AIJAZ A. KHAN, WERNER H. BAUR. AND M. A. Q. KHAN 2065 We would like to thank Dr K. L. Loening for sug- gesting the von Baeyer name of photoaldrin and the Computer Center of the University of Illinois at Chicago for the computing facilities. References AMMON, H. L. & JENSEN, L. H. (1967). Acta Cryst. 23, 805. BARTELL, L. S. (1959). J. Amer. Chem. Soc. 81, 3496. BARTELL, L. S. (1962). Tetrahedron, 17, 177. BAUR, W. H. (1970). In Handbook of Geochemistry, Hy- drogen, 1-A-(l-5). Berlin: Springer. BAUR, W. H. (1972). Acta Cryst. B28, 1456. BAUR, W. H. & KHAN, A. A. (1970). Acta Cryst. B26, 1584. DOYLE, P. A. & TURNER, P. S. (1968). Acta Cryst. A24, 390. International Tables for X-ray Crystallography (1962). Vol. III. Birmingham: Kynoch Press. KARLE, J. & HAUPa'MAN, M. (1956). Aeta Cryst. 9, 635. KHAN, A. A. & BAUR, W. H. (1972). Acta Cryst. B28, 683. KHAN, M. A. Q., SUTHERLAND, D. J., ROSEN, J. D. & CAREY, W. F. (1970). J. Econ. Entomol. 63, No. 2, 470. MACDONALD, A. C. & TROTTER, J. (1965). Acta Cryst. 19, 456. ROSEN, J. D. (1967). Chem. Commun. p. 189. SHIMANOUCHI, H. (~ SASADA, Y. (1970). Acta Cryst. B26, 563. SUTHERLAND, D. J. 8z ROSEN, J. D. (1968). Mosq. News. 28, 155. Aeta Cryst. (1972). B28, 2065 The Crystal Structure of Diammonium Hydrogen Phosphate, (NH4)2HPO4 By AIJAZ A. KHAN Department of Geological Sciences, University of Illinois at Chicago, Chicago, Illinois 60680, U.S.A. AND J. P. Roux AND WmLIAM J. JAMES Department of Chemistry and the Graduate Center for Materials Research, University of Missouri-Rolla, Rolla, Missouri 65401, U.S.A. (Received 30 December 1971) (NH4)2HPO4 crystallizes in the space group P21/c with cell constants a = 11"043 (6), b = 6.700 (3), c = 8.031 (4)/~, fl = 113"42 (3) ° and Z= 4, and is isomorphous with (NHa)2HAsO4. The structure is refined using three-dimensional data collected on a four-circle automatic diffractometer. The final R value is 0.015 for 649 observed reflections. The positions of the hydrogen atoms indicate that only four among five N. • • O contact distances (less than 3.2/~ from each NH4 + ion) represent the actual N-H... O bonds. The length of the O-H..-O bond is 2.615 (1) A and differs significantly from 2.669 (13)/~ found in the isomorphous arsenate. It is suggested that this is a consequence of the size difference between the ps+ and As 5+ ions. Introduction In an earlier paper (Khan, Straumanis & James, 1970) the crystal structure of (NH4)2HAsO4 is reported. However, it was not possible to determine the positions of the hydrogen atoms during that study and the hydro- gen bonding suggested was based only on the contacts between the heavier atoms. Since each NH + ion in the structure is coordinated by five oxygen atoms, there is a certain amount of ambiguity regarding the N-H...O bonding. The cell dimension data and the space group reported