11
INFRARED
SPECTROSCOPY
(SPEKTROSKOPI INFRAMERAH)
11-1 INTRODUCTION (PENDAHULUAN)
Infrared radiation was
discovered in 1800 by Sir William Herschel who reported his findings on certain
experiments in heat radiation to the Royal Society. At that time scientists did
not have a clear understanding of the nature of radiation. Herschel’s
experiment consisted of resolving sunlight into its spectrum with a glass prism
and placing thermometers at successive positions throughout the spectrum
(Figure 11-1). The thermometers placed beyond the red end of the solar spectrum
registered the most marked rise in temperature demonstrating that more heat
existed outside and beyond the red region that within. This simple experiment
was both the discovery of the infrared region (infra meaning beyond) and the
construction of the first infrared spectrometer. In further experiments
Herschel measured the absorption of this new radiation by numerous substances
including sea water, distilled water, and, oddly enough, gin and brandy.
Unfortunately he was not aware of the potential of his discovery, namely, that
his absorption could reveal information about the molecular structure of
organic compounds. Before the significance of infrared absorption was to be
appreciated, the theory of the nature of radiation had to be better understood.
Almost a century passed before the necessary theory, techniques, and
instrumentation for infrared analysis were developed. The infrared region lies
between the visible and radio portion of the electromagnetic spectrum (Figure
11-2). The most useful range lies between 4000 and 400 cm-1.
Radiasi inframerah ditemukan pada tahun 1800 oleh William Herschel
yang melaporkan penemuannya pada eksperimen tertentu di dalam radiasi panas
kepada masyarakat Kerajaan. Pada waktu itu ilmuwan tidak mempunyai suatu
pemahaman yang jelas tentang radiasi alam. Eksperimen Herschel terdiri dari
memecahkan cahaya matahari ke dalam spektrumnya dengan suatu kaca prisma dan
menempatkan termometer pada posisi berurutan sepanjang seluruh spektrum (Gambar
11-1). Termometer yang ditempatkan di luar akhir spektrum merah matahari menunjukkan
kenaikan temperature yang paling tinggi dan di luar daerah merah yang di dalam.
Eksperimen sederhana ini kedua-duanya merupakan penemuan daerah inframerah (infra
berarti di luar) dan konstruksi yang pertama spektrometer inframerah. Di dalam
eksperimen Herschel lebih lanjut
mengukur penyerapan radiasi baru oleh banyak unsur yang mencakup air
laut, air suling, bahan lainnya, gin dan semacamnya. Sayangnya ia tidak peduli
akan potensi penemuannya, yakni bahwa absorpsinya bisa mengungkapkan informasi
tentang struktur molekul dari campuran organik. Sebelum arti absorpsi
inframerah diapresiasi, teori radiasi alami telah harus lebih baik dipahami.
Hampir satu abad dilewati teori yang perlu, teknik, dan instrumentasi untuk analisa
inframerah telah dikembangkan. Daerah Inframerah berada antara sinar tampak dan
spektrum elektromagnetik radio ( Gambar 11-2). Daerah paling besar yang digunakan berada antara 4000 dan 400 cm-1.
Beginning in 1903
William W. Coblentz, a young graduate student at Cornell University, improved
the experimental techniques and set about at long last to measure the
absorption spectra of pure substances. For two years he mapped spectra by a
point-to-point plotting of values and finally in 1905 published the first
collection of accurate infrared spectra of 131 substances. With more than half
a century’s progress since Coblentz, we now understand the basic theory of
infrared spectroscopy that he so energetically tried to postulate. Organic
chemists in 1930’s seriously began to consider infrared spectroscopy as a
possible method to identify compounds through their functional groups. By 1935
a few of the larger chemical companies has invested in infrared spectrometers
for organic quantitative analysis.
