The Who, What, When, Where and Why of Chemistry
Chemistry is not a world unto itself. It is woven firmly into the fabric of the rest of the world, and various fields, from literature to archeology, thread their way through the chemist's text.
John Tierney had an op-ed piece in the NY Times on Tuesday about the recent National Academy of Sciences report on bias toward women in science. He dismisses their findings of bias, and pins the reason for the underrepresentation of women in research universities on "they don't want to". Most girls, he opines, like the soft sciences, because they are concrete and people oriented, while boys prefer the abstract and "things". Perhaps, but you can be movivated by the concrete, be people oriented and still do "hard science". Check out the letters in response to his piece (full disclosure, one of the letters is mine). Martha Pollack's response regarding engineering was wonderful - people oriented hard science exists.
My quantum mechanics class had a problem last week aimed at figuring out the color of a porphyrin molecules. Porphyrins are nitrogen-containing ring shaped chelating molecules (here is a picture) and are ubiquitous in biological systems. An iron bound to a porphyrin is the heme in hemoglobin, when a magnesium is bound, it is a key piece of chlorophyll.
The color of porphyrin should not be a mystery, as long as you know some Greek. The name comes from the Greek for purple, and indeed these compounds have vivid red-violet hues.
I asked the students to compute the energy needed to excite one of porphyrin's 18 pi electrons from the highest occupied level to the lowest unoccupied level, assuming that they could model the compound as 18 independent electrons trapped in a square 1000 pm on a side. The answer in the back of the book gave the absorbtion wavelength of 588 nm, which is precisely what you would expect for a purple compound (absorbing visible yellow light). It seemed too good to be true, for such a simple model to give such a good anwer and it was! There is an error in the answer, and the actual value is not in the visible at all, suggesting that the porphyrin is colorless!
The problem was an apt one for me to be grading this morning, as I was waiting to donate some of my own hemes in the form of whole blood at the college's blood drive.
We're looking at the Schrödinger equation to start the term in physical chemistry. It is, of course, an eignevalue equation. The term is really a pastiche of German and English, or perhpas a quasi-translation of the German term, eigenwert. The prefix "eigen" is best translated for quantum mechanics as "characteristic". Chemists often use the eigenvalues to "characterize" or "classify" the wavefunctions or states of a systems. For example, the 1s orbital takes its designation from two eigenvalues of the wavefunction: n=1 and l=0.
"Among those ... trying to acquire a general acquaintance with Schrödinger's wave mechanics there must be many who find their mathematical equipment insufficient to follow his first great problem to determine the eigenvalues and eigenfunctions for the hydrogen atom. " Nature 23 July 192 Culture of Chemistry returns with the new term!