I found J coupling data for hydronium using DFT and GIAO but I need help making sense of the data. Can someone with help? enter image description here

  • $\begingroup$ Without knowing the atom numbers, it's gonna be pretty hard to know what's going on. My (educated) guess is that atom #4 is the oxygen, so it's coupled equally to the 3 other hydrogens, which are each equally coupled to each other. $\endgroup$ – Geoff Hutchison May 24 '16 at 3:55
  • $\begingroup$ The oxygen is atom #4. $\endgroup$ – pocketed_the_rocket May 24 '16 at 4:37
  • $\begingroup$ Can you tell me what the negative values mean? $\endgroup$ – pocketed_the_rocket May 24 '16 at 4:40
  • $\begingroup$ All couplings may be positive or negative, and the relative sign can provide useful structural insights. It arises from the relative energies of the mixed spin states, most commonly influenced by Fermi contact mechanism. 1JOH are typcially negative; 1JCH are typically positive for instance. The sign of coupling does not change the appearance of a first order spectrum. If you need further clarification, you should consider asking another question. $\endgroup$ – long May 24 '16 at 22:54
  • $\begingroup$ We are using the J coupling data to justify the trends we see for the spin echo experiment. We used varying pHs (all acidic) and found that T1 (spin-lattice) relaxation was lowered. I'm still an undergrad but I do know that adding a proton is going to affect the magnetic moment of the molecule, I just need to justify it. All the couplings for water were negative whereas the only the hydronium oxygen couplings were negative. Do you think you can give me any insight as to why this is? Even pointing me in the right direction would be greatly appreciated. $\endgroup$ – pocketed_the_rocket Jun 1 '16 at 0:39

I read this as indicating 2JH-H = 9.6Hz and 1JO-H = -163.8Hz. That's a large O-H coupling.

For water, experimental values have been reported as 2JH-H = -7.3Hz and 1JO-H = -80.6Hz 1, and calculated values vary, but within an order of magnitude. Gas phase studies for 1JO-H also provide values around -80Hz. I can't explain the large discrepancy here; perhaps there is a significant change in bond angle for hydronium? Or perhaps a poorly refined calculation set?

  • $\begingroup$ I went to the solvation tab in Guassview 5.0 and set our model to be IEFPCM with water as our solvent. We wanted to mimic our chemical environment Could that have any anything to do with the discrepancy? . Also, we are limited on resources and could only use an augmented triple zeta basis set. $\endgroup$ – pocketed_the_rocket Jun 1 '16 at 3:35

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