Supporting Information. melamine as m/z 85 and m/z 126 species - namely the good match with the ionization energy

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1 Supporting Information Identification of m/z 85 and m/z 126 While a high level of confidence can be established for the assignment of cyanourea and melamine as m/z 85 and m/z 126 species - namely the good match with the ionization energy (IE) values and the previous detection of these species in this reactive system - eliminating possible alternatives to m/z 85 and m/z 126 photoions needs to be addressed. Alternative species to these detected photoions were identified in the NIST WebBook 1 using a molecular weight search for masses 85 and 126 containing any combination of C, H, N and O atoms. In order to confirm or exclude possible species, IE values for the parent ions were calculated at the M06/6-31+G(d,p) level of theory and are reported as free energies at 298 K, including zero-point energies (Table S1 and Table S2). Uncertainties in the M06 IE values are estimated to be ±0.2 ev. Known IE values are reported wherever possible and comparison to the M06 calculated values are in good agreement with the literature values. It should be noted that the DCA - anion is believed to be the reactive center for the hypergolic reaction mechanism, with the BMIM + cation acting primarily as a spectator prior to ignition. 2 The possible m/z 85 species from the literature are seen in Table S1. The experimental IE value determined for m/z 85 is 10.2±0.2 ev. From Table S1, the possible chemical species that could match this value have M06 calculated IE values ranging from 10.0 ev to 10.3 ev. Of these, 1-isocyanatopropane, cyanourea, 3-methoxypropionitrile, ethoxyacetonitrile and 2- hydroxybutyronitrile are possible candidates for m/z 85. These possible m/z 85 structures can be seen in Scheme 1, and the transformation of the DCA - to cyanourea can occur through simple acid-catalyzed addition of H 2 O, whereas the formation of the other m/z 85 species would likely require a complex, multi-step mechanism. 1

2 The evidence supporting the formation of cyanourea upon reaction of BMIM + DCA - + HNO 3 includes the matching experimental and theoretical (M06) IE values, the identification of cyanourea in this study and in a previous study, 2 and the similar bonding motif present in both DCA - and cyanourea yet absent in the other possible m/z 85 species. This leads to the conclusion that cyanourea is the most likely candidate for the product of BMIM + DCA - + HNO 3 detected at m/z 85 under our experimental conditions. Table S1. Theoretical (M06) and literature 3 values for ionization energy (IE) of parent species (m/z 85). Species are listed in increasing IE values (M06). The experimental IE value is 10.2±0.2 ev. molecular formula Ionization Energy (M06, ev) Ionization Energy (WebBook, ev) species name n-vinylacetamide C 4 H 7 NO ,5-dihydro-2-methyloxazole C 4 H 7 NO butenamide C 4 H 7 NO 8.9 cis-2-butenoic acid amide C 4 H 7 NO 8.9 trans-crotonamide C 4 H 7 NO 9.0 methacrylamide C 4 H 7 NO 9.0 1,2-dihydro-3H-1,2,4-triazol-3-one C 2 H 3 N 3 O pyrrolidinone C 4 H 7 NO H-azetidin-2-one, 2-methyl- C 4 H 7 NO cyclopropanecarboxamide C 4 H 7 NO isocyanatopropane C 4 H 7 NO 10.0 cyanourea C 2 H 3 N 3 O methoxypropionitrile C 4 H 7 NO 10.1 ethoxyacetonitrile C 4 H 7 NO hydroxybutyronitrile C 4 H 7 NO 10.3 cyanoacetic acid C 3 H 3 NO acetone cyanohydrin C 4 H 7 NO methyl cyanoformate C 3 H 3 NO

