(E)-N′-[4-(4-Chloro­benzo­yloxy)-3-methoxy­benzyl­idene]­isonicotino­hydrazide

Han, J.-R.; Wang, H.-J.; Zhen, X.-L.; Tian, X.

doi:10.1107/S1600536807043395

(E)-N′-[4-(4-Chlorobenzoyloxy)-3-methoxybenzylidene]isonicotinohydrazide

J.-R. Han, H.-J. Wang, X.-L. Zhen and X. Tian

In the title compound, C₂₁H₁₆ClN₃O₄, the vanillin group makes dihedral angles of 83.04 (6) and 53.42 (6)° with the mean planes of the terminal chlorobenzene and pyridine rings, respectively. The packing is stabilized by intermolecular N—H

O hydrogen bonds and weak C—H

O interactions that link adjacent molecules into chains.

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Supporting information

	Crystallographic Information File (CIF) https://doi.org/10.1107/S1600536807043395/hb2532sup1.cif Contains datablock I
	Structure factor file (CIF format) https://doi.org/10.1107/S1600536807043395/hb2532Isup2.hkl Contains datablock I

CCDC reference: 663730

checkCIF report

Key indicators

Single-crystal X-ray study
T = 294 K
Mean (C-C) = 0.003 Å
R factor = 0.043
wR factor = 0.111
Data-to-parameter ratio = 15.4

checkCIF/PLATON results

No syntax errors found



Alert level C
PLAT242_ALERT_2_C Check Low       Ueq as Compared to Neighbors for         C3
PLAT250_ALERT_2_C Large U3/U1 Ratio for Average U(i,j) Tensor ....       2.48

   0 ALERT level A = In general: serious problem
   0 ALERT level B = Potentially serious problem
   2 ALERT level C = Check and explain
   0 ALERT level G = General alerts; check

   0 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   2 ALERT type 2 Indicator that the structure model may be wrong or deficient
   0 ALERT type 3 Indicator that the structure quality may be low
   0 ALERT type 4 Improvement, methodology, query or suggestion
   0 ALERT type 5 Informative message, check

Comment top

The synthesis and structure of Schiff bases have attracted much attention in biology and chemistry (Kahwa et al., 1986). One aim is to develop protein and enzyme mimics (Santos et al., 2001). As part of an investigation of the coordination properties of Schiff bases functioning as ligands, we report the synthesis and structure of the title compound, (I).

In (I) (Fig. 1), the vanillin group (C8—C13/C15/O2/O3) is nearly planar, with an r.m.s. deviation for fitted atoms of 0.0283 Å. This plane makes dihedral angles of 53.42 (6)° and 83.04 (6)° with the pyridine ring (C17—C21/N3) and the terminal benzene ring (C1—C6), respectively. The dihedral angle between the pyridine ring and the benzene ring is 43.89 (10)°. Otherwise, all bond lengths and angles are within their normal ranges (Allen et al., 1987).

The crystal packing is stabilized by intermolecular N—H···O and C—H···O hydrogen bonds that link adjacent molecules into one-dimensional extended (Table 1, Fig. 2).

Related literature top

For general background, see: Kahwa et al. (1986); Santos et al. (2001). For reference structural data, see: Allen et al. (1987).

Experimental top

An anhydrous ethanol solution (50 ml) of 4-formyl-2-methoxyphenyl 4-chlorobenzoate (2.91 g, 10 mmol) was added to an anhydrous ethanol solution (50 ml) of isonicotinohydrazide (1.37 g, 10 mmol) and the mixture stirred at 350 K for 5 h under nitrogen, giving a white precipitate. The product was isolated, recrystallized from ethanol and then dried in a vacuum to give the pure compound in 75% yield. Colourless blocks of (I) suitable for X-ray analysis were obtained by slow evaporation of an acetonitrile solution.

