Phase transitions in n-alkyl­ammonium di­hydrogenphosphates and -arsenates and ferroelastic n-hexyl- and n-octyl­ammonium di­hydrogen­arsenate

-Hexyl­ammonium di­hydrogenarsenate, (C₆­H₁₆N)[AsO₂(OH)₂], and -octyl­ammonium di­hydrogenarsenate, (C₈H₂₀N)[AsO₂(OH)₂], are both ferroelastic at room temperature. The samples used in this study were not subjected to a phase transition after they had been crystallized. The structures are monoclinic (P2₁/n) and isostructural with the corresponding di­hydrogenphosphates. Each sample contained two domains and each structure was refined as a twin. There are strong hydrogen bonds between di­hydrogenarsenates and moderate hydrogen bonds between di­hydrogenarsenates and -alkyl­ammonium groups. The hydrogen-bond distances correspond well to those observed in the di­hydrogenphosphates. All the atoms except two H atoms exist in pairs linked by the lost symmetry operations derived from the prototypic space group P2/b2₁/n2₁/a. Each of these two different H atoms is involved in an asymmetric hydrogen bond between an oxy­gen pair. These oxy­gens are supposed to change their roles as hydrogen-bond donors and acceptors during the ferroelastic switching. The phase-transition sequences are affected by interactions between the neighbouring organic chains in the structure.

S1. Comment

%T Ferroelasticity and complicated phase transition sequences were discovered in the series of the related dihydrogenphosphates C1ADP-C12ADP by Kroupa \& Fuith (1993, 1994). The phase transition in C3ADP was also investigated by Iwata et al. (1996). Regarding the phase transitions, the compounds C$n$ADP can be divided into three groups: a) C1ADP-C4ADP b) the compounds with $n$$>$5 and $n$ even and c) $n$$γeq$5 and $n$ odd.

Each of the compounds of group a) undergoes a continuous ferroelastic phase transition. In contrast to group a) each of the compounds in groups b) and c) undergoes a discontinuos ferroelastic phase transition. In addition, the compounds of the latter two groups b) and c) undergo other phase transitions when they are heated from the room temperature phase up to the ferroelastic phase transition. The hysteresis effects are common. (In the case of C9ADP, the original room-temperature phase does not even recover after cooling, Kroupa \& Fuith, 1993.) The compounds in group c) undergo more complicated phase transitions. The differences between the phase transition sequences of the first-time and at least once heated crystals are more prominent in group c) than in group b).

The ferroelastic phase transitions in the dihydrogenarsenates occur at considerably higher temperatures (by tens of K) than in the corresponding dihydrogenphosphates. C6ADA undergoes a ferroelastic phase transition at $σim$378 K, C7ADA at $σim$364 K and C8ADA melts at $σim$380 K before a phase transition takes place (Kroupa \& Fuith, 1994).

Until now, the structural studies were performed only in the series of the dihydrogenphosphates C2ADP-C10ADP. C2ADP, C3ADP and C4ADP were studied by Kasatani et al. (1998), C3ADP by Fábry, Císa\v rová \& Kroupa et al. (2000a), C4ADP by Fábry, Císa\v rová \& Kroupa et al. (2000b), C5ADP and C6ADP by Kasatani et al. (1999), C5ADP, C6ADP, C9ADP by Fábry et al. (2000), C7ADP and C8ADP by Fábry et al. (1997), and C10ADP by Oliver et al. (1998). The reasons for different packing of $n$-alkylammonium chains were explained by Fábry, Pet\v r\'ι\v cek, C\'ι sa\v rová \& Kroupa et al. (2000) by dominancy of the hydrogen-bonding of the ammonium groups to the double layers of the dihydrogenphosphates. Until now, however, no structure determination of any of the dihydrogenarsenates was performed. σch

The aim of this study was a structure determination of C6ADA and C8ADA in order to find out structural differences between these compounds and the corresponding dihydrogenphosphates, and to find out the reason for differences in the phase transitions.

The twinning matrix which relates the diffractions $h$$_{1}$$k$$_{1}$$l$$_{1}$ and $h$$_{2}$$k$$_{2}$$l$$_{2}$ from the dominant and minor domains, respectively, is analogous to that in the dihydrogenphosphates. The diffractions $h$$_{1}$$k$$_{1}$$l$$_{1}$ and $h$$_{2}$$k$$_{2}$$l$$_{2}$ are expressed in the reciprocal space basis of the first domain and a, c, $βeta$ mean the lattice parameters.

$$πmatrix{h_{2} & k_{2} & l_{2} χr} = πmatrix{h_{1} & k_{1} & l_{1} χr} πmatrix{-1 & 0 & 0 χr 0 & -1 & 0 χr 2 {a οver c}χosβeta & 0 & 1 χr} $$

The structures are similar to C6ADP and C8ADP which are depicted elsewhere. Figs. 1 and 2 depict the closest C$χdots$C intermolecular contacts in C6ADA. The refined values of the minor domain proportion are in good accordance with the values which were calculated from the intensities of well separated pertinent diffractions from the minor and the prevailing domains. The differences are due to imprecision in the reading of the crystal shape, and consequently in absorption correction, as well as in shielding of the domains which may be unevenly distributed within the specimen. The interatomic distances are normal - Tables 1 and 2 - Allen \& Kennard, 1993. The hydrogen-bond distances and angles regarding the double layers of dihydrogenarsenates are comparable to those in the dihydrogenphosphates C6ADP and C8ADP (Table 3). The hydrogen bonds N$χdots$O are shorter by $σim$0.033 AA in the dihydrogenarsenates than in the dihydrogenphosphates. The N$χdots$O distances are more evenly distributed in C6ADA and C8ADA than in the corresponding dihydrogenphosphates. The absolute values of the atomic displacement vectors - a measure of the distorsion of a structure from a prototypic phase (Abrahams \& Keve, 1971) - for the non-hydrogen atoms are in the range 0.01-0.14 AA.

