The ¹H NMR spectra of three isomers with molecular formula C₄H₉Br are shown here. Which isomer produces which spectrum?
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Verified step by step guidance

1. Analyze the molecular formula C4H9Br, which indicates the presence of four carbons, nine hydrogens, and one bromine atom. This suggests the compound is an alkyl bromide.
2. Examine the 1H NMR spectrum provided. The spectrum shows three distinct signals corresponding to three different types of hydrogen environments.
3. Interpret the signal at approximately 1 ppm labeled as '3 H'. This is likely a methyl group (-CH3) attached to a carbon that is not directly bonded to an electronegative atom, as the chemical shift is low.
4. Interpret the signal at approximately 2 ppm labeled as '2 H'. This is likely a methylene group (-CH2-) adjacent to the bromine atom, as the chemical shift is slightly deshielded due to the electronegativity of bromine.
5. Interpret the signal at approximately 0.9 ppm labeled as '6 H'. This suggests two equivalent methyl groups (-CH3) in a symmetrical environment, likely part of an isopropyl group structure. Combine this information to deduce the isomer that matches the spectrum.

Key Concepts

1H NMR Spectroscopy

Proton Nuclear Magnetic Resonance (1H NMR) spectroscopy is a powerful analytical technique used to determine the structure of organic compounds. It provides information about the number of hydrogen atoms in different environments within a molecule, indicated by peaks in the spectrum. The position of these peaks (measured in parts per million, PPM) reveals the electronic environment of the protons, while the area under each peak corresponds to the number of protons contributing to that signal.

Chemical Shifts

Chemical shifts in 1H NMR spectra are indicative of the electronic environment surrounding hydrogen atoms in a molecule. Different functional groups and molecular structures cause protons to resonate at different frequencies, resulting in distinct chemical shifts. For example, protons adjacent to electronegative atoms or in alkyl chains will appear at different PPM values, allowing chemists to infer structural information about the isomers being analyzed.

Integration and Multiplicity

Integration in 1H NMR refers to the area under the peaks, which quantifies the number of protons contributing to each signal. Multiplicity, on the other hand, describes the splitting of NMR signals due to neighboring protons (n+1 rule), providing insight into the number of adjacent hydrogen atoms. Together, integration and multiplicity help in deducing the connectivity and arrangement of atoms in the isomers, which is crucial for identifying which spectrum corresponds to each isomer of C4H9Br.