Organic Chemistry Laboratory I
Spectroscopy Experiment:  Infrared, Nuclear Magnetic Resonance and Mass Spectroscopy
Experiment Description and Background

Each student will be assigned an unknown number that corresponds to one of the compounds on the spectroscopy experiment master unknown list.  (See "Unknown Assignments" on the main page  for your specific lab section to find your assigned unknown number).  IR, NMR and mass spectral data for each of the unknowns is posted by unknown number under the "spectral data" section.   Students will retrieve the spectral data posted on the website that corresponds to his or her assigned unknown.  Students will be required to read the experiment background and interpret the spectral data as outlined in the experiment procedure section and during spectroscopy workshops to identify the unknown.  Attendance at one spectroscopy workshop will be required.  Spectroscopy workshops are scheduled for the week of November 26-30, 2007.  (See workshop schedule). During the workshop, background information on spectroscopy will be discussed as well as approaches for interpreting spectral data.  Students should bring the spectra corresponding to their unknown to the workshop.  Upon identification of the unknown, students will be required to complete the spectroscopy worksheet.  There is no pre-lab, quiz, notebook or technique grade for this experiment.  The overall experiment grade will be determined based on attendance at one workshop and the spectroscopy worksheet. 

Overview of Spectroscopy
Infrared (IR), Nuclear Magnetic Resonance (NMR), Ultraviolet (UV) and Mass (MS) Spectroscopy are four analytical techniques that are commonly used to elucidate the structure of organic molecules.  IR, NMR and UV spectroscopy each uses different frequencies of electromagnetic radiation to provide different kinds of structural information about the molecules being analyzed.  Mass spectroscopy does not rely on electromagnetic radiation but rather involves ionizing molecules via other energy sources to determine the mass of a molecule.  Table 1 lists four different types of spectroscopy, the frequencies of the electromagnetic spectrum utilized (for IR, NMR and UV), and the kind of structural information each spectral analysis provides.

Type of Spectroscopy
Frequency of EM Spectrum
(Hertz (Hz))
(meters (m))
Structural Information
IR Spectroscopy
5 X 1014  -  3 X 1012
(infrared region)
1 X 10-6  -  1 X 10-4
Bond types, Functional Groups
NMR Spectroscopy
1 X 1011  -  1 X 109
(radio wave region) 
1 X 10-2  - 1.0
Carbon Types, Carbon Skeleton
UV Spectroscopy
  2.5 X 1016  -  2 X 1015
(ultraviolet region)
  1 X 10-8  -  3.8 X 10-7 Pi and Conjugated System
Mass Spectroscopy
Molecular Mass
Isotopes of Cl, Br

Table 1:  Structual Information of Organic Compounds obtained from Different types of Spectroscopy

Infrared Spectroscopy
In IR spectroscopy, a sample molecule is exposed to radiation with a frequency in the infrared region of the electromagnetic spectrum.  Only a portion of the infrared region of the electromagnetioc spectrum(2.5 X 10-6 - 2.5 X 10-5m) is used in conventional IR spectroscopy.  Specific frequencies of radiation within the infrared region of the spectrum are absorbed by individual bonds in the molecule, causing the bonds to vibrate and rotate.  The frequency of the radiation that is absorbed by the molecule is recorded as a peak or absorbance in the spectrum.  These peaks are characteristic of certain types of bonds.  Table 2 lists various functional group classes, specific bonds associated with that functional group, and the characteristic position of the peak in the IR spectrum corresponding to that bond.  Click on the link to view a sample spectrum containing peaks corresponding to the bond type indicated.

Absorbance Range
FunctionalGroup/Bond Type
Intensity of Absorption
2850-2960 cm-1
Csp3-H of alkanes and alkyl groups
3020-3100 cm-1
Csp2-H of alkenes and alkenyl groups
1640-1680 cm-1
Csp2=Csp2 of alkene
3300 cm-1
Csp-H of alkynes
2100-2260 cm-1
CºC of alkynes
CºC of nitriles
500-800 cm-1
Csp3-X, X = halogen of alkyl halide
3200-3650 cm-1
O-H of alcohol, phenol, carboxylic acid
strong, broad
1660-2000 cm-1
1450-1600 cm-1
Csp2=Csp2 of aromatic
1680-1850 cm-1
C=O of carbonyls 
ketones, aldehydes, esters, carboxylic acids, amides
See more specific ranges
3300-3500 cm-1
1030-1230 cm-1
N-H of amines
C-N of amines
1540 cm-1
N=O of nitro group (-NO2)

