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884 - IR Spectroscopy-62-70

1. Infrared spectroscopy is mainly used for qualitative analysis to identify unknown compounds by examining their functional groups. It revolutionized organic chemistry in the 1950s by decreasing compound identification time significantly. 2. Infrared spectroscopy can also be used for quantitative analysis, but it is less accurate and precise than UV-Vis spectroscopy due to more complex spectra, narrower bands, and limitations of IR instruments. However, it has good selectivity since compounds have unique IR spectra. 3. Modern IR instruments can search databases of reference spectra to identify unknown compounds based on their strongest and secondary absorption bands and overall spectral fingerprint.

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266 views9 pages

884 - IR Spectroscopy-62-70

1. Infrared spectroscopy is mainly used for qualitative analysis to identify unknown compounds by examining their functional groups. It revolutionized organic chemistry in the 1950s by decreasing compound identification time significantly. 2. Infrared spectroscopy can also be used for quantitative analysis, but it is less accurate and precise than UV-Vis spectroscopy due to more complex spectra, narrower bands, and limitations of IR instruments. However, it has good selectivity since compounds have unique IR spectra. 3. Modern IR instruments can search databases of reference spectra to identify unknown compounds based on their strongest and secondary absorption bands and overall spectral fingerprint.

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Applications of IR

1.) Qualitative Analysis (Compound Identification)


- main application
- Use of IR, with NMR and MS, in late 1950’s revolutionized
organic chemistry
 decreased the time to confirm compound identification 10- 1000 fold
 examine what functional groups are present by looking at
group frequency region- 3600 cm-1 to 1200 cm-1
Abbreviated Table of Group Frequencies for Organic Groups
Bond Type of Compound Frequency Range, cm-1 Intensity
C-H Alkanes 2850-2970 Strong
C-H Alkenes H 3010-3095 Medium
C C
675-995 strong

C-H Alkynes C H 3300 Strong


C

C-H Aromatic rings 3010-3100 Medium


690-900 strong
0-H Monomeric alcohols, phenols 3590-3650 Variable
Hydrogen-bonded alchohols, phenols 3200-3600 Variable, sometimes broad
Monomeric carboxylic acids 3500-3650 Medium
Hydrogen-bonded carboxylic acids 2500-2700 broad
N-H Amines, amides 3300-3500 medium
C=C Alkenes 1610-1680 Variable
C=C Aromatic rings 1500-1600 Variable
C C Alkynes 2100-2260 Variable
C-N Amines, amides 1180-1360 Strong
C N Nitriles 2210-2280 Strong
C-O Alcohols, ethers,carboxylic acids, esters 1050-1300 Strong
C=O Aldehydes, ketones, carboxylic acids, esters 1690-1760 Strong
NO2 Nitro compounds 1500-1570 Strong
1300-1370
Fingerprint Region (1200-700 cm-1)
- region of most single bond signals
- many have similar frequencies, so affect each other & give pattern
characteristics of overall skeletal structure of a compound
- exact interpretation of this region of spectra seldom possible because
Fingerprint Region
of complexity
- complexity  uniqueness
Computer Searches
- many modern instruments have reference IR spectra on file
(~100,000 compounds)
- matches based on location of strongest band, then 2nd strongest band, etc
overall skeletal structure of a compound
- exact interpretation of this region of spectra seldom possible because of
complexity
- complexity  uniqueness

Bio-Rad SearchIT database of


~200,000 IR spectra
2.) Quantitative Analysis
- not as good as UV/Vis in terms of accuracy and precision
► more complex spectra

► narrower bands (Beer’s Law deviation)

► limitations of IR instruments (lower light throughput, weaker detectors)

► high background IR

► difficult to match reference and sample cells

► changes in e (A=ebc) common

- potential advantage is good selectivity, since so many compounds have


different IR spectra
► one common application is determination of air contaminants.

Contaminants Concn, ppm Found, ppm Relative error, %


Carbon Monoxide 50 49.1 1.8

Methylethyl ketone 100 98.3 1.7

Methyl alcohol 100 99.0 1.0

Ethylene oxide 50 49.9 0.2


chloroform 100 99.5 0.5
Example : The spectrum is for a substance with an empirical formula of C3H5N. What is
the compound?

Aliphatic Nitrile or alkyne No aromatics


hydrogens group One or more
alkane groups

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