Structure description top

The crystal packing is stabilized by intermolecular N—H···O and C—H···O hydrogen bonds that link adjacent molecules into one-dimensional extended (Table 1, Fig. 2).

For general background, see: Kahwa et al. (1986); Santos et al. (2001). For reference structural data, see: Allen et al. (1987).

Computing details top

Data collection: SMART (Bruker, 1999); cell refinement: SAINT (Bruker, 1999); data reduction: SAINT (Bruker, 1999); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997a); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997a); molecular graphics: SHELXTL (Sheldrick, 1997b); software used to prepare material for publication: SHELXTL (Sheldrick, 1997b).

Figures top

	Fig. 1. The structure of (I) with displacement ellipsoids for non-H atoms drawn at the 30% probability level.
	Fig. 2. Partial packing diagram for (I), with H bonds drawn as dashed lines.

(E)—N'-[4-(4-Chlorobenzoyloxy)-3-methoxybenzylidene]isonicotinohydrazide top

Crystal data top

C₂₁H₁₆ClN₃O₄	F(000) = 848
M_r = 409.82	D_x = 1.372 Mg m⁻³
Monoclinic, P2₁/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 2900 reflections
a = 21.746 (6) Å	θ = 3.2–26.2°
b = 12.011 (3) Å	µ = 0.23 mm⁻¹
c = 7.700 (2) Å	T = 294 K
β = 99.472 (5)°	Block, colourless
V = 1983.8 (9) Å³	0.16 × 0.16 × 0.14 mm
Z = 4

Data collection top

Bruker SMART APEX CCD diffractometer	4047 independent reflections
Radiation source: fine-focus sealed tube	2457 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.039
φ and ω scans	θ_max = 26.4°, θ_min = 1.0°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −27→25
T_min = 0.938, T_max = 0.969	k = −14→14
10889 measured reflections	l = −9→7

Refinement top

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)] = 0.043	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.111	H-atom parameters constrained
S = 1.02	w = 1/[σ²(F_o²) + (0.0372P)² + 0.6687P] where P = (F_o² + 2F_c²)/3
4047 reflections	(Δ/σ)_max = 0.001
263 parameters	Δρ_max = 0.17 e Å⁻³
0 restraints	Δρ_min = −0.23 e Å⁻³

Crystal data top

C₂₁H₁₆ClN₃O₄	V = 1983.8 (9) Å³
M_r = 409.82	Z = 4
Monoclinic, P2₁/c	Mo Kα radiation
a = 21.746 (6) Å	µ = 0.23 mm⁻¹
b = 12.011 (3) Å	T = 294 K
c = 7.700 (2) Å	0.16 × 0.16 × 0.14 mm
β = 99.472 (5)°

Data collection top

Bruker SMART APEX CCD diffractometer	4047 independent reflections
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	2457 reflections with I > 2σ(I)
T_min = 0.938, T_max = 0.969	R_int = 0.039
10889 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.043	0 restraints
wR(F²) = 0.111	H-atom parameters constrained
S = 1.02	Δρ_max = 0.17 e Å⁻³
4047 reflections	Δρ_min = −0.23 e Å⁻³
263 parameters

Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > 2σ(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Cl1	0.37851 (3)	0.67006 (8)	0.39409 (12)	0.0980 (3)
O1	0.68083 (8)	0.62360 (17)	0.3371 (3)	0.0733 (6)
O2	0.64129 (6)	0.47384 (13)	0.1858 (2)	0.0493 (4)
O3	0.68737 (7)	0.56737 (15)	−0.0796 (2)	0.0604 (5)
O4	1.00413 (7)	0.13543 (13)	0.31364 (19)	0.0476 (4)
N1	0.90937 (7)	0.26639 (15)	0.1402 (2)	0.0401 (4)
N2	0.96127 (7)	0.23334 (15)	0.0708 (2)	0.0410 (4)
H2	0.9660	0.2559	−0.0322	0.049*
N3	1.14908 (10)	0.0721 (2)	−0.1253 (4)	0.0751 (7)
C1	0.56095 (11)	0.6848 (2)	0.3981 (3)	0.0527 (6)
H1	0.5937	0.7319	0.4420	0.063*
C2	0.50132 (12)	0.7084 (2)	0.4265 (3)	0.0589 (7)
H2A	0.4938	0.7707	0.4914	0.071*
C3	0.45322 (11)	0.6391 (2)	0.3581 (3)	0.0552 (7)
C4	0.46278 (10)	0.5459 (2)	0.2630 (3)	0.0557 (7)
H4	0.4296	0.5002	0.2167	0.067*
C5	0.52281 (10)	0.5215 (2)	0.2373 (3)	0.0481 (6)
H5	0.5302	0.4580	0.1748	0.058*
C6	0.57183 (9)	0.59036 (19)	0.3037 (3)	0.0405 (5)
C7	0.63702 (10)	0.5675 (2)	0.2801 (3)	0.0442 (6)
C8	0.70153 (9)	0.44466 (19)	0.1576 (3)	0.0440 (6)
C9	0.73453 (10)	0.3666 (2)	0.2639 (3)	0.0578 (7)
H9	0.7181	0.3364	0.3577	0.069*
C10	0.79233 (10)	0.3328 (2)	0.2315 (3)	0.0567 (7)
H10	0.8149	0.2798	0.3036	0.068*
C11	0.81639 (9)	0.37765 (19)	0.0923 (3)	0.0396 (5)
C12	0.78249 (9)	0.45669 (18)	−0.0154 (3)	0.0407 (5)
H12	0.7987	0.4866	−0.1097	0.049*
C13	0.72453 (9)	0.49122 (19)	0.0170 (3)	0.0413 (5)
C14	0.70980 (14)	0.6149 (3)	−0.2264 (4)	0.0845 (10)
H14A	0.7477	0.6548	−0.1863	0.127*
H14B	0.6791	0.6651	−0.2865	0.127*
H14C	0.7177	0.5568	−0.3055	0.127*
C15	0.87629 (9)	0.33998 (18)	0.0491 (3)	0.0390 (5)
H15	0.8905	0.3708	−0.0477	0.047*
C16	1.00436 (9)	0.16643 (18)	0.1624 (3)	0.0387 (5)
C17	1.05475 (9)	0.13263 (18)	0.0612 (3)	0.0407 (5)
C18	1.11589 (10)	0.1288 (2)	0.1437 (4)	0.0613 (7)
H18	1.1266	0.1456	0.2627	0.074*
C19	1.16111 (12)	0.0994 (3)	0.0448 (5)	0.0795 (9)
H19	1.2024	0.0985	0.1006	0.095*
C20	1.08999 (12)	0.0745 (2)	−0.2004 (4)	0.0613 (7)
H20	1.0804	0.0555	−0.3189	0.074*
C21	1.04152 (10)	0.10353 (19)	−0.1141 (3)	0.0472 (6)
H21	1.0006	0.1034	−0.1733	0.057*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Cl1	0.0569 (4)	0.1276 (7)	0.1220 (7)	0.0410 (5)	0.0513 (5)	0.0189 (6)
