Coupling constant in 1H NMR with phosphorus

Question

I observed a doublet with peaks at 5.6336 and 5.5886 ppm with the NMR instrument running at 300 MHz, and got a $J_\ce{PH}= \pu{13.5 Hz}$. Is this correct for this molecule (ethoxycarbonylmethyl(triphenyl)phosphonium bromide)? This value is larger than the hydrogen coupling constants for the C-C bonds. Is this larger coupling constant a consequence of the phosphorous?

Answer 1

Yes, this is the ²J_P-H coupling. You could verify this (if you had access to a NMR spectrometer) by running a ¹H{³¹P} experiment; that is, a proton with ³¹P decoupling. This doublet would collapse back to a singlet. You would also notice some changes to the phenyl protons, as they all have some coupling to the ³¹P also. A ²J coupling of 13.5Hz is very typical for this type of ¹H-³¹P coupling.

The magnitude of ¹H-³¹P couplings varies enormously, depending on bond order, oxidation state of ³¹P and bonding arrangement.

• ¹J_PH of PH₃ = ~175Hz.
• ¹J_PH of HP(O)(OMe)₂ = ~1200Hz
• ²J_PH P(CHx)y are typically 1-20Hz

It is sometimes observed in ³¹P-1H (and ³¹P-¹³C) that high bond order couplings are larger than lower bond order couplings (ie ³J_PH > ²J_PH)