Microwave Radiation and biomolecular excitation

Microwave Radiation and biomolecular excitation

Post by laird_tay.. » Sat, 14 Jan 1995 03:53:29





> Its been awhile since Pchem, but does anyone know if microwave radiation
> can "e*" or be absorbed by specific atoms in a macromolecule?  
> similar to NMR, albeit without the signal emission, so that the energy is
> absorbed by the atom? I know they use microwave radiation to do
> spectroscopy on determing the moments of inertia of small gas molecules,
> but would be interested if microwave radiation could be 'tuned' as it
> were to specific frequencies to e* atoms in a larger molecule...I
> understand this would  be vibrational energy levels and they are not as
> distinct as other energy quanta...but any thoughts would be appreciated..
> it is just something I have been wondering about. thanks Tim

The definitive seminal work that defines all this was written by Gerhard
Hertzberg, a Canadian with the Research Council of Canada, during WWII.
It's a 2-volume set.  Volume I is on diatomic molecules -- I have the 2nd
edition, republished in 1950 by Van Nostrand Reinhold (no ISBN available).
Volume II is "Infrared and Raman Spectra of Polyatomic Molecules"  I have
never actually seen a copy, but know at least one physicist who says he
has one, and says it's terrific.  Apparently the treatment is timeless...

Basically, electron-orbital transitions are e*d by near-IR through
far-UV energy quanta (photons are reputed to do nicely).  Vibrational
spectra (both compressive/tensile, and torsional rotation of double-bond
structures) are typically in the mid- to near-IR range, and rotational
of single-bond structures are in the microwave to far-IR range, at least
for diatomic molecules.  The same rules of thumb probably apply loosely
to polyatomic molecules; Hertzberg's second volume probably names names
and gives numbers.  The technology of efficient excitation (required for
hassle-free storage) and saturation limits (joules/mole before the wheels
start coming off chemically) are another matter about which I know nearly
nothing.  Happy reading!


 
 
 

Microwave Radiation and biomolecular excitation

Post by Kenneth R Fost » Sat, 14 Jan 1995 22:49:39



: > Its been awhile since Pchem, but does anyone know if microwave radiation
: > can "e*" or be absorbed by specific atoms in a macromolecule?  ...

: The definitive seminal work that defines all this was written by Gerhard
: Hertzberg...

: Basically, electron-orbital transitions are e*d by near-IR through
: far-UV energy quanta (photons are reputed to do nicely).  Vibrational
: spectra (both compressive/tensile, and torsional rotation of double-bond
: structures) are typically in the mid- to near-IR range, and rotational
: of single-bond structures are in the microwave to far-IR range, at least
: for diatomic molecules.  The same rules of thumb probably apply loosely
: to polyatomic molecules; Hertzberg's second volume probably names names
: and gives numbers.  The technology of efficient excitation (required for
: hassle-free storage) and saturation limits (joules/mole before the wheels
: start coming off chemically) are another matter about which I know nearly
: nothing.  Happy reading!

This is true, of course.  On the other hand, these effects are observable
only in gases at very low pressure, i.e. near-vacuum, and generally pertain
to millimeter waves and above. The original poster did not specify
exactly what he had in mind but (given the environmental concerns about
microwaves used in communications) he may have had in mind molecules
in aqueous solution, exposed to microwaves (say) in the frequency range 300 MHz
to (say) 10 GHz.  The answer there is unequivocally no.

There have been scattered reports of resonance-type absorption
of microwaves near 1 GHz by DNA in aqueous solution (see, for example,
Edwards et al, Phys. Rev. Lett. 53:1284-1287 (1984) but these claims
were nonreproducible and probably due to artifact.  See the editorial
"Physicists retreat again" by Frank-Kamenetskii, Nature 328:108 (1987).