From owner-chemistry@ccl.net Mon Feb 9 07:29:00 2015 From: "Cornie Van Sittert Cornie.VanSittert^nwu.ac.za" To: CCL Subject: CCL: Homo-lumo gap significance Message-Id: <-51014-150209071726-22653-c4BcJChSHSuQQdOWJxH0ig(~)server.ccl.net> X-Original-From: "Cornie Van Sittert" Content-Type: multipart/mixed; boundary="=__Part6550E424.0__=" Date: Mon, 09 Feb 2015 14:16:52 +0200 Mime-Version: 1.0 Sent to CCL by: "Cornie Van Sittert" [Cornie.VanSittert/./nwu.ac.za] This is a MIME message. If you are reading this text, you may want to consider changing to a mail reader or gateway that understands how to properly handle MIME multipart messages. --=__Part6550E424.0__= Content-Type: multipart/alternative; boundary="=__Part6550E424.1__=" --=__Part6550E424.1__= Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: quoted-printable Good afternoon, =20 I was wondering if anybody could help me. I would like to ask you about = the HOMO-LUMO energy gap. =20 I have three transition states to compare, TS1, TS2 and TS3. The HOMO-LUMO = was calculated for each on the whole system, so I have my HOMO on my Nu- = and my LUMO on my electrophile. For the HOMO-LUMO energy gap, I subtracted = the LUMO energy from the HOMO energy and got the absolute value (column = 2). Column 3 is the HOMO-LUMO energy gap within the transition state = structure. =20 In the table below I compare the HOMO-LUMO energy gaps, and it seems that = at the transition state (TS, column 3), the N9 has the biggest energy gap, = followed by N3, and then N7. I read that all transition states reach the = most stable form, which is the TS with the largest HOMO-LUMO energy gap. = This follows the trend in the lab where we made the N9>N3>>N7. The = HOMO-LUMO energy gap of transition state minus the HOMO-LUMO energy gap of = reactants (column 4) shows the N9 From what I read in literature, the HOMO-LUMO energy gap for the transition= state should be as big as possible so that the total HOMO-LUMO energy gap = is small: (column 4) E(total) gap =3D E(reactants) gap - E(transition state) gap =20 pathway HOMO-LUMO gapR HOMO-LUMO gapTS HOMO-LUMO gap(TS-R) N3 0.15009 0.12572 0.02439 N7 0.14983 0.12340 0.02643 N9 0.14973 0.12767 0.02206 =20 =20 The question that no one seems to answer is if these values (shown above) = are significantly different from each other. =20 Is the difference on the 3rd decimal place in the eigenvalues for the TS = significant? or would all the reactions occur? When can the difference be = taken as significant? =20 Kind regards, Cornie van Sittert =20 =20 =20 Vrywaringsklousule / Disclaimer: http://www.nwu.ac.za/it/gov-man/disclaimer= .html --=__Part6550E424.1__= Content-Type: text/html; charset=US-ASCII Content-Transfer-Encoding: quoted-printable Content-Description: HTML
Good afternoon,
 
I was wondering if anybody could help me.  I would like to ask = you about the HOMO-LUMO energy gap.
 
I have three transition states to compare, TS1, TS2 and TS3. The = HOMO-LUMO was calculated for each on the whole system, so I have my HOMO = on my Nu- and my LUMO on my electrophile. For the HOMO-LUMO energy = gap, I subtracted the LUMO energy from the HOMO energy and got the = absolute value (column 2).  Column 3 is the HOMO-LUMO energy gap = within the transition state structure.
 