3 Scheme 1. Structures of the dicyanamide anion and possible m/z 85 species. Note the similar NCNCN bonding motif present in cyanourea but absent in the other possible structures. From the structure of melamine (Scheme 2), it can be seen that the structure of DCA - is incorporated into the melamine essentially intact, and melamine has been previously identified as a product in the reaction of BMIM + DCA - + HNO 3 4 and in dicyanamide polymerization. 5 Similar to the above analysis to confirm the m/z 85 peak seen in the experiments, possible products with m/z 126 are listed in Table S2. Scheme 2. Structure of the dicyanamide anion (left) and melamine (right). Note the similar NCNCN bonding motif present in both species. 3

4 Table S2. Theoretical (M06) and literature 3 values for ionization energy (IE) of parent species (m/z 126). Species are listed in increasing IE values (M06). The experimental IE value is 8.7±0.2 ev. species name molecular formula Ionization Energy (M06, ev) 2,4-dihydro-2,4,5-trimethyl-3H-pyrazol-3-one C 6 H 10 N 2 O 7.6 1,3-dimethyl-5-methoxypyrazol C 6 H 10 N 2 O methoxy-1,3-dimethyl-1H-pyrazole C 6 H 10 N 2 O furancarboxylic acid, hydrazide C 5 H 6 N 2 O ,6-diamino-4(1H)-pyrimidone C 4 H 6 N 4 O 7.9 melamine C 3 N 6 H n-propyl-2-pyrazolin-5-one C 6 H 10 N 2 O 8.7 Ionization Energy (WebBook, ev) thymine C 5 H 6 N 2 O methyluracil C 5 H 6 N 2 O ,3-diazabicyclo[2.2.2.]oct-2-ene,2-oxide C 6 H 10 N 2 O methyluracil C 5 H 6 N 2 O methyluracil C 5 H 6 N 2 O The experimental IE value determined for m/z 126 is 8.7±0.2 ev. From Table S2, the possible chemical species that could match this value have M06 calculated IE values ranging from 8.5 ev to 8.8 ev, i.e., that for melamine, 3-n-propyl-2-pyrazolin-5-one and thymine. While the calculated IE values allow for the elimination of many of the possible m/z 126 species (outside the range of 8.5 to 8.9 ev), we cannot exclude the possibility of the formation of thymine and 3- n-propyl-2-pyrazolin-5-one as sources for the m/z 126 photoion signal. Again, the structural similarity of DCA - to melamine, along with the complexity of the mechanism required to produce thymine or 3-n-propyl-2-pyrazolin-5-one from BMIM + DCA - + HNO 3, lead to the conclusion that melamine is the most likely product at m/z

5 Species Profile The RTIL aerosol reaction with HNO 3 is assumed here to take place under pseudo-firstorder conditions in BMIM + DCA - (i.e., [HNO 3 ]>>[BMIM + DCA - ]). BMIM + DCA - + HNO 3 BMIM + NO - 3 (m/z 201) + HDCA (m/z 67) k 1 (1) HDCA + HNO 3 Products k 2 (2) The expected time profiles of [BMIM + DCA - ] and [HDCA] can be shown to be given by: [BMIM + DCA - ] = [BMIM + DCA - ] o exp(-k 1 [HNO 3 ]t) (3) [HDCA] = k 1 [HNO 3 ][BMIM + DCA - ] o (exp(-k 2 [HNO 3 ]t) - exp(-k 1 [HNO 3 ]t))/(k 1 [HNO 3 ]- k 2 [HNO 3 ]) (4) where [BMIM + DCA - ] o is the effective initial amount of the ionic liquid, BMIM + DCA -, that is available in the aerosol particle for reaction, and k 1 and k 2 are the phenomenological rate coefficients for the above two steps in Equations (1) and (2), respectively. The term [HNO 3 ]t represents the extent of exposure that the aerosol experiences over the flow-tube reaction length. Equation (4) predicts a simple bi-exponential signal profile for the initial product HDCA formed via Equation (1) and consumed via Equation (2). The time profiles for secondary products, such as Products in Equation (2), which may also react with HNO 3 can easily be derived and shown to be multi-exponential in nature. The HDCA and other subsequent products are detected by sampling the exposed aerosols using the heated copper block vaporizer inside the mass spectrometer. The signal profile in Figure S1 for m/z 67 corresponding to HDCA clearly is not bi-exponential, as predicted by Equation 4, with a very large baseline value at zero HNO 3 - exposure. Furthermore, at large exposures, the signal does not fall off exponentially as evidenced on the semi-logarithmic plot in Figure S1. This would imply additional source terms 5

