Elusidasi Struktur. (2 SKS) P3: Spektroskopi Infra Merah (IR) Genap 2021

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Elusidasi Struktur

(2 SKS)

P3: Spektroskopi Infra Merah (IR)

Genap | 2021

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Materi:

P Materi Tanggal

1. Pendahuluan

2. Spektroskopi UV-Vis

3. Spektroskopi IR 25.03.2021

4. Spektroskopi IR (2) 5. Spektroskopi MS 6. Spektroskopi MS (2) 7. Review

8. UTS

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Spektroskopi IR

Hadi | Elusidasi Struktur | 2021 3

(1). acetophenone (2). cyclohexanone (3) 1,4-dioxane (4) δ-valerolactone Prinsip dasar:

identifikasi gugus fungsional

(4)

IR radiation:

 IR dekat: 0,7 – 2,5 μm; menengah: 2.5 – 50 μm; jauh: 50 – 1000 μm.

(5)

Basic principle:

 Energi IR mampu menyebabkan molekul ber- vibrasi pada tingkat vibrasi tertentu.

 Deteksi gugus fungsional.

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elektronik  vibrasi  rotasi

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Jenis vibrasi

 Inti-inti atom terikat oleh ikatan kovalen mengalami getaran (vibrasi/osilasi)

 Jenis vibrasi

1. Vibrasi Regangan 2. Vibrasi Tekuk

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Vibrasi regangan

 Vibrasi yang menyebabkan perubahan terus menerus pada jarak ikatan

 Jenis:

1. simetris (symmetrical

stretching)

2. asimetris (asymmetrical

stretching)

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Vibrasi Tekuk

 Vibrasi yang menyebabkan perubahan sudut ikatan.

 Jenis:

1. Rocking 2. Scissoring 3. Twisting 4. Wagging

(10)

Contoh vibrasi:

 Molekul linear



3n-5 jenis vibrasi.

 Molekul non-linear



3n-6 jenis vibrasi.

 Contoh: H

₂

O

Mempunyai 3 atom non linear  3n – 6

= 3 (3) – 6 = 3 jenis vibrasi

(11)

Tingkat energi vibrasi

asymmetric stretch

symmetric stretch

bend

(12)

Dasar Spektroskopi IR

 Bila ikatan bergetar, maka energi vibrasi secara terus menerus dan secara periodik berubah dari energi kinetik ke energi potensial dan sebaiknya.

 Hukum Hooke: Jumlah energi total pegas adalah sebanding dengan:

– frekuensi vibrasi dan tetapan gaya (k) pegas

– massa (m₁ dan m₂) dari dua massa yang terikat.

(13)

Deskripsi matematis energi vibrasi

13 Hadi | Elusidasi Struktur | 2021

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Energi Vibrasi: momen dipol

 Hanya senyawa dengan momen dipol tertentu yang dapat menyerap radiasi IR.

 Ikatan yang tidak menyerap :

1. Alkena dan alkuna

tersubtitusi yang simetris 2. Molekul diatomik yang

simetris

(15)

Energi Vibrasi: momen dipol

 Beberapa gugus fungsi

menyerap kuat radiasi infra merah:

1. Gugus karbonil merupakan absorber terkuat

2. Begitu juga dengan –O-H dan –C-O-

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Instrumentasi

 Komponen utama spektrometer IR :

– Sumber radiasi IR – Monokromator

– Sel sampel – Detektor

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Sumber radiasi

 Sumber harus memancarkan radiasi yang harus cukup intens untuk deteksi sampel

 Stabil

 Memiliki panjang gelombang yang diinginkan .

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INCANDESCENT LAMP:

tungsten filament Longer life

Hadi | Elusidasi Struktur | 2021

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Monokromator

 Memilih frekuensi yang diinginkan dari sumber.

 Ada dua jenis:

– Prism Monochromator

– Grating Monochromator

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Sel sampel

 Material berisi sampel harus transparan terhadap radiasi IR

 Sel harus sangat sempit  0,01 – 1 mm

 Contoh: Garam seperti natrium klorida dan kalium bromida banyak digunakan

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Detektor

 Bolometer

 Thermocouple

 Photo conductivity cell

 Semiconductor detectors

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Double beam spektrometer IR

21 Hadi | Elusidasi Struktur | 2021

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Preparasi sampel

 Menggunakan window material, berupa alkali halida seperti NaCl, atau KBr

 Teknik pelet KBr: padatan sampel digerus dalam mortal kecil bersama dengan kristal KBr kering dalam jumlah sedikit sekali (0,5 – 2 mg cuplikan + 100 mg KBr kering). Campuran

tersebut kemudian dipress dengan alat penekan hidrolitik hingga menjadi pelet yang transparan.

