A comparative study of two polymorphs of bis­(1-hy­droxy-2-methyl­propan-2-aminium) carbonate

Alkanolamines have been known for their high CO₂ absorption for over 60 years and are used widely in the natural gas industry for reversible CO₂ capture. In an attempt to crystallize a salt of (RS)-2-(3-benzoyl­phen­yl)propionic acid with 2-amino-2-methyl­propan-1-ol, we obtained instead a polymorph (denoted polymorph II) of bis­(1-hy­droxy-2-methyl­propan-2-aminium) carbonate, 2C₄H₁₂NO⁺·CO₃²⁻, (I), suggesting that the amine group of the former compound captured CO₂ from the atmosphere forming the aminium carbonate salt. This new polymorph was characterized by single-crystal X-ray diffraction analysis at low temperature (100 K). The salt crystallizes in the monoclinic system (space group C2/c, Z = 4), while a previously reported form of the same salt (denoted polymorph I) crystallizes in the triclinic system (space group P, Z = 2) [Barzagli et al. (2012). ChemSusChem, 5, 1724–1731]. The asymmetric unit of polymorph II contains one 1-hy­droxy-2-methyl­propan-2-aminium cation and half a car­bonate anion, located on a twofold axis, while the asymmetric unit of polymorph I contains two cations and one anion. These polymorphs exhibit similar structural features in their three-dimensional packing. Indeed, similar layers of an alternating cation–anion–cation neutral structure are observed in their mol­ecular arrangements. Within each layer, carbonate anions and 1-hy­droxy-2-methyl­propan-2-aminium cations form planes bound to each other through N—H...O and O—H...O hydrogen bonds. In both polymorphs, the layers are linked to each other via van der Waals inter­actions and C—H...O contacts. In polymorph II, a highly directional C—H...O contact (C—H...O = 156°) shows as a hydrogen-bonding inter­action. Periodic theoretical density functional theory (DFT) calculations indicate that both polymorphs present very similar stabilities.

S1. Introduction

Alkanolamines, such as mono­ethano­lamine, di­ethano­lamine, methyldi­ethano­lamine and 2-amino-2-methyl­propan-1-ol, have been known for their high CO₂ absorption for over 60 years (Yang et al., 2008). They are used widely in the natural gas industry for industrial applications in CO₂ reversible capture from natural gas extraction and gas refinery (Barzagli et al., 2012). In an attempt to crystallize a salt of (RS)-2-(3-benzoyl­phenyl)­propionic acid with 2-amino-2-methyl­propan-1-ol, we obtained instead a polymorph (denoted polymorph II) of bis­(1-hy­droxy-2-methyl­propan-2-aminium) carbonate, (I), suggesting that the amine group of the former compound captured CO₂ from the atmosphere forming the aminium carbonate salt. In 2012, a previous polymorph (denoted polymorph I) was crystallized in the triclinic system (Barzagli et al., 2012).

S2. Experimental

S2.1. Synthesis and crystallization

A solution of 2-amino-2-methyl­propan-1-ol (0.2 ml, 2 mmol) in aceto­nitrile (0.5 ml) was added to a solution of 2-(3-benzoyl­phenyl)­propionic acid (32 mg, 0.125 mmol) in aceto­nitrile (0.5 ml). Colourless crystals of bis­(1-hy­droxy-2-methyl­propan-2-aminium) carbonate, (I), suitable for X-ray diffraction analysis were obtained after 4 d by slow evaporation of the solution at room temperature.

S2.2. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. H atoms bonded to O and N atoms were observed in the difference Fourier synthesis. They were placed at the observed positions and allowed to refine freely with an isotropic displacement parameter. The remaining H atoms were positioned geometrically and allowed to ride on their respective parent atoms, with C—H = 0.98 Å and U_iso(H) = 1.5U_eq(C) for methyl H atoms, and C—H = 0.99 Å and U_iso(H) = 1.2U_eq(C) for methyl­ene H atoms.

