Dear Thao,

I think that you have the charge wrong and, consequently, multiplicity. You have specified that the TS has charge 0 and multiplicity doublet. I think that the sulphur should be negative (taking into account your molecule). Please, check the structure (by Lewis) and be sure that charge is correct.

On the other hand, if the charge do not solve your problem you can try:

1) Add integral(grid=ultrafine) or increase basis set.

2) Try QTS3 (taking into account that you know more or less the TS structure). If not, QTS2.

3)Re-scan the surface to get a better TS guest. You can do it automatically (even doing 3D surface " very expensive but very illustrative").

I hope it helps you,

Best regards,

Joaquim

Hi Joaquim,

Thank you very much for your advice. I attached here the original structure of the reactants.  Because the Nitrogen has 4 bonds instead of 3, the multiplicity should be 2. Now I am confused about the charge, is it positive or neutral? Also with the reactants, I put charge 0 , multiplicity 2 and Gaussian did not complain about it.  Normally if charge or multiplicity are wrong, Gaussian will terminate with error. So I wonder what is your opinion about this?

Thank you once again for your help.

Dear Thao,

You are right ( I did not see that N+). Even so, it is rare that gaussian indicate it as doublet. Please, could you attach the reactant optimized? This night I will try to check the structures.

dear Joaquim,

Here is the reactants structures. Thank u very much

Dear Thao,

In the reactant structure you have the same problem. The input should be a singlet with a charge of +1, due that the partial charge on sulphur should be 0.

On the other hand, I wrote the reaction which I think you want to investigate with the lone pairs. As you could see, the sulphur has a partial charge of 0 during all reaction. For this reason, your inputs must have a global charge of +1 and being singlet.

For normal reaction and simple molecules (group P), in general, you will have singlets. If you study UV excitations or metals complex or very specific reactions (where you will know the multiplicity because it will be a main characteristics of your reaction), then you will have other multiplicities.

In addition, to see all of this with numbers, I recommend you to use the NBO analysis and then, check the charge distribution of each atom. Also, it is important to check it because in DFT you can not be sure that always your functional distributes correctly the charges.

A thing that I like to do for the first time I study a molecule (or in rare or questionable situations) is to compare the DFT with MP2 to be sure that the functional works correctly with my compound.

I hope it helps you,

Joaquim

[Edited] In the link, you can draw molecules (by Lewis structure) and you could check the charges. There are many other that could help you with this.

http://cssp.chemspider.com/StructureSearch.aspx

Thank you very much Joaquim. I will correct the structure and hope it will be right. I am very appreciate your help . 

Sincerely,

If you find some problem or question, I will try to help you.

Best regards,

Joaquim

Thao,

Is the reaction you're studying supposed to be a radical or a radical-cation reaction? Because only then should the reactants and TS be doublets. I'd go by that criterion  ("ordinary" molecules and ions are of course singlets) rather than counting bonds to particular atoms, which seems to have led you astray.  Note that in "real life", radical reactions and radical-cation reactions usually need an initiation step.  Does your reaction need an initiation step?

Given the drawings, I think you meant to have charge +1 multiplicity 1.  If you were using GaussView and neglected to set the charge to +1, GaussView will automatically assume a zero charge and multiplicity 2, as multiplicity 2 is GaussView's default for odd-electron systems.  Essentially, GaussView is assuming that the reactant and TS are neutral radicals.  Gaussian will only complain if there is a mismatch between the number of electrons in the system and whether that number is odd or even according to charge and multiplicity.  If you had manually changed the gjf file  to charge 0 multiplicity 1, with an odd number of electrons in the system, then you would get an error in Gaussian.  

So mostly I concur with Joaquim, but for different reasons.  

Thank you very much David

Dear colleagues,

I have managed to obtain the imaginary frequency according to your advice. However, I have one small question: How big is the imaginary frequency enough? It is because my imaginary frequency is only about -78 cm-1. 

Thank you

Dear Thao,

Of course it depend on the system and the weight of the atoms involved. Usually it is said that should be above 100 cm^1, even so, I have never seen this "rule" writed in a book or paper (I suppose that it is a common value for the common organic reactions).

In order to be sure if it is a real TS, you should check:

-1st) Check the imaginary vibration representation. This vibration at the extreme positions should be similar from reactants to products. Follow your chemical intuition.

-2nd) You can perform a IRC calculation in order to ensure that it is a TS, by finding the near intermediates or reactants and products. Sometimes this calculation can give you a problems, then, you can try to save the structures of your vibration slightly displaced from the center and running an optimization with it. It should give similar results.

-3rd) Try to find the TS with the same guess (or from the same TS) with other method or basis set. For example, if you have been found the TS with a B3LYP/aug-cc-pvDz, then you can try to do:

   a) B3LYP/aug-cc-pVnZ  where n=T or Q.

   b) Change the method to MP2/aug-cc-pVnZ where n=T or Q.

I hope it helps you,

Joaquim

Dear Thao,

whenever you have difficulty in prediction of initial guess for TS, try QST2 method, where you have to provide reactant and product structures. QST2 will give you TS according to ur minima....try QST methods....it will help you....