Factors Governing the Three-Dimensional Hydrogen-Bond Network

Poly(m-Phenylene Isophthalamide) and a Series of Its Model Compounds (4): Similarity in ... three-dimensional hydrogen-bonded network structure, diffe...
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J. Phys. Chem. B 2006, 110, 20858-20864

Factors Governing the Three-Dimensional Hydrogen-Bond Network Structure of Poly(m-Phenylene Isophthalamide) and a Series of Its Model Compounds (4): Similarity in Local Conformation and Packing Structure between a Complicated Three-Arm Model Compound and the Linear Model Compounds Piyarat Nimmanpipug,† Kohji Tashiro,*,‡ and Orapin Rangsiman§ Department of Chemistry, Faculty of Science, Chiangmai UniVersity, Chiangmai 50200, Thailand, Department of Future Industry-Oriented Basic Science and Materials, Toyota Technological Institute, Tempaku, Nagoya 468-8511, Japan, and Department of Chemistry, Faculty of Science, Mahidol UniVersity, Rama 6 Road, Bangkok 10400, Thailand ReceiVed: April 3, 2006; In Final Form: August 15, 2006

Crystal structure of a three-arm model compound of poly(m-phenylene isophthalamide) (PMIA), N,N′,N′′triphenyl trimesamide Φ(CONHΦ)3, has been analyzed by the X-ray diffraction method. The torsional angles around the bonds connecting the amide group and the central benzene ring are 24-34°, almost the same as those observed for many kinds of aromatic amide compounds, reflecting mainly the intramolecular energetic balance between the amide and benzene groups. On the other hand, the torsional angles around the bonds connecting the amide group and the outer benzene ring were found to distribute over a wide range of 2-51° due to the additional effect of intermolecular interactions. This is the first example to show experimentally clearly the role of intra- and intermolecular interactions in the control of torsional angle around the benzeneamide linkage. The hydrogen bonds are formed between the amide groups of the neighboring molecules, resulting in the construction of three-dimensional network structure. The local packing structure of the threearm compound was found to be essentially the same as those observed for PMIA and the linear model compounds, indicating a characteristic structural feature of the meta-linkage-type aromatic amide compounds. The energy calculation was made using the software Polymorph Predictor to extract the energetically most stable crystal structure, which was compared successfully with the X-ray analyzed structure.

Introduction Poly(m-phenylene isophthalamide) (PMIA), which has metatype benzene-amide linkages in the skeletal chain, shows a three-dimensional hydrogen-bonded network structure, different from the stacked two-dimensional sheets structure of poly(pphenylene terephthalamide) (PPTA) with para-type benzeneamide linkages.1-3 As reported in a series of papers,4-6 we have investigated the factors governing this unique three-dimensional network structure (or jungle-gym-type structure) of PMIA on the basis of X-ray structure analysis and computer simulation made for a series of model compounds of PMIA having a basic chemical structure R-φ-NHCO-φ-CONH-φ-R or R-φCONH-φ-NHCO-φ-R, where φ is a meta-type phenylene ring and R is an end group. By investigating the thusaccumulated information of structure, it was found that the molecular conformation is closely related with the modes of intermolecular hydrogen bonds. If the molecular shape is flat, we have three types of molecular shape, as illustrated in Figure 1: TT, TC, and CC, where T and C indicate, respectively, the trans and cis forms with respect to the C(b)-C(b)-N-C(O) or C(b)-C(b)-C(O)-N linkages, where C(b) is a benzene carbon atom and C(O) is a carbonyl carbon atom. However, the benzene-amide linkage is twisted more or less by about 30° due to the effect of intramolecular nonbonded interactions.2,4-6 * Corresponding author. E-mail: [email protected]. † Chiangmai University. ‡ Toyota Technological Institute. § Mahidol University.

Figure 1. Illustration of molecular shapes constructed by a combination of T (trans) and C (cis) bonds about benzene-amide group.