O1	0.0400 (9)	0.0843 (14)	0.0974 (15)	−0.0070 (9)	0.0166 (10)	−0.0315 (12)
O2	0.0293 (7)	0.0608 (11)	0.0622 (10)	0.0054 (7)	0.0206 (7)	−0.0098 (9)
O3	0.0457 (9)	0.0760 (12)	0.0637 (11)	0.0246 (9)	0.0210 (8)	0.0184 (9)
O4	0.0464 (9)	0.0593 (10)	0.0398 (9)	0.0102 (8)	0.0145 (7)	0.0067 (8)
N1	0.0286 (9)	0.0563 (12)	0.0387 (10)	0.0085 (8)	0.0152 (8)	−0.0018 (9)
N2	0.0327 (9)	0.0600 (12)	0.0347 (10)	0.0126 (8)	0.0181 (8)	0.0042 (9)
N3	0.0526 (14)	0.0872 (18)	0.095 (2)	0.0181 (12)	0.0398 (14)	−0.0006 (15)
C1	0.0491 (13)	0.0569 (16)	0.0538 (15)	0.0057 (12)	0.0134 (12)	−0.0055 (12)
C2	0.0636 (16)	0.0619 (17)	0.0565 (16)	0.0238 (14)	0.0255 (14)	0.0012 (13)
C3	0.0401 (13)	0.0753 (18)	0.0558 (16)	0.0239 (13)	0.0250 (12)	0.0166 (14)
C4	0.0349 (12)	0.0714 (18)	0.0634 (17)	0.0033 (12)	0.0155 (11)	0.0032 (14)
C5	0.0397 (12)	0.0547 (15)	0.0530 (14)	0.0055 (11)	0.0166 (11)	−0.0040 (12)
C6	0.0340 (11)	0.0504 (14)	0.0397 (12)	0.0087 (10)	0.0137 (9)	0.0036 (11)
C7	0.0363 (12)	0.0549 (15)	0.0433 (13)	0.0027 (11)	0.0122 (10)	0.0000 (12)
C8	0.0263 (10)	0.0564 (15)	0.0533 (14)	0.0044 (10)	0.0180 (10)	−0.0032 (12)
C9	0.0440 (13)	0.0779 (18)	0.0588 (16)	0.0128 (13)	0.0297 (12)	0.0178 (14)
C10	0.0409 (12)	0.0773 (18)	0.0569 (15)	0.0208 (12)	0.0233 (11)	0.0211 (14)
C11	0.0277 (10)	0.0522 (14)	0.0410 (12)	0.0061 (10)	0.0118 (9)	−0.0009 (11)
C12	0.0328 (11)	0.0508 (14)	0.0425 (13)	0.0022 (10)	0.0174 (10)	0.0017 (11)
C13	0.0316 (11)	0.0481 (14)	0.0455 (13)	0.0074 (10)	0.0098 (10)	0.0008 (11)
C14	0.079 (2)	0.098 (2)	0.084 (2)	0.0399 (18)	0.0359 (17)	0.0445 (19)
C15	0.0307 (10)	0.0513 (14)	0.0378 (12)	0.0022 (10)	0.0141 (9)	−0.0001 (11)
C16	0.0313 (10)	0.0463 (13)	0.0407 (13)	0.0033 (10)	0.0127 (9)	−0.0002 (11)
C17	0.0334 (11)	0.0440 (13)	0.0479 (14)	0.0080 (9)	0.0156 (10)	0.0060 (11)
C18	0.0390 (13)	0.0826 (19)	0.0621 (17)	0.0163 (13)	0.0079 (12)	0.0034 (15)
C19	0.0325 (13)	0.102 (2)	0.106 (3)	0.0201 (14)	0.0184 (15)	0.004 (2)
C20	0.0636 (17)	0.0648 (18)	0.0632 (17)	0.0102 (14)	0.0331 (14)	−0.0056 (14)
C21	0.0420 (12)	0.0512 (14)	0.0523 (15)	0.0075 (11)	0.0195 (11)	−0.0009 (12)

