Introduction

The need for materials that combine large nonlinear optical characteristics with resistance to physical and chemical attacks has led to the synthesis and investigation of semi-organic compounds [1, 2].

The advantages of high optical nonlinearity and chemical flexibility of organic materials, on the one hand, and the thermal stability and mechanical robustness of inorganic materials, on the other hand, confer to these compounds’ properties which make them suitable for applications in nonlinear optics (NLO) [3, 4]. Amino acids and strong inorganic acids are good raw materials to produce semi-organic crystals [5,6,7,8,9,10,11]. Amino acids possess good optical properties as they contain a proton-donor carboxyl (–COOH) group and a proton-acceptor amino (–NH2) group leading to extensive hydrogen bond networks [12, 13]. Except glycine, all amino acids have chiral symmetry and several compounds crystallize in non-centrosymmetric space groups [11, 12, 14].

Searching for improved properties, a great amount of work was performed involving the synthesis, structural studies and optical properties of organic–inorganic compounds derived directly from natural amino acids or some amino acid salts. However, there is no systematic study allowing to evidence the effect of the different amino acid ligands (cations) on the thermodynamic properties and thermal stability of the complexes formed with a particular inorganic acid. Besides, the information concerning the correlation existing between composition, crystal structure and thermochemical properties of these materials is rather scarce. To experimentally investigate the thermal behavior, differential thermal analysis (DTA), differential scanning calorimetry (DSC) or thermogravimetry (TG) have been generally used. Tao et al. [15] carried out DSC, DTA and TG measurements in order to investigate the thermal behavior of a series of amino acid salts [AA]X (X stands for Cl−, NO −3 , BF −4 , PF6−, 1/2SO 2−4 , CF3COO−) having amino acid or amino ester acid cations, some of these compounds being considered ionic liquids (ILs) due to their low melting temperatures (< 373.15 K). Thermal properties of diglycine hydrobromide were studied in powder form by recording the TGA/DTA response curve in the temperature range 301.15–1073.15 K [12]. The thermogravimetric and differential thermal analyses of mono-l-alaninium nitrate (MAN) were performed between 323.15 and 673.15 K by Vimalan et al. [5]. DTA and TG have been also used by Pal et al. [6] for the investigation of mixed crystals of two nonlinear optical materials, l-arginine hydrochloride monohydrate (LAHCl) and l-arginine hydrobromide monohydrate.

Instead, there are only a few studies reporting information on relevant experimental thermodynamic functions of these compounds, suitable for evaluation of their phase stability. To our best knowledge, except two works (our recent study on the formation and combustion enthalpies of l-alanine nitrate and l-alanine ethyl ester nitrate [16] and the research on the enthalpies of formation of several structurally similar glycine-based sulfate/bisulfate amino acid reported by Zhu [17]), there are no other experimental studies on the thermodynamic data of formation of organic–inorganic compounds having amino acid cation. Particularly, except for our data reported in the above-mentioned paper [16] there are no other experimental values of combustion and formation enthalpies mentioned in the literature for the amino acid nitrates (AANO3).

Continuing our previous investigations on the thermodynamic properties of the amino acids [18,19,20] and their derivatives [16] the goal of the present paper is to perform a systematic study of the thermodynamic stability of a series of nitrates (AANO3) synthesized in our laboratory having cations derived from the α-amino acids glycine (Gly), l-proline (Pro) and l-glutamine (Gln) and to provide new experimental thermodynamic data regarding their enthalpies of combustion and formation. Of particular interest to us is to evidence the strong interplay between thermodynamic behavior, composition and structural dynamics by using techniques such as combustion calorimetry and DSC, as well as FTIR, Raman and UV–Vis spectroscopy.

Experimental

Material synthesis and chemical identification

The nitrates of glycine, l-glutamine and l-proline were prepared starting from the corresponding amino acids: glycine (Merck, assay > 99.7%), l-glutamine (Merck, assay > 99%), l-proline (Merck, assay > 99%) and nitric acid (HNO3 from Merck, 65%, density 1.4 kg L−1) taken in stoichiometric ratio 1:1. The required amounts were estimated taking into account the following general reaction:

$$\text{RCH} \left( \text{NH}_{2} \right) \text{COOH} + \text{HNO}_{3} \to \text{RCH} \left( \text{NH}_{3}^{ + } \right) \text{COOH} \cdot \text{NO}_{3}^{ - }$$
(1)

