IR Spectrum

Orgo 2 (Part of Exam 1)

What type of light is used in IR spectroscopy and what does it study?

Infrared light is used in IR spectroscopy and it studies the vibrational modes of molecules

Where does IR radiation fall on the electromagnetic spectrum?

Between microwaves (10^-2 m) and visible light (400-700 nm)

Give a real-world example of infrared radiation

Some variety of night vision googles produce images in total absence of visible light by detecting the IR radiation emitted by warm objects

Define spectroscopy.

The study of the interaction between matter and electromagnetic radiation

Write the equation relating wavelength, frequency, and the speed of light

Describe the relationship between frequency and wavelength.

Inverse relationship; as frequency increases wavelength decreases and vice versa

What is the relationship between photon energy and frequency?

Directly proportional; as frequency increases energy increases

Which type of electromagnetic radiation has the highest energy?

Gamma rays

What is the IR radiation wavelength range?

700nm - 1mm

What does an IR spectrometer measure?

measures the absorption of infrared radiation by a sample as a function of a wavenumber.

What happens when a molecule absorbs IR light?

It transitions to a higher vibrational energy state, as the absorbed energy matches one of its vibrational frequencies

In IR, bonds are modeled as what mechanical analogy?

Springs

Name the two basic types of vibrational motion.

Bending and stretching

What defines a symmetric stretch?

When two bonds are stretching in phase with each other. (same direction)

Describe an asymmetric stretch

When two bonds stretch out of phase with each other. (opposing directions)

Give an example of in-plane bending

scissoring vibration

What does it mean that vibrational energies are quantized?

It means that a molecule can only vibrate at a specific energy level, not an arbitrary one

What condition must be met for IR absorption?

The vibration must cause a change in the molecule’s dipole moment

What occurs if IR energy is too low to match the energy gap?

Nothing occurs

Why are radio waves safe compared to UV light?

Radio waves are lower in frequency, thus lower in energy compared to UV light; therefore, they don’t have enough energy to break chemical bonds and such, which UV light does to cause damage to us.

What does the y-axis in an IR spectrum represent?

Percent transmittance

What does a peak on an IR spectrum signify?

It signifies a specific bond or functional group in a molecule

Define wave number in IR spectroscopy.

Wavenumber is a measure of the frequency of radiation expressed as reciprocal centimeters, and calculated as the reciprocal of wavelength

What do higher wave numbers indicate in IR spectroscopy?

Higher wavenumber means more energy is required, indicating lighter atoms or stronger bonds

State the useful IR range for organic chemists.

4000-1500 cm^-1

What characterizes a strong IR absorption?

Large change in dipole moment/ nearing 0 for % transmittance

Where does the carbonyl stretch appear in IR spectra?

1650-1820 centered at ~1700

Why is the carbonyl stretch important for IR analysis?

It’s important because it is a strong characteristic for absorption that helps identify carbonyl-containing functional groups

What is the fingerprint region in IR?

< 1400 cm^-1 in IR spectra, with complex absorption unique to each compound.

Why is it called the fingerprint region?

Just like the human finger, that region is completely unique to each compound

What is the functional group region in IR?

>1500 is the functional group region

At what wave number do O–H stretches appear?

3200-3600 cm-1

Which bond vibrations appear around 2,100–2,300 cm-1?

Alkynes and Nitriles

Where are bending vibrations typically seen in IR?

In the fingerprint region

What is another term for the functional group region?

Diagnostic region

Where do N–H stretches typically appear?

3350-3500 cm^-1

What is the typical IR position for C=C double bond stretches?

1600-1700 cm^-1

Why do chemists focus on the functional group region instead of the fingerprint

region?

The functional group provides a major characteristic that helps a chemist determine the general form of the molecule, with the fingerprint region being too unique to allow us to identify any bond/groups.

What bonds give peaks around 2,700–3,300 cm-1?

Carboxylic acid (Number One Answer)

N-H of amine/amide

alcohol

C-H

Why do functional groups absorb at characteristic IR frequencies?

(quantized) Because the frequency of a bond’s vibration is determined by the specific strength of the bond and the masses of the atoms involved (Hooke’s law)

What analogy is used to describe bond vibrations in IR?

a spring connected to 2 masses (weights)

State Hooke’s law as applied to IR spectroscopy.

vibrational frequency of a bond is proportional to the square root of the spring constant divided by the mass of the atoms involved

What does "k" represent in Hooke’s law for IR?

spring constant

What effect does decreasing atomic mass have on vibrational frequency?

Decreasing atom mass = increases vibrational frequency

Why does a C–H stretch absorb at higher frequency than a C–C stretch?

C-H absorbs at a higher frequency because the hydrogen atom has a significantly lower mass than the carbon atom.

How does bond stiffness affect IR absorption?

Bond stiffness (increase in k) means a higher wavenumber

Rank sp³, sp², and sp C–H stretches by frequency.

highest frequency ————————————- lowest frequency

sp, sp², sp³

Why is an sp C–H stretch highest in frequency?

Because it has the highest s-character

What determines IR peak intensity?

The change in dipole moment during vibrational transition

Why does a polar bond show strong IR absorption?

It possesses a large dipole moment change

Why can symmetric alkenes have weak or missing C=C IR peaks?

Because it would be a zero net dipole change

How does molecular symmetry affect IR intensity?

It would decrease intensity

What causes O–H stretches to be so intense in IR?

High IR intensity, hydrogen bonding, high polarity, large dipole moment

At what wavenumber do carbonyl stretches appear, and why are they intense?

~1700 cm^-1, large dipole moment

Explain why sp bonds are stronger than sp² or sp³.

