Subject: RE QUESTION ON COMPUTING STABILITY

From: "E. Lewars" <[email-masked]>
Date: Wed, 22 Nov 1995 17:39:43 -0500 (EST)

1995 Nov 22
Errol Lewars  Chem Dept Trent U, Peterborough, Ontario, CANADA

Hello,

With regard to the recent question by Thorsten Koch "How do I measure the
stability of organic molecules? ...I compare absolute electronic energy with...
a known and more or less stable isomer."  There were also queries about the
relevance of frontier orbitals and calculated IR frequencies.

  *Stability* can refer to (1) the tendency of an isolated molecule to react,
perforce by isomerizing or by dissociating, or it can refer to (2) the tendency
of a molecule to react with another (possibly identical) molecule.

  Considering (1):
  The calculated freqs (i.e. the calc IR spectrum) provide information about
the reactivity of the isolated molecule.  If there are no imaginary ("negative")
freqs, the molecule can exists *as far as that particular level of calc is
concerned*, that is, it is a relative minimum on the potential energy surface;
it sits in a well and jiggles its all-real vibrational modes, escaping from
the well at a rate that, ignoring wierd things like tunneling, depends (at
least approximately) on the height of the barrier separating it from the
nearest other well (or nearest plateau, for dissociation).
  A low-freq vibration corresponding to bond cleavage should make one suspect
that at some higher computational level this freq will become imaginary, and
the bond will actually break.  A low-freq "vibration" that coresponds to a
torsional mode would, if it were coaxed into being imaginary at a higher level,
correspond to a conformational change: the molecule might still exist, but might
not, say, be flat as in the input geom, but rather puckered (e.g. flat D4h
cyclobutane may prefer to be D2d).
  The stability of a real molecule (a relative minimum) can be quantified by
calculating the rate at which it goes over the lowest barrier separating it
from its isomerization or dissoaciation product(s).  Of course, if it's
substantially the lowest-energy species on the PES then you don't have to worry
about this.  Stability is most intelligible if expressed as half-life at some
temperature (like room temp, where most of us live).

  Considering (2):
  Comparison of the frontier orbitals (HOMO and LUMO) with those of known,
similar molecules is probably wothwhile, since reaction with electrophiles
and nucleophiles will be governed (to the ubiquitious first approximation)
by the energies of these orbitals.

Some references to calulations on the stabilities/existence of exotic organic
molecules are:
Oxirene and substituted oxirenes--
Vacek,..., Schaefer,..., J Phys Chem 98 (1994) 8660.
Fowler, Galbraith, Vacek and Schaefer, J Am Chem Soc 116 (1994) 9311.
Scott, Nobes, Schaefer and Radom, J Am Chem Soc 116 (1994) 10159

N6 (e.g. hexaazabenzene), N8, etc--
Engelke, J Am Chem Soc  115 (1993) 2961
Engelke, J Org Chem 57 (1992) 4841
Engelke and Stine, J Phys Chem 94 (1990) 5689 ["Is N8 Cubane Stable?]
Engelke, J Phys Chem 94 (1990) 6924                            /|\
Engelke, J Phys Chem 96 (1992) 10789                            |
Engelke, J Phys Chem 93 (1989) 5722                             |
                                                             STABLE
Benzooxirene--
Lewars, THEOCHEM 1996

CO2 polymers--
Lewars, THEOCHEM 1996