Decomposition of protonated formic acid: One transition state—Two product channels

Abstract

The unimolecular chemistry of protonated formic acid, [HCOOH]H ϩ , has been investigated by analyzing the fragmentation of metastable ions (MI) during flight in a sector mass spectrometer, and by proton transfer to formic acid in a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. High level ab initio calculations have been used to model the relevant parts of the potential energy surface (PES). In addition, ab initio direct dynamics calculations have been conducted, tracing out 60 different reaction trajectories. The only stable isomer in the mass spectrometric experiments is HC(OH) 2 ϩ , which is the precursor to both observed ionic products, HCO ϩ and H 3 O ϩ , via the same saddle point of the potential energy surface. The detailed motion of the dissociating molecule during passage of the post-transition state region of the PES therefore determines which product ion is formed. After passing the TS a transient HC(O)OH 2 ϩ molecule is first formed. High total energy increases the probability that the nascent water molecule will have sufficient speed to escape the HCO ϩ moiety. Otherwise, typically at low energies, the two units recombine, upon which intra-complex proton transfer is very likely. Eventually, this will give the more stable H 3 O ϩ . (J Am Soc Mass Spectrom 2004, 15, 982-988)

Strategic examination of the classical catalysis of formic acid decomposition for intermittent hydrogen production, storage and supply: A review

“…If it is assumed that all four reactions are important steps in formic acid decomposition, then, in combining those two schools of thought, one can see all four steps as being very important to the hydrogen formation step, hence the proposed scheme of elementary steps for HCOOH decomposition is in the order of (4-5 or 2-3). The distribution ratio of HCO + :H 3 O + is 7:3 and the reactions in equations ( 4)- (5) show that the decomposition of protonated metastable ions of HCOOH give hydroxonium and formyl ions [120,121]. The difference in heat of reaction values for both steps was reported to have some measure of thermochemical variations during their product formation steps.…”

Gas‐phase acylium ion transfer reactions mediated by a proton shuttle mechanism

“…Model calculations at the MP2/6‐31+G(d,p) level for formic acid reveal that this [1,3] H‐migration is characterized by a high energy barrier of 51 kcal mol −1 , a value within 1 kcal mol −1 of that previously calculated at the G2 level50. This high energy barrier is similar to barriers encountered for tautomerization processes in simple gas‐phase ionic systems through [1,3]‐H migration in ionic systems51–53.…”

“…The mechanism shown in Scheme implies hydroxy‐protonation of acetic acid that is calculated to be ∼13 kcal mol −1 less stable than I 47. Two possibilities can be considered for generating the hydroxy‐protonated acid: Intramolecular [1,3]‐H migration as shown in Scheme .Model calculations at the MP2/6‐31+G(d,p) level for formic acid reveal that this [1,3] H‐migration is characterized by a high energy barrier of 51 kcal mol −1 , a value within 1 kcal mol −1 of that previously calculated at the G2 level50. This high energy barrier is similar to barriers encountered for tautomerization processes in simple gas‐phase ionic systems through [1,3]‐H migration in ionic systems51–53.…”

Dissociative Recombination of Protonated Formic Acid: Implications for Molecular Cloud and Cometary Chemistry

“…Given the initial gas mixture in the ion source, DCOOD and D 2 , ions with m/z = 50 injected into the ring should have been protonated formic acid with the precise structure DC(OD) + 2 . In support of this Sekigushi et al (2004) concluded from mass-spectrometric investigation and ab initio calculations that protonation occurs only on the carbonyl oxygen of formic acid. Some contamination by ions produced via reactions amongst DCOOD fragments cannot be completely ruled out.…”