In the table below I compare the HOMO-LUMO energy gaps, and it = seems that at the transition state (TS, column 3), the N9 has the biggest = energy gap, followed by N3, and then N7. I read that all transition states = reach the most stable form, which is the TS with the largest HOMO-LUMO = energy gap. This follows the trend in the lab where we made the N9>N3>= ;>N7. The HOMO-LUMO energy gap of transition state minus the HOMO-LUMO = energy gap of reactants (column 4) shows the N9<N3<N7 trend. = Articles say that reactions will follow the one with the smallest gap, and = this agrees with my experimental work.
From what I read in literature, the HOMO-LUMO energy gap for the = transition state should be as big as possible so that the total HOMO-L= UMO energy gap is small: (column 4)
E(total) gap =3D E(reactants) gap - = E(transition state) gap
 

pathway

HOMO-LUMO gapR

HOMO-LUMO gapTS

HOMO-LUMO gap(TS-R)=

N3

0.15009

0.12572

0.02439

N7

0.14983

0.12340

0.02643

N9

0.14973

0.12767

0.02206
 
 
The question that no one seems to answer is if these values = (shown above) are significantly different from each other.
 
Is the difference on the 3rd decimal place in the eigenvalues for the = TS significant? or would all the reactions occur?  When can the = difference be taken as significant?
 
Kind regards,
Cornie van Sittert
 
 
 

Vrywaringsklousule / Disclaimer: http://www.nwu.ac.za/it/gov-man/disclaimer.html

--=__Part6550E424.1__=-- --=__Part6550E424.0__=-- From owner-chemistry@ccl.net Mon Feb 9 08:14:01 2015 From: "Kaushik Hatua kaushikhatua|a|yahoo.in" To: CCL Subject: CCL: PES for excited state Message-Id: <-51015-150209080239-13222-eEubSNeP5hOe9v3UgT+2Og=-=server.ccl.net> X-Original-From: Kaushik Hatua Content-Type: multipart/alternative; boundary="_DC9ACE5A-4F52-4ACE-9D2E-729C4A5A3ADA_" Date: Mon, 9 Feb 2015 18:32:20 +0530 MIME-Version: 1.0 Sent to CCL by: Kaushik Hatua [kaushikhatua_._yahoo.in] --_DC9ACE5A-4F52-4ACE-9D2E-729C4A5A3ADA_ Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="Windows-1252" Hi all I like to calculate excited PES by TDDFT / CCSDT along certain bond length.= Optimized ground state is triplet. Can anybody help me in this regard. Sent from Nokia Lumia = --_DC9ACE5A-4F52-4ACE-9D2E-729C4A5A3ADA_ Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset="Windows-1252"
Hi all
I like to calculate excited PES by TDDFT= / CCSDT along certain bond length. Optimized ground state is triplet. Can = anybody help me in this regard.

