Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S1600536808021880/bi2290sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S1600536808021880/bi2290Isup2.hkl |
CCDC reference: 702423
Key indicators
- Single-crystal X-ray study
- T = 293 K
- Mean (C-C) = 0.009 Å
- R factor = 0.060
- wR factor = 0.160
- Data-to-parameter ratio = 12.9
checkCIF/PLATON results
No syntax errors found
Alert level B PLAT241_ALERT_2_B Check High Ueq as Compared to Neighbors for O4 PLAT242_ALERT_2_B Check Low Ueq as Compared to Neighbors for N3 PLAT412_ALERT_2_B Short Intra XH3 .. XHn H4 .. H9B .. 1.75 Ang.
Alert level C PLAT094_ALERT_2_C Ratio of Maximum / Minimum Residual Density .... 2.17 PLAT230_ALERT_2_C Hirshfeld Test Diff for O3 -- N3 .. 5.29 su PLAT230_ALERT_2_C Hirshfeld Test Diff for O4 -- N3 .. 6.56 su PLAT232_ALERT_2_C Hirshfeld Test Diff (M-X) Cu1 -- N4 .. 5.40 su PLAT241_ALERT_2_C Check High Ueq as Compared to Neighbors for O2 PLAT242_ALERT_2_C Check Low Ueq as Compared to Neighbors for Cu1 PLAT341_ALERT_3_C Low Bond Precision on C-C Bonds (x 1000) Ang ... 9 PLAT154_ALERT_1_C The su's on the Cell Angles are Equal (x 10000) 2000 Deg. PLAT480_ALERT_4_C Long H...A H-Bond Reported H1 .. O3 .. 2.65 Ang. PLAT480_ALERT_4_C Long H...A H-Bond Reported H4B .. O4 .. 2.66 Ang.
Alert level G PLAT860_ALERT_3_G Note: Number of Least-Squares Restraints ....... 1 PLAT199_ALERT_1_G Check the Reported _cell_measurement_temperature 293 K PLAT200_ALERT_1_G Check the Reported _diffrn_ambient_temperature . 293 K PLAT793_ALERT_4_G Check the Absolute Configuration of N2 ..... R
0 ALERT level A = In general: serious problem 3 ALERT level B = Potentially serious problem 10 ALERT level C = Check and explain 4 ALERT level G = General alerts; check 3 ALERT type 1 CIF construction/syntax error, inconsistent or missing data 9 ALERT type 2 Indicator that the structure model may be wrong or deficient 2 ALERT type 3 Indicator that the structure quality may be low 3 ALERT type 4 Improvement, methodology, query or suggestion 0 ALERT type 5 Informative message, check
A mixture of copper(II) nitrate hemi(pentahydrate) (1 mmol) and N-(2-hydroxy-4-methoxybenzyl)bisethylenetriamine (1 mmol) in 20 ml methanol was refluxed for two hours. The resulting solution was cooled and filtered and the filtrate was evaporated naturally at room temperature. Two day later, blue blocks were obtained with a yield of 16 %. Elemental analysis calculated: C 41.60, H 5.07, N 14.93 %; found: C 41.51, H 5.08, N 14.85 %.
H atoms bound to C atoms were placed in calculated positions with C—H = 0.93–0.97 Å and refined as riding with Uiso(H) = 1.2 or 1.5Ueq(C). The H atoms bound to N4 were also placed in calculated positions with N—H = 0.90 Å and allowed to ride with Uiso(H) = 1.2Ueq(N). Atom H1 was located in a difference Fourier map and its position was refined with the N—H distance restrained to 0.90 (1) Å and with Uiso = 0.05 Å2.
Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT-Plus (Bruker, 2001); data reduction: SAINT-Plus (Bruker, 2001); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).
