IR Chemistry

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IR chemistry in easy way.

Transcript of IR Chemistry

  • Spectroscopyseeing the unseeable

    Using electromagnetic radiation as a probe to obtain information about atoms and molecules that are too small to see.

    Electromagnetic radiation is propagated at the speed of light through a vacuum as an oscillating wave.

  • electromagnetic relationships:

    = c 1/

    E = h E

    E = hc/ E 1/

    = wave length

    = frequency

    c = speed of light

    E = kinetic energy

    h = Plancks constant

    c

  • some electromagnetic radiation ranges

    Approx. freq. range Approx. wavelengths

    Hz (cycle/sec) meters

    Radio waves 104 - 1012 3x104 - 3x10-4

    Infrared (heat) 1011 - 3.8x1014 3x10-3 - 8x10-7

    Visible light 3.8x1014 - 7.5x1014 8x10-7 - 4x10-7

    Ultraviolet 7.5x1014 - 3x1017 4x10-7 - 10-9

    X rays 3x1017 - 3x1019 10-9 - 10-11

    Gamma rays > 3x1019 < 10-11

  • Provides information about the vibrations of functional groups in a molecule

    Infrared Spectroscopy

    Therefore, the functional groups present in a molecule can be deduced from an IR

    spectrum

  • Infrared radiation

    = 2.5 to 17 m

    = 4000 to 600 cm-1

    These frequencies match the frequencies of covalent bond stretching and bending vibrations. Infrared spectroscopy can be used to find out about covalent bonds in molecules.

    IR is used to tell:

    1. what type of bonds are present

    2. some structural information

  • IR source sample prism detector

    graph of % transmission vs. frequency

    => IR spectrum

    4000 3000 2000 1500 1000 500

    v (cm-1)

    100

    %T

    0

  • E2

    E2

    DE = hnE2

    E1

    DE = hn

    Frequency

    Intensity

    nFrequency

    Intensity

    n

    Absorbance Emission

    7

  • The functional group concept of organic chemistry

    IR spectroscopy can identify functional groups!

  • Some Important and Characteristic Infrared Absorption Frequencies and Wavelengths for Some Common Stretching Motions

    Atom Group Typical of Frequency (cm-1) Wavelength (m) O-H (free) Alcohols (dilute) 3550-3650 cm-1 2.8 m O-H (H bonded) Alcohols

    (concentrated) Carboxylic acids

    3200-3400 cm-1 3.0 m

    C C H Acetylene (CH) 3300 cm-1 3.0 m C C H Benzene (CH),

    Ethylene (CH) 3010-3100 cm-1 3.3 m

    C_____C_____H Ethane (CH) 2950-3000 cm-1 3.5 m C C Acetylene 2100-2260 cm-1 4.5 m C N Nitriles 2000-2300 cm-1 4.5 m C O Carbonyl 1650-1750 cm-1 5.5- 6.0 m C C Alkene 1620-1680 cm-1 6.0 m C C Alkane 600-1500 cm-1 6.7-17 m C O Alcohols,

    Ethers 1000-1300 cm-1 10-7.7 m

    In general we will only be using the data in an IR spectrum for stretching vibrations which have energies higher than 1620 cm-1. Although the bands at lower energy are known and assigned, the region below 1620 cm-1 is very congested with single bond stretches of two heavy atoms (see C-C and C-O in table) and C-H bends and are beyond the scope of what we want to do.

  • IR spectra of ALKANES

    CH bond saturated

    (sp3) 2850-2960 cm-1

    + 1350-1470 cm-1

    -CH2- + 1430-1470

    -CH3 + and 1375

    -CH(CH3)2 + and 1370, 1385

    -C(CH3)3 + and 1370(s), 1395 (m)

  • IR of ALKENES

    =CH bond, unsaturated vinyl

    (sp2) 3020-3080 cm-1

    + 675-1000

    RCH=CH2 + 910-920 & 990-1000

    R2C=CH2 + 880-900

    cis-RCH=CHR + 675-730 (v)

    trans-RCH=CHR + 965-975

    C=C bond 1640-1680 cm-1 (v)

  • IR spectra BENZENEs

    =CH bond, unsaturated aryl

    (sp2) 3000-3100 cm-1

    + 690-840

    mono-substituted + 690-710, 730-770

    ortho-disubstituted + 735-770

    meta-disubstituted + 690-710, 750-810(m)

    para-disubstituted + 810-840(m)

    C=C bond 1500, 1600 cm-1

  • IR spectra ALCOHOLS & ETHERS

    CO bond 1050-1275 (b) cm-1

    1o ROH 1050

    2o ROH 1100

    3o ROH 1150

    ethers 1060-1150

    OH bond 3200-3640 (b)

  • nn--Hexane: CHHexane: CH33CHCH22CHCH22CHCH22CHCH22CHCH33

    11--Hexene: CHHexene: CH22=CHCH=CHCH22CHCH22CHCH22CHCH33

    1650 cm1650 cm--11

    3100 cm3100 cm--11

    ??

  • IR: Masses, Atoms and Springs

    F = force, restoring back to equilibrium position

    k = characteristic stretching constant

    x = displacement from the equilibrium position

    A Model: Picture the atoms of a diatomic molecule as point masses connected by springs (bonds).

