The salt (l-ProH···l-Pro)(I3) (I) crystallizes in the monoclinic polar space group P21 (Table 1). The asymmetric unit contains one formula unit (Fig. 1). Selected bond lengths and the angle in the triiodide anion are listed in Table 2, while geometric parameters of hydrogen bonds are provided in Table 3. The packing diagram in the structure of (I) is shown in Fig. 2. The intramolecular bond lengths of protonated l-ProH and zwitterionic l-Pro have typical values and are similar to those of (l-ProH···l-Pro)(I) (Table 2). The bond lengths and the angle of the I3− ion are as expected (see [1, 2]).
Table 2
Selected bond lengths and the angle of I3− ion (in Å and °) for (l-ProH···l-Pro)(I3) (I) and (l-ProH···l-Pro)(I) [9] for comparison.
| Bonds | A | B | A [9] | B [9] |
| C1-O1 | 1.294(5) | 1.279(4) | 1.292(2) | 1.274(2) |
| C1-O2 | 1.218(5) | 1.226(4) | 1.217(2) | 1.234(2) |
| C1-C2 | 1.517(4) | 1.526(6) | 1.514(3) | 1.516(2) |
| C2-C3 | 1.526(6) | 1.547(6) | 1.518(3) | 1.523(3) |
| C3-C4 | 1.489(8) | 1.511(10) | 1.527(3) | 1.523(3) |
| C4-C5 | 1.506(8) | 1.471(8) | 1.515(3) | 1.526(3) |
| C5-N1 | 1.495(6) | 1.484(5) | 1.506(5) | 1.512(3) |
| N1-C2 | 1.499(5) | 1.497(4) | 1.497(3) | 1.499(2) |
| I1-I2 | 2.9213(5) | | | |
| I2-I3 | 2.8805(5) | | | |
| I1-I2-I3 | 178.525(14) | | | |
The dimeric cation (l-ProH···l-Pro) in the structure of (I) is formed due to a strong hydrogen bond O1A-H1A···O1B with an O···O distance of 2.458(4) Å (Table 3). This value is close to that of (l-ProH···l-Pro)(I) (2.454(2) Å) [10]. In (I), carbonyl atoms have a trans-arrangement relative to the hydrogen bond in contrast to (l-ProH···l-Pro)(I), where they are in a cis-arrangement. The NH2+ groups of A- and B-moieties form hydrogen bonds with the nearest oxygen atoms, but not with the triiodide anion, while there are five C-H···I type contacts (Table 3). Some of them may be considered as weak hydrogen bonds. Notably, the triiodide anions in the structure of (I) are not connected to each other by halogen bonds.
Table 3
Hydrogen bond parameters (in Å and °) for (l-ProH···l-Pro)(I3) (I).
| D-H···A | D-H | H···A | D···A | DHA |
| O1A-H1A···O1B | 0.93 | 1.55 | 2.458(4) | 166 |
| N1A-H11A···O1B i | 0.91 | 1.90 | 2.783(4) | 164 |
| N1A-H12A···O2A | 0.91 | 2.24 | 2.727(4) | 113 |
| N1A-H12A···O2B ii | 0.91 | 2.05 | 2.869(4) | 148 |
| N1B-H11B···O1A iii | 0.91 | 1.99 | 2.865(4) | 162 |
| N1B-H12B···O1A | 0.91 | 1.99 | 2.870(4) | 162 |
| N1B-H12B···O2A iv | 0.91 | 2.08 | 2.865(4) | 144 |
| N1B-H12B···O2B | 0.91 | 2.19 | 2.685(4) | 113 |
| C3A-H31A···I2 | 0.99 | 3.06 | 4.035(5) | 169 |
| C3A-H32A···I1 v | 0.99 | 2.94 | 3.867(5) | 157 |
| C4B-H42B···I3 vi | 0.99 | 3.07 | 4.052(5) | 170 |
| C5B-H51B···I2 vi | 0.99 | 3.15 | 3.993(5) | 143 |
| C5B-H52B···I2 iv | 0.99 | 3.14 | 3.975(5) | 143 |
| Symmetry code: (i) -x + 2, y-1/2, -z; (ii) x, y-1, z; (iii) -x + 2, y + 1/2, -z + 1; (iv) x, y + 1, z; |
(v) -x + 1, y + 1/2, -z; (vi) -x + 1, y + 1/2, -z + 1.
