Re: Word? steric

Eugene Leitl (
Wed, 11 Sep 1996 13:05:38 +0200 (MET DST)

On Tue, 10 Sep 1996, John K Clark wrote:

> On Tue, 10 Sep 1996 (Anton Sherwood) Wrote:
> [ Q: what is steric? ]
> Steric means the space filling properties of a molecule. If all molecules had
> an exact shape and were perfectly rigid, things would be pretty

Often, measurement of atom sizes are compared to measuring a grapefruit
with a pair of calipers: the size determined depends on pressure applied.
Repulsion component gets exponentially up with increased squeeze, though,
so the grapefruit kernel turns (almost) infinitely incompressible (at least
for room temperature conditions, at several MK fusion can occur) very soon.

> straightforward, but of course they are not, a Nanotechnology engineer would
> ignore this at his peril. It's not always a disadvantage however, just as in

Most often it is a disadvantage, e.g. consider the space cone occupied by
the STM tip + cantilever. There is complementary demand for not blocking
out access to the site & for the maximum mechanical rigidity of the tip
itself and support structure. If one thinks of the suface deposited ("3d
nanolithography printer"), which shields access to the reaction site, the
problems are obviously nontrivial. E.g. trying to access the same site with
two or more tips is obviously _very_ difficult, so a hollow tip would be
nice, but this has negative impact upon tip solidity... (btw Heckl's group
in Munich has recently learned to fabricate hollow tubes with a few nm
inner channel aperture, which are _not_ buckytubes. It's not in the press,

The tube approach suggests "molecular Gatling guns" (thanks, Jason),
where an inert precursor gets activated (e.g. by a laser pulse or a
electrical field spike) i.e. turned into a reactive moiety only shortly
before use, repeated in a rapid succession.

> our Macro world, if something doesn't quite fit you can often give it a good
> whack and make it fit. Chemists also have a term called "Steric Hindrance",

Unfortunately, a hard enough whack will rupture bonds. Additionally, e.g.
blue or UV light bear sufficient energy to (photo)dissotiate bonds, if
absorbed. You kill your circuitry if you look at it too sharply. This
requires molecular circuitry to be shielded from shortwave radition, either
by using red-orange-greenish light and/or embedding it in shielding matrix,
e.g. engineered proteins. The matrix is needed for autoassembly anyway,
so it's convenient.

> it refers to the slowing of a chemical reaction because some part of a
> molecule mechanically gets in the way. This is very important in large

Interspecies reactions require collisions. E.g. methane is kinetically
inert in oxygen up to certain temperature/pressure, since it is
spherically shielded by four hydrogens. An instable molecule can be often
stabilized, if more bulky residues are substituted for the original ones.
Another instance of it is crystal lattice photochemistry, which typically
yields different products from the solvated ones. Reactive species are
often made infinetely stable by embeddinig them in inert matrix, e.g.
argon under cryogenic conditions. Stuff must be diluted, but it's usually
enough to be seen by spectroscopy.

> biological molecules and depending on the circumstances it can be a help or a
> hindrance to the Nanotechnology engineer, but either way he must have a

Mostly, it's hindrance.

> understanding of it. There is a lot more about it in Nanosystems.

Yes. A worthwhile book to read, though a bit laconic on chemistry.