 Tablet cuplikan tipis tersebut kemudian

ditempatkan di tempat sel spektrofotometer

(23)

Senyawa yang mampu berinteraksi dg radiasi IR

 Hanya vibrasi yang meyebabkan perubahan moment dipole molekul dan memiliki absorpsi frekuensi pada daerah IR

 Molekul gas, misal O₂, H₂, dan Cl₂, tidak memiliki momen dipole, sehingga tidak ada serapan IR yang terjadi

 SO₂ dan CO₂memiliki moment dipole, sehingga memberikan serapan IR

 Setiap gugus fungsi memiliki serapan IR tersendiri

 Semakin kompleks struktur molekul, semakin kompleks serapan IR-nya

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Spektra IR

 Intensitas absorpsi (%T) vs bilangan gelombang (cm^-1).

 Bilangan gelombang:

,

(25)

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Identifikasi senyawa

 Prinsip:



Membandingkan spektrum.



Tidak ada dua sampel akan memiliki spektrum yang IR identik.

 Kriteria: contoh dan referensi harus diuji dalam kondisi yang sama, seperti keadaan fisik, suhu, pelarut, dll

 Kelemahan: enantiomer tidak dapat dibedakan

(spektrum adalah identik)

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Syarat spektra

 adequately resolved and of adequate intensity.

 spectrum of a pure compound.

 The spectrophotometer should be calibrated.

 The method of sample handling must be specified.

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Interpretasi spektra

 Pengamatan pertama kali ditujukan pada puncak yang berada di daerah bilangan gelombang 4000-1500 cm^-1

 determinasi gugus fungsi,

 Daerah sebelah kanan 1500 cm^-1 disebut dengan

daerah sidik jari (fingerprint region). Daerah sidik jari akan sangat khas untuk masing-masing senyawa.

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Daerah "Sidik Jari“ ( fingerprint )

 1500-700 cm

^-1

 perbedaan kecil dalam struktur dan komposisi molekul menghasilkan perubahan puncak yang signifikan di wilayah ini.

 wilayah ini membantu untuk mengidentifikasi suatu senyawa yang tidak diketahui

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Absorption regions

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IR bands

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A comparison of the shapes of the absorption bands for the O-H and N-H groups

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Typical infrared absorption regions

2.5 4 5 5.5 6.1 6.5 15.4

4000 2500 2000 1800 1650 1550 650

FREQUENCY (cm^-1) WAVELENGTH (mm)

O-H C-H N-H

C=O C=N Very

few bands

C=C

C-Cl C-O C-N X=C=Y C-C

(C,O,N,S) C N

C C

N=O N=O *

(36)

O-H

2.5 4 5 5.5 6.1 6.5 15.4

4000 2500 2000 1800 1650 1550 650

O-H C-H N-H

C=O C=N Very

few bands

C=C

C-Cl C-O C-N X=C=Y C-C

(C,O,N,S) C N

C C

N=O N=O*

(37)

The O-H stretching region

 O-H



3600 cm

^-1

(alcohol, free)

 O-H



3300 cm

^-1

(alcohols and acid, H-bonding)

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3600 3300

H-BONDED FREE

broadens

shifts

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The O-H: bonded

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The O-H: free

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The O-H: as acid

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O-H OH H-bond

C-H

C-O CH₂

ALCOHOL

,

Cyclohexanol

(42)

N-H

2.5 4 5 5.5 6.1 6.5 15.4

4000 2500 2000 1800 1650 1550 650

O-H C-H N-H

C=O C=N Very

few bands

C=C

C-Cl C-O C-N X=C=Y C-C

(C,O,N,S) C N

C C

N=O N=O*

(43)

The N-H stretching region

 N-H  3300 - 3400 cm

^-1

 Primary amines give two peaks

 Secondary amines give one peak

 Tertiary amines give no peak

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N H

H N H

symmetric asymmetric

H

(44)