The salt [(l-ProH)3(l-Pro)](I3)3 (II) also crystallizes in the monoclinic polar P21 space group (Table 1). Selected bond lengths and angles are listed in Table 4, while the geometric parameters of hydrogen bonds are provided in Table 5. The salt (II) with a formal composition of [(l-ProH)3(l-Pro)](I3)3 has an unusual structure. In Fig. 3 the molecular motif consisting of A-, B-, C-, and D-moieties is shown. The B- and C-moieties form a pseudocentrosymmetric dimer via a very strong hydrogen bond O1C-H1C-O1B with an O···O distance of 2.427 Å (Table 5). The bond lengths C1C-O1C (1.274(4) Å) and C1B-O1B (1.270(4) Å) are alike within the error limits and are characteristic for dimers with very short O···O distances. This dimer, in turn, forms hydrogen bonds with cationic A- and D-moieties via O1A-H1A···O2B and O1D-H1D···O2C hydrogen bonds (Table 5).
Bond lengths C1A-O1A, C1A-O2A and C1D-O1D, C1D-O2D (Table 4) are characteristic for carboxylic groups. So, the cationic part in the structure of (II) can be presented as a tetramer [l-ProH···(l-Pro-H-l-Pro)···l-ProH]3+ which is balanced by three triiodide anions: (I1-I2-I3)−, (I4-I5-I6)− and (I7-I8-I9)−. The bond lengths and angles of triiodide anions are shown in Table 4. These three triiodide anions are connected to each other by supramolecular halogen bonds I3···I4 and I6···I7 (Fig. 4). Although the average bond lengths (2.9203 Å, 2.9251 Å, 2.9241 Å, respectively) are similar to each other, it is worth noting that for the middle triiodide anion (I4-I5-I6)− surrounded on both sides, the bond lengths (2.9235 Å and 2.9268 Å) are closer than in the other two cases (Table 4).
Table 4
Selected bond lengths and angles (in Å and °) for [(l-ProH)3(l-Pro)](I3)3 (II).
| Bonds | A | B | C | D |
| C1-O1 | 1.304(5) | 1.270(4) | 1.274(4) | 1.303(5) |
| C1-O2 | 1.207(5) | 1.231(5) | 1.232(5) | 1.209(5) |
| C1-C2 | 1.512(5) | 1.524(4) | 1.525(4) | 1.512(5) |
| C2-C3 | 1.517(5) | 1.540(5) | 1.539(6) | 1.520(5) |
| C3-C4 | 1.530(5) | 1.540(5) | 1.529(5) | 1.518(5) |
| C4-C5 | 1.506(6) | 1.516(6) | 1.510(6) | 1.507(6) |
| C5-N1 | 1.509(5) | 1.484(5) | 1.494(5) | 1.498(5) |
| N1-C2 | 1.502(5) | 1.504(4) | 1.501(5) | 1.491(5) |
| I1-I2 | 2.9757(4) | I4-I5 | 2.9235(4) | |
| I2-I3 | 2.8649(4) | I5-I6 | 2.9268(4) | |
| I7-I8 | 2.8535(4) | I3···I4 | 3.6663(6) | |
| I8-I9 | 2.9948(4) | I6···I7 | 3.6146(6) | |
| I1-I2-I3 | 174.864(13) | I2-I3···I4 | 167.19(1) | |
| I4-I5-I6 | 177.434(14) | I3···I4-I5 | 168.74(1) | |
| I7-I8-I9 | 176.175(13) | I5-I6···I7 | 172.62(1) | |
| | | I6···I7-I8 | 173.21(1) | |
The NH2+ groups form five N-H···I type hydrogen bonds with anions and two N-H···O type hydrogen bonds (excluding one intramolecular bond) with the carbonyl oxygen atoms O2A and O2D of neighboring A- and D- l-prolinium cations. Additionally, there are five C-H···I type short contacts that can be considered weak hydrogen bonds. All iodine atoms that form N-H···I and C-H···I contacts are terminal atoms of triiodide anions.
Table 5
Hydrogen bond parameters (in Å and °) for [(l-ProH)3(l-Pro)](I3)3 (II).