6 for m/z 67 signal in our experimental setup. Since no initial HDCA is expected to be present in the flow-tube, we propose here that both the large non-zero m/z 67 signal at zero exposure and the non-exponential decaying signal are a result of baseline source terms due to HDCA production within the detector by the thermal vaporizer. Thermal decomposition of the unreacted RTIL and/or products may occur at the vaporizer, whose subsequent photoionization and detection will lead to additional peaks in the recorded mass spectrum. Vaporization of unreacted BMIM + DCA - would be detected at m/z 139 as a result of the dissociative ionization to BMIM + + DCA. + e -. 6 Dissociative fragmentation of BMIM + DCA to BMIM: + HNCNCN + + e - is ruled out based on the M06 appearance energy (AE) value for HNCNCN + of 11.2±0.2 ev (see Table 1, main manuscript) that is 1.0 ev higher than the experimental AE value of 10.2 ev for m/z 67. However, thermolysis could lead to carbene formation (BMIM: + HDCA), 7 which would then be detected upon vaporization at m/z 138 and 67, respectively. Therefore, this inherent detector production of the HDCA reaction product signal must be included to fit the observed m/z 67 signal, leading to a finite m/z 67 signal at zero HNO 3 -exposure. The observed HDCA signal will then be a sum given by: Signal (m/z 67) [HDCA] + (k decomp /(k decomp +k vap )) ([BMIM + DCA - ] o exp(-k 1 [HNO 3 ]t) + [BMIM + DCA - ] bulk ) (5) The first term is the reactive contribution to m/z 67 signal (Equation (4)) and the second term represents the detector production sources. Here, k decomp /(k decomp +k vap ) is the fractional rate for decomposition of RTIL at the detector, with k decomp as the first-order decomposition rate coefficient, k vap the effective rate coefficient for evaporation and [BMIM + DCA - ] bulk the fraction of RTIL in the aerosol (i.e., the aerosol core) not available for HNO 3 reaction. In another words it is assumed that diffusion of HNO 3 within the aerosol is slow, and the reaction is restricted in a 6

7 region close to the surface, and therefore the core of the particle (represented by some fraction of the total volume of the particle) will not be available for reaction. In addition to k 1, k 2 and [BMIM + DCA - ] o, as the adjustable parameters to fit Equation (4), modeling of Equation (5) now includes fitting with k decomp /(k decomp +k vap ) and [BMIM + DCA - ] bulk, where [BMIM + DCA - ] tot (= [BMIM + DCA - ] bulk + [BMIM + DCA - ] o ) represents the total amount of RTIL in the aerosol particle. Since BMIM + NO - 3 formed in Equation (1) is expected to be a stable product, no further reaction of it with HNO 3 is expected. The temporal profile of BMIM + NO - 3 growth can be shown to be given by a single exponential of the form: [BMIM + NO 3 - ] = [BMIM + DCA - ] o (1- exp(-k 1 [HNO 3 ]t)) (6) At the heater block of our detector, the BMIM + NO - 3 may vaporize or undergo thermolysis: BMIM + NO BMIM: + HNO 3. Subsequent monitoring of detector-produced HNO 3 at m/z 63 provides a unique method to follow the temporal behavior of BMIM + NO - 3 production in the flow-tube reactor. (Note, monitoring of m/z 138 signal in not unique to this reaction, since it may also arise from thermolysis of the unreacted RTIL; BMIM + DCA - + BMIM: + HDCA). Figure S2 shows a very fast rise in m/z 63 signal (with a half-life rise-exposure < 1 x 10-5 atm sec) that is essentially flat beyond 2 x 10-5 atm sec exposure values compared to the decaying m/z 67 signal over the entire 4 x 10-4 atm sec exposure range in Figure S1, which implies k 1 >> k 2. On the other hand, if k 2 >> k 1 were to be the case, which would also be consistent with the measured initial m/z 67 decay, a slow growth in the m/z 63 signal over the entire range of HNO 3 exposures used would be expected, and this is not the case in Figure S2. The conclusion that k 1 >> k 2 is further supported by examining the m/z 46 signal, which has a similar profile to the m/z 63 signal, where the m/z 46 (NO + 2 ) signal results from the dissociative fragmentation of the m/z 63 (HNO + 3 ) signal. 7