S3. Results and discussion

The asymmetric unit of polymorph II of the title salt, (I), contains one 1-hy­droxy-2-methyl­propan-2-aminium cation and half a carbonate anion located on a twofold axis (Fig. 1). The formula unit '2(C₄H₁₂NO)·CO₃' contains ten hydrogen-bond donors for only five hydrogen-bond acceptors. Whereas all donors belong to the cations, three of five acceptors are found in the anion and the remaining two in the cations. In the crystal structure of polymorph II, the cation is bonded, via four hydrogen bonds, to three anions. Two anions are linked to the cation by a single N—H···O hydrogen bond, while the third is connected to the cation by one N—H···O and one O—H···O hydrogen bond. The carbonate anion is linked to six cations through eight hydrogen bonds. It is connected to four cations through single N—H···O hydrogen bonds and to the two other cations via both O—H···O and N—H···O hydrogen bonds, forming neutral layers of alternated cation–anion–cation planes which are parallel to the ab plane (Fig. 2). These layers, located at z = 1/4 and 3/4, inter­act through van der Waals inter­actions and C—H···O hydrogen bonds, leading to an infinite three-dimensional network (Fig. 3 and Table 2).

The asymmetric unit of polymorph I (Barzagli et al., 2012) of (I) contains two 1-hy­droxy-2-methyl­propan-2-aminium cations and one carbonate anion. The formula unit contains eight hydrogen-bond donors in two cations for only four hydrogen-bond acceptors, i.e. three in the anion and one in one cation. Accordingly, the two independent cations are fundamentally distinct from one another in terms of their hydrogen-bond inter­actions. The first cation is bonded to three anions and one cation, linked to two of the three anions by N—H···O hydrogen bonds and to the third by one O—H···O hydrogen bond, and linked to another equivalent cation through inversion centre symmetry by two N—H···O hydrogen bonds. Similar to polymorph II, the second cation in polymorph I is bonded to three anions, linked to two by single N—H···O hydrogen bonds and to the third by N—H···O and O—H···O hydrogen bonds. In the crystal structure of polymorph I, the carbonate anion is also surrounded by six cations, linked to four through N—H···O hydrogen bonds and to a fifth by two hydrogen bonds (N—H···O and O—H···O), similar to polymorph II. The main difference with polymorph II is seen in the link to the sixth cation via only an O—H···O hydrogen bond in polymorph I. All these hydrogen bonds connect anions and cations to form neutral layers of a alternating cation–anion–cation structure, here parallel to the (011) plane (Fig. 2). These layers are inter­connected through van der Waals inter­actions and C—H···O contacts (Fig. 3). The formation of the hydrogen-bonded cation–anion–cation structure in both polymorphs I and II can be regarded as a consequence of the 2:1 cation–anion ratio and the higher concentration of hydrogen-bond donors relative to hydrogen-bond acceptors in their crystal structures.

In the crystal structures of polymorphs I and II, the centroids of the cations form edge-sharing o­cta­hedra around the anions. These o­cta­hedra, which are oriented in a similar manner in both polymorphs, form similar layers (Figs. 4 and 5), as outlined before. In polymorph II, the six centroid-to-centroid distances between the anion and the cations of each o­cta­hedron range from 4.101 to 4.738 Å. In polymorph I, five centroid-to-centroid distances vary from 4.007 to 4.651 Å, corresponding to anion–cation hydrogen-bonding inter­actions similar to those present in polymorph II. The sixth centroid-to-centroid distance, equal to 5.602 Å, is the longest and corresponds to a different relative orientation between the sixth cation and the central anion with respect to that found in polymorph II. This is propably the source of the greatest deformation of the o­cta­hedra in polymorph I compared with those of the polymorph II (Figs. 4 and 5), as observed by the angles formed between the centroid of the central anion and those of the cations in trans positions with respect to the anion (156.05, 166.99 and 168.7° for polymorph I, compared with 170.47 and 173.80° for polymorph II). In both polymorphs, each layer comprises one plane formed by the centroids of the anions (anionic plane), which is located between two planes formed by the centroids of the cations (cationic planes) (Fig. 6). In polymorph II, the layer thickness (4.918 Å) is larger than that of polymorph I (4.216 Å) (Fig. 6). In addition, the inter­layer distance in polymorph II (4.327 Å) is greater than in polymorph I (3.969 Å), while the surface occupied by one '2(C₄H₁₂NO)·CO₃' formula unit in the layers of the former (33.5 Ų) is smaller than in those of the latter (39.3 Ų) (Figs. 4 and 5). As a consequence, the layers in polymorph II are denser and more distant with respect to each other than those in polymorph I, leading to a crystalline density difference of 4.5% between the two polymorphs (1.287 Mg m⁻³ at 100 K for polymorph II and 1.232 Mg m⁻³ at 173 K for polymorph I).

