3658
J. Phys. Chem. 1994,98, 3658-3663
Thermal Reactions of Protonated Formic Acid Clusters (HCOOH)a+ (n = 1-3) Wan Yong Feng and Cbava Lifshitz'vt Department of Physical Chemistry and The Fritz Haber Research Center for Molecular Dynamics, The Hebrew University of Jerusalem, Jerusalem, 91 904, Israel Received: January 28, 1993'
Thermal reactions of (HCOOH)nH+ (n = 1-3) with a series of base molecules B were studied using a selected ion flow tube. Reactions observed include association, ligand switching, and proton transfer and occur at gas collision rates for all the bases having proton affinities (PAS)higher than formic acid. Branching ratios between the various reaction products depend on the proton affinity of the neutral reagent, but specific chemical reactions were observed for different neutrals having similar PAS. Thermochemical calculations suggest the evaporation of the formic acid neutral dimer in some ion/molecule reactions of (HCOOH)sH+. The proton binding abilities of the two groups-C=O and OH-in formic acid play a role in some of the end products which have been observed. The reaction of (HCOOH)sH+ with acetonitrile leads to a mixed trimer in which protonated formic acid is the core ion, while reaction with ammonia leads to a mixed trimer in which protonated ammonia is the core ion.
Introduction Properties of clusters incorporating several molecules about a proton are of interest, since they bridge the gap between gasphase and liquid-solution-phase chemistry. Ion/molecule reactions of proton bound clusters have attracted attention recently.1-3 Solvation is known to have marked effects on reaction rates in some instances.3 We have studied recently reactions of ions with closed solvation shells: (CH3OH)pH+? ( C H ~ C N ) Z H +(CH3,~ OCH3)2H+,5and(CH&OCH&H+.S Contrary to the protonated methanol trimer, the proton bound dimers of acetonitrile,dimethyl ether, and acetone have alkyl blocking groups; they do not possess hydrogens which can form stable hydrogen-bonding networks beyond the dimer. While the reactions of protonated methanol trimer were found to be intrinsically fast? those of the alkylblocked proton bound dimers were not? indicating that they were characterized by double-well potential energy surfaces with intermediate barriers.69 In the present study we have concentrated on proton bound clusters of formic acid, (HCOOH),H+ (n = 1-3). Formation, reactivity, and structures of these clusters have been explored, using a modified SIFT (selected ion flow tube) t e c h n i q ~ e . ~ ? ~ Protonated formic acid clusters are interesting since they possess potentially two proton binding sites-the OH and the C-0 groups. The effect of stepwise solvation on ion reactivity is of great interest?JoJl and we have studied reactions of the monomer, dimer, and trimer with a series of base molecules having different proton affinities.
Experimental Section The SIFT apparatus employed has been described in detail.I2 Briefly, reactant ions are generated in a suitable ion source. They are mass-selected by a quadrupole mass filter and injected into the flow tube by a helium carrier gas using a Venturi inlet. A neutral reactant is introduced into the flow tube at an appropriate distance downstream to ensure laminar flow. A detector quadrupole filter analyzes the reactant and product ions. We were unable to inject the protonated dimer and trimer from the ion source into the flow tube. We injected the monomer (HCOOH)H+ instead, which was converted to the dimer and To whom correspondneceshould be addressed. t Archie and Marjorie Sherman Professor of Chemistry.
'Abstract published in Advance ACS Abstracts, March 1 , 1994.
0022-3654/94/2098-3658$04.50/0
3
]HCOOH)~H+
Y
2 n=l
4
5
The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 3659
Thermal Reactions of Protonated Formic Acid Clusters
TABLE 1: Rate Constants and Branching Ratios for Reactions of (HCOOH)JI+ (n = 1-3) ~
neutral B CH30Ha
PA, kcal/mol 181.9
n
kemd
kcd
1
2.29
2.11
2 3 1 2 3
2.09 2.01 2.90 2.71 2.60
1.88 1.81 2.83 2.47 2.33
CH3CHO
186.6
CH3CN
188.6
1 2 3
4.07 3.62 3.40
3.88 3.41 3.23
CHjCOCH3
198.4
1
2.72
2.90
2 3
2.46 2.31
2.47 2.31
203
1 2 3
2.65 2.30 2.23
1.35 1.11 1.02
209.2
1 2 3
1.34 1.17 0.90
NH3
208.3
1 2 3
1.38 1.31 1.29
2.24 2.09 2.03
CHiNHz
219.6
1 2 3
2.06 1.97 1.88
1.87 1.67 1.60
227.8
2.24 1.82
1.61 1.40
1.77
1.32
+ B at 300 K branching ratio, %
(CHaOH)H+(98.7) (HCOOH)(CH3OH)H+(1.3) (HCOOH)(CH3OH)H+(100) (HCOOH)z(CHaOH)H+ (100) (CHaCHO)H+(100) (HCOOH)(CH,CHO)H+(100) (HCOOH)&CH3CHd)H+