Dimulai tahun 1903 William W. Coblentz, seorang siswa lulusan muda
pada Universitas Cornell, meningkatkan teknik percobaan dan memperbaiki tentang
yang lama untuk mengukur spektra absorpsi dari unsur murni. Selama dua tahun ia
memetakan spektra menjadi kumpulan titik ke titik dari nilai dan akhirnya pada
tahun 1905 dipublikasikan kumpulan pertama spektra inframerah yang akurat dari
131 unsur. Dengan lebih dari separuh kemajuan abad semenjak Coblentz, kita sekarang memahami teori dasar
dari spektroskopi inframerah yang telah ia
cobakan dengan penuh semangat sebagai dalil. Ahli kimia organik tahun 1930-an
dengan serius mulai untuk mempertimbangkan spektroskopi inframerah sebagai
metoda yang mungkin untuk mengidentifikasi campuran berdasarkan gugus fungsinya.
Pada tahun 1935 beberapa perusahaan besar kimia telah menanam modal terhadap spektrometer
inframerah untuk analisis kuantitatif organik.
The major industrial
impetus to the field came early in World War II when it was shown that infrared
spectroscopy offered the most accurate and rapid method of analyzing the C4
hydrocarbon fraction of interest in the production of synthetic rubber. The
boom had started and commercial production of instruments was initiated during
1943. Availability of instrumentation stimulated infrared studies and the
number of papers published rocketed sky high. Such basic research has nowadays
enabled the ordinary bench chemist not only to record his own spectra but to interpret
them in term of structure. Infrared spectroscopy has gained worldwide
acceptance by chemists to the extent that it is considered standard laboratory
practice.
Daya dorong industri yang utama kepada berbagai bidang datang awal
Perang Dunia II ketika ditunjukkan bahwa spektroskopi inframerah menawarkan
metoda yang cepat dan akurat untuk meneliti fraksi hidrokarbon C4 yang menarik
minat untuk produksi karet sintetis. Kenaikan harga secara tiba-tiba terjadi
dan produksi instrumen komersial telah diresmikan pada 1943. Ketersediaan
instrumentasi merangsang pembelajaran inframerah dan banyaknya dokumen yang
dipublikasikan seperti meroket setinggi langit. . Riset dasar seperti itu saat
ini memungkinkan ahli kimia yang biasa duduk tidak hanya untuk merekam spektranya
tetapi juga menginterpretasikannya dalam hal struktur. Spektroskopi inframerah
telah memperoleh penerimaan di seluruh dunia oleh ahli kimia secara luas yang dipertimbangkan untuk
standar praktek laboratorium.
11-2 INFRARED
THEORY
Vibrational
Spectra. Molecular vibrations
can occur by two different mechanisms. Firstly, quanta of infrared radiation
can excite atom to vibrate directly-the absorption of infrared radiation gives
rise to the infrared spectrum. Secondly, quanta of visible light can achieve
the same result indirectly-the Raman Effect.
Spektra Getaran. Getaran molekul dapat terjadi dengan dua
mekanisme berbeda. Pertama-tama, kuanta radiasi inframerah dapat meningkatkan
atom untuk bergetar secara langsung, absorpsi radiasi inframerah memberi
kenaikan kepada spektrum inframerah tersebut. Kedua, kuanta cahaya tampak dapat
mencapai hasil yang sama secara tidak langsung-Efek Raman.
Most organic molecules
are fairly large and their resultant vibrational spectra are complex. To
introduce the basic concepts governing vibrational spectra a simple diatomic
covalent bond will be considered as a spring with the atomic masses at either
end (Figure 11-3). The stiffness f the spring is described by a force
constant, k. If such simple a simple system is put into motion (by stretching
and releasing), the induced vibrations of the masses are adequately described
by Hooke’s law of simple harmonic motion.
Banyak molekul organik yang cukup besar dan jumlah spektra
getarannya kompleks. Untuk memperkenalkan konsep dasar yang mengatur spektra
getaran suatu ikatan kovalen dwiatom sederhana akan dianggap sebagai suatu
massa-atom pada akhir salah satunya ( Gambar 11-3). Kekakuan f diuraikan oleh
suatu tetapan kekuatan, k. Jika sederhana seperti itu suatu sistem sederhana
memasuki gerakan ( dengan memotong dan melepaskan), penyebab getaran dari massa
cukup banyak diuraikan oleh Hukum Hooke melalui gerak harmonik sederhana.
Frequency motion,
Frekuensi gerak
where
is the reduced
mass, that is the harmonic mean of individual masses or
dimana
adalah penurunan massa,
yaitu harmoni rata-rata dari massa individu
Atau
To a first approximation, this assumption
of harmonic forces in agreement with actual conditions in real molecules.
