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Introduction
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	C_sp3-H of alkanes and alkyl groups	medium-strong
3020-3100 cm^-1	C_sp2-H of alkenes and alkenyl groups	medium-strong
1640-1680 cm^-1	C_sp2=C_sp2 of alkene	medium
3300 cm^-1	C_sp-H of alkynes	strong
2100-2260 cm^-1	CºC of alkynes CºC of nitriles	weak-medium
500-800 cm^-1	C_sp3-X, X = halogen of alkyl halide	strong
3200-3650 cm^-1	O-H of alcohol, phenol, carboxylic acid	strong, broad
1660-2000 cm^-1 1450-1600 cm^-1	C_sp2=C_sp2 of aromatic	weak medium
1680-1850 cm^-1	C=O of carbonyls ketones, aldehydes, esters, carboxylic acids, amides See more specific ranges	strong
3300-3500 cm^-1 1030-1230 cm^-1	N-H of amines C-N of amines	medium medium
1540 cm^-1	N=O of nitro group (-NO₂)	strong

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)	Liquids
Mull	Solid
KBr Pellet; ATR	Solid
Solution	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^-1where 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

Introduction
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.

Solvent	Molecular Formula
carbon tetrachloride	CCl₄
deuterated chloroform	CDCl₃
deuterated acetone	C₃D₆O
deuterated dimethylsulfoxide (DMSO)	C₂D₆SO
deuterated methanol	CD₃OD

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.

Range	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.
Anywhere	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

Type of Spectroscopy	Frequency of EM Spectrum (Hertz (Hz))	Wavelength (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