| D-H···A | D-H | H···A | D···A | DHA |
| O1A-H1A···O2B i | 0.70(5) | 1.97(5) | 2.674(4) | 173(7) |
| O1C-H1C···O1B i | 1.19 | 1.35 | 2.427(3) | 146 |
| O1D-H1D···O2C ii | 0.70(5) | 1.99(5) | 2.664(4) | 162(6) |
| N1A-H11A···O2A | 0.91 | 2.21 | 2.692(4) | 113 |
| N1A-H12A···I1 | 0.91 | 2.82 | 3.690(3) | 147 |
| N1B-H11B···O2A iii | 0.91 | 2.03 | 2.818(4) | 144 |
| N1B-H12B···I4 iii | 0.91 | 2.91 | 3.690(3) | 144 |
| N1C-H11C···O2D iv | 0.91 | 2.06 | 2.836(4) | 142 |
| N1C-H12C···I6 v | 0.91 | 2.93 | 3.698(3) | 143 |
| N1D-H11D···I9 vi | 0.91 | 3.08 | 3.856(4) | 144 |
| N1D-H12D···I9 iii | 0.91 | 2.82 | 3.638(4) | 150 |
| C2A-H2A···I1 | 1.00 | 3.11 | 3.831(4) | 130 |
| C3A-H31A···I7 iii | 0.99 | 3.09 | 3.844(5) | 134 |
| C3A-H32A···I7 vi | 0.99 | 3.04 | 3.873(5) | 143 |
| C2D-H2D···I3 | 1.00 | 3.12 | 3.953(4) | 141 |
| C3D-H31D···I3 vi | 0.99 | 3.09 | 3.821(5) | 131 |
| Symmetry code: (i) x, y, z + 1; (ii) x, y, z-1; (iii) -x + 1, y-1/2, -z + 1; (iv) -x + 2, y-1/2, -z + 1; |
(v) -x + 1, y-1/2, -z + 2; (vi) -x + 1, y + 1/2, -z + 1; (vii) x, y + 1, z.
The infrared spectra of (I) and (II) are shown in Fig. 5. A tentative assignment of peaks is given in Table 6.
In the high-frequency region of (I), one can expect absorptions caused by stretching modes of NH2+, CH, and CH2 groups. The stretching mode ν(OH) of the protonated A-moiety is assumed to be in the low-frequency region [19]. The strong absorption band at 3123 cm− 1 is assigned to ν(NH), while the peaks at 3004, 2975, and 2943 cm− 1 to ν(CH). The peak at 1707 cm− 1 is characteristic for ν(C = O) of a carboxylic group, while the rather strong one at 1577 cm− 1 is assigned to the deformation vibrations of NH2+ groups. Peaks at 1446, 1419, 1376, 1346, and 1321 cm− 1 are assigned to the deformation vibrations of CH2 groups. The peak at 1419 cm− 1 may also be caused by νs(COO−). Those in the 1200 − 800 cm− 1 range are superimposed with a broad absorption centered ca. 990 cm− 1 which is likely caused by ν(OH) stretching mode (see [19]).
In the high-frequency region of (II), one can expect absorptions caused by stretching modes of NH2+, CH, CH2 groups as well as OH groups of carboxyl groups of A- and D- l-prolinium cations. The strong absorption band near 3000 cm− 1 is assigned to ν(NH) of NH2+ (peak at 3133 cm− 1) and ν(OH) (the peak at 3075 cm− 1). Expected values of ν(OH) based on the correlation ν(OH) vs. R(O···O) [18] for hydrogen bonds O1A-H1A···O2B (2.674 Å) and O1D-H1D···O2C (2.664 Å) are close to the observed ones. Weak peaks at 3006, 2982, 2936 cm− 1 are assigned to ν(CH). The strong absorption band at 1715 cm− 1 is characteristic for ν(C = O) of a carboxyl group. We assign it to ν(C = O) of the carboxyl groups of A- and D- l-prolinium cations. The position of weak absorption at 1653 cm− 1 is characteristic for νas(COO−). We assign it to the (l-Pro-H-l-Pro) dimeric cation. The weakness is likely caused by its pseudocentrosymmetric nature. The bands at 1552 and 1448 cm− 1 are also characteristic for proline and originated from δ(NH2) and δ(CH2), respectively. In the lower range there are peaks due to other deformation vibrations of CH2 and NH2 groups, as well as ring vibrations. The characteristic feature is also the absorption band at 1242 cm− 1, which we attribute to the ν(C-OH) of the carboxyl groups of A- and D- l-prolinium cations. More interesting is that these peaks are superimposed on half of the broad band in the 1400 − 500 cm− 1 region. Such a feature in this region is characteristic for ν(OH) of very strong hydrogen bonds in dimeric cations. We assign it to the ν(OH) of the (l-Pro-H-l-Pro) dimeric cation. For strong hydrogen bonds (R(O···O) = 2.40–2.58 Å) the correlation of ν(OH)(cm− 1) vs. R(O···O) (Å) [19] has approximately linear character: ν(OH) = 12500R-29875. For R(O···O) = 2.427 Å the expected value of ν(OH) is ca. 460 cm− 1, which corresponds well to the center of the mentioned broad band.
