From owner-chemistry@ccl.net Wed Jun 29 12:07:01 2022 From: "Gary Breton gbreton:_:berry.edu" To: CCL Subject: CCL: Chemical reaction insights resulting from geometry optimizations Message-Id: <-54739-220629085157-19127-mEoY284a/x0IXczRZW8B/Q : server.ccl.net> X-Original-From: "Gary Breton" Date: Wed, 29 Jun 2022 08:51:55 -0400 Sent to CCL by: "Gary Breton" [gbreton]^[berry.edu] Hi everyone, There is a long history of computational modeling of elimination reactions that occur via the E2 and E1cb reaction mechanisms. The E2 mechanism is a concerted mechanism in which a base removes a beta-hydrogen atom while a leaving group (e.g., a halogen) departs more-or-less simultaneously to give rise to an alkene product. In the 2-step E1cb mechanism, the base removes the beta-hydrogen completely, and the resulting anion then "ejects" the leaving group to form the alkene product. While I am still perusing some of the earlier literature, I was surprised to find that no one has reported that attempts at modeling the anion "intermediate" in the E1cb route (devoid of the base and detached proton) using DFT methods (which have been the preferred methods in the literature for modeling these reactions [I am using wB97XD/aug-cc-pvtz]), results in spontaneous ejection of the leaving group during the optimization process to form the alkene. Thus, the bare anion is not a stationary point on the PES. Some competitions can be set up comparing preference for leaving group loss, etc. that appear to mirror experimental results. I'm still working on some of these interesting findings. Two questions: 1. Since I am not dealing with stationary points, how legitimate is it to compare, for example, the preference for ejection of a Cl- leaving group over a F- leaving group from a modeled (non-optimized) anion? NBO calculations have been employed that support why elimination of the Cl is preferred. 2. I have been trying to find other instances where attempted geometry optimizations of this sort have lead to chemical reaction insights due to spontaneous "reactivity" during the optimization process (e.g., rearrangements, eliminations, etc.). My searches have come up empty so far and was hoping some of you might lead me in the proper direction. Thanks for reading and any suggestions that may be offered. Gary Breton Berry College From owner-chemistry@ccl.net Wed Jun 29 16:36:00 2022 From: "Marcel Swart marcel.swart.:.gmail.com" To: CCL Subject: CCL: Chemical reaction insights resulting from geometry optimizations Message-Id: <-54740-220629161915-21125-d1pT6wPlW1fuAMkkHpDuiQ*o*server.ccl.net> X-Original-From: Marcel Swart Content-Type: multipart/alternative; boundary="Apple-Mail=_F9D8272F-E5F4-4DC1-B4C6-420F0AD37652" Date: Wed, 29 Jun 2022 22:19:06 +0200 Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3696.100.31\)) Sent to CCL by: Marcel Swart [marcel.swart-,-gmail.com] --Apple-Mail=_F9D8272F-E5F4-4DC1-B4C6-420F0AD37652 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=utf-8 Matthias Bickelhaupt has been working on this for ages. Just two obvious = references which I know of, I=E2=80=99m sure there must be more: F. M. Bickelhaupt =E2=80=93=E2=80=93=E2=80=93 "Base-Induced = 1,4-Elimination: Insights from Theory and Mass Spectrometry" =E2=80=93=E2=80= =93=E2=80=93 Mass Spectrom. Rev. 2001, 20, 347-361 F. M. Bickelhaupt, G. J. H. Buisman, L. J. de Koning, N. M. M. = Nibbering, E. J. Baerends =E2=80=93=E2=80=93=E2=80=93 "Gas-Phase = Base-Induced 1,4-Eliminations: Occurrence of Single-, Double- and = Triple-Well E1cb Mechanisms" =E2=80=93=E2=80=93=E2=80=93 J. Am. Chem. = Soc. 1995, 117, 9889-9899 In particular note the first one, section V: "Mechanism of = 1,4-Elimination: E1cb Versus E2=E2=80=9D. What I remember from my postdoc time with him (I wasn=E2=80=99t involved = in elimination, but in Sn2) is that indeed the leaving group (e.g. F=E2=80= =93) moves around the whole molecule and forms a FHF(-) moiety during = the optimizations. MS Marcel Swart ICREA Research Professor at University of Girona (UdG) Director of Institut de Qu=C3=ADmica Computacional i Cat=C3=A0lisi, UdG Univ. Girona, Campus Montilivi (Ci=C3=A8ncies) c/ M.A. Capmany i Farn=C3=A9s, 69 17003 Girona, Spain www.marcelswart.eu marcel.swart_-_gmail.com > On 29 Jun 2022, at 14:51, Gary Breton gbreton:_:berry.edu = wrote: >=20 >=20 > Sent to CCL by: "Gary Breton" [gbreton]^[berry.edu] > Hi everyone, >=20 > There is a long history of computational modeling of elimination = reactions that occur via the E2 and E1cb reaction mechanisms. The E2 = mechanism is a concerted mechanism in which a base removes a = beta-hydrogen atom while a leaving group (e.g., a halogen) departs = more-or-less simultaneously to give rise to an alkene product. In the = 2-step E1cb mechanism, the base removes the beta-hydrogen completely, = and the resulting anion then "ejects" the leaving group to form the = alkene product. >=20 > While I am still perusing some of the earlier literature, I was = surprised to find that no one has reported that attempts at modeling the = anion "intermediate" in the E1cb route (devoid of the base and detached = proton) using DFT methods (which have been the preferred methods in the = literature for modeling these reactions [I am using = wB97XD/aug-cc-pvtz]), results in spontaneous ejection of the leaving = group during the optimization process to form the alkene. Thus, the = bare anion is not a stationary point on the PES. Some competitions can = be set up comparing preference for leaving group loss, etc. that appear = to mirror experimental results. I'm still working on some of these = interesting findings. >=20 > Two questions: >=20 > 1. Since I am not dealing with stationary points, how legitimate is = it to compare, for example, the preference for ejection of a Cl- leaving = group over a F- leaving group from a modeled (non-optimized) anion? NBO = calculations have been employed that support why elimination of the Cl = is preferred. >=20 > 2. I have been trying to find other instances where attempted = geometry optimizations of this sort have lead to chemical reaction = insights due to spontaneous "reactivity" during the optimization process = (e.g., rearrangements, eliminations, etc.). My searches have come up = empty so far and was hoping some of you might lead me in the proper = direction. >=20 > Thanks for reading and any suggestions that may be offered. >=20 > Gary Breton > Berry College >=20 >=20 >=20 > -=3D This is automatically added to each message by the mailing script = =3D- > To recover the email address of the author of the message, please = change>=20>=20>=20 > Subscribe/Unsubscribe:=20>=20>=20 > Job: http://www.ccl.net/jobs=20 > Conferences: = http://server.ccl.net/chemistry/announcements/conferences/ >=20>=20>=20>=20 >=20 --Apple-Mail=_F9D8272F-E5F4-4DC1-B4C6-420F0AD37652 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=utf-8
Matthias Bickelhaupt has been working on this for ages. Just = two obvious references which I know of, I=E2=80=99m sure there must be = more:

F. M. = Bickelhaupt =E2=80=93=E2=80=93=E2=80=93 "Base-Induced 1,4-Elimination: = Insights from Theory and Mass Spectrometry" =E2=80=93=E2=80=93=E2=80=93&nb= sp;Mass Spectrom. Rev. 2001, 20, 347-361
F.= M. Bickelhaupt, G. J. H. Buisman, L. J. de Koning, N. M. M. Nibbering, = E. J. Baerends =E2=80=93=E2=80=93=E2=80=93 "Gas-Phase Base-Induced = 1,4-Eliminations: Occurrence of Single-, Double- and Triple-Well = E1cb Mechanisms" =E2=80=93=E2=80=93=E2=80=93 J. Am. Chem. = Soc. 1995, 117, 9889-9899

In particular note the first one, = section V: "Mechanism of 1,4-Elimination: E1cb Versus E2=E2=80=9D.
What I remember from my postdoc time with him (I wasn=E2=80=99= t involved in elimination, but in Sn2) is that indeed the leaving group = (e.g. F=E2=80=93) moves around the whole molecule and forms a FHF(-) = moiety during the optimizations.

MS

Marcel Swart
ICREA Research Professor at University of Girona = (UdG)
Director of Institut de Qu=C3=ADmica Computaciona= l i Cat=C3=A0lisi, UdG

Univ. Girona, Campus = Montilivi (Ci=C3=A8ncies)
c/ M.A. Capmany i Farn=C3=A9s, = 69
17003 Girona, Spain

www.marcelswart.eu
marcel.swart_-_gmail.com


On 29 Jun 2022, at 14:51, Gary Breton gbreton:_:berry.edu <owner-chemistry_-_ccl.net> wrote:


Sent to CCL by: "Gary  Breton" [gbreton]^[berry.edu]
Hi = everyone,

There is a long history of = computational modeling of elimination reactions that occur via the E2 = and E1cb reaction mechanisms.  The E2 mechanism is a concerted = mechanism in which a base removes a beta-hydrogen atom while a leaving = group (e.g., a halogen) departs more-or-less simultaneously to give rise = to an alkene product.  In the 2-step E1cb mechanism, the base = removes the beta-hydrogen completely, and the resulting anion then = "ejects" the leaving group to form the alkene product.

While I am still perusing some of the earlier literature, I = was surprised to find that no one has reported that attempts at modeling = the anion "intermediate" in the E1cb route (devoid of the base and = detached proton) using DFT methods (which have been the preferred = methods in the literature for modeling these reactions [I am using = wB97XD/aug-cc-pvtz]), results in spontaneous ejection of the leaving = group during the optimization process to form the alkene.  Thus, = the bare anion is not a stationary point on the PES.  Some = competitions can be set up comparing preference for leaving group loss, = etc. that appear to mirror experimental results.  I'm still working = on some of these interesting findings.

Two = questions:

1.  Since I am not dealing = with stationary points, how legitimate is it to compare, for example, = the preference for ejection of a Cl- leaving group over a F- leaving = group from a modeled (non-optimized) anion?  NBO calculations have = been employed that support why elimination of the Cl is preferred.

2.  I have been trying to find other = instances where attempted geometry optimizations of this sort have lead = to chemical reaction insights due to spontaneous "reactivity" during the = optimization process (e.g., rearrangements, eliminations, etc.). =  My searches have come up empty so far and was hoping some of you = might lead me in the proper direction.

Thanks= for reading and any suggestions that may be offered.

Gary Breton
Berry College



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