However, quantum theory governs molecular motion and restricts the energy
stored in the vibration, Ev, such that only certain energy transitions are
allowed, as determined by a quantum number, v.
Untuk
suatu perkiraan pertama, asumsi kekuatan harmoni ini sesuai persetujuan dengan kondisi
nyata di dalam molekul nyata. Bagaimanapun, teori kuantum mengatur gerakan
molekular dan membatasi penyimpanan energi pada getaran, Ev, seperti halnya transisi energi tertentu dibolehkan,
yang ditentukan oleh suatu bilangan kuantum, v.
where v = 0,1,2,3,....etc. For instance, if a molecule were to
undergo a transition from the lowest level (v
= 0) to the first level (v = 1) by
absorption of infrared radiation, the frequency of that exciting radiation
would be given by the Bohr principle, hv
= E1 – E2.
Dimana
Now Equation 11-2 gives
Eo = ½ hv and E1 = 3/2 hv
Therefore, by subtitution:
(E1 - Eo)
/h = v
In
summary, the absorption of infrared radiation causes the excitation of the
molecule to higher vibrational levels and is quantized. The normal vibration
has the same frequency as the electromagnetic radiation. The absorption process
can occur only if there is a change in the magnitude and direction of the
dipole moment of the bond.
In
the same manner, a transition from the lowest level (v = 0) to the second level (v
= 2) would occur at a vibrational frequency, 2v. In musical terminology, v
is the fundamental vibration frequency and 2v
the overtone frequency.
According
to Boltzmann’s distribution law almost all molecules are in the lowest
vibrational energy level (v = 0) at
room temperature. Therefore, the vibrational transitions in a molecule are
restricted to those occuring from the lowest level. However, a selection rule
forbids transitions which change the quantum number, v, by more than unit. This rule limits the obsevale transitions to
the fundamental frequencies, v, that
is, vibrational spectra should exhibit only characteristic vibrational
frequencies corresponding to the various bonds within the molecule. This
selection rule is obeyed only for perfectly harmonic vibrations. In practice,
molecular vibrations are not stricly harmonic and the selection rule fails as
is evident by the appearance of weak overtone vibrations.
Evaluation
of Equation 11-1 for O-H bond (Table 11-1) indicates that the hydrogen atom
bond to an oxygen atom vibrates back and forth approximately 1014
times per second in the direction of the linkage.
In
summary, the O-H bond vibrates 1.11 x 1014 cycles per second and the
electromagnetic radiation required to excite this particular vibration must
have a frequency of approximately 1.11 x 1014 Hz or 3700 cm-1.
Experience has demonstrated that all alcohol spectra contain a characteristic absorption
at approximately 3600 cm-1. If the vibrations of the C-C linkage are
calculated in a similar manner, the result is 3.5 x 1013 Hz
corresponding to a frequency of 1100 cm-1. Now in the case of the
C=C linkage, the masses of the atoms involved are the same (i.e., µ = 6), but the force constant, k, is greater, and the vibrational
frequency is 4.9 x 1013 Hz, corresponding to a radiation of 1640 cm-1.
The force constant of the C
C
linkage is even greater and the absorption band is at 2100 cm-1.
This series illustrates the potential
power of infrared to distinguish between combinations of the same two
atoms linked differently.
Polyatomic
Vibrational Spectra. In a polyatomic molecule, the atoms or covalent bonds
are not rigidly linked together and are able to vibrate from their position of
rest. In addition, there are the bond angles enclosed by the various individual
diatomic bonds which result in a powerful qualitative method of describing the
vibrations of polyatomic molecules.
Table
11-1 Reduced Mass and Force
Contains for Various
Atom Pairs
|
Atom Pair
|
Force Constanta
|
|
|
C
C
|
4.5
|
6
|
|
C
C
|
9.6
|
6
|
|
C
C
|
15.6
|
6
|
|
C
O
|
5.0
|
6.85
|
|
C=O
|
12.1
|
6.85
|
|
C
H
|
5.1
|
0.923
|
|
O
H
|
7.7
|
0.941
|
|
C
N
|
5.8
|
6.46
|
|
N
H
|
6.4
|
0.933
|
|
C
N
|
17.7
|
6.46
|
a Expressed in 105
dynes/cm.
b Expressed in units
of mH, where mH = mass of a hydrogen atom
= 1.67339 x 10-24 g.