The differences in the phase-transition sequences between the members of the family C$n$ADP with $n$$λeq$4 and with $n$$γeq$5 (which depend also on the parity of $n$) and in C$n$ADA are affected by the number of the C$χdots$C and C$χdots$N contacts and their distances. As it was already pointed out by Fábry et al. (1997) the closest C$χdots$C distances correspond well to the van der Waals contacts (Weast \& Astle, 1980). However, in C5ADP, C7ADP and C9ADP the closest C$χdots$C distances are as short as 3.7 AA in contrast to those in the dihydrogenphosphates with $n$ even which are about 3.8 AA long (in C4ADP 3.9 AA). It should be noted that C5ADP, C7ADP and C9ADP are the compounds where hysteresis effects are the most prominent as it was pointed in the Introduction. On the other hand, in C$n$ADP with $n$$λeq$4, the chains are so short that sufficient number of carbons can not get into interaction.

The larger dihydrogenarsenates would cause the $n$-alkylammonium chains in the dihydrogenarsenates to be more distant from each other than in the pertinent dihydrogenphosphates (Table 4). The average difference between the corresponding C$χdots$C distances in C6ADA - C6ADP and C8ADA - C8ADP are 0.065 and 0.062 AA, respectively.

The phase transition phenomena observed in these compounds can be explained at least qualitatively by the distances between the atoms in the neighbouring chains as well as by the number of these contacts. The closer the $n$-alkylammonium chains are, the more complicated phase transitions are observed. Not surprisingly, the phase transitions in the dihydrogenarsenates take place at higher temperatures than in the analogous dihydrogenphosphates. It is because the molecules in the dihydrogenarsenates have more space and thermal agitation must be more intensive to bring about these molecules into interaction which would provoke a phase transition.

From this point of view, on the phase transitions it may also be predicted that not only in C9ADP (Kroupa \& Fuith, 1993), but also in other C$n$ADP with $n$ odd and $n$$γeq$11, the room-temperature phase would not recover after the crystals were heated and cooled. It is because the longer chains would be less able to restore the original low-temperature arrangement.

As in the dihydrogenphosphates, the ferroelastic switching in C6ADA and C8ADA is concomitant to hopping of the hydrogens HO41 and HO22 from the donor (O41, O22, respectively) to the acceptor oxygens (O42, O21, respectively) - Tab. 3.

S2. Experimental

Crystallization of the n-alkylamine and H₃AsO₄ in methanol solutions (Kroupa & Fuith, 1994). Crystals which appeared single-domained under the polarization microscope were selected for a diffractometer measurement.

S3. Refinement

The structures are superstructures and ferroelastic structures as well. They can be related to the prototypic space group P 2/b 2₁/n 2₁/a. Therefore the crystals were expected to be twinned though the samples were chosen which seemed to be single-domained when viewed in the polarization microscope.

The domain proportion f was a refined parameter which converged to the value to -0.006 (4) and 0.026 (4) for C6ADA and C8ADA, respectively. (Therefore in case of C6ADA the domain proportion was set to 0.0 and this parameter was not refined.)

The domain proportion was also determined from well separated diffractions. In case of C6ADA there were used 19 pairs of the diffraction 004 measured at various azimuthal angles. The determined values of the domain proportion was 0.007 (3). In case of C8ADA there were used 31 pairs of different diffractions, among them also the reflections of the type 004. The values of the domain proportion determined in this way resulted in 0.009 (3).

All hydrogen atoms could be found in the difference Fourier maps. Most of them were refined to tolerable positions, nevertheless they all were restrained during refinement in order to prevent the H atoms to be misplaced.

The bond distances as well as the angles in which the H atoms were involved were restrained. The O—H, N—H and C—H bond lengths were restrained to 0.85, 0.90 and 0.95 Å, respectively.

Computing details

For both compounds, data collection: Enraf-Nonius Software (Enraf-Nonius, 1989); cell refinement: Enraf-Nonius Software; data reduction: JANA2000 (Petříček & Dušek, 2000); program(s) used to solve structure: JANA2000; program(s) used to refine structure: JANA2000; molecular graphics: ORTEPIII (Burnett & Johnson, 1996). Software used to prepare material for publication: JANA98 (Petříček & Dušek, 1998) for C6ADA; JANA98 (Petříček & Dušek, 2000) for C8ADA.

Figures

	Fig. 1. View of the fragment of C6ADA along the a axis with C—C and C—N intermolecular contacts up to 4.2 Å and 50% probability displacement ellipsoids (ORTEPIII; Burnett & Johnson, 1996).
	Fig. 2. View of the fragment of C6ADA along the c axis with C—C and C—N intermolecular contacts up to 4.2 Å and 50% probability displacement ellipsoids (ORTEPIII; Burnett & Johnson, 1996).

