Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270112008384/ky3010sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270112008384/ky3010Isup2.hkl | |
Chemical Markup Language (CML) file https://doi.org/10.1107/S0108270112008384/ky3010Isup3.cml | |
Portable Document Format (PDF) file https://doi.org/10.1107/S0108270112008384/ky3010sup4.pdf |
CCDC reference: 879436
D-(-)-α-Phenylglycine hydrazide (3.30 g, 20 mmol) was added to a mixture of triethyl orthobenzoate (4.57 g, 20 mmol) and 0.1 g p-toluenesulfonic acid in xylene (20 ml) and kept under reflux for 3 h (monitored by thin-layer chromatography). After cooling, the mixture was washed with water (30 ml), dried over MgSO4 and then concentrated under reduced pressure. The oily residue was subjected to column chromatography (silica gel, eluent: hexane–AcOEt, 1:2 v/v), yielding 3,5-diphenyl-4,5-dihydro-1,2,4-triazin-6(1H)-one (2.60 g). This crude product was dissolved in ethanol (50 ml) and left in solution for 10 d at room temperature. Yellow needles of (I) were filtered off and dried in air [yield 0.35 g, 14%; m.p. 494–495 K, reference 491–493 K (Camparini et al., 1978)].
Based on the solid-state geometry, the molecular structure of (I) was optimized using standard density functional theory (DFT) employing the B3LYP hybrid function (Becke, 1988; 1993; Lee et al., 1988) at the 6-311++G** level of theory. All species corresponded to minima at the B3LYP/6-311++G** level with no imaginary frequencies. All calculations were performed using the GAUSSIAN09 program package (Frisch et al., 2010). Further details are given in the Supplementary material.
All H atoms were generated in idealized positions and then refined in riding mode. For aromatic C atoms, C—H = 0.93 Å and Uiso(H) = 1.2Ueq(C). For the amine NH group, N—H = 0.86 Å and Uiso(H) = 1.2Ueq(N).
Data collection: CrysAlis CCD (Oxford Diffraction, 2008); cell refinement: CrysAlis RED (Oxford Diffraction, 2008); data reduction: CrysAlis RED (Oxford Diffraction, 2008); 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: SHELXL97 (Sheldrick, 2008).
C15H11N3O | F(000) = 1040 |
Mr = 249.27 | Dx = 1.333 Mg m−3 |
Monoclinic, C2/c | Melting point: 495 K |
Hall symbol: -C 2yc | Mo Kα radiation, λ = 0.71073 Å |
a = 22.975 (5) Å | Cell parameters from 2174 reflections |
b = 5.5835 (10) Å | θ = 3.5–25.0° |
c = 21.550 (5) Å | µ = 0.09 mm−1 |
β = 116.06 (3)° | T = 293 K |
V = 2483.5 (9) Å3 | Plate, colourless |
Z = 8 | 0.22 × 0.18 × 0.15 mm |
Oxford Xcalibur diffractometer | 703 reflections with I > 2σ(I) |
Radiation source: fine-focus sealed tube | Rint = 0.084 |
Graphite monochromator | θmax = 25.0°, θmin = 3.5° |
Detector resolution: 1024 x 1024 with blocks 2 x 2 pixels mm-1 | h = −27→27 |
ω scans | k = −5→6 |