Sent from Nokia Lumia
= --_DC9ACE5A-4F52-4ACE-9D2E-729C4A5A3ADA_-- From owner-chemistry@ccl.net Mon Feb 9 12:52:00 2015 From: "Igors Mihailovs igors.mihailovs0{}gmail.com" To: CCL Subject: CCL: Homo-lumo gap significance Message-Id: <-51016-150209125050-7064-fqHKJEPYcNv5KIGfWuESsw__server.ccl.net> X-Original-From: Igors Mihailovs Content-Type: multipart/alternative; boundary=001a11347ba8ce8891050eab6855 Date: Mon, 9 Feb 2015 19:50:23 +0200 MIME-Version: 1.0 Sent to CCL by: Igors Mihailovs [igors.mihailovs0:_:gmail.com] --001a11347ba8ce8891050eab6855 Content-Type: text/plain; charset=UTF-8 Dear Mr. van Sittert, Which units are Your eigenvalues/gap values in? Possibly hartrees? You could possibly convert these gap values to joules and compare with k_B*T, as suggests, for instance, plain Arrhenius formula... With best wishes, Igors Mihailovs Institute of Solid State Physics University of Latvia 2015-02-09 14:16 GMT+02:00 Cornie Van Sittert Cornie.VanSittert^nwu.ac.za < owner-chemistry+*+ccl.net>: > Good afternoon, > > I was wondering if anybody could help me. I would like to ask you about > the HOMO-LUMO energy gap. > > I have three transition states to compare, TS1, TS2 and TS3. The HOMO-LUMO > was calculated for each on the whole system, so I have my HOMO on my Nu- > and my LUMO on my electrophile. For the HOMO-LUMO energy gap, I subtracted > the LUMO energy from the HOMO energy and got the absolute value (column > 2). Column 3 is the HOMO-LUMO energy gap within the transition state > structure. > > In the table below I compare the HOMO-LUMO energy gaps, and it seems that > at the transition state (TS, column 3), the N9 has the biggest energy gap, > followed by N3, and then N7. I read that all transition states reach the > most stable form, which is the TS with the largest HOMO-LUMO energy gap. > This follows the trend in the lab where we made the N9>N3>>N7. The > HOMO-LUMO energy gap of transition state minus the HOMO-LUMO energy gap of > reactants (column 4) shows the N9 will follow the one with the smallest gap, and this agrees with my > experimental work. > From what I read in literature, the HOMO-LUMO energy gap for the > transition state should be as big as possible so that the total HOMO-LUMO > energy gap is small: (column 4) > E(total) gap = E(reactants) gap - E(transition state) gap > > > pathway > > HOMO-LUMO gapR > > HOMO-LUMO gapTS > > HOMO-LUMO gap(TS-R) > > N3 > > 0.15009 > > 0.12572 > > 0.02439 > > N7 > > 0.14983 > > 0.12340 > > 0.02643 > > N9 > > 0.14973 > > 0.12767 > > 0.02206 > > > The question that no one seems to answer is if these values (shown above) > are significantly different from each other. > > Is the difference on the 3rd decimal place in the eigenvalues for the TS > significant? or would all the reactions occur? When can the difference be > taken as significant? > > Kind regards, > Cornie van Sittert > > > > > Vrywaringsklousule / Disclaimer: *http://www.nwu.ac.za/it/gov-man/disclaimer.html > * > --001a11347ba8ce8891050eab6855 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
Dear Mr. van Sittert,

Which units are Yo= ur eigenvalues/gap values in? Possibly hartrees? You could possibly convert= these gap values to joules and compare with k_B*T, as suggests, for instan= ce, plain Arrhenius formula...
With best wish= es,
Igors Miha= ilovs
Institute of Solid State Physics
Universi= ty of Latvia


2015-02-09 14:16 GMT+02:00 Cornie Van Sitter= t Cornie.VanSittert^nwu.ac.za <owne= r-chemistry+*+ccl.net>:
Good afternoon,
=C2=A0
I was wondering if anybody could help me.=C2=A0 I would like to ask yo= u about the HOMO-LUMO energy gap.
=C2=A0
I have three transition states to compare, TS1, TS2 and TS3. The HOMO-= LUMO was calculated for each on the whole system, so I have my HOMO on my N= u- and my LUMO on my electrophile. For the HOMO-LUMO energy gap,=C2=A0I sub= tracted the LUMO energy from the HOMO energy and got the absolute value (co= lumn 2).=C2=A0 Column 3=C2=A0is the HOMO-LUMO energy gap within the transit= ion state structure.
=C2=A0
In the table below=C2=A0I compare the HOMO-LUMO energy gaps, and it se= ems that at the transition state (TS, column 3), the N9 has the biggest ene= rgy gap, followed by N3, and then N7. I read that all transition states rea= ch the most stable form, which is the TS with the largest HOMO-LUMO energy = gap. This follows the trend in the lab where we made the N9>N3>>N7= . The HOMO-LUMO energy gap of transition state minus the HOMO-LUMO energy g= ap of reactants (column 4) shows the N9<N3<N7 trend. Articles say tha= t reactions will follow the one with the smallest gap, and this agrees with= =C2=A0my experimental work.
From what=C2=A0I read in literature, the HOMO-LUMO energy gap for the = transition state should be as big as possible so that the total=C2=A0HOMO-L= UMO energy gap is small: (column 4)
E(total) gap =3D E(reactants) gap - = E(transition state) gap
=C2=A0

pathway

HOMO-LUMO gapR

HOMO-LUMO gapTS<= /p>

HOMO-LUMO gap(TS-R)

