When I think back on all the things I learned in grad school. . .well, let's say that not all of it has come in handy. Chemistry, like all the other sciences, long ago split into all kinds of sub-specialties, so it's no wonder that I haven't had to worry much about Tanabe-Sugano diagrams (to pick a representative example from inorganic chemistry.) Nor has normal coordinate analysis featured much in my work of the past twenty years, since I'm not some sort of theoretical spectroscopist. And, thank God, I haven't had to sit down and do any quantum mechanics since the day of my final exam in that well-known rite-of-passage course.
But I haven't had to use things that are nominally in my area of expertise, either. Electron spin resonance, for example - that's something that free radical chemists care about, but I did a whole post-doctoral year doing free radical chemistry, and never did the subject come up. Makes me glad that I didn't spend any more time learning it.
How about chiral aldol chemistry? That's close to home. It's organic synthesis, my very own subfield, and it was the subject of the first question I was asked during my PhD orals. Have I ever done a chiral aldol reaction? Not a one, and I don't have any plans to. Was my time well spent learning all the various theories about how they work? Doubts have crept in.
Moving even closer to how I earn my living, how about all those med-chem graphs and equations they try to teach you? My first year in the business, they sent me (and a number of other folks) off to a well-known summer short course in medicinal chemistry, to teach us the ropes. Now, I can't pretend that I didn't learn anything useful there, although it was all material that I was going to learn anyway. But those equations, those fine equations for pharmacokinetic behavior, for clearance and absorption and distribution. . .I haven't had call for one of them since.
I mean, I think about those phenomena all the time, but not in mathematical terms. The real systems are just too messy for that, and most of the time we don't understand what's going on, anyway. I can just see myself back in that classroom, copying these things down. Did I have the suspicion right there that I'd never write them down again, or did that take a little while longer?