7417 measured reflections | l = −25→25 |
2174 independent reflections |
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)] = 0.047 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.050 | H-atom parameters constrained |
S = 0.96 | w = 1/[σ2(Fo2) + (0.005P)2] where P = (Fo2 + 2Fc2)/3 |
2174 reflections | (Δ/σ)max < 0.001 |
172 parameters | Δρmax = 0.14 e Å−3 |
0 restraints | Δρmin = −0.15 e Å−3 |
C15H11N3O | V = 2483.5 (9) Å3 |
Mr = 249.27 | Z = 8 |
Monoclinic, C2/c | Mo Kα radiation |
a = 22.975 (5) Å | µ = 0.09 mm−1 |
b = 5.5835 (10) Å | T = 293 K |
c = 21.550 (5) Å | 0.22 × 0.18 × 0.15 mm |
β = 116.06 (3)° |
Oxford Xcalibur diffractometer | 703 reflections with I > 2σ(I) |
7417 measured reflections | Rint = 0.084 |
2174 independent reflections |
R[F2 > 2σ(F2)] = 0.047 | 0 restraints |
wR(F2) = 0.050 | H-atom parameters constrained |
S = 0.96 | Δρmax = 0.14 e Å−3 |
2174 reflections | Δρmin = −0.15 e Å−3 |
172 parameters |
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 > 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 | ||
N1 | 0.44615 (14) | 0.8833 (5) | 0.41540 (14) | 0.0605 (8) | |
H1 | 0.4478 | 0.9898 | 0.4449 | 0.073* | |
N2 | 0.39386 (14) | 0.8916 (5) | 0.35228 (17) | 0.0639 (9) | |
C3 | 0.39204 (17) | 0.7233 (7) | 0.30839 (17) | 0.0466 (10) | |
N4 | 0.43901 (13) | 0.5557 (5) | 0.32306 (13) | 0.0506 (8) | |
C5 | 0.48875 (18) | 0.5468 (6) | 0.38142 (18) | 0.0429 (9) | |
C6 | 0.49620 (18) | 0.7238 (7) | 0.43659 (18) | 0.0520 (10) | |
O7 | 0.54187 (10) | 0.7374 (4) | 0.49572 (10) | 0.0723 (8) | |
C8 | 0.53908 (17) | 0.3675 (7) | 0.39300 (17) | 0.0464 (9) | |
C9 | 0.52554 (16) | 0.1926 (6) | 0.34245 (17) | 0.0637 (11) | |
H9 | 0.4856 | 0.1932 | 0.3038 | 0.076* | |
C10 | 0.5702 (2) | 0.0194 (6) | 0.34868 (18) | 0.0698 (11) | |
H10 | 0.5601 | −0.0949 | 0.3141 | 0.084* | |
C11 | 0.6302 (2) | 0.0123 (7) | 0.4058 (2) | 0.0844 (14) | |
H11 | 0.6604 | −0.1055 | 0.4103 | 0.101* | |
C12 | 0.64313 (19) | 0.1846 (8) | 0.45502 (19) | 0.0870 (14) | |
H12 | 0.6832 | 0.1843 | 0.4935 | 0.104* | |
C13 | 0.59834 (19) | 0.3603 (7) | 0.44954 (17) | 0.0754 (12) | |
H13 | 0.6086 | 0.4737 | 0.4844 | 0.090* | |
C14 | 0.33639 (16) | 0.7274 (7) | 0.24217 (17) | 0.0511 (10) | |
C15 | 0.29077 (18) | 0.9059 (6) | 0.22270 (18) | 0.0622 (10) | |
H15 | 0.2967 | 1.0335 | 0.2526 | 0.075* | |
C16 | 0.23607 (18) | 0.9021 (7) | 0.1598 (2) | 0.0746 (12) | |
H16 | 0.2062 | 1.0264 | 0.1479 | 0.089* | |
C17 | 0.22605 (17) | 0.7126 (7) | 0.11469 (17) | 0.0732 (12) | |
H17 | 0.1895 | 0.7063 | 0.0724 | 0.088* | |
C18 | 0.27223 (18) | 0.5333 (6) | 0.13456 (18) | 0.0764 (12) | |
H18 | 0.2664 | 0.4045 | 0.1051 | 0.092* | |
C19 | 0.32689 (17) | 0.5412 (6) | 0.19740 (18) | 0.0670 (11) | |
H19 | 0.3574 | 0.4190 | 0.2093 | 0.080* |
U11 | U22 | U33 | U12 | U13 | U23 | |
