The Molecular Surface Structure of Ammonium and Potassium

ARTICLE pubs.acs.org/JPCC

The Molecular Surface Structure of Ammonium and Potassium Dinitramide: A Vibrational Sum Frequency Spectroscopy and Quantum Chemical Study Martin Rahm,‡,§ Eric Tyrode,† Tore Brinck,‡ and C. Magnus Johnson*,† †

Surface and Corrosion Science and ‡Physical Chemistry, Royal Institute of Technology (KTH), SE-10044, Stockholm, Sweden Competence Centre for Energetic Materials (KCEM), Gammelbackav€agen 6, SE-69151, Karlskoga, Sweden

§

ABSTRACT: Vibrational sum frequency spectroscopy (VSFS) and quantum chemical modeling have been employed to investigate the molecular surface structure of ammonium and potassium dinitramide (ADN and KDN) crystals. Identification of key vibrational modes was made possible by performing density functional theory calculations of molecular clusters. The surface of KDN was found to be partly covered with a thin layer of the decomposition product KNO3, which due to its low thickness was not detectable by infrared and Raman spectroscopy. In contrast, ADN exhibited an extremely inhomogeneous surface, on which polarized dinitramide anions were present, possibly together with a thin layer of NH4NO3. The intertwined use of theoretical and experimental tools proved indispensable in the analysis of these complex surfaces. The experimental verification of polarized and destabilized dinitramide anions stresses the importance of designing surface-active polymer support, stabilizers, and/or coating agents, in order to enable environmentally friendly ADN-based solid-rocket propulsion.

’ INTRODUCTION Growing environmental concerns over hazardous chemicals used in rocket propellant formulations, such as ammonium perchlorate and hydrazine, have intensified research on alternative green propellants.1
4 Ammonium dinitramide (ADN) and potassium dinitramide (KDN) are two energetic oxidizers based on the exotic dinitramide anion (DN
, N(NO2)2
, 1). ADN is by far the most promising candidate for future propellant formations, as it enables both reduced environmental hazards and improved performance.1,2 Despite almost two decades of academic and industrial research into dinitramide salts, an adequate ADN-based solid-rocket propellant is yet to be presented. The reasons for this are primarily ADN’s anomalous solid-state behavior, and its reactivity toward many chemical environments. Some approaches to the problems, which are currently looked into by various institutions worldwide, are chemical and adhesive coating, stabilizing additives, and various ADN-prilling processes.5
7 Fortunately, after a series of recent computational studies,8
10 the wealth of experimental data11
16 can now be understood in greater detail. It has become increasingly clear that surface processes are of paramount importance for the thermal decomposition of ADN and KDN and for many other dinitramide salts.8
10,14,17,18 Polarized dinitramide anions protruding from crystal surfaces have been theoretically predicted to be the reason for the surprisingly low activation barriers in the solid state.8,9 Quantum chemical modeling of ADN and KDN clusters suggests that such structures are thermodynamically favored on the surface, ahead of the nonpolarized bulklike (X-ray) structure. Such polarized anions, which are twisted relative to the resonance stabilized and semiflat bulk structure, have longer and weaker r 2011 American Chemical Society

nitrogen
nitrogen bonds. Consequently their vibrational characteristics differ significantly from the bulk. Scheme 1 shows the structures of the free dinitramide anion (1, DN
), and some ADN clusters, together with one selected key vibrational mode. As it is likely that surface chemistry plays a key role for the chemical reactivity of these compounds, detailed knowledge of surface structure is important. To verify the theoretical predictions, we decided to investigate both ADN and KDN with surface sensitive vibrational sum frequency spectroscopy (VSFS). The main solid-state decomposition product for both ADN and KDN is nitrate (NO3
),9,11,14,17,19,20 and nitrate has previously been detected on the surface of KDN using X-ray photoelectronic spectroscopy.18 Analyzing KDN is advantageous, as it exhibits a solid-state behavior similar to ADN, while having a significantly simpler chemical structure and decomposition chemistry (due to the lack of hydrogen).9 However, despite the simplified chemical picture, the behavior of KDN is still complex. A thorough differential scanning calorimetry (DSC) study of KDN has shown it to exhibit complicated eutectic, fusion, and liquefaction processes in the solid state.17 Very few sum frequency spectroscopy studies of energetic materials have been performed, and to the best of our knowledge the technique has not been applied to examinations of the molecular surface structure of ADN or KDN. However, two Received: October 20, 2010 Revised: April 11, 2011 Published: May 10, 2011 10588