(C6ADA) n-hexylammonium dihydrogenarsenate

Crystal data

(C6H13NH3)[As(O)2(OH)2]	F(000) = 1008
Mr = 243.13	Dx = 1.555 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
a = 9.3105 (7) Å	Cell parameters from 25 reflections
b = 29.929 (2) Å	θ = 12.6–13.9°
c = 7.453 (6) Å	µ = 3.26 mm^−1
β = 90.64 (7)°	T = 292 K
V = 2077 (2) Å^3	Plate, colourless
Z = 8	0.43 × 0.20 × 0.09 mm

Data collection

Enraf-Nonius CAD-4-MACHIII-PC diffractometer	Rint = 0.038
ω–2θ scans	θmax = 26.5°
Absorption correction: gaussian (Templeton & Templeton, 1978)	h = −11→11
Tmin = 0.482, Tmax = 0.749	k = 0→37
4613 measured reflections	l = 0→9
4290 independent reflections	3 standard reflections every 3600 min
2505 reflections with I > 3σ(I)	intensity decay: 7.0%

Refinement

Refinement on F	0 constraints
Least-squares matrix: full with fixed elements per cycle	H-atom parameters constrained
R[F^2 > 2σ(F^2)] = 0.036	Weighting scheme based on measured s.u.'s w = [σ^2(Fo) + 0.0001(Fo)^2]^-1
wR(F^2) = 0.047	(Δ/σ)max = 0.001
S = 1.89	Δρmax = 1.86 e Å^−3
4290 reflections	Δρmin = −2.43 e Å^−3
362 parameters	Extinction correction: Becker & Coppens, 1974, type I, Lorentz. iso.
58 restraints	Extinction coefficient: 0.000029 (2)

Special details

Experimental. All the samples contained domains which could be reproducibly moved if the stress was exerted. In contrast to the crystals of C6ADA and C8ADA the domains could be called forth by stress not as easily as in C7ADA.

Refinement. The structure was refined as a twin. domain fraction f: calculated from the refinement: 0.026 (4) calculated from 31 pairs of measured intensities of well separated refelctions icluding the reflections 004 which were measured at different azimuthal angles: 0.009 (3)

The twinning matrix is given in _diffrn_reflns_transf_matrix_ items

The H atoms except of those which are bonded to the O atoms were restrained by the distfix and anglefix functions of JANA2000: The values for distfix were 0.85(0.022) A ng. for O—H distances. The values for distfix were 0.90(0.022) A ng. for N—H distances. The values for distfix were 0.95(0.03) A ng. for methyl-H distances. The values for distfix were 0.95(0.013) A ng. for methylene-H distances. The values for anglefix were 109.47(1.00) °.

