From owner-chemistry@ccl.net Fri Feb 25 08:18:00 2011 From: "ashutosh gupta ashu1809() gmail.com" To: CCL Subject: CCL: UV-VIS spectra values obtained computationally are different (TDDFT & CIS) for different concentrations Message-Id: <-44019-110225081720-3525-ed08AOL+UMvUd2a7tvtIdQ * server.ccl.net> X-Original-From: ashutosh gupta Content-Type: multipart/alternative; boundary=20cf3054a2e9a69a93049d1b2589 Date: Fri, 25 Feb 2011 18:47:08 +0530 MIME-Version: 1.0 Sent to CCL by: ashutosh gupta [ashu1809]-[gmail.com] --20cf3054a2e9a69a93049d1b2589 Content-Type: text/plain; charset=ISO-8859-1 Hi Andrei, Thank you very much for such a nice and lucid explanation. indeed the extinction coefficients of bands at 400 nm has low intensity (100 units) whereas at 280nm, it has value of some 30,000 units. What i am worried about is that TDDFT is considered to better than CIS. my TDDFT results are showing values around 400 nm but not at 280 nm, whereas CIS is giving the same as 280 nm. could it be that i am missing out higher level of excitation in case of TDDFT (however S1,S2,S3 values are 399,394,356 nm(s)). why CIS & TDDFT results are varying so much. Is it something to do with theory part? thanks again for your kind reply and your help. thanks indeed. best wishes& kind regards. ashu. 2011/2/25 asp asp99++yandex.ru > > Sent to CCL by: asp [asp99*|*yandex.ru] > Hi, Ashu, > it looks like that some king of interaction is taking place in your system. > It could be assosiation of compound molecules or formation of complexes with > the solvent leading to concentartion-dependent absorption band position. If > there is an equlibrium in the solution, then it is likely that at lower > concentation you will have more unreacted (isolated) molecules of the > compound. The results of calculation also give you values for an isolated > molecule, so it is more reasonable to compare values obtatined at lower > concentrations. > > However, there is a possibility that you are regestering to completly > separete bands corresponding to different electron transitions in the > molecule. What are the extinction coefficients for your experimental bands? > It is likely that the band at 400 nm has low intensity and you simply could > not detect it at low concentration. > How this helps to figure things out, > Andrei > > 25.02.2011, 05:05, "ashutosh gupta ashu1809 : gmail.com" ccl.net>: > > Hi,Greetings. > > > > I have a UV-Vis spectra of a compound at different concentrations. (10-2M > to 10-5M) > > > > At higher concentration( ~10-2 M), the experimental peaks match well with > TDDFT/6-31+G* values (400nm), whereas at lower concentration (~10-5M), the > experimental peaks match with CIS/6-31+G* values (280 nm). > > > > The IR values obtained experimentally and computationally matches well > (for DFT values). > > > > I would be thankful if someone could help me in knowing as to which value > (TDDFT or CIS) to consider for assigning UV-VIS spectra computationally. > > > > thanking you. > > ashu> > > --20cf3054a2e9a69a93049d1b2589 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Hi Andrei,
Thank you very much for such a nice and lucid explanation.

indeed the=A0extinction coefficients of bands at 40= 0 nm has low intensity (100 units) =A0whereas at 280nm, it has value of som= e 30,000 units.

What i am worried about is that TDDFT is considered to = better than CIS.
my TDDFT results are showing values around 400 n= m but not at 280 nm, whereas CIS is giving the same as 280 nm.

could it be that i am missing out higher level of excitation= in case of TDDFT (however S1,S2,S3 values are 399,394,356 nm(s)).

why CIS & TDDFT results are varying so much. Is it som= ething to do with theory part?

thanks again for your kind reply and your help.

thanks indeed.
best wishes& kind regards.
ashu.

2011/2/25 asp asp99+= +yandex.ru <owner-chemistry|ccl.net><= br>

Sent to CCL by: asp [asp99*|*yandex.ru]
Hi, Ashu,
it looks like that some king of interaction is taking place in your system.= It could be assosiation of compound molecules or formation of complexes wi= th the solvent leading to concentartion-dependent absorption band position.= If there is an equlibrium in the solution, then it is likely that at lower= concentation you will have more unreacted (isolated) molecules of the comp= ound. The results of calculation also give you values for an isolated molec= ule, so it is more reasonable to compare values obtatined at lower concentr= ations.

However, there is a possibility that you are regestering to completly separ= ete bands corresponding to different electron transitions in the molecule. = What are the extinction coefficients for your experimental bands? It is lik= ely that the band at 400 nm has low intensity and you simply could not dete= ct it at low concentration.
How this helps to figure things out,
Andrei

25.02.2011, 05:05, "ashutosh gupta ashu1809 : gmail.com" <owner-chemistry^ccl.net>:
> Hi,Greetings.
>
> I have a UV-Vis spectra of a compound at different concentrations. (10= -2M to 10-5M)
>
> At higher concentration( ~10-2 M), the experimental peaks match well w= ith TDDFT/6-31+G* values (400nm), whereas at lower concentration (~10-5M), = the experimental peaks match with CIS/6-31+G*=A0values (280 nm).
>
> The IR values obtained experimentally and computationally matches well= (for DFT values).
>
> I would be thankful if someone could help me in knowing as to which va= lue (TDDFT or CIS) to consider for assigning UV-VIS spectra computationally= .
>
> thanking you.
> ashu



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