Is the Proton Affinity of Nitric Acid Larger Than the Proton Affinity of Methyl Nitrate? A Direct Experimental Answer

Abstract

A direct experimental approach based on the evaluation of the ligand exchange equilibrium H20-N02+ + CH30H CH3(H)O-N02+ + H 2 0 has been exploited for the determination of APA = PA(HN03) -PA(CH30N02) at 298 K. The result, APA = 4.0 f 1.2 kcal mol-', is sufficiently accurate to provide firm experimental support to the counterintuitive prediction, based on high-level ab initio calculations, that the PA of HN03 exceeds that of CH3-ON02 by 5.6 f 5 kcal mol-'. Combination of the experimental APA with the known PA of CH30N02 gives PA(HNO3) = 182.0 f 2.3 kcal mol-', in excellent agreement with the theoretically computed value, 182.5 f 3 kcal mol-'. An explanation of the considerably lower PA(HN03) value derived from earlier ICR bracketing experiments is offered, based on the R(H)O-N02+ vs RON02H+ isomerism. whose role in determining the observed PA trend along the RON02 series (R = H, CH3, C2H5) is discussed.

Gas-Phase Proton Affinity of Nitric Acid and Its Esters. A Mass Spectrometric and ab Initio Study on the Existence and the Relative Stability of Two Isomers of Protonated Ethyl Nitrate

“…The results obtained using d-labeled species suggest a cyclic transition state, leading to the formation of the most stable isomer of protonated nitric acid. 11,16 Although isomer 2c cannot be prepared in the pure state, being invariably formed together with 1c from process 1, several criteria can be adopted for its characterization by CAD spectrometry in the mixed populations. Assuming that the fragment of m/z ) 64 originates exclusively from 2c, whereas those of m/z ) 31 and 45 originate only from 1c, one can utilize the intensity ratio R 1 ) i 64 /i 45 , or R 2 ) i 64 /i 31 .…”

“…On the basis of the above considerations, we have undertaken a joint mass spectrometric and theoretical study aimed at ascertaining whether gas-phase protonation of C 2 H 5 ONO 2 gives isomeric ions of structure 1 and 2, establishing their relative stability and reevaluating the PA of ethyl nitrate by experimental and theoretical methods more reliable than the "bracketing" technique 18 employed in earlier measurements. 16,19 Methodology Structural Analysis. The techniques employed include mass-analyzed ion kinetic energy (MIKE) and collisionally activated dissociation (CAD) spectrometry.…”

“Ipso” Aromatic Nitration in the Gas Phase

“…[8][9][10] The reactivity of (CH 3 OH-NO 2 ) + toward aromatics has been previously investigated by a combination of mass spectrometric and radiolytic techniques, [11][12][13] complemented by a recent theoretical study. 14 We have followed the same experimental approach 15 to investigate the gas-phase nitration of substrates selected for their recognized ability to undergo nitrodealkylation in condensed media.…”

Gas-phase nitronium ion affinities.

“…The spectra were recorded at temperatures ranging from 400 to 50°C, as measured by a thermocouple inserted into the source block, using gaseous mixtures of NO2 and the two ligands (L1 and L2), whose composition was optimized to obtain the highest intensity of the L1(NO')L2 dimers, at total pressures in the 0.1-to 0. The study of the complexes formed in the gas phase by NO2 is currently the focus of considerable fundamental interest (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11) and, in addition, has a direct bearing on areas ranging from mechanistic organic chemistry (12)(13)(14)(15)(16)(17)(18)(19) to atmospheric chemistry, clustering and nucleation phenomena, intracluster reactivity (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30), etc. In view of the considerable interest of the problem and of the scant experimental information currently available (31-33), we have undertaken a systematic study of the binding energy (BE) of NO+ to representative ligands, aimed at the construction of a gas-phase nitronium ion-affinity scale.…”