In order to estimate the relative stability of the two polymorphs, periodic theoretical calculations at the Density Functional level of Theory (DFT) have been performed on both structures. The geometry optimizations (unit-cell parameters and atomic positions) of these two crystal phases leading to energy minima were achieved using the B3LYP hybrid exchange/correlation functional with the 6–31G** basis set (Vosko et al., 1980; Becke, 1993) and the CRYSTAL14 program (Dovesi et al., 2014). Using dispersion corrections (Grimme, 2006), the two polymorphs were observed to have almost the same stability [ΔE(II–I) = −0.2 kJ mol⁻¹ per '2(C₄H₁₂NO)·CO₃' formula unit, in line with their close structural similarity. The unit-cell volume of each polymorph decreases after geometry optimizations; this decrease is more important in polymorph I, whose structure was determined at higher temperature (T = 173 K) than in polymorph II (T = 100 K).

In polymorph II, the bonding angles of the carbonate anion [119.62 (11) and 120.8 (2)°] are very close to the formal value of 120°. The O2—C5 bond length [1.2934 (17) Å] is slightly longer than the O3—C5 bond length [1.283 (3) Å]. This is probably due to the fact that the O2 atom is an acceptor of three hydrogen bonds, two involving the NH₃⁺ ammonium group of the organic cation (N1—H2···O2 and N1—H3···O2) and one involving the hy­droxy group (O1—H1···O2), while the O3 atom is an acceptor of two N1—H4···O3 hydrogen bonds (Table 2).

A correlation between observed C—O bond lengths and the number of classical hydrogen bonds which are accepted by the O atom is also observed in polymorph I. The O3—C9 bond length [1.298 (1) Å] is longer than the O4—C9 [1.274 (1) Å] and O5—C9 [1.275 (1) Å] bond lengths. This is probably due to the fact that the O3 atom is an acceptor of three hydrogen bonds, two involving the NH₃⁺ ammonium group of the organic cation (N2—H23N···O3 and N1—H11N···O3) and one involving the hy­droxy group (O2—H2···O3), while the O4 and O5 atoms are acceptors of only two hydrogen bonds each (N2—H22N···O4 and N2—H21N···O4, and N1—H13···O5 and O1—H1···O5).

Comparing the intra­layer inter­actions of the two polymorphs, we note that the N—H···O hydrogen-bond lengths are comparable. Indeed, for polymorph I (Barzagli et al., 2012), the N···O distances vary between 2.685 (2) and 2.800 (2) Å, while in polymorph II, this distance ranges from 2.7403 (19) to 2.8536 (18) Å (Table 2). With regard to O—H···O hydrogen bonds, polymorph II has one [O1—H1···O2ⁱⁱⁱ: O1···O2 = 2.738 (2) Å; see Table 2 for symmetry codes], while polymorph I has two [O···O = 2.598 (2) and 2.758 (2) Å]. Inter­layer C—H···O contacts are present in both polymorphs. The C···O distance in polymorph II [C4···O1 = 3.498 (2) Å] is shorter than those present in polymorph I [C···O = 3.629 (2)–3.739 (2) Å].

In order to analyse the crystal structures of carbonates, a search was made of the Cambridge Structural Database (CSD, Version of 2015; Groom & Allen, 2014) using the CONQUEST software. After excluding organometallic compounds, the search resulted in 40 structures containing the carbonate anion that can be divided into three groups. The first group includes three structures formed by the carbonate anion and alkali cations. The second contains structures consisting of the carbonate anion, organic cations and neutral organic molecules, which are cocrystallized. The last group comprises four crystal structures formed by the carbonate anion and organic cations only [CSD refcodes BGUDCB10 (Pinkerton & Schwarzenbach, 1978), GUANCB (Adams & Small, 1974), YAXNOC (Nowakowska et al., 2012) and YENGOP (Barzagli et al., 2012), the last being polymorph I discussed in this paper]. Similar to polymorph II, in the crystal structures of all four of these compounds, the carbonate anions and the cations form layers of an alternating cation–anion–cation neutral structure. Within each layer, carbonate anions and cations are connected through hydrogen bonds. Similar to polymorphs I and II, in BGUDCB10, GUANCB and YAXNOC, the carbonate anion is linked to six cations. In BGUDCB10, the carbonate anion is connected to the cations through ten N—H···O hydrogen bonds (two one-point connections and four two-point connections). In GUANCB, the carbonate anion is bonded to the cations through eight N—H···O hydrogen bonds (four one-point connections and two two-point connections) and in YAXNOC it is linked to the cations through six N—H···O hydrogen bonds (six one-point connections). In BGUDCB10, YAXNOC and YENGOP, the cations form edge-sharing o­cta­hedra around the anions such as in polymorph II, while the o­cta­hedra are sharing commun corners in the GUANCB structure. [The Results and discussion section might benefit from being broken into separate sections with titles, for example, "DFT calculations"]