Appropriate amounts of amino acid were first dissolved in deionized water, and then the solutions were slowly added to the solution of nitric acid on stirring. The resulting mixtures were allowed to evaporate at room temperature for 24 h and then were kept in the oven at 333 K for 12 h. Purity of the synthesized salt was further improved by successive recrystallization processes (see Table 1). GlyNO3 and GlnNO3 are solids, and ProNO3 is gel at room temperature. All samples were dried under vacuum at 333 K before starting the experiments. The water content in the samples was determined by using a Metrohm 848 TitrinoPlus Karl Fischer titrator (Table 1). Elemental analysis of AANO3 was performed using a EuroVector model EA3000 CHNS elemental analyzer, the results being included in Table S1 (Supplementary Material). The elemental analysis indicated that the contents of C, H and N are in good agreement with the values based on the structure formula.

Table 1 Sample table
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Apparatus and methods

FTIR and Raman spectroscopy

The various functional groups present in the samples were identified and confirmed by recording the FTIR spectra at room temperature, in the 4000–525 cm−1 range, using a Thermo Scientific Nicolet iS10 FTIR spectrophotometer with attenuated total reflection (ATR) method. Sixty-four scans were accumulated for each spectrum. All ATR spectra were collected with a spectral resolution of 4 cm−1.

Unpolarized Raman spectra of the glycine, glutamine and proline nitrate materials were recorded by means of a LabRam HR spectrometer (Jobin–Yvon–Horiba) over 50–4000 cm−1 range. The 514-nm line of an Ar+ laser was used as exciting radiation through a 50 LWD objective of an Olympus microscope in a backscattering geometry and at a confocal hole of 200 μm. The spectral resolution was better than 2 cm−1. Spectra of glycine, l-proline and l-glutamine were collected for comparative purposes. The resulting spectra were background-corrected and curve-fitted by using Igor software, version 6.2 [21]. Fitting residuals were kept below 2%. Incident laser power below 8 mW was used to avoid local heating and graphitization effects.

UV–Vis spectrophotometry

The UV–Vis spectrum of ProNO3 was recorded with an Ocean Optics High-Resolution Fibber Optic Spectrometer HR 4000 in the range 200–1100 nm. The optical resolution was ± 0.27 nm. For GlyNO3 and GlnNO3 the spectra were obtained with UV–Vis Jasco V 550 spectrophotometer (the resolution of ± 1 nm and wave length accuracy ± 0.3 nm, in a 10-mm quartz cell and 60-mm-diameter integrating sphere) with scanning speed of 1000 nm/min. The optical band gap energy is estimated using the formula Eg = h ν, and the data are also presented in Table 1. The h is the Plank’s constant (h = 6.626 × 10−34 J s), and ν is frequency of light (ν = c/λ, c = 3 × 109 m s−1, and λ is wavelength measured in nm).

Polarimetry

Considering the importance of the chiral properties of molecules in NLO applications, as well as in the field of ionic liquids, polarimetric measurements have to be carried out. The measurements of optical rotation were performed with a 341 PerkinElmer polarimeter using a 1-cm path length glass cell. Determinations were made for solutions of the three nitrates in deionized water, at 25 °C, in D line of sodium. The values of specific rotation [α] 25D are obtained.

Combustion calorimetry

A Parr Instruments model 6200 microprocessor-controlled isoperibol oxygen bomb calorimeter was used. Samples were burned by using the semi-micro-combustion kit of the calorimeter provided with a small bomb, inner volume 22 mL. The bomb was placed into the bucket, which contained 450 mL of water, filled from an automatic pipet. The temperature was measured with a high-precision electronic thermometer using a specially designed thermistor sensor. Measurements were taken with resolution of 0.0001 K. Purity of the oxygen used for combustion was 99.998%.

The energy equivalent of the calorimeter, εcalor = (2339.6 ± 3.6) J K−1 (twice the standard deviation of the mean), was derived from combustion of the standard benzoic acid (CAS 65-85-0) supplied by Parr Company, from ten calibration experiments, using samples weighing between 0.10 and 0.19 g. The massic energy of combustion under certificate conditions of benzoic acid was Δ benzoic acidu° = − 26,434 J g−1.

