Hydrogen Bonding
In addition to the general solvent effects discussed earlier, specific solute solvent interactions may occur, the most important of which is hydrogen bonding. Hydrogen bonding is recognized as a relatively strong, specific interaction of the form A H---B between molecules or between suitable portions of the same molecule. Proton chemical shifts are extremely sensitive to hydrogen bonding. In almost all cases, formation of a hydrogen bond causes the resonance of the bonded proton to become...
Isotope Effects
Small but significant changes in chemical shifts are often found on isotopic substitution. Such a change between two isotopomers molecules that differ only in substitution of an isotope is called an isotope effect and is given the symbol A. The primary isotope effect refers to a change of chemical shift in the nuclide being observed, for example, 1H 2H 3H or 14N 15N. The primary isotope effect is usually negligible except for strong hydrogen bonds hydrogen bond energy of about 50-100 kj mole ,...
Radiation Damping
Although we usually make every effort to maximize the NMR signal in order to be able study small samples, there are occasions on which the signal is so strong that it causes difficulties. We know that the bulk magnetization induces a current in the rf coil that provides the NMR signal. However, this current generates an additional magnetic field at the sample, which opposes the applied rf field and drives the magnetization back to equilibrium more rapidly than normal relaxation. Although this...
Internal and External References
Two types of reference are used in NMR Internal and external. An internal reference is a compound giving a sharp NMR line that is dissolved directly in the sample solution under study. The reference substance is then dispersed uniformly at a molecular level through the sample. The magnetic field acts equally on the sample and reference molecules, so that Eq. 4.6 and the other relations derived before are completely valid. Provided the reference compound does not react chemically with the...
Info Nmm
FIGURE 1.3 'H NMR spectrum of 4-hydroxy-4-methyl-2-pentanone diacetone alcohol at 60 MHz. Assignments of lines S ppm 1.23, CH3 2 2.16, CH3C 0 2.62, CH2 4.12, OH. See Chapter 4 for definition of the 8 scale. nuclide at a time. Each of the spectral lines can be assigned to one of the functional groups in the sample, as indicated in the figure. The step function shown along with the spectrum is an integral, with the height of each step proportional to the area under the corresponding spectral...
Scalar Relaxation
The indirect spin spin coupling can provide a relaxation mechanism by two separate processes. As we saw in Chapter 7, J is a tensor, and its aniosotropy can cause relaxation as molecules tumble, just as with CSA. However, for most nuclei, values of J and the resultant anisotropy are generally much smaller than chemical shift aniosotropy expressed in Hz hence J anisotropy is only very rarely of any significance as a relaxation mechanism. The second mechanism results from the scalar coupling...
Effects Of Molecular Asymmetry
As we have seen, the chemical shift of a nucleus depends on its immediate environment. When two nuclei are in environments that are not equivalent by symmetry, we can anticipate that their chemical shifts will be different, with the magnitude of the difference depending on details of the environment. In many instances it is clear from the molecular structure whether two or more nuclei are in equivalent or nonequivalent environments, but in many other instances certain asymmetric features are...
Spin Decoupling And Double Resonance
The presence of spin coupling provides much of the richness of NMR spectra, not only because of the information on bonding and conformation that we can infer from the magnitudes of coupling constants but also because spin information can be transferred from one nucleus to another by means that we shall explore in later chapters. Likewise, in solids, magnetic dipolar coupling provides valuable information on internuclear distances and molecular motions, as we discuss in Chapter 7. Nevertheless,...
Notation
A3B or A3X depending on magnetic field and hydrogen bonding effect on Oqh see Section 4.5 aa'bb' or aa'xx' depending on magnetic field and assuming no coupling between rings a3mxy a3x2 assuming rapid internal rotation Generally, no significance is attached to the order in which the nuclei are given. For example in b either the hydrogen or fluorine nuclei could be designated A likewise, d could be called an A3B or AB3 system. Nuclei with different magnetogyric ratios e.g., 'H and 13C clearly...
Acquisition Time
The FID must be sampled at a rate adequate to cover the spectral width, but how long should the data be acquired That depends on both the desired resolution and considerations of signal noise ratio. The FID for a given spectral component decays with a time constant T2 , which depends on the natural transverse relaxation time of the component and the inhomogeneity in the magnetic field. Thus, even acquisition of the FID for an unlimited time leads, on Fourier FIGURE 3.7 a Depiction of...
PROBLEMS Thp
6.1 Describe the following spin systems as AB, etc. CH2 CHF PF3 cubane CH3CHOHCH3 H2 chlorobenzene n-propane. Assume free rotation about single bonds. 6.2 Show that for N coupled nuclei there are l 2N N 1 coupling constants. 6.3 Verify that ip2 in Eq. 6.30 gives the value of E2 in Eq. 6.29. 6.4 Which of the following spectra result from AB systems Spectra are given as frequency relative intensity . a 100 1 , 108 2.3 , 120 2.3 , 128 1 b 100 1 , 104 2.4 , 113 2.4 , 120 1 c 100 1 , 110 4 , 114 4 ,...
Cdc13 Solvent Where On H Nmr
FIGURE 5.4 l3C NMR spectrum at 15 MHz with broadband 'H decoupling of a sample with the molecular formula C5H,0O. ,3C chemical shifts 146.14, 110.77, 70.99, and 29.38 ppm. The triplet centered near 78 ppm is from the solvent CDC13. chemical shift correlation chart, Figure 4.3, deduce the structure of the molecule giving rise to the spectrum. 5.7 Deduce the structures of the molecules giving rise to the 1H NMR spectra in Figs. 5.5 to 5.9. 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 FIGURE 5.5 'H NMR...
Symmetry Of Spin Wave Functions
The presence of symmetry in a molecule imposes severe restrictions on many chemical and spectral properties. Often the existence of symmetry permits considerable simplification in the analysis of NMR spectra. We speak of a symmetry operation 91 as an operation that can be applied to a molecule to leave it in a configuration that is physically indistinguishable from the original configuration. With regard to wave functions, it is ip2 that corresponds to a physically measurable quantity, not ip...