N1 | 0.065 (2) | 0.071 (3) | 0.046 (2) | −0.0064 (19) | 0.0255 (19) | −0.0181 (18) |
N2 | 0.055 (2) | 0.070 (2) | 0.063 (2) | −0.0054 (18) | 0.0222 (18) | −0.0133 (19) |
C3 | 0.047 (3) | 0.048 (3) | 0.046 (3) | −0.013 (2) | 0.021 (2) | −0.016 (2) |
N4 | 0.0455 (19) | 0.064 (2) | 0.0406 (19) | 0.0046 (17) | 0.0171 (16) | −0.0003 (17) |
C5 | 0.057 (3) | 0.038 (3) | 0.043 (2) | −0.004 (2) | 0.030 (2) | −0.001 (2) |
C6 | 0.061 (3) | 0.048 (3) | 0.059 (3) | −0.007 (2) | 0.037 (2) | −0.003 (3) |
O7 | 0.0738 (19) | 0.0888 (19) | 0.0481 (16) | −0.0022 (15) | 0.0211 (13) | −0.0169 (15) |
C8 | 0.052 (2) | 0.054 (3) | 0.031 (2) | −0.005 (2) | 0.016 (2) | 0.000 (2) |
C9 | 0.065 (3) | 0.069 (3) | 0.064 (3) | 0.012 (2) | 0.035 (2) | 0.000 (2) |
C10 | 0.088 (3) | 0.071 (3) | 0.063 (3) | 0.005 (3) | 0.045 (3) | −0.003 (3) |
C11 | 0.086 (4) | 0.097 (4) | 0.083 (4) | 0.013 (3) | 0.048 (3) | 0.025 (3) |
C12 | 0.074 (3) | 0.113 (4) | 0.064 (3) | 0.013 (3) | 0.021 (3) | 0.011 (3) |
C13 | 0.076 (3) | 0.089 (3) | 0.054 (3) | 0.008 (3) | 0.022 (2) | −0.001 (2) |
C14 | 0.045 (3) | 0.050 (3) | 0.057 (3) | 0.001 (2) | 0.022 (2) | −0.005 (2) |
C15 | 0.074 (3) | 0.062 (3) | 0.055 (3) | 0.012 (2) | 0.033 (2) | 0.000 (2) |
C16 | 0.082 (3) | 0.078 (3) | 0.074 (3) | 0.008 (3) | 0.043 (3) | −0.002 (3) |
C17 | 0.067 (3) | 0.083 (3) | 0.064 (3) | 0.009 (3) | 0.023 (2) | 0.006 (3) |
C18 | 0.073 (3) | 0.068 (3) | 0.074 (3) | 0.005 (3) | 0.019 (2) | −0.020 (2) |
C19 | 0.054 (3) | 0.069 (3) | 0.065 (3) | −0.005 (2) | 0.014 (2) | −0.020 (2) |
N1—C6 | 1.365 (3) | C11—C12 | 1.364 (4) |
N1—N2 | 1.364 (3) | C11—H11 | 0.9300 |
N1—H1 | 0.8600 | C12—C13 | 1.389 (4) |
N2—C3 | 1.321 (3) | C12—H12 | 0.9300 |
C3—N4 | 1.357 (3) | C13—H13 | 0.9300 |
C3—C14 | 1.438 (4) | C14—C19 | 1.369 (3) |
N4—C5 | 1.276 (3) | C14—C15 | 1.371 (3) |
C5—C8 | 1.466 (4) | C15—C16 | 1.386 (4) |
C5—C6 | 1.497 (4) | C15—H15 | 0.9300 |
C6—O7 | 1.247 (3) | C16—C17 | 1.386 (4) |
C8—C13 | 1.372 (4) | C16—H16 | 0.9300 |
C8—C9 | 1.393 (3) | C17—C18 | 1.383 (4) |
C9—C10 | 1.372 (3) | C17—H17 | 0.9300 |
C9—H9 | 0.9300 | C18—C19 | 1.385 (4) |
C10—C11 | 1.388 (4) | C18—H18 | 0.9300 |
C10—H10 | 0.9300 | C19—H19 | 0.9300 |
C6—N1—N2 | 126.8 (3) | C10—C11—H11 | 121.2 |
C6—N1—H1 | 116.6 | C11—C12—C13 | 122.1 (4) |
N2—N1—H1 | 116.6 | C11—C12—H12 | 119.0 |
C3—N2—N1 | 115.3 (3) | C13—C12—H12 | 119.0 |
N2—C3—N4 | 123.5 (3) | C8—C13—C12 | 120.4 (4) |
N2—C3—C14 | 115.7 (4) | C8—C13—H13 | 119.8 |
N4—C3—C14 | 120.8 (4) | C12—C13—H13 | 119.8 |
C5—N4—C3 | 122.2 (3) | C19—C14—C15 | 118.2 (3) |
N4—C5—C8 | 119.5 (4) | C19—C14—C3 | 119.0 (4) |
N4—C5—C6 | 119.8 (4) | C15—C14—C3 | 122.8 (4) |
C8—C5—C6 | 120.6 (3) | C16—C15—C14 | 122.1 (4) |
O7—C6—N1 | 120.6 (4) | C16—C15—H15 | 119.0 |
O7—C6—C5 | 127.1 (4) | C14—C15—H15 | 119.0 |