Therefore, a combination of a whole molecular shape (TT, CT, and CC) with the twisting angles around the benzene-amide linkages may generate the various types of molecular conformation. In the actual crystal lattice, however, such a combination is limited to several types only, and we have a close relationship between the whole molecular shape and the intermolecular hydrogen bonding mode as listed in Figure 1. For example, the

10.1021/jp062058r CCC: $33.50 © 2006 American Chemical Society Published on Web 09/20/2006

H Bond Structure of Poly(m-phenylene isophthalamide)

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Figure 2. Crystal structure of TPT viewed along the various directions.

model compounds with CT-type molecular conformation tend to exhibit the three-dimensional hydrogen bonding network of (a-b) type, that is, the intermolecular hydrogen bonds are formed alternatively along the a and b axes when the structures are viewed along the c axis. This hydrogen bond network structure is similar to that of PMIA. It is desirable to answer such a question whether this relation between molecular conformation and packing structure mode is universal or not. In the present paper, we have investigated the crystal structure of a PMIA model compound with more complicated chemical formula, N,N′,N′′-triphenyl trimesamide (TPT). The TPT has three benzamide groups extending from the central benzene ring. The benzene-amide sequence itself is

TABLE 1: Crystallographic Data of TPT TPT formula molecular weight crystal system space group a/Å b/Å c/Å β/° V/Å3 Z1b Z2b R Rw

C27H21N3O3 435.483 monoclinic P21/n 10.0520(6)a 27.0730(20) 16.0540(10) 101.007(3) 4288.5(5) 2 8 0.076 0.062

a The numerical value in the parenthesis is a standard deviation; for example, 10.0520(6) ) 10.0520 ( 0.0006. b Z1 and Z2 are the numbers of molecules in an asymmetric unit and in a unit cell, respectively.

similar to those of model compounds of PMIA and PMIA itself. It is expected that TPT may take the three-dimensional hydrogen bond network similar to those of PMIA and its model

compounds investigated so far,4-6 but in a much more complicated manner. We have synthesized this compound and analyzed the molecular and crystal structures by performing the X-ray structure analysis. At the same time, we have searched the energetically most stable packing structure by means of computer simulation technique. As already demonstrated in our previous papers,4-6 the packing structure of TPT was expected to be also reproducible reasonably by means of the software Polymorph Predictor. In fact, the thus-predicted packing structure was found to be comparable with the X-ray result, as can be seen below.

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TABLE 2: Fractional Atomic Coordinates and Equivalent Isotropic Temperature Factors of TPT

atom

x/a

y/b

z/c

U (iso)

atom

x/a

y/b

z/c

U (iso)

O(1) O(2) O(3) O(4) N(5) O(6) O(7) N(8) N(9) N(10) N(11) N(12) C(13) C(14) C(15) C(16) C(17) C(18) C(19) C(20) C(21) C(22) C(23) C(24) C(25) C(26) C(27) C(28) C(29) C(30) C(31) C(32) C(33) C(34) C(35) C(36) C(37) C(38) C(39) C(40) C(41) C(42) C(43) C(44) C(45) C(46) C(47) C(48) C(49) C(50) C(51) C(52) C(53) C(54)

0.4877(9) -0.0206(1) -0.0295(1) 0.0197(1) -0.5006(1) 0.4711(1) -0.4763(1) 0.0061(1) 0.5397(1) -0.2519(1) 0.2476(1) 0.0362(1) 0.1989(1) 0.1737(1) 0.4697(2) -0.0329(1) -0.2415(1) 0.1355(2) -0.1304(2) 0.3984(1) -0.1056(1) -0.3275(1) 0.3717(2) -0.3015(1) 0.0587(2) 0.2604(2) 0.6370(2) -0.4435(2) 0.3498(2) -0.6138(1) -0.1900(2) -0.6890(2) -0.1122(2) 0.3108(1) -0.0975(2) -0.1490(2) 0.3747(1) -0.6464(2) -0.2323(2) 0.1345(1) 0.2570(2) -0.8029(2) 0.6141(2) 0.3788(1) 0.7541(2) 0.2539(2) -0.1875(2) -0.1503(1) 0.1345(1) 0.2475(2) 0.1196(2) -0.2982(1) -0.7570(2) -0.2866(2)

0.1408(4) 0.1349(4) 0.3619(4) 0.2473(4) 0.1734(4) 0.3655(3) 0.2438(4) 0.1748(5) 0.1872(5) 0.3825(4) 0.3876(4) 0.1827(4) 0.2853(5) 0.2348(6) 0.1785(6) 0.1719(6) 0.1964(6) 0.1503(7) 0.2120(6) 0.2665(6) 0.2625(6) 0.2312(6) 0.2168(6) 0.2816(5) 0.2195(6) 0.2012(5) 0.1544(6) 0.2175(6) 0.3544(5) 0.1503(6) 0.2968(6) 0.1717(6) 0.1002(6) 0.3015(6) 0.1502(6) 0.3502(6) 0.5162(5) 0.1042(6) 0.4346(6) 0.4652(5) 0.4399(5) 0.1470(8) 0.1041(6) 0.4655(4) 0.17428(7) 0.5423(5) 0.1727(6) 0.4517(5) 0.5163(5) 0.1703(7) 0.0995(7) 0.4667(6) 0.0798(6) 0.1448(8)

