrogen atom with a methyl group provokes downfield shifts of 44.7 and 64.5 ppm, respectively. Further substitution of a second hydrogen atom with a methyl causes downfield shifts by 31 and 64.9 ppm, respectively. 

The most important information in XPS originates from the small shifts of the binding energy induced by changes in the chemical environment of the atoms. For instance a positive charge, i.e. a lack of electrons, in the electron cloud creates an additional potential which the photoelectron has to overcome when it leaves the atom. Thus the binding energy EB increases by AEB which shows in the XPS-spectrum as a shift of the photoelectron line to lower 

There is quite a number of theoretical approaches to the understanding of experimentally observed chemical shifts. It was early realized that chemical shifts could be related to the formal oxidation state of the element under study. Further investigations revealed that the effective charge q (A) of an atom A in a molecule is the important parameter and numerous correlations based on the equation 

In many cases more refined molecular orbital models give a better agreement between theory and experiment. Self-consistent field- 80 81 or CNDO-82 calculations as well as other ab initio calculations 83 84) were performed and the results of several different approaches for phosphorous compounds were critically evaluated by M. Pelavin 77). For the nitrogen compounds two different linear relationships, one for cations and for neutral molecules, the other for anoins, were observed 82), a phenomenon which might be explained with a Madelung contribution. 

Even though correlations of binding energy with atomic charge are very useful in structure determination and studies of chemical bonds, one has to keep in mind that these models are approximations and the conclusions drawn are only valid within the specific limitations of each theoretical approach. Further sophisticated calculations 8S,90) and correlations with ther- 

We will now begin exploring the three characteristics of every signal in an NMR spectrum. The first characteristic is the location of the signal, called its chemical shift (8), which is defined relative to the frequency of absorption of a reference compound, tetramethylsilane (TMS). 

In practice, deuterated solvents used for NMR spectroscopy typically contain a small amount of TMS, which produces a signal at a lower frequency than the signals produced by most organic compounds. The frequency of each signal is then described as the difference (in hera) between the resonance frequency of the proton being observed and that of TMS, divided by the operating frequency of the spectrometer. 

For example, when benzene is analyzed using an NMR spectrometer operating at 300 MHz, the protons of benzene absorb a frequency of rf radiation that is 2181 Hz larger than the frequency of absorption of TMS. The chemical shift of these protons is then calculated in the following way  

Notice that the chemical shift of the protons is a constant, regardless of the operating frequency of the spectrometer. That is precisely why chemical shifts have been defined in relative terms, rather than absolute terms (hertz). If signals were reported in hertz (the precise frequency of rf radiation absorbed), then the frequency of absorption would be dependent on the strength of the magnetic field and would not be a constant. 

In actual fact, there is an excess of only about 1 in tO nuclei in the lower energy level so that, when conducting an NMR experiment, we are working with just t in 10 of the NMR-active nuclei  

The first thing we need to consider when discussing the NMR effect is why equation (4.1) contains the term B ff rather than B. This is called the chemical shift effect, and to understand this key aspect of NMR spectra we need to consider what happens when we place our NMR sample in a magnetic field. The atomic nuclei in our sample adopt one of a number of possible alignments, depending upon their spin. We also, however, have to 

The really important aspect to all of this is that nuclei in similar chemical environments exhibit similar chemical shifts. Thus protons ( H nuclei) attached to a carbon atom bonded to oxygen, H-C-O, show a characteristic chemical shift (3.5-A.5 ppm), while protons attached to a carbon atom bonded to nitrogen, H-C-N, have a different chemical shift range (2.5-3.5 ppm) and, since the carbon is attached to the less electronegative N atom, resonate at lower frequency. We can therefore use chemical shifts to our great advantage when interpreting NMR spectra. 

A transition that occurs at 100 Hz on a 100 MHz NMR spectrometer will occur at 500 Hz on a 500 MHz instrument, although the giving rise to the signal is exactly the same. 

In CDCI3 or DMSO-t/g we normally choose tetramethylsilane (TMS), (CH3)4Si, as our reference, and this is given a reference value of 0 Hz so that, in the above example, the signals which occur at 100 Hz and 500 Hz, at magnetic fields of 100 MHz and 500 MHz respectively, have a chemical shift of 8 1.0 in both cases. 

If an atom is located in a magnetic held, the electrons of the atom will circulate in the direction of the applied magnetic held. This movement produces a tiny magnetic held at the nucleus that is in opposition to the applied magnetic held [77], Consequently, the magnetic held at the nucleus is, for this reason normally, less than the applied held by a dimensionless fraction, o, called the shielding constant. That is, this additional held is directly proportional to the externally applied held  

the electrons that surround each nucleus act to slightly perturb the magnetic held at the spin site. This causes the Larmor precession frequency to be modihed by the chemical environment of the spin. This effect, called the chemical shift, is described by the equation 

It is evident that this effect modihes the Larmor frequency such that 

The Physical Chemistry of Materials Energy and Environmental Applications 

In practice, it is common to express the chemical shift of a peak in the spectrum in terms of the relative difference in frequency from some reference peak. The chemical shift in parts per million (ppm) is, therefore, defined as 

So far as NMR is concerned, molecules present in dilute solutions form independent entities that do not interact noticeably between themselves. Alternatively, within a given molecule, the electronic and steric environment of each nucleus creates a very weak local magnetic field which shields it more or less from the action of the external field Bq. Each atom has a particular environment - at least if there are no specific elements of symmetry in the molecule. According to the Larmor equation, these very weak local variations in the intensity of the field will affect the frequency of resonance as compared to what would be seen in a vacuum. This effect is called shielding or deshielding of the nuclei. 