The samples were weighed using a Mettler Toledo microbalance with an accuracy of ± 2×10 −6 g. The water content in the samples measured by Karl Fischer titration was used to make the appropriate corrections for the combustion results. For combustion of proline nitrate we checked if this compound is hygroscopic, by determining the water content (using Karl Fischer titration) before combustion experiment and then after 15 min, period of time when the sample was maintained in the presence of the deionized water from the bomb. No change in the water content in the samples has been registered. The buoyancy effect was taken into account in the experiments, and the masses were corrected using densities listed in Table S4 in Supplementary Material. The water volume added to the bomb for dissolution of combustion gases was 0.5 cm3. The bomb was flushed with purified oxygen for 30 s, then sealed and filled with oxygen. The samples were burned in oxygen at a pressure of 3.04 MPa. The final solution from the bomb was analyzed for the presence of nitric acid (about 20% from the total nitrogen) by titration with 0.0709 N Na2CO3 solution. The heat due to nitric acid formation was obtained using the value of the molar internal energy of formation of nitric acid solution (0.1 M), Δfu°HNO3,aq = − 58.8 kJ mol−1 [22].

Auxiliary materials—cotton and Ni–Cr filament—were used in combustion measurements. The corrections for the auxiliary materials, q(cotton) and q(ign), were calculated from the mass of cotton and the mass of the filament, the combustion enthalpy Δcu°(cotton) = − (16,240 ± 4.8) J g−1 (determined in our laboratory, the value being in agreement with Ref. [23]) and Δcu°Ni–Cr) = − 5.86 kJ g−1 (certified by the fabricant). The experimental values of massic energy of combustion, Δcu, were brought to the standard state (T = 298.15 K and P = 105 Pa), the corrections being made following the procedure recommended by Hubbard et al. [24] and using the data from Tables S4a, S4b, S4c from Supplementary Material.

DSC measurements

The thermal behavior of the samples under non-isothermal regime was investigated by scanning calorimetry (DSC) using a computer-controlled PerkinElmer power-compensated DSC (model 8500) calorimeter.

Samples were placed in aluminum sealed pans, while an empty pan was used as reference. Temperature and heat flow rate scale of the DSC was calibrated by measuring high-purity indium (Tfus = 429.8 K and ΔHfus = 28.4 J g−1). The DSC curves of the studied compounds were recorded at a heating rate of 10 K min−1 under nitrogen of purity > 99.996% with a flow rate of 20 mL min−1. The Pyris Software for Windows was used in order to calculate the thermal effects (melting/decomposition enthalpies).

Results and discussion

Vibrational modes

IR and Raman spectroscopies have proved useful in identification of the amino acid and their derivatives, as well as for studying changes in the hydrogen bond lengths, which govern structural properties of these compounds [25, 26]. IR and Raman spectra of the (Gly/Pro/Gln)NO3 along with the corresponding amino acids Gly, Pro and Gln are illustrated in Fig. 1, while positions and band assignments [25,26,27,28,29,30,31,32,33,34] of the bands are listed in Table S2 in Supplementary Material.

Fig. 1
figure 1

Vibrational spectra of the (Gly/Pro/Gln)NO3 and of the corresponding AA: a IR and b Raman (*liquid phase)

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Upon HNO3 reaction with Gly, splitting (Δ = νa[CH2] − νs[CH2]) between νa(CH2) and νs(CH2) (see inset of Fig. 1b) increases to 63 cm−1 in comparison with Gly where Δ is of 33 cm−1. Smaller splitting value [35] recorded for GlnNO3 indicates protonation of the terminal amine group and/or variation of the hydrogen bond strength under NO −3 influence. This finding is also supported by the lack of the IR and Raman bands at 3400 cm−1 due to the νa(NH2) for the GlnNO3.

Evidence beyond doubt of the NH +3 groups consists in their IR finger prints, i.e., deformation, δ(NH +3 ), and torsion modes, τ(NH +3 ), at 2116 cm−1 for Gly and 2038 cm−1 for Gln (Fig. 1a). Hydrogen atoms of the NH +3 group, giving rise to three bonds N+-H···O, cause different spectral features in the IR spectrum within 2600–3400 cm−1 of Gly in comparison with the ones of GlyNO3 (Fig. 1a).

All the solid samples (Gly/Gln)NO3 and the ionic liquid ProNO3 [36] investigated here show evidence of the ν1(NO3) modes at about 1040 cm−1 in their IR and Raman spectra.