8 The fast rise in m/z 63 (and 46) signal of Figure S2 suggests that the reaction represented by Equation (1) has gone to completion relative to the reaction represented by Equation (2). Therefore for HNO 3 exposures greater than 2 x 10-5 atm sec, Equation (5) can be simplified to show that the m/z 67 signal may be approximated as: Signal (m/z 67) S [BMIM + DCA - ] tot exp(-k 2 [HNO 3 ]t) + (k decomp /(k decomp +k vap )) [BMIM + DCA - ] tot (1- S) (7) where S is the fraction of the total [BMIM + DCA - ] tot available near the surface for reaction, i.e., S = [BMIM + DCA - ] o /([BMIM + DCA - ] o +[ BMIM + DCA - ] bulk ). In freely fitting Equation (7) to the data points of Figure S1, large errors were noticed in the computed values for S and (k decomp /(k decomp +k vap ). To improve the quality of the fit, the value of (k decomp /(k decomp +k vap ) was set in the range from 0.1 to 0.9, to yield fitted S values, respectively, in the range from 0.28±0.02 to 0.78±0.02, with a nominal value of k 2 = (4.14±0.32) x molec -1 cm 3 s -1. The indicated errors are 1 standard deviation in the precision of the fit. For the purpose of computing k 2 for the HDCA reaction in the RTIL particle, no correction is made to the effective HNO 3 concentration that is available within the RTIL particle since the intrinsic Henry s Law constant for HNO 3 in BMIM + DCA - is not known, and it is further assumed in the analysis that the nitric acid remains in the molecular form even though it may also ionize to H + - and NO 3 (as discussed in the main manuscript). The response curves, i.e., the product photoion current as a function of HNO 3 exposure (10-4 atm sec), for several species are shown in Figure S3. The growth rate of m/z 85 is slower than that of 126, which suggests that this is not an ion fragment signal of 126. On the other hand, the growth rate of m/z 44 is similar to that of 85 at 11.0 ev (i.e., below the CO 2 or N 2 O IE), implying it is a fragment signal. The growth rate of m/z 42 at 11.0 ev is similar to the growth rate of m/z 43 at 14.3 ev and both appear to have their origin from possible cyanourea thermal decomposition. Figure S2 also shows the product curves for m/z 138 from 8

