Reactivity of Phosphate Monoester Monoanions in Aqueous Solution. 1. Quantum Mechanical Calculations Support the Existence of “Anionic Zwitterion” MeO⁺(H)PO₃^2- as a Key Intermediate in the Dissociative Hydrolysis of the Methyl Phosphate Anion¹

Abstract

The dissociative hydrolysis reaction of the methyl phosphate monoanion has been studied for the reactant species CH(3)OPO(3)H(-) (1) and CH(3)OPO(3)H(-) x H(2)O (1a) in the gas and aqueous phases by density functional theory (B3LYP) calculations. Nonspecific solvation effects were taken into account with the polarizable continuum model PCM either by solvating the gas-phase reaction paths or by performing geometry searches directly in the presence of the solvation correction. In agreement with previous theoretical studies, our gas-phase calculations indicate that proton transfer to the methoxy group of 1 is concerted with P-O bond cleavage. In contrast, optimizations performed with the PCM solvation model establish the existence of the tautomeric form CH(3)O(+)(H)PO(3)(2-) (2) as an intermediate, indicating that proton transfer and P-O bond cleavage become uncoupled in aqueous solution. The dissociative pathway of 1a is energetically favored over the dissociative pathway of 1 only when the added water molecule plays an active catalytic role in the prototropic rearrangement 1 <--> 2. In that case, it is found that the collapse (via P-O bond cleavage) of the hydrated zwitterionic form CH(3)O(+)(H)PO(3)(2-) x H(2)O (2a) is rate-determining. This collapse may occur by a stepwise mechanism through a very short-lived metaphosphate intermediate (PO(3)(-)), or by a concerted S(N)2-like displacement through a loose metaphosphate-like transition state. The present calculations do not allow a distinction to be made between these two alternatives, which are both in excellent agreement with experiment. The present study also reveals that PO(3)(-) reacts selectively with CH(3)OH and H(2)O nucleophiles in aqueous solution. However, the observed selectivity of metaphosphate is governed by solvation effects, not nucleophilicity (water being much more effective than methanol in capturing PO(3)(-)). This arises from a better solvation of the addition product H(2)O(+)PO(3)(2-) as compared to CH(3)O(+)(H)PO(3)(2-).

Gaseous H5P2O8− Ions: A Theoretical and Experimental Study on the Hydrolysis and Synthesis of Diphosphate Ion

“…The dissociative mechanism, which proceeds via a six-membered-ring transition state involving the water molecule, is found to be favored more than the associative mechanism, in both the gas and solution phase. In agreement with previous theoretical results, [8] DFT calculations carried out using the hybrid functional B3 LYP on the dissociative hydrolysis reaction of CH 3 OPO 3 H À indicated that, in the gas phase, proton transfer to the methoxy group is concerted with PÀO cleavage. The dissociative pathway of the CH 3 Keywords: ab initio calculations · cluster compounds · diphosphate anions · gas-phase chemistry · mass spectrometry · phosphorus transition state.…”

“…However, it was reported that in the CH 3 PO 3 H À dissociative hydrolysis, the less strained six-membered ring transition state is lower in energy than the corresponding four-membered transition state and facilitates the process. [7][8] The same influence of the water molecule was found in the [PO 3 …”

“…On the release of H 2 PO 4 À and H 3 PO 4 from both III b and III c at the B3LYP/6-31 + G* level of theory we calculate that these processes are barrier-free and endothermic by 29. 8 Figure 7 we report the optimized geometry at the B3LYP/6-31 + G* level of theory of cluster IVa together with that of the hydrated metaphosphate anion here labeled [PO 3 À cluster whose optimized geometry has already been described. [18] For this species our calculations indicate an endothermicity of 6.3 kcal mol À1 with no activation barrier.…”

The Diphosphate Monoanion in the Gas Phase: A Joint Mass Spectrometric and Theoretical Study

“…At low radio-frequency pulse intensity, the CAD spectrum of H 3 P 2 O 7 À ions displayed only the PO 3 À fragment ion at m/z 79, corresponding to the loss of an H 3 PO 4 molecule. If radio-frequency pulses of higher intensity were used, the abundance of the ion at m/z 79 increased and small amounts of the ion at m/z 159 were observed.…”

“…Under low collision-energy conditions, the spectrum displayed the fragment at m/z 159, HP 2 O 6 À , corresponding to the loss of a water molecule. At slightly higher collision energies, it displayed the ion at m/z 79, PO 3 À , corresponding to the loss of H 3 PO 4 . As the collision energies were increased, this ion became the most important fragmentation channel with a relative intensity of 62.4 % at 30 eV.…”