Question

Sketch the proton chemical shifts in ppm and \(\mathrm{Hz}\) as well as the integral you would expect for each of the following substances at \(60 \mathrm{MHz}\). (The spin-spin splitting of the resonance lines evident in Figures 9-23 and 9-27, but not seen in Figure \(9-31\), can be safely neglected with all of the compounds listed.) a. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCH}_{2} \mathrm{OCH}_{3}\) b. \(\mathrm{CH}_{2} \mathrm{COC}\left(\mathrm{CH}_{3}\right)_{3}\) c. \(\mathrm{HCOC}\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHO}\) d. e. \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CCl}_{2}\) f. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{COC} \equiv \mathrm{CH}\) g. \(\left(\mathrm{CH}_{2} \mathrm{Cl}\right)_{3} \mathrm{CCO}_{2} \mathrm{H}\) h.* cis-1-methyl-4-tert-butyl-1,2,2,3,3,4,5,5,6,6-decachlorocyclohexane

Short answer

Identify proton environments, estimate chemical shifts in ppm, convert to Hz at 60 MHz, and find integration ratios.

Step-by-step solution

Determine the Chemical Environment

Identify the unique hydrogen environments in each molecule to determine their chemical shifts. For example, in \(\left(\text{CH}_3\right)_3 \text{CCH}_2 \text{OCH}_3\), identify the \(\text{CCH}_2\) group and the \(\text{OCH}_3\) group.

Estimate Chemical Shifts in ppm

Using standard NMR spectroscopy tables, estimate the chemical shifts in ppm. For example, a methyl group attached to an oxygen (\(\text{OCH}_3\) group) typically resonates around 3.2-3.8 ppm.

Calculate Shifts in Hz at 60 MHz

Convert the chemical shifts from ppm to Hz using the formula \[ \text{Frequency in Hz} = \text{Chemical shift (ppm)} \times \text{Spectrometer frequency (MHz)}.\] For example, a shift of 3.2 ppm at 60 MHz would be \(3.2 \times 60 = 192\ \text{Hz}\).

Determine Integration Ratios

Estimate the integration (area under the peak) for each type of proton. The integration corresponds to the number of protons giving rise to that signal. In \(\left(\text{CH}_3\right)_3 \text{CCH}_2 \text{OCH}_3\), there are nine protons in the \(\text{CH}_3\) groups and two protons in the \(\text{CH}_2\) group, leading to integration ratios of about 9:2.

Example E Analysis

For \(\left(\text{CH}_3\right)_2 \text{C} = \text{CCl}_2\), the chemical shifts: Alkenyl protons \(\approx 5.3\ \text{ppm}\), methyl groups \(\approx 1.5\ \text{ppm}\). Frequency at 60MHz: alkenyl protons \(318\ \text{Hz}\) and methyl groups \(90\ \text{Hz}\). Integration ratio 1:6.

Key concepts

Chemical Shift

In NMR Spectroscopy, the **chemical shift** reveals vital information about the electronic environment around a proton. The shift is measured in parts per million (ppm). This indicates how much the magnetic field experienced by a nucleus is affected by nearby atoms and electrons. For example, in methoxy (\(\text{OCH}_3\) groups), the electronegative oxygen atom deshields the proton shifting its resonance downfield to about 3.2-3.8 ppm.
Understanding where a peak falls within this range helps chemists infer the structure of a molecule.

Proton Environment

The **proton environment** refers to the unique chemical surroundings of proton groups in a molecule. It determines the distinct NMR signals observed.
For instance, when analyzing a compound like \(\left(\text{CH}_3\right)_3 \text{CCH}_2 \text{OCH}_3\), each unique hydrogen environment — such as the \(\text{CCH}_2\) and \(\text{OCH}_3\) groups — will produce separate signals. Recognizing these environments allows chemists to map out molecular structures based on proton behavior.

Integration Ratios

**Integration ratios** correlate with the area under each NMR signal. They indicate the relative number of protons contributing to that signal, offering insights into a molecule’s structure. For instance, in \(\left(\text{CH}_3\right)_3 \text{CCH}_2 \text{OCH}_3\), nine protons in the \(\text{CH}_3\) groups contribute to one signal, while two protons in the \(\text{CCH}_2\) group contribute to another.
This results in an integration ratio of 9:2, clearly showing the proportion of protons in each environment.

Spectrometer Frequency

**Spectrometer frequency**, measured in MHz, influences NMR spectroscopy readings by determining the precision of chemical shift conversion.
To convert chemical shifts from ppm to Hz, multiply by the spectrometer frequency. For example, a 3.2 ppm shift on a 60 MHz spectrometer equates to \(3.2 \times 60 = 192\ \text{Hz}\).

• Higher frequencies offer increased resolution.
• Lower frequencies are more common in early models.

This calculation bridges the abstract notion of ppm values with tangible measurable frequencies.

NMR Chemical Shift Tables

Chemical shift tables, or **NMR chemical shift tables**, are critical tools in predicting where protons might resonate on the NMR spectrum. These tables list common functional groups and their typical chemical shift ranges.
For example, methyl protons \(\text{(CH}_3\) attached to an electronegative atom like oxygen can be expected to fall between 3.2 and 3.8 ppm.
Using these tables, chemists can quickly assess and predict shifts, streamlining the identification of molecular structures and confirming hypotheses about unknown compounds.