Fig. 1. The asymmetric unit of the title compound drawn with 30% probability displacement ellipsoids for the non-hydrogen atoms. |
[Cu(C13H20N3O2)(NO3)] | Z = 2 |
Mr = 375.87 | F(000) = 390 |
Triclinic, P1 | Dx = 1.587 Mg m−3 |
Hall symbol: -P 1 | Mo Kα radiation, λ = 0.71073 Å |
a = 7.2012 (10) Å | Cell parameters from 2739 reflections |
b = 10.095 (2) Å | θ = 2.2–25.0° |
c = 11.581 (2) Å | µ = 1.42 mm−1 |
α = 69.15 (2)° | T = 293 K |
β = 89.73 (2)° | Block, blue |
γ = 89.95 (2)° | 0.43 × 0.28 × 0.22 mm |
V = 786.8 (3) Å3 |
Bruker APEXII CCD diffractometer | 2739 independent reflections |
Radiation source: fine-focus sealed tube | 1896 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.029 |
ϕ and ω scans | θmax = 25.0°, θmin = 2.2° |
Absorption correction: multi-scan (SADABS; Bruker, 2001) | h = −8→8 |
Tmin = 0.569, Tmax = 0.730 | k = −11→12 |
4891 measured reflections | l = −13→13 |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.060 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.160 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.00 | w = 1/[σ2(Fo2) + (0.107P)2 + 0.9393P] where P = (Fo2 + 2Fc2)/3 |
2739 reflections | (Δ/σ)max = 0.016 |
213 parameters | Δρmax = 0.91 e Å−3 |
1 restraint | Δρmin = −0.42 e Å−3 |
[Cu(C13H20N3O2)(NO3)] | γ = 89.95 (2)° |
Mr = 375.87 | V = 786.8 (3) Å3 |
Triclinic, P1 | Z = 2 |
a = 7.2012 (10) Å | Mo Kα radiation |
b = 10.095 (2) Å | µ = 1.42 mm−1 |
c = 11.581 (2) Å | T = 293 K |
α = 69.15 (2)° | 0.43 × 0.28 × 0.22 mm |
β = 89.73 (2)° |
Bruker APEXII CCD diffractometer | 2739 independent reflections |
Absorption correction: multi-scan (SADABS; Bruker, 2001) | 1896 reflections with I > 2σ(I) |
Tmin = 0.569, Tmax = 0.730 | Rint = 0.029 |
4891 measured reflections |
R[F2 > 2σ(F2)] = 0.060 | 1 restraint |
wR(F2) = 0.160 | H atoms treated by a mixture of independent and constrained refinement |
S = 1.00 | Δρmax = 0.91 e Å−3 |
2739 reflections | Δρmin = −0.42 e Å−3 |
213 parameters |
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger. |
x | y | z | Uiso*/Ueq | ||
Cu1 | 0.72893 (13) | 0.60509 (7) | 0.61951 (6) | 0.0396 (3) | |
C2 | 0.7556 (9) | 0.9968 (6) | 0.1737 (5) | 0.0428 (13) | |
C3 | 0.7606 (11) | 1.1062 (6) | 0.2212 (6) | 0.0533 (16) | |
H3 | 0.7675 | 1.2001 | 0.1682 | 0.064* | |
C4 | 0.7551 (10) | 1.0737 (7) | 0.3470 (6) | 0.0511 (16) | |