Since each type of chemical bond in a
molecule involves different values of force constants and reduced masses,
absorption of radiation will occur over a range of frequencies. Thus, if
infrared radiation of successive frequencies is passed through a substance, a
series of absorption bands is recorder the active fundamental modes of
vibration. These can be subdivided into the following classes.
1.
Stretching vibrations are those in which two bonded
atoms continuously oscillate, changing the distance between them without
altering the bond axis or bond angles. They are either isolated vibrations
(e.g., the methylene group,
).
Coupled vibrations are symetrical or unsymetrical (asymetric) as illustrated
(Figure 11-4).
In the symetric case both hydrogen atoms move away from
the carbon simultaneously while in the asymetric case one hydrogen moves toward
the carbon while the other moves away. Stretching vibrations generally require
higher energies than bending and are denoted by the Greek symbolnu, v, followed by the chemical group in
parentheses afterwards, that is, v(C=O)
= 1600 cm-1 indicates that the fundamental stretching vibration of
the carbonyl group is observed at 1600 cm-1.
2.
Bending vibrations are characterized by a
continuously changing angle between two bonds. Bending modes of aromatic C-H
groups for instance, which take place in the plane of the phenyl nucleus, are
denoted by the symbol delta,
(C-H)
while those which occur out of the plane are denoted by the symbol gamma,
(C-H).
This nomenclature also applies to alkenes and alkynes.
3.
Wagging vibrations result when a nonlinear
three-atom structural unit oscillates back and forth in the equilibrium plane
formed by the atoms and their two bonds (Figure 11-5). Such vibrations are
denoted by the symbol omega,
(CH2).
4.
Rocking vibrations occur where the same structural
unit oscillates back and forth out of the equilibrium plane (Figure 11-5). The
symbol to denote this particular mode of vibration is rho,
(CH2).
5.
Twisting vibrations occur when the same structural
unit rotates around the bond which joins it to the rest of the molecule (Figure
11-5). Such vibrations are recorder by the symbol tau,
(CH2).
6.
Scissoring vibrations occur when two nonbonded atoms
move back and forth toward each other (Figure 11-5). These are denoted by the
symbol s(CH2).
Apart from these fundamental modes of vibration, harmonic
and combination vibrations may also occur. Harmonic vibrations posses
frequencies which represent approximately integral multiples of the fundamental
frequency, for example, 2v or 2
.
Frequencies of combination vibrations are composed of the sum of (v +
),
or the difference between (v -
)
the frequencies of two or more fundamental or harmonic vibrations.
If the origin and theory of the infrared spectrum are
kept in mind it is apparent that the spectrum represents a veritable wealth of
information about the basic characteristics of the molecule, namely, the nature
of the atoms, their spatial arrangement, and their chemical linkage forces. It
is for this reason that the infrared spectrum gained recognition as being a
“fingerprint of the molecule”.
11-3 SAMPLE
PREPARATION
In general the
amount of sample necessary to obtain a good infrared spectrum is the order of 1
of 5 mg. Techniques have been developed to handle the sample in any of its
three possible phases solid, liquid and gas.
Solid Substances
As solid state spectra. Solid state forces such as intermolecular hydrogen
bonding render such spectra somewhat unreliable for diagnostic purposes. For succesful examination of solids
the following points must be clearly understood. First, the particle size
should be less than the wavelength of the infrared radiation (1 µm), otherwise
pronounced scattering of the incident high occurs. This restriction can easly
be overcome by very through grinding or even better by using a mechanical ball
and mill technique. Second, these small particles must now be suspended in a
medium of similar refractive index. In practice, grinding is continued in Nujol
(medicinal paraffin) to obtain a “mull”. The mull is the placed between two
polished sodium chloride plates and placed in the spectrophotometer. Nujol has
a relatively simple spectrum consisting only of v(CH) at 2950 cm-1, δ(CH)
asy. For methylene and methyl groups at 1450 cm-1 and δ(CH sym. For methyl groups only at 1380
cm-1.