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Scheme 1. Calculated Harmonic Vibrational Frequencies for the Symmetric Out of Phase NO2 Stretch in the Dinitramide Anion (DN
) and in Some Clusters of ADNa

a

Polarized anions are predicted at ∼1255 cm
1.

VSFS investigations treat the surface structure of single crystals of the related explosive β-HMX.21,22 It was found that the transitions were split and blue-shifted compared to the bulk due to the presence of both free and buried CH2 and NO2 groups at the surface. Additionally, an observation of great importance was the presence of surface deposits of the noncentrosymmetric phase δ-HMX, which because of its bulk contribution to the SF signal could distort the surface SF spectrum of β-HMX appreciably. In addition, a significant number of bulk studies of ADN and KDN have been performed with linear spectroscopic techniques. Important results include detailed vibrational assignments based on comparisons between theoretical calculations, infrared and Raman spectra,23
25 decomposition investigations of KDN,17 and phase changes of ADN.20

’ VIBRATIONAL SUM FREQUENCY SPECTROSCOPY (VSFS) Vibrational sum frequency spectroscopy is a coherent secondorder nonlinear laser spectroscopy technique, which can be used to investigate the molecular structure of all interfaces accessible by the laser beams. For a thorough description of the underlying theories, the reader is referred to reference literature.26
30 The second-order nature of VSFS results in the unique property that it only detects molecules with a net orientation. Thus, under the electric dipole approximation, VSFS is a truly surface sensitive technique, and only probes the molecules residing at the interface between two centrosymmetric media, such as a gas and a liquid/ solid. This property makes it possible to distinguish the very few water molecules in the surface region in a glass of water from the excess of water molecules in the bulk, for example. However, it is important to note that media with no inversion symmetry, such as nonlinear crystals, generate additional SF signal from the bulk material. The technique involves the spatial and temporal overlap of a visible beam at a fixed frequency and a tunable infrared (IR) beam at the sample, which results in the generation of a beam having a frequency equal to the sum of the frequencies of the incident beams. In order to generate a peak in the sum frequency spectrum, the molecular vibrations need to be both IR and Raman active in addition to possessing a net orientation. The sum frequency signal is proportional to the square of the nonlinear polarization induced in the surface region, according to eq 1 ð2Þ

ð2Þ

ISF
jPSF j2
jχeff j2 Ivis IIR

ð1Þ

where ISF is the sum frequency intensity, P(2) SF is the induced is the 27 element macrosurface nonlinear polarization, χ(2) eff scopic second-order nonlinear susceptibility including the Fresnel factors, and Ivis and IIR are the intensities of the visible and infrared beams, respectively. The second-order susceptibility contains a basically frequency independent nonresonant contribution from the substrate (χ(2) NR), and resonant contributions from the n molecular vibrations (χ(2) R,n) contributing to the SF signal, according to eq 2. ð2Þ

χð2Þ ¼ χNR þ

∑n χð2Þ R, n

ð2Þ

The resonant contribution of χ(2) can be expressed in terms of the molecular hyperpolarizability averaged over all orientations, Æβ(2)Ræ, following eq 3 ð2Þ

χR ¼

N ð2Þ Æβ æ ε0 R

ð3Þ

where N is the number of molecules contributing to the SF signal and ε0 is the dielectric permittivity. Thus, in isotropic bulk media with no net orientation, Æβ(2)Ræ is zero and no SF signal is generated. However, at an interface where the symmetry is broken and the molecules possess a net orientation, Æβ(2)Ræ has a nonzero value, resulting in the generation of sum frequency signal. The hyperpolarizability can be expressed in terms of the IR (μc) and Raman (Rab) transition moments, as described in eq 4 βð2Þ