H4 | 0.7594 | 1.1475 | 0.3772 | 0.061* | |
C5 | 0.7434 (8) | 0.9336 (6) | 0.4323 (5) | 0.0355 (12) | |
C6 | 0.7281 (8) | 0.8240 (6) | 0.3825 (5) | 0.0379 (12) | |
C7 | 0.7387 (9) | 0.8620 (6) | 0.2511 (6) | 0.0430 (14) | |
H7 | 0.7337 | 0.7905 | 0.2181 | 0.052* | |
C8 | 0.7388 (8) | 0.9117 (6) | 0.5646 (5) | 0.0364 (12) | |
C9 | 0.7516 (10) | 1.0377 (7) | 0.6051 (6) | 0.0503 (15) | |
H9A | 0.6518 | 1.0342 | 0.6614 | 0.075* | |
H9B | 0.7428 | 1.1235 | 0.5342 | 0.075* | |
H9C | 0.8682 | 1.0357 | 0.6455 | 0.075* | |
C10 | 0.7303 (11) | 0.7615 (7) | 0.7821 (5) | 0.0519 (16) | |
H10A | 0.6546 | 0.8307 | 0.8011 | 0.062* | |
H10B | 0.8577 | 0.7716 | 0.8046 | 0.062* | |
C11 | 0.6598 (11) | 0.6117 (8) | 0.8535 (6) | 0.0625 (19) | |
H11A | 0.6895 | 0.5850 | 0.9404 | 0.075* | |
H11B | 0.5260 | 0.6079 | 0.8456 | 0.075* | |
C12 | 0.6772 (13) | 0.3710 (8) | 0.8405 (7) | 0.0637 (19) | |
H12A | 0.5426 | 0.3719 | 0.8441 | 0.076* | |
H12B | 0.7233 | 0.3152 | 0.9219 | 0.076* | |
C13 | 0.7406 (11) | 0.3068 (7) | 0.7474 (6) | 0.0562 (17) | |
H13A | 0.8729 | 0.2881 | 0.7557 | 0.067* | |
H13B | 0.6766 | 0.2177 | 0.7620 | 0.067* | |
N1 | 0.7187 (7) | 0.7844 (5) | 0.6477 (4) | 0.0402 (11) | |
N2 | 0.7485 (8) | 0.5147 (5) | 0.8029 (4) | 0.0415 (11) | |
H1 | 0.871 (2) | 0.517 (7) | 0.816 (6) | 0.050* | |
N3 | 0.2005 (9) | 0.5570 (6) | 0.6866 (6) | 0.0550 (14) | |
N4 | 0.6991 (8) | 0.4070 (5) | 0.6214 (5) | 0.0500 (13) | |
H4A | 0.5822 | 0.3937 | 0.6006 | 0.060* | |
H4B | 0.7773 | 0.3921 | 0.5664 | 0.060* | |
O1 | 0.7593 (8) | 1.0387 (5) | 0.0493 (4) | 0.0616 (13) | |
O2 | 0.7129 (9) | 0.6914 (4) | 0.4468 (4) | 0.0633 (15) | |
O3 | 0.0805 (9) | 0.5939 (7) | 0.6146 (6) | 0.0869 (18) | |
O4 | 0.3604 (9) | 0.5871 (7) | 0.6519 (7) | 0.096 (2) | |
O5 | 0.1656 (9) | 0.4935 (8) | 0.7984 (6) | 0.098 (2) | |
C1 | 0.7476 (17) | 0.9303 (8) | −0.0054 (6) | 0.080 (3) | |
H1A | 0.8518 | 0.8674 | 0.0206 | 0.121* | |
H1B | 0.7488 | 0.9741 | −0.0938 | 0.121* | |
H1C | 0.6346 | 0.8776 | 0.0207 | 0.121* |
U11 | U22 | U33 | U12 | U13 | U23 | |
Cu1 | 0.0575 (5) | 0.0319 (4) | 0.0322 (4) | −0.0024 (3) | 0.0031 (3) | −0.0149 (3) |
C2 | 0.040 (3) | 0.038 (3) | 0.048 (3) | −0.004 (2) | 0.000 (3) | −0.013 (3) |
C3 | 0.076 (5) | 0.029 (3) | 0.046 (3) | 0.007 (3) | −0.016 (3) | 0.000 (2) |
C4 | 0.070 (4) | 0.035 (3) | 0.048 (3) | −0.004 (3) | 0.013 (3) | −0.015 (3) |