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

	x	y	z	Uiso*/Ueq
As1	0.08376 (3)	0.30239 (1)	0.25269 (4)	0.0232 (1)
O11	−0.0423 (3)	0.34309 (9)	0.2279 (4)	0.0414 (9)
O21	0.1177 (2)	0.29244 (8)	0.4672 (2)	0.0351 (8)
O31	0.2201 (2)	0.31701 (8)	0.1270 (3)	0.0341 (7)
O41	0.0102 (2)	0.25386 (8)	0.1756 (3)	0.0367 (8)
As2	0.57995 (3)	0.30006 (1)	0.26243 (4)	0.0237 (1)
O12	0.4614 (3)	0.34303 (9)	0.2641 (3)	0.0410 (9)
O22	0.6188 (3)	0.29041 (9)	0.0427 (3)	0.0388 (9)
O32	0.7232 (2)	0.31610 (9)	0.3762 (3)	0.0372 (8)
O42	0.5067 (2)	0.25360 (8)	0.3410 (2)	0.0325 (7)
N1	0.1936 (3)	0.6915 (1)	0.2670 (3)	0.030 (1)
C11	0.1091 (4)	0.6498 (1)	0.2468 (5)	0.033 (1)
C21	0.2039 (4)	0.6089 (1)	0.2545 (5)	0.038 (1)
C31	0.1159 (4)	0.5662 (1)	0.2491 (5)	0.042 (1)
C41	0.2055 (4)	0.5238 (1)	0.2531 (5)	0.043 (1)
C51	0.1164 (5)	0.4815 (2)	0.2506 (6)	0.052 (2)
C61	0.2062 (7)	0.4390 (2)	0.2550 (7)	0.068 (2)
N2	0.7013 (3)	0.69112 (9)	0.2308 (4)	0.030 (1)
C12	0.6139 (4)	0.6498 (1)	0.2494 (5)	0.033 (1)
C22	0.7070 (4)	0.6088 (1)	0.2443 (5)	0.037 (1)
C32	0.6168 (4)	0.5661 (1)	0.2501 (5)	0.042 (1)
C42	0.7049 (4)	0.5235 (1)	0.2466 (5)	0.043 (1)
C52	0.6152 (5)	0.4812 (2)	0.2495 (6)	0.052 (2)
C62	0.7047 (6)	0.4389 (2)	0.2480 (7)	0.067 (2)
H1N1	0.234 (2)	0.6945 (9)	0.377 (2)	0.04 (1)*
H2N1	0.133 (3)	0.7145 (8)	0.246 (3)	0.08 (2)*
H3N1	0.262 (2)	0.6915 (9)	0.185 (3)	0.04 (1)*
H1C11	0.043 (2)	0.650 (1)	0.342 (2)	0.05 (1)*
H2C11	0.059 (2)	0.652 (1)	0.136 (2)	0.06 (1)*
H1C21	0.266 (2)	0.610 (1)	0.155 (2)	0.05 (1)*
H2C21	0.259 (2)	0.608 (1)	0.362 (2)	0.04 (1)*
H1C31	0.055 (2)	0.564 (1)	0.350 (2)	0.05 (1)*
H2C31	0.059 (2)	0.564 (1)	0.142 (2)	0.06 (1)*
H1C41	0.267 (2)	0.524 (1)	0.152 (2)	0.06 (1)*
H2C41	0.262 (2)	0.525 (1)	0.361 (2)	0.05 (1)*
H1C51	0.058 (3)	0.480 (1)	0.145 (2)	0.08 (2)*
H2C51	0.056 (3)	0.483 (1)	0.352 (2)	0.06 (1)*
H1C61	0.137 (4)	0.416 (1)	0.245 (5)	0.12 (2)*
H2C61	0.261 (3)	0.435 (1)	0.362 (3)	0.12 (2)*
H3C61	0.269 (3)	0.438 (1)	0.153 (4)	0.08 (2)*
H1N2	0.764 (2)	0.6937 (9)	0.318 (2)	0.04 (1)*
H2N2	0.638 (3)	0.7138 (8)	0.236 (3)	0.07 (1)*
H3N2	0.747 (2)	0.6917 (9)	0.126 (2)	0.05 (1)*
H1C12	0.562 (2)	0.653 (1)	0.358 (2)	0.04 (1)*
H2C12	0.548 (2)	0.650 (1)	0.151 (2)	0.032 (9)*
H1C22	0.756 (2)	0.610 (1)	0.133 (2)	0.05 (1)*
H2C22	0.775 (2)	0.607 (1)	0.340 (2)	0.05 (1)*
H1C32	0.559 (2)	0.568 (1)	0.354 (2)	0.05 (1)*
H2C32	0.556 (2)	0.563 (1)	0.148 (2)	0.05 (1)*
H1C42	0.757 (2)	0.526 (1)	0.138 (2)	0.06 (1)*
H2C42	0.770 (2)	0.524 (1)	0.347 (2)	0.04 (1)*
H1C52	0.554 (2)	0.483 (1)	0.147 (2)	0.06 (1)*
H2C52	0.559 (3)	0.483 (1)	0.354 (2)	0.06 (1)*
H1C62	0.768 (3)	0.439 (1)	0.348 (4)	0.13 (3)*
H2C62	0.643 (3)	0.414 (1)	0.257 (4)	0.09 (2)*
H3C62	0.758 (3)	0.436 (1)	0.141 (4)	0.10 (2)*
HO11	−0.116 (3)	0.336 (1)	0.282 (5)	0.07 (1)*
HO41	0.007 (4)	0.252 (1)	0.059 (2)	0.07 (1)*
HO12	0.382 (3)	0.335 (2)	0.224 (6)	0.11 (2)*
HO22	0.613 (4)	0.2626 (6)	0.015 (4)	0.05 (1)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
As1	0.0206 (2)	0.0292 (2)	0.0197 (2)	−0.0019 (2)	0.0002 (1)	0.0035 (1)
O11	0.025 (1)	0.038 (2)	0.061 (2)	0.004 (1)	0.005 (1)	0.015 (1)
O21	0.048 (1)	0.039 (2)	0.018 (1)	−0.006 (1)	−0.0037 (9)	0.002 (1)
O31	0.022 (1)	0.057 (2)	0.023 (1)	−0.010 (1)	0.0018 (9)	0.000 (1)
O41	0.046 (1)	0.043 (2)	0.021 (1)	−0.019 (1)	−0.001 (1)	−0.002 (1)
As2	0.0204 (2)	0.0304 (2)	0.0201 (2)	−0.0024 (2)	−0.0026 (1)	−0.0019 (1)
O12	0.023 (1)	0.038 (2)	0.061 (2)	0.005 (1)	−0.008 (1)	−0.010 (1)
O22	0.058 (2)	0.036 (2)	0.022 (1)	−0.009 (1)	0.006 (1)	−0.004 (1)
O32	0.020 (1)	0.060 (2)	0.031 (1)	−0.009 (1)	−0.0078 (9)	0.004 (1)
O42	0.044 (1)	0.034 (1)	0.020 (1)	−0.014 (1)	−0.0045 (9)	0.0014 (9)
N1	0.030 (2)	0.031 (2)	0.028 (1)	0.002 (1)	−0.001 (1)	0.000 (1)
C11	0.031 (2)	0.034 (2)	0.032 (2)	0.002 (2)	−0.007 (2)	−0.001 (2)
C21	0.037 (2)	0.033 (2)	0.043 (2)	0.004 (2)	−0.004 (2)	−0.005 (2)
C31	0.039 (2)	0.038 (3)	0.048 (2)	−0.002 (2)	−0.002 (2)	0.000 (2)
C41	0.048 (3)	0.035 (3)	0.046 (2)	−0.003 (2)	−0.001 (2)	−0.002 (2)
C51	0.057 (3)	0.042 (3)	0.055 (3)	−0.009 (2)	0.000 (2)	−0.002 (2)
C61	0.104 (4)	0.039 (3)	0.062 (3)	−0.004 (3)	−0.001 (3)	−0.001 (2)
N2	0.031 (2)	0.029 (2)	0.031 (1)	0.005 (1)	−0.001 (1)	0.000 (1)
C12	0.028 (2)	0.035 (2)	0.035 (2)	0.000 (2)	0.000 (2)	0.000 (2)
C22	0.033 (2)	0.038 (3)	0.040 (2)	−0.002 (2)	−0.003 (2)	0.002 (2)
C32	0.037 (2)	0.040 (3)	0.049 (2)	−0.004 (2)	−0.002 (2)	−0.002 (2)
C42	0.051 (3)	0.034 (3)	0.043 (2)	−0.006 (2)	0.001 (2)	0.002 (2)
C52	0.060 (3)	0.040 (3)	0.055 (3)	−0.011 (2)	−0.003 (3)	0.003 (2)
C62	0.102 (4)	0.036 (3)	0.064 (3)	−0.004 (3)	0.005 (3)	0.003 (2)