Another common method of suspending the
solid particles is to intimately mix the substances with potassium bromide. The
resulting mixture is compressed under high presure to form a disc which can be
placed directly into the spectrophotometer.
As Solution Spectra. Separation of molecules can be achieved easily by
dissolving them in a suitable solvent. In such an environment
of solvent molecules we can deal with the molecule as an individual entity.
Accurate quantitative work demands a dilute solution. By varying the
concentration of alcohols and phenols in particular, a degree of control is
exercised over the association or intermolecular hydrogen bonding between these
molecules. In progressively more dilute solution they will exhibit polymeric,
trimeric, dimeric, and finally monomeric hydroxyl strechting frequencies, v(OH).
Choice of solvent depends on the region
of the spectrum of most interest. All solvents have some absorptions in the
4000 to 650 cm-1 region. By using “window areas”, that is,
transparent areas of the solvent, the whole spectrum may be covered. For
instance, the most common use the carbon tetrachloride (CCl4) is
from 4000 to 1300 cm-1 and for carbon disulfide (CS2),
1300 to 660 cm-1. Double been operation may cancel solvent
absorption (assuming the same pathlengths are used), but only to a certain
degree.
Sodium chloride cells are employed, the
most useful thickness being 0.1 mm and 0.5 mm. Typical concentrations for
oxygen containing substances are 5 to 10% in the 0.1 mm cell and 1 to 2% in the
0.5 mm cell.
Finally, a word of warning regarding
solvent effects. Shifts in the absorption frequencies may result on changing
from one solvent to another. Measurements in nonpolar CCl4 are
usually regarded as the reference value. Water is not at all a suitable solvent
due to its intense absorption spectrum even in thin films throughout the entire
infrared region. In any case, sodium chloride cell windows would gradually
dissolve if water were employed as solvent.
Liquid
Substances
As Pure Liquid Spectra. These can be easily examined either as thin films
pressed between two NaCl plates or in cells with known pathlengths ranging from
0.01 to 0.1 mm. Spectra of pure liquids often show strong intermolecular
hydrogen bonding and association effects.
Gases
Gaseous samples are measured in cells
with long pathlengths; 10 cm is common. For trace amounts, very long paths are needed
which can be achieved only with mirrors mounted at the ends of the cell to give
multiple reflections.
11-4 INFRARED ABSORPTION SPECTRA
It is not always convenient to reproduce
spectra in a scientific paper or book, so parameters are quoted which allow the
reader to visualize, reconstruct, or compare with his own data.
Position. This is normally quoted as the wave number of
maximum absorption, v(X-Y) cm-1,
where X and Y represent the two atoms in question.
Half-Band Width. The apparent half-band width, ∆v1/2, is cited as the width in cm-1 at
half-height.
Intensity. The apparent molar absorptivity measured at the
peak maximum, ϵa, is given
by Beer,s law:
ϵa =
Integrated Intensity. A logical extention of the apparent molar absorptivity, ϵa, is to measure the area under the peak. The
integrated intensity, B, is given by
Figure 11-6 shows the calculations involved for a carbonyl
absorption band. These parameters vary with the type of instrument employed and
the conditions under which the recording is made. This variation is greatest
with sharp peaks. Two important factors for such variation are the scanning
speed and the slit width. Quantitative spectra must be scanned more slowly than is necessary for qualitative analyses. The error stems from
the inability of the detecting system to respond to rapid changes in
absorption.
In
the past the effect of slit width on prism infrared specrtrophotometers has
hindered the transfer of data from one laboratory to another. To a large extent
the advent of grating spectrophotometers has removed this barier. The v(C=O) band of phenyl benzoate, shown in
Figure 11-7, illustrates the effect of slit width. The areas under the peaks,
however, are the same in each case. For quantitative analyses with prism
instruments the same slit width should be used for recording the spectra of the
unknown and calibration solutions. If possible, resolving power high enough to
give no slit width error should be used, although, in general, only grating
spectrophotometers will be capable of this.