C5 | 0.032 (3) | 0.033 (3) | 0.048 (3) | −0.006 (2) | 0.013 (2) | −0.022 (2) |
C6 | 0.045 (3) | 0.033 (3) | 0.035 (3) | −0.006 (2) | −0.003 (2) | −0.011 (2) |
C7 | 0.050 (4) | 0.038 (3) | 0.048 (3) | −0.005 (3) | 0.015 (3) | −0.025 (3) |
C8 | 0.031 (3) | 0.030 (3) | 0.051 (3) | 0.001 (2) | 0.006 (2) | −0.018 (2) |
C9 | 0.058 (4) | 0.046 (3) | 0.060 (4) | −0.001 (3) | −0.001 (3) | −0.034 (3) |
C10 | 0.077 (5) | 0.048 (3) | 0.037 (3) | 0.001 (3) | 0.002 (3) | −0.022 (3) |
C11 | 0.064 (5) | 0.078 (5) | 0.053 (4) | −0.009 (4) | 0.010 (3) | −0.033 (4) |
C12 | 0.087 (6) | 0.053 (4) | 0.053 (4) | −0.005 (4) | −0.007 (4) | −0.021 (3) |
C13 | 0.072 (5) | 0.044 (4) | 0.053 (4) | 0.000 (3) | 0.016 (3) | −0.020 (3) |
N1 | 0.046 (3) | 0.044 (3) | 0.038 (2) | −0.001 (2) | 0.005 (2) | −0.025 (2) |
N2 | 0.046 (3) | 0.049 (3) | 0.032 (2) | −0.003 (2) | 0.002 (2) | −0.018 (2) |
N3 | 0.055 (4) | 0.055 (3) | 0.057 (4) | −0.002 (3) | 0.004 (3) | −0.022 (3) |
N4 | 0.055 (3) | 0.046 (3) | 0.049 (3) | −0.005 (2) | 0.006 (2) | −0.017 (2) |
O1 | 0.098 (4) | 0.042 (2) | 0.040 (2) | −0.006 (2) | −0.007 (2) | −0.0085 (19) |
O2 | 0.125 (5) | 0.029 (2) | 0.036 (2) | −0.005 (2) | 0.000 (2) | −0.0107 (17) |
O3 | 0.065 (4) | 0.108 (5) | 0.078 (4) | −0.005 (3) | 0.020 (3) | −0.022 (3) |
O4 | 0.060 (4) | 0.100 (5) | 0.147 (6) | 0.018 (3) | −0.040 (4) | −0.067 (5) |
O5 | 0.071 (4) | 0.137 (6) | 0.076 (4) | 0.003 (4) | −0.010 (3) | −0.026 (4) |
C1 | 0.155 (9) | 0.052 (4) | 0.038 (3) | −0.008 (5) | −0.003 (4) | −0.020 (3) |
Cu1—N1 | 1.952 (5) | C10—C11 | 1.529 (10) |
Cu1—N2 | 1.997 (5) | C10—H10A | 0.970 |
Cu1—N4 | 2.004 (5) | C10—H10B | 0.970 |
Cu1—O2 | 1.880 (4) | C11—N2 | 1.453 (9) |
Cu1—O3i | 2.535 (6) | C11—H11A | 0.970 |
Cu1—O4 | 2.676 (7) | C11—H11B | 0.970 |
C2—C7 | 1.342 (8) | C12—N2 | 1.451 (9) |
C2—O1 | 1.350 (7) | C12—C13 | 1.511 (10) |
C2—C3 | 1.398 (9) | C12—H12A | 0.970 |
C3—C4 | 1.374 (9) | C12—H12B | 0.970 |
C3—H3 | 0.930 | C13—N4 | 1.481 (9) |
C4—C5 | 1.410 (8) | C13—H13A | 0.970 |
C4—H4 | 0.930 | C13—H13B | 0.970 |
C5—C6 | 1.422 (8) | N2—H1 | 0.90 (1) |
C5—C8 | 1.468 (8) | N3—O3 | 1.169 (9) |
C6—O2 | 1.284 (7) | N3—O4 | 1.220 (9) |
C6—C7 | 1.433 (8) | N3—O5 | 1.248 (8) |
C7—H7 | 0.930 | N4—H4A | 0.900 |
C8—N1 | 1.310 (7) | N4—H4B | 0.900 |
C8—C9 | 1.508 (8) | O1—C1 | 1.449 (9) |
C9—H9A | 0.960 | C1—H1A | 0.960 |
