Chapter 2: Spectroscopy (C4475473)

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1 Absorption spectroscopy

Absorption spectroscopy measures the absorption of radiation, such that the detector captures the light that remains after absorption of particular frequencies. It thus has a colored background, with discrete black lines [representing absorbed frequencies]. An example is IR spectroscopy.

© 2014 MR. SHUM

IR spectroscopy is absorption spectroscopy relating to the infrared region of the electromagnetic spectrum. Remember from  that electromagnetic waves have a constantly changing electrical and magnetic field [both perpendicular to another], therefore a photon [also] has a changing/reversing electric field. Because unlike charges repel, polar bonds will be stretched and squeezed back and forth, in accordance with a resonance frequency. In order to create resonance, the frequency of the reversing electric field [of the photon] should be the same as the resonant frequency of the polar bond. This also, is the frequency at which the molecule absorbs the most energy, and correlates with a dip in the IR spectrum, where absorbance [on the y-axis] is plotted against wavenumber [on the x-axis].

© 2014 MR. SHUM

Remember from  that resonant frequency is, through the relationship [mathjax]v=f\lambda[/mathjax], related to the velocity, which is defined by [mathjax]v=\sqrt{\dfrac{elastic}{inertial}}[/mathjax], and therefore, in turn, related to the elastic and inertial components [of its polar bond]. Since [mathjax]f\propto\sqrt{elastic}[/mathjax], the elastic component increases resonant frequency, which makes sense because elasticity is tendency to snap back [to the resting position]. In contrast, since [mathjax]f\propto \dfrac{1}{\sqrt{inertial}}[/mathjax], the inertial component decreases resonant frequency, which makes sense because inertia is the tendency to retain present motion, and therefore, not reverse it (as discussed ). In molecules, inertia is related to molecular mass, and elasticity is related to bond strength. Therefore, lighter molecules, and molecules with stronger bonds, have greater resonant frequencies. Intensity is dependent on change in dipole moment. For example:

  • Carbonyls ([mathjax]-CO-[/mathjax]) have a heavy oxygen atom (
    ), and so have a
    low resonant frequency
    . Separately, carbonyl is quite polar, and so therefore exhibits a large change in the dipole (
    ), so therefore has
    strong intensity
    .
    As a result, carbonyl has a sharp dip (strong intensity) at [mathjax]1700cm^{-1}[/mathjax] (low resonant frequency)
  • Alcohols ([mathjax]-OH[/mathjax]) have a light weight hydrogen (
    ), and a strong partial bond (
    ), and so have
    high resonant frequency
    , . Note their peaks are given in a range, as they are very broad, since hydrogen bonding can cause the hydrogen signals to couple

The unit reciprocal centimers, is a measurement of the wavenumber, which is analogous to frequency, but rather than being the number of waves per second, is the number of waves per unit distance [in this scenario, centimers].

Fingerprint region of the spectrum, is the portion of the spectrum between [mathjax]600 - 1400cm^{-1}[/mathjax] that can identify each compound.

Non-polar molecules may display on IR spectroscopy by detecting other vibrations, but show as far weaker signals.

Formative learning activityMaps to RK2.A
What is absorption spectroscopy?

2 Mass spectroscopy

3 1H NMR spectroscopy

Nuclear magnetic resonance (NMR) use any nuclei with an odd atomic number (number of protons, discussed ) or an odd mass number (number of nucleons), most commonly a proton [in proton NMR]. This odd proton exhibits nuclear spin, thereby producing a [magnetic] moment around the nucleus. When placed in an external magnetic field, the nucleus can either align (1) with the [external] magnetic field, which is where the north pole of the nucleus points towards the south pole of the external magnet, and is therefore a lower energy state; or (2) against [vv., and therefore related to a higher energy state]. The stronger the [external] magnetic field, the greater the energy difference between the two [with/lower and against/higher] states. If a photon has energy that is equal to this energy difference, and strikes the low energy nucleus; the nucleus is inverted to the high-energy state, and is known as being in resonance. An NMR spectrometer has a source, which emits photons of a constant frequency, and changes the [external] magnetic field strength until resonance is detected.

