Typical Values Of Chemical Shifts And Coupling Constants

1.3.1 Typical Values of JH Chemical Shifts

The chemical shift scale can be roughly divided into regions that correspond to specific chemical environments (olefinic, aromatic, etc.). Knowing these regions gives you a useful first guess as to the interpretation of a resonance, but you must keep in mind that more than one functional group might contribute in an additive fashion to the chemical shift. For example, we can estimate the chemical shift of a CH2 group situated between an olefin and a carbonyl group (C=C-CH2-C=O) as follows: A CH3 group next to an olefin or carbonyl resonates at 2.1 ppm (see below under "b"). This represents a downfield shift of 1.25 ppm from a "hydrocarbon" CH3 group (0.85 ppm, under "a" below). Thus we can estimate the shift for this CH2 as follows:

1.2 CH2 in hydrocarbon environment +2.5 effect of neighboring C=C or C=O (+1.25 ppm) times 2 3.7 total: predicted chemical shift of C=C-CH2-C=O

If we saw a resonance at 3.7 ppm, our first guess would be a proton on a singly oxygenated carbon, -CH2-O- (part "d" below), but it is dangerous to get "locked into" that idea because the possibility exists of smaller effects adding together, as shown in the example above.

(a) "Hydrocarbon": attached to an sp3-hybridized carbon and many bonds away from any unsaturation or electronegative atom. The same differences between methyl, methylene, and methine are observed in all other environments.

(b) a to a carbonyl, olefin, or aromatic group: H-C-C=O or H-C-C=C: 2.1.

(c) Next to a nitrogen: H-C-N (attached to an sp3-hybridized carbon with one single bond to nitrogen): 2.6.

(d) Next to an oxygen: H-C-O (attached to an sp3-hybridized carbon with one single bond to oxygen):

(e) "Olefinic": H-C=C: 5-6 ppm (where C=C is not part of an aromatic ring). Resonance effects can shift out of this range: up to 1 ppm upfield for electron-donating groups (e.g., H-C=C-O-) and 1 ppm downfield for electron-withdrawing groups (e.g., H-C=C-C=O). This is a result of increased or decreased electron density at the carbon bearing the proton in resonance structures such as H-C--C=O+- (electron donation: vinyl ether) and H-C+-C=C-O- (electron withdrawal: «^-unsaturated ketone).

(f) "Anomeric": H-C(-O)-O (attached to an sp3-hybridized carbon that has two single bonds to oxygen): 5-6 ppm.

(g) "Aromatic": attached to carbon of a benzene, furan, pyrrole, pyridine, indole, naphthalene, and so on, ring: generic 7-8 ppm. The effect of substituents due to resonance effects (strongest at ortho position):

1. electron-rich carbon (e.g., ortho or para to O or N of phenol, aniline, phenolic ether, or in an electron-rich heteroaromatic: pyrrole, furan): 6-7 ppm;

2. electron-poor carbon (e.g., ortho or para to C=O or NO2, or in the two or four position of pyridine): 8-9 ppm.

(i) Carboxylic acid: HO-C(=O) or phenolic: HO-C(aromatic): 12-14 ppm.

Note that there are other types of protons not listed here that can fall into the same chemical shift ranges listed above. The above categories are simply the most common ones. Also, through-space ("anisotropic") effects of unsaturated groups (C=C, C=O, and aromatic rings) can change chemical shifts from the above categories in ways that depend on conformation.

1.3.2 Typical Values of JH-JH Coupling Constants (J)

A superscipt preceding the letter J refers to the number of bonds between the two nuclei: 3 J means three-bonds or vicinal (H-C-C-H) and 2J means two bonds or geminal (H-C-H). Sometimes a subscript is used to clarify which types of nuclei are coupled: JHH means proton-to-proton coupling.