Table 2: 
Characterisic IR Absorbances Ranges for Various Bond Types
Sample Preparation and the Infrared Spectrophotometer
There are a variety of methods for preparing samples for IR analysis.  The most common methods of sample prepartion for IR analysis are listed in Table 3.   Liquid samples will typically be run in this course using sodium chloride plates (i.e. salt plates).  The sample is prepared by placing a few drops of the liquid compound on one salt plate and placing a second salt plate on top of the first.  Solid samples will be prepared for IR analysis using potassium bromide pellets.  Pellets are prepared by mixing the solid sample with KBr, placing the mixture in a mini-press and compressing the mixture for form a small disc or pellets.  ATR analysis is a faster and simpler way to run solid samples.  Other sample preparation methods (mull, solution) are available but will not be used in this course.  A mull is prepared by grinding a solid sample with mineral oil to form a paste which is then placed between salt plates for analysis.  The solutions of samples, either liquids or solids can analyzed using special solution cells.

Sample Prep Method
Compound State
Neat (undiluted, salt plates) 
KBr Pellet; ATR
Liquid or Solid

Table 3:
  Sample Preparation Methods for IR Spectroscopy
Interpretation of the IR Spectrum
Interpretation of IR spectra involves correlating peaks in a spectrum with known ranges for specific bond or functional group types.  Use the four general guidelines bulleted below for interpreting the IR spectrum of your unknown.  More specifiic guidelines are outlined in the experiment procedure.. 
  • Focus on Peaks between 1500-3600cm-1.  Peaks below 1500cm-1 generally cannot be specifcally interpreted.
  • Focus on the major peaks in the spectrum with absorbances of 50% of scale or greater.  Weaker absorbances generally cannot be specifically interpreted.
  • The most characteristic regions of the IR spectrum are 3000-3500 cm-1where OH (very broad and intense) and NH (sharper than OH) peaks appear and 1650-1850cm-1 where carbonyl peaks appear.   These regions will provide the most information about the structure of the compound.
  • The absence of peaks can be just as characteristic as the presence of peaks.  For example, a peak at 1750cm-1 indicates a carbonyl containing functional group is present in the molecule.  If no peak appears between 1650-1850cm-1, then no carbonyl containing functionl groups are present.
Step by step guidelines for specific interpretation of the IR spectrum of  the unknown are outlined in the experimental procedure section.  Follow these steps to determine the major functional groups in your unknown. 

NMR Spectroscopy
NMR Spectroscopy is an analytical method for determining the structure of organic compounds.  Specifically, NMR spectroscopy provides information about the carbon skeleton of an organic molecule.  Only elements with an odd number of protons or neutrons in its nucleus can be analyzed by NMR spectroscopy.  The most useful type of NMR analyses are proton (1H) and carbon-13 (13C) NMR spectroscopy. (Carbon-13 is an isotope of the more naturally abundant carbon 12). Only proton NMR spectroscopy will be studied in this experiment. Samples for analysis by proton NMR spectroscopy are inserted into a "probe" and exposed to radio frequencies that induce protons to resonante.  The resonance energy associated with the specific proton in a molecule gives rise to a peak in the NMR spectrum.  Peaks in an NMR spectrum are often referred to as "resonances".  In proton NMR spectroscopy, only H atoms give rise to peaks in the spectrum. Every proton or hydrogen atom in the spectrum will correspond specifically to a hydrogen atom in the molecule being analyzed. 

Sample Preparation
Samples (both solids and liquids) for one-dimensional NMR spectroscopic analysis are usually dissolved in a solvent, although there are also solid state instruments that allow for solids to be analyzed without any solvent..  Since all hydrogen atoms in the sample will be detected by the spectrophotometer, only organic solvents which do not contain hydrogens atoms can be used in NMR spectroscopy, otherwise peaks from the solvent will appear in the spectrum.  Frequently, deuterated organic solvents are used, which means all of the H atoms of those solvents are replaced by deuterium (D), the 2H isotope of H, which is not detectable in NMR spectroscopy.  Some common solvents that are used for NMR spectroscopic analysis are listed in Table 4.  Compounds must be completely soluble in the solvent for solution NMR analysis.
Molecular Formula
carbon tetrachloride
deuterated chloroform
deuterated acetone
deuterated dimethylsulfoxide (DMSO)
deuterated methanol

Table 4:  Deuterated Solvents Used in NMR Sectroscopy

Appearance of the Proton NMR Spectrum
Shown below in Figure 1 is a typical, one-dimensional  proton NMR spectrum. (The term “spectrum” is the singular form of the word; “spectra” is the plural form of the word.)   Two-dimensional spectra or NMR spectra of other atoms (i.e., 13C ) look different.  For proton NMR, “peaks” originate from the baseline of the spectrum.  Only the H atoms of an organic molecule give rise to peaks in a proton NMR spectrum.  Peaks appear at different positions along the baseline (horizontal axis, frequency) depending on what type of H atom is responsible for giving rise to that peak. The term “resonance” is also used to describe a peak in an NMR spectrum.