for (NHa)2HPO4 by Smith, Lehr & Brown (1957) indicated that this phosphate is isomorphous with the corresponding arsenate. A crystal structure study of (NHa)2HPO4 was therefore undertaken to find the actual hydrogen bonding from the NH~ ions by determining the positions of the hydrogen atoms. Further it was deemed desirable to make a comparative study of the two isomorphous structures. Such a comparison might reveal any pos- sible influence on the structure of a replacement of a ps+ ion with an As 5+ ion. Experimental From a commercially supplied crystalline sample of (NH4)2HPO4, an almost spherical crystal of diameter 0.40 mm was selected for the X-ray work. The intensity data were collected on a four-circle automatic dif- fractometer using Ag K~ radiation. The details of the experimental procedures are the same as described in some earlier publications (Khan, Baur & Forbes, 1972; Baur & Khan, 1970). Lattice parameters were deter- mined from 12 carefully centered reflections. The intensities were collected in two of the four equivalent quadrants up to sin 0/2=0.54 A -1. These were aver- aged after applying the Lorentz-polarization correc- tions and were converted to Fo values. Absorption corrections were neglected. 1440 non-unique reflec- tions were averaged to 773; of these 124 were considered
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`2066 THE CRYSTAL STRUCTURE OF DIAMMONIUM HYDROGEN PHOSPHATE to be zero, which resulted in a total of 649 observed reflections. Crystal data are: a= 11.043 (6), b=6.700 (3), c= 8-031 (4) A, fl= 113.42 (3) °, V=545.3 A 3, Z=4, Dx= 1.608, Dm=l'619 g.cm -3 (Schiff, 1859), F.W. 131-97. The space group is P21/c. The programs were the same as those used by Khan, Baur & Forbes (1972). Atomic scattering factors for P, N, O- and H were taken from the International Tables for X-ray Crystallography (I 962). Starting with the positional parameters for the non- hydrogen atoms as in (NH4)2HAsO4 (Khan et al., 1970) the structure was refined by the least-squares method. The refinement converged to a conventional R equal to 0.05, when all the seven atoms were refined anisotropically. A difference Fourier synthesis, at this stage revealed the positions of the nine hydrogen atoms in the structure. The final refinement with isotropic temperature factors for the hydrogen atoms and anisotropic factors for the heavier atoms converged to an R of 0.015. The weighted R is 0.020. Results and discussion Positional and anisotropic thermal parameters for the nonhydrogen atoms in (NH4)zHPO4 are listed in Table 1 and the root-mean-square thermal displace- ments for these atoms along the principal axes and their orientations relative to the a, b and c axes are given in Table 2. Positional and isotropic temperature factors for the hydrogen atoms are given in Table 3. Observed and