A day in pchem lecture: NMR, lululemon yoga pants and tattoos

By lululemon athletica
(Flickr: Yoga Journal Conference)
 [CC BY 2.0], via Wikimedia Commons

It's the end of term, two more 90 minute lectures left in my introductory quantum chemistry and spectroscopy course.  We've done the basics of wave functions and expectation values, we've looked at linear variation theory and written code to do Hückel MO calculations, we've covered rotational and vibrational and rotational-vibrational spectroscopy.  So what to do with these last few days?  The quantum mechanics of NMR.

I kicked off today's lecture by looking at magnetic field strengths, what's the earth's magnetic field (5 μT) or of a refrigerator magnet (5 mT), compared to the superconducting magnets used in NMR, which are on the order of 10T. (1T is one tesla.)  This led to a quick review of the risks in MRI, which aren't about the energy of the radiation used (which is billions of times lower than X-rays), but more about the interactions of the high magnetic fields, the radiofrequencies and metals.

A hand shot up and student who is an EMT describes a patient whose tattoo started burning during an MRI.  I pointed out this is a known phenomenon, and while most inks don't pose an issue, it should discourage you from DIY tattooing.  Then a student asked, "Is it true you can't wear lululemon pants when you have an MRI?"

I admitted this was out of my zone, but promised to follow up.

I can now report that yes, wearing lululemon pants — or any clothing with metallic microfibers, such as those great antimicrobial t-shirts — in an MRI can lead to serious burns, particularly in patients that have been sedated or are otherwise unconscious and unable to signal their discomfort.  Even non-ferromagnetic materials presents problems in the MRI as eddy currents can develop around them, creating little induction heaters.  Loops of all sorts, even skin to skin contact between a patient's own body parts can lead to heating and subsequent burns.  And tattoos with large loops in them?  They can heat as well.

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Other things I learned this afternoon.  You can levitate a frog with a 16T field (thank you Wikipedia), and neutron stars have magnetic fields on the megaTesla scale.

Shedding some light on chemistry: Mole Day and the Year of Light

[If you want to participate in some science about science blogs, see the bottom of this post!]

It's October and there is lots of science to celebrate.  Chemists in the US and elsewhere are celebrating Mole Day on Friday (October 23 at 6:02 pm) to honor Avogadro's number (6.02 x 10²³ items are in a mole -- it's the chemist's version of a dozen).  It's also the International Year of Light, and while you might think that light is the purview of physicists, it's an element of chemistry as well.  I suggested in a recent essay that one might want to celebrate the year of light on the 10th October at 3 in the afternoon (3 x 10¹⁰  is the speed of light in cm/sec)

I've written two pieces on the relationship between chemistry and light for the celebration.  The first for Nature Chemistry, The Enlightenment of Chemistry, looks at the two-way relationship between chemistry and light.  Light is  not just an energy source for doing chemistry, but the production of light in various ways has pushed chemistry forward.  The full text is here.

The second, for the UN's Year of Light blog celebrates the October 27th anniversary of Bunsen's and Kirchhoff's publication on the spectroscope and atomic emission spectra — and the role the spectroscope played in not only filling out the periodic table, but in confirming the periodicity of the table.

"Hunting for new elements spectroscopically meant you didn’t actually need to have any of it in your lab or even on your planet, as long as you could observe the light from a burning sample. In 1868 several chemists and astronomers independently observed a faint line in the spectrum of the sun, and assigned it to a new element, helium, which as far as they knew did not exist on earth. It would take nearly 30 years for two Swedish chemists to confirm that it was present on earth — by matching the spectrum with that of a gas found in a uranium ore. (The helium to be found on earth comes from radioactive decay.)" — read the rest here.

Want to participate in some science to celebrate?

Help us do science! I’ve teamed up with researcher Paige Brown Jarreau to create a survey of the Culture of Chemistry's readers. By participating, you’ll be helping me improve the blog and contributing to SCIENCE on blog readership. You will also get science art from Paige's Photography for participating, as well as a chance to win a t-shirt, a $50 Amazon gift card and other perks!   It should only take 10-15 minutes to complete. You can find the survey here: http://bit.ly/mysciblogreaders. 

Fiat Lux 1: On Fire for Quantum Mechanics

My sailboat's name is the Fiat Lux — "let there be light" in Latin — drawing from both my theological and scientific personae. I sail a Laser, an Olympic class racing dingy, which is an apt boat for a quantum mechanic. The ability to amplify light by stimulating an existing emission process was first predicted by quantum mechanics, then the apparatus to actually do it was built. Laser is really an acronym: Light Amplification by Stimulated Emission of Radiation. The radiation is electromagnetic radiation, not the radioactive radiation.

There's been a smattering of conversation about light production around my house this weekend between sailing the Laser, setting off fireworks and observing fireflies. One of my teen guests wondered how the fire in fire flies was different from the fire in fireworks. All light is not created in quite the same way....though there are some fundamental similarities.

There are really two fires in fireworks, the thermal explosives that send them skyward, and the "rockets red glare" — the glittering burst of color in the sky. The heat from the thermal explosion (usually blackpowder or a similar substance) is what trigger the colors.

If you ever done a flame test, putting a solid substance or a concentrated solution on a wire loop and placing it in a flame to see what color is produced, you've done the same chemistry. The extreme heat excites electrons in an atom or molecule, and as they fall back down to their lowest energy, or ground state, emitting a photon (a bit of light) that just exactly matches the difference in energy between the excited state and the ground state. An orange flame meant you had sodium on the wire, while a violet flame suggested potassium. More properly this technique is called atomic emission spectroscopy.

For atoms the picture you usually see in a high school text of this process is of a ladder, where electrons are shown moving from rung to rung. The larger the distance between the two rungs (or states) the higher the energy of the photon emitted. If the distance corresponds to photons in the visible region, you see a color, otherwise you have to use something fancier to figure out the energy of the photons being released.

Different atoms have different spacings between states and so the colors they emit when heated to high temperatures are likewise different. There are in fact many states, and so many types of photons can be emitted, but few are in the visible region.

If you click here, you can see a simulation of the photons you'd expect to see when an excited sodium atom returns to the ground state. Are you surprised that sodium can be used for yellow-orange in fireworks? Some urban legends suggest that lead (or radioactive barium) are used in fireworks, but if you look at the line spectrum of lead you can see why it can't be true -- there is no rung to rung jump in lead that corresponds to a visible photon. So a lead firework would be invisible! (Lead used to be used to make the fireworks "crackle"...)

(And it's true that barium salts are used in fireworks, but they are not radioactive. There are no naturally occurring radioactive isotopes of barium.)

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Read more:

An article at C&E News on the chemistry of fireworks

Related posts:

Jello lasers
Romancing the stone (lasers in the plot line of a romance novel)

Image from Wikimedia.