# Characterize adenine using "UV-Vis" (ultraviolet-visible), "IR" (infrared), "MS" (mass spectroscopy), and ""^(13) "C" "NMR" (nuclear magnetic resonance) data?

Apr 27, 2016

Warning! Very long answer!

#### Explanation:

For reference, here is the structure of adenine.

Ultraviolet Spectrum

The UV spectrum measures the absorption of energy in the 200 nm to 400 nm region as nonbonding or $\pi$ electrons in a conjugated $\pi$ system jump from one molecular orbital to another (typically $\pi \to {\pi}^{\text{*}}$ or $\text{nb"->pi^"*}$ transitions).

The spectrum is a plot of the intensity of the absorption against the wavelength.

(from www.researchgate.net)

The peaks at 205 nm and 260 nm tell us only that the molecule has a conjugated $\pi$ system.

Infrared Spectrum

An infrared spectrum measures the absorption of energy as different bonds in the molecule vibrate in various ways.

It often gives information about the functional groups in a molecule.

The spectrum is usually a plot of the transmittance (or % transmission) of the light against the energy measured in wavenumbers (tildeν = E/(hc)).

(from webbook.nist.gov)

The broad band from $\text{2000-3500 cm"^"-1}$ tells us that there is strong hydrogen bonding among the molecules.

The peaks at $\text{3300 and 3100 cm"^"-1}$ are $\text{N-H}$ stretches, while that at $\text{2950 cm"^"-1}$ is probably $\text{C-H}$ stretch.

The peak at $\text{1600 cm"^"-1}$ is an ${\text{NH}}_{2}$ bending vibration.

Mass Spectrum

Mass spectroscopy is a technique that fragments a molecule into ions and sorts them based on their mass-to-charge ($m / z$) ratio.

The masses of the fragments often give us useful information on the structure of the molecule.

In a mass spectrogram, the relative abundance of each ion is plotted against its $\text{m/z}$ ratio.

(from www.researchgate.net)

The peak at mass 135 is the molecular ion ($\text{M}$). It gives the molecular mass of adenine.

Other major peaks are at

$108 = \text{M - HCN}$

$81 = \text{M - 2HCN}$

$65 = {\text{M – 2HCN – CH"_2"N}}_{2}$

$54 = \text{M - 3HCN}$

$28 = \text{HCN}$

This tells you that you have very stable nitrogen-containing aromatic rings.

$\text{^13"C}$ NMR Spectra

A sample is placed in a magnetic field and irradiated with a radiofrequency that causes the $\text{^13"C}$ atoms to absorb energy and "flip" their nuclear spin.

Each atom absorbs an energy that depends on its environment in the molecule, so the spectrum tells us how many "different" carbon atoms are in the molecule.

The spectrum is a plot of signal intensity against the frequency measured in "ppm".

The 50 MHz $\text{^13"C}$NMR spectrum of adenine shows 5 peaks, corresponding to the 5 carbon atoms in adenine.