Figure 1:  Proton NMR Spectrum of Ethyl Acetate

The proton NMR spectrum of ethyl acetate is given in Figure 3  There are three distinct peaks or resonances in the spectrum.  Two of the peaks are split or have multiplicities greater than 1.  The peak at 0 ppm corresponds to TMS, the internal standard and is not "counted" as a peak.  The triplet at 1.1-1.35 ppm corresponds to the Hc hydrogens of the CH3 group of ethyl acetate.  This peak is upfield of the singlet with a chemical shift of 2.0 ppm. The singlet corresponds to the Ha hydrogens of the methyl group of ethyl acetate.  Finally, the quartet at 4.0-4.4 ppm, downfield of the singlet and the triplet,  corresponds to the Hb hydrogens of the other methyl group of ethyl acetate.

Tetramethylsilane (TMS) or other “internal standards” are used to calibrate the x-aixis of the spectrum to set the scale for thespectrum.  TMS is used most frequently and always gives rise to a peak at zero.      The units of x-axis of the spectrum are parts per million (ppm) or hertz (Hz).  The ppm units are more commonly used and run  from 1-10 ppm or 1-12 ppm in a typical proton spectrum.  The specific ppm unit where a peak appears in the spectrum is referred to as the chemical shift of the proton(s) that give rise to that peak.  The far left side of the spectrum is referred to as downfield and the far right side of 
the spectrum is referred to as upfield.  The vertical axis is a measure of peak intensity.  The area under the peak (or peak height and width) represents the relative number of H atoms that correspond to that peak.

Chemical Shift (d)
The chemical shift of a proton is the position of the peak in the NMR spectrum that corresponds to that proton.  The chemical shift is the parts per million (ppm) unit at which the peak appears, “shifted from” the zero point defined by the TMS internal standard.  The chemical shift itself is reported in ppm units.  The chemical shift of a proton is dependent on what kind of atom the proton is bonded to and on the kinds of other atoms that are near to the proton in the structure of the compound being analyzed.  Protons can be categorized into seven chemical shift ranges.  The Table 5 given below defines these ranges.  

Kind of Proton
0-1.5 ppm
H atoms bonded to sp3 carbons where the sp3 carbons are only bonded to other sp3 carbons and hydrogen  (alkanes)
1.5-2.5 ppm
H atoms bonded to sp3 carbons where the sp3 carbon is bonded to at least one sp2 carbon and no heteroatoms (allylic, benzylic, alpha-H)
2.5-4.5 ppm
H atoms bonded to an sp3 carbon that is also bonded to at least one heteroatom
4.5-6.5 ppm
H atoms bonded to sp2 carbons of alkenes (not aromatic sp2 carbons)
6.5-8.5 ppm
H atoms bonded to sp2 carbons of an aromatic ring
10-12 ppm
H atom bonded to an sp2 carbon atom of the carbonyl group of an aldehyde or H atom bonded to the sp3 oxygen of a carboxylic acid.
H atom directly bonded to a heteroatom other than the oxygen atom of a carboxylic acid.  Show up as a broad singlet (eg. OH of alcohol or phenol; NH of amine or amide)

  Table 5:  Chemical Shift Ranges of Various Proton Types in Proton NMR Spectroscopy
Equivalent and Non-Equivalent Protons
Protons bonded to the same atom usually appear together as one peak.  Protons bonded to the same kind of atom (eg., protons bonded to alkane type sp3 carbons) appear in the same chemical shift range, but  will  have different specific chemical shifts due to differences in the environment of protons.  Protons bonded to exactly the same atom that appear together as one peak are considered to be equivalent protons.  Protons that are in different enviirnoments (i.e., bonded to different atoms) are consdiered to be non-equivalent protons.  Consider the designation of protons of ethyl acetate in the example below. 


