IR Spectroscopy Flashcards
spectroscopy
the use of electromagnetic radiation to probe matter
what does the amount of energy absorbed depend on
the change in the dipole moment
how does a large change in the dipole affect amount of energy absorbed
larger amount of energy is absorbed and it exhibits stronger absorption peaks
how are nonpolar bonds affected by infrared radiation
they do not absorb infrared radiation because there is no change in the dipole moment
how are relatively nonpolar bonds like C-C bonds affected by IR spectroscopy
absorb less energy and exhibit weak absorption peaks
acronym for remembering increasing —> decreasing electronegativity
FONClBrISCH
what is the dipole
difference in electronegativity
zone 1 range
3600 - 2800 cm-1
what groups does zone 1 include
carboxylic acids, alcohols, amines, C-H
what is zone 2 range
2300-2100 cm-1
what groups does zone 2 contain
alkynes, nitriles
what range does zone 3 have
1850 - 1600 cm-1
what groups does zone 3 contain
carbonyls, alkenes, benzene
zone 4 range
< 1400 cm-1
what groups does zone 4 have
C-O, -N, -X single bonds
characteristic of O-H and N-H vibrations
intense and broad signal around 3300 range in zone 1
characteristics of C-H vibrations
triple peaked sharp medium signal near 2900-3000 range in zone 1
how to tell the difference between IR spectroscopy absorption pattern for alcohols vs carboxylic acids
- carboxylic acids have a broader O-H section due to extensive hydrogen bonding, while alcohols have both broad and sharp O-H sections
- carboxylic acids have an intense zone 3 absorption for C=O, but alcohols do not
characteristics of primary amine zone 1 absorption
twin set of peaks around 3360 and 3280 (associated with different vibrational modes available for hydrogen bonds) plus a sharp intense C-H spike around 3000
characteristic zone 1 and 2 IR absorption for terminal alkyne
shark intense peaks at 3300 mark (or to the left of 3000 mark)
characteristic zone 1 and 2 IR absorption for terminal alkenes
sharp little peak just to the left of 3000 mark, and steep three peaks just to the right of 3000 mark
characteristics zone 1 IR absorption for alkane
nothing in zone 1, three sharp and intense peaks just to the right of 3000 mark
characteristics of zone 1 and 2 IR absorption for aldehyde C-H bonds
nothing to the left of 3000, two distinct peaks around 2700 and 2800
characteristics of zone 2 IR absorption for internal alkyne
weak dipole moment at 2050
characteristics of zone 2 IR absorption for internal alkynes
sharp medium intensity peak at 2100
characteristics of zone 2 IR absorption for terminal C N triple bonds
sharp intense peak around 2100
zone 3 IR absorption characteristics for C=O
sharp intense peak
how does resonance affect wave numbers
less double bond character means lower wave number
what kind of resonance delocalization do aldehydes and ketones have
none
what kind of resonance delocalization do esters have
minor resonance delocalization
what kind of resonance delocalization do amides have
significant resonance delocalization of carbonyl p bond
how does carbonyl bond strength compare to aldehydes/ketones/esters
it is weaker
zone 3 IR absorption characteristics of alkenes
no C=C absorption for trans alkenes, but small sharp spike in 1600 range for cis alkenes; presence of a halide produces a large dipole with several intense sharp signals
zone 3 IR absorption characteristics for benzene
- overtones in 2000-1900 range (not in zone 1 or zone 2)
- 2-3 peaks at 1600, 1400, and 1450 in zone 3 due to C-C stretching in aromatic ring
how is IR spectroscopy used
absorption of IR radiation causes covalent bonds to change from a lower vibration state to a higher one
function of IR spectroscopy
determining the kinds of functional groups in a molecule
wavenumbers
the frequency of electromagnetic radiation expressed as the number of waves per cm, with units cm-1
