What is a frequency lock system in an NMR spectrometer? Describe the two types of lock systems.

Name: Problem 23, Chapter 19: Principles of Instrumental Analysis
Uploaded: 2019-09-13T05:31:37-08:00
Duration: 9 min 26 s
Channel: Ian Kaigh
Description: What is a frequency lock system in an NMR spectrometer? Describe the two types of lock systems.

step 1 Number 17 from the Smith Organic Chemistry textbook. And this problem gives us four molecules an

What is a frequency lock system in an NMR spectrometer?...

Key Concepts

Frequency Lock System

A frequency lock system in NMR is used to maintain the homogeneity and stability of the magnetic field during an experiment. This system continuously monitors a reference frequency – typically from a nucleus with a known chemical shift – and adjusts the magnet’s field strength if any drift is detected. This stabilization is crucial for acquiring high-resolution spectra over long acquisition times, as even slight fluctuations can lead to errors in resonance frequencies and ultimately affect the quality of the data.

Internal (Solvent) Lock

An internal lock system uses a deuterated solvent that is already present in the sample as the reference nucleus. The spectrometer continuously observes the deuterium signal, and any deviations from its expected frequency trigger real?time adjustments to the magnetic field. This method is convenient because it does not require any extra hardware or a separate sample compartment, and it is widely used in routine experiments where the sample is prepared in a deuterated medium.

External Lock System

An external lock system employs a separate reference compound placed in a dedicated channel or in a coaxial insert alongside the sample. This external source provides a stable signal that is monitored to correct for magnetic field drifts. Such systems are particularly useful when the sample itself lacks a suitable deuterium signal or when different experimental conditions require the use of an external standard to avoid interference with the sample’s resonances.

Transcript

00:01 Number 17 from the smith organic chemistry textbook.

00:04 And this problem gives us four molecules and asks us to describe the proton nmr spectrum of each compound.

00:12 So specifically how many nmr signals are present, the splitting pattern for each of those signals, and the approximate chemical shifts.

00:22 And so in order to do this, the first thing that we should do is identify the types of unique protons in each molecule.

00:34 Because remember, unique protons are going to give rise to different signals, but protons that are the same will give rise to the same signal.

00:45 So looking at a, these three protons are going to give one signal, and then these two protons are different than those three.

00:56 And then this methyl group at the end here is different as well.

01:01 And so we will have three different signals, three types of signals.

01:07 And so the first one that i've underlined in red, so those protons, the carbon that they're attached to, is not attached to any other carbon.

01:19 And so they don't have any neighboring protons.

01:22 And so they're going to show as a singlet.

01:32 Okay, so they'll be a singlet.

01:35 And they should show around 3 ppm.

01:42 So they are on a carbon that is attached.

01:47 They're on a carbon that's attached to an oxygen, which is a pretty electronegative.

02:00 Okay, sorry, my eraser's not working.

02:04 It looks like.

02:06 Okay, there we go.

02:07 So they're on a carbon that's attached to an oxygen, which is a pretty electronegative atom.

02:14 And so they're deshielded.

02:15 And so they'll show somewhere around 3 ppm.

02:19 So then looking at the green underlined protons, they have three neighboring protons.

02:25 So they will be split into a quartet.

02:32 And we should expect them to show at about 3 .5.

02:37 Ppm.

02:40 So again, they're on a carbon that's attached to an oxygen, so they're going to be pretty deshielded.

02:47 Okay, and then lastly, the blue protons are going to be split into a triplet by their two neighboring protons, and we would expect them to show at about 1 ppm.

03:08 So they might see a little bit of deshielding by that oxygen, but not much.

03:13 Okay, and so i'm just going to do that for each of these three remaining molecules.

03:19 So in b, actually, all of these protons are different.

03:25 So each of these is going to give rise to its own signal.

03:29 And then i'll use black, i suppose, to underline these.

03:33 So we're going to have four different signals here.

03:36 So first, our red methyl group has two neighbors.

03:40 So we should expect a triplet.

03:43 And that's probably going to be at about one.

03:49 So not much deshielding there.

03:52 So then next looking at the methylene group next to it, that's going to be a quartet because that methyl group that we just described in red is going to be splitting it.

04:03 And so this is alpha to a carbonyl.

04:06 So it's going to be somewhat deshielded, so 2 ppm or so.

04:13 So then moving on to the blue underlined proton.

04:17 That has six neighbors, and so it is going to show as a septet, so seven peaks.

04:32 And that's attached to an oxygen, and so it's going to be pretty deshielded, so 3 .5 ppm or so.

04:43 And then lastly, the one that i underlined in black.

04:46 So that's six protons because it's two methyl groups...