There are three different types of protons (hydrogens) associated with ethyl acetate, designated as Ha, Hb and Hc in the structure at the left.  The designation of these three types of atoms is determined based on the fact that each of these types of hydrogens are bonded to different atoms in the structure of ethyl acetate.  The Ha hydrogens are non-equivalent with the Hb and Hc hydrogens and it would be expected that the NMR spectrum of ethyl acetate would exhibit three distinct peaks, each peak corresponding to the three types of hydrogens in the molecule.  The three Ha hydrogens are equivalent and would be expected to appear as one peak in the NMR spectrum.  Likewise, the three Hc protons are equivalent and the two Hb protons are equivalent

Multiplicity  (Splitting) of Peaks

Peaks corresponding to a proton or group of equivalent protons in an NMR spectrum do not always show up as a single peak.  Instead, peaks may be split into specific, multiple parts, where all the parts are considered to make up the peak or resonance for that proton.  The split peaks have specific characteristics .  In general, peaks in an NMR spectrum may appear in five ways, shown in Figure 2.
Figure 2:
  Examples of Different Types of Peak Multiplicities in Proton NMR Spectroscopy

Peaks corresponding to a specific proton, or group of protons are split into multiple parts due to the presence of other non-equivalent hydrogen atoms that are three bonds or less away from the proton.  These hydrogens that are three bonds away from the proton are referred to as neighboring hydrogens.  The extent to which a peak is split into multiple parts is referred to as its multiplicity (m).  The multiplicity (m) of a peak for a particular proton(s) will be split into n+1 parts, where n = the # of neighboring hydrogen atoms.

m = n +1

m = multiplicity
n = # of non-equivalent "neighboring" hydrogens (3 bonds away)

Equivalent  hydrogens will not split
each other and are not counted when determining "n".  Peaks with a multiplicity of one are referred to as singlet (one peak), a multiplicity of two, doublet (two peaks), multiplicity of three, triplet (three peaks), multiplicity of four, quartet (four peaks) or multiplicity of greater than four, multiplet (five or more peaks).  An example analysis of proton multiplicities in provided below.  The ratios of the peak heights or areas for these different multiplicities are very characteristic.  The two peaks of a doublet appear in a 1:1 ration, the three peaks of a triplet appear as a 1:3:1 ratio, the four peaks of a quartet appear as a 1:2:2:1 ratio.  The ratio of peaks that make up a multiplet will vary.

Figure 3:  Proton NMR Spectrum of an Alcohol

The proton NMR spectrum of an alcohol in Figure 3 at the left has three "singlets", two sharp and one broad.  The two sharp singlets correspond to equivalent aromatic protons (at ~7.4 ppm) and protons bonded to a heteroatom (at ~ 4.7 ppm).  The low, broad “singlet corresponds to the OH hydrogen of the alcohol.

Doublets appear when there is one neighboring hydrogen atom (n =1; m =1+1=2, doublet).  A doublet is present in the NMR spectrum of isopropyl alcohol (Figure 4, below right).  The doublet corresponds to the six, equivalent hydrogen atoms of the two methyl groups labeled as Ha protons in the structure.  The Ha protons are split into two parts by the one, neighboring  Hb proton. The Hb proton appears as a multiplet (split into seven parts parts) due  to the six neighboring Ha hydrogens. The Hc proton does not appear in the spectrum.

Protons in a molecule that are not near neighboring hydrogen atoms will appear as singlets in the proton NMR spectrum (n=0, m=0 + 1= 1, singlet).  Hydrogen atoms directly bonded to heteroatoms (OH, NH etc) usually appear as singlets as well, however the shape of the peak for these H atoms is usually low and broad as opposed to tall and sharp (with the exception of carboxylic acid OH hydrogens.  These are usually sharp).  Hydrogen atoms bonded to heteroatoms do not cause splitting (even if they are "neighboring") not do these hydrogen get split.  Sometimes, protons bonded to heteroatoms do not appear in the spectrum at all or are masked underneath other peaks. 

Figure 4:  Proton NMR Spectrum of Isopropanol

Integration of Peaks
The relative area under each peak corresponds to the relative number of protons that correspond to that peak.  Integration measures the relative areas under peaks in the same spectrum.  The integration of peaks is typically provided in two ways, using integration lines or by numerical ratios.  The two methods of integration are illustrated Figure 5 in the (numerical method; proton NMR spectrum of isopropyl alcohol) and Figure 6 (line method, 2-fluoroaniline) .  For the line method, the rise of the line must be measured with a ruler.  The ratio of the length of the rises for each peak corresponds to the ratio of different types of protons.  Integration is not always provided in an NMR spectrum.