Two
specral regions are distinguished, the region of group frequencies and the
fingerprint region. The group frequency region lies approximatelybetween 4000
cm-1 and 1400 cm-1. In this region the principal
absorptions bands may be assigned to vibration units consisting of only two
atom, and the frequency is characteristic of their masses and the force
constant of their linkage. This simplification ignores the rest of the
molecule. The funcional groups of organic molecules illustrate such vibrational
units. To a first approximation the frequency of their fundamental stretching
mode, v(X-Y), is independent of the
influence of the rest of the molecule (i.e., all alcohols have v(O-H) at approximately 3600 cm-1).
Such influences do reveal themselves on careful study and can offer valuable
evidence as to the nature of the neighboring atoms.
The
fingerprint region extends from 1400 cm-1 to 400 cm-1. Absorption
bands found here are related to vibrations of the molecule as a whole, each
atom exerting a mutual influence on the others (i.e., combination bands). The
resulting bands are, of course, unique to a particular molecule and can be used
for the unambigous identification of that molecule provided you have a standard
spectrum of that substance previously recorded on file. On exception to this
rule is that a series of compounds such as long chain fatty acids can give
almost identical spectra.
11-5 CORRELATION
CHART
In
the molecular diagnoses of vibrational frequencies or absorption bands it is
extremely useful to refer to tabulated values of the various fuctional groups
and their associated characteristic group frequency ranges (Cholthup chart,
Figure 11-8). This chart provides at a glance the expected range of frequencies
for a particular funcional group. To supplement this general data a series of
more descriptives tables (Tables 11-2 through 11-26) has been prepared to deal
with each major funcional group in greater detail and the factors affecting the
absorption bands position.
11-6 INTERPRETATION
OF INFRARED SPECTRA
In
almost every subject there are certain basic “alphabet-like” essential facts
that must be mastered early and infrared spectroscopic identification is no
exception. Although extensive correlation tables have systematized and
rationalized experimental findings, they have by no means removed the necessity
of having almost reflex command of such elementary essentials as identification
of funcional groups.
To
refresh the memory the infrared spectrum consist of to major partitions the
groups frequency region and fingerprint region. The strange of the infrared
method lies in the fact that each functonal group has a characteristic
absorption band int his so called”group frecuency region” while direct comparison of fingerprint regions
between unknown and standard reference spectra give the true unambiguous identification. Initially, however,
concentration is placed on “seeing” the various functional groups present in
the molecule. This facility in turn demands the use of corellation tables
listening such as values until
frecuent usage commits them the memory. Many of the unknown spectra to be
interpreted into molecular structure are not entirely deboid of some background
information such as the exact molecular
weigh (from mass spectrometry) which can be tranlated into a molecular formula,
CxHyOz, this imoportant piece of information
might also have come from the more classical and still widely used combustion
analysis. Today the chemist no longer
concentrates on a solution to this problem from one particular method
alone but by a joint attack by many associated technique (UV, NMR, MS, m.p.,
etc.).
Double
Bond Equivalent. for practical purposes
let us assume that besides the unknown spectrum you are also given the
molecular formula. Possession of this information alone allows the calculation
of the “Double Bond Equivalent” (hereafter called D.B.E.) of the molecule; that
is, the number of double bonds (or their equivalent) the molecule possesses
relative to the appropriate saturated hydrocarbon. For example, what is the
D.B.E. of C6H10? From the general formula, CnH2n+2
for the homologous series of saturated hydrocarbons, the C6 member
should have H14. Therefore, the original formula, C6H10,
is four hydrogens short of that prediction. Now, if it takes the removal of two
hydogen atoms from two adjacent carbon atoms to create a double bonds or two
D.B.E. A triple bond corresponds to the shortage of four hydrogens, or 2 D.B.E.
A.D.B.E. of two can suggest two double bonds, or one double bond and one rong
or one triple bond; three possible stuctures from this information alone are
If
the molecular formula contains an oxygen atom or atoms, the simply subtitute CH2
for each one and calculate as before. For example,
C6H5O à C6H6
+ CH2=C7H8
∆
= H8 = 4 D.B.E.
CnH2n+2 à C7H16
The
number of D.B.E. that a molecule possesses enables you to search the spectrum
to statisfy this criterion; for example, one carbonyl, two double bonds, and
one ring might be a possible solution.
