Topic 4d: Molecular Structure and Absorption Spectra

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Spectral properties are used to determine the structure of molecules and ions.

Ultraviolet (UV), infrared (IR), and nuclear magnetic resonance (NMR) are particularly important.

Molecules can be raised from a lower energy state (ground state) to a higher energy state (excited state) by the absorption of a photon (quantum of energy) of electromagnetic radiation of a permitted wavelength.

Hooke’s law: characterizes a stretching vibration between 2 nuclei by predicting the frequency of an absorbance using 2 main terms—force constant and reduced mass.

force constant: represents the strength of the bond, is directly related to the wavenumber of the absorbance—the higher the force constant, the higher the wavenumber. Triple bonds are the strongest and therefore have the highest force constant.

Reduced mass: (m1m2)/(m1 +m2), inversely related to wavenumber

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When an organic compound absorbs infrared radiation, bonds within that molecule are stretched. This phenomenon is used to gain information about the molecular structure.

The chemical bond between 2 atoms is often compared to a spring with a mass on each end.

Just as a spring wound tighter will contract faster after being stretched, a double bond is stronger than a single bond, and a triple bond is the strongest. The heavier the mass, the less the spring will stretch.

Different functional groups have different characteristic absorption frequencies. Typically, the greater the polarity of the bond, the stronger its IR absorption.

infrared spectrum is plotted with x-axis giving the infrared frequencies in units of wavenumbers (1/λ; high wavenumbers=high frequencies because λ and f are inversely related) and y-axis giving % transmission (which frequencies pass through without being absorbed, 100% transmission).

Diagnostic region of the spectrum: 1500-4000 cm-1, most organic functional groups absorb in this region as follows

2500-4000 cm-1

3300 cm-1 amine groups (NH) and hydroxyl group (OH)

3000 cm-1 carboxylic acids (COOH)

1900-2500 cm-1

Triple bonds are found in this region

1500-1900 cm-1

1750 cm-1 carbonyl functional groups (C=O, COOH).

400-1500 cm-1

known as the fingerprint region.

The C-O stretch presents as a band around 1100, diagnostic of ethers, esters, and alcohols.

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UV spectroscopy

absorption of UV + visible rays results in electrons becoming excited; transition metals give off bright colors in the visible spectrum after absorbing those wavelengths; compounds that absorb only UV rays will appear colorless or white.

π systems are often known as chromophores, because they are responsible for the color in many organic compounds.

after absorbing a wavelength, if that wavelength corresponds to the energy gap (E) between HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied orbital), this causes the electron to reach an excited state

conjugation increases the wavelength of light absorbed because conjugation decreases the HOMO-LUMO gap; less energy required= longer wavelength as wavelength and energy are inversely related

The wavelength of light absorbed is dependent on the degree of conjugation in the chromophore, and this absorption can tell us about the nature of the π system in an unsaturated molecules

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NMR

Theory behind NMR: Nuclei have spin; proton nuclei can have spin (+ charge that is perpendicular to magnetic field) which is either up or down.

When that nucleus is placed in a magnetic field, the spin will either align or oppose the applied magnetic field, resulting in a possible low energy state (α) and a possible high energy state (β).

The α state can flip to the β state, and when this occurs the proton is in resonance with the external magnetic field.

When the spin comes back down to its low energy state, each proton's resonance energy is recorded by the NMR spectrometer as a frequency and a signal is plotted on the spectrum.

The spectrum can be analyzed to determine the electronic structure directly around the proton, equivalent protons, and nearby protons.

Shielding: the magnitude of the effective magnetic field experienced by the proton is decreased as a result of electrons (from electronegative atoms) near a proton nucleus creating their own induced magnetic field in the opposite direction of the effective magnetic field.

Magnetic field is directly proportional to the frequency, and thus there is a reduction in the frequency of resonance.

chemical shift (ppm): the position of the proton signals in the final spectrum; NMR signals are plotted as a function of frequency relative to the standard of TMS. As such, change in the frequency of resonance affects the chemical shift.

tetramethylsilane (TMS): the NMR standard, thermally stable, liquid at room temperature, has 12 identical protons resulting in high intensity of absorption, and the protons are shielded.

Deshielded protons have chemical shifts at higher frequencies on the spectrum; the more deshielded (the electron cloud is distorted away from the atom due to the presence of an electronegative atom pulling) the proton, the further downfield (to the left) in a NMR spectrum it will appear.

Equivalent protons: different protons in the same environment; there will only be one signal for all equivalent protons.

Integrating the area under a signal in NMR spectrum can tell you how many equivalent protons that signal corresponds to.

Spin-spin splitting phenomenon: splits in the NMR signals due to interactions with magnetic fields produced by neighboring protons. The degree of splitting depends on the number of adjacent hydrogens, so the signal will be split into n + 1 peaks (n= the number of neighboring/interacting protons).

ppm establishes a universal scale for proton NMR ranging from 0-10ppm. However this scale obscures that frequency increases to the left (downfield shift, protons shift to the left as they are deshielded).

0-2ppm: sp3 carbons, TMS (Standard at 0)

2-3ppm: sp carbon

3-6ppm: sp2 carbons

6-9ppm: aromatic carbons

9-10ppm: Aldehydes

10-13ppm: carboxylic acid

The above is a double doublet pattern, arising from protons in close proximity to one another having different coupling constants.