calculated structure factors are compared in Table 4. Fig. 1 shows the structure of (NH4)2HPO4 when viewed along the b axis. Table 5 contains selected interatomic bond lengths and bond angles. The structure as described for the isomorphous (NH4)zHAsO4 (Khan et al., 1970), consists of PO4 and NH4 tetrahedra held together by O-H...O and N-H-..O bonds. The interatomic distances and angles have the usual values. Of the four P-O bond lengths within the PO 4 tetrahedron, one is longer than the remaining three, typical of a (PO3OH) group. The average values of the O-P-O angles which, respectively, contain and do not contain the P-O(1) bond [the P-O(H) bond] are smaller (107.2 °) and larger (111.7 °) than the ideal tetrahedral angle. The oxygen-oxygen contacts in the PO4 tetrahedron range between 2.444 and 2.542 A. Table 2. (NH4)2HPO4 root-mean-square thermal displacements along principal axes and their orientations relative to a, b and c Angle with Axis Displacement [100] [010] P 1 0.133 (1) ~ 104 (2) ° 19 (2) ° 2 0.145 (1) 135 (7) 91 (4) 3 0.149 (1) 49 (7) 71 (2) O(1 ) 1 0.152 (2) 90 (3) 9 (13) 2 0.158 (2) 80 (1) 99 (13) 3 0-229 (2) 10 (1) 88 (1) 0(2) 1 0.146 (2) 90 (2) 30 (2) 2 0.174 (2) 148 (5) 105 (3) 3 0.190 (2) 58 (5) 115 (2) 0(3) 1 0.151 (2) 32 (1) 121 (1) 2 0.193 (2) 94 (3) 110 (5) 3 0.205 (2) 58 (1) 38 (3) 0(4) 1 0.152 (2) 52 (5) 40 (6) 2 0.163 (2) 120 (5) 57 (6) 3 0.184 (2) 127 (3) 71 (3) N(I) 1 0.159 (3) 22 (7) 107 (9) 2 0.172 (3) 106 (9) 162 (9) 3 0.190 (2) 105 (4) 94 (5) N(2) 1 0.149 (3) 85 (6) 12 (7) 2 0.165 (2) 147 (6) 80 (7) 3 0.183 (2) 123 (6) 97 (3) [OOll 73 (2) ° 111 (7) 152 (6) 98 (13) 164 (7) 103 (1) 63 (2) 40 (4) 63 (4) 103 (2) 146 (5) 121 (6) 95 (3) 115 (3) 25 (3) 99 (4) 91 (6) 9 (4) 82 (3) 98 (6) 12 (5) Table 3. Positional and isotropic temperature factors for the H atoms in (NH4)2HPO 4 x y z B H(1) 0.179 (2) 0.876 (3) 0.137 (3) 3.1 (5)/~, °2 H(2) 0.513 (2) 0.160 (2) 0"115 (2) 1.3 (4) H(3) 0.418 (1) 0.000 (3) 0.115 (2) 1.1 (4) H(4) 0.496 (2) 0.121 (2) 0.285 (3) 1.8 (4) H(5) 0.392 (2) 0.205 (3) 0.127 (2) 1.2 (4) H(6) 0.170 (2) 0.405 (3) 0.186 (2) 1.9 (4) H(7) 0.125 (i) 0.498 (3) 0.315 (2) 0.5 (3) H(8) 0.161 (2) 0.292 (3) 0.348 (2) 0.9 (4) H(9) 0.043 (2) 0.351 (2) 0-197 (2) 0.8 (4) Each NH + ion has five N...O contacts which are smaller than 3.4 A, and the coordination of the oxygen atoms around the N atoms is very similar to that in the isomorphous arsenate. The average values of these N(1)... O and N(2)... O lengths are 2.895 and 2-890 A respectively in the phosphate. Corresponding values Table 1. (NH4)zHPO4 positional parameters in fractions of the cell edges and thermal parameters (× 104), with their standard deviations The form of the anisotropic thermal ellipsoid is exp [- (ill lh 2 -bf122 k2 +fl3312 + 2fll2hk + 2,813h1+ 2f123kl)]. x y z 1311 [322 1333 1312 1313 P 0-24913 (3) 0"89110 (5) 0"43043 (4) 40 (1) 80 (1) 78 (1) 2 (1) 22 (1) O(1) 0"2039 (1) 0.9678 (2) 0"2277 (1) 98 (1) 102 (3) 94 (2) 3 (1) 39 (1) 0(2) 0"2644 (1) 0.0823 (1) 0"5394 (1) 64 (1) 108 (2) 113 (2) -10 (1) 38 (1) 0(3) 0"3767 (1) 0"7735 (1) 0"4845 (1) 54 (1) 159 (3) 136 (2) 22 (1) 34 (1) 0(4) 0"1389 (1) 0.7602 (1) 0"4409 (1) 49 (1) 111 (2) 110 (2) -7 (1) 23 (1) N(I) 0.4517 (1) 0.1190 (2) 0.1608 (2) 50 (2) 128 (4) 129 (3) 3 (2) 31 (2) N(2) 0-1217 (1) 0.3857 (2) 0.2620 (2) 52 (2) 99 (4) 114 (3) -2 (2) 26 (2) 1323 -4(1) 2 (2) - 23 (2) - 1 (2) 5 (2) - 1 (3) - 6 (3)
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`AIJAZ A. KHAN, J. P. ROUX AND WILLIAM J. JAMES 2067 Table4. (NH4)2HPO4 0bserved and calculated structure (NH4)zHPO~ is 0.63 A -~ which, according to our factors (x 5) experience, is the normal value for the H atom peaks in other structures investigated by the X-ray method. .: ........ ~. ,. o -t ;: ,~o~,~ , ......... o, ~, o .. ~, ............ ,. , ,. ,. ,. ~.~ o~ ............ o, , ~ 7o . , The isotropic temperature factors for the H atoms ..... : ........ ~ ................ ~ -' ;: ;°, : ,1. ....... ; :~ ,~ ..... belonging to the NH + groups are not large and have an :,,,!~:~ ! , : -. ; : .... ; 2 A 2. These considerations do not " ' ' average value of 1 ~ l/ 1~ ~-~ o1 oq 9 g~ 2b b -I ~e ~7 L- 15 -5 98 q9 11 : ............. : .... t;t .-, :~ ;: :~ ,.," ,:: ,, .......... ~ ~.. 1, :l ,o" ,'; "7 fa i hi h th NH2- .................. vor a model n w c e ions are in a state of "'" !i ....... 'i .... ............ o o' .... ........... ::::o :z;t : . 16 Z6 *2 ',* ~o t ,v~',~,; ~:-' ,~'~ ,~ : ; t;: t;: .... v ,. :. ,,." ~ .... ,~ ............... ,.o , -o to to 11 z 17 .... z* * g¢o ~ll ~, u g~ Jl ~ 4 ~ J~ ? -5 ?g ?] 9 -I tz lg 12 -~ 64 o~, 8 .t. 7. ~= ................. :: ....... I-o ~o ; .......... 13 .... tt~ z~ xz t, ~- -~ t .~o ,re o "~ z n~ o7 to 11 , 1 l~, tt ~ it i~ -~ 49 b~ b-6 t g~ ~,-? 13 12 II 36 36 ........... r "' t~: ..... 2 .................... -I .... .- ., .- ~ -~ .,, .. = • ~. ~. a -,, ~e sn 5 ~a u no 59 ~ z 1o1 1oo ...... I ................ ; ...... ",~ °' 71 2 o ..... ....................... ~ ,o I ~ ,, -"; .... ,, ~,~ 40 1 [ 31 av 1 -t ¢3 g* ~ 4 t3b tJo |4 ) .e ~.o i 4 )7 35 1 -3 tO* tO) ,. ., ~ tu 15 .5 ~, l zt az ~, -7 3e' 3e o i a~ ,~ t:, 17 tz ,,. ,~, ~. 0 0 glo 119 zo -'~ IB go T -5 rib 117 17 -3 37 J8 0 -~, 99 100 .. to ...................... : o ..... l ,~ .................. -,, ~o~ ~e~ xl z u/ oo t -t l~ t. ,, z s* ~,. 5 a at zl ~ ~ tz tz it -,, 75 ~ [~ o IZI lib I -') to0 tel 7 J 7a 78 9 4~ ~5 1 b* ~. -.i 28 4 l • l~ -d .~o )~ 1 iO~ l~t 11 ~ 5~ 50 u ,. , ~.. J -5 8 8 1o 176 171 ~,, ,,z ; -i ~,o ,,~ 1 tsu t.,, LJ o it tt l; -1 ao l~ ~, -~ 5~ 5,~ e o a; z~ t~, ,, -~ .,. ~t ~ u ~. b -~ , tn Zt -~ tZ to t~: -t ,, I 57 56 -I ~,. ,.o t I -* ~L zw, ~ ~u z .., ,.. ~ -o t,.1 t~l 1 -,. z~ zz ~, c z,,5 t~.i ts ~ le 18 -* .... l, ~/u Vd-'I .'