Use
of Group Frequencies. The search for functional groups by looking for their
characteristic stretching vibrations in the “group frequency region” then
begins. It should be mentioned, however, that, if impossible, confirmation of
your findings should also be sought; for example v(CH) = 2980 cm-1 could be either –CH3 or –CH2-
but the appearance of the δ(CH) at 1370 cm-1 confirms the pressence
of the methyl group. In the case of a band at 3300 cm-1, one might
suspect a terminal acetylene. Confirmation can be easily provided by the
appearance of the v(O-H) because of
inexperience in recognizing band contours (v(O-H)
tends to be broad and dependent on dillution), the lack of any v(C-O) at 1000 cm-1 would
have made you re-think your proposed assignment. Only practice can provide that
intuitive power of making correct assignments the first time.
Particular
emphasis should be placed on the significance of the assignment of the presence
of a carbonyl function, v(C=O) at
1720 cm-1. The presence of a carbonyl group can be attributed to any
of the following chemical classes: (a) ketone, (b) aldehyde, (c) ester, (d)
lactone, (e) anhydride, (f) carboxylic acid.
To
differentiate between these is a matter of elimination. For instance, if the
compound is an aldehyde, there should be two highly characteristic v(C-H) bands at 2700 and 2800 cm-1.
Esters, in addition to the v(C=O),
should also exhibit a characteristic ester band at 1200 cm-1, v(C-O). Lactone show complex band
patterns in the carbonyl stretching region, usualy doublets. Anhydrides show
much more complex band patterns in the v(C=O)
region and at higher values than lactones (molecular formula require O4).
In the case of carboxylic acids the presence of a very broad v(O-H) centered around 3000 cm-1
dominates the spectrum. In the event that none of the above applies, the
compound is a ketone and the detailed search continues to discover its close
environment (ring size, conjugation, etc).
Use
of Band Intensity. Spectra presented on the following pages are quite often
recorded in solution in CCl4. The calculation of apparent
absortivity is possible yet another criterion in the identification of an
absorption band. The first step is to convert the % transmittance values for
the base line and peak maximum to absorbance by use of the following diagram
and then apply the general equation
ϵα
=
11-7 REPRESENTATIVE
SPECTRA
Let
us first examine the infrared spectra of some representative compounds and
correlate absorption bands with molecular structure. We will begin with a
simple hydrocarbon, n-hexane, and
then modify the molecule in several ways and observe the effect on the program.
The infrared spectrum of n-hexane
(Figure 11-9 exhibits vibrational absorption bands which are characteristic of
aliphatic hydrocarbons, that is, asymetricand symetric C-H stretching (avanged
at 2899 cm-1), -CH2- scissoring (1471 cm-1)
and C-CH3 bending (1381 cm-1). The weak band near 725 cm-1
is caused by bending vibrations of the –(CH2)4- unit.
The 2-methylpentane molecule
represents a branched chain isomer of n-hexane. Let us consider how branching
affects the absorption spectrum (Figure 11-10). The C-H stretching and –CH2-
scissoring vibrations are not changed significantly compared to the straight
chain analogue. However, the C-CH3 bending vibrations indicate a
strong “doublet” at 1377 cm-1 and 1361 cm-1. This doublet
and a band at 1166 cm-1 are characteristic of the isopropyl group.
If the saturated hydrocarbon is rearranged into the form of an unstrained
ring, the C-H stretching vibration is essentially unchanged and the –CH2-
scissoring vibration is displaced slightly to longer wavelength. For example,
the spectrum of cyclohexane (Figure
11-11) exhibits C-H stretching vibration at 2865 cm-1 and –CH2-
scissoring at 1451 cm-1 compared to the respective 2899 cm-1
and 1471 cm-1 values for n-hexane.
When the ring compound is sterically strained, the C-H stretching vibrations
move ti higher frequency. For example, for bromocyclopropane the symetric and
asymetric C-H stretching vibrations appear at 3077 cm-1 and 2985 cm-1,
respectively.
When two hydrogens are removed from n-hexane
to give hexene-1, the spectral
effect is shown in Figure 11-12. Two classes of C-H stretching vibrations are
apparence.