IR bands at about 1700 cm−1 and 1200 cm−1 (Fig. 1a and Table S3 in Supplementary Material) indicate C=O and COH stretching modes of the COOH groups from (Gly/Pro/Gln)NO3.

Low vibrational bands (< 600 cm−1) in the Raman spectra of (Gly/Pro/Gln)NO3 in Fig. 1b are assignable to skeletal deformations of COOH groups, lattice modes (< 200 cm−1), vibrational and translational (105 cm−1 [31]) motions of NO −3 and hydrogen bonds within 50–300 cm−1 region [31, 37]. Thus, fitting of the Raman band within 450–540 cm−1 domain reveals that torsional mode τ(NH +3 ) at 491 cm−1 for Gly undergoes splitting into 494 cm−1 and 498 cm−1 in case of GlyNO3 (see Fig. 1b) pointing out the existence of two types of N+–H···O bonds due to NH +3 interactions with both COO− and NO −3 . Moreover, intense Raman band at 122 cm−1, attributed to in-phase vibrations of atoms involved in ionic N–H···O hydrogen bonds [37], might indicate the hyperpolarizability of the GlyNO3 crystals, providing a structure without center of symmetry.

UV–Vis spectra

The UV–Vis spectra for AANO3 are shown in Fig. 2. These show a wide transparency window as there is no absorption in the region between 350 and 1050 nm. The values of the UV cutoff wavelengths are shown in Table 2.

Fig. 2
figure 2

Absorption spectra of the synthesized AANO3

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Table 2 Characteristics of the UV–Vis spectra corresponding to the nitrogen derivatives
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The smallest value of the cutoff wavelength of 337 nm is recorded for the simplest amino acid nitrate, namely GlyNO3. The width of the transparency window decreases in succession: ProNO3 > GlnNO3 > GlyNO3. Moreover, wide transparency windows and band gap energy values (Eg) point out suitability of these materials for optoelectronic applications.

Polarimetric measurements

Polarimetry was used for checking chirality of the asymmetrical carbon atom in the nitrates synthesized in the laboratory.

The calculated specific rotation values were as follows: [α] 25D  = − 39.8° mol−1 for ProNO3 (solution in water and c = 1 g 100 mL−1) and [α] 25D  = + 7.76° mol−1 for GlnNO3 (solution in water and c = 2.5 g 100 mL−1). The chirality of these compounds is preserved during the processes of preparation and purification. Conversely, the optical rotation value for GlyNO3 is 0 (solution in water and c = 20.4 g 100 mL−1) despite the hyperpolarizability of the GlyNO3 crystals noticed in the low-frequency Raman spectrum in Fig. 1b.

Combustion measurements

The molar standard combustion enthalpies ΔcH om of the compounds are calculated from the constant-volume combustion energy ΔcU om :

$$\Delta_{\text{c}} H_{\text{m}}^{o} = \Delta_{\text{c}} U_{\text{m}}^{o} + \Delta nRT$$
(2)

where Δn is the change in gas mole number (products vs. reactants) during the combustion reaction.

The following combustion reactions have been taken into account:

GlyNO3 combustion:

$${\text{C}}_{2} {\text{H}}_{6} {\text{N}}_{2} {\text{O}}_{5} \left( s \right) + {\text{O}}_{2} \left( g \right) \to 2{\text{CO}}_{2} \left( g \right) + 3{\text{H}}_{2} {\text{O}}\left( l \right) + {\text{N}}_{2} \left( g \right)\Delta n = 2$$
(3)

GlnNO3 combustion:

$${\text{C}}_{5} {\text{H}}_{11} {\text{N}}_{3} {\text{O}}_{6} \left( s \right) + 4.75{\text{O}}_{2} \left( g \right) \to 5{\text{CO}}_{2} \left( g \right) + 5.5{\text{H}}_{2} {\text{O}}\left( l \right) + 1.5{\text{N}}_{2} \left( g \right)\Delta n = 1.75$$
(4)

ProNO3 combustion:

$${\text{C}}_{5} {\text{H}}_{10} {\text{N}}_{2} {\text{O}}_{5} \left( l \right) + 5{\text{O}}_{2} \left( g \right) \to 5{\text{CO}}_{2} \left( g \right) + 5{\text{H}}_{2} {\text{O}}\left( l \right) + {\text{N}}_{2} \left( g \right)\Delta n = 1$$
(5)

Tables S4a, S4b, S4c from Supplementary Material show the individual values of the combustion experiments of AANO3. A total of over ten runs were performed for every material, but some runs were rejected because of doubt about combustion completeness. In runs used in data calculation, there was no evidence of soot formation in the bomb. Five runs were retained for GlyNO3, GlnNO3 and ProNO3.

