Spectroscopy Topic 2

Question 1

How to calculate reduced mass?

Answer 1

Question 2

How to calculate frequency of vibration?

Answer 2

k-force constant

Question 3

How we get higher IR absorption frequesncy (wavenumber)?

Answer 3

By increasing force constant and by lowering atomic mass

Question 4

What are the subregions of IR spectroscopy?

Answer 4

4000-2500 cm^-1 X-H bonds

2500-1900 cm^-1 triple bonds

1900-1500 cm^-1 double bonds

1500-400 cm^-1 fingerprint region

Question 5

What is absorption band of alcohols and phenols in IR?

Answer 5

3200-3600 cm^-1, very broad

Question 6

What is absorption band of carboxylic acids in IR?

Answer 6

2500-3000 cm^-1, very broad

Question 7

What is absorption band of amines and amides in IR?

Answer 7

3300-3500 cm^-1, broad

1⁰ amines/amides show two bands

Question 8

What is absorption band of C-H bond in IR?

Answer 8

sp³<3000 cm^-1

sp² and sp >3000 cm^-1

Question 9

What is meanind of aldehydic?

Answer 9

Having a character of aldehyde

Question 10

What is absorption band of aldehydic C-H bonds in IR?

Answer 10

2700-2900 cm^-1, often shows two bands

Question 11

What is absorption band of S-H bonds in IR?

Answer 11

2550-2600 cm^-1, weak and not very broad

Question 12

What is absorption band of C≡C and C≡N bonds in IR?

Answer 12

2100-2260 cm^-1 , medium band

Question 13

What is absorption band of C=C bonds in IR?

Answer 13

For non-conjugated 1620-1680 cm^-1, weak-medium band. Conjugation lowers by 40 cm^-1

Question 14

What is absorption band of C=O bonds in IR?

Answer 14

1800-1650cm^-1, strong absorption band

Question 15

What is absorption band of acid chlorides in IR?

Question 16

What is absorption band of anhydrides in IR?

Answer 16

1750-1820 cm^-1, 2 bands

Question 17

What is absorption band of esters in IR?

Answer 17

1735-1750 cm^-1

Question 18

What is absorption band of aldehydes in IR?

Answer 18

1720-1740 cm^-1

Question 19

What is absorption band of ketones in IR?

Answer 19

1705-1720 cm^-1

Question 20

What is absorption band of carboxylic acids in IR?

Answer 20

1700-1725 cm^-1

Question 21

What is absorption band of amides in IR?

Answer 21

1670-1700 cm^-1

Question 22

Why absorption band of C=O depends on functional groups?

Answer 22

The stronger C=O bond the hugher the absorption band

Question 23

How conjugation afects absorption?

Answer 23

it lowers by ~ 20 cm^-1, except for amides where it’s increased by ~ 15 cm^-1

Question 24

What is the efect of ring size on cyclic ketone absorption for C=O stretch?

Answer 24

As the ring size gets smaller, bond strength increases due to the increased p character of bonds in smaller rings. The external C=O bond increases in s character to compensate, which results in stronger bonds and higher frequency.

Question 25

What is absorption band of C-O in IR?

Answer 25

1000-1300 cm^-1, medium to strong absorption

Question 26

What is absorption band of N-O stretch in NO₂ in IR?

Answer 26

medium to strong absorption 1300-1400 cm^-1, and strong absorption 1500-1600 cm^-1

Question 27

What is absorption band of C-Cl bonds in IR?

Answer 27

600-800 cm^-1, weak

Question 28

What is the absorption for organic molecules in UV/Vis spectroscopy?

Answer 28

above 200 nm, due to the presence of conjugated double bonds/ delocalisation. The electrons generally transition between π and π* MOs in this case.

Question 29

What is the absorption for unconjugated carbonyl compounds in UV/Vis spectroscopy?

Answer 29

Around 300 nm, due to an n - π* transition. This is usually very weak and therefore of limited diagnostic use.

Question 30

What is the absorption for benzene in UV/Vis spectroscopy?

Answer 30

Benzene has a conjugated π-system that gives a reasonable absorption at 255 nm. This can be shifted to higher wavelength by substituents that modulate electron distribution through the molecule.

Question 31

What the presence of a significant UV absorption above approximately 200 nm indicates?

Answer 31

It indicates a conjugated π-system within the molecule