Geometric parameters (Å, º) top

Cl1—C3	1.733 (2)	C8—C9	1.368 (3)
O1—C7	1.190 (3)	C8—C13	1.383 (3)
O2—C7	1.351 (3)	C9—C10	1.382 (3)
O2—C8	1.407 (2)	C9—H9	0.9300
O3—C13	1.358 (3)	C10—C11	1.377 (3)
O3—C14	1.422 (3)	C10—H10	0.9300
O4—C16	1.223 (2)	C11—C12	1.390 (3)
N1—C15	1.275 (3)	C11—C15	1.468 (3)
N1—N2	1.384 (2)	C12—C13	1.388 (3)
N2—C16	1.343 (3)	C12—H12	0.9300
N2—H2	0.8600	C14—H14A	0.9600
N3—C20	1.320 (3)	C14—H14B	0.9600
N3—C19	1.334 (4)	C14—H14C	0.9600
C1—C2	1.379 (3)	C15—H15	0.9300
C1—C6	1.388 (3)	C16—C17	1.501 (3)
C1—H1	0.9300	C17—C18	1.377 (3)
C2—C3	1.372 (4)	C17—C21	1.378 (3)
C2—H2A	0.9300	C18—C19	1.385 (3)
C3—C4	1.371 (3)	C18—H18	0.9300
C4—C5	1.383 (3)	C19—H19	0.9300
C4—H4	0.9300	C20—C21	1.380 (3)
C5—C6	1.379 (3)	C20—H20	0.9300
C5—H5	0.9300	C21—H21	0.9300
C6—C7	1.484 (3)

C7—O2—C8	116.01 (17)	C10—C11—C12	119.98 (18)
C13—O3—C14	117.02 (17)	C10—C11—C15	121.4 (2)
C15—N1—N2	114.13 (16)	C12—C11—C15	118.58 (18)
C16—N2—N1	120.79 (17)	C13—C12—C11	120.33 (19)
C16—N2—H2	119.6	C13—C12—H12	119.8
N1—N2—H2	119.6	C11—C12—H12	119.8
C20—N3—C19	116.4 (2)	O3—C13—C8	116.20 (17)
C2—C1—C6	119.8 (2)	O3—C13—C12	125.38 (19)
C2—C1—H1	120.1	C8—C13—C12	118.4 (2)
C6—C1—H1	120.1	O3—C14—H14A	109.5
C3—C2—C1	119.5 (2)	O3—C14—H14B	109.5
C3—C2—H2A	120.3	H14A—C14—H14B	109.5
C1—C2—H2A	120.3	O3—C14—H14C	109.5
C4—C3—C2	121.8 (2)	H14A—C14—H14C	109.5
C4—C3—Cl1	119.3 (2)	H14B—C14—H14C	109.5
C2—C3—Cl1	118.9 (2)	N1—C15—C11	122.04 (18)
C3—C4—C5	118.6 (2)	N1—C15—H15	119.0
C3—C4—H4	120.7	C11—C15—H15	119.0
C5—C4—H4	120.7	O4—C16—N2	124.75 (17)
C6—C5—C4	120.6 (2)	O4—C16—C17	121.97 (19)
C6—C5—H5	119.7	N2—C16—C17	113.27 (18)
C4—C5—H5	119.7	C18—C17—C21	118.3 (2)
C5—C6—C1	119.73 (19)	C18—C17—C16	120.1 (2)
C5—C6—C7	122.7 (2)	C21—C17—C16	121.62 (19)
C1—C6—C7	117.6 (2)	C17—C18—C19	118.3 (3)
O1—C7—O2	123.2 (2)	C17—C18—H18	120.8
O1—C7—C6	125.0 (2)	C19—C18—H18	120.8
O2—C7—C6	111.79 (19)	N3—C19—C18	124.1 (3)
C9—C8—C13	121.59 (18)	N3—C19—H19	118.0
C9—C8—O2	119.25 (19)	C18—C19—H19	118.0
C13—C8—O2	119.07 (19)	N3—C20—C21	124.1 (3)
C8—C9—C10	119.8 (2)	N3—C20—H20	118.0
C8—C9—H9	120.1	C21—C20—H20	118.0
C10—C9—H9	120.1	C17—C21—C20	118.9 (2)
C11—C10—C9	119.9 (2)	C17—C21—H21	120.6
C11—C10—H10	120.1	C20—C21—H21	120.6
C9—C10—H10	120.1