N1—C6—C5 | 112.3 (3) | C15—C16—C17 | 119.8 (4) |
C13—C8—C9 | 117.9 (4) | C15—C16—H16 | 120.1 |
C13—C8—C5 | 124.9 (4) | C17—C16—H16 | 120.1 |
C9—C8—C5 | 117.2 (3) | C16—C17—C18 | 117.8 (4) |
C10—C9—C8 | 121.1 (4) | C16—C17—H17 | 121.1 |
C10—C9—H9 | 119.4 | C18—C17—H17 | 121.1 |
C8—C9—H9 | 119.4 | C17—C18—C19 | 121.5 (4) |
C9—C10—C11 | 121.0 (4) | C17—C18—H18 | 119.3 |
C9—C10—H10 | 119.5 | C19—C18—H18 | 119.3 |
C11—C10—H10 | 119.5 | C18—C19—C14 | 120.6 (4) |
C12—C11—C10 | 117.5 (4) | C18—C19—H19 | 119.7 |
C12—C11—H11 | 121.2 | C14—C19—H19 | 119.7 |
C6—N1—N2—C3 | 1.2 (5) | C8—C9—C10—C11 | 0.3 (5) |
N1—N2—C3—N4 | −1.6 (5) | C9—C10—C11—C12 | −0.5 (5) |
N1—N2—C3—C14 | 178.8 (3) | C10—C11—C12—C13 | 0.7 (6) |
N2—C3—N4—C5 | 0.7 (5) | C9—C8—C13—C12 | 0.7 (5) |
C14—C3—N4—C5 | −179.7 (3) | C5—C8—C13—C12 | −178.9 (3) |
C3—N4—C5—C8 | −177.5 (3) | C11—C12—C13—C8 | −0.9 (5) |
C3—N4—C5—C6 | 0.7 (5) | N2—C3—C14—C19 | −171.6 (3) |
N2—N1—C6—O7 | 179.0 (3) | N4—C3—C14—C19 | 8.8 (5) |
N2—N1—C6—C5 | 0.1 (4) | N2—C3—C14—C15 | 5.8 (5) |
N4—C5—C6—O7 | −179.9 (4) | N4—C3—C14—C15 | −173.8 (3) |
C8—C5—C6—O7 | −1.8 (5) | C19—C14—C15—C16 | 0.1 (5) |
N4—C5—C6—N1 | −1.0 (4) | C3—C14—C15—C16 | −177.3 (3) |
C8—C5—C6—N1 | 177.1 (3) | C14—C15—C16—C17 | 0.5 (6) |
N4—C5—C8—C13 | 170.8 (3) | C15—C16—C17—C18 | −0.5 (5) |
C6—C5—C8—C13 | −7.3 (5) | C16—C17—C18—C19 | 0.0 (5) |
N4—C5—C8—C9 | −8.9 (4) | C17—C18—C19—C14 | 0.6 (5) |
C6—C5—C8—C9 | 173.0 (3) | C15—C14—C19—C18 | −0.7 (5) |
C13—C8—C9—C10 | −0.5 (4) | C3—C14—C19—C18 | 176.9 (3) |
C5—C8—C9—C10 | 179.2 (3) |
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···O7i | 0.86 | 1.93 | 2.787 (3) | 171 |
C13—H13···O7 | 0.93 | 2.21 | 2.871 (5) | 127 |
C15—H15···N2 | 0.93 | 2.45 | 2.757 (5) | 99 |
C9—H9···N4 | 0.93 | 2.41 | 2.740 (5) | 101 |
C19—H19···N4 | 0.93 | 2.46 | 2.801 (5) | 102 |
Symmetry code: (i) −x+1, −y+2, −z+1. |
Experimental details
Crystal data | |
Chemical formula | C15H11N3O |
Mr | 249.27 |
Crystal system, space group | Monoclinic, C2/c |
Temperature (K) | 293 |
a, b, c (Å) | 22.975 (5), 5.5835 (10), 21.550 (5) |
β (°) | 116.06 (3) |
V (Å3) | 2483.5 (9) |
Z | 8 |
Radiation type | Mo Kα |
µ (mm−1) | 0.09 |
Crystal size (mm) | 0.22 × 0.18 × 0.15 |
Data collection | |
Diffractometer | Oxford Xcalibur diffractometer |
Absorption correction | – |
No. of measured, independent and observed [I > 2σ(I)] reflections | 7417, 2174, 703 |
Rint | 0.084 |
(sin θ/λ)max (Å−1) | 0.595 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.047, 0.050, 0.96 |
No. of reflections | 2174 |
No. of parameters | 172 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.14, −0.15 |
Computer programs: CrysAlis CCD (Oxford Diffraction, 2008), CrysAlis RED (Oxford Diffraction, 2008), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008).