In Table 3 are given the derived molar quantities from the mean standard massic energy of combustion for each compound. ΔcU °m and ΔcH °m are the standard molar energy and enthalpy of combustion, respectively, and ΔfH °m is the calculated standard molar enthalpy of formation. As recommended [24, 39], the uncertainties associated with these molar quantities are twice the overall standard deviation of the mean, and they include the uncertainties associated with the calibration experiments (uncertainties of massic energy of combustion of benzoic acid and energetic equivalent of the calorimeter) and auxiliary substances.

Table 3 Thermochemical data at T = 298.15 K (p° = 0.1 MPa) for AANO *3
Full size table

Enthalpies of formation of the studied nitrates are calculated according to Eq. (6) from the corresponding enthalpies of combustion and taking into account the known enthalpies of formation of CO2 and H2O: ΔfH °m CO2(g) = − 393.51 ± 0.13 kJ mol−1 and ΔfH °m H2O(l) = − 285.83 ± 0.042 kJ mol−1 [38].

$${\triangle}_{{\rm f}} {\text{H}}_{{\rm m}}^\circ = n_{{{{\rm CO}}_{2} }} \triangle_{{\rm f}} H_{{\rm m}}^\circ {\text{CO}}_{2} \left( {\text{g}} \right) + \, n_{{{{\rm H}}_{2} {\text{O}}}} \triangle_{{\rm f}} H_{{\rm m}}^\circ {\text{H}}_{2} {\text{O}}\left( l \right) - \triangle_{\text{c}} H_{{\rm m}}^\circ$$
(6)

The uncertainty assigned to ΔfH °m includes the uncertainties of the enthalpies of formation of the reaction products H2O and CO2.

In Table 3 are also included the thermochemical data of AlaNO3 previously reported by us [16], for comparison. Among these amino acid nitrates, the highest enthalpy of formation value (− 764.4 kJ mol−1) belongs to ProNO3 and the lowest one (− 1011.3 kJ mol−1) is the formation enthalpy of GlnNO3. The influence of the chemical structure of the amino acid cation on the thermochemical properties of the synthesized derivatives has been evidenced. The trend in the variation of the enthalpy of formation in the series of the studied amino acid nitrates is as follows: GlnNO3 > GlyNO3 > AlaNO3 > ProNO3. Since the cationic form was IR depicted for all the AANO3 under discussion, the latter succession obeys hydrogen bonds involving N-containing groups, i.e., five different hydrogen bonds of the NH +3 and NH2 groups from GlnNO3, three hydrogen bonds of the NH +3 from GlyNO3 and AlaNO3 and only two hydrogen bonds of the NH +2 in ProNO3.

In Fig. 3 our data for the enthalpies of formation of the AANO3 are given together with the values of the corresponding amino acids from the literature [40,41,42,43,44,45].

Fig. 3
figure 3

Enthalpies of formation of the AANO3 studied in this work (filled bars) and the data of the corresponding AA taken from the literature (white bars)

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More negative enthalpies of formation of AANO3 than those of the corresponding amino acids point out that nitrate compounds are more stable. The contribution of the additional chemical bonds to the energetic parameters affects their enthalpies of formation. The enthalpy of formation of GlyNO3 is about 273.6 kJ mol−1 lower than that of Gly, the difference being comparable with the difference of about 237.6 kJ mol−1 found between the enthalpies of formation of ProNO3 and Pro. The GlyNO3–Gly and ProNO3–Pro pairs contain N+-H···O bonds, in contrast to GlnNO3–Gln pair which contains also two N–H···O bonds of the amide group [46] with different bond strengths. Hence, in the case of GlnNO3–Gln pair the difference is smaller, indicating that nitrate formation does not change too much the energetic contribution of the hydrogen bonds.

DSC runs

The DSC curves of all samples have been recorded in the temperature range 200–523 K. Distinct thermal behavior of AANO3 is illustrated in Fig. 4.