Figure 5:  Numerical Intergation of Isopropanol

Figure 6:  Line Intergration of 2-Fluoroaniline

Mass Spectroscopy
Mass spectroscopy is an analytical technique used for determining the molecular weight of organic compounds.  Mass spectroscopy also detects the presence of significant isotopes of atoms in molecules, most notably, bromine and chlorine.  In a mass spectrophotometer, a molecule is introduced into a vacuum chamber.  The molecule (M) is then bombarded with high speed electrons which induce a loss of a single electron from the molecule.  The net result is that a radical cation (M.+) is generated.  The radical cation is then delivered to the detector which sends a signal to the recorder to generate a peak. The specific position of the peak in the mass spectrum is an indicator of the original compound’s mass to charge ratio (m/z) which can be correlated to the compound's molecular weight.

Figure 7:  Sample Mass Spectrum
Appearance of the Mass Spectrum
The mass spectrum is a graph (Figure 7).  Along the x-axis is the mass/charge ratio, m/z.  Along the y-axis is the relative abundance (RA), also called relative intensity (RI).  Peaks appear as lines in the spectrum. Lines correspond to the molecular weight of the original molecule analyzed (M) or pieces (fragments) of the original molecule analyzed.  The position of the line along the x-axis indicates the molecular weight of the molecule or fragment that corresponds to that line.  The height of the peak corresponds to the amount of the molecule or fragment of an exact molecular weight that is present relative to other fragments of the molecule.

Interpretation of Mass Spectra
The peak in the spectrum that corresponds to the original molecule, M, is called the parent peak or molecular ion.  This peak is often, but not always the tallest peak in the spectrum.  The parent peak is usually identified as the peak that is at at least 50% relative intensity and has the largest m/z value.  The tallest peak in the spectrum is called the base peak. 

For molecules with a charge of +1 (typical for chemical ionization and electrom impact mass spectroscopy), the m/z value of the parent peak corresponds to the molecular weight of the molecule, M, if all atoms of the molecule are composed of the most abundant isotopes of the individual atoms that make up that molecule. Many atoms that are common in organic molecules have multiple isotopes, however usually only one isotope is highly naturally abundant.  Table 6 lists the most important atoms in a mass spectral analysis of  organic compounds.

Relative Abundance

   Table 6:  Natural abundances of Atoms Common to Organic Molecules.

For example, an organic molecule like methane, CH4, has a  parent peak with a molecular weight of 16, where the carbon atom of methane is 12C, the most abundant isotope of carbon.  Carbon-12 has a relative abundance of 98.895% and the height of the parent peak that contains this isotope will correspond to 98.89% relative intensity or "relative abundance".  There will also be another small peak in the spectrum at m/z =17 that corresponds to methane where the carbon atom is 13C, the carbon 13 isotope.  This peak will have a relative abundance of 1.1%.  And finally there will be a third peak in the spectrum at 18, corresponding to methane with 14C, but the relative abundance will be so small it may not be detectable in the spectrum.  The peak at 16 is called M+, the peak at 17 is called M++1, and the peak at 18 is called M++2.

For organic molecules that contain C, H, and O, the mass spectrum will typically contain an M+ and a very small M++1 peak, due to 13C, but no significant M++2, M++3 etc…because the statistical probability that two very low abundance atoms like 13C and 14C will occur in the same molecule is unlikely.  However, since chlorine and bromine have multiple isotopes of significant abundance,  molecules that contain these halogens will exhibit significant M++2 peaks.  Consider chlorobenzene.  There will be a parent peak due to the molecular mass from the six 12C atoms and the one 35Cl atom at m/z =112.  Additional M++1 , M++2 etc will appear due to the 13C and 37Cl isotopes.  The relative intensities of the peaks that arise due to the less abundant isotopes will be much smaller. For molecules containing chlorine, the ratio of the M+ peak and the M+ + 2 peak is 75 :25.  For bromine the ratio of the M+ peak to the M++2 peak is ~50:50.

Figure 8:  Mass Spectrum of Chlorobenzene

Molecular Formula
Relative Intensity
12C6H535Cl  (M+)
12C5 13CH535Cl  (M++1)
12C61H537Cl (M++2)
12C5 13CH537Cl (M++3)

Table 7:  Specific Molecular Formulas, with isotopes for chlorobenzene and the expected m/z values for observed peaks