Table 6
Wavenumbers (cm− 1) and assignment of peaks in the infrared spectra of (l-ProH···l-Pro)(I3) (I) and [(l-ProH)3(l-Pro)](I3)3 (II).
| (I) | (II) | Assignment |
| 3123 | 3133 | ν(NH) |
| | 3075 | ν(OH) |
| 3004;2975;2943 | 3006;2982;2936 | ν(CH) |
| 2880;2739;2553; 2448 | 2875;2733 | Combi |
| 1707 | 1715 | ν(C = O) |
| 1647 | 1653 | νas(COO−) |
| 1577 | 1552 | δ(NH2) |
| 1446;1419 | 1448 | δ(CH2) |
| 1376;1346 | 1369 | ω(CH2) |
| 1321 | 1330 | τ(CH2) |
| 1300 | 1306 | τ(CH) |
| 1279 | 1282;1266 | δ(CH) |
| 1231 | 1242 | ν(C-OH) |
| 1188;1163 | 1190;1169 | ω(NH2) |
| 1081;1050;1024 | 1088;1044 | |
| 987; 919;860 | 987;950;921;900;860 | |
| 788;765;722 | 754;737;662 | |
DFT calculations of the electronic structure of (I, II) crystals
The grid parameters for calculating electronic properties were 3×3×2 (I) and 2×3×1 (II), k-point set in the Brillouin region for crystals. In a Kohn–Sham computation, the approximate functional used to determine the exchange-correlation energy (𝐸𝑥𝑐) has a significant impact on the accuracy of the final findings. The electronic structure simulations were performed based on the DFT theory by OTFG (On-the-fly generation) ultrasoft pseudopotentials. The relativistic treatment was Koelling-Harmon, energy range – 10 eV, separation compares 0.005 1/Å. Band energy tolerance is within 1.0 x 10−5 eV per atom. The DOS and PDOS were calculated.
The inhomogeneous electron densities in solids and the slow valence electron density fluctuations in space make using the generalized gradient approximation (GGA) in the PBE scheme for computing electronic characteristics an excellent method.
We calculated the energy band structures with the directions with high first Brillouin zone equilibrium points, including Z→G→Y→A→B→D→E→C for both (I) and (II) crystals. A direct transition energy for (I, II) crystals, which appears between the highest valence band value and the lowest conduction band value of the Brillion region at the symmetry point B, is 2.281 eV (I) and 1.641 eV (II), and an indirect band gap at Y→G range is 1.631 eV (II) (Fig. 6). For the (l-ProH···l-Pro)(I3) (I) crystal indirect transition was not observed.
As is known, the density of states (DOS) of a system describes the number of states occupied at each energy level in statistical and solid-state physics. Composition of the calculated energy bands can be resolved with the help of partial density of states (PDOS) and total density of states (TDOS) diagrams.
Figures 7 and 8 demonstrate the total density of states for the valence and conduction bands. In these figures, the zero tick mark on the energy scales (the top of the valence band) indicates the position of the Fermi level. To obtain a measure of the contribution of different atomic states to the band structure, as well as to their possible hybridizations, a comprehensive analysis of the partial density of states was carried out.
From the supercell calculations, the PDOS for the different elements O (2s2, 2p4), N (2s2, 2p3) and I (5s2 4d10 5p5) in the (l-ProH···l-Pro)(I3) (I) and [(l-ProH)3(l-Pro)](I3)3 (II) crystals are extracted and shown in Figs. 7 and 8. These diagrams allow us to conclude that the main contribution near the edge of the conduction band for both crystals is made by the I (5p) states, which hybridize with the N (2p) and O (2p) states. In the presence of states I (5p), the band gap for (I) is Eg = 2.281 eV, and for (II) it is Eg = 1.631 eV.
Salt (II) has a complicated composition [(l-ProH)3(l-Pro)](I3)3 containing a peculiar tetrameric cation [l-ProH···(l-Pro-H- l-Pro)···l-ProH]. In addition, the presence of triiodide anions with their supramolecular halogen bonds leads to a decrease in the bandgap compared to the salt (l-ProH···l-Pro)(I3) (I).
Bandgap measurements
The type of transition selected based on DFT-calculations and the bandgap were estimated from the UV-Vis diffuse reflectance data (Fig. 9) using Tauc expression and Kubelka-Munk function [20–22]. The bandgap for the direct transition (I) is 2.04 eV while for the indirect one it is 1.51 eV (II).