CH₃ CH₂ CH₂ CH₂ NH₂ NH₂

NH₂ scissor

CH₂

CH₃

PRIMARY AMINE aliphatic

1-Butanamine

(45)

NH CH₂ CH₃ NH

benzene Ar-H

CH₃

SECONDARY AMINE

N-Ethylbenzenamine

(46)

N CH₃

CH₃

no N-H

benzene

CH₃

Ar-H Ar-H

-CH₃

TERTIARY AMINE

N,N-Dimethylaniline

(47)

C-H

47

2.5 4 5 5.5 6.1 6.5 15.4

4000 2500 2000 1800 1650 1550 650

O-H C-H N-H

C=O C=N Very

few bands

C=C

C-Cl C-O C-N X=C=Y C-C

(C,O,N,S) C N

C C

N=O N=O*

(48)

The C-H stretching region

 BASE VALUE  3000 cm

^-1

 C-H sp stretch   3300 cm

^-1

 C-H sp

³

stretch  < 3000 cm

^-1

 C-H aldehyde, two peaks (both weak)

 ~ 2850 and 2750 cm

^-1

3000 divides UNSATURATED

SATURATED

(49)

3000

-C-H

=C-H 3100

3300

=C-H=

2900 2850 2750

-CH=O (weak)

increasing CH bond strength sp³- 1s sp²- 1s

sp - 1s

increasing frequency (cm^-1)

aldehyde

increasing s character in bond

CH BASE VALUE = 3000 cm^-1

Stronger bonds have larger force constants and absorb at higher frequencies

(50)

The C-H bending region

 CH

₂

bending  ~ 1465 cm

^-1

 CH

₃

bending (asym)  ~ 1460 cm

^-1

 CH

₃

bending (sym)  ~ 1375 cm

^-1

(51)

C

H H

C

H H

C

H H C

H H

C

H H

C

H H

Scissoring Wagging

Rocking Twisting

Bending Vibrations

~1465 cm^-1

~720 cm^-1

~1250 cm^-1

in-plane out-of-plane

Methylene group bending vibrations

(52)

CH₃ CH₂ CH₂ CH₂ CH₂ CH₃

CH

stretch

CH₂ bend

CH₃ bend

CH₂ rocking

ALKANE

Hexane

(53)

C  N and C  C

53

2.5 4 5 5.5 6.1 6.5 15.4

4000 2500 2000 1800 1650 1550 650

O-H C-H N-H

C=O C=N Very

few bands

C=C

C-Cl C-O C-N X=C=Y C-C

(C,O,N,S) C N

C C

N=O N=O*

(54)

The triple bond stretching region

 The cyano group often gives a strong, sharp peak due to its large dipole moment.

CN  2250 cm

^-1

 The carbon-carbon triple bond gives a sharp peak, but it is often weak due to a lack of a dipole.

CC  2150 cm

^-1

This is especially true if it is at the center of a symmetric molecule.

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CH₃ CH₂ C N CN

NITRILE BASE = 2250

Propanenitrile

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2.5 4 5 5.5 6.1 6.5 15.4

4000 2500 2000 1800 1650 1550 650

O-H C-H N-H

C=O C=N Very

few bands

C=C

C-Cl C-O C-N X=C=Y C-C

(C,O,N,S) C N

C C

N=O N=O*

C=O

(57)

The carbonyl stretching region

 This region stretches from about 1800 to 1650 cm

^-1 - RIGHT IN THE MIDDLE OF THE SPECTRUM

 BASE VALUE



1715 cm

^-1

(ketone)

 The bands are very strong !!! due to the large C=O dipole moment.

 C=O is often one of the strongest peaks in the spectrum

(58)

CH₃ C CH₂ CH₃ O

KETONE

C=O C-H

overtone 2x C=O

CH bend

BASE = 1715 1715

C=O

2-Butanone

(59)

C R

O H

C R

O

O C R O

C R

O

Cl R C O

OR' R C

O

R R C

O

NH₂ C

R

O

OH

1690 1710

1715 1725

1735 1800

1810 and 1760

BASE VALUE acid

chloride ester aldehyde

carboxylic

acid amide

ketone

anhydride

( two peaks )

EACH DIFFERENT KIND OF C=O COMES AT A DIFFERENT FREQUENCY

C=O is sensitive to its environment

THESE VALUES ARE WORTH LEARNING

all are +/- 10 cm^-1⁵⁹

(60)

C=C

2.5 4 5 5.5 6.1 6.5 15.4

4000 2500 2000 1800 1650 1550 650

O-H C-H N-H

C=O C=N Very

few bands

C=C

C-Cl C-O C-N X=C=Y C-C

(C,O,N,S) C N

C C

N=O N=O*

(61)

The C=C stretching region

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 C=C double bond at 1650 cm

^-1

is often weak or not even seen.