Geometric parameters (Å, º)

As1—O11	1.700 (3)	C41—H1C41	0.95 (2)
As1—O21	1.653 (2)	C41—H2C41	0.95 (2)
As1—O31	1.645 (2)	C51—H1C51	0.95 (2)
As1—O41	1.703 (2)	C51—H2C51	0.95 (2)
As2—O12	1.695 (3)	C61—H1C61	0.94 (3)
As2—O22	1.706 (2)	C61—H2C61	0.95 (3)
As2—O32	1.645 (2)	C61—H3C61	0.97 (3)
As2—O42	1.658 (2)	N2—H1N2	0.87 (2)
N1—C11	1.484 (5)	N2—H2N2	0.90 (3)
C11—C21	1.510 (5)	N2—H3N2	0.89 (2)
C21—C31	1.518 (6)	C12—H1C12	0.95 (2)
C31—C41	1.519 (6)	C12—H2C12	0.95 (2)
C41—C51	1.514 (6)	C22—H1C22	0.95 (2)
C51—C61	1.522 (7)	C22—H2C22	0.95 (2)
N2—C12	1.488 (5)	C32—H1C32	0.95 (2)
C12—C22	1.502 (5)	C32—H2C32	0.95 (2)
C22—C32	1.531 (6)	C42—H1C42	0.95 (2)
C32—C42	1.517 (6)	C42—H2C42	0.96 (2)
C42—C52	1.517 (6)	C52—H1C52	0.95 (2)
C52—C62	1.515 (7)	C52—H2C52	0.94 (2)
O11—HO11	0.83 (3)	C62—H1C62	0.95 (3)
O41—HO41	0.87 (2)	C62—H2C62	0.94 (3)
O12—HO12	0.83 (3)	C62—H3C62	0.95 (3)
O22—HO22	0.86 (2)	O11—O32^i	2.588 (3)
N1—H1N1	0.90 (2)	O21—O22^ii	2.543 (3)
N1—H2N1	0.90 (3)	O31—O12	2.579 (3)
N1—H3N1	0.89 (2)	O41—O42^iii	2.504 (3)
C11—H1C11	0.94 (2)	N1—O22^iv	2.960 (4)
C11—H2C11	0.95 (2)	N1—O32^v	2.771 (3)
C21—H1C21	0.95 (2)	N1—O42^vi	2.747 (4)
C21—H2C21	0.95 (2)	N2—O21^v	2.841 (3)
C31—H1C31	0.95 (2)	N2—O31^iv	2.784 (3)
C31—H2C31	0.95 (2)	N2—O41^vi	2.814 (4)
O11—As1—O21	111.0 (1)	O12—As2—O22	106.3 (1)
O11—As1—O31	106.5 (1)	O12—As2—O32	107.4 (1)
O11—As1—O41	107.5 (1)	O12—As2—O42	111.3 (1)
O21—As1—O31	117.3 (1)	O22—As2—O32	111.4 (1)
O21—As1—O41	104.2 (1)	O22—As2—O42	106.8 (1)
O31—As1—O41	110.2 (1)	O32—As2—O42	113.4 (1)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	D···A	D—H···A
O11—HO11···O32^i	0.83 (3)	2.588 (3)	174 (4)
O41—HO41···O42^iii	0.87 (2)	2.504 (3)	178 (4)
O12—HO12···O31	0.83 (3)	2.579 (3)	176 (4)
O22—HO22···O21^vii	0.86 (2)	2.543 (3)	175 (3)
N1—H3N1···O22^iv	0.89 (2)	2.960 (4)	160 (2)
N1—H1N1···O32^v	0.90 (2)	2.771 (3)	160 (2)
N1—H2N1···O42^vi	0.90 (3)	2.747 (4)	167 (2)
N2—H1N2···O21^v	0.87 (2)	2.841 (3)	170 (2)
N2—H3N2···O31^iv	0.89 (2)	2.785 (3)	159 (2)
N2—H2N2···O41^vi	0.90 (3)	2.814 (4)	161 (2)

Symmetry codes: (i) x−1, y, z; (iii) x−1/2, −y+1/2, z−1/2; (iv) −x+1, −y+1, −z; (v) −x+1, −y+1, −z+1; (vi) −x+1/2, y+1/2, −z+1/2; (vii) x+1/2, −y+1/2, z−1/2.

(C8ADA) n-octylammonium dihydrogenarsenate

Crystal data

(C8H17NH3)[As(O)2(OH)2]	F(000) = 1136
Mr = 271.19	Dx = 1.479 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
a = 9.307 (1) Å	Cell parameters from 25 reflections
b = 34.936 (4) Å	θ = 11–14°
c = 7.488 (1) Å	µ = 2.79 mm^−1
β = 90.58 (1)°	T = 292 K
V = 2434.6 (5) Å^3	Plate, colourless
Z = 8	0.41 × 0.26 × 0.12 mm

Data collection

Enraf-Nonius CAD-4-MACHIII-PC diffractometer	Rint = 0.015
ω–2θ scans	θmax = 26°
Absorption correction: gaussian (Templeton & Templeton, 1978)	h = −11→11
Tmin = 0.348, Tmax = 0.717	k = 0→42
5743 measured reflections	l = 0→9
4771 independent reflections	3 standard reflections every 3600 min
3125 reflections with I > 3σ(I)	intensity decay: 5.0%