1. In freely rotating alkyl groups (e.g., CH3-CH2-): 7.0 Hz

2. In benzene rings: 3JHH = 7.5, 4JHH = 1.5, 5Jhh = 0.7 Hz

3. In a pyridine ring: J2,3 = 5.5, J3,4 = 7.6, J3,5 = 1.6, J2,5 = 0.9, J2,6 = 0.4 Hz

4. In a furan (pyrrole) ring: J2,3 = 1.8 (2.6), J34 = 3.4 (3.5), J2,4 = 0.9 (1.3), J2,5 = 1.5 (2.1) Hz

5. In a chair cyclohexane ring: Ji,2 = 12 (ax-ax), 3 Hz (eq-ax or eq-eq)

6. In a chair six-membered ring sugar, J1j2(eq-ax) = 4, J1j2(ax-ax) = 9 Hz.

7. In an isolated olefin C1H-C2^=C3H-C4H: J2,3 = 8-12(cis), 14-17(trans), J1,2 = 7 Hz

2. On a saturated (sp3) carbon: 12-15 Hz (12.5 in a cyclohexane chair)

1. Isolated olefin QH-C2H=C3H-C4H: Ju and J2,4 ("allylic") 0-3; JM ("bis-allylic") 1-2 Hz

2. "W" coupling (saturated chain in rigid planar W conformation):

1.3.3 Typical Values of 13C Chemical Shifts

13 C chemical shifts are more sensitive to steric crowding effects and less sensitive to through-space effects of double bonds than 1H chemical shifts. Increasing the substitution of a carbon (CH3 to CH2 to CH to C) leads to downfield shifts of about 10 ppm in each step.

(a) Carbonyl (C=O) shifts are far downfield (155-210 ppm) and the peaks are generally weak due to slow relaxation of quaternary carbons (except aldehydes, which are not quaternary). Ketones and aldehydes: 200-210 (isolated), 190-200 (a,ftunsaturated), Carboxylic acids, esters, amides: 170-180, Urethanes (NC(O)O): 150-160.

(b) Aromatic carbons are typically 120-130 ppm for unsubstituted positions (i.e., CH) and 136-150 at the position of alkyl substitution (weak quaternary peak). Strong electron-withdrawing groups (O, N, NO2, F) can shift the substituted (ipso) carbon to 150-160. Substituents that can donate to the ring by resonance (O, N) shift the ortho carbons and, to a lesser extent, the para carbon upfield to 110-120. Likewise, substituents that are electron-withdrawing by resonance (CO, CN) shift the ortho and para carbons downfield to 130-140. Meta carbons are unaffected because resonance structures cannot place + or - charges at these positions. Nitro (NO2) is unusual in that it shifts the ortho carbon upfield about 5 ppm and the para carbon downfield about 6 ppm. At the point of attachment of the substituent ("ipso" carbon) the range is 130-140 for "neutral" substituents and farther downfield (150-160) for electron-withdrawing substituents (e.g., O).

(d) Olefinic carbons (isolated C=C) fall in the same range as aromatic CH: 120-130. Substitution pulls this value downfield: a quaternary olefinic carbon resonates in the range of 140 ppm. They can also be shifted by resonance effects when electron withdrawing or donating groups are attached, just like in aromatic systems. For example, a quaternary ft carbon of an a, ^-unsaturated ketone resonates in the 170-180 ppm range, making it easy to confuse with an ester carbonyl carbon. This is due to the resonance structure: -C^+-C^=C-O-.

(e) Anomeric carbons of sugars (O-C-O) and in acetals and ketals: 90-110.

(f) Singly oxygenated carbons (C-O singlebond): 50-85. CH2OH carbon is in the upfield range (60-70) and quaternary carbons in the downfield range (75-85). A methoxy group (CH3O) is even farther upfield: 50-60.

(g) Carbons with a single bond to nitrogen: 50-70.

(h) Saturated carbons with no nearby electronegative atoms or double bonds: 10-50, with CH3 on the upfield side and quaternary carbons on the downfield side. Strained rings (cyclobutane, cyclopropane) show significant upfield shifts.

1.4 FUNDAMENTAL CONCEPTS OF NMR SPECTROSCOPY 1.4.1 Spin

The atomic nucleus can be viewed as a positively charged sphere that is spinning on its axis. This spin is an inherent property of the nucleus, and because charge is being moved it creates a small magnetic field aligned with the axis of spinning. Thus we can consider the nucleus as a tiny, permanent bar magnet. Because different isotopes of a given atom (e.g., 12C, 13C, 14C) have different numbers of neutrons in the nucleus, they have different magnetic properties. For this reason we only talk about specific isotopes in NMR: 1H, 19F, 11B, and so on, and our attention is focused on the nucleus of these isotopes. The nucleus of each isotope has the following intrinsic properties:

1. Magnetogyric ratio, y. This is essentially the strength of the nuclear magnet. Different nuclei have different magnet strengths; for example, the 13C nuclear magnet is only one-fourth as strong as the 1H nuclear magnet, and the 15N nuclear magnet has only one-tenth of the strength of the 1H magnet. The y is the same for every nucleus of a given type (e.g., 19F), regardless of its position within a molecule.