0.2747(7) 0.2715(7) 0.2632(7) 0.0377(6) 0.1033(7) 0.2570(7) 0.0311(7) 0.1066(8) 0.3932(9) 0.2432(8) 0.2396(8) 0.3890(8) 0.1806(1) 0.1666(1) 0.3148(1) 0.3103(1) 0.2101(1) 0.4358(1) 0.2707(1) 0.2859(1) 0.2826(1) 0.1631(1) 0.2729(1) 0.1768(1) 0.0977(1) 0.2139(1) 0.4394(1) 0.0907(1) 0.2452(9) 0.0509(1) 0.2370(1) -0.0210(1) 0.0641(1) 0.2383(1) 0.0501(1) 0.2488(9) 0.2525(9) 0.0790(1) 0.2558(1) 0.2228(9) 0.2402(1) -0.0628(1) 0.4407(1) 0.2563(9) 0.4864(1) 0.2346(1) -0.0136(1) 0.3293(1) 0.2207 (9) 0.4876(1) 0.4318(1) 0.1969(1) 0.0337(1) -0.0634(1)

0.0403(6) 0.0509(7) 0.0431(7) 0.0509(7) 0.0350(8) 0.0473(7) 0.0530(8) 0.0302(8) 0.0314(8) 0.0345(8) 0.0330(8) 0.0390(8) 0.0360(1) 0.0328(9) 0.0290(1) 0.0370(1) 0.0370(1) 0.0320(1) 0.0326(9) 0.034(1) 0.032(1) 0.030(1) 0.030(1) 0.031(1) 0.037(1) 0.0283(9) 0.041(1) 0.032(1) 0.033(1) 0.037(1) 0.030(1) 0.045(1) 0.054(1) 0.033(1) 0.034(1) 0.033(1) 0.0604(9) 0.0470(9) 0.043(1) 0.0480(8) 0.034(1) 0.053(1) 0.049(1) 0.0455(8) 0.052(1) 0.062(1) 0.047(1) 0.0582(9) 0.0593(9) 0.045(1) 0.058(1) 0.061(1) 0.059(1) 0.064(1)

C(55) C(56) C(57) C(58) C(59) C(60) C(61) C(62) C(63) C(64) C(65) C(66) H(46) H(49) H(40) H(37) H(44) H(11) H(20) H(13) H(26) H(9) H(45) H(57) H(59) H(58) H(43) H(33) H(47) H(54) H(65) H(60) H(8) H(66) H(61) H(56) H(52) H(48) H(10) H(24) H(21) H(17) H(5) H(32) H(42) H(55) H(53) H(38) H(12) H(51) H(50) H(63) H(64) H(62)

-0.8348(2) -0.1321(2) 0.8460(2) 0.7075(2) 0.8244(2) -0.2105(2) -0.2794(2) 0.2169(2) 0.3440(2) 0.3293(2) -0.2988(2) -0.1976(2) 0.2543 0.0505 0.0497 0.4594 0.4616 0.1602 0.4731 0.1416 0.2433 0.5227 0.7750 0.9311 0.8892 0.6933 0.5306 -0.0528 -0.1820 -0.3456 -0.3686 -0.2178 0.0460 -0.1824 -0.3225 -0.0762 -0.3555 -0.1056 -0.3426 -0.3581 -0.0284 -0.2618 -0.4580 -0.6658 -0.8596 -0.9135 -0.7800 -0.5922 0.0195 0.0435 0.2563 0.4227 0.3940 0.2054

