Flashcards in Vibrational (Infrared) spectroscopy Deck (37)

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1

## What does the absorption involve?

### Absorption of infrared energy involves the vibrations of chemical bonds, vibration means lots of things: stretch, bend etc

2

## What unit used on spectra?

### Typically uses wavenumber on the “energy” axis – 4000 cm -1 – 400 cm -1, corresponding to the change in energy 12 kJ mol-1

3

## Vibration of a chemical bond?

###
Treat the chemical bond as a spring, If the spring is extended or compressed, there is a restoring force, F, Simple harmonic motion: Hookes Law, Force, F, is proportional to the extension, x F = -kx

Energy, E = 1⁄2 k x2

k is the force constant of the spring

4

## What is force constant?

### related to the stiffness of the spring (or strength of bond); how easy it is to set into motion

5

## Typical molecular vibrational energies?

### 10-19 - 10-20 J, infrared frequencies

6

## Energy level of a harmonic oscillator?

###
Energy levels are equally separated

All vibrational levels have degeneracy =1

The lowest energy possible, v=0, is NOT zero

This is called zero point energy

7

## Gross selection rule in vibrational spectroscopy?

### The dipole moment of a molecule must change during the vibration

8

## Specific selection rule in vibrational spectroscopy?

### Only transitions between adjacent energy levels can occur, delta v = ±1

9

## Homonuclear diatomics?

### Not IR active

10

## Heteronuclear diatomics?

### IR active

11

## Polyatomics?

### Some modes IR active some IR inactive

12

## How do we find the number of vibrational modes?

###
We use a 3-D geometry to define molecules in space, the position of each atom in a molecule can be given in terms of three measurements,

1 atom = 3 degrees of freedom

2 atoms = 6 degrees of freedom

3 atoms = 9 degrees of freedom

N atoms = 3N degrees of freedom

A degree of freedom (DOF) is an independent mode of position or motion in a molecule

13

## Translational motion?

### Always 3 modes of translational motion (doesn’t matter how many atoms)

14

## Rotational motion?

###
2 cases

Non linear, 3 axes = 3 DOF

Linear, 2 axes = 2 DOF

rotation about the x axes does not change positions of any atoms

15

## Vibrational motion?

###
The other degrees of freedom (to make total up to 3N) are taken up by vibrational motion within molecules

Total Degrees of freedom = 3N for N atoms

Always 3 trans.+ either 2 or 3 rot = 5 or 6 (trans. + rot.)

Linear (3N – 5) vibrational modes

Non linear (3N – 6) vibrational modes

An independent mode of vibration (one that does not influence another mode) is called a normal mode

16

## Vibrational spectroscopy for complex molecules?

### As the number of atoms increase, the number of peaks increases rapidly, Usually not possible to assign all the peaks but can use the fingerprint region to identify characteristic frequencies

17

## Effect of isotopic substitution?

### As for rotational spectroscopy, E vibrational depends on reduced mass so will change if different isotopes are present, k depends on the electron distribution in the bond - adding neutrons to a nucleus has little effect on electron density, assume isotopic substitution does not change the force constant

18

## How does changing from H to D affect the reduced mass?

### The bond vibration of HCl gives an absorption at 2990 cm–1, replacing H by deuterium, D (2H), changes the reduced mass, this gives the D–Cl absorption at lower wavenumber, 2140 cm-1, Largest shifts occur for H → D but significant for other elements – can be used to confirm the presence or otherwise of elements in a compound

19

## The selection rule for anharmonic motion?

### The selection rule delta v = ± 1 strictly works only for harmonic motion, weak bands can be seen at 2hv0 3hv0 etc these are overtone bands

20

## Motion of a bond?

###
The motion of a bond is not harmonic. At large displacements, atoms repel (at short distances) or bonds break (at long distances).

Anharmonicity effects --> separation between levels is not constant

21

## More realistic motion of bond?

### Anharmonic not harmonic as classically thought

22

## Anharmonic effect on the curve of energy levels?

###
Energy – distance curve isn’t a true parabola - ”Morse” curve

If push the atoms together, repulsion increases - steeper slope

If we pull the atoms apart, the bond breaks - dissociation energy

23

## What does solution of the Schrödinger equation for the Morse potential show for the selection rule?

### The selection rule delta v = ± 1 is not strictly obeyed, overtones appear with delta v = ± 2 and delta v = ± 3

24

## What does solution of the Schrödinger equation for the Morse potential show for the vibrational levels?

### Vibrational levels are not equally spaced, separation gets smaller with increasing v and becomes = 0 at the limit

25

## Equation for better description of energy levels?

### Ev = (v + 1⁄2) hv – A (v + 1⁄2)2 hv

26

## Why So Many Peaks for Vibrational-Rotational Spectra?

### Transitions occur simultaneously between vibrational and rotational levels

27

## What does IR detect?

### Detects changes in the vibrational energy of molecules

28

## How can diatomic molecules be treated?

### Diatomic molecules can be treated as harmonic or (better) anharmonic oscillators which allows calculation of bond force constant and bond dissociation energy

29

## What do triatomic molecules show?

### Triatomic molecules show overtone and combination bands

30