Introduction to spectroscopy 1-infrared spectroscopy
Spectroscopy At this point in the course, we have learned many
organic reactions that can be used to synthesize organic molecules.
How do you determine whether the product that you have obtained in
an organic reaction is the product that you had expected?Also, how
would you determine the structure of unknown organic compounds that
are isolated from natural sources? Definition Spectroscopy:
It is an analytical technique concerned withstudying of the
interaction of electromagnetic radiation and matter in orderto
determine the structure. Types of Spectroscopy Infrared
spectroscopy (IR)
measures the bond vibration frequencies in a molecule and is used
to determine the functional group Mass spectrometry (MS) fragments
the molecule and measures the masses Nuclear magnetic resonance
spectroscopy (NMR) detects signals from hydrogen atoms and can be
used to distinguish isomers electromagnetic relationships:
c Wavelength ():Crest-to-crest distance between waves Frequency
(v):Number of cycles per second number of cycles/sec or s-1 or
Hertz (Hz) electromagnetic relationships:
Electromagnetic radiation displays the properties of both particles
and waves The particle component is called a photon The energy (E)
component of a photon is proportional to the frequency. Where h is
Plancks constant and n, is the frequency in Hertz (cycles per
second) E = hn The speed of light, c, is constant, the frequency,
n, is inversely proportional to how long the oscillation is, or
wavelength: c ___ n = c = 3 x 1010 cm/s l hc ___ E = hn = l
Electromagnetic (EM) Spectrum
Frequency, n in Hz ~1019 ~1017 ~1015 ~1013 ~1010 ~105 Wavelength, l
~.0001 nm ~0.01 nm 10 nm 1000 nm 0.01 cm 100 m Energy (kcal/mol)
> 300 300-30 300-30 ~10-4 ~10-6 g-rays X-rays UV IR Microwave
Radio nuclear excitation (PET) core electron excitation (X-ray
cryst.) electronic excitation (p to p*) molecular vibration
molecular rotation Nuclear Magnetic ResonanceNMR (MRI) Visible High
frequency (u) Short wavelength (l)
= High energy Low frequency (u) Long wavelength (l) = Low energy
The Spectrum and Molecular Effects
Absorption Spectra Organic compound exposed to electromagnetic
radiation, can absorb energy of certain wavelengths. Changing
wavelengths to determine which are absorbed and which are
transmitted produces an absorption spectrum Energy absorbed is
shown as dips in spectrum Infrared Absorption of Ethyl Alcohol
CH3CH2OH Infrared Spectroscopy Infrared radiation 1. what type of
bonds are present
IR is used to tell: 1. what type of bonds are present 2. some
structural information Infrared Spectroscopy
Infrared spectroscopy is the measurement of the wavelength and
intensity of the absorption of mid-infrared light by a sample.
Mid-infrared is energetic enough to excite molecular vibrations to
higher energy levels. The wavelength of infrared absorption bands
is characteristic of specific types of chemical bonds, and infrared
spectroscopy finds its greatest utility for identification of
organic and organometallic molecules. The high selectivity of the
method makes the estimation of an analyte in a complex matrix
possible. The IR Spectroscopic Process
As a covalent bond oscillates due to the oscillation of the dipole
of the molecule a varying electromagnetic field is produced The
greater the dipole moment change through the vibration, the more
intense the EM field that is generated The IR Spectroscopic
Process
When a wave of infrared light encounters this oscillating EM field
generated by the oscillating dipole of the same frequency, the two
waves couple, and IR light is absorbed The coupled wave now
vibrates with twice the amplitude IR beam from spectrometer coupled
wave EM oscillating wave from bond vibration Dipole Changes During
Vibrations Absorption of infrared radiation is thus confined
largely to molecular species that have small energy differences
between various vibrational and rotational states. In order to
absorb infrared radiation, a molecule must undergo a net change in
dipole moment as a consequence of its vibrational or rotational
motion. The dipole moments is determined by the magnitude of the
charge difference and the distance between the two occurs during
the vibration or rotation of homonuclear species such as O2, N2, or
Cl2; consequently centers of Types of Molecular Vibrations:
charge. No net change in dipole moment, such compounds cannot
absorb in the infrared A polar bond is usually IR-active A nonpolar
bond in a symmetrical molecule will absorb weakly or not at all
Types of Molecular Vibrations: Vibrations fall into the basic
categories of stretching and bending. A stretching vibration
involves a continuous change in the interatomic distance along the
axis of the bond between two atoms. Bending vibrations are
characterized by a change in the angle between two bonds and are of
four types: scissoring, rocking, wagging, and twisting. Different
Types of Vibration
Symmetric Stretch Asymmetric Stretch Scissoring Rocking Wagging
Twisting Stretch Vibration or oscillation along the line of the
bond
Bend Vibration or oscillation not along the line of the bond H C H
C symmetric asymmetric H C H C H C H C twist wag rock scissor out
of plane in plane What is the Infrared? Just below red in the
visible region
Wavelengths usually m More common units are wavenumbers, or cm-1,
the reciprocal of the wavelength in centimeters ( cm-1) Wavenumbers
are proportional to frequency and energy Energy required to distort
covalent bonds (stretching, compression, wagging, etc.) Light that
is absorbed shows up as a peak in absorbance or a dip in
transmittance. Absorption of Infrared light in the wavelength range
of 2
Absorption of Infrared light in the wavelength range of 2.5x10-6 m
to 25x 10-6 m (2.5x10-4 cm to 25x 10-4 cm)=(2.5 m to 25 m) causes
bond vibrations Specific wavelengths (or frequencies) can be
correlated with specific functional groups Wave Number => (cm-1)
=> proportional to energy n Remember Energy is inversely
proportional to wavelength but proportional to wavenumber n Number
of possible modes (1) If Linear molecule: 3N 5
Vibrational Modes Number of possible modes (1) If Linear molecule:
3N 5 Example: Carbon dioxide is a linear molecule and thushas 3 x 3
5 = 4 modes (2) If Nonlinear molecule: 3N 6 Example : Triatomic
molecule such as water, sulfur dioxide, or nitrogen dioxide have 3
x 3 6 = 3 vibrational modes Infrared Instruments An infrared
spectrophotometer is;
an instrument that passes infrared light through an organic
molecule and produces a spectrum that contains a plot of the amount
of light transmitted on the vertical axis against the wavelength of
infrared radiation on the horizontal axis. *In infrared spectra the
absorption peaks point downward because the vertical axis is the
percentage transmittance of the radiation through the sample.