N3

0.15009

0.12572

0.02439

N7

0.14983

0.12340

0.02643

N9

0.14973

0.12767

0.02206
=C2=A0
=C2=A0
The question that no one=C2=A0seems to answer is if these values (show= n above) are significantly different from each other.
=C2=A0
Is the difference on the 3rd decimal place in the eigenvalues for the = TS significant? or would all the reactions occur?=C2=A0 When can the differ= ence be taken as significant?
=C2=A0
Kind regards,
Cornie van Sittert
=C2=A0
=C2=A0
=C2=A0

Vrywaringsklousule / Disclaimer: http://www.nwu.ac.za/it/gov-man/disclaim= er.html


--001a11347ba8ce8891050eab6855-- From owner-chemistry@ccl.net Mon Feb 9 13:51:01 2015 From: "Tom Albright talbright1234=-=gmail.com" To: CCL Subject: CCL: Homo-lumo gap significance Message-Id: <-51017-150209134948-30400-1rivS2WCoojV8OKI6LgONA/./server.ccl.net> X-Original-From: Tom Albright Content-Type: multipart/alternative; boundary=Apple-Mail-9-671464680 Date: Mon, 9 Feb 2015 12:49:39 -0600 Mime-Version: 1.0 (Apple Message framework v1085) Sent to CCL by: Tom Albright [talbright1234]_[gmail.com] --Apple-Mail-9-671464680 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=us-ascii The units are in Hartrees. HOWEVER, to categorically say that the = HOMO-LUMO gap is a measure of thermodynamic or kinetic stability is not = true. Let me take a simple example: cyclobutadiene (in its ground state) = is very reactive and one can rightfully say that it is a consequence of = a small HOMO-LUMO gap. On the other hand tetrakis(t-butyl)cyclobutadiene = can be isolated, crystallized and stored indefinitely at room = temperature and its HOMO-LUMO gap is similar to that in the parent = molecule. You have three different transition states - should be done is = to carefully compare the bonding in each. See, for example, Albright, = Burdett and Whangbo, "Orbital Interactions in Chemistry, 2nd edition, J. = Wiley 2013). On Feb 9, 2015, at 11:50 AM, Igors Mihailovs igors.mihailovs0{}gmail.com = wrote: > Dear Mr. van Sittert, >=20 > Which units are Your eigenvalues/gap values in? Possibly hartrees? You = could possibly convert these gap values to joules and compare with = k_B*T, as suggests, for instance, plain Arrhenius formula... > With best wishes, > Igors Mihailovs > Institute of Solid State Physics > University of Latvia >=20 >=20 > 2015-02-09 14:16 GMT+02:00 Cornie Van Sittert = Cornie.VanSittert^nwu.ac.za : > Good afternoon, > =20 > I was wondering if anybody could help me. I would like to ask you = about the HOMO-LUMO energy gap. > =20 > I have three transition states to compare, TS1, TS2 and TS3. The = HOMO-LUMO was calculated for each on the whole system, so I have my HOMO = on my Nu- and my LUMO on my electrophile. For the HOMO-LUMO energy gap, = I subtracted the LUMO energy from the HOMO energy and got the absolute = value (column 2). Column 3 is the HOMO-LUMO energy gap within the = transition state structure. > =20 > In the table below I compare the HOMO-LUMO energy gaps, and it seems = that at the transition state (TS, column 3), the N9 has the biggest = energy gap, followed by N3, and then N7. I read that all transition = states reach the most stable form, which is the TS with the largest = HOMO-LUMO energy gap. This follows the trend in the lab where we made = the N9>N3>>N7. The HOMO-LUMO energy gap of transition state minus the = HOMO-LUMO energy gap of reactants (column 4) shows the N9 =46rom what I read in literature, the HOMO-LUMO energy gap for the = transition state should be as big as possible so that the total = HOMO-LUMO energy gap is small: (column 4) > E(total) gap =3D E(reactants) gap - E(transition state) gap > =20 > pathway >=20 > HOMO-LUMO gapR >=20 > HOMO-LUMO gapTS >=20 > HOMO-LUMO gap(TS-R) >=20 > N3 >=20 > 0.15009 >=20 > 0.12572 >=20 > 0.02439 >=20 > N7 >=20 > 0.14983 >=20 > 0.12340 >=20 > 0.02643 >=20 > N9 >=20 > 0.14973 >=20 > 0.12767 >=20 > 0.02206 >=20 > =20 > =20 > The question that no one seems to answer is if these values (shown = above) are significantly different from each other. > =20 > Is the difference on the 3rd decimal place in the eigenvalues for the = TS significant? or would all the reactions occur? When can the = difference be taken as significant? > =20 > Kind regards, > Cornie van Sittert > =20 > =20 > =20 > Vrywaringsklousule / Disclaimer: = http://www.nwu.ac.za/it/gov-man/disclaimer.html >=20 >=20 With Best Regards Tom Albright --Apple-Mail-9-671464680 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=us-ascii The = units are in Hartrees. HOWEVER, to categorically say that the HOMO-LUMO = gap is a measure of thermodynamic or kinetic stability is not true. Let = me take a simple example: cyclobutadiene (in its ground state) is very = reactive and one can rightfully say that it is a consequence of a small = HOMO-LUMO gap. On the other hand tetrakis(t-butyl)cyclobutadiene can be = isolated, crystallized and stored indefinitely at room temperature and = its HOMO-LUMO gap is similar to that in the parent molecule. You have = three different transition states - should be done is to carefully = compare the bonding in each. See, for example, Albright, Burdett and = Whangbo, "Orbital Interactions in Chemistry, 2nd edition, J. Wiley = 2013).
On Feb 9, 2015, at 11:50 AM, Igors Mihailovs = igors.mihailovs0{}gmail.com wrote:

