CCL: Minima

Yes, there are indeed different minima, which are characterized by all positive frequencies,
if they are well separated from each other. For instance, taking as an example the
bimolecular nucleophilic substitution (Sn2) reaction, between
Cl(-) + CH3Br => CH3Cl + Br(-).
In this reaction (in the gas-phase !), there are several minima: 
reactants, reactant-complex, product-complex, products.
(note that I mean here the reactants and products at infinite separation)
It is fairly easy to distinguish between them, and draw an energy
profile for this reaction (see e.g. J.Comput.Chem. 2007, 28, 1551 and
references therein).

Note that these minima are connected along the energy profile by the
backside pathway reaction coordinate. (You might check this with IRC
calculations)

Note that for the abovementioned reaction, the product complex is
lowest in energy, e.g. the global minimum along the Sn2 pathway.
However, sometimes there are also competing pathways, e.g. 
substitution vs. elimination, in which case you obtain minima along
either (or both) pathways. See e.g. 
Bento, A. P.; Solà, M.; Bickelhaupt, F. M. J. Chem. Theor. Comp. 2008, 4, 929-940

Hope this helps.

Marcel.
On Aug 29, 2008, at 7:07 PM, Neelum Seeburrun nseeburrun**gmail.com wrote:


Sent to CCL by: "Neelum  Seeburrun" [nseeburrun!=!gmail.com]
Dear All
The problem I am having is that there are different types of minima: local,global,true and genuine. All of the minima give real values of vibrational frequencies. So how to distingush between them?
Your help or references will be appreciated.
Neelum
nseeburrun ~~ gmail.com



-= This is automatically added to each message by the mailing script =-
To recover the email address of the author of the message, please change
the strange characters on the top line to the ^^ sign. You can also
look up the X-Original-From: line in the mail header.

E-mail to subscribers: CHEMISTRY^^ccl.net or use:

E-mail to administrators: CHEMISTRY-REQUEST^^ccl.net or use

Subscribe/Unsubscribe: 

Before posting, check wait time at: http://www.ccl.net


Search Messages: http://www.ccl.net/htdig  (login: ccl, Password: search)

If your mail bounces from CCL with 5.7.1 error, check:




===================================
dr. Marcel Swart

ICREA researcher at
Institut de Química Computacional
Universitat de Girona

Parc Científic i Tecnològic
Edifici Jaume Casademont (despatx A-27)
Pic de Peguera 15
17003 Girona
Catalunya (Spain)

tel
+34-972-183240
fax
+34-972-183241
e-mail
web
===================================