Structure description

Alkanolamines, such as mono­ethano­lamine, di­ethano­lamine, methyldi­ethano­lamine and 2-amino-2-methyl­propan-1-ol, have been known for their high CO₂ absorption for over 60 years (Yang et al., 2008). They are used widely in the natural gas industry for industrial applications in CO₂ reversible capture from natural gas extraction and gas refinery (Barzagli et al., 2012). In an attempt to crystallize a salt of (RS)-2-(3-benzoyl­phenyl)­propionic acid with 2-amino-2-methyl­propan-1-ol, we obtained instead a polymorph (denoted polymorph II) of bis­(1-hy­droxy-2-methyl­propan-2-aminium) carbonate, (I), suggesting that the amine group of the former compound captured CO₂ from the atmosphere forming the aminium carbonate salt. In 2012, a previous polymorph (denoted polymorph I) was crystallized in the triclinic system (Barzagli et al., 2012).

The asymmetric unit of polymorph II of the title salt, (I), contains one 1-hy­droxy-2-methyl­propan-2-aminium cation and half a carbonate anion located on a twofold axis (Fig. 1). The formula unit '2(C₄H₁₂NO)·CO₃' contains ten hydrogen-bond donors for only five hydrogen-bond acceptors. Whereas all donors belong to the cations, three of five acceptors are found in the anion and the remaining two in the cations. In the crystal structure of polymorph II, the cation is bonded, via four hydrogen bonds, to three anions. Two anions are linked to the cation by a single N—H···O hydrogen bond, while the third is connected to the cation by one N—H···O and one O—H···O hydrogen bond. The carbonate anion is linked to six cations through eight hydrogen bonds. It is connected to four cations through single N—H···O hydrogen bonds and to the two other cations via both O—H···O and N—H···O hydrogen bonds, forming neutral layers of alternated cation–anion–cation planes which are parallel to the ab plane (Fig. 2). These layers, located at z = 1/4 and 3/4, inter­act through van der Waals inter­actions and C—H···O hydrogen bonds, leading to an infinite three-dimensional network (Fig. 3 and Table 2).

The asymmetric unit of polymorph I (Barzagli et al., 2012) of (I) contains two 1-hy­droxy-2-methyl­propan-2-aminium cations and one carbonate anion. The formula unit contains eight hydrogen-bond donors in two cations for only four hydrogen-bond acceptors, i.e. three in the anion and one in one cation. Accordingly, the two independent cations are fundamentally distinct from one another in terms of their hydrogen-bond inter­actions. The first cation is bonded to three anions and one cation, linked to two of the three anions by N—H···O hydrogen bonds and to the third by one O—H···O hydrogen bond, and linked to another equivalent cation through inversion centre symmetry by two N—H···O hydrogen bonds. Similar to polymorph II, the second cation in polymorph I is bonded to three anions, linked to two by single N—H···O hydrogen bonds and to the third by N—H···O and O—H···O hydrogen bonds. In the crystal structure of polymorph I, the carbonate anion is also surrounded by six cations, linked to four through N—H···O hydrogen bonds and to a fifth by two hydrogen bonds (N—H···O and O—H···O), similar to polymorph II. The main difference with polymorph II is seen in the link to the sixth cation via only an O—H···O hydrogen bond in polymorph I. All these hydrogen bonds connect anions and cations to form neutral layers of a alternating cation–anion–cation structure, here parallel to the (011) plane (Fig. 2). These layers are inter­connected through van der Waals inter­actions and C—H···O contacts (Fig. 3). The formation of the hydrogen-bonded cation–anion–cation structure in both polymorphs I and II can be regarded as a consequence of the 2:1 cation–anion ratio and the higher concentration of hydrogen-bond donors relative to hydrogen-bond acceptors in their crystal structures.

