IR Spectroscopy

Question 1

Infrared Radiation

Answer 1

2.5um to 25um correspond to bond vibrations.

Question 2

IR Units

Answer 2

Wavenumber (cm-1): Reciprocal of wavelength.

Question 3

Stretching Frequencies

Answer 3

Heavier the atoms? Lower frequency. Stronger the bond? Higher the frequency.

Question 4

Stretching Vibrations

Answer 4

Symmetrical stretch (2853cm-1) and asymmetrical stretch (2926 cm-1)

Question 5

Bending Vibrations

Answer 5

Scissoring (1450 cm-1) and Rocking (720 cm-1) are in plane. Wagging (1250cm-1) and Twisting (1250cm-1) out of plane.

Question 6

Infrared Absorption

Answer 6

Happens if IR frequency matches vibrational frequency and if there is an overall change in dipole moment (IR active).

Question 7

IR Spectrum

Answer 7

Spectrum different for two molecules (unless enantiomers), Functional groups in region 1600-3500cm-1.

Question 8

Carbon-Carbon Stretch

Answer 8

Single bond (1200cm-1), Double (1660cm-1), Triple (<2200cm-1). More conjugation? Lower frequency!

Question 9

Carbon-Hydrogen Stretch

Answer 9

sp3 (2800-3000cm), sp2 (3000-3100cm), sp (3300cm). NOTE MORE S CHARACTER MEANS STRONGER THE FREQUENCY!

Question 10

Alkane

Answer 10

Band between 2800-3000cm-1. No band at 1600-2500cm-1? No other groups.

Question 11

Alkene

Answer 11

Band for C-C double bond and C-H sp2 unsaturated stretch. VALUES ON OTHER NOTECARDS!

Question 12

Alkyne

Answer 12

Band for C-C triple bond and C-H sp unsaturated stretch. VALUES ON OTHER NOTECARDS!

Question 13

Alcohol

Answer 13

Broad O-H band around 3300cm-1.

Question 14

Amines (Contains N)

Answer 14

Primary? Peak with two sharp spikes. Secondary? One broad and one sharp. Tertiary? Neither. (Look around 3300cm-1).

Question 15

Carbonyl (C-O double bond present)

Answer 15

Around 1710cm-1. Depends on group.

Question 16

Ketones (C-O double bond)

Answer 16

C-O double bond strong around 1715cm-1.

Question 17

Aldehyde (C-O double bond, near Hydrogen)

Answer 17

2 bands around 2720 and 2820 cm-1. C=O stretch 1725cm-1.

Question 18

Carboxylic Acids (COOH)

Answer 18

VERY broad around 2500-3500cm-1. (C=O at 1710cm-1)

Question 19

Amides (two N-H groups)

Answer 19

Two N-H bands around 1690cm-1

Question 20

Ester (C-O double bond OR)

Answer 20

C=O stretch around 1735cm-1

Question 21

Nitriles (C-N triple bond)

Answer 21

2200-2300cm-1

Question 22

Ketones (Carbonyl molecule, two alkyl groups)

Answer 22

1710cm-1, conjugation lowers density to 1685cm-1. Ring strain increases stretching

Question 23

Aldehydes (Carbonyl molecule, one alkyl group)

Answer 23

1725cm-1, C-H stretch from 2710-2810cm-1.

Question 24

Benzene Ring

Answer 24

Peak at 1600cm-1 (small)