C9—H9B | 0.960 | C1—H1B | 0.960 |
C9—H9C | 0.960 | C1—H1C | 0.960 |
C10—N1 | 1.493 (7) | ||
O2—Cu1—N1 | 93.98 (19) | N2—C11—H11A | 110.0 |
O2—Cu1—N2 | 179.4 (3) | C10—C11—H11A | 110.0 |
N1—Cu1—N2 | 85.7 (2) | N2—C11—H11B | 110.0 |
O2—Cu1—N4 | 95.0 (2) | C10—C11—H11B | 110.0 |
N1—Cu1—N4 | 167.3 (2) | H11A—C11—H11B | 108.4 |
N2—Cu1—N4 | 85.4 (2) | N2—C12—C13 | 108.5 (6) |
O2—Cu1—O4 | 93.8 (3) | N2—C12—H12A | 110.0 |
N1—Cu1—O4 | 87.8 (2) | C13—C12—H12A | 110.0 |
N2—Cu1—O4 | 86.7 (2) | N2—C12—H12B | 110.0 |
N4—Cu1—O4 | 82.8 (2) | C13—C12—H12B | 110.0 |
C7—C2—O1 | 124.9 (5) | H12A—C12—H12B | 108.4 |
C7—C2—C3 | 119.6 (6) | N4—C13—C12 | 109.0 (6) |
O1—C2—C3 | 115.4 (5) | N4—C13—H13A | 109.9 |
C4—C3—C2 | 119.5 (6) | C12—C13—H13A | 109.9 |
C4—C3—H3 | 120.3 | N4—C13—H13B | 109.9 |
C2—C3—H3 | 120.3 | C12—C13—H13B | 109.9 |
C3—C4—C5 | 123.1 (6) | H13A—C13—H13B | 108.3 |
C3—C4—H4 | 118.5 | C8—N1—C10 | 120.5 (5) |
C5—C4—H4 | 118.5 | C8—N1—Cu1 | 126.7 (4) |
C4—C5—C6 | 116.8 (5) | C10—N1—Cu1 | 111.3 (4) |
C4—C5—C8 | 118.2 (5) | C12—N2—C11 | 118.0 (6) |
C6—C5—C8 | 124.9 (5) | C12—N2—Cu1 | 108.8 (4) |
O2—C6—C5 | 124.9 (5) | C11—N2—Cu1 | 106.1 (4) |
O2—C6—C7 | 116.7 (5) | C12—N2—H1 | 112 (4) |
C5—C6—C7 | 118.3 (5) | C11—N2—H1 | 108 (4) |
C2—C7—C6 | 122.5 (5) | Cu1—N2—H1 | 103 (4) |
C2—C7—H7 | 118.7 | O3—N3—O4 | 119.1 (6) |
C6—C7—H7 | 118.7 | O3—N3—O5 | 120.6 (7) |
N1—C8—C5 | 120.8 (5) | O4—N3—O5 | 120.2 (7) |
N1—C8—C9 | 119.6 (5) | C13—N4—Cu1 | 108.5 (4) |
C5—C8—C9 | 119.6 (5) | C13—N4—H4A | 110.0 |
C8—C9—H9A | 109.5 | Cu1—N4—H4A | 110.0 |
C8—C9—H9B | 109.5 | C13—N4—H4B | 110.0 |
H9A—C9—H9B | 109.5 | Cu1—N4—H4B | 110.0 |
C8—C9—H9C | 109.5 | H4A—N4—H4B | 108.4 |
H9A—C9—H9C | 109.5 | C2—O1—C1 | 117.9 (5) |
H9B—C9—H9C | 109.5 | C6—O2—Cu1 | 127.1 (4) |
N1—C10—C11 | 107.4 (5) | N3—O4—Cu1 | 167.6 (6) |
N1—C10—H10A | 110.2 | O1—C1—H1A | 109.5 |
C11—C10—H10A | 110.2 | O1—C1—H1B | 109.5 |
N1—C10—H10B | 110.2 | H1A—C1—H1B | 109.5 |
C11—C10—H10B | 110.2 | O1—C1—H1C | 109.5 |
H10A—C10—H10B | 108.5 | H1A—C1—H1C | 109.5 |
N2—C11—C10 | 108.5 (6) | H1B—C1—H1C | 109.5 |
Symmetry code: (i) x+1, y, z. |
D—H···A | D—H | H···A | D···A | D—H···A |
N2—H1···O5i | 0.90 (1) | 2.15 (2) | 3.013 (8) | 161 (6) |
N2—H1···O3i | 0.90 (1) | 2.65 (6) | 3.134 (8) | 115 (5) |
N4—H4A···O2ii | 0.90 | 2.43 | 3.316 (9) | 168 |
N4—H4B···O3ii | 0.90 | 2.29 | 3.157 (8) | 162 |
