Direct Experimental Determination of the Energy Barriers for Methyl Cation Transfer in the Reactions of Methanol with Protonated Methanol, Protonated Acetonitrile, and Protonated Acetaldehyde:  A Low Pressure FTICR Study

Abstract

Methyl cation transfer reactions between methanol and protonated methanol, protonated acetonitrile, and protonated acetaldehyde have been investigated experimentally by low-pressure FT-ICR mass spectrometry. The temperature dependencies of the rate constants for these reactions were determined in an Arrhenius-type analysis to obtain activation energies, enthalpies, and entropies of activation. The enthalpies of activation were determined to be −16.9 ± 0.6, −16.5 ± 0.6, and −18.4 ± 0.7 kJ mol-1 for the methanol/protonated methanol, methanol/protonated acetonitrile, and methanol/protonated acetaldehyde reactions, respectively. These values agree quite well with ab initio-calculated values. The entropies of activation were found to be quite similar for all three reactions within experimental uncertainty, which is expected due to the similar transition-state structures for all reactions. Ab initio potential energy surfaces calculated at the MP2/6-311G** level and basis set are reported for the three reactions. For the methanol/protonated acetonitrile and methanol/protonated acetaldehyde reactions, isomerization of the initially produced proton-bound dimer to a methyl-bound complex is suggested prior to methyl cation transfer. The barrier for the first isomerization is predicted to be significantly lower than the barrier for methyl cation transfer such that it does not interfere with the experimental determination of the latter.

Competing rearrangement reactions in small gas-phase ionic complexes: The internal SN2 and nitro-nitrite rearrangements in nitroalkane proton-bound pairs

“…Calculations [6][7][8][9][10][11] and experiments [12][13][14][15][16] on the methanol proton-bound dimer indicated that isomerization proceeds via an internal S N 2-type reaction. Work from our laboratory and that of other labs also showed that the family of methyl-substituted nitrile-alcohol proton-bound pairs [9,11,[17][18][19][20] and dimethyl ether protonbound dimer [21] loses water and methanol in a similar fashion.…”

Dynamics and mechanism of structural diffusion in linear hydrogen bond

“…The results on the smallest shared-proton complexes (n ¼ 1), obtained at the same level of accuracy, are summarized in Table 2 [64] which agrees with ab initio calculations at MP2/6-311G(d,p) level of theory. [65] The stabilities of the protonated H-bonds in the CH 3 OH þ 2 (CH 3 OH) complexes are substantially higher than in (CH 3 OH) 2 ; DE for (CH 3 OH) 2 were reported in the range of À13 and À17 kJ/mol, depending upon the methods used. [66,67] For the CH 3 OH þ 2 (CH 3 OH) complexes, RIMP2/TZVP calculations predicted slightly shorter H-bond distances (R OAO ¼ 2.38 and 2.39 Å , respectively), and about 2 kJ/mol lower interaction energies (DE ¼ À148.3 and -147.1 kJ/mol, respectively).…”

The Gas‐Phase Formation of Methyl Formate in Hot Molecular Cores

“…In the low-pressure ion cyclotron resonance (ICR) experiments at Waterloo (Fridgen et al 2001), additional attempts were made to study the reaction of protonated methanol with neutral formaldehyde. Gaseous formaldehyde and the vapor from degassed methanol were introduced to the ICR cell via heated precision leak valves at (1 2) ; 10 À8 torr for formaldehyde and 5 ; 10 À9 torr for methanol.…”