From owner-chemistry@ccl.net Sun Mar 13 01:05:00 2011 From: "Rajagopala Reddy Seelam rajagopalaseelam]~[gmail.com" To: CCL Subject: CCL:G: query regarding crystal structure Message-Id: <-44120-110313010232-27453-5yEiJFSpWRD6xA5X8eka8w|,|server.ccl.net> X-Original-From: "Rajagopala Reddy Seelam" Date: Sun, 13 Mar 2011 01:02:30 -0500 Sent to CCL by: "Rajagopala Reddy Seelam" [rajagopalaseelam]*[gmail.com] Hello CCL subscribers, I have xyz coordinates of a crystal structure solved from X-Ray Diffraction Studies. I want to calculate excited states of this molecule using g03. Please answer me following few queries. 1) I am in confusion in deciding to go for either a single point energy calculation or an optimization. If I optimize the geometry, the resulting geometry is not as same as the structure obtained from crystal structure. Rather it will be isomer for the crystal structure. Which is the best way to proceed for crystal structure ? 2) I am getting imaginary frequencies for this structure at single point energy. In principle we should not get any imaginary frequencies for stable molecule. What is workaround for this problem. Thank you so much best wishes Rajagopala Reddy Seelam From owner-chemistry@ccl.net Sun Mar 13 06:15:00 2011 From: "Radoslaw Kaminski rkaminski.rk[A]gmail.com" To: CCL Subject: CCL:G: query regarding crystal structure Message-Id: <-44121-110313061356-24932-uUtQ0sDhHdMOb7NI6DtZKA..server.ccl.net> X-Original-From: Radoslaw Kaminski Content-Type: multipart/alternative; boundary=0016363b9ce07fdf5d049e5a73e3 Date: Sun, 13 Mar 2011 11:13:48 +0100 MIME-Version: 1.0 Sent to CCL by: Radoslaw Kaminski [rkaminski.rk(-)gmail.com] --0016363b9ce07fdf5d049e5a73e3 Content-Type: text/plain; charset=ISO-8859-1 Hi, Concerning crystal-field-constrained optimization you can try to use the QM/MM method. I have used it for the calculation of excited states in molecular crystals and it works very well. You can check it out in: R. Kaminski. M. S. Schmokel, P. Coppens, J. Phys. Chem. Lett., 2010, 1, 2349. The reprint is on my web-site: http://www.chem.uw.edu.pl/people/RKaminski/publications.html Cheers, Radek 2011/3/13 Rajagopala Reddy Seelam rajagopalaseelam]~[gmail.com < owner-chemistry%ccl.net> > > Sent to CCL by: "Rajagopala Reddy Seelam" [rajagopalaseelam]*[gmail.com] > Hello CCL subscribers, > I have xyz coordinates of a crystal structure solved from X-Ray Diffraction > Studies. I want to calculate excited states of this molecule using g03. > Please answer me following few queries. > > 1) I am in confusion in deciding to go for either a single point > energy calculation or an optimization. If I optimize the geometry, > the resulting geometry is not as same as the structure obtained > from crystal structure. Rather it will be isomer for the crystal > structure. Which is the best way to proceed for crystal structure ? > > 2) I am getting imaginary frequencies for this structure at single > point energy. In principle we should not get any imaginary frequencies > for stable molecule. What is workaround for this problem. > > Thank you so much > best wishes > Rajagopala Reddy Seelam> > > -- Radoslaw Kaminski, M.Sc. Eng. Ph.D. Student Crystallochemistry Laboratory Department of Chemistry University of Warsaw Pasteura 1, 02-093 Warszawa, Poland http://www.chem.uw.edu.pl/people/RKaminski/ --0016363b9ce07fdf5d049e5a73e3 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Hi,

Concerning crystal-field-constrained optimization you can try to= use the QM/MM method. I have used it for the calculation of excited states= in molecular crystals and it works very well. You can check it out in: R. = Kaminski. M. S. Schmokel, P. Coppens, J. Phys. Chem. Lett., 2010, 1, 2349. = The reprint is on my web-site: http://www.chem.uw.edu.pl/people/RKaminski/pub= lications.html

Cheers,

Radek


2011/3/13 Ra= jagopala Reddy Seelam rajagopalaseelam]~[gmail= .com <o= wner-chemistry%ccl.net>

Sent to CCL by: "Rajagopala Reddy Seelam" [rajagopalaseelam]*[gmail.com]
Hello CCL subscribers,
I have xyz coordinates of a crystal structure solved from X-Ray Diffraction= Studies. I want to calculate excited states of this molecule using g03. Pl= ease answer me following few queries.

1) I am in confusion in deciding to go for either a single point
=A0 energy calculation or an optimization. If I optimize the geometry,
=A0 the resulting geometry is not as same as the structure obtained
=A0 from crystal structure. Rather it will be isomer for the crystal
=A0 structure. Which is the best way to proceed for crystal structure ?
2) I am getting imaginary frequencies for this structure at single
=A0 point energy. In principle we should not get any imaginary frequencies=
=A0 for stable molecule. What is workaround for this problem.

Thank you so much
best wishes
Rajagopala Reddy Seelam



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--
Radoslaw Kaminski, M.Sc= . Eng.
Ph.D. Student
Crystallochemistry Laboratory
Department of C= hemistry
University of Warsaw
Pasteura 1, 02-093 Warszawa, Poland
h= ttp://www.chem.uw.edu.pl/people/RKaminski/
--0016363b9ce07fdf5d049e5a73e3-- From owner-chemistry@ccl.net Sun Mar 13 19:29:00 2011 From: "=?ISO-8859-1?Q?Oldamur_Holl=F3czki?= holloczki{}gmail.com" To: CCL Subject: CCL: NHC-structure optimization! Message-Id: <-44122-110313163115-9799-/W6zokgh1+wZ8/FT8mSVJw(a)server.ccl.net> X-Original-From: =?ISO-8859-1?Q?Oldamur_Holl=F3czki?= Content-Type: text/plain; charset=ISO-8859-1 Date: Sun, 13 Mar 2011 21:31:08 +0100 MIME-Version: 1.0 Sent to CCL by: =?ISO-8859-1?Q?Oldamur_Holl=F3czki?= [holloczki-#-gmail.com] Dear Kavitha, I think the electronic structure you obtained represents well that of the carbene, so you do not need to worry. I think the method/base you used usually give sufficient description for NHCs, although there can be exceptions, of course. Best wishes, Oldamur 2011/3/12 Kavitha velappan kavitha.velappan(a)gmail.com : > Dear CCL subscribers, > > I tried to optimize the structure of N-heterocyclic carbene at the > B3LYP/6-311++G** level. The optimized structure correspond to that of sp3 > hybridized carbene(!!!) - obtained by NBO analysis. The lone pair of > electron on carbene carbon delocalized into the antibonding orbital of the > adjacent N-C bond. Is there a way to get the structure corresponding to sp2 > hybridized carbene (avoiding the possibility of delocalization)? Whether > optimization using a larger basis set/other methods will solve this? > > Any help would be greatly appreciated. > Kavitha >