ПОВЕРХНОСТЬ. РЕНТГЕНОВСКИЕ, СННХРОТРОННЫЕ И НЕЙТРОННЫЕ HC^EMOBÄHHü, 2007, < 2, с. 73-79
UDK 539.2
REFINEMENT OF HYDROGEN POSITIONS IN (NH4)2Se04 © 2007 r. L. S. Smirnov1, G. Melnyk2, N. Zink2, K. Wozniak3, P. Dominiak3,
6
A. Pawlukojc1, 4, A. Loose5, |L. A. Shuvalov|'
1I.M. Frank Laboratory of Neutron Physics, JINR, Dubna, Moscow district, Russia 2Johannes Gutenberg-Universität, Mainz, Institut für Anorganische und Analytische Chemie, Mainz, Germany
3University of Warsaw, Warsaw, Poland 4Institute of Nuclear Chemistry and Technology, Warsaw, Poland 5BENSC, Hahn-Meitner Institute, Berlin, Germany 6Shubnikov Institute of Crystallography RAS, Moscow, Russia Submitted to Edition 15.06.2006
The crystal structure of ammonium selenate has been studied by means of single crystal X-ray and neutron diffraction with the purpose of the refinement of hydrogen positions. The refined hydrogen positions obtained by single crystal neutron diffraction show that N-H bond lengths form a regular tetrahedron in an ammonium ion. The single crystal X-ray diffraction data show that N-H bond lengths are shorter than those obtained by neutron diffraction and are not equal between themselves. Thus, the comparison of the results of X-ray and neutron diffraction allows one to suggest that the shorter N-H bond lengths obtained by X-ray diffraction reflect the distribution of the electron charge density of ammonium ions within the (NH4)2SeO4 crystal lattice.
INTRODUCTION
Until recently ammonium selenate, (NH4)2SeO4 or ASE, has been known as a compound which has a mono-clinic crystal structure at room temperature as determined by X-ray diffraction in [1]. The appeared communication [2] informed that a phase transition above room temperature, i.e. at 380 to 400 K, possibly via an intermediate phase was observed in (NH4)2SeO4. The high temperature crystal structure of ASE was not determined in this communication. On the other hand, the authors indicated that they did not observe any phase transition from 300 up to 5 K.
The crystal structure of ammonium selenate as determined in the X-ray investigation [1] has a monoclinic unit cell with C2/m sp. gr., Z = 4, and lattice parameters a = 12.152(6) À, b = 6.418(3) À and c = 7.711(4) À, P = 115.50(12)° at room temperature. The results of the X-ray study [1] showed that the ammonium groups NH4(1) and NH4(2) are deformed.
The objective of the current investigation is the refinement of hydrogen positions in ASE with the help of X-ray and neutron single crystal diffraction.
EXPERIMENTAL RESULTS AND DISCUSSION
The investigated ASE single crystal was prepared using the reaction of synthesis in an aqueous medium:
CuSeO4 + ( NH4 )2S — CuS + ( NH4 )2SeO4.