2. "Spin." This determines the number of quantum states available for the nucleus.

spin-0 no magnetic properties spin-/ 2 states: 1/2,-1/2

etc.

For example, a spin-/ nucleus can be viewed as having two quantum states: one with the spin axis at a 45° angle to the external magnetic field and one with the spin axis at a 135° angle to the external field. A spin-1 nucleus can be viewed as having three possible states: 45°, 90°, and 135°. In this book we will be concerned primarily with spin-/ nuclei.

Here are some examples showing the composition of the nucleus (p = protons, n =

12C(6p + 6n) 1H (1p + 0n) 2H (1p + 1n) 16O (8p + 8n) 3H (1p + 2n) 14N (7p + 7n) 18O (8p + 10n) 13C (6p + 7n) 17O (8p + 9n) 15N (7p + 8n) 19F (9p + 10n) 29Si (14p + 15n) _31P (15p + 16n)_

Note that there is a pattern: Nuclei with an even number of protons and neutrons (even-even) have spin zero; "odd-even" and "even-odd" nuclei tend to be spin-/; and "odd-odd" nuclei tend to have a spin greater than 1/2. This is just a rule of thumb (e.g., 17O violates the "rule"). Nuclei with spin greater than 1/2 are more difficult to observe than spin-/ nuclei because they have a "nuclear quadrupole moment" that makes their NMR peaks very broad. For this reason, most NMR work is focused on the spin-/ nuclei. Because NMR is usually done in deuterated solvents (D2O, CD3OD, etc.), we will have to occasionally consider the effects of a spin-1 (three quantum states) nucleus.

1.4.2 Precession

When we place a spin-/ nucleus in a strong external magnetic field, the nucleus wants to align itself with the magnetic field, just like a compass needle moves to align with the earth's magnetic field. But because the nucleus is spinning (i.e., it has an intrinsic property of angular momentum), it cannot simply change its angle with the magnetic field from 45° to 0°. The torque it experiences from the external magnetic field instead causes the spin axis to "wobble" or precess around the magnetic field direction. This is analogous to a spinning top or gyroscope, which responds to the torque produced by the earth's gravitational field by describing a circle with its spin axis. The precession rate of the nucleus in a magnetic field is the resonant frequency referred to in the name "nuclear magnetic resonance." The precession rate is in the range of radio frequency, tens or hundreds of megahertz, or millions of rotations per second. In this classical model the torque exerted on the nucleus is proportional to both the laboratory magnetic field strength, Bo, and to the strength of the nuclear magnet, y. The rate of precession is proportional to the torque, so we have:

It cannot be emphasized too much that the resonant frequency in NMR is proportional to the magnetogyric ratio, y, and to the laboratory magnetic field strength, Bo. This relationship forms the basis of nearly every phenomenon observed in NMR. There are two ways to measure the precession rate: the angular velocity, mo, in units of radians per second and the frequency, vo, in units of cycles per second or hertz. In this book we will use frequencies in hertz. This frequency is sometimes called the Larmor frequency, and the zero subscript refers to this fundamental frequency, which results from the laboratory magnetic field interacting with the nucleus' magnetic field.

As an example, consider a proton (1H nucleus) in a 7.05 T laboratory magnetic field:

Such a magnet would be called a "300 MHz" magnet because the 1 H nucleus precesses at a rate of 300 MHz in this magnet. NMR magnets are almost never described in tesla but rather by their XH resonance frequency. This can be confusing because if you are observing 13 C nuclei on a 500-MHz NMR instrument, you are operating at a resonant frequency of 125 MHz, not 500 MHz. Because the resonant frequency for a given magnet (NMR magnets have a fixed magnetic field strength) is proportional to the magnetogyric ratio, y, of the nucleus being observed, the NMR frequencies for different nuclei will always be in the same ratio: the ratio determined by their relative y values.

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