0.1008(7) 0.5020(7) 0.1420(9) 0.0742(7) 0.0927(9) 0.0717(7) 0.5184(7) 0.0694(7) 0.1409(9) 0.0899(8) 0.0949(9) 0.5334(7) 0.5777 0.5341 0.4478 0.5335 0.4469 0.3723 0.2795 0.3102 0.1665 0.2174 0.2088 0.1559 0.0724 0.0392 0.0907 0.0840 0.2075 0.1615 0.0760 0.0368 0.1597 0.5679 0.5427 0.5154 0.4554 0.4286 0.3697 0.3063 0.2736 0.1620 0.1562 0.2031 0.1614 0.0841 0.0476 0.0890 0.2132 0.0848 0.2056 0.1542 0.0681 0.03420

-0.0356(1) 0.3421(1) 0.5338(1) 0.4909(1) 0.5357(1) 0.0129(1) 0.2116(1) 0.4787(1) 0.5356(1) 0.5313(1) -0.0509(1) 0.2834(2) 0.2348 0.2090 0.2127 0.2595 0.2688 0.2340 0.3263 0.1501 0.2055 0.4209 0.4923 0.5610 0.5719 0.4928 0.4099 0.1099 -0.0243 -0.1086 -0.0861 0.0223 0.1596 0.2951 0.1718 0.3923 0.1457 0.3708 0.2312 0.1449 0.3230 0.1999 0.1537 -0.0420 -0.1117 -0.0658 0.0516 0.1281 0.4156 0.3950 0.4875 0.5721 0.5644 0.4755

0.057(1) 0.088(1) 0.062(1) 0.066(1) 0.071(2) 0.084(1) 0.087(1) 0.075(1) 0.063(2) 0.066(2) 0.074(1) 0.098(2) 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050

Experimental Section Synthesis. TPT was synthesized with condensation reaction for a pair of trimesoyl chloride and aniline. The single crystals were prepared from the acetone solution at room temperature by solvent evaporation method. The thus-prepared single crystal had a needle shape.

X-ray Structure Analysis. X-ray diffraction was measured by using a MAC Science DIP 3000 diffractometer system. The incident X-ray beam was a Mo KR line (λ ) 0.71073 Å) from the MAC Science SRA-18X X-ray generator (50 kV and 200 mA). The oscillation amplitude was ∆ω ) 4° in the full range of ω ) 0-96°. The indexing of the observed reflections,

H Bond Structure of Poly(m-phenylene isophthalamide)

Figure 3. Torsional angles of TPT molecules, where two molecules are included as one crystallographic asymmetric unit.

estimation of the unit cell parameters, and integration of the reflection intensities were made by using the software DENZO, and the scaling of the thus-evaluated reflection intensities was made by the software SCALEPACK.7,8 The DENZO indexed the observed reflections and refined the lattice parameters and geometrical parameters of the measurement system such as the rotational axis of the sample, the center position of the

J. Phys. Chem. B, Vol. 110, No. 42, 2006 20861 oscillation, etc. with a limited number of chosen frames. The SCALEPACK adjusted the intensity scale between the successive frames, from which the exact structure factors were obtained, and refined the lattice parameters furthermore by using the whole data set. The crystal structure was solved by using the software maXus (Nonius BV, Delft, The Netherlands), which consisted of a set of software necessary for the determination of the space group symmetry and initial models, the least-squares refinement of the structure, etc. The direct method was used to find out the initial models, where the software SIR92 developed by Altmare et al. was used.9 Least-squares refinement was made on the basis of the full matrix method by using the quantity ∑w(|Fo|2 - |Fc|2)2 as a minimized function with the weight w ) exp[FA‚sin2 θ/λ2]/[σ2(Fo) + FB‚Fo2], where σ2(Fo) was the square of standard deviation of the observed structure factor Fo and the coefficients FA and FB were set to the values 0.0 and 0.03, respectively. The reflections satisfying the cutoff condition of |Fo| > 3σ(|Fo|) were used in the least-squares refinement. Because no detectable effect was actually found, the absorption

Figure 4. Hydrogen bond networks observed for TPT crystal. The outer benzene rings are erased and the central benzene rings are displaced by pink spheres for clarity.

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Figure 6. Comparison of local packing structure between TPT and PMIA.