Although it would appear that all nuclei [of the same atom] would resonate at the same frequency, this is not so. This is because of the presence of electrons around the nucleus, which have their own magnetic field rotating in the opposite direction [to the magnetic field of the nucleus]. This is known as nuclear shielding, and increases the required [external] magnetic field strength, before resonance can be achieved. Note therefore, that electron withdrawing groups (discussed ) will deshield protons, and therefore reduce the magnetic field strength necessary to cause resonance.

An NMR spectrum plots signal intensity [on the y-axis] against chemical shift [on the x-axis]. Note that the chemical shift increases from the right to the left. Nevertheless, the larger [to the LHS] chemical shift, indicates a weaker magnetic field strength. Therefore, to the left is known as downfield. It follows from , that protons attached to a carbon, which is in turn attached to electron withdrawing groups, will have increased deshielding of protons. This reduces magnetic field strength required [to cause resonance], and therefore peaks downfield. For example, in the image , oxygen is strongly electron withdrawing, meaning that the hydrogen [of the carbon] attached to the oxygen, will be far downfield, at around 4 ppm.

© 2014 MR. SHUM

Spin-spin splitting is where there are several peaks made in the same cluster. It occurs due to neighboring hydrogens (hydrogens attached to an adjacent atom, usually a carbon) which are stereochemically different. Hydrogens are stereochemically equivalent if they are enantiotropic to another, meaning [its, usually, carbon is] attached to the same functional groups.

For example, the hydrogens on the end of propane ([mathjax]C_{8}H_{8}[/mathjax]) are enantiotropic, but they are stereochemically different from the two hydrogens on the middle carbon, because the middle carbon only has two hydrogen attachments, whereas the end carbons have three hydrogen attachments.

© 2014 MR. SHUM

The number of peaks equals the number of sets of stereochemically different hydrogens. Each peak splits into [mathjax]n+1[/mathjax] peaks, where [mathjax]n[/mathjax] is the number of hydrogens on non-[stereochemically different] neighboring atoms.

For example, in ethyl acetate (in the image ), there are 3 peaks [as there are 3 sets of stereochemically different hydrogens]; and the red hydrogens have 4 peaks because it is attached to a carbon that has 3 hydrogens [then, adding 1], and green hydrogens with 1 peak because its attached carbon has no hydrogens [then, adding 1], and blue hydrogens with 3 peaks because its attached has 2 hydrogens [then, adding 1].

The clusters that contain the greater number of hydrogens will have a greater area under the curve [of the related peaks]. As this can be difficult to observe, an integral trace is used, which is a light line that traverses across the page [from left to right]. As it reaches each peak, it rises in proportion with the area under the curve, and stays at this new height.

© 2014 MR. SHUM

Digital trace is where a number is written by the rise, which indicates the proportion of each rise [to another], but not the exact number of hydrogens. For example, a rise of two indicates twice as many hydrogens, as a rise of one.

There is always a peak at [mathjax]0 ppm[/mathjax], as it corresponds to the peak due to the calibration chemical, which is usually tetramethylsilane ([mathjax]Si[/mathjax]  surrounded by four methyl groups). It is used because all of its hydrogens is enantiotropic, meaning there will only be one peak. In addition, silicon is not electron withdrawing, meaning there is increased shielding of protons, to the point that a very strong magnetic field strength is going to be required, and hence a signal upfield, far to the right.

Carbon-13 NMR is analogous to proton NMR, but spin-spin splitting can be ignored. Note that carbon-12 cannot be used, because it has even atomic and mass numbers, thus does not have nuclear spin, and therefore does not show up in an NMR. Additionally, in a proton NMR, deuterium does not show up, as it has a different magnetic moment.




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Chapter 2: Spectroscopy - Organic chemistry - Pre-med science - MR. SHUM'S CLASSROOM