Rapid Gas-Phase Reactions. Amines and Boron Trifluoride. II. Pressure Dependence of Rate Constant

Abstract

Relative rate constants for reaction of monomethylamine and trimethylamine with boron trifluoride are reported for the pressure range 0.04-600 Torr. The limiting ratio at low pressure is about 0.06, and the extrapolated high-pressure ratio is of the order 4. The half-pressure for quasi-bimolecular behavior of the monomethylamine-boron trifluoride system is in the vicinity of 70 Torr. When the data are combined with those of Kistiakowsky, et a!., individual pressure-dependent rate constants can be estimated. The amine-BFs series has loose activated complexes and low energy for redistribution among internal modes, making it an unusual candidate for theoretical unimolecular study.

A Systematic Exploration of B–F Bond Dissociation Enthalpies of Fluoroborane-Type Molecules at the CCSD(T)/CBS Level

“…Beginning with BF 3 , which is perhaps the most widely studied of these compounds, a diverse range of reactions involving this molecule have been reported. From the perspective of its reaction in the gas phase, representative examples of such studies include its reaction with (i) catecholate and related anions [2], (ii) the gas phase ion chemistry of BF 3 /CH 4 mixtures [3], and (iii) the reaction of ammonia and methylamine [4,5]. From the perspective of the reactions of BF 3 in solution (where it is often used in the form of its etherate adduct), it has been shown to play a role in facilitating various polymerization reactions [6][7][8], dehydration reactions of alcohols [9,10], esterification reactions [11][12][13], alkylation reactions [14,15], as well as the synthesis of syn-fluorohydrins from epoxides [16].…”

Bimolecular rates of association of amines with boron trifluoride: Theoretical consideration

“…This increasing fall-off tendency with a decreasing number of degrees of freedom, other things being equal, is in accord with unimolecular theory. The high-pressure rate constants were estimated [4,5] utilizing for all the data [1,4,5] values of the order of the collision frequency in the case of CHsNH,+BF, and values lower by factors of 3 and 4 for the (CH3)ZNH and (CHJSN systems, respectively.' Thereby the activated complex for the first system is "loose," and that of the other two are "almost loose.…”

“…No amount of juggling of A H permitted agreement below 0.1 torr, that is, a suitable leveling off of the rate constant ratio, while still retaining reasonable agreement a t higher pressures. Since the reactions in [4,5] below 0.1 torr may be partially heterogeneous [4], such a behavior is not expected.…”