I'm trying to simulate the 13C NMR spectrum of cyclopentane-1,3-dione (PubChem CID: 77466; CAS 3859-41-4; ChemSpider ID: 69875; SDBS No: 15258; SMILES:
C1CC(=O)CC1=O). There are three equivalence classes of carbons: The two directly attached to the oxygens (≈197 ppm, call this class 1), the one carbon between those two (≈105 ppm, call this class 2) and the "other two" (≈31.3 ppm, class 3).
I've been using Gaussian 16 via the guidance provided by Dean Tantillo's group at UC Davis, the chemical shift repository (CHESHIRE) in particular first performing molecular structure optimization at some level (e.g. B3LYP/6-31+G(d,p)) and then calculating shieldings (at mPW1PW91/6-311+G(2d,p) with solvent corrections,
scrf=(solvent=chloroform, smd)). But I keep getting shielding values that are radically off.
In particular, regardless of the settings I arrive at isotropic shielding values for classes 1, 2, 3, of ≈-34, ≈136, ≈144. Now of course observed chemical shifts and shieldings are often related by a linear scaling, per the Tantillo group's work (and others), which means that it's very odd that classes 2 & 3 here (the non-Oxygen-attached carbons) have nearly identical shielding values.
I am new to chemistry and DFT methods, so I worry I might be something wrong, but it might also be the case that this is just a "hard" molecule for DFT methods to get right. I wanted to know:
Are there any higher levels of theory that I should try / better methods for shielding calculation that would be worth assessing?
Is there any way of analyzing the output from Gaussian16 to get a hint that things might be going awry?
Is there something that just makes this a "hard" molecule to calculate? I get the impression that the contributions in liquid state from various rotamers, etc. is relatively minor as it seems quite rigid (although I have not quantified this).