Understanding Molecular Structure Through Spectroscopic Analysis
Expert reviewed •23 November 2024• 4 minute read
Introduction
Spectroscopic analysis is a powerful tool in organic chemistry that allows chemists to determine molecular structures through various analytical techniques. This article explores a comprehensive example of structure determination using multiple spectroscopic methods.
The Analytical Problem
We are presented with a molecule having a molecular ion peak of 108 m/z, and we'll use multiple spectroscopic techniques to determine its structure.
Analysis of Spectral Data
Mass Spectrometry Analysis
The mass spectrum shows several key features:
Molecular ion peak (M+) at 108 m/z
Significant M+2 peak
The M+2 peak pattern indicates the presence of a chlorine atom in the molecule
The ratio of M+ to M+2 peaks is approximately 3:1, characteristic of chlorine
Infrared Spectroscopic Data
The IR spectrum reveals:
Strong absorption at ≈3000 cm⁻¹ (C-H stretch)
No significant peaks in the carbonyl or hydroxyl regions
Multiple peaks in the fingerprint region (below 1500 cm⁻¹)
Carbon-13 NMR Analysis
The ¹³C NMR spectrum shows:
Four distinct carbon environments
Chemical shifts indicating sp² hybridized carbons
Peaks consistent with an aromatic system
Proton NMR Analysis
The ¹H NMR spectrum indicates:
Multiple signals in the aromatic region (δ 7-8 ppm)
Integration pattern suggesting five protons
Splitting patterns consistent with para-substitution
Structure Determination
Based on the spectral evidence:
The molecular mass of 108 and M+2 peak pattern indicates chlorobenzene (C₆H₅Cl)
The IR spectrum confirms aromatic C-H stretching
The ¹³C NMR shows the expected number of unique carbons