N1—C6 | 1.365 (3) | C3—N4 | 1.357 (3) |
N1—N2 | 1.364 (3) | N4—C5 | 1.276 (3) |
N2—C3 | 1.321 (3) | C5—C6 | 1.497 (4) |
C6—N1—N2 | 126.8 (3) | C5—N4—C3 | 122.2 (3) |
C3—N2—N1 | 115.3 (3) | N4—C5—C6 | 119.8 (4) |
N2—C3—N4 | 123.5 (3) | N1—C6—C5 | 112.3 (3) |
N4—C5—C8—C13 | 170.8 (3) | N2—C3—C14—C19 | −171.6 (3) |
C6—C5—C8—C13 | −7.3 (5) | N4—C3—C14—C19 | 8.8 (5) |
N4—C5—C8—C9 | −8.9 (4) | N2—C3—C14—C15 | 5.8 (5) |
C6—C5—C8—C9 | 173.0 (3) | N4—C3—C14—C15 | −173.8 (3) |
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1···O7i | 0.86 | 1.93 | 2.787 (3) | 170.5 |
C13—H13···O7 | 0.93 | 2.21 | 2.871 (5) | 126.9 |
C15—H15···N2 | 0.93 | 2.45 | 2.757 (5) | 99.2 |
C9—H9···N4 | 0.93 | 2.41 | 2.740 (5) | 100.8 |
C19—H19···N4 | 0.93 | 2.46 | 2.801 (5) | 101.5 |
Symmetry code: (i) −x+1, −y+2, −z+1. |
1,2,4-Triazine derivatives display a broad spectrum of biological activity and have numerous applications in many fields. They are thus compounds of general interest (Neunhoeffer, 1996). They are applied in medicine as potential antibacterial and antifungal agents, in the agrochemical industry as plant-protection materials, and as components of commercial dyes (Abdel Hamide, 1997; Freidinger et al., 1993; Ackerman, 2007; Bettati et al., 2002). A wide range of synthetic procedures have been reported for the related 1,2,4-triazin-6-ones. They are commonly prepared from acid hydrazides (Zhao et al., 2003), amides (Blass et al., 2002) or iminoesters (Martinez-Teipel et al., 2001; Kammoun et al., 2000), or from small heterocyclic azirine structures (Nishiwaki & Saito, 1970). Recently, whilst investigating the reactions of optically active α-aminocarboxylic acid hydrazides with triethyl orthoesters, we obtained two groups of products: five-membered 2-(1-amino-1-phenylmethyl)-1,3,4-oxadiazoles and six-membered 5-substituted 3-phenyl-1,2,4-triazin-6-ones (Kudelko et al., 2011). One of the by-products, separated from the post-reaction mixture in low yields, was the title compound, (I). A literature survey revealed that similar compounds are usually constructed via oxidation of the appropriate 4,5-dihydro-1,2,4-triazin-6(1H)-ones with the use of mild oxidizing agents such as 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ, Miesel, 1982). We believe that (I) was formed by the air-oxidation of 4,5-dihydro-3,5-diphenyl-1,2,4-triazin-6(1H)-one, as it was exposed to the atmosphere whilst standing in ethanol solution for 10 d. A search of the Cambridge Structural Database (CSD; ConQuest Version 1.13; Allen, 2002) afforded only a few examples of 3,5-disubstituted-1,2,4-triazin-6(1H)-one derivatives (Buscemi et al., 2006; Garg & Stoltz, 2005; Sanudo et al., 2006; Trávníček et al., 1995). Therefore, the synthesis and structural characterization of (I) are reported herein.