The refinement of the crystal structure of (NH4)2SeO4 was carried out by single crystal X-ray and neutron diffraction. The X-ray diffraction and neutron diffraction
measurements were performed at room temperature on the Kuma4CCD X-ray diffractometer (Warsaw University, Chemistry Department, Warsaw, Poland) and the E5 neutron four-circle diffractometer (BENSC HMI, Berlin, Germany), respectively. The conditions of the measurements, including experimental temperatures, used wavelengths, refined space group, obtained lattice parameters, number of measured reflections, number of unique reflections used for refining of atomic
Table 1. The conditions of measurements
Parameter
Temperature Wavelength Space group
a b c
P
Number of reflections Unique reflections R1
(NH4)2SeO4
X-ray
290 K 0.71073 Â
C2/m 12.127(2) Â 6.4200(9) Â 7.700(1) Â 115.44(2)° 4911 721 0.074
Neutron
290 K 0.9019 Â C2/m 12.4319(43) Â 6.5744(36) Â 7.8845(31) Â 115.513(18)° 1055 874 0.0677
Table 2a. The atomic positions in (NH4)2SeO4 (X-ray diffraction refinement)
Atom X y z Occupancy Ueq, Â2
Se 0.175(1) 0.0 0.218(1) 0.5 0.018(1)
O(1) 0.322(2) 0.0 0.361(4) 0.5 0.033(1)
O(2) 0.097(3) 0.0 0.344(4) 0.5 0.034(1)
O(3) 0.144(2) 0.209(3) 0.086(5) 1.0 0.030(1)
N(1) 0.517(3) 0.0 0.246(5) 0.5 0.028(1)
N(2) 0.842(3) 0.0 0.261(5) 0.5 0.023(1)
H(1) 0.458(5) 0.0 0.288(7) 0.5 0.03(1)
H(2) 0.907(7) 0.0 0.266(9) 0.5 0.06(2)
H(3) 0.568(5) 0.0 0.366(8) 0.5 0.03(1)
H(4) 0.855(5) 0.0 0.372(8) 0.5 0.03(1)
H(5) 0.525(3) -0.106(5) 0.189(5) 1.0 0.034(9)
H(6) 0.800(4) -0.103(6) 0.200(5) 1.0 0.030(8)
Table 2b. The atomic positions in (NH4)2SeO4 (current neutron diffraction)
Atom X y z Occupancy Ueq, Â2
Se 0.175(2) 0.0 0.218(3) 0.5 0.0202(4)
O(1) 0.322(3) 0.0 0.360(5) 0.5 0.0360(8)
O(2) 0.096(3) 0.0 0.344(5) 0.5 0.0367(9)
O(3) 0.144(2) 0.209(4) 0.086(3) 1.0 0.0323(5)
N(1) 0.517(2) 0.0 0.245(3) 0.5 0.0317(5)
N(2) 0.842(2) 0.0 0.261(3) 0.5 0.0274(4)
H(1) 0.437(7) 0.0 0.262(1) 0.5 0.057(2)
H(2) 0.923(8) 0.0 0.253(1) 0.5 0.056(2)
H(3) 0.585(7) 0.0 0.384(1) 0.5 0.0614(2)
H(4) 0.856(6) 0.0 0.402(9) 0.5 0.0463(2)
H(5) 0.527(9) 0.126(1) 0.1757(9) 1.0 0.0718(2)
H(6) 0.791(6) 0.126(1) 0.1946(8) 1.0 0.0573(1)
Table 3. The thermal parameters for atoms in (NH4)2SeO4 (current neutron diffraction)
Atom U11 U22 U33 U23 U13 U12
Se 0.0179(7) 0.0255(1) 0.0178(7) 0.0 0.0084(6) 0.0
O(1) 0.0208(1) 0.0544(2) 0.0279(1) 0.0 0.0060(1) 0.0
O(2) 0.0295(1) 0.0607(3) 0.0243(1) 0.0 0.0158(1) 0.0
O(3) 0.0368(1) 0.0266(11) 0.0315(9) 0.0058(8) 0.0129(8) -0.0002(9)
N(1) 0.0377(9) 0.0292(1) 0.0295(8) 0.0 0.0159(7) 0.0
N(2) 0.0303(9) 0.0286(1) 0.0235(7) 0.0 0.0118(7) 0.0
H(1) 0.0428(3) 0.0734(6) 0.0557(4) 0.0 0.0210(3) 0.0
H(2) 0.0566(4) 0.0696(6) 0.0569(4) 0.0 0.0378(4) 0.0
H(3) 0.0436(3) 0.0963(8) 0.0460(4) 0.0 0.0207(3) 0.0
H(4) 0.0465(3) 0.0613(5) 0.0349(3) 0.0 0.0212(2) 0.0
H(5) 0.1169(6) 0.0515(4) 0.0583(3) 0.0098(3) 0.0485(4) -0.0041(4)
H(6) 0.0677(33) 0.0473(3) 0.0520(3) 0.0104(2) 0.0211(2) 0.0158(3)
/r
Fig. 1. The (NH4)2SeO4 unit cell as a projection on the (ac) plane.
c
positions and the refinement results, are presented in table 1.
The experimental X-ray and neutron data are processed using the programs [3, 4]. The atomic positions in (NH4)2SeO4 and Ueq equivalent isotropic displacement parameters at 290 K refined by means of X-ray diffraction are presented in table 2a. The Ueq were defined as one third of the trace of the orthogonalized Uj tensor.