Dear Mr. van Sittert,

Which units are Your = eigenvalues/gap values in? Possibly hartrees? You could possibly convert = these gap values to joules and compare with k_B*T, as suggests, for = instance, plain Arrhenius formula...
With = best wishes,
Igors Mihailovs
Institute of Solid State = Physics
University of = Latvia


2015-02-09 14:16 GMT+02:00 Cornie Van = Sittert Cornie.VanSittert^nwu.ac.za = <owner-chemistry]*[ccl.net>:
Good afternoon,
 
I was wondering if anybody could help me.  I would like to ask = you about the HOMO-LUMO energy gap.
 
I have three transition states to compare, TS1, TS2 and TS3. The = HOMO-LUMO was calculated for each on the whole system, so I have my HOMO = on my Nu- and my LUMO on my electrophile. For the HOMO-LUMO energy = gap, I subtracted the LUMO energy from the HOMO energy and got the = absolute value (column 2).  Column 3 is the HOMO-LUMO energy = gap within the transition state structure.
 
In the table below I compare the HOMO-LUMO energy gaps, and it = seems that at the transition state (TS, column 3), the N9 has the = biggest energy gap, followed by N3, and then N7. I read that all = transition states reach the most stable form, which is the TS with the = largest HOMO-LUMO energy gap. This follows the trend in the lab where we = made the N9>N3>>N7. The HOMO-LUMO energy gap of transition = state minus the HOMO-LUMO energy gap of reactants (column 4) shows the = N9<N3<N7 trend. Articles say that reactions will follow the one = with the smallest gap, and this agrees with my experimental = work.
=46rom what I read in literature, the HOMO-LUMO energy gap for = the transition state should be as big as possible so that the = total HOMO-LUMO energy gap is small: (column 4)
E(total) gap =3D = E(reactants) gap - E(transition state) gap
 

pathway

HOMO-LUMO = gapR

HOMO-LUMO = gapTS

HOMO-LUMO = gap(TS-R)

N3

0.15009

0.12572

0.02439

N7

0.14983

0.12340

0.02643

N9

0.14973

0.12767

0.02206
 
 
The question that no one seems to answer is if these values = (shown above) are significantly different from each other.
 