The molecular structure of (I) and the atomic numbering scheme are presented in Fig. 1 and the packing arrangement in the crystal state is presented in Fig. 2. The main intermolecular interaction consists of centrosymmetric dimers formed by pairs of N—H···O hydrogen bonds (Table 2). The molecular structure of (I) consists of three rings: A (the phenyl ring containing atoms C8–C13), B (the triazine ring containing atoms N1/N2/C3/N4/C5/C6) and C (the phenyl ring containing atoms from C14–C19). These are nearly coplanar in the crystalline state, with angles between the ring planes for A/B and B/C being similar [8.6 (2) and 8.4 (2)°, respectively]. The A and C planes are arranged in a mutually cis position. The twists (described by torsion angles) around the C3—C14 and C5—C8 bonds are less than 10°. These small deformations from planarity probably result from the intermolecular interactions present in the crystal lattice. A density functional theory (DFT) study predicts a completely planar conformation (Cs point-group symmetry) as the preferred one for the isolated molecule of (I). See Supplementary materials for further details of the DFT calculations.
The near planarity of the system favours the formation of intramolecular hydrogen bonds and π-electron delocalization. The molecular structure of (I) contains four weak intramolecular hydrogen bonds (Fig. 1, Table 2); one C—H···O interaction, which forms a six-membered ring, and three C—H···N interactions that form five-membered rings, denoted quasi-rings. The resulting rings can be investigated as molecular patterns of intramolecular resonance assisted by hydrogen bonds. The position of the extra ring formed by the substituent interacting through the hydrogen bond is found to influence both the strength of that hydrogen bond and the local aromaticity of the polycyclic aromatic hydrocarbon skeleton. Relatively speaking, a greater loss of aromaticity of the ipso-ring (phenyl ring) can be observed for these kinked-like structures because of the greater participation of π-electrons from the ipso-ring in the formation of the quasi-ring (Krygowski et al., 2010; Palusiak et al., 2009).
The harmonic oscillator model of aromaticity (HOMA) is a leading method for the quantitative determination of cyclic π-electron delocalization in chemical compounds. It is based on the geometric criterion of aromaticity, which stipulates that bond lengths in aromatic systems lie between values that are typical for single and double bonds (Kruszewski & Krygowski, 1973; Krygowski, 1993). Therefore, HOMA = 0 for a model non-aromatic system (e.g. the Kekulé structure of benzene) and HOMA = 1 for a system with all bonds equal to the optimal value, assumed to be realised for fully aromatic systems. The HOMA value (based on B3LYP/6-311++G** optimized geometries) calculated for ring A (0.960) is lower than that of ring C (0.979). This loss of aromaticity of ring A is caused by both the electron-withdrawing properties of the neighbouring carbonyl group and the above-mentioned interactions with the quasi-rings formed by intramolecular hydrogen bonds, in particular by C13—H13···O7. Breaking the intramolecular hydrogen bonds by a twist of 90° around the C5—C8 bond results in an increase in energy of 5.65 kcal mol-1 (1 kcal mol-1 = 4.184 kJ mol-1), based on the B3LYP/6–311++G** calculations, as well as an increase in the aromaticity of ring A (HOMA = 0.989). The aromaticity of ring C remains almost unchanged (HOMA = 0.978).
Similar sequences of values were obtained for a twist about the C3—C14 bond. In this case the increase in energy is 5.22 kcal mol-1, and the HOMA value rises to 0.988 for ring C and decreases to 0.958 for ring A. Finally, in a hypothetical conformation without intramolecular hydrogen bonds, viz. with both phenyl groups perpendicular to the triazine ring, the energy is higher by 11.75 kcal mol-1 and the HOMA index takes the same value for rings A and C (0.989).
Note that there are no significant differences between the values of the bond lengths and angles of (I) in the solid state and those found for the calculated planar structure; the differences do not exceed 0.02 Å for bond distances and 2° for bond angles. All bond distances and angles are normal (Table 1) and are in good agreement with the geometry of 3,5-disubstituted-1,2,4-triazin-6(1H)-one derivatives (Buscemi et al., 2006; Garg & Stoltz, 2005; Sanudo et al., 2006; Trávníček et al., 1995).