9 BMIM + DCA - and BMIM + NO 3 - as a result of thermolysis at the detector, and m/z 139 from BMIM + DCA - ion pair vaporization, which are consistent with our analysis for Equation (5). Gas Phase and Condensed Phase Energetics Table S3. Enthalpy, free energy and entropy contributions to the reactions 1A-4A and 1B-4B of Ref 39 (in the main manuscript) using input geometries from the SI in Ref 39, calculated for the gas phase at the M06/6-31+G(d,p) level of theory and in the condensed phase using the SMD- GIL model at the M06/6-31+G(d,p) level of theory (values are relative to initial reactants). Calculated energies are reported at T = 298 K in both kj/mol and kcal/mol and Ref 39 enthalpy values are in kcal/mol. M06 (gas) (kj/mol) M06 (gas) (kcal/mol) Ref 39 H o reaction Hº Gº -T Sº Hº Gº -T Sº (kcal/mol) HNO 3 + DCA - 2A A 2A3A TS A 3A A 3A4A TS A 4A avg = 47.2 avg = HNO 3 + DCA - 2B B 2B3B TS B 3B B 3B4B TS B 4B avg = 94.1 avg = 22.5 SMD-GIL (liq) (kj/mol) SMD-GIL (liq) (kcal/mol) reaction Hº Gº -T Sº Hº Gº -T Sº HNO 3 + DCA - 2A A 2A3A TS A 3A A 3A4A TS A 4A avg = 46.9 avg = HNO 3 + DCA - 2B B 2B3B TS B 3B B 3B4B TS B 4B avg = 93.7 avg =

10 Figure S1. Ion current versus HNO 3 exposure for m/z 67 signal. Lines are fit to data; exponential (straight line) and according the analysis of Equation (5) (curved line). Error bar represents typical uncertainty of datum. 10

11 Figure S2. m/z 63 (and 46) ion curve resulting from HNO 3 formation from BMIM + NO 3 -, m/z 138 from BMIM + DCA - and BMIM + NO 3 - as a result of thermolysis at the detector, and m/z 139 from BMIM + DCA - ion pair vaporization. 11

12 Figure S3. m/z 42, 44, 85, and 126 ion curves as a function of HNO 3 exposure at 11.0 ev photon energy, product curves of HNCO (m/z 43 at 14.3 ev) as a function of HNO 3 exposure, and CO 2 (m/z 44 at 13.5 ev) and 15 N 14 NO (m/z 45 at 14.3 ev) as a function of H 15 NO 3 exposure. 12

13 References (1) Molecular Weights. In NIST Chemistry WebBook, NIST Standard Reference Database Number 69; Linstrom, P. J., Mallard, W. G., Eds.; National Institute of Standards and Technology: Gaithersburg MD, (retrieved July 16, 2015). (2) Chambreau, S. D.; Schneider, S.; Rosander, M.; Hawkins, T.; Gallegos, C. J.; Pastewait, M. F.; Vaghjiani, G. L. Fourier Transform Infrared Studies in Hypergolic Ignition of Ionic Liquids. J. Phys. Chem. A 2008, 112, (3) Lias, S. G., Ionization Energy Evaluation. In NIST Chemistry WebBook Eds. Linstrom, P. J., Mallard, W. G.; NIST Standard Reference Database Number 69; National Institute of Standards and Technology, Gaithersburg, MD, (retrieved July 16, 2015). (4) Chingin, K.; Perry, R. H.; Chambreau, S. D.; Vaghjiani, G. L.; Zare, R. N. Generation of Melamine Polymer Condensates upon Hypergolic Ignition of Dicyanamide Ionic Liquids. Angew. Chemie Int. Ed. 2011, 50, (5) Wooster, T. J.; Johanson, K. M.; Fraser, K. J.; MacFarlane, D. R.; Scott, J. L. Thermal Degradation of Cyano Containing Ionic Liquids. Green Chem. 2006, 8, (6) Koh, C.; Liu, C.-L.; Harmon, C.; Strasser, D.; Golan, A.; Kostko, O.; Chambreau, S. D.; Vaghjiani, G. L.; Leone, S. R. Soft Ionization of Thermally Evaporated Hypergolic Ionic Liquid Aerosols. J. Phys. Chem. A 2011, 115, (7) Chambreau, S. D.; Schenk, A. C.; Sheppard, A. J.; Yandek, G. R.; Vaghjiani, G. L.; Maciejewski, J.; Koh, C. J.; Golan, A.; Leone, S. R. Thermal Decomposition Mechanisms of Alkylimidazolium Ionic Liquids with Cyano-Functionalized Anions. J. Phys. Chem. A 2014, 118,