Is the difference on the 3rd decimal place in the eigenvalues for = the TS significant? or would all the reactions occur?  When can the = difference be taken as significant?
 
Kind regards,
Cornie van Sittert
 
 
 

Vrywaringsklousule / Disclaimer: http://www.nwu.ac.za/it/gov-man/disclaimer.html =



With = Best Regards
Tom Albright



= --Apple-Mail-9-671464680-- From owner-chemistry@ccl.net Mon Feb 9 19:27:01 2015 From: "Tymofii Nikolaienko tim_mail[A]ukr.net" To: CCL Subject: CCL: Homo-lumo gap significance Message-Id: <-51018-150209165845-28271-rB9+XqN85PWl0pFcJ/iElQ ~ server.ccl.net> X-Original-From: Tymofii Nikolaienko Content-Type: multipart/alternative; boundary="------------020908010600040905000503" Date: Mon, 09 Feb 2015 23:58:18 +0200 MIME-Version: 1.0 Sent to CCL by: Tymofii Nikolaienko [tim_mail#%#ukr.net] This is a multi-part message in MIME format. --------------020908010600040905000503 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 8bit Can I try to exaggerate this discussion a bit? It is well known that a concept of 'orbital' has historically originated > from Hartree-Fock approximation, and that in fact molecular orbital is no more than a mathematical tool for building an approximate wavefunction. Some authors have advocated the viewpoint that in reality orbitals simply do not exist! For example: * Martín Labarca, Olimpia Lombardi "Why orbitals do not exist?" [Foundations of Chemistry, 2010, Volume 12, Issue 2, pp 149-157, DOI 10.1007/s10698-010-9086-5 ] * J. F. Ogilvie, "The nature of the chemical bond---1990: There are no such things as orbitals!" [J. Chem. Educ., 1990, 67 (4), p 280, DOI: 10.1021/ed067p280 ] With that in mind, *do HOMO and LUMO have any importance,* - except for adding another 'beautiful picture' to some paper with HOMO and/or LUMO isosurfaces and withOUT any discussion of what /_consequences_/ does their shapes imply , - for understanding physical properties and chemical reactivity ? Similar doubts about an importance apply to the *HOMO-LUMO gap*, but here I'd like to recall that, ofr example, replacing ONE (filled) orbital (HOMO) in a Slatter determinant with ONE unfilled orbital (e.g., LUMO) does NOT produce even approximate wavefunction of an excited state, since the crudest approximation to that wavefunction comes with CIS method, where a linear combination of (many!) singly substituted determinants is used in order to get (a not so good) wavefunction of an excited state. So, there seems to be no /physical /ground for associating the HOMO-LUMO gap with characteristic wavelengths in a UV-Vis-like spectra. So, *why to care about HOMO / LUMO* unless we are not dealing with solids ?... Best regards, Tymofii, a physicist ;) 09.02.2015 20:49, Tom Albright talbright1234=-=gmail.com wrote: > The units are in Hartrees. HOWEVER, to categorically say that the > HOMO-LUMO gap is a measure of thermodynamic or kinetic stability is > not true. Let me take a simple example: cyclobutadiene (in its ground > state) is very reactive and one can rightfully say that it is a > consequence of a small HOMO-LUMO gap. On the other hand > tetrakis(t-butyl)cyclobutadiene can be isolated, crystallized