The crystal structures of the brown–yellow and orange polymorphs of the title compound, 4-[(2-nitrophenyl)diazenyl]phenol, C
12H
9N
3O
3, have been determined and their visible reflection spectra recorded. Both structures adopt a stacking arrangement with interstack hydrogen bonds.
Ab initio and semi-empirical (
AM1 and
INDO-
CISD) calculations were performed in order to rationalize the difference in colour. It can be attributed neither to the subtle distinctions in molecular geometry nor to the effect of intermolecular electrostatic interactions. The most probable origin of this difference is the mixing of intramolecular
n π* and intermolecular charge-transfer excitations.
Supporting information
CCDC references: 170196; 170197
Compound (I) was prepared according to the established procedure of Elbs et
al. