Espectroscopía

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ESPECTRO ELECTROMAGNÉTICO

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excites vibrational energy levels

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An absorption band due to a particular functional group always appears in the same general region.

wavenumber = v

APARIENCIA DEL ESPECTRO IR

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Generalizations

1

2c M

k

k = force constant of bond (proportional to bond strength)

M = reduced mass of atoms (proportional to mass of atoms)

1. stronger bonds absorb at higher wavenumber (larger k)

2. stretches absorb at higher wavenumber than bends (larger k)

3. bonds to H absorb at higher wavenumber (smaller mass)

4. polar bonds have more intense absorptions (smaller %T)

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Enlaces sencillos a Hidrógeno

O-H

3550-3200 cm-1 intense (polar) very broad (H bonding)

O H O

H

decreasing bond strength

> N-H

3400-3200 less intense less broad NH2 has 2 NH has 1

> CspH

3300

> Csp2H

3100 -3000

> Csp3H 3000 -2850

CH

O

> 2830 -2700 two bands

3000 cm-1 divides sp2 and sp3

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Triples enlaces

C N C C

2260-2200 more intense

2150-2100 less intense less polar

For symmetrical alkynes, this band can be very weak.

CH3CH2 C C CH2CH3

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Double Bond Region

C

O

C

N

C

C

~1700 cm-1 very intense (polar) very useful

lower wavenumber (uncommon)

1660-1640 less intense (nonpolar)

four bands near 1600, 1580, 1500, and 1450 variable position and intensity (use caution)

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Csp3-H (<3000)

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3000 divides Csp2-H and Csp3-H

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Csp-H

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Csp2-H Csp3-H

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2. 13C Magnetic Resonance Spectroscopy

Apariencia

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Cuántas señales debe dar? Simule el espectro

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CH3

CH3

CH3

CH3

CH3

CH3

A cuál xileno corresponde?

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Chemical shift position on x-axis info about C to which the H is bonded

multiplicity number of peaks in group info about nearby H’s

Integral area under a peak is proportional to the number of H’s

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Inductive Effects

X C HElectronegative X pulls electrons away from H; H is deshielded; appears more downfield

C is slightly more electronegative than H

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Table 14.1

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Simule el espectro del siguiente compuestos

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DU = 1

Qué compuesto es? Cómo se calcula el IDH?

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