N4—H4B···O4ii | 0.90 | 2.66 | 3.175 (9) | 118 |
Symmetry codes: (i) x+1, y, z; (ii) −x+1, −y+1, −z+1. |
Experimental details
Crystal data | |
Chemical formula | [Cu(C13H20N3O2)(NO3)] |
Mr | 375.87 |
Crystal system, space group | Triclinic, P1 |
Temperature (K) | 293 |
a, b, c (Å) | 7.2012 (10), 10.095 (2), 11.581 (2) |
α, β, γ (°) | 69.15 (2), 89.73 (2), 89.95 (2) |
V (Å3) | 786.8 (3) |
Z | 2 |
Radiation type | Mo Kα |
µ (mm−1) | 1.42 |
Crystal size (mm) | 0.43 × 0.28 × 0.22 |
Data collection | |
Diffractometer | Bruker APEXII CCD diffractometer |
Absorption correction | Multi-scan (SADABS; Bruker, 2001) |
Tmin, Tmax | 0.569, 0.730 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 4891, 2739, 1896 |
Rint | 0.029 |
(sin θ/λ)max (Å−1) | 0.595 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.060, 0.160, 1.00 |
No. of reflections | 2739 |
No. of parameters | 213 |
No. of restraints | 1 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.91, −0.42 |
Computer programs: APEX2 (Bruker, 2004), SAINT-Plus (Bruker, 2001), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008).
D—H···A | D—H | H···A | D···A | D—H···A |
N2—H1···O5i | 0.90 (1) | 2.15 (2) | 3.013 (8) | 161 (6) |
N2—H1···O3i | 0.90 (1) | 2.65 (6) | 3.134 (8) | 115 (5) |
N4—H4A···O2ii | 0.90 | 2.43 | 3.316 (9) | 168.4 |
N4—H4B···O3ii | 0.90 | 2.29 | 3.157 (8) | 162.4 |
N4—H4B···O4ii | 0.90 | 2.66 | 3.175 (9) | 117.5 |
Symmetry codes: (i) x+1, y, z; (ii) −x+1, −y+1, −z+1. |
Schiff bases have been studied as ligands for a long time due to instant and enduring popularity from their easy synthesis and versatility in complexes. They play an important role in the development of coordination chemistry as well as inorganic biochemistry, catalysis, optical materials and so on (Garnovskii et al., 1993; Huang et al., 2002). Considerable attention has been focused on the syntheses and structures of CuII and NiII complexes. The NiII complexes with multidentate Schiff-base ligands have aroused particular interest because Ni can exhibit several oxidation states and may provide the basis of models for active sites of biological systems. On the other hand, the main attention in the optically active Schiff-base complexes is concentrated on their catalytic abilities in stereoselective synthesis (Bhadbhade & Srinivas, 1993; Bunce et al., 1998).