Higher s-character leads to shorter and stronger bonds

What two factors determine IR frequency according to Hooke’s law?

Spring’s constant (k) and mass (m)

How does bond polarity relate to IR peak intensity?

Higher bond polarity = greater/stronger IR peak intensity

Why do carbonyl peaks appear stronger than alkene peaks?

The difference in atoms; carbonyl is highly polar, while an alkene is not

What happens if IR photon energy doesn’t match bond vibration energy?

Nothing happens (no vibrational excitation occurs)

Summarize how Hooke’s law explains IR spectra.

Vibrational frequency is determined by the bond strength and mass of the molecule involved

Which IR region should be ignored during initial spectrum analysis?

The fingerprint region

What does a broad, intense IR peak from ~3,600 to ~3,000 cm-1 indicate?

Possibly an O-H stretching

At what approximate wavenumber do carbonyl (C=O) stretches appear?

~1700cm^-1

How does an O–H stretch of a carboxylic acid differ from an alcohol’s O–H stretch?

Carboxylic acids tend to be broader and overlaps C-H stretch 2500-3300cm^-1

Where do sp³ C–H stretches appear in relation to 3,000 cm-1?

Right below 3000 cm^-1

Where do sp² C–H stretches appear in IR?

Right above 3000 cm^-1

Define the fingerprint region in IR spectroscopy.

1500-500cm^-1, a unique section for molecules

What is the first step in analyzing an IR spectrum effectively?

What are the rest of the steps?

1. Look at peaks above 1400 cm^-1

2. Recognize the easy-to-spot peaks

3. Look at ~1700. Is there a peak?

4. Draw a line at 3000 cm^-1. Sp3 C-H stretches on the right, sp2 C-H stretches on the left, and sp C-H stretches far left.

Why can O–H and sp³ C–H stretches overlap in IR spectra?

Some O-H stretches are stronger and extended to the sp3 C-H bonds, merging with those peaks like carboxylic acids.

Why do we draw a vertical line at 3,000 cm-1 in IR analysis?

To clearly indicate the sp/sp2/sp3 C-H stretches

Why is the O–H stretch broad in IR spectroscopy?

Because of hydrogen bonding and its constant movement, which means it varies in its strength

What type of interaction causes O–H peak broadening in IR?

hydrogen bonding

At what IR range do N–H stretches appear?

3300-3500

Why are N–H stretches less intense and narrower than O–H stretches?

Because N-H stretches are less polar than O-H stretches

How can IR reveal the number of hydrogens attached to nitrogen?

With how many peaks are

Primary = 2 peaks

Secondary = 1 peak

Tertiary = 0 peaks

Why do two N–H peaks appear in IR for a nitrogen with two hydrogens?

Because there are 2 identical N-H bond stretches in 2 distinct ways

What happens in IR if nitrogen has no attached hydrogens?

Then there are no peaks/signals

Can IR differentiate between amines and amides using N–H stretches?

Not really, no

Why are N–H stretches sharper than O–H stretches?

Weaker bonds = Sharper peaks.

N is less electronegative than O, which means the N-H bond weaker

What explains two peaks for N–H stretches with two hydrogens?

asymmetric and symmetric vibrational stretching modes

At what approximate IR frequency does a carbonyl (C=O) stretch appear?

1700

Name a functional group that contains a carbonyl (C=O).

ketones

aldehydes

carboxylic acids

esters

amides

acid chlorides

How does the IR C=O stretch of a ketone compare to an amide?

Ketone frequency is higher than amide (1715 > 1650-1690)

Why does resonance in an amide lower its C=O IR frequency?

There is a decrease in bond strength, more single bond character than double bond character

What resonance effect stabilizes an amide?

Nitrogen’s delocalized lone pair, pi electron overlapping pi orbital of carbonyl group

What pattern indicates conjugation in a molecule?

double bond, single bond, double bond, single bond (double = multiple bond)

How does conjugation affect a carbonyl’s IR frequency?

lowers carbonyl IR frequency = Increase single bond character = lowers k

Why does conjugation lower IR energy requirements for carbonyl stretches?

resonance delocalization = decreases bond stretch = Increases length of bond = lowers k = lowers frequency = lowers energy

Which has a higher C=O stretch frequency: a conjugated ketone or an unconjugated ketone?

An unconjugated ketone has a higher frequency

Explain why conjugation reduces IR frequency using molecular orbital theory.

Conjugation is the mixing of pi orbitals, pi bonds order decreases = weaker bond (e- sharing weakens the bond, which lowers the force needed to stretch)

At what wavenumber range would you expect to observe a carbon-carbon double bond stretch (C=C) in an IR spectrum?

1600-1680 cm^-1

Describe how you would distinguish between sp³ and sp² C–H stretches using IR spectroscopy.

sp3 below 3000 cm^-1 sp2 right above 3000 cm^-1

What type of C–H stretch would you expect to see from a hydrogen on a benzene ring?

Sp2 C-H stretch

Identify the type of functional group responsible for an intense, sharp peak around 2200–2250 cm-1.

Nitrile

How can you distinguish a nitrile from an alkyne in an IR spectrum?

How intense the peaks are; Nitrile peaks are strong, while alkynes are weak

Explain why nitrile IR peaks are more intense than those of alkynes.

Greater dipole moment because nitriles are asymmetric

What is the hybridization of the carbon involved in a carbon-nitrogen triple bond?

sp

Describe the signature IR feature of an aldehyde group.

Fermi doublet at 2720 and 2820 cm^-1

What distinguishes an aromatic compound from a normal alkene in IR spectroscopy?

Out-of-plane C-H bends

A broad absorption around 3400 cm-1 is observed. What type of functional group is likely present?

Alcohol