Hi, how are you taking into account the temperature? It is a calculation in some solvent and you are changing the dielectric constant? Cheers, Reynier

Hi, I agree with Reynler. The most obvious way to include temperature effects is to do a molecular dynamics simulation at different temperatures and try to calculate NMR properties based on those trajectories (for example in a QM/MM fashion). But of course, this is much easier to write down than to carry out... Anyhow, this might have a larger influence than the correction due to ro-vibrational corrections.

The latter can also be calculated but this is behind the standard temperature correction you get from a GAUSSIAN calculation. I am not saying it is not possible with GAUSSIAN but you have to look out how to do. I have no idea if it is possible with GAUSSIAN, but there has to be software for it. Search for NMR and ro-vibrational corrections in the literature. But be aware that these kinds of calculations will be somewhat more expensive than a standard NMR calculation...

Hi, how you gave different values of the temperature in your calculations? You can send the used keywords?

i suggest that you must make sure of the samples and what do you want to calculate? what do you want to know?. C13-NMR or H-NMR?

1H chemical shifts in NMR: Part 201

Anisotropic and steric effects in halogen substituent chemical shifts (SCS),

a modelling and ab-initio investigation

Raymond J. Abraham1*, Mehdi Mobli1 and Richard J.Smith2

So Their is Lot of information . However I think it will be very nice just to follow this Links .

http://www.gaussian.com/g_tech/g_ur/k_nmr.htm

In different solvents this methods gives different values, but at different temperature and same solvents the values are comes same.

KEYWORDS are

nmr=(giao,spinspin) b3lyp/6-31g(d,p) scrf=(iefpcm,solvent=chloroform)

And temperature = 220 or 298.5 and 300k

Do you know the change of chemical shift experimentally? dielectric constant of chloroform do not change a lot with T, at 0 degress: 5.5 ----------20 degrees: 4.8---------- 100degress: 3.7 maybe your experimental change in chemical shifts with T can not be explained only by a change in dielectric constant(?) How much is the change in dielectric constant in those "different solvents"...

Dear Reynier Suardiaz

How can we change dielctric constant in calculation methods???? BY changing solvent only or by any other keywords????

Thank you miss Olga Lavinda

I am looking for NMR values.

sorry, I didn't see your last post. There is not a big change in T and not a big change in dielectric constant. Then, what do you expect? Maybe you have to take into consideration other stuff in addition to T in a PCM model. Which are the experimental results? Maybe, if you have experimental changes in your chemical shifts at different temperatures, is not due to a change that you can take into account within a PCM calculation. Which are the different solvents you mentioned? Do you have experimental results?

are you including the keyword "read" and defining a temperature?

this have been taken from G03:

HF/6-31++G(d,p) SCF=Tight SCRF=(PCM,Read,Solvent=Cyclohexane) Test

PCM SP calculation on hydrogen fluoride

0,1

H

F 1 R

R=0.9161

TABS=300.0

ALPHA=1.21

TSNUM=70

anyway I am not sure, if you change the temp here, you will obtain a meaningful result. You also can give the vale of dielectric constant (see gaussian manual) and then giving the value of dielectric constant of chloroform at different T can be a way to have a part of the effect of a change in T but only a part. Anyway, as I said before I don think dielectric constant of chloroform changes a lot from 298.5 to 300k. You will obtain basically the same result. If you are comparing different solvents, e.g. water. dielectric constant is 80 :-o there is a change! Why do you suspect there is a change in chemical shifts, experiment?

Dear Reynier Suardiaz,

thank you so much for your much attention,

I have experimental as well as calculated results, I'll attach calculated data files after some time.

Thank you so much Mr. Aleksandr Sahakyan. I think vibrational correction might be useful. I'll try it.

I would recommend starting with a conformational analysis on your molecule, either in a systematic MM approach or following an educated guess and scanning the potential energy surface along the dihedral angle between the substituent and the pyrrole ring. Then, geometry optimizations initiated from energy minima should and frequency calculations should yield multiple conformations. Keep al the conformers within about 2 kcal/mol of free energy difference wrt. the most stable one and perform GIAO NMR computations on each conformer. Plot the shielding tensors (or the computed chemical shifts e.g. wrt TMS) against experiment. Check for deviations from a linear dependence. See if other conformers have similar (reversed) deviations from linearity. Perform a Boltzmann average on chemical shifts (based on free energies computed in the previous frequency runs) and check the linearity, again. Depending on the energetics of conformational isomerism and the vicinity of the incriminated protons in each conformation, you may evidence a conformational equilibrium.