IR Spectroscopy

12.6, 12.7, 12.8

What portion of infrared is utilized by organic chemist

Mid portion infrared inbetween near infrared and far infrared

What values are λ and v usually given in?

• λ within the IR region are usually given in micrometers (1 μm = 10^–6 m)

• v is given in wavenumbers (v^→)rather than hertz

  • A wavenumber is the reciprocal of wavelength in centimeters w/ the unit of 1/ (λ (cm))

What’s the useful IR region

A wave number of 4000 to 400 corresponding to energies of 48.0 kJ/mol to 4.80 kJ/mol

What are the type of allowed molecular vibrations?

• Symmetric Strething - both bonds pull outward/inward at the same time

• Antisymmetric Stretching - One bond lengthens while the other shortens

• In - plane bending - Atom move within the plane of the molecule, changing the bond angle

• Out - of - plane bending - Atoms move above and below the molcular plane, chaging bond angles in 3D

Why does an organic molecule absorb some wavelengths of IR radiation but not others?

Because the amount of energy a molecule contains is not continously variable but is quantized (the energy a molecule contains is constant), meaning it will only react (stretch or bend) at specific frequencies corresponding to specific energy levels

• When a molecule is irradiated w/ light, energy is absorbed if the frequency of the radiation matches the frequency of the vibration bonds naturally are in

• The result of this energy absorption is an increased amplitude for the vibration causing more stretching/compressing

• Since the frequency absorbed by a molecule corresponds to a specific molecular motion, we can find what kinds of motions a molecule has by measuring its IR spectrum and what type of functional groups are present

Do bonds in a molcule have a constant length?

No, a bond length is the average length of bonds at a specific frequency that are alternately stretching and contracting

Why is complete interpretation of IR spectrum difficult

• Organic molecules have many different bond stretching and bending motions, with each of those bonds having different absorptions

• This limits the use of IR in lab to pure samples of fairly small molecules because little can be learned from IR about large, complex molecules

How can the complexity of IR be a good thing?

Because the IR can act as an unique fingerprint of a compound. If two samples have identical IR spectra, they’re almost certainly identical compounds

• IR spectrum have a finger print region that is always different

How can we still get structural information from an IR then?

Via functional groups as they have specific absorption frequencies

• In addition to interpreting absorptions that are present in an IR spectrum, it’s also possible to get structural information by noticing which absorptions aren’t present

What is Dr.Waas Brown Cat w/ blue eyes

Barnaby born in 2012

What is Dr.Wass White and brown cat with golden eyes

Molly, born in April 2024

When organic molecules absorb infrared radiation, where do they undergo their level of transitions

At the level of bonds

• These are vibrational transitions involving strtching or bending of one or more bonds

When reading an infrared spectrum, what is on the x and y axis?

X axis - wavenumbers

• this unit is directly proportional to energy undlike wavelength whish is inversely proportional to energy

• The higher the energy of absorption, the “stiffer” the “spring” of the bond meaning it stretches inward and out more while lower energies are looser

What is unique about an IR spectra?

The peaks point downward

In an IR spectra, what are the most important numbers?

3000 cm^-1 and 1700 cm^-1

• At 3000 cm^-1 C - H bond stretching occurs

• At 1700 cm^-1 carbonyl group bond stretching occurs

When learning to interpret IR spectra, how should we approach it?

By looking at as many IR spectra as possible and interpret what you see instead

What is the archetype for a relatively featureless IR spectrum, and why is it important

A typical saturated hydrocarbon (think of hexane)

• showcases the absence of functional groups

  • Contains no multiple bonds

What do peaks just below 3000 represent

Sign of SP3 Carbons with C - H bond stretching, represents C - H stretching frequencies

• Can identify your compound is a saturated hydrocarbon

What do peaks just below the 1500 wavenumber region on an IR spectra represent?

Finger print region peaks

• characteristic for reach individual compound; it’s fingerprint

What do peaks just above 3000 represent

• C - H stretching frequencies of an unsaturation carbon carbon molecule

• Sign of Sp2 hybridized Carbons typical of either an alkene or an aromatic compound

What do peaks between 1650 and 1750 represent?

Carbonyl compounds stretching

What bond stretching occurs at 1050 cm^-1?

• C - O stretching frequency in an ester, used to distinguish an ester from a ketone or aldehyde, not neccisarily an alcohol

• In fingerprint region

How do you identify an alcohol containing compound on an IR

At around 3400 cm^-1, there will be a smooth parabolic peak indicating the stretching of a C - OH bond

• There will always be a peak at 1050 as well to indicate C - O stretching, but to classify it as an alcohol and not an ester it should be missing its carbonyl peak

What bond is stretching in a large valley peak around the 3000 area

O - H stretching of a carboxylic acid

• It may completely cover the C - H stretching region

• For a carboxylic acid you should also see a C=O stretch of the carbonyl

How to use the japanese resource to identify functional groups from an IR spectrum

• Refer to list of functional groups at the beginning of chapter 3

• Make up a molecule that contains this group that is similar to the aktiv questions

• Name the compound or find its molecular formula

• Look up the name in SDBS or even w/ a google search and its IR spectrum

• Look for key peaks comparing to page 399 in 12.7 as needed

What bond stretching occurs at 3300

Alkynes

What bond stretching occurs from 600 - 800

C - Cl

What bond stretching occurs at 500 - 600

C - Br

What bond stretching occurs at 2200

Nitrile