In the crystal structures of polymorphs I and II, the centroids of the cations form edge-sharing o­cta­hedra around the anions. These o­cta­hedra, which are oriented in a similar manner in both polymorphs, form similar layers (Figs. 4 and 5), as outlined before. In polymorph II, the six centroid-to-centroid distances between the anion and the cations of each o­cta­hedron range from 4.101 to 4.738 Å. In polymorph I, five centroid-to-centroid distances vary from 4.007 to 4.651 Å, corresponding to anion–cation hydrogen-bonding inter­actions similar to those present in polymorph II. The sixth centroid-to-centroid distance, equal to 5.602 Å, is the longest and corresponds to a different relative orientation between the sixth cation and the central anion with respect to that found in polymorph II. This is propably the source of the greatest deformation of the o­cta­hedra in polymorph I compared with those of the polymorph II (Figs. 4 and 5), as observed by the angles formed between the centroid of the central anion and those of the cations in trans positions with respect to the anion (156.05, 166.99 and 168.7° for polymorph I, compared with 170.47 and 173.80° for polymorph II). In both polymorphs, each layer comprises one plane formed by the centroids of the anions (anionic plane), which is located between two planes formed by the centroids of the cations (cationic planes) (Fig. 6). In polymorph II, the layer thickness (4.918 Å) is larger than that of polymorph I (4.216 Å) (Fig. 6). In addition, the inter­layer distance in polymorph II (4.327 Å) is greater than in polymorph I (3.969 Å), while the surface occupied by one '2(C₄H₁₂NO)·CO₃' formula unit in the layers of the former (33.5 Ų) is smaller than in those of the latter (39.3 Ų) (Figs. 4 and 5). As a consequence, the layers in polymorph II are denser and more distant with respect to each other than those in polymorph I, leading to a crystalline density difference of 4.5% between the two polymorphs (1.287 Mg m⁻³ at 100 K for polymorph II and 1.232 Mg m⁻³ at 173 K for polymorph I).

In order to estimate the relative stability of the two polymorphs, periodic theoretical calculations at the Density Functional level of Theory (DFT) have been performed on both structures. The geometry optimizations (unit-cell parameters and atomic positions) of these two crystal phases leading to energy minima were achieved using the B3LYP hybrid exchange/correlation functional with the 6–31G** basis set (Vosko et al., 1980; Becke, 1993) and the CRYSTAL14 program (Dovesi et al., 2014). Using dispersion corrections (Grimme, 2006), the two polymorphs were observed to have almost the same stability [ΔE(II–I) = −0.2 kJ mol⁻¹ per '2(C₄H₁₂NO)·CO₃' formula unit, in line with their close structural similarity. The unit-cell volume of each polymorph decreases after geometry optimizations; this decrease is more important in polymorph I, whose structure was determined at higher temperature (T = 173 K) than in polymorph II (T = 100 K).

In polymorph II, the bonding angles of the carbonate anion [119.62 (11) and 120.8 (2)°] are very close to the formal value of 120°. The O2—C5 bond length [1.2934 (17) Å] is slightly longer than the O3—C5 bond length [1.283 (3) Å]. This is probably due to the fact that the O2 atom is an acceptor of three hydrogen bonds, two involving the NH₃⁺ ammonium group of the organic cation (N1—H2···O2 and N1—H3···O2) and one involving the hy­droxy group (O1—H1···O2), while the O3 atom is an acceptor of two N1—H4···O3 hydrogen bonds (Table 2).

A correlation between observed C—O bond lengths and the number of classical hydrogen bonds which are accepted by the O atom is also observed in polymorph I. The O3—C9 bond length [1.298 (1) Å] is longer than the O4—C9 [1.274 (1) Å] and O5—C9 [1.275 (1) Å] bond lengths. This is probably due to the fact that the O3 atom is an acceptor of three hydrogen bonds, two involving the NH₃⁺ ammonium group of the organic cation (N2—H23N···O3 and N1—H11N···O3) and one involving the hy­droxy group (O2—H2···O3), while the O4 and O5 atoms are acceptors of only two hydrogen bonds each (N2—H22N···O4 and N2—H21N···O4, and N1—H13···O5 and O1—H1···O5).