    As a first approximation use Hookes Law

    F = -kx

  • n =1

    2pk

    mrmr =

    m1m2m1 + m2

    IR Stretching Frequencies of two bonded atoms:

    n = frequencyk = spring strength (bond stiffness)mr = reduced mass (~ mass of largest atom)

    What Does the Frequency, n, Depend On?

  • Internuclearseparation

    Heavy atomsand /or

    weak bonds

    Internuclearseparation

    Light atomsand/or

    strong bonds

    X Y X Y

    X Y X Y

    X YX Y

    PE

    r re re

    a b

    c d

    F = dPE/dr

    Vibrations, potential energy and motion

  • IR Stretching Frequencies: What Do they Depend On?

    Directly on the strength of the bonding between the two atoms (n ~ k)

    Inversely on the reduced mass of the two atoms (v ~ 1/m)

    Expect: n will increase with increasing bond strength (bond order) and decreasing mass

  • Examples of stretching frequencies and correlations with bond strengths (bond order)

    CC--CC

    C=CC=C

    C C

    Bond strength*

    350350

    600600

    840840

    Bond order

    11

    22

    33

    nn

    2200 cm2200 cm--11

    1600 cm1600 cm--11

    1000 cm1000 cm--11

    *In kJ/mol*In kJ/mol For same reduced mass!For same reduced mass!

  • Quantum mechanics: The frequency (n) depends on the energy gap between vibrational levels

    E = hn = hc/l (cm-1)

    The natural frequency (8.67 x 1013 s-1) is absorbed selectively

    Only the natural frequency will be absorbed

  • The absorption intensity depends on how efficiently the energy of an electromagnetic wave of frequency n can be transferred to the

    atoms involved in the vibration

    The greater the change in dipole moment during a vibration, the higher the intensity of absorption of a photon

    What does the absorption intensity depend on?

  • Dipole Moment Must Change during for a vibration to be IR active!

    In order to interact strongly with the EM radiation, the motion of the molecule must be such that the dipole

    moment changes.

    Which of the vibrations are IR active?

  • What is the intensity of an IR signal of: O2 or N2 or H2?

    Ans: In order to absorb infrared radiation, a molecular vibration must cause a change in the dipole moment of

    the molecule

    O2, N2 and H2 DO NOT ABSORB IR LIGHT!

    The are not Greenhouse gases

  • Does O=C=O absorb IR light?

    Ans: vibrations of O=C=O which cause a change in the dipole moment of the molecular absorb IR light

    vibrations of O=C=O which do not cause a change in the dipole moment of the molecular DO NOT absorb IR light

    No dipoleNo dipolegeneratedgenerated

    Dipole Dipole generatedgenerated

  • Which should have a higher stretching frequency, CO, CO+, or CO-? Why?

    Ans: The higher the bond order, the stronger the bond and the higher the frequency for the IR stretch.

    Bond order: CO = 3, CO+ = 5/2, CO- = 5/2

    CO will have the higher stretching frequency

    CO+ and CO- will have similar, lower frequencies

    CO = (s)2(p)4 CO+ = (s)2(p)3 CO- = (s)2(p)4(p*)1

  • region of infrared that is most useful lies between2.5-16 mm (4000-625 cm-1)

    depends on transitions between vibrational energy states

    stretching

    bending

    Infrared Spectroscopy: further review

  • Stretching Vibrations of a CH2 Group

    Symmetric Antisymmetric

  • Bending Vibrations of a CH2 Group

    In plane In plane

  • Bending Vibrations of a CH2 Group

    Out of plane Out of plane

  • 2000200035003500 30003000 25002500 1000100015001500 500500

    Wave number, cmWave number, cm--11

    Infrared Spectrum of Hexane

    CHCH33CHCH22CHCH22CHCH22CHCH22CHCH33

    CCH stretchingH stretchingbendingbending bendingbending

    bendingbending

  • 2000200035003500 30003000 25002500 1000100015001500 500500

    Wave number, cmWave number, cm--11

    Infrared Spectrum of 1-Hexene

    HH22C=CHCHC=CHCH22CHCH22CHCH22CHCH33

    HHCCC=CC=CHH

    C=CC=C

    HH22C=CC=C

  • Structural unit Frequency, cm-1

    Stretching vibrations (single bonds)

    sp CH 3310-3320sp2 CH 3000-3100sp3 CH 2850-2950sp2 CO 1200sp3 CO 1025-1200

    Infrared Absorption Frequencies

  • Structural unit Frequency, cm-1

    Stretching vibrations (multiple bonds)

    Infrared Absorption Frequencies

    C C 1620-1680

    C N

    C C 2100-2200

    2240-2280

  • Structural unit Frequency, cm-1

    Stretching vibrations (carbonyl groups)

    Aldehydes and ketones 1710-1750

    Carboxylic acids 1700-1725

    Acid anhydrides 1800-1850 and 1740-1790

    Esters 1730-1750

    Amides 1680-1700

    Infrared Absorption Frequencies

    C O

  • Structural unit Frequency, cm-1

    Bending vibrations of alkenes

    Infrared Absorption Frequencies

    CH2RCH

    CH2R2C

    CHR'cis-RCH

    CHR'trans-RCH

    CHR'R2C

    910-990

    890

    665-730

    960-980

    790-840

  • Structural unit Frequency, cm-1

    Bending vibrations of derivati