~*, .... ~41 g~ ",J 'lgt 1-' ~ ]--'~ 2'.. ................ g45 Z5 -I ]1 ]] b ] 58 ........... ,8 8 -b I0 4 ,, ,,,. . ~9 ~-5 IV ~ '~ ~t ~z -1 ~,~ 8 Z too ,~,~ tt 3 13 11 e ~,. .. , ,.= z -z t~t t~o ~ -- ,.. ,~. -', z. z,~ 7 .- 6, 6 -¢, 51 57 -1 .~) ~,~ to-1 oo o,, ~. ~u ,.? o -o I t 15 ~n 7 ,.o q -,. nz eo -,~ e,¢ i. ,~-z ~t lo ~ t.z ~. i~ t ~o ~, o z ZuO zoo tt -,, zt zz ~, z 06 6* -t 3.tO ~,~'~ ~', -I ,ee~ .o,, z; o |~ 15 to g z5~ tsz to ~ z'~ ~o .) -z 167 1'~0 1~ .- e, ~- ' ~" °" I ............. t,: ....................... o ..... ..................................... o ~, .-oi 1o 11 31 1 t~ 1~ xd -z ~ Lg 7 -~ la t9 O -z ~5 ~b Lt ~5 33 6 t )7 3e .. ~.. -~ . 1 o ,~1 zou zo -,~ re5 ~.,~' I. o uo 1o -,. 51 5,~ 7 t le .... !2 ....... ..... : 671 .............. -o ,~o ~ z~ -* ~ ~1. to ,.~ .o o o t g ~ el z ,,~ e ~o 50 7 -z 37 ~s ~o 5. 5~ 1 t~,v tot at ~.- e 'o t07 tO'~ t~ "" 9, ~- ~ t~u to t,3 tz~ ~ n ~, ~o 6 ,, ~ ~ u -3 5'. 5~ 8 ~ tO3 tog 11 t~,z 14. 9 ', disorder (dynamic or static). , At the same time there is not enough evidence to ; suggest that the short N(1)...O(1) or N(2)...O(1) distances are a result of bifurcated bonding. The i shortest contact which the oxygen atom O(1) has with i any H atom of the NH + group is 2.50 A from H(8). 'i The angle N(2)-H(8)...O(1) is 114 (1) °. On the other ,~ hand the hydrogen bonding suggested in Table 5 ,,~ involves normal H...O contacts and the angles N-H. • • O are also large, indicating that the N-H- • • O lg : bonds are more or less linear, and this is particularly ,i true for the bonds frcm the N(2) atom. This conclusion 11 ~: is also supported on the basis of the electrostatic bond- 'i strength considerations. The bond strengths received ,~ by the oxygen atoms O(1), O(2), 0(3) and 0(4) are "i respectively 2.08, 1.75, 1.75, 1.75 v.u. The oxygen : atom O(1) which acts as a donor in the only O-H. • .O ; bond in the structure, is saturated and is not well ,7 :i qualified to act as an acceptor of a hydrogen bond. , It was pointed out earlier (Khan & Baur, 1972) that t~ , like other cations, the effective radius of tl.e NH + ion : increases with an increase in coordination number (C.N.). The NH + ion can, in some crystal structures, i replace an alkali atom of comparable size, such as ,; K +, Rb + or Cs +. Nevertheless the ammonium ions in 8 : these structures tends to have a smaller C.N. than the t! replaced alkali atom and thus exhibits a tendency to 5 to engage in N-H.-.O bonding. It is not surprising, 7 ,g 11~ Table 5. Interatomie bond distances, bond angles and hydrogen bonding in (NH4)2HPO4 in the arsenate are 2.888 and 2.880 A. The angles (a) PO4 tetrahedron O...