Refinement

Refinement on F	0 constraints
Least-squares matrix: full with fixed elements per cycle	H-atom parameters constrained
R[F^2 > 2σ(F^2)] = 0.043	Weighting scheme based on measured s.u.'s w = [σ^2(Fo) + 0.0001(Fo)^2]^-1
wR(F^2) = 0.070	(Δ/σ)max = 0.001
S = 3.19	Δρmax = 1.36 e Å^−3
4771 reflections	Δρmin = −1.07 e Å^−3
431 parameters	Extinction correction: Becker & Coppens, 1974, type I, Lorentz. iso.
70 restraints	Extinction coefficient: 0.000069 (3)

Special details

Refinement. The structure was refined as a twin. domain fraction f: calculated from the refinement: 0.026 (4) calculated from 31 pairs of measured intensities of well separated reflections including the reflections 004 which were measured at different azimuthal angles: 0.01 (1)

The twinning matrix is given in _diffrn_reflns_transf_matrix_ items

The H atoms except of those which are bonded to the O atoms were restrained by the distfix and anglefix functions of JANA2000: The values for distfix were 0.85(0.022) A ng. for O—H distances. The values for distfix were 0.90(0.022) A ng. for N—H distances. The values for distfix were 0.95(0.03) A ng. for methyl-H distances. The values for distfix were 0.95(0.013) A ng. for methylene-H distances. The values for anglefix were 109.47(1.00) °.