Comparing the intra­layer inter­actions of the two polymorphs, we note that the N—H···O hydrogen-bond lengths are comparable. Indeed, for polymorph I (Barzagli et al., 2012), the N···O distances vary between 2.685 (2) and 2.800 (2) Å, while in polymorph II, this distance ranges from 2.7403 (19) to 2.8536 (18) Å (Table 2). With regard to O—H···O hydrogen bonds, polymorph II has one [O1—H1···O2ⁱⁱⁱ: O1···O2 = 2.738 (2) Å; see Table 2 for symmetry codes], while polymorph I has two [O···O = 2.598 (2) and 2.758 (2) Å]. Inter­layer C—H···O contacts are present in both polymorphs. The C···O distance in polymorph II [C4···O1 = 3.498 (2) Å] is shorter than those present in polymorph I [C···O = 3.629 (2)–3.739 (2) Å].

In order to analyse the crystal structures of carbonates, a search was made of the Cambridge Structural Database (CSD, Version of 2015; Groom & Allen, 2014) using the CONQUEST software. After excluding organometallic compounds, the search resulted in 40 structures containing the carbonate anion that can be divided into three groups. The first group includes three structures formed by the carbonate anion and alkali cations. The second contains structures consisting of the carbonate anion, organic cations and neutral organic molecules, which are cocrystallized. The last group comprises four crystal structures formed by the carbonate anion and organic cations only [CSD refcodes BGUDCB10 (Pinkerton & Schwarzenbach, 1978), GUANCB (Adams & Small, 1974), YAXNOC (Nowakowska et al., 2012) and YENGOP (Barzagli et al., 2012), the last being polymorph I discussed in this paper]. Similar to polymorph II, in the crystal structures of all four of these compounds, the carbonate anions and the cations form layers of an alternating cation–anion–cation neutral structure. Within each layer, carbonate anions and cations are connected through hydrogen bonds. Similar to polymorphs I and II, in BGUDCB10, GUANCB and YAXNOC, the carbonate anion is linked to six cations. In BGUDCB10, the carbonate anion is connected to the cations through ten N—H···O hydrogen bonds (two one-point connections and four two-point connections). In GUANCB, the carbonate anion is bonded to the cations through eight N—H···O hydrogen bonds (four one-point connections and two two-point connections) and in YAXNOC it is linked to the cations through six N—H···O hydrogen bonds (six one-point connections). In BGUDCB10, YAXNOC and YENGOP, the cations form edge-sharing o­cta­hedra around the anions such as in polymorph II, while the o­cta­hedra are sharing commun corners in the GUANCB structure. [The Results and discussion section might benefit from being broken into separate sections with titles, for example, "DFT calculations"]

Synthesis and crystallization

A solution of 2-amino-2-methyl­propan-1-ol (0.2 ml, 2 mmol) in aceto­nitrile (0.5 ml) was added to a solution of 2-(3-benzoyl­phenyl)­propionic acid (32 mg, 0.125 mmol) in aceto­nitrile (0.5 ml). Colourless crystals of bis­(1-hy­droxy-2-methyl­propan-2-aminium) carbonate, (I), suitable for X-ray diffraction analysis were obtained after 4 d by slow evaporation of the solution at room temperature.

Refinement details

Crystal data, data collection and structure refinement details are summarized in Table 1. H atoms bonded to O and N atoms were observed in the difference Fourier synthesis. They were placed at the observed positions and allowed to refine freely with an isotropic displacement parameter. The remaining H atoms were positioned geometrically and allowed to ride on their respective parent atoms, with C—H = 0.98 Å and U_iso(H) = 1.5U_eq(C) for methyl H atoms, and C—H = 0.99 Å and U_iso(H) = 1.2U_eq(C) for methyl­ene H atoms.

Computing details

Data collection: (CrysAlis PRO; Agilent, 2014); cell refinement: (CrysAlis PRO; Agilent, 2014); data reduction: (CrysAlis PRO; Agilent, 2014); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: Mercury (Macrae et al., 2006); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015).

