CCL: vinyl group twist

 Sent to CCL by: Detlev Conrad Mielczarek [detlevcm .. googlemail.com]
 Good evening David.
 There really is no good alternative to breaking at least one of the bonds.
 Or if you didn't, you would end up with orbitals in an "x
 configuration" crossing each other's paths.
 Now you could consider breaking both bonds and going through a quintet
 state, but the energy barrier for such a transition should be much
 higher.
 Crossing from singlet to triplet and then back from triplet to singlet
 does not cause any problems.
 Breaking the bond initially will require an energy input.
 However, reforming the bond will liberate energy. - This bond
 formation also resembles a free radical termination in some ways which
 is generally considered to be barrierless.
 As a result, you really have an energy barrier/opposition to twisting
 the molecule and breaking the initial bond, while then continuing the
 rotation and reforming the bond will bring you back to the original
 lowest energy ground state.
 Incidentally, there is a lot of literature on this, such as for example:
 sciencedirect.com/topics/chemistry/bond-rotation
 I couldn't find anything on halogen bond rotation, but a quick google
 search turned up the following on bond dissociation energies, which
 should give you a good idea of the relative strengths of these bonds.
 Are carbon—halogen double and triple bonds possible?
 Robert Kalescky, Elfi Kraka, Dieter Cremer
 27 February 2014
 International Journal of Quantum Chemistry
 10.1002/qua.24626
 Detlev
 On Thu, 1 Sept 2022 at 18:25, David Shobe shobedavid===gmail.com
 <owner-chemistry(-)ccl.net> wrote:
 >
 > Detlev--
 >
 > I think you are correct about the triplet state being the lowest electronic
 state in twisted vinyl-X compounds, but would the reaction really proceed
 through *two* intersystem crossings (singlet to triplet then back to singlet)?
 >
 > --David Shobe
 >
 > On Mon, Aug 29, 2022, 4:09 PM Detlev Conrad Mielczarek
 detlevcm{:}googlemail.com <owner-chemistry++ccl.net> wrote:
 >>
 >>
 >> Sent to CCL by: Detlev Conrad Mielczarek [detlevcm%googlemail.com]
 >> If you are twisting through a double bond, I would expect at least one
 of the bonds to break (forming. Asingle bond) and see the transition progress
 from a singlet through a triplet state before reforming the double bond and
 becoming s singlet again.
 >> (The ORCA manual contains an example of twisting an ethene molecule.)
 >>
 >> In this case, you would need to ideally treat this with a
 multi-reference method such as CASSCF.
 >>
 >> Running a transition state search in a single multiplicity will
 invariably lead to incorrect results in such cases, even if some methods seem to
 work through them.
 >>
 >> You may be able to use two separate calculations to obtain a valid
 energy value for the transition state.
 >>
 >> Detlev
 >>
 >>
 >>
 >>
 >>           Original Message
 >>
 >>
 >> > From: owner-chemistry++ccl.net
 >> Sent: 29 August 2022 21:21
 >> To: detlevcm++googlemail.com
 >> Reply to: chemistry++ccl.net
 >> Subject: CCL: vinyl group twist
 >>
 >>
 >> CCL group--
 >>
 >>
 >> I am trying to find transition states for twisting the double bonds in
 vinyl-X compounds (X is any group, not necessarily halogen), with the torsion
 angle going from 0° to 180°, through a transition state that should
 have a torsion angle of approximately 90°. Of course, I expect these to be
 high-energy transition states. I have tried both opt=ts with a few values of X,
 and once tried opt=qst3.
 >>
 >>
 >>
 >> The opt=qst3 method generated something weird, in which the vinyl group
 had a linear H-C=C angle (instead of 120°). The other distal H on the
 vinyl's methylene group had a H-C=C angle of 96°  It is a transition state
 (1 imag freq), but definitely not the one I am looking for.
 >>
 >>
 >> Using opt=(usually along with noeigentest), I invariably end up with a
 much-lower-energy transition state involving torsion of the vinyl group as a
 whole in the vinyl-X molecule, or with a torsion of some subgroup within X.
 >>
 >>
 >> Is it normal for attempts to calculate a high-energy transition state
 to end up with a different, lower-energy transition state? More importantly, is
 there any way to avoid this phenomenon and get the desired transition state?
 >>
 >>
 >> --David Shobe>> E-mail to subscribers: CHEMISTRY++ccl.net or
 use:>>
 >> E-mail to administrators: CHEMISTRY-REQUEST++ccl.net or use>>
 >>
 --
 Detlev Conrad Mielczarek
 émail privé: detlevcm(-)googlemail.com
 téléphone: 0033 (0)648 151 995