Molecular Dynamics Simulations, Reaction Pathway and Mechanism Dissection, and Kinetics Modeling of the Nitric Acid Oxidation of Dicyanamide and Dicyanoborohydride Anions

Direct
dynamics simulations of HNO3 with dicyanamide
anion DCA– (i.e., N­(CN)2–) and dicyanoborohydride anion DCBH– (i.e., BH2(CN)2–) were performed at the B3LYP/6-31+G­(d) level of theory in an attempt
to elucidate the primary and secondary reactions in the two reaction
systems. Guided by trajectory results, reaction coordinates and potential
energy diagrams were mapped out for the oxidation of DCA– and DCBH– by one and two HNO3 molecules,
respectively, in the gas-phase and in the condensed-phase ionic liquids
using the B3LYP/6-311++G­(d,p) method. The oxidation of DCA– by HNO3 is initiated by proton transfer. The most important
pathway leads to the formation of O2N–NHC­(O)­NCN–, and the latter reacts with a second HNO3 to produce O2N–NHC­(O)­NC­(O)­NH–NO2–(DNB–). The oxidation of DCBH– by HNO3 may follow a similar mechanism
as that of DCA–, producing two analogue products:
O2N–NHC­(O)­BH2CN– and
O2N–NHC­(O)­BH2C­(O)­NH–NO2–. Moreover, two new, unique reaction pathways
were discovered for DCBH– because of its boron-hydride
group: (1) isomerization of DCBH– to CNBH2CN– and CNBH2NC– and
(2) H2 elimination in which the proton in HNO3 combines with a hydride-H in DCBH–. The Rice–Ramsperger–Kassel–Marcus
(RRKM) theory was utilized to calculate reaction kinetics and product
branching ratios. The RRKM results indicate that the formation of
DNB– is exclusively important in the oxidation of
DCA–, whereas the same type of reaction is a minor
channel in the oxidation of DCBH–. In the latter
case, H2 elimination becomes dominating. The RRKM modeling
also indicates that the oxidation rate constant of DCBH– is higher than that of DCA– by an order of magnitude.
This rationalizes the enhanced preignition performance of DCBH– over DCA– with HNO3.