Figure 5. Comparison of local packing structure between TPT and BPMPIA.

correction for the observed intensity was not included in the structural refinement. The reliability of the structure analysis was evaluated by the reliability factors defined by the following equations: R ) ∑||Fo|2 - |Fc|2|/∑|Fo|2 and Rw ) [∑w(|Fo|2 |Fc|2)2/(∑w|Fo|2)2]1/2. Results and Discussion Molecular and Crystal Structures of TPT. Figure 2 shows the X-ray analyzed crystal structure of TPT. The crystallographic data is listed in Table 1. The fractional atomic coordinates are shown in Table 2. As shown in Figure 2, the molecule takes the almost regular triangle shape and has approximately the 3-fold rotation axis perpendicular to the molecular plane. The torsional angles around the benzene-amide bonds are shown in Figure 3. The three torsional angles around the bonds between the central benzene ring and amide group are about 24-34°, almost in the range of the values observed for many model compounds discussed in the previous papers.4-6 The torsional angles between the outer benzene ring and amide group are not systematic, but they spread in a wide range of 4-53° depending on the location of the outer benzene rings. This is a quite clear example to see the effects of intramolecular and intermolecular interactions on the torsional angles between the benzene and amide group. Basically, the torsional angle is determined by the local interactions between the benzene ring and amide group, as discussed in detail in the previous papers.2,4-6 This can be typically seen for the three torsional angles about the central benzene ring of the TPT molecule, because this inner part is

guarded from the external effect due to the neighboring molecules by being surrounded by the outer benzene groups. However, the torsional angle is not determined by the intramolecular interactions only but sensitively affected by the intermolecular forces. The torsional angle about the outer benzene ring is a good example to show this situation. When the packing mode of these molecules is investigated, it is found that the TPT molecules are packed in a complicated manner. The intermolecular hydrogen bonds are formed in the three-dimensonal directions. In Figure 4 are shown the hydrogen bond structure between the neighboring amide groups, where the outer benzene rings are erased and the central benzene rings are displaced by pink spheres for clarity. When the structure is viewed along the b axis, the hydrogen bonds are formed alternately along the a and c axes. When the structure is viewed along the c axis, the hydrogen bonds are found to be formed along the a and b axes in the layer planes consisting of aggregated molecules. When viewed along the a axis, the zigzag-type hydrogen bond structure is seen within the bc plane. In this way, the hydrogen bonds of three different amide groups of a molecule are formed in the three-dimensional network mode. Comparison of Packing Structure among the Various Compounds. The local packing structure of TPT molecules is found to be close to that observed for PMIA and some of its model compounds. In fact, as shown in Figure 5, the local structure is very close to each other when the structure is compared between TPT and PMIA model compounds. In this figure, the local packing structure of TPT is compared with that of bis-p-methyl-phenol isophthalamide (BPMPIA). The molecular shape of BPMPIA is of a CT type, as illustrated in Figure 1. The TPT takes also the CT type shape. Besides, the intermolecular distance and the molecular orientation are also very similar, although the TPT has another benzamide unit as a pendant group. The similarity can be seen also for the hydrogen bonding mode. In Figure 6 is compared the local structure between TPT and PMIA. Quite good coincidence can be seen also in the local conformation between these two molecules. Therefore, it may be said as a general tendency that the relationship between the molecular shape and local packing mode can be applied also to the case of a triangular model compound. In other words, the packing structure of TPT may be constructed by overlapping the local packing structures observed for a linear model compound (BPMPIA) and the parent polymer (PMIA) by rotating them every 120° around the normal

H Bond Structure of Poly(m-phenylene isophthalamide)

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Figure 8. Comparison in packing structure of TPT between the X-ray analyzed result (blue color) and the computer simulation (red color). Figure 7. Schematic illustration of crystal structure of TPT, which can be assumed as an overlap of local structures of PMIA or BPMPIA with three different orientations.

to the molecular planes, as illustrated in Figure 7, resulting in the totally complicated crystal structure. Computer Simulation of Packing Structure of TPT. In the previous papers,5,6 we performed the energy calculation of the crystal structure for various types of model compounds and found the relatively good correspondence between the computersimulated crystal structure and the actually observed crystal structure. In this calculation, we used the software Polymorph Predictor (Accelrys Inc.),10,11 which was checked to be able to predict the molecular packing structure at relatively high degree of reasonableness. We have applied this method to the present case of TPT to check the usefulness of this software and to know the factors to govern the packing mode of this complicated substance. At first, we constructed a single molecule of TPT and obtained the energetically stable structure through the energy minimization, in which COMPASS was used as the force field12 and the atomic charges were assigned automatically by this COMPASS. By assuming the space group symmetry P21/a, which is different from the actually observed space group P21/n but is equivalent to the latter when the unit cell coordinates are exchanged, the energetically most stable packing structures were extracted by searching a tremendously large number of plausible crystal structure models built up with the Monte Carlo method. In the calculation, we assumed only one TPT molecule is included in a unit cell, although the two molecules are contained in an asymmetric unit in the actual unit cell structure, as seen in Figure 2. This is because the structural prediction becomes quite complicated and hard in the case of two molecules because the total number of independent variables is quite large, as known from the complicated chemical formula, and so the