RRβ μγ ωn
ωIR
iΓn

ð4Þ

where ωIR is the frequency of the infrared laser beam, ωn is a vibrational transition frequency, i is the imaginary unit, and Γn is the inverse relaxation time. From eq 4, it is evident that when ωIR approaches ωn and is in resonance with a molecular vibration, the hyperpolarizability and thus the macroscopic susceptibility as well as the SF intensity increase, resulting in a peak in the SF spectrum. For materials possessing isotropy in the surface plane, such as liquids, there are four different polarization combinations of the laser beams yielding a nonzero SF signal, namely, SSP, SPS, PSS, and PPP. The letters refer to the polarizations of the SF, vis, and IR beams, respectively, where S indicates that the beam is polarized perpendicularly to the plane of incidence and P indicates that the beam is polarized parallel to this plane. 10589

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Table 1. Calculated Products of IR and Raman Spectraa νs(NO2) out of phase

νs(NO2) out of phase

νs(NO2) in phase

νa(N3)

(inkl. νa(N3))

(inkl. νa(N3))b

(inkl. νs(N3))

1027 (136)

1200 (4549)

DN
1

DN


1329 (1918)

otherc 1448 (3738) 1511 (6467)

TS1

(DN
twisted)

1047 (353)

1256 (7713)

1365 (1899)

1418 (2224) 1555 (4594)

ADN 2

ADN dimer (Ci symmetric)

3

ADN dimer

νs(NO2) out of phase

νs(NO2) out of phase

νs(NO2) in phase

νa(N3)

(inkl. νa(N3))

(inkl. νa(N3))b

(inkl. νs(N3))

other c

1023 (2326)

1174 (8189)

1251 (9556)

1305 (1035)

1393
1770

1052 (730) 4

ADN dimer

5

ADN dimer

1326 (427)

1029 (75) 1052 (687)

1173 (5214)

1263 (18322)

1020 (130)

1177 (7941)

1240 (21178)

1058 (606) 6

ADN trimer

ADN tetramer

1012 (161)

1189 (965)

1035 (121)

1212 (5128)

1261 (8028)

8

KDN dimer (Ci symmetric)

9

KDN dimer

11 12

1166 (1490)

1044 (3687) 1046 (456)

1212 (3764) 1235 (17113)

1255 (9387)

1297 (1980)

1356 (1995) νs(NO2) out of phase

νs(NO2) out of phase

νs(NO2) in phase

νa(N3)

(inkl. νa(N3))

(inkl. νa(N3))b

(inkl. νs(N3))

otherc

999 (116)

1208 (4468)

1240 (15953)

1320 (134)

1427
1602

1345 (770)

997 (139)

1208 (4897)

1319 (361)

1408
1578

KDN dimer

1064 (673) 998 (134)

1220 (21503) 1163 (1634)

1358 (2410) 1310 (8780)

1470
1583

1045 (170)

1209 (4272)

1318 (90)

KDN trimer

KDN tetramer

14

NO3


15

AN trimer

AN tetramer

KN 17

1392
1712

1317 (2255) 1322 (814)

KDN dimer

AN

16

1360
1710c

1330 (228)

1030 (179)

1029 (536)

1200 (377)

1043 (220)

1217 (8091)

1240 (17410)

KN trimer

1331 (1960)

1422
1595

1335 (1684)

1061 (362) 13

1394
1710

1325 (1254)

1059 (251) 10

1304 (373) 1311 (1822)

1058 (460)

KDN

1409
1713

1343 (1943)

1053 (895) 7

1304 (1564) 1331 (1979)

1349 (1074)

1041 (453)

1206 (2265)

1336 (1107)

1042 (1387)

1214 (930)

1341 (225)

1048 (1095) 1058 (1358)

1226 (8930) 1234 (34980)

1348 (3665) 1350 (435)

νs(NO3)

νa(NO3)

1004 (2452)

1286 (1750)

1028 (1091)

1289 (1414)

1445
1586

otherc

1359 (10351)

1062 (11)

1367 (616)

1028 (1046)

1290 (769)

1033 (429)

1293 (3009)

