Chem HH W11 Notes - Dr. Masato Koreeda Date: January 5, 2011 Topic: _IR Spectroscopy_ page 1 of 3. fingerprint region

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1 hem W11 Notes - Dr. Masato Koreeda Date: January 5, 2011 Topic: _I Spectroscopy_ page 1 of 3. Infra-red (I) Spectroscopy (Ege s book: Section 12.2; hapter 3 of the textbook arwood/laridge) Uses the light with ν = ~ cycles/s (i.e., 1 ~ 10 kcal/mol) I. I Spectrum Fourier Transform (FT) System 2.5 µ 7 µ 15 µ 25 µ wavelength ( = 10-4 cm) 100 % transparency 0 4,000 3,000 1, cm wave numbers fingerprint region A. Δ = hv = hc/λ v = 1 x 10 4 = wavenumbers (cm -1 ) λ # of waves per cm wavelength in microns (µ) [1 µ = 10-4 cm = 10-6 m] h = Planck s constant; c = velocity of the light λ = wavelength 1 cm -1 corresponds to 2.86 cal/mol 4,000 cm kcal/mol 400 cm kcal/mol wave numbers are proportional to the energy B. The 1,430 ~ 500 cm -1 (7 ~ 20 µ) region: Fingerprint region Used for confirming the identity of compounds. Not useful for the identification of functional groups.. 4,000 ~ 1,430 cm -1 (2.5 ~ 7 µ) region: Most of the characteristic stretching vibration peaks appear in this region. Useful for the identification of functional groups such as -, N- (or N 2 ), =, =N,, N, etc. D. I spectra are recorded as: (1) neat/liquid (liquid samples; Diamond AT FT I*); ground solid (Diamond AT FT I) (2) KBr disc (solid samples) [for best results, the disc has to be transparent] (3) sometimes in solution (in l 3, l 3 or dioxane) (4) even as a gas *AT FT I = Attenuated Total-eflection Fourier Transform I

2 hem W11 Notes - Dr. Masato Koreeda Date: January 5, 2011 Topic: _I Spectroscopy_ page 2 of 3. II. I absorption peaks. Molecular vibrations are detected. The two types of molecular vibrations are: (1) Stretching vibrations (often designated as v ) e.g., v= These are generally stronger than bending vibrations and more reliable in identifying functional groups. (2) Bending or deformation vibrations (often designated as δ ). e.g., (1) Stretching vibrations scissoring wagging twisting rocking Most of these peaks show up in the 1,400 ~ 400 cm -1. Not so useful; unreliable for the identification of functional groups. armonic oscillator model (based on ooke s law) m 1 m 2 masses of atoms 1 and 2 F = kx = m d 2 x/dt 2 (force) v = 1 f (m1 + m2) 2πc m1 m2 f : force constant (indication of how strong the bond is) (a) v for absoption rel. force const. (b) Because of v = 1 f (m1 + m2) 2πc m1 m2 v 2200 cm -1 1,600 v for v- is larger than that of v-. v = ~1600 cm i.e., in addition to the force constant, v ~700 cm masses of the two atoms (m 1 and m 2 ) have to be taken into consideration. A better use of this formula is the comparison of isotopically labeled compounds, e.g., v-: 2900 cm -1 vs v-d: 2050 cm -1 The force constants for - and -D are virtually identical; this difference in wavenumers comes from that in the masses ( vs D).

3 hem W11 Notes - Dr. Masato Koreeda Date: January 5, 2011 Topic: _I Spectroscopy_ page 3 of 3. III. A Few Key Issues oncerning I Stretching Vibrations (1) Stronger intensity expected for polar bonds Bonds that cause large dipole moment changes upon vibration strong stretching vibration absorption the detection of I peaks is based on the change in the dipole moments upon vibration in each bond. (2) igher v higher energy required for stretching of the bond. note: Make sure to compare within the same combination of atoms. Examples: (a) ν= 1715 cm cm -1 more singlebonded = 1655 cm -1 even more singlebonded = (b) various = functional groups (i) The resonance effect makes = more single bonded lower cm -1 for ν= Z ' Z ' Z ' (ii) If Z is a highly electronegative atom, the inductive effect through the -Z sigma bond makes the = bond shorter, i.e., stronger = bond Z ' higher cm -1 for ν= It is the fine balance of these two opposing effects that determines the ν= of each of -(=)-Z-. acid halides l 1800 cm -1 esters/lactones ν= ~1735 cm cm -1 The inductive effect F by the ' oxygen atom is a slightly more significant inductive effect contributor than the resonance more significant effect by the ' lone pair ' ketones ν= ~1715 cm -1 ' amides ν= ~1660 cm -1 N ' " N ' " An extremely significant contributor...

4 hem W11 Notes - Dr. Masato Koreeda Date: January 5, 2011 Topic: Major I Peaks page 1 of 2. Important I Peaks Use only the cm -1 region. This is where most of the characteristic stretching vibration absorptions appear. I cm -1 (1) ν-: 3600 ~ 3200 cm -1 broad and very strong Intermolecular -bonding lower cm ~ 3500 cm -1 : dimeric; 3400 ~ 3200 cm -1 : polymeric Intramolecular -bonding lower cm -1 & sharper band (3) ν -: 3310~3200 cm -1 strong; should also see ν (2140~2100 cm -1 )

5 hem W11 Notes - Dr. Masato Koreeda Date: January 5, 2011 Topic: Major I Peaks page 2 of 2. II ~ 2700 cm -1 : ν- (medium to weak intensity) aromatic and alkenic ν-: 3100~3010 cm -1 alkanic (sp 3 ) ν-: 2960~2850 cm -1 IV ~ 1620 cm -1 : various ν=; very strong acid anhydrides: 1850~1800 and 1790~1740 cm -1 (two peaks) acid halides: 1830~1780 cm -1 esters and six-membered lactones: 1750~1735 cm -1 aldehydes: 1730~1720 cm -1 (slightly higher cm -1 than corresponding ketone ν= ketones: 1715 cm -1 (1720~1705 cm -1 ) carboxylic acid: 1720~1700 cm -1 amides: 1680~1640 cm -1 Notes: (1) onjugation lowers ν= by 30~20 cm cm cm cm -1 3 (2) ing strain raises ν= 1715 cm cm cm -1 α,β-unsaturated ketone 1740 cm -1 aryl ketone 1775 cm cm -1 V ~ 1500 cm -1 : various ν= (intensity variable) & ν=n (strong) Arom ν=: often 2~3 or more bands at ~1600, 1500, 1400 cm -1 ; can be very strong