NBO charge

NBO charge

Postby Sabyasachi » Sun Aug 06, 2017 2:07 am

Dear all,

I'm new in this community and it is my first question.

I am dealing with tetrahydrofuran (THF) and methylated THF (at 3 position) and using G09 I optimized both followed by NBO calculations (b3lyp/6-31g*).
I've attached two figures herewith. All atoms with their NBO charges are shown.
THF with NBO charge
thf.png (47.85 KiB) Viewed 689 times
Methylated THF with NBO charge
me-thf.png (69.29 KiB) Viewed 689 times

If you look at the C-3, i.e. to which methyl is attached in the 2nd fig, before and after methylation a reduction in negative charge is seen,
as if, methyl group is quenching the negative charge. But as we all know, methyl is electron donating group.

I tried with benzene-toluene, thiophene-methylated thiophene systems, but everywhere this feature is found to occur. Even other charges like
Mulliken, Baldwin charges, show the similar trend.

So, I guess this effect is not inductive (-I/+I) of alkyl group. But I am not able to explain what it actually is.

Any suggestion/ comment will be greatly appreciated

Thank you.

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Re: NBO charge

Postby ericg » Mon Aug 07, 2017 8:50 am

These charges are reasonable if you consider the differing electronegativities of the H and C atoms and their influence on sigma bond polarity.

If you examine the C-C and C-H bonds in the NBO output, you'll see that the former is essentially apolar (50%/50% contributions from two hybrids) whereas the latter is polar (~60%/40%, polarized toward C). The difference in these polarities accounts for the differing charges (by about 0.2e) at C. H is in fact a stronger sigma electron donor than methyl.

My sense is that the electron-donating character of the methyl group is best understood in terms of pi-type delocalizing interactions, not sigma-type induction. I ran a calculation on toluene and counted the number of electrons in the pi and pi* NBOs of the aromatic ring. I got 6.0043e, revealing the net donation of 0.0043e from the out-of-plane CH bonds of the methyl group into the ring pi system. Although the transfer of this amount of electron density may seem small, it will stabilize toluene by approximately 0.0043 a.u., or 2.7 kcal/mol, which is sufficiently large to have important chemical consequences.

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