CCL: Level of Theory for zwitterionic species

 Sent to CCL by: Justin Finnerty [justin.finnerty(~)uni-oldenburg.de]
 On Thu, 2009-11-05 at 10:11 -0500, Sarah Ann Kebbell
 sarah.kebbell2_-_mail.dcu.ie wrote:
 > Sent to CCL by: "Sarah  Ann Kebbell"
 [sarah.kebbell2^-^mail.dcu.ie]
 > Dear CCLers,
 >
 > just wondering if anybody has an previous experience in locating
 intermediates that are zwitterionic in nature and if so, is B3LYP/6-31G(d) a
 sufficient level of theory to locate zwitterions at or do I need to change the
 theory/basis set to do so?
 >
 > Many thanks,
 > Sarah
 Dear Sarah,
 myself, Rainer Koch and a number of others have worked with the
 well-known experimentalist Prof Curt Wentrup in the field of reactive
 intermediates.  We have successfully used B3LYP/6-31G(d) to predict the
 base geometry of a range of formally zwitterionic intermediates.  This
 was always combined with improved energy predictions using single-point
 calculations with large basis-sets and/or better theories.  As Tanja
 said, solvation can be a major factor and we have had to use both the
 implicit solvation model (eg SCRF) and more rarely explicit solvation.
 However, the improvement in computing resources now means we use the
 6-311++G** basis set for both geometry optimistation and
 thermochemistry.  We are finding this to give much improved results for
 determining reaction pathways that had formerly eluded us.  We are also
 investigating alternative functionals, such as MPW1K, as possible
 alternatives to the venerable B3LYP.
 One point that I see is that the zwitterionic character is not readily
 apparent in the molecular orbital picture, particularly for gas-phase
 calculations.  It usually requires full NBO style analysis to get a more
 "chemist'S intuitive" picture with formal charges on atoms.  However,
 I,
 myself, prefer the MO picture where you consider "zwitterions" as
 having
 volumes with increased or decreased electron density rather than the
 notion of formal charges.
 Cheers
   Justin
 --
 Dr Justin Finnerty
 Rm W3-1-165         Ph 49 (441) 798 3726
 Carl von Ossietzky Universität Oldenburg