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

Abstract

Direct dynamics simulations of HNO 3 with dicyanamide anion DCA − (i.e., N(CN) 2 − ) and dicyanoborohydride anion DCBH − (i.e., BH 2 (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 HNO 3 molecules, respectively, in the gasphase and in the condensed-phase ionic liquids using the B3LYP/ 6-311++G(d,p) method. The oxidation of DCA − by HNO 3 is initiated by proton transfer. The most important pathway leads to the formation of O 2 N−NHC(O)NCN − , and the latter reacts with a second HNO 3 to produce O 2 N−NHC(O)NC(O)NH− NO 2 − (DNB − ). The oxidation of DCBH − by HNO 3 may follow a similar mechanism as that of DCA − , producing two analogue products: O 2 N−NHC(O)BH 2 CN − and O 2 N−NHC(O)BH 2 C(O)NH−NO 2− . Moreover, two new, unique reaction pathways were discovered for DCBH − because of its boron-hydride group: (1) isomerization of DCBH − to CNBH 2 CN − and CNBH 2 NC − and (2) H 2 elimination in which the proton in HNO 3 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, H 2 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 HNO 3 .

Unsupervised Reaction Pathways Search for the Oxidation of Hypergolic Ionic Liquids: 1-Ethyl-3-methylimidazolium Cyanoborohydride (EMIM+/CBH–) as a Case Study

“…Compared to bimolecular collisions, unimolecular dissociation guarantees reactions (given enough simulation time and excess energy) and thus has the potential of being more effective in detecting reaction pathways. These intermediates for dynamics simulations (IDS) either have been reported by previous research (e.g., IDS1, the hydrogen bond complex between HNO 3 and CBH – ; IDS2, NO 3 – attached to the carbon atom on HCBH; IDS3, migration of the −NO 2 from oxygen to nitrogen in IDS2; and IDS4, NO 3 – attached to the boron atom of an isomer of HCBH) or are proposed according to chemical intuition (e.g., ISD5, migration of the −NO 2 from oxygen to carbon in IDS2; ISD6, migration of the −BH 3 from carbon to oxygen and the transition from nitro to nitrite in IDS5). It is important to note that this list is by no means exhaustivethe trajectories initiated from IDSs of different bond connections sample diverse chemical configurations and aim to detect potential products which will be employed to search for intermediates (section ).…”

“…The transition from v370 to v345 involves crossing a barrier of 5.2 kcal/mol, same as the barrier between v331 and v370 . HCN is a common product formation for similar reactions, like nitric acid with dicyanoamide or dicyanoborohydride. , The reaction energy of the pHCN pathway is −5.3 kcal/mol. The mechanism of the system transiting from v370 to i186 has been explained in the previous paragraph. Instead of forming v75 , which leads to the formation of NO, NO 2 , N 2 O, HNO 2 , and HONO (1–5), the N–N bond could break, and the −NO 2 moiety moves down the molecule to form an O–B bond.…”

“…Instead of forming v75, which leads to the formation of NO, NO The mechanisms seen here in the MEPs have significant overlap with similar ionic liquid (anion)−oxidant reactions in the literature. For example, studies of nitric acid and dicyanamide 39,41 as well as nitric acid and dicyanoborohydride 39,40 have both found proton migration, −ONO 2 attachment to the carbon, and −NO 2 migration as key first steps in the reaction. In the current system, all three mechanisms are seen in a number of transition states connecting intermediates.…”

Structures, proton transfer and dissociation of hydroxylammonium nitrate (HAN) revealed by electrospray ionization tandem mass spectrometry and molecular dynamics simulations

“…The oB97XD/6-31+G(d,p) density functional was chosen for the trajectory simulations, as this functional is able to provide a good description of IL reaction dynamics. 47 A quadratically convergent SCF method was opted in to integrate trajectories (i.e., SCF = XQC) 48 in case the conventional first-order SCF algorithm failed to converge within the default cycles. Trajectories were terminated at 2-3 ps when the trajectory either reached a geometry of stable product…”

“…Trajectories were analyzed using in-house custom programs written for these purposes. 47,50 3. Experimental findings 3.1 HANÁNO 3 À formed in negative electrosprays…”