Molecular Spectroscopy - Advanced Analytical Chemistry - Lecture Slides, Slides of Analytical Chemistry

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Molecular Spectroscopy

Spectroscopy

(^) = methods based on the

(EM) and matterinteraction of electromagnetic radiation

Electromagnetic Radiation = form of energy with both wave and particle properties

EM moves through space as a wave

terms of electric vectorbest understood inEM with matter are Most interactions of

Dispersion CurveA Typical

showing

(^) η (^) as

frequencya function of

(^) ν

( νννν )

(η)

Normally

(^) η (^) increases with

(^) ν (^) and this is

referred to as “Normal Dispersion”

When absorption occurs, the nature of the interaction changes

η slows downelectric vector interacts with the medium &^ is a measure of the extent to which the

For a given frequency and medium, a larger η (^) means more interaction with electric field

polarizability & the medium is said to have greater

(^) i.e., is more able to follow

the electric vector

Dispersion CurveA Typical η 2) regions where ν increases with absorbtion bandprior to an

This is referred to as normal dispersion

( νννν )

(η)

Dispersion CurveA Typical η 3) regions where ν decreases with absorption bandsvery sharply at

because the nature of the interaction changesThis is referred to as anomalous dispersion

( νννν )

(η)

Refractive indices (

η ) for various substances at

589 nm (the sodium D line)

Substance

η

air

water, 20

(^) o C

NaCl crystal

benzene

quartz (fused)

glass (crown)

ethyl alcohol

carbon disulfide

Refraction

(^) = change in velocity of EM as it

goes from one medium to another

to surfaceNormal

Velocity largerMedium 1 (air)

(^) η (^) = 1.

Velocity smallerMedium 2 (glass)

(^) η (^) = 1.

rayIncident

Ф 1 Ф 2

ray Refracted

directionOriginal

normalRay bent toward

Critical Angle

Ф

C )

to surface Normal

Medium 2 (glass)Medium 1 (air)

90 ray isIncident o to

grazing incidencethis is callednormal

Ф 1 Ф 2

ray Refracted

Maximum value of

(^) Ф 2

Ф (^2) = (^) Ф C At 90

o incidence sin

(^) Ф (^1) = 1.

η 1

sin (^) Ф C = ----- η 2

2. Fiber Optics1) RefractometersImportant for:

• waves must have similarWave Interaction - interaction between waves

(^) ν (^) but can be out of

phase (i.e., they start in different places)

• wave yPrinciple of superposition = vectors add

(^1) +y

2 formed by adding y

1 & y

2 by

vector addition

Interference

(^) - amplitude of the resulting

wave depends on phase difference

(^) α (^1)

α 2

⇒Interference Constructive waves add

Destructive Interference

(^) ⇒

(^) waves cancel

At (^) α (^1)

α (^2) = 0

o adding of waves gives

Maximum Constructive Interference

(^0) o

180 o 360 o 540 o 720 o 900 o

is zeroWave 1 & Wave 2difference betweenPhase angleResultant waveWave 2Wave 1

(^) α (^1)

α (^2) = 0 o

Amplitude

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When

(^) α (^1)

α (^2) = 180

o or 540

o adding of waves

gives Maximum Destructive Interference

(^0) o

180 o 360 o 540 o 720 o 900 o

Resultant waveWave 2 Wave 1

is 180Wave 1 & Wave 2difference betweenPhase angle

o (α (^1)

α (^2) = 180

)o

Amplitude

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Fourier Analysis

(^) – mathematical process of

various component frequencies.and a computer to figure out amplitudes ofNMR. Requires complex mathematicsTransform (FT) methods like FTIR and FTintegral and is important in all Fourierfrequencies. This requires a Fouriervarious frequencies into their individualresolving a combination of waves of