All variations in a affect the resonant frequency of the corresponding nucleus. This phenomenon is called a chemical shift. As many chemical shifts are observed as there are molecules containing different shielding constants a. Consequently large molecules lead to complex spectra as a result of their numerous 7 values. For a nucleus i for which 7=1/2, the Larmor equation becomes, on the introduction 

Concerning the proton H, expression 15.9 leads to a difference of 1 Hz for the resonance frequency when the surrounding field varies of 2.3 x 10 T, while a variation of 10 T (1 gauss) induces a shift of 4258 Hz. For this reason the setting 

FIGURE 4.10. Signal of neat chloroform with spinning side bands produced by spinning rate of (a) 6 Hz and (b) 14 Hz. From Bovey, F.A. (1969). NMR Spectroscopy. New York Academic Press, with permission. 

The oscillations seen only in scanned (CW) spectra at the low-frequency end of a strong sharp peak are called ringing (Fig. 4.11). These are beat frequencies resulting from passage through the absorption peak. 

Traces of ferromagnetic impurities cause severe broadening of absorption peaks because of reduction of 

FIGURE 4.11. Ringing (or wiggles) seen after passage through resonance in a scanned spectrum. Direction of scan is from left to right. 

T2 relaxation times. Common sources are tapwater, steel wool, Raney nickel, and particles from metal spatulas or fittings (Fig. 4,12). These impurities can be removed by filtration. 

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Information from an n.m.r. spectrum is classified into the chemical shift, <5 (the relative shift from a standard [Me Si for H, CC13F for which is rendered independent of the field), and the coupling constants, J, which are determined directly from the spectra. 

ESR Electron spin (paramag- Chemical shift of splitting of Chemical state of adsorbed... 

NMR Nuclear magnetic resonance [223, 224] Chemical shift of splitting of nuclear spin states in a magnetic field H [225], C [226, 227], N [228], F [229], 2 Xe [230] Other Techniques Chemical state diffusion of adsorbed species... 

MS Mossbauer Spectroscopy [233-236] Chemical shift of nuclear energy states, usually of iron Chemical state of atoms... 

Figrue BTl 1.1 shows the range of radiolfequencies where resonances may be expected, between 650 and 140 MHz, when Bq = 14.1 T, i.e. when the H resonance frequency is 600 MHz. There is one bar per stable isotope. Its width is the reported chemical shift range (Bl.11.5) for that isotope, and its height corresponds to the log of the sensitivity at the natural abundance of the isotope, covering about six orders of magnitude. The... 

Figure Bl.11.6. and chemical shifts in phenol, relative to benzene in each case. Note that 5 (H or C) approximately follows 6 (the partial charge at C).

B1.11.5.3 CHEMICAL SHIFTS ARISING FROM MORE DISTANT MOIETIES... 

Figure Bl.11.7. chemical shifts in [10]-paracyclophane. They have values on either side of the 1.38 ppm found for large polymethylene rings and, thus, map the local shielding and deshielding near the aromatic moiety, as depicted in the upper part of the figure.

Enonnous numbers of chemical shifts have been recorded, particularly for FI and Many algoritlnns for the prediction of shifts have been extracted from these, so that the spectra of most organic componnds can be predicted at a useful level of accuracy, usmg data tables available in several convenient texts [12, F3,14 and 15]. Alternatively, computer programs are available that store data from 10 -10 spectra and then use direct... 

It is also usually possible to remove all the couplings from a particular isotope, e.g. H, provided that one only wishes to observe the spectrum from another isotope, e.g. Either the decoupling frequency is noise-modulated to cover the relevant range of chemical shifts, or else the same decoupling is achieved more efficiently, and with less heating of the sample, by using a carefiilly designed, continuous sequence of... 

Similar experiments exist to correlate the resonances of different types of nucleus, e.g. C with H, provided that some suitable couplings are present, such as It is necessary to apply pulses at both the relevant frequencies and it is also desirable to be able to detect either nucleus, to resolve different peak clusters. Detection tlirough the nucleus with the higher frequency is usually called reverse-mode detection and generally gives better sensitivity. The spectrum will have the two different chemical shift scales along its axes... 

Jameson C J and Mason J 1987 The chemical shift Multinuclear NMR ed J Mason (New York Plenum) oh 3... 

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