University:
York University
Course:
CHEM 1000 | Chemical Structure
Academic year:

2021
Views:

63

Pages:

2
Author:

Niklas

14.3 Signal Characteristics - Wavenumber f (Wavenumber) for a stretching vibration depends on: Bond Strength Mass of Atoms (Bonded together) Force constant (Bond strength) Wavenumber = (1/2πc) * √(f/μ) Reduced mass (μ) = (m₁m₂) / (m₁ + m₂) 1) C-H ~ 3000 cm⁻¹ C-D ~ 2200 cm⁻¹ C-O ~ 1100 cm⁻¹ C-Cl ~ 700 cm⁻¹ 2) C≡N ~ 2200 cm⁻¹ C=N ~ 1600 cm⁻¹ C-N ~ 1100 cm⁻¹ DIAGNOSTIC REGION Bonds to H Triple bonds Double bonds Single bonds FINGERPRINT REGION (< 1500 cm⁻¹) Difficult to analyze More peaks Focus on Diagnostic Region Ignore Fingerprint Region Stretching Frequency: sp > sp² > sp³ C-H sp³ ~ 2900 cm⁻¹ sp² ~ 3100 cm⁻¹ sp ~ 3300 cm⁻¹ Alkane 3000 3000 2800 Wavenumber (cm⁻¹) Alkene 3100 3000 2800 Wavenumber (cm⁻¹) Alkyne 3300 3100 3000 2800 Wavenumber (cm⁻¹) Resonance delocalization of e- affects the strength of a covalent bond → wavenumber stretching signal Ex: Ketone Conjugated Ketone O O || || C=C-C=O C-C=C-C=O Remember! On exams! Carbonyl stretch Conjugated carbonyls have a LOWER stretching frequency Ex: Ester Conjugated unsat. ester O O || || C=O-C C=C-C=O-C 1600 - 1850 cm⁻¹ - Check for DB 2100 - 2300 cm⁻¹ - Check for TP 2700 - 4000 cm⁻¹ - Check for X-H Bonds Analyze wavenumber, intensity, and shape of each signal