Absorption of radiation lowers the percentage transmittance value.
Since all bonds in an organic molecule interact with infrared
radiation, IR spectra provide a considerable amount of structural
data. How does it work? A molecule is exposed to IR.
It starts to undergo distortions based on the types of bonds
present. The spectrum represents the presence of chemical groups
(Functional groups). The IR Spectrum Each stretching and bending
vibration occurs with a characteristic frequency as the atoms and
charges involved are different for different bonds The y-axis on an
IR spectrum is in units of % transmittance In regions where the EM
field of an osc. bond interacts with IR light of the same n
transmittance is low (light is absorbed) In regions where no osc.
bond is interacting with IR light, transmittance nears 100% The
x-axis of the IR spectrum is in units of wavenumbers, n, which is
the number of waves per centimeter in units of cm-1(Remember E = hn
or E = hc/l) How are these interpreted?
The spectrum is divided into two distinct sections. 4000cm-1 to
1300cm-1 is the functional group region. 1300cm-1 400cm-1 is the
fingerprint region. Bands in these areas can be used as evidence of
certain functional groups. Fingerprint region tends to have small
bands, that are difficult to individually resolve. Summary of IR
Absorptions
Bonds to H Triple bonds Double bonds Single Bonds CC 2200 cm-1
(weak or absent if internal)
The IR Spectrum The detection of different bonds 1-Lighter atoms
will allow the oscillation to be faster higher energy. This is
especially true of bonds to hydrogen C-H, N-H and O-H 2-Stronger
bonds will have higher energy oscillations Triple bonds > double
bonds > single bonds in energy C-C cm-1 C=C cm-1 CC cm-1(weak or
absent if internal) 3-As opposed to chromatography or other
spectroscopic methods, the area of a IR band (or peak) is not
directly proportional to concentration of the functional group
producing the peak 4-The intensity of an IR band is affected by two
primary factors: a.Whether the vibration is one of stretching or
bending b. Electronegativity difference of the atoms involved in
the bond c. For both effects, the greater the change in dipole
moment in a given vibration or bend, the larger the peak. d. The
greater the difference in electronegativity between the atoms
involved in bonding, the larger the dipole moment, Hence polar
bonds such as O-H and C=O give more intense absorptions than
non-polar C-H, C-C and C=C bonds. e. Typically, stretching will
change dipole moment more than bending Infrared Group
Analysis
Since most types of bonds in covalent molecules have roughly the
same energy, i.e., C=C and C=O bonds, C-H and N-H bonds they show
up in similar regions of the IR spectrum Remember all organic
functional groups are made of multiple bonds and therefore show up
as multiple IR bands (peaks) Infrared Group Analysis How to Analyze
an IR Spectrum;Look for whats there and whats not there
C-H absorption The wavenumber will tell you sp3(C-C), sp2(C=C), sp
(CC) and perhaps aldehyde. Carbonyl (C=O) absorption Its presence
means the compound is an aldehyde, ketone, carboxylic acid, ester,
amide, anhydride or acyl halide. Its absence means the compound
cannot be any of the carbonyl-containing compounds. O-H or N-H
absorption This indicates either an alcohol, N-H containing amine
or amide, or carboxylic acid. CC and CN absorptions Be careful:
internal triple bonds often do not show up in IR spectra. C=C
absorption Can indicate whether compound is alkene or aromatic.
EXAMPLE 2-aminopentane Amines - Primary Shows the N-H stretch for
NH2 as a doublet between cm-1 (symmetric and anti-symmetric modes)
EXAMPLE pyrrolidine Amines Secondary N-H band for R2N-H occurs at
cm-1 as a single sharp peak weaker than O-H Tertiary amines (R3N)
have no N-H bond and will not have a band in this region Examples
of Infrared Spectra O-H Stretch of a Carboxylic Acid
This O-H absorbs broadly, cm-1, due to strong hydrogen bonding
Carbonyl Stretching Exact absorption characteristic of type of
carbonyl compound The C=O bond of simple ketones, aldehydes, and
carboxylic acids absorb around 1710 cm-1 Usually, its the strongest
IR signal Carboxylic acids will have O-H also Aldehydes have two
C-H signals around 2700 and 2800 cm-1 Practice Q: Identify the
chemical species represented in this IR spectra? Types of peaks
V-shape peak Broad U-shape peak
-NH bond for 2o amine (R2NH) Broad U-shape peak -OH bond W-shape
peak -NH bond for 1o amine (RNH2) Sharp & strong peak C=O
at1710 cm-1 Strengths and Limitations
IR alone cannot determine a structure Some signals may be ambiguous
The functional group is usually indicated The absence of a signal
is definite proof that the functional group is absent
Correspondence with a known samples IR spectrum confirms the
identity of the compound Web References
Top Related