and > stored indefinitely at room temperature and its HOMO-LUMO gap is > similar to that in the parent molecule. You have three different > transition states - should be done is to carefully compare the bonding > in each. See, for example, Albright, Burdett and Whangbo, "Orbital > Interactions in Chemistry, 2nd edition, J. Wiley 2013). > On Feb 9, 2015, at 11:50 AM, Igors Mihailovs > igors.mihailovs0{}gmail.com wrote: > >> Dear Mr. van Sittert, >> >> Which units are Your eigenvalues/gap values in? Possibly hartrees? >> You could possibly convert these gap values to joules and compare >> with k_B*T, as suggests, for instance, plain Arrhenius formula... >> With best wishes, >> Igors Mihailovs >> Institute of Solid State Physics >> University of Latvia >> >> >> 2015-02-09 14:16 GMT+02:00 Cornie Van Sittert >> Cornie.VanSittert^nwu.ac.za >> >: >> >> Good afternoon, >> I was wondering if anybody could help me. I would like to ask >> you about the HOMO-LUMO energy gap. >> I have three transition states to compare, TS1, TS2 and TS3. The >> HOMO-LUMO was calculated for each on the whole system, so I have >> my HOMO on my Nu- and my LUMO on my electrophile. For the >> HOMO-LUMO energy gap, I subtracted the LUMO energy from the HOMO >> energy and got the absolute value (column 2). Column 3 is the >> HOMO-LUMO energy gap within the transition state structure. >> In the table below I compare the HOMO-LUMO energy gaps, and it >> seems that at the transition state (TS, column 3), the N9 has the >> biggest energy gap, followed by N3, and then N7. I read that all >> transition states reach the most stable form, which is the TS >> with the largest HOMO-LUMO energy gap. This follows the trend in >> the lab where we made the N9>N3>>N7. The HOMO-LUMO energy gap of >> transition state minus the HOMO-LUMO energy gap of reactants >> (column 4) shows the N9> will follow the one with the smallest gap, and this agrees >> with my experimental work. >> From what I read in literature, the HOMO-LUMO energy gap for the >> transition state should be as big as possible so that the >> total HOMO-LUMO energy gap is small: (column 4) >> E(total) gap = E(reactants) gap - E(transition state) gap >> >> pathway >> >> >> >> HOMO-LUMO gap_R >> >> >> >> HOMO-LUMO gap_TS >> >> >> >> HOMO-LUMO gap_(TS-R) >> >> >> >> N3 >> >> >> >> 0.15009 >> >> >> >> 0.12572 >> >> >> >> 0.02439 >> >> >> >> N7 >> >> >> >> 0.14983 >> >> >> >> 0.12340 >> >> >> >> 0.02643 >> >> >> >> N9 >> >> >> >> 0.14973 >> >> >> >> 0.12767 >> >> >> >> 0.02206 >> >> >> >> The question that no one seems to answer is if these values >> (shown above) are significantly different from each other. >> Is the difference on the 3rd decimal place in the eigenvalues for >> the TS significant? or would all the reactions occur? When can >> the difference be taken as significant? >> Kind regards, >> Cornie van Sittert >> >> Vrywaringsklousule / Disclaimer: >> _http://www.nwu.ac.za/it/gov-man/disclaimer.html >> _ >> >> > > With Best Regards > Tom Albright > > > --------------020908010600040905000503 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Can I try to exaggerate this discussion a bit?