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

	x	y	z	Uiso*/Ueq
As1	0.08370 (3)	0.294625 (9)	0.25268 (4)	0.0233 (1)
O11	−0.0431 (2)	0.32941 (7)	0.2281 (3)	0.0429 (9)
O21	0.1175 (2)	0.28668 (6)	0.4665 (3)	0.0361 (7)
O31	0.2207 (2)	0.30771 (7)	0.1290 (3)	0.0346 (7)
O41	0.0103 (2)	0.25285 (7)	0.1759 (3)	0.0395 (8)
As2	0.57989 (3)	0.292651 (9)	0.26326 (4)	0.0238 (1)
O12	0.4614 (2)	0.32965 (7)	0.2659 (3)	0.0403 (9)
O22	0.6182 (3)	0.28446 (8)	0.0442 (3)	0.0414 (9)
O32	0.7228 (2)	0.30587 (7)	0.3759 (3)	0.0358 (7)
O42	0.5069 (2)	0.25254 (6)	0.3412 (2)	0.0296 (7)
N1	0.1942 (3)	0.70009 (7)	0.2668 (3)	0.0285 (9)
C11	0.1113 (3)	0.6641 (1)	0.2480 (4)	0.032 (1)
C21	0.2053 (3)	0.6291 (1)	0.2551 (5)	0.036 (1)
C31	0.1166 (4)	0.5927 (1)	0.2482 (5)	0.038 (1)
C41	0.2066 (4)	0.5563 (1)	0.2528 (5)	0.039 (1)
C51	0.1187 (4)	0.5199 (1)	0.2499 (5)	0.040 (1)
C61	0.2067 (4)	0.4835 (1)	0.2526 (5)	0.039 (1)
C71	0.1180 (4)	0.4472 (1)	0.2504 (6)	0.050 (1)
C81	0.2051 (6)	0.4108 (1)	0.2530 (7)	0.064 (2)
N2	0.7031 (3)	0.69985 (8)	0.2305 (3)	0.0296 (9)
C12	0.6156 (3)	0.6644 (1)	0.2494 (4)	0.032 (1)
C22	0.7080 (3)	0.6290 (1)	0.2435 (5)	0.033 (1)
C32	0.6179 (4)	0.5926 (1)	0.2486 (5)	0.040 (1)
C42	0.7069 (4)	0.5563 (1)	0.2482 (5)	0.038 (1)
C52	0.6181 (4)	0.5198 (1)	0.2490 (5)	0.041 (1)
C62	0.7053 (4)	0.4834 (1)	0.2466 (5)	0.038 (1)
C72	0.6183 (4)	0.4470 (1)	0.2485 (6)	0.049 (1)
C82	0.7049 (5)	0.4108 (1)	0.2476 (6)	0.061 (2)
H2N1	0.132 (2)	0.7197 (6)	0.253 (3)	0.06 (1)*
H1N1	0.235 (2)	0.7012 (7)	0.376 (2)	0.034 (9)*
H3N1	0.262 (2)	0.7017 (7)	0.184 (2)	0.019 (8)*
H1C1	0.044 (2)	0.6669 (9)	0.342 (2)	0.035 (9)*
H2C1	0.061 (2)	0.6630 (9)	0.137 (2)	0.04 (1)*
H1C2	0.268 (2)	0.634 (1)	0.159 (2)	0.06 (1)*
H2C2	0.259 (2)	0.6279 (8)	0.363 (2)	0.026 (8)*
H1C31	0.052 (2)	0.591 (1)	0.345 (2)	0.05 (1)*
H2C31	0.064 (2)	0.5906 (8)	0.139 (2)	0.022 (8)*
H1C41	0.263 (3)	0.556 (1)	0.153 (4)	0.05 (1)*
H2C41	0.266 (2)	0.5574 (9)	0.357 (2)	0.035 (9)*
H1C51	0.060 (3)	0.5186 (9)	0.139 (4)	0.05 (1)*
H2C51	0.057 (2)	0.5226 (9)	0.350 (2)	0.037 (9)*
H1C61	0.267 (3)	0.4832 (9)	0.144 (4)	0.04 (1)*
H2C61	0.266 (2)	0.486 (1)	0.356 (2)	0.07 (1)*
H1C71	0.057 (4)	0.445 (1)	0.145 (5)	0.09 (2)*
H2C71	0.060 (2)	0.448 (1)	0.354 (2)	0.06 (1)*
H1C81	0.265 (3)	0.411 (1)	0.357 (3)	0.09 (2)*
H2C81	0.147 (3)	0.3886 (8)	0.253 (4)	0.09 (2)*
H3C81	0.264 (3)	0.411 (1)	0.148 (3)	0.11 (2)*
H1N2	0.765 (2)	0.7002 (9)	0.322 (2)	0.06 (1)*
H2N2	0.641 (3)	0.7193 (6)	0.241 (3)	0.06 (1)*
H3N2	0.751 (2)	0.7022 (7)	0.128 (2)	0.031 (9)*
H1C12	0.565 (2)	0.6694 (9)	0.356 (2)	0.04 (1)*
H2C12	0.550 (2)	0.6633 (8)	0.152 (2)	0.031 (9)*
H1C22	0.767 (2)	0.634 (1)	0.143 (2)	0.07 (1)*
H2C22	0.767 (2)	0.6261 (7)	0.347 (2)	0.015 (7)*
H1C32	0.560 (2)	0.594 (1)	0.352 (2)	0.07 (1)*
H2C32	0.558 (2)	0.5920 (8)	0.145 (2)	0.026 (9)*
H1C42	0.763 (2)	0.5557 (9)	0.143 (2)	0.05 (1)*
H2C42	0.772 (3)	0.5565 (9)	0.356 (4)	0.04 (1)*
H1C52	0.558 (2)	0.519 (1)	0.145 (2)	0.05 (1)*
H2C52	0.562 (3)	0.5223 (9)	0.348 (4)	0.05 (1)*
H1C62	0.764 (2)	0.4844 (9)	0.144 (2)	0.033 (9)*
H2C62	0.763 (2)	0.486 (1)	0.351 (2)	0.06 (1)*
H1C72	0.555 (3)	0.446 (1)	0.147 (4)	0.05 (1)*
H2C72	0.563 (2)	0.4490 (9)	0.354 (2)	0.04 (1)*
H1C82	0.768 (3)	0.412 (1)	0.348 (3)	0.12 (2)*
H2C82	0.645 (3)	0.3889 (8)	0.256 (4)	0.09 (2)*
H3C82	0.760 (3)	0.409 (1)	0.142 (3)	0.08 (2)*
HO11	−0.118 (3)	0.320 (1)	0.274 (5)	0.10 (2)*
HO41	0.009 (3)	0.2552 (9)	0.061 (2)	0.04 (1)*
HO12	0.389 (2)	0.3195 (9)	0.220 (4)	0.03 (1)*
HO22	0.603 (4)	0.2621 (5)	0.008 (4)	0.06 (1)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
As1	0.0215 (2)	0.0273 (2)	0.0211 (2)	−0.0022 (1)	0.0013 (1)	0.0030 (1)
O11	0.024 (1)	0.041 (2)	0.064 (2)	0.003 (1)	0.002 (1)	0.015 (1)
O21	0.046 (1)	0.039 (1)	0.024 (1)	−0.006 (1)	−0.002 (1)	−0.001 (1)
O31	0.025 (1)	0.052 (2)	0.027 (1)	−0.010 (1)	0.0027 (9)	0.006 (1)
O41	0.045 (1)	0.052 (2)	0.021 (1)	−0.017 (1)	0.000 (1)	−0.005 (1)
As2	0.0223 (2)	0.0275 (2)	0.0216 (2)	−0.0021 (1)	−0.0016 (1)	−0.0008 (1)
O12	0.025 (1)	0.030 (1)	0.065 (2)	0.003 (1)	−0.009 (1)	−0.006 (1)
O22	0.064 (2)	0.037 (2)	0.023 (1)	−0.008 (1)	0.007 (1)	−0.002 (1)
O32	0.025 (1)	0.047 (1)	0.036 (1)	−0.008 (1)	−0.0077 (9)	0.012 (1)
O42	0.047 (1)	0.020 (1)	0.022 (1)	−0.010 (1)	−0.0017 (9)	0.0003 (9)
N1	0.031 (1)	0.026 (2)	0.028 (1)	0.003 (1)	0.000 (1)	−0.001 (1)
C11	0.033 (2)	0.028 (2)	0.034 (2)	−0.001 (2)	−0.005 (2)	−0.004 (2)
C21	0.034 (2)	0.030 (2)	0.044 (2)	0.006 (2)	−0.003 (2)	−0.008 (2)
C31	0.040 (2)	0.026 (2)	0.049 (2)	−0.002 (2)	0.002 (2)	−0.004 (2)
C41	0.037 (2)	0.034 (2)	0.045 (2)	0.000 (2)	−0.001 (2)	−0.003 (2)
C51	0.044 (2)	0.030 (2)	0.046 (2)	−0.001 (2)	0.001 (2)	−0.004 (2)
C61	0.048 (2)	0.029 (2)	0.040 (2)	−0.002 (2)	−0.003 (2)	−0.002 (2)
C71	0.059 (3)	0.031 (2)	0.060 (3)	−0.007 (2)	0.003 (2)	0.000 (2)
C81	0.094 (4)	0.032 (3)	0.067 (3)	−0.001 (3)	−0.005 (3)	0.004 (2)
N2	0.029 (1)	0.030 (2)	0.031 (1)	0.002 (1)	0.003 (1)	−0.003 (1)
C12	0.028 (2)	0.029 (2)	0.038 (2)	−0.001 (2)	0.000 (2)	−0.002 (2)
C22	0.031 (2)	0.028 (2)	0.040 (2)	0.000 (2)	−0.003 (2)	−0.005 (2)
C32	0.036 (2)	0.032 (2)	0.051 (2)	0.001 (2)	−0.003 (2)	−0.004 (2)
C42	0.041 (2)	0.028 (2)	0.043 (2)	−0.001 (2)	0.003 (2)	0.001 (2)
C52	0.047 (2)	0.031 (2)	0.045 (2)	−0.001 (2)	−0.002 (2)	0.001 (2)
C62	0.048 (2)	0.028 (2)	0.039 (2)	−0.002 (2)	0.002 (2)	0.001 (2)
C72	0.060 (3)	0.034 (3)	0.052 (3)	−0.005 (2)	0.001 (2)	0.003 (2)
C82	0.093 (4)	0.026 (3)	0.064 (3)	−0.002 (2)	0.005 (3)	0.001 (2)