(H-N-H)...O also compare well with those found in the arsenate. Whenever the coordination number of an NH + ion in crystal structures is higher than 4, it is difficult to postulate a correct N-H--.O bonding from the positions of N and O atoms alone. The difficulty increases with the increase in the coor- dination number. In the isomorphous structures of (NH4)2HPO4 and (NH4)zHAsO4, the first four N...O contacts around each of the two NH2- ions have nearly tetrahedral values for the O-N-O angles and therefore represent the N-H...O bonds. The hydro- gen positions determined in (NH4)2HPO4 confirm this conclusion (Table 5). However, the fifth N...O contact, smaller than 3.2 A for each NH + ion, was thought to be a result of dynamic or static disorder of the ammonium ion or that each N atom in addition to three normal N-H...O bonds also formed one bi- furcated bond (Khan et al., 1970). The average value of the electron density representing the H atoms in P-O(I) 1.587 (1) A P-O(2) 1.522 (1) P-O(3) 1-519 (1) P-O(4) 1.530 (1) Mean 1.539 O(1)-P-O(2) 103.61 (6) ° O(1)-P-O(3) 109.84 (5) O(1)-P-O(4) 108.01 (5) O(2)-P-O(3) 113.19 (5) O(2)-P-O(4) 111.07 (5) O(3)-P-O(4) 110.77 (5) O(1)-O(2) 2.444 (2) A O(1)-O(3) 2.542 (2) O(1 )-0(4) 2.522 (2) 0(2)-0(3) 2.539 (2) 0(2)-0(4) 2.516 (2) 0(3)-0(4) 2-509 (2) (b) O-H.. • O bonding O(1)-H(1)--.O(4)=2"615 (1) A O(1)-H(I)=0.91 (2) A /_ O(1)-H(1)..-0(4)= 168 (2) ~
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`2068 THE CRYSTAL STRUCTURE OF DIAMMONIUM HYDROGEN PHOSPHATE (c) N-H" ".O bonding N(1)-H(2)...0(3) N(1)-H(3)...0(3) N( 1 )-H(4) • • • 0(3) N(1)-H(5)''-0(2) N(I) ........ 0(1) N(2)-H(6)--O(2) N(Z)-H(7)--O(4) N(Z)-H(8)--O(2) N(Z)-H(9)--O(4) N(2) O(1 ) Table 5 (cont.) N-H H...0 N-H...O/N...0 Z_N-H...0 0"93 (1)/~ 1"87 (1) ,~, 2"789 (1) ,~ 172 (1) ° 0"89 (2) 2-07 (2) 2"946 (2) 166 (1) 0-92 (2) 1"96 (2) 2"817 (2) 154 (1) 0"84 (2) 1-93 (2) 2"763 (2) 175 (2) -- -- 3.158 (1) -- 0.96 (1) 1.86 (1) 2.822 (1) 175 (2) 0"86 (2) 2-00 (2) 2-860 (2) 178 (1) 0"91 (2) 2"06 (2) 2"965 (2) 175 (1) 0"85 (1) 1"96 (1) 2"814 (1) 176 (2) - - 2"989 (2) - /_O..-(HNH)...O H(2)-N(1)-H(3)..O(3) 103.80 (5) ° H(2)-N(1)-H(4)..O(3) "0(2) .0(3) • 0(2) • O(2) 0(3) 0(3) 0(3) H(2)-N(1)-H(5). 0(3) H(3)-N(1)-H(4). 0(3) H(3)-N(1)-H(5). 0(3) I-I(4)-N(1)-H(5). 0(2). • H(6)-N(2)-H(7). 0(2)" • H(6)-N(2)-H(8). 0(2). • H(6)-N(2)-H(9). 0(4)" • H(7)-N(2)-H(8). 0(4). • H(7)-N(2)-H(9). 0(2)" • H(8)-N(2)-H(9). /_ H-N-H 110 (1) ° 106 (1) 91.84 (4) 106 (1) 96.98 (5) 115 (2) 131.46 (6) 110 (2) 116"68 (5) 111 (1) 106.28 (5) • 0(4) 105 (1) 108-00 (5) • 0(2) 111 (1) 105.32 (4) • 0(4) 110 (1) 110.84 (5) • 0(2) 108 (2) 108.34 (5) • 0(4) 112 (1) 113.68 (4) • O(4) 11o (1) 110.28 (5) --.) a , ~0(3) 0(3) .. "Y JE . ""Y". ~/~/H(9) 88,~ ..-" 0(2)"-..~(~u ~'~'16 ~ '-" I "'"-./.35 H(6) ~)-" 42 ...-~~-" /I O / ' o(2) o / H(1) 08 Fig. 1. (NI"I4)2HPO4, viewed along [010]. The heights of selected atoms are indicated in fractional coordinates (x 100).
