J. Phys. Chem. 1994, 98, 11902-11908
11902
Gas Phase Acidity of N202H2 and Reactions of Nz02H- with Aliphatic Alcohols W. E. Wentworth, C. F. Batten,? and E. Desai D'Sa Department of Chemistry, University of Houston, Houston, Texas 77204-5641
E. C . M. Chen* School of Natural and Applied Sciences, University of Houston-CL, Houston, Texas 77058 Received: July 15, 1994; In Final Form: September 12, 1994@
We report the atmospheric pressure gas phase ion-molecule chemistry of the N202H- (mlz = 61) anion which is formed when certain hydrogen atom containing molecules are added to an N20/Ar mixture in an atmospheric pressure ionization mass spectrometer (63Ni-APIMS) source at pressures of -1 atm and temperatures ranging from 373 to 600 K. By using the bracketing method, the gas phase acidity of N202H2 is found to be about the same as that of m-nitrotoluene Waci,j(g) 5 365 k c d m o l . The rate constant for proton abstraction from nitromethane is comparable to other proton transfer reactions, kt = (2 f 0.4) x cm3 molecule-' s-l. N202H- reacts with aliphatic alcohols to form esters rather than by proton abstraction. The rate constant for esterification is 10-100 times lower than the rate constant for proton abstraction from nitromethane. SCF-MO calculations show that there are at least six local minima in the potential energy surface involving N202-. The structure with two oxygens bound to one nitrogen with a CzVgeometry, " 0 2 , corresponds to the global minimum. It is the only form with a calculated vertical electron affinity which is greater than 3.49 eV and will not undergo photodetachment at that energy as has been observed experimentally. Similar results are obtained for NzOzH-. The calculated heat of formation of the CzVform of N202H- is -37.32 kcallmol.
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Introduction Nitrogen oxides have long been recognized for their important role in the production of acid rain. Nitrous oxide has been used as a chemical ionization reagent in negative ion chemical ionization mass spectrometry' and as a sensitization reagent in electron capture detectors.'S2 Fundamental studies of the kinetic reactions of nitrogen oxide anions have been r e p ~ r t e d . ~The -~ properties of ionic clusters has been recently reviewed.'O The review emphasized the importance of the photoelectron spectra of N202- formed from different reagents measured by Posey and Johnson.ll Most recently, NO has been identified as an important molecule in biological processes. l 2 Kinetic studies of NO autooxidation in aqueous solution "suggest that the oxidizing intermediates formed are neither the gas phase oxidation product NO2 nor the common nitrogen oxides N2O3. NOf, or OzNO-, but rather an uncharacterized NxOyn- species."12 In spite of the importance of these species to diverse areas of study, many questions concerning the structure, kinetic mechanisms, and stability of some of the simplest anions of nitrogen oxides remain unanswered. The importance of NO to biological processes can be better appreciated by consulting the feature article in Chemical and Engineering News.12 This is further emphasized by noting that, in one half year volume of Chemical Abstracts under the subject of biological studies of NO, there are three pages of listings. Some specific references with regard to the importance of this paper to the general area will be given in the conclusions. We report the atmospheric pressure gas phase ion-molecule chemistry of the N202H- (mlz = 61) anion which is formed when certain hydrogen atom containing molecules are added to an N2O/Ar mixture in an atmospheric pressure ionization mass
' Deceased March 1994.
* To whom correspondence should be addressed.
@
Abstract published in Advance ACS Abstracts, October 15, 1994.
spectrometer (63Ni-APIMS)source at pressures of 1 atm and temperatures ranging from 373 to 600 K. Aliphatic hydrocarbons produced N202H-, while aromatic hydrocarbons did not. The gas phase acidity of NzOzH2 was determined by kinetic bracketing studies. During the course of these determinations, an ion-molecule reaction of N202H- with aliphatic alcohols was observed. We present the results of a systematic study of these reactions. Previously, we reported the observation of N202- ( d e = 60) when N20 was added to Ar in an M I source but did not suggest a structure for the anion.' The N202- anion we have observed fits the general formula for some of the biologically significant species discussed above, and the reactions of the associated anion, NzOzH-, may be relevant to those studies. We consider the differences in the mass spectra obtained in the API system and those obtained under different conditions of temperature, pressure, and N20 concentrations in terms of the fundamental kinetic and thermodynamic properties proposed by other^.^-^ The specific systems used in the enhanced electron capture detection2 and negative ion chemical i o n i z a t i ~ n ~will ~,'~ be discussed. Photoelectron spectra of mass-selected N202-, produced when different gas mixtures were exposed to low-energy electrons and then supersonically expanded, provide evidence for the existence of at least three different structures for this anion." In order to characterize the structures of the various forms of the anion N202-, we have calculated the heats of formation for the postulated species and compare these values with the energetics reported by Posey and Johnson." The calculations were made using the default PM3 parameters in the UHF/SCF subroutine of the HyperChem software. Different values of the heats of formation were obtained for the various structures. The structure with the lowest value of the heat of formation corresponds to a chemically bound form of N202- with a C2" structure analogous to that of CO3-. These calculations agree
0022-3654/94/2098-11902$04.50/0 0 1994 American Chemical Society
Reactions of N202H- with Aliphatic Alcohols with the experimental energetics given by Posey and Johnson and support their conclusions concerning the structures of the three different forms of N202-. We report calculations for the neutral complexes and negative ions related to the gas phase acidity of H2N202. These calculated values support our experimental gas phase acidity obtained from the AFT-MS studies. The calculated global minimum for N202H- also corresponds to the CzVgeometry. Experimental Section All measurements were made with a SpectrEL quadrupole mass spectrometer (Extranuclear Labs. Pittsburgh, PA) having a nominal mass range from 2 to 560 amu. The standard electronics were modified to allow measurement of either positive or negative ions from an external high-pressure ionization source. The standard ion source block and filament holder were removed and replaced with a cylindrical stainless steel wire mesh accelerating lens. The entire vacuum chamber enclosing the mass spectrometer was pumped by two oil diffusion pumps: the standard 680 L s-' pump and a supplemental 280 L s-l pump. In these experiments, the ionization region was maintained at 1 atm and the mass analyzer region at
