From owner-chemistry@ccl.net Sun Aug 31 06:25:01 2008 From: "Marcel Swart marcel.swart++icrea.es" To: CCL Subject: CCL: Minima Message-Id: <-37684-080831061858-9273-AIhF2efcaSuyWuUVhGDxwg^^server.ccl.net> X-Original-From: Marcel Swart Content-Type: multipart/alternative; boundary=Apple-Mail-7--797240484 Date: Sun, 31 Aug 2008 11:40:11 +0200 Mime-Version: 1.0 (Apple Message framework v753.1) Sent to CCL by: Marcel Swart [marcel.swart ~ icrea.es] --Apple-Mail-7--797240484 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=ISO-8859-1; delsp=yes; format=flowed Yes, there are indeed different minima, which are characterized by =20 all positive frequencies, if they are well separated from each other. For instance, taking as =20 an example the bimolecular nucleophilic substitution (Sn2) reaction, between Cl(-) + CH3Br =3D> 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 =20 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.; Sol=E0, M.; Bickelhaupt, F. M. J. Chem. Theor. Comp. =20 2008, 4, 929-940 Hope this helps. Marcel. On Aug 29, 2008, at 7:07 PM, Neelum Seeburrun nseeburrun**gmail.com =20 wrote: > > Sent to CCL by: "Neelum Seeburrun" [nseeburrun!=3D!gmail.com] > Dear All > The problem I am having is that there are different types of =20 > minima: local,global,true and genuine. All of the minima give real =20 > values of vibrational frequencies. So how to distingush between them? > Your help or references will be appreciated. > Neelum > nseeburrun ~~ gmail.com > > > > -=3D This is automatically added to each message by the mailing =20 > script =3D- > To recover the email address of the author of the message, please =20 > change> Conferences: http://server.ccl.net/chemistry/announcements/=20 > conferences/ > > Search Messages: http://www.ccl.net/htdig (login: ccl, Password: =20 > search)> > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D dr. Marcel Swart ICREA researcher at Institut de Qu=EDmica Computacional Universitat de Girona Parc Cient=EDfic i Tecnol=F2gic Edifici Jaume Casademont (despatx A-27) Pic de Peguera 15 17003 Girona Catalunya (Spain) tel +34-972-183240 fax +34-972-183241 e-mail marcel.swart^^icrea.es marcel.swart^^udg.edu web http://www.icrea.es/pag.asp?id=3DMarcel.Swart http://iqc.udg.edu/~marcel =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D --Apple-Mail-7--797240484 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=ISO-8859-1 Yes, there are indeed different minima, which are characterized by all = positive=A0frequencies,
if they are well separated from each other. = For instance, taking as an example = the
bimolecular=A0nucleophilic substitution (Sn2) reaction, = between
Cl(-) + CH3Br =3D> CH3Cl + Br(-).
In this = reaction (in the gas-phase !), there are several = minima:=A0
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.=A0
substitution vs. elimination, in which case you obtain = minima along
either (or both) pathways. See = e.g.=A0
Bento, A. P.; Sol=E0, 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=A0 Seeburrun" = [nseeburrun!=3D!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.
nseeburrun ~~ = gmail.com



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dr. Marcel = Swart

ICREA = researcher=A0at
Institut de Qu=EDmica = Computacional
Universitat de = Girona

Parc Cient=EDfic i = Tecnol=F2gic
Pic de Peguera = 15
Catalunya = (Spain)

tel
http://iqc.udg.edu/~marcel<= /span>



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