Figures

	Fig. 1. The asymmetric unit in polymorph II, showing displacement ellipsoids at the 50% probability level for the non H-atoms. H atoms are represented by spheres of arbitrary radii. [Symmetry code: (ii) −x, y, −z + 1/2.]
	Fig. 2. Intermolecular N—H···O and O—H···O hydrogen-bonding interactions between the 1-hydroxy-2-methylpropan-2-aminium cations and the carbonate anions, lying on (left) the ab plane of polymorph II and (right) the (011) plane of polymorph I. The H atoms of CH3 and CH2 groups have been omitted for clarity. Hydrogen bonds are represented by dashed lines.
	Fig. 3. The packing of (left) polymorph II and (right) polymorph I in projection along the a axis. The H atoms of CH3 and CH2 groups which are not engaged in hydrogen-bonding interactions have been omitted for clarity. Hydrogen bonds are represented by dashed lines.
	Fig. 4. A layer of edge-sharing octahedra in polymorph II. The octahedra are formed by the centroids of cations around the central anion. The surface occupied by one '2(C4H12NO)·CO3' formula unit in the layer is delimited by white dashed lines.
	Fig. 5. A layer of edge-sharing octahedra in polymorph I. The octahedra are formed by the centroids of cations around the central anion. The surface occupied by two '4(C4H12NO)·2CO3' formula units in the layer is delimited by white dashed lines.
	Fig. 6. A schematic representation of two layers. d1 is the distance between two cationic planes belonging to adjacent layers, d2 is the distance between two anionic planes belonging to adjacent layers and d3 is the thickness of the layer (see text).

Bis(1-hydroxy-2-methylpropan-2-aminium) carbonate

Crystal data

2C4H12NO^+·CO3^2^−	F(000) = 528
Mr = 240.30	Dx = 1.287 Mg m^−3
Monoclinic, C2/c	Cu Kα radiation, λ = 1.54184 Å
a = 10.6114 (9) Å	Cell parameters from 2163 reflections
b = 6.3184 (7) Å	θ = 8.2–75.9°
c = 19.063 (2) Å	µ = 0.87 mm^−1
β = 104.075 (9)°	T = 100 K
V = 1239.7 (2) Å^3	Block, colourless
Z = 4	0.18 × 0.12 × 0.07 mm

Data collection

Oxford SuperNova diffractometer	1296 independent reflections
Radiation source: sealed X-ray tube	1089 reflections with I > 2σ(I)
Detector resolution: 10.4508 pixels mm^-1	Rint = 0.045
ω scans	θmax = 76.9°, θmin = 4.8°
Absorption correction: analytical [CrysAlis PRO (Agilent, 2014), based on expressions derived by Clark & Reid (1995)]	h = −13→13
Tmin = 0.895, Tmax = 0.950	k = −7→7
4510 measured reflections	l = −23→23

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	H-atom parameters constrained
R[F^2 > 2σ(F^2)] = 0.053	w = 1/[σ^2(Fo^2) + (0.0927P)^2 + 1.3673P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.159	(Δ/σ)max < 0.001
S = 1.09	Δρmax = 0.37 e Å^−3
1296 reflections	Δρmin = −0.30 e Å^−3
90 parameters

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
O3	0.5000	0.7255 (3)	0.7500	0.0185 (4)
O2	0.60837 (11)	0.4212 (2)	0.77210 (7)	0.0176 (4)
O1	0.79741 (13)	0.1198 (2)	0.59807 (8)	0.0208 (4)
N1	0.80245 (14)	0.5035 (2)	0.69501 (8)	0.0158 (4)
C5	0.5000	0.5224 (4)	0.7500	0.0145 (5)
C1	0.74396 (17)	0.4943 (3)	0.61461 (10)	0.0157 (4)
C3	0.62788 (17)	0.6436 (3)	0.59678 (11)	0.0204 (4)
H11	0.5885	0.6398	0.5447	0.031*
H10	0.5637	0.5990	0.6231	0.031*
H12	0.6567	0.7880	0.6112	0.031*
C4	0.84836 (17)	0.5593 (3)	0.57641 (10)	0.0190 (4)
H8	0.8126	0.5546	0.5240	0.029*
H7	0.8776	0.7034	0.5909	0.029*
H9	0.9221	0.4616	0.5898	0.029*
C2	0.69656 (17)	0.2684 (3)	0.59443 (10)	0.0185 (4)
H5	0.6426	0.2222	0.6273	0.022*
H6	0.6405	0.2690	0.5446	0.022*
H3	0.738 (2)	0.489 (4)	0.7197 (13)	0.017 (5)*
H4	0.869 (3)	0.407 (5)	0.7072 (17)	0.035 (7)*
H1	0.822 (3)	0.073 (5)	0.6410 (19)	0.041 (8)*
H2	0.842 (3)	0.643 (5)	0.7078 (17)	0.038 (8)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O3	0.0159 (8)	0.0142 (9)	0.0258 (10)	0.000	0.0062 (7)	0.000
O2	0.0141 (7)	0.0189 (7)	0.0223 (7)	0.0031 (5)	0.0094 (5)	0.0022 (5)
O1	0.0217 (7)	0.0202 (7)	0.0221 (8)	0.0035 (5)	0.0087 (5)	−0.0011 (5)
N1	0.0132 (7)	0.0184 (8)	0.0189 (8)	0.0006 (6)	0.0099 (6)	0.0002 (6)
C5	0.0135 (11)	0.0170 (12)	0.0159 (12)	0.000	0.0090 (9)	0.000
C1	0.0138 (8)	0.0173 (9)	0.0187 (9)	0.0004 (6)	0.0091 (6)	0.0004 (6)
C3	0.0159 (8)	0.0217 (9)	0.0260 (10)	0.0025 (7)	0.0096 (7)	0.0036 (7)
C4	0.0170 (8)	0.0215 (9)	0.0225 (9)	−0.0012 (7)	0.0124 (7)	−0.0006 (7)
C2	0.0153 (8)	0.0205 (9)	0.0205 (9)	−0.0004 (7)	0.0060 (6)	−0.0006 (7)