Fig. 4
figure 4

DSC curves of AANO3

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For comparison in Table 4 are also included the Tonset values of melting and/or decomposition reported by different authors from DTA or DSC/TG measurements. A very good agreement of the decomposition temperatures of GlyNO3 and ProNO3 obtained in our work with the data reported by Tao et al. [15] from TGA measurements could be noticed (Table 4). Concerning the melting point of GlyNO3, the value of 419 K obtained by us is very near the value of 418.85 K obtained by Dhas [4] from DTA/TG data when working at a heating rate of 10 K min−1 (like in our conditions of measurements).

Table 4 Thermodynamic parameters for melting/decomposition of AANO3 from DSC measurements
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The initial temperature of decomposition and the total enthalpy of decomposition are also important parameters when assessing the thermal stability of amino acids and their derivatives. Thermal stability, referring to the initial temperature of decomposition, decreases in succession GlyNO3 > GlnNO3 > ProNO3 due to longer side chains of the L-Gln and L-Pro in comparison with Gly [47]. The same order is recorded in the case of the decomposition enthalpies.

Thermal stability of these compounds is ruled by breaking bonds at different positions in accordance with their side chains. Despite the same number of carbon atoms, the side chain of L-Pro consists in a pyrrolidine ring (C4H9N), while an amide (C4H9N2O) represents the side chain of L-Gln. Moreover, the lack of the νa(NH2) modes at 3402 cm−1 in the IR and Raman spectra of GlnNO3 in contrast to L-Gln (Table S2 in Supplementary Material) points out the participation of NH2 of the amide group in additional N–H···O bonds formation. Tao et al. [36] also reported different thermal stabilities of amino acid derivatives in accordance with the thermal stability of the corresponding amino acids. Hence, the thermal stability scale based on the decomposition parameters could be explained by the fact that the less stable cations presented here are those of Gln and L-Pro. Although AA loss carboxyl group by decomposition, the presence of the heteroatoms in the side chains (here L-Pro and L-Gln) can hinder losing of the carboxyl groups and trigger further cleavage of other bonds from side chains [47].

In Fig. 5, the total heats of decomposition of the GlyNO3, ProNO3 and GlnNO3 are compared with the heats of decomposition values of the corresponding amino acids obtained in our previous works [43, 44, 48]  by using the same experimental conditions. For comparison, the data for AlaNO3 [16] and Ala [47, 48] are also included in Fig. 5.

Fig. 5
figure 5

Total decomposition enthalpies of the studied AANO3 (empty triangles) and of the corresponding AA (filled circles)

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One can observe, without exception, a shift to lower values for nitrates, the result being explained by the formation of the N+–H···O and N–H···O bonds with NO −3 . It should be noted that the exothermic decomposition is influenced by the presence of an additional functional group that determines the decreasing of the decomposition temperatures and changes in the developed energy.

Conclusions

In the present paper a systematic study of the thermodynamic stability of a series of nitrates (AANO3) synthesized in our laboratory having cations derived from the α-amino acids glycine (Gly), l-proline (Pro) and l-glutamine (Gln) has been performed. The strong interplay between composition, structural dynamics and thermodynamic behavior has been evaluated by using techniques as combustion calorimetry and DSC, as well as FTIR, Raman and UV–Vis spectroscopy.

For the first time the combustion and formation enthalpies of the synthesized nitrates have been measured by combustion calorimetry. A critical comparison of the formation enthalpy of nitrate compounds with that of the corresponding amino acids indicates increased stability of the derivatives. At the same time, the influence of the cation structure (type of the side chain R and number and type of the hydrogen bonds) on the thermochemical stability of the nitrates was emphasized.

The initial temperature of decomposition and the total enthalpy of decomposition obtained from DSC measurements show without exception, a shift to lower values of both these parameters for nitrates, the result being explained by the formation of the N+–H···O and N–H···O bonds with NO −3 . The exothermic decomposition evidenced by DSC measurements is influenced by the presence of an additional functional group and length side chain that determines the decreasing of the decomposition temperatures and changes in the developed energy.

Large transmittance in the visible region and band gap energy value obtained from UV–Vis spectra suggest that these materials are suitable for optoelectronic applications.

Supplementary material

The elemental analysis of AANO3, the NMR spectra, IR and Raman data (band position and assignments) of the (Gly/Pro/Gln)NO3 and of the corresponding amino acids, as well as the combustion experiments results of the amino acid nitrates, are included in Supplementary Material.