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`AIJAZ A. KHAN, J. P. ROUX AND WILLIAM J. JAMES 2069 therefore, that in (NH4)2HPO4-type structures, the NH4 groups (C.N. = 5) form normal N-H... O bonds. The average N-H length is 0.90 It and compares well with the values determined by X-ray methods. For example, it is 0.93 A in (NH4)3PO4 (Mootz & Wunder- lich, 1970). The tetrahedral angles H-N-H in (NH4)2HPO4 do not show a severe distortion of the NH + groups. A significant difference which exists in the isomorph- ous structures of (NH4)2HPO4 and (NH4)2HAsO4 is in the length of the hydrogen bond O(1)-H(1)...0(4). This bond is shorter in the phosphate, 2.615 (1)/k, than in the arsenate, 2.669 (13)/k and the difference is nearly four times the combined errors. If the electro- negativity (2.1 for P and 2.0 for As) or the size (ionic radius for ps+ =0.35 and for As s+ =0.47.30 of an X atom in HXO4 groups (X=P or As) can influence the length of a O-H... O bond, it is likely that the bonds from the OH groups will be influenced more than the O-H...O bonds from any H20 molecule present in the structure. Two pairs of isomorphous phosphates and arsenates containing OH groups have been studied carefully by the X-ray method: Na2HPO4.7H20 and Na2HAsO4.7H20 (Baur & Khan, 1970) and NH4H2PO4 and NH4H2AsO4 (Khan & Baur, unpublished work). The O-H...O bond lengths in these four structures, given in the same order as above, are 2.658 (8), 2.662 (5), 2.490 (2) and 2.517 (3)/~ and indicate that the O-H... O bond lengths in phosphates are either equal or slightly shorter than in the isomorphous arsenates. Unlike these structures where the oxygen atoms of the (OH) groups take part in more than one hydrogen bond by also accepting bonds either from (H20) mole- cules or from NH~ ions, in (NH4)2HPO4 or in (NH4)2HAsO4, the oxygen atom O(1) of the OH group is involved in only one O-H...O bond. There is, therefore, some reason to expect that the O-H...O bond in this isomorphous pair of structures will be influenced most when ps+ is replaced by As 5+. Baur (1970) has observed an inverse relation between the O-H...O bond length and the difference, Ap, in the bond strengths received by the donor and the acceptor atoms: Ap-=Pdonor--Pacceptor, where Paonor and Paeeeptor represent the bond strengths received by the donor and the acceptor atoms. According to this relation a shorter O-H..-O bond in the phosphate indicates an increase in Ap. Assuming that the bond strength is also a function of the interionic distance in addition to the charge and coordination number, the observed difference may be explained as a consequence of an increase in Pdo,or due to closer proximity of the donor atom to the ps+ ion than to the As 5+ ion. Since in this structure the bonds link one PO 3- (or AsOa4 -) group to another it has been implied here that the changes in Pao,or have more pronounced effect on the O-H...O bond lengths than the changes in the Paeeeptor. Every atom in the structure (not considering the hydrogen atoms) displays the anisotropic character of the thermal vibrations. The anisotropy is minimum for the P atom and maximum for the O(1) atom (Table 2). The major component of the maximum r.m.s, thermal displacement of the O(1) atom is along the a axis and in a direction perpendicular to the O-H...O bonds. Computer time provided by the Computer Center of the University of Illinois at Chicago is gratefully acknowledged. References BAUR, W. H. (1970). Trans. ACA, 6, 129. BAUR, W. H. & KHAN, A. A. (1970). Acta Cryst. B26, 1584. International Tables for X-ray Crystallography (1962). Vol. III. Birmingham: Kynoch Press. KHAN, A. A. 8¢. BAUR, W. H. (1972). Acta Cryst. B28, 683. KHAN, A. A., BAUR, W. H. & FORBES, W. C. (1972). Acta Cryst. B28, 267. KHAN, A. A., STRAUMANIS, M. E. & JAMES, W. J. (1970). Acta Cryst. B26, 1889. MOOTZ, D. t% WUNDERLICH, H. (1970). Acta Cryst. B26, 1826. SCHIFF, H. (1859). Liebigs Ann. 112, 88. SMITH, J. P., LEHR, J. R. & BROWN, W. E. (1957). Acta Cryst. 10, 709
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`Merck Exhibit 2207, Page 5
`Mylan Pharmaceuticals Inc. v. Merck Sharp & Dohme Corp.
`IPR2020-00040
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