Geometric parameters (Å, º)

As1—O11	1.703 (2)	C51—H1C51	0.99 (3)
As1—O21	1.652 (2)	C51—H2C51	0.95 (2)
As1—O31	1.648 (2)	C61—H1C61	0.99 (3)
As1—O41	1.709 (2)	C61—H2C61	0.95 (2)
As2—O12	1.699 (2)	C71—H1C71	0.97 (4)
As2—O22	1.707 (2)	C71—H2C71	0.95 (2)
As2—O32	1.634 (2)	C81—H3C81	0.96 (3)
As2—O42	1.666 (2)	C81—H1C81	0.95 (2)
N1—C11	1.480 (4)	C81—H2C81	0.94 (3)
C11—C21	1.506 (5)	N2—H1N2	0.89 (2)
C21—C31	1.517 (5)	N2—H2N2	0.89 (2)
C31—C41	1.523 (5)	N2—H3N2	0.89 (2)
C41—C51	1.512 (5)	C12—H1C12	0.95 (2)
C51—C61	1.510 (5)	C12—H2C12	0.95 (2)
C61—C71	1.515 (6)	C22—H1C22	0.95 (2)
C71—C81	1.507 (6)	C22—H2C22	0.95 (2)
N2—C12	1.490 (4)	C32—H1C32	0.95 (2)
C12—C22	1.508 (5)	C32—H2C32	0.95 (2)
C22—C32	1.524 (5)	C42—H1C42	0.95 (2)
C32—C42	1.512 (5)	C42—H2C42	1.01 (3)
C42—C52	1.521 (5)	C52—H1C52	0.95 (2)
C52—C62	1.508 (5)	C52—H2C52	0.92 (3)
C62—C72	1.509 (5)	C62—H1C62	0.95 (2)
C72—C82	1.500 (6)	C62—H2C62	0.95 (2)
O11—HO11	0.84 (3)	C72—H1C72	0.95 (3)
O41—HO41	0.87 (1)	C72—H2C72	0.95 (2)
O12—HO12	0.83 (2)	C82—H1C82	0.95 (3)
O22—HO22	0.84 (2)	C82—H2C82	0.95 (3)
N1—H2N1	0.90 (2)	C82—H3C82	0.95 (2)
N1—H1N1	0.90 (1)	O11—O32^i	2.588 (3)
N1—H3N1	0.89 (2)	O21—O22^ii	2.552 (3)
C11—H1C11	0.95 (2)	O31—O12	2.571 (3)
C11—H2C11	0.95 (1)	O41—O42^iii	2.513 (3)
C21—H1C21	0.95 (2)	N1—O22^iv	2.974 (4)
C21—H2C21	0.95 (2)	N1—O32^v	2.785 (3)
C31—H1C31	0.95 (2)	N1—O42^vi	2.735 (3)
C31—H2C31	0.95 (1)	N2—O21^v	2.843 (3)
C41—H1C41	0.92 (3)	N2—O31^iv	2.803 (3)
C41—H2C41	0.95 (2)	N2—O41^vi	2.809 (4)
O11—As1—O21	110.5 (1)	O12—As2—O22	106.3 (1)
O11—As1—O31	106.3 (1)	O12—As2—O32	107.7 (1)
O11—As1—O41	107.4 (1)	O12—As2—O42	111.6 (1)
O21—As1—O31	116.8 (1)	O22—As2—O32	111.5 (1)
O21—As1—O41	104.7 (1)	O22—As2—O42	106.6 (1)
O31—As1—O41	110.9 (1)	O32—As2—O42	112.9 (1)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	D···A	D—H···A
O11—HO11···O32^i	0.84 (3)	2.588 (3)	174 (5)
O41—HO41···O42^iii	0.87 (2)	2.513 (3)	165 (3)
O12—HO12···O31	0.83 (3)	2.571 (3)	168 (3)
O22—HO22···O21^vii	0.84 (2)	2.552 (3)	163 (3)
N1—H3N1···O22^iv	0.89 (2)	2.975 (4)	165 (2)
N1—H1N1···O32^v	0.90 (2)	2.785 (3)	164 (2)
N1—H2N1···O42^vi	0.90 (3)	2.735 (3)	162 (2)
N2—H1N2···O21^v	0.89 (2)	2.843 (3)	166 (3)
N2—H3N2···O31^iv	0.89 (2)	2.803 (3)	153 (2)
N2—H2N2···O41^vi	0.89 (3)	2.809 (4)	163 (2)

Symmetry codes: (i) x−1, y, z; (iii) x−1/2, −y+1/2, z−1/2; (iv) −x+1, −y+1, −z; (v) −x+1, −y+1, −z+1; (vi) −x+1/2, y+1/2, −z+1/2; (vii) x+1/2, −y+1/2, z−1/2.