The atomic positions in (NH4)2SeO4 and Ueq equivalent isotropic displacement parameters at 290 K refined by neutron diffraction are presented in table 2b. The Ueq is defined as one third of the trace of the or-
cq
thogonalized Uj tensor.
The anisotropic displacement parameters obtained by neutron diffraction at 290 K for (NH4)2SeO4 are presented in table 3.
The Se-O(I) and N(1)-H(J) and N(2)-H(J) bond lengths in (NH4)2SeO4 determined with the help of recent X-ray and neutron diffraction measurements are presented in table 4. A comparison of average Se-O(I), N(1)-H(J) and N(2)-H(J) bond lengths shows that the bond lengths determined by X-ray diffraction are shorter than those determined by neutron diffraction.
The comparison points to that there takes place a change in the electronic charge density if a chemical
bond between SeO42 and NH+ ions in the crystal lattice is formed. The Se-O(I) average bond lengths determined by X-ray diffraction in [1] are longer than those obtained in a recent X-ray study by 0.012 A and are shorter than those obtained in the neutron diffraction study by 0.028 A. The comparison of the averages of the N(1)-H(J) and N(2)-H(J) bond lengths with those obtained in [1] cannot be correct as the determination of hydrogen positions in [1] is just approximate. The authors [1] determined the hydrogen positions from the difference map and they were not involved in a full-matrix least squares refinement. The final refinement was
carried out with fixed H positions obtained from the Fo-Fc differential Fourier maps.
A comparison of the N(2)-H(J) average bond length from [1] with that from a present X-ray study shows that they differ by 0.126 A. It is interesting to compare the obtained average Se-O(I) and N-H(I) bond lengths
—2 +
with those calculated for free SeO4 and NH4 ions.
The configuration calculations for free SeO42 and
NH+ ions was carried out with the GAUSSIAN98 program [5]. On the basis of the Hartree-Fock method and 6-31G* electron wave functions there are accepted the HF/6-31G* model. The calculated Se-O and N-H bond lengths are presented in table 4. The Se-O and N-H bond lengths obtained from the neutron diffraction ex-
Fig. 2. The (NH4)2SeO4 unit cell as a projection on the (bc) plane.
Fig. 3. The (NH4)2SeO4 unit cell as a projection on the (ab) plane.
periment are longer than those calculated for free ions by 0.018 and 0.032 À, respectively.
However, analysis of current single crystal X-ray and neutron diffraction shows an insignificant spread in
Table 4. The bond lengths for NH4(1), NH4(2) and SeO4 ions
Atoms X-ray, À (from [1]) Neutron, À X-ray, À
Se-O(1) 1.645(8) 1.682(4) 1.638(2)
Se-O(2) 1.644(8) 1.667(4) 1.628(3)
Se-O(3) 1.643(5) 1.667(3) 1.630(2)
Mean 1.644(7) 1.672(4) 1.632(2)
SeO2-
Se-O
N(1)-H(1) 0.890 1.057(4) 0.91(6)
N(1)-H(3) 0.936 1.052(4) 0.86(6)
N(1)-H(5) 0.861 1.029(8) 0.84(4)
Mean 0.896 1.046(5) 0.87(5)
N(2)-H(2) 0.947 1.045(6) 0.78(8)
N(2)-H(4) 0.922 1.047(8) 0.80(6)
N(2)-H(6) 0.94 1.035(7) 0.85(4)
Mean 0.936 1.042(7) 0.81(6)
NH+
N-H
Calculation, À
1.654
1.012
the values of bond lengths for Se-O(I), N(1)-H(I) and N(2)-H(J) prompting the conclusion that in a (NH4)2SeO4 each ammonium ion can be considered as a nearly regular tetrahedron.
A (NH4)2SeO4 unit cell is presented in fig. 1 as a projection on the (ac) plane, in fig. 2 as a projection on the (bc) plane and in fig. 3 as a projection on the (ab) plane.
It is interesting to carry out a comparison between differential Fourier electronic charge density and differential Fourier nuclear density maps obtained by X-ray and neutron diffraction, respectively. The corresponding Fourier maps for NH4(1) and NH4(2) groups are presented in figs. 4 and 5, respectively.
The differential Fourier
Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.