Geometric parameters (Å, º)

O3—C5	1.283 (3)	C1—C2	1.532 (3)
O2—C5	1.2934 (17)	C3—H11	0.9800
O1—C2	1.412 (2)	C3—H10	0.9800
O1—H1	0.85 (3)	C3—H12	0.9800
N1—C1	1.508 (2)	C4—H8	0.9800
N1—H3	0.92 (3)	C4—H7	0.9800
N1—H4	0.92 (3)	C4—H9	0.9800
N1—H2	0.99 (3)	C2—H5	0.9900
C1—C3	1.523 (2)	C2—H6	0.9900
C1—C4	1.523 (2)
C2—O1—H1	109 (2)	C1—C3—H10	109.5
C1—N1—H3	110.0 (15)	H11—C3—H10	109.5
C1—N1—H4	109.5 (19)	C1—C3—H12	109.5
H3—N1—H4	115 (2)	H11—C3—H12	109.5
C1—N1—H2	109.3 (18)	H10—C3—H12	109.5
H3—N1—H2	107 (2)	C1—C4—H8	109.5
H4—N1—H2	106 (2)	C1—C4—H7	109.5
O3—C5—O2^i	119.62 (11)	H8—C4—H7	109.5
O3—C5—O2	119.62 (11)	C1—C4—H9	109.5
O2^i—C5—O2	120.8 (2)	H8—C4—H9	109.5
N1—C1—C3	108.29 (14)	H7—C4—H9	109.5
N1—C1—C4	107.95 (14)	O1—C2—C1	114.11 (14)
C3—C1—C4	111.69 (15)	O1—C2—H5	108.7
N1—C1—C2	108.64 (14)	C1—C2—H5	108.7
C3—C1—C2	108.84 (14)	O1—C2—H6	108.7
C4—C1—C2	111.33 (14)	C1—C2—H6	108.7
C1—C3—H11	109.5	H5—C2—H6	107.6
N1—C1—C2—O1	−71.47 (18)	C4—C1—C2—O1	47.3 (2)
C3—C1—C2—O1	170.81 (14)

Symmetry code: (i) −x+1, y, −z+3/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C4—H8···O1^ii	0.98	2.58	3.498 (2)	156
N1—H3···O2	0.92 (3)	1.94 (3)	2.8536 (18)	173 (2)
N1—H4···O3^iii	0.92 (3)	1.84 (3)	2.7403 (19)	169 (3)
O1—H1···O2^iv	0.85 (3)	1.90 (4)	2.738 (2)	168 (3)
N1—H2···O2^v	0.99 (3)	1.84 (3)	2.823 (2)	171 (3)

Symmetry codes: (ii) −x+3/2, −y+1/2, −z+1; (iii) x+1/2, y−1/2, z; (iv) −x+3/2, y−1/2, −z+3/2; (v) −x+3/2, y+1/2, −z+3/2.