C15—N1—N2—C16	172.0 (2)	C14—O3—C13—C8	−179.0 (2)
C6—C1—C2—C3	−1.2 (4)	C14—O3—C13—C12	0.4 (4)
C1—C2—C3—C4	0.5 (4)	C9—C8—C13—O3	179.6 (2)
C1—C2—C3—Cl1	−179.74 (19)	O2—C8—C13—O3	3.2 (3)
C2—C3—C4—C5	0.5 (4)	C9—C8—C13—C12	0.2 (3)
Cl1—C3—C4—C5	−179.16 (18)	O2—C8—C13—C12	−176.2 (2)
C3—C4—C5—C6	−1.0 (4)	C11—C12—C13—O3	−179.8 (2)
C4—C5—C6—C1	0.4 (3)	C11—C12—C13—C8	−0.4 (3)
C4—C5—C6—C7	179.8 (2)	N2—N1—C15—C11	174.10 (19)
C2—C1—C6—C5	0.7 (3)	C10—C11—C15—N1	−1.7 (3)
C2—C1—C6—C7	−178.7 (2)	C12—C11—C15—N1	−178.8 (2)
C8—O2—C7—O1	0.2 (3)	N1—N2—C16—O4	−4.8 (3)
C8—O2—C7—C6	−179.50 (18)	N1—N2—C16—C17	175.74 (18)
C5—C6—C7—O1	−178.8 (2)	O4—C16—C17—C18	−40.3 (3)
C1—C6—C7—O1	0.5 (4)	N2—C16—C17—C18	139.2 (2)
C5—C6—C7—O2	0.8 (3)	O4—C16—C17—C21	138.9 (2)
C1—C6—C7—O2	−179.79 (19)	N2—C16—C17—C21	−41.6 (3)
C7—O2—C8—C9	98.1 (3)	C21—C17—C18—C19	2.0 (4)
C7—O2—C8—C13	−85.4 (3)	C16—C17—C18—C19	−178.8 (2)
C13—C8—C9—C10	0.0 (4)	C20—N3—C19—C18	0.2 (5)
O2—C8—C9—C10	176.5 (2)	C17—C18—C19—N3	−1.4 (5)
C8—C9—C10—C11	−0.1 (4)	C19—N3—C20—C21	0.3 (4)
C9—C10—C11—C12	−0.1 (4)	C18—C17—C21—C20	−1.5 (3)
C9—C10—C11—C15	−177.1 (2)	C16—C17—C21—C20	179.2 (2)
C10—C11—C12—C13	0.3 (3)	N3—C20—C21—C17	0.4 (4)
C15—C11—C12—C13	177.5 (2)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N2—H2···O4ⁱ	0.86	2.03	2.807 (2)	150
C19—H19···O1ⁱⁱ	0.93	2.52	3.420 (3)	162

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

Experimental details

Crystal data
Chemical formula	C₂₁H₁₆ClN₃O₄
M_r	409.82
Crystal system, space group	Monoclinic, P2₁/c
Temperature (K)	294
a, b, c (Å)	21.746 (6), 12.011 (3), 7.700 (2)
β (°)	99.472 (5)
V (Å³)	1983.8 (9)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	0.23
Crystal size (mm)	0.16 × 0.16 × 0.14

Data collection
Diffractometer	Bruker SMART APEX CCD
Absorption correction	Multi-scan (SADABS; Sheldrick, 1996)
T_min, T_max	0.938, 0.969
No. of measured, independent and observed [I > 2σ(I)] reflections	10889, 4047, 2457
R_int	0.039
(sin θ/λ)_max (Å⁻¹)	0.625

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.043, 0.111, 1.02
No. of reflections	4047
No. of parameters	263
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.17, −0.23

Computer programs: SMART (Bruker, 1999), SAINT (Bruker, 1999), SHELXS97 (Sheldrick, 1997a), SHELXL97 (Sheldrick, 1997a), SHELXTL (Sheldrick, 1997b).