IR Spectroscopy

Question 1

What is spectroscopy and how does it help with molecules?

Answer 1

spectroscopy is the study of the interaction of electromagnetic radiation (fancy way of saying light) with matter. using IR, which a part of the electromagnetic spectrum, you can determine and understand the structure of molecules

Question 2

What is light and how does it behave?

Answer 2

light can be looked at as a wave and as a particle

waves are described using speed of light = frequency x wavelength; frequency and wavelength have an inverse relationship

particles are described using energy of a photon = planck’s constant x frequency; where frequency and energy of a photon have a direct relationship

Question 3

Explain IR in terms of spectroscopy.

Answer 3

IR has a wavelength from 800nm to 10^6 nm and is invisible to our eyes but can be felt (warm)

Question 4

What is an IR Spectrometer and how does it help determine a structure of a molecule?

Answer 4

an IR Spectrometer has a plate where a sample is kept and an amount of IR is zapped through the sample. the detector underneath the sample then determines the amount of IR that went through the sample also giving the amount of IR that was absorbed by the sample. therefor the amount of IR absorbed can help us determine the structure of the molecule in the sample

Question 5

What are vibrational motions?

Answer 5

Stretching: which is lengthening of a bond (stretch and compress)

Bending: which is the movement of a bond in-to or out-of the plane

Question 6

What does it mean when they say vibrational motions are “quantized”?

Answer 6

this means they have a specific energy, which means that each vibrational motions changes the energy of the molecule

only certain energy levels are allowed, therefore if we want to go from one vibrational motion to another, we have to zap with IR radiation (which is energy) and the energy from the IR radiation has to match the difference in energy necessary to change to the new vibrational motion

nothing will happen to the molecule if you zap with an insufficient amount of energy, it will stay the same because energy is quantized

Question 7

Explain the IR Spectrum.

Answer 7

the IR Spectrum’s Y-Axis is the percent transmittance which is the amount of light going through a certain functional group of a molecule. The spectrum has peaks and those are when molecules are absorbing photons of light. The spectrum is measured (X-Axis) in wavenumbers which is inverse centimeters. The spectrum is typically measured from 4500 cm-1 to 500 cm-1

Question 8

What is the difference between strong and weak absorptions?

Answer 8

strong absorptions are those whose percent transmittance is near zero (near the bottom of the spectrum)

weak absorptions are those whose percent transmittance are near 100% (near the top of the spectrum)

Question 9

What are some common absorption frequencies?

Answer 9

Alcohols appear around 3200-3600 cm-1

Carbonyls appear around 1700 cm-1

Question 10

What are the regions of the IR spectrum?

Answer 10

The fingerprint region is the region from 1400 to 500 cm-1. This region is called the fingerprint region because each molecule has a unique set of peaks in that region that identifies it. Just like each human being has a fingerprint and no two fingerprints are exactly alike. This can also be applied to molecules in this region, no two molecules will have the same fingerprint region.

The functional group region (AKA diagnostic region) is between 4500 to 1400 cm-1 and identifies the functional groups. Two molecules could have very similar regions in the functional group region

Question 11

What is an overview for where you would see stretches in the IR Spectrum?

Answer 11

around 3600 you will see alcohols (OH), around 3300-3500 you will see NH, and around 2700-3300 you will see CH

triple bonds will be found from 2000-2500

double bonds (C=C, C=0) will be found 1500-2000

below 1500 gets you into the fingerprint region and we do not focus attention in this region

Question 12

How does frequency change in terms of IR Spectrum with different molecules?

Answer 12

using hooke’s law, you can see that as a mass gets smaller, the frequency of a spring will get larger. as the spring constant gets larger, then the spring will become stiffer meaning the frequency will increase

Question 13

How does hybridization affect frequency?

