Multiwfn forum

alexandrepinto

Member

Registered: 2019-02-28

Posts: 1

Acid Dissociation Constant

Hello, my name is Alex, I'm trying to study a deprotonation process using DFT, for that I'm optimizing protonated and deprotonated structures using M062X and gen keyword (3-21g/6-31+(d)) (https://pubs.acs.org/doi/abs/10.1021/jp3089947 and I'm using M062X instead of B3LYP because it gave better energetic results in my case) in the gas phase and with the SMD solvent model, this way I should be able to calculate pKa values. However, the system I'm using has 3 identical solvent exposed groups capable of deprotonation and altough these being covalently attached to a carbon nanotube they have great conformational flexibility, so I assume the deprotonation process of all groups is somewhat complex. Can I have your advice on this please? Also, does multiwfn software offer a way to evaluate acid dissociation? Is there any problem in using M062X functional, should I use D3 corrections? Thank you for your time!

sobereva

Tian Lu (Multiwfn developer)

From: Beijing

Registered: 2017-09-11

Posts: 1,768

Website

Re: Acid Dissociation Constant

Using M06-2X to evaluate pKa is a good choice. D3 correction should not detectably affect the pKa result, since the influence of D3 for M06-2X calculation is marginal, and present problem has little correlation with weak interaction.

Multiwfn doesn't directly provide a way of predicting pKa, but many quantities that can be calculated by Multiwfn have direct relationship with pKa. For example, atomic charge of the hydrogen, electrostatic potential at the nuclear position of the hydrogen (without contribution from this proton) and so on. My collaborators have published an article Journal of Computational Chemistry 2018, 39, 117–129 to discuss relationship between pKa and a batch of molecular descriptors (many of them can be easily calculated by Multiwfn).

In fact I don't exactly understand what is the central problem in your question. If the functionalized carbon nanotube have multiple sites that can occur deprotonation, you can calculate respective pKa for each site in usual way.