1035 (1447) 1036 (482)

1304 (992) 1321 (1026)

νs(NO3)

νa(NO3)

1379
1718

1391
1720

νa(NO3)

1038 (147)

1334 (1898)

1434 (1)

1038 (164)

1338 (3781)

1447 (3792)

1039 (466)

1352 (738)

1449 (7332)

10590

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Table 1. Continued 18

KN

νs(NO3)

νa(NO3)

νa(NO3)

KN tetramer

1037 (613)

1323 (2426)

1400 (1353)

1045 (17) 1046 (36)

1344 (2187) 1366 (254)

1416 (2876) 1438 (2023)

1049 (530)

1380 (1062)

1460 (3863)

a

Analytical B3LYP/6-31þG(d,p) frequencies have been scaled by 0.97. Intensities are shown within parentheses. b Corresponds to polarized (twisted) dinitramide anions. c A multitude of closely spaced lattice, N
H, and N
O vibrations that have not been analyzed in detail. Higher wavenumbers corresponding to H-stretching have been omitted (>2600 cm
1).

The four polarization combinations probe different elements of the χ(2) tensor as follows SSP : χð2Þ YYZ SPS : χð2Þ YZY PSS : χð2Þ ZYY PPP : χð2Þ XXZ þ χð2Þ XZX þ χð2Þ ZXX þ χð2Þ ZZZ where χ(2)XXZ= χ(2)YYZ, χ(2)XZX = χ(2)YZY = χ(2)ZXX= χ(2)ZYY far away from electronic transitions, as in these experiments. The polarization combinations SPP, PSP, and PPS probe the elements χ(2)XYZ, χ(2)YXZ, χ(2)YZX, χ(2)XZY, χ(2)ZXY, and χ(2)ZYX, which all equal zero unless the surface plane is nonisotropic or the bulk structure is noncentrosymmetric.

’ EXPERIMENTAL SECTION Materials. Ammonium dinitramide (batch ADN-0546172, >99.2%) and potassium dinitramide (batch KDN-502, >99.5%) were kindly supplied by Eurenco Bofors. Ammonium nitrate (AN, >99%, Sigma-Aldrich) and potassium nitrate (KN, >99.0, Sigma-Aldrich) were recrystallized in water and 2-propanol, respectively, before use. 2-Propanol (99.8%) was purchased from Sharlau Chemie. Crystallization of ADN and KDN. One hundred milligrams of ADN and KDN, respectively, were added to mixtures of 1.0 mL of 2-propanol and 0.1 mL of distilled water. After the mixture was stirred, any nondissolved salt was removed through filtration. The solutions were left to evaporate slowly under dry air. KDN crystallized as sheets with a size of approximately 2  2 mm and less than 1 mm thick, and ADN formed smaller and more needleshaped crystals. These crystals were obtained after re-crystallization in pure 2-propanol, using seeding crystals from the first crystallizations. The final crystals were stored under dry air, as well as in saturated solution of 2-propanol. The qualities of all samples were investigated using a light polarizing stereomicroscope. Since optically anisotropic crystals rotate the plane of light, they appear either light or dark upon rotation, making cracks or polycrystallinity easy to detect.31 All crystal batches were shown to contain exclusively single crystals. When ADN or KDN were instead stored under drying conditions (e.g., using drying gel or vacuum) the crystals disintegrated over time to form polycrystalline solids or powders (these were not used for measurements). Apparatus. A detailed description of the sum frequency spectrometer is found elsewhere32 and here only the most

important information is provided. The heart of the sum frequency spectrometer is a Nd:YAG laser (Ekspla PL2143A) with a fundamental output wavelength of 1064 nm, a pulse length of 24 ps, a repetition frequency of 20 Hz, and an output energy of 30 mJ. It is used to pump an optical parametric generator/optical parametric amplifier (OPG/OPA) from Laservision, generating two output beams. One has a fixed wavelength of 532 nm and the other one is tunable on the infrared region, covering the spectral range 1000
4000 cm
1. In these experiments, the IR beam had an energy in the range 10
70 μJ, and the visible beam 800 μJ. The bandwidth of the IR beam was