 C=C benzene ring shows peak(s) near 1600 and 1400 cm

^-1

, one or two at each value -

CONJUGATION LOWERS THE VALUE.

 When C=C is conjugated with C=O it is stronger and comes at a lower frequency

(62)

CH₂ CH CH₂ CH₂ CH₂ CH₃ ALKENE

oops C=C

=C-H

C-H

aliphatic

C-H bend

1-Hexene

(63)

C-O, N=O, and C-X

2.5 4 5 5.5 6.1 6.5 15.4

4000 2500 2000 1800 1650 1550 650

O-H C-H N-H

C=O C=N Very

few bands

C=C

C-Cl C-O C-N X=C=Y C-C

(C,O,N,S) C N

C C

N=O N=O*

(64)

The C-O stretching region

 The C-O band appears in the range of 1300 to 1000 cm

^-1

 Look for one or more strong bands appearing in this range!

 Ethers, alcohols, esters and carboxylic acids

have C-O bands

(65)

CH₃ CH₂ CH₂ CH₂ O CH₂ CH₂ CH₂ CH₃ ETHER

C-O

BASE = 1100

CH₂ CH₃ bending C-H

Dibutyl Ether

(66)

The N=O stretching region

 N=O stretching  1550 and 1350 cm

^-1

asymmetric and symmetric stretchings

 Often the 1550 cm

^-1

peak is stronger than the

other one

(67)

CH₃ CH

CH₃ NO₂

NITROALKANE

N=O

N=O C-H

gem-dimethyl

2-Nitropropane

(68)

The C-X stretching region

 C-Cl 785 to 540 cm

^-1

, often hard to find amongst the fingerprint bands!!

 C-Br and C-I appear outside the useful range of infrared spectroscopy.

 C-F bonds can be found easily, but are not that

common.

(69)

C

Cl Cl

Cl

C-Cl

Carbon Tetrachloride

Often used as a solvent for IR spectra.

When it is used, spectra show C-Cl absorptions.

(70)

Simple approach to the analysis of spectrum at a glance (General Rules)

 Look if carbonyl group is present?

The C=O gives strong absorption in the region 1820- 1660 cm

^-1

 If C=O is present, check for the presence of

following group...

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Acids (O-H)

a broad band near 3400-2400 cm^-1

Amides (N-H)

medium absorption near 3400 cm^-1 & some times double dips with equivalent halves

Ester (C-O)

strong absorption near 1300-1000 cm^-1.

Anhydrides (C-O)

gives two C-O absorption near 1810-1760 cm^-1

Aldehyde (C-H)

show two weak absorption near 2850 and 2750 cm^-1.

Ketones (C-R)

If none of the above is present, conforms the presence of ketones

(72)

If C=O is absent,

 Alcohols and Phenols: Check for O-H, show broad absorption near 3400-3300 cm

^-1

. Confirm this by finding C-O near 1300-1000 cm

^-1.

 Amines: N-H stretch, show medium absorption near 3400 cm

^-1

 Ethers: C-O near 1300

^-1

-1000cm

^-1

, and also absence

of O-H near 3400 cm

^-1

(73)

 Double bonds and / or Aromatic ring:

– C=C is a weak absorption near 1650 cm

^-1

– Medium to strong absorption 1600-1450 cm

^-1

– Aromatic C-H occurs to left of the 3000 cm

^-1

– Aliphatic C-H occurs to right of 3000 cm

^-1

 Triple bonds:

– C≡N medium, sharp absorption near 2250 cm

^-1

– C≡C weak sharp absorption near 2150 cm

^-1

(74)

 Nitro groups

– Two absorptions at 1600-1530 cm

^-1

and 1390- 1300 cm

^-1

 Hydrocarbons

– None of the preceding is found

– Major absorptions are in C-H region near 3000 cm

^-1

– Very simple spectrum

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