It is well known that a concept of 'orbital' has historically originated from Hartree-Fock approximation, and that in fact molecular orbital
is no more than a mathematical tool for building an approximate wavefunction.
Some authors have advocated the viewpoint that in reality orbitals simply do not exist! For example:
* Martín Labarca, Olimpia Lombardi "Why orbitals do not exist?" [Foundations of Chemistry, 2010, Volume 12, Issue 2, pp 149-157,  DOI 10.1007/s10698-010-9086-5 ]
* J. F. Ogilvie, "The nature of the chemical bond—1990: There are no such things as orbitals!" [J. Chem. Educ., 1990, 67 (4), p 280, DOI: 10.1021/ed067p280 ]

With that in mind, do HOMO and LUMO have any importance, - except for adding another 'beautiful picture' to some paper with HOMO and/or LUMO
isosurfaces and withOUT any discussion of what consequences does their shapes imply , -  for understanding physical properties and chemical reactivity ?

Similar doubts about an importance apply to the HOMO-LUMO gap, but here I'd like to recall that, ofr example, replacing ONE (filled) orbital (HOMO) in
a Slatter determinant with ONE unfilled orbital (e.g., LUMO) does NOT produce even approximate wavefunction of an excited state, since the
crudest approximation to that wavefunction comes with CIS method, where a linear combination of (many!) singly substituted determinants is used
in order to get (a not so good) wavefunction of an excited state. So, there seems to be no physical ground for associating the HOMO-LUMO gap with
characteristic wavelengths in a UV-Vis-like spectra.

So, why to care about HOMO / LUMO unless we are not dealing with solids ?...

Best regards,
Tymofii,
a physicist ;)






09.02.2015 20:49, Tom Albright talbright1234=-=gmail.com wrote:
The units are in Hartrees. HOWEVER, to categorically say that the HOMO-LUMO gap is a measure of thermodynamic or kinetic stability is not true. Let me take a simple example: cyclobutadiene (in its ground state) is very reactive and one can rightfully say that it is a consequence of a small HOMO-LUMO gap. On the other hand tetrakis(t-butyl)cyclobutadiene can be isolated, crystallized and stored indefinitely at room temperature and its HOMO-LUMO gap is similar to that in the parent molecule. You have three different transition states - should be done is to carefully compare the bonding in each. See, for example, Albright, Burdett and Whangbo, "Orbital Interactions in Chemistry, 2nd edition, J. Wiley 2013).
On Feb 9, 2015, at 11:50 AM, Igors Mihailovs igors.mihailovs0{}gmail.com wrote:

Dear Mr. van Sittert,

Which units are Your eigenvalues/gap values in? Possibly hartrees? You could possibly convert these gap values to joules and compare with k_B*T, as suggests, for instance, plain Arrhenius formula...
With best wishes,
Igors Mihailovs
Institute of Solid State Physics
University of Latvia


2015-02-09 14:16 GMT+02:00 Cornie Van Sittert Cornie.VanSittert^nwu.ac.za <owner-chemistry]*[ccl.net>:
Good afternoon,
 
I was wondering if anybody could help me.  I would like to ask you about the HOMO-LUMO energy gap.
 
I have three transition states to compare, TS1, TS2 and TS3. The HOMO-LUMO was calculated for each on the whole system, so I have my HOMO on my Nu- and my LUMO on my electrophile. For the HOMO-LUMO energy gap, I subtracted the LUMO energy from the HOMO energy and got the absolute value (column 2).  Column 3 is the HOMO-LUMO energy gap within the transition state structure.
 
In the table below I compare the HOMO-LUMO energy gaps, and it seems that at the transition state (TS, column 3), the N9 has the biggest energy gap, followed by N3, and then N7. I read that all transition states reach the most stable form, which is the TS with the largest HOMO-LUMO energy gap. This follows the trend in the lab where we made the N9>N3>>N7. The HOMO-LUMO energy gap of transition state minus the HOMO-LUMO energy gap of reactants (column 4) shows the N9<N3<N7 trend. Articles say that reactions will follow the one with the smallest gap, and this agrees with my experimental work.
From what I read in literature, the HOMO-LUMO energy gap for the transition state should be as big as possible so that the total HOMO-LUMO energy gap is small: (column 4)
E(total) gap = E(reactants) gap - E(transition state) gap
 

pathway

HOMO-LUMO gapR

HOMO-LUMO gapTS

HOMO-LUMO gap(TS-R)

N3

0.15009

0.12572

0.02439

N7

0.14983

0.12340

0.02643

N9

0.14973

0.12767

0.02206
 
 
The question that no one seems to answer is if these values (shown above) are significantly different from each other.
 
Is the difference on the 3rd decimal place in the eigenvalues for the TS significant? or would all the reactions occur?  When can the difference be taken as significant?
 
Kind regards,
Cornie van Sittert
 
 
 

Vrywaringsklousule / Disclaimer: http://www.nwu.ac.za/it/gov-man/disclaimer.html



With Best Regards
Tom Albright



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