Infrared spectroscopy Probing molecular structure based on bond vibrations Unlike UV-visible spectroscopy, which examines transitions between electronic states (promotion of electrons from one orbital to another), we are interested in absorption of light to induce bond vibration. This is also quantized
hν = ΔEexcited-ground
IR terminology As with UV-vis we are interest in: 1) What is the energy of a transition (E=hυ)? υ
) •instead of l, usually express as wavenumber ( ~ ~ υ
= 1/λ = υ/c 2) How likely is the transition to occur (ε)– this is measured by how intense the peak is, usually expressed as %tranmittance (low %t=intense)
Balls and springs
Hooke’s Law:
1 υ= 2πc
K µ
m1 ⋅ m 2 µ= m1 + m 2
reduced mass
What Hooke’s Law tells us… C-H oop
1. The larger the force constant (i.e. the
stronger the bond), the higher the frequency of vibration and the higher the frequency of light absorbed to cause this vibration. All else being equal:
C-H rocking 1000-1300 cm-1
H H
H
H C-H 3036 cm-1
H H
C-C 1479 cm-1
νsingle bonds < νdouble bonds < νtriple bonds 2. The smaller the masses of the balls (atoms), the higher the frequency:
νC-H > νC-D >> νC-O
D D
D
D
C-H 2381 cm-1
D D
C-C 1330 cm-1
Overtones and Combination bands vibrating strings fundamental υ
combination bands
2 • υ
H H
H
H
H H
3 • υ
observe bands that are integer values of fundamental υ (overtones)
Combination bands from two different vibrations coupled together = υ1+ υ2 Overtone and combination bands lower intensity than fundamental frequencies. Occur at slightly lower than integral frequencies
Dipole moments
no C=C band
C=C band
CH3
CH3
Only vibrations that give rise to changes in dipole will be “IR active” (allowed transitions). +q
-q d
+q
-q d
Dipole moment (µ) = q • d
C=O band (~1700 cm-1) O
large Δµ
More dipoles….
High symmetry lowers ε (lower A) N
Vibrational modes Linear triatomic
Bent triatomic
νs A
νs stretch B
IR-active only if A≠B
x
x
νas stretch degenerate
Scissoring or bending
νas IR-active: 2350 cm-1 in CO2
Methane
νs stretch
rocking
νas stretch
scissoring
twisting
wagging
Alkanes CH3 (C20H42)
CH3 CH stretch
CH2 rock ~720 cm-1
CH2 rock
weak CC (pseudosymmetric as chains get longer)
no CH2 rock? branching increases CC bands
Alkanes CH2 rocking CH3 CH2 rock
(C20H42)
CH3
weak CC (pseudosymmetric as chains get longer)
CH stretch
H
CH2 rock ~720 cm-1
HH
H
H
H H
H
CH2 rock: concerted motion increases in intensity as linear chain length increases without branching
Cycloalkanes
No CH2 rocking
CH3 groups only present in side chains
Infrared spectroscopy - SFU
Infrared spectroscopy Probing molecular structure based on bond vibrations Unlike UV-visible spectroscopy, which examines transitions between electron...