TABLE 3: Comparison in Torsional Angles of TPT between X-ray Analysis and Computer Simulation torsional angles

observed

calculated

inner benzene-amide linkages

25°, 29°, 33°, 35°, 40°, 40° 4°, 7°, 18°, 37°, 43°, 53°

19°, 19°, 38°

outer benzene-amide linkages

11°, 30°, 34°

probability of hitting the correct answer becomes remarkably low. Figure 8 indicates the thus-obtained energetically lowest crystal structure model in comparison with the X-ray analyzed structure. The unit cell parameters are a ) 5.01 Å, b ) 27.85 Å, c ) 14.91 Å, and β ) 95.01°, which are essentially the same as those of the actual one (a ) 10.05 Å, b ) 27.07 Å, c ) 16.05 Å, and β ) 101.01°), although the a axial length is a half the actual value and the c axis is a little longer than the actual value. As seen in Figure 8, the packing mode is almost the same as the X-ray structure. The molecular parameters, in particular the torsional angles around the linkage between benzene and amide group, are also found to correspond well to those of the actual ones, as listed in Table 3. In the previous papers, we pointed out the importance of good balance between van der Waals interactions and electrostatic interactions for determining the packing structure as well as the molecular conformation. This situation is considered to work effectively also in the present case of TPT. Conclusions The crystal structure of a new compound TPT has been investigated in detail by the X-ray analysis. The threedimensional hydrogen bonding network is formed in a complicated manner, but the local structure was found to be similar to those observed for PMIA and its model compounds discussed in the previous papers. It can be said that the intimate relation between the molecular shape (CT, TT, and CC) and the packing

20864 J. Phys. Chem. B, Vol. 110, No. 42, 2006 mode of the molecules is applicable also to the system of more complicated geometry such as TPT. The energy calculation supported this consideration reasonably. Acknowledgment. This work was supported by MEXT (Ministry of Education, Culture, Sports, Science, and Technology) “Collaboration with Local Communities” Project (20052009). Supporting Information Available: File describing the X-ray structure analysis results of N,N′,N′′-triphenyl trimesamide. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kakida, H.; Chatani, Y.; Tadokoro, H. J. Polym. Sci., Polym. Phys. Ed. 1976, 14, 427.

Nimmanpipug et al. (2) Tashiro, K.; Kobayashi, M.; Tadokoro, H. Macromolecules 1977, 10, 413. (3) Northolt, M. G. Eur. Polym. J. 1974, 10, 799. (4) Nimmanpipug, P.; Tashiro, K.; Maeda, Y.; Rangsiman, O. J. Phys. Chem. B 2002, 106, 6842. (5) Tashiro, K.; Nimmanpipug, P.; Rangsiman, O. J. Phys. Chem. B 2002, 106, 12884. (6) Nimmanpipug, P.; Tashiro, K.; Rangsiman, O. J. Phys. Chem. B 2003, 107, 8343. (7) Otowinoski, W. D. S.; Minor, W. Methods Enzymol. 1997, 276. (8) Otowinoski, W. D. S.; Minor, W. In Macromolecular Crystallography, Part A; Carter, C. W., Jr., Sweet, R. M., Eds.; Academic Press: London, 1997; p 307. (9) Altmare, A.; Cascarano, G.; Gianovasso, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435. (10) Luesen, F. J. J. J. Cryst. Growth 1996, 166, 900. (11) Lommerse, J. P. M.; Motherwell, W. D. S.; Ammon, H. L.; Dunitz, J. D.; Gavezzotti, A.; Hofmann, D. W. M.; Luesen, F. J. J.; Mooij, W. T. M.; Orice, S. L.; Schweizer, B.; Schmidt, M. U.; van Eijck, B. P.; Verwer, P.; Williams, D. E. Acta Crystallogr., Sect. B 2000, 56, 697. (12) Sun, H. J. Phys. Chem. B 1998, 102, 7338.