Answer 13

since sp3 hybridized carbons have longer bonds than sp2 and sp, it will have the unstiffest spring.

sp2 will have shorter bonds, meaning that the bonds are stronger, which is analogous to a stiffer spring which means a higher frequency and higher energy.

sp has the shortest bonds, which means the strongest bonds, stiffest spring, and will absorb a much higher frequency photon

Question 14

Why are some peaks tall and some shorter in a IR spectrum?

Answer 14

this has to do with quantum mechanics, but we will only look at it basically. meaning the symmetry of an alkene will determine the intensity of the peaks. this means the less symmetric an alkene is, the more intense of a peak you will get.

Question 15

Why are carbonyl stretches so intense?

Answer 15

since carbonyls are more polarized, the more polarized a bond is the more intense of a peak you will get

oxygen is a more electronegative atom than carbon, so the electron density is pulled towards the oxygen making it more polarized. similarly with the oxygen and hydrogen in an alcohol

Question 16

What are the differences between alcohols and carboxylic acids in the IR Spectrum?

Answer 16

alcohols are TYPICALLY nice and smooth (can be messy), broad but not as broad as carboxylic acid stretches

Question 17

Why are alcohol (OH) stretches so broad?

Answer 17

due to the hydrogen in the OH bond being able to hop around to and from other molecules or solvent molecules. how? because of hydrogen bonding networks between the solvent and carboxylic acid molecules, this allows hydrogen to jump around, and jumping around makes the signal broad

Question 18

Where will you find a NH stretch and what is the difference between NH and OH stretches?

Answer 18

NH stretch can occur on amines and amides and will be found between 3300-3500 cm-1

NH is less intense and not as broad as OH; NH bond is less polar than OH bond in alcohols and carboxylic acids and the hydrogen bonding network in amines and amides are weaker than those in alcohols and carboxylic acids

Question 19

What changes to the IR Spectrum do you see with different types of amines and amides?

Answer 19

amines and amides have no differences in the 3300-3500 region of the spectrum

if the nitrogen is connected to 2 hydrogens, you will see 2 peaks in the spectrum due to the capability of 2 different vibrational modes at the same time. same for 1 and same for 0 hydrogens connected to nitrogen.

Question 20

What is conjugation and how does it affect carbonyl stretches?

Answer 20

conjugation is looking at molecules pi electrons and searching for a repeating pattern of double-single-double

if a group is conjugated, it will lower the frequency meaning it will lower the energy

the energy portion: the non-conjugated molecule has a larger energy gap between the homo and lumo, the more conjugated has a smaller gap. the smaller gap is because of the conjugation so it takes less energy for photon of light to be absorbed

Question 21

Where will you find alkene (C=C stretches) on the IR spectrum?

Answer 21

around 1500 region

Question 22

Where will you find alkyne and nitrile stretches, where the differences between the two? where will you see a terminal alkyne?

Answer 22

if you see a stretch between 2210-2260 you will 100% have an alkyne or nitrile. alkynes are weak in intensity compared to nitriles

a terminal alkyne will have a stretch around 3300

Question 23

Where will you find aromatic and aldehyde CH stretches?

Answer 23

aromatic is a cyclic molecule which is conjugated and can be found around 3000-3100 (similar to alkenes)

aldehydes have 2 unique peaks at 2720 and 2820 at moderate intensity

Question 24

What causes a molecule to have a stronger C=C signal?

Answer 24

an electronegative atom on the molecule will cause a dipole moment which increases the signal strength

if the electronegative atoms are connected to 1 carbon and not the other, it will cause it to increase the signal strength compared to if a electronegative atom was connected to both (14.5)

Question 25

What can cause a carbonyl to have a lower wavenumber?

Answer 25

conjugation and being next a double bond because of resonance (14.3)

Question 26

What causes a C=C to be more intense?

Answer 26

dipole moments, a dipole moment will promote an unsymmetrical molecule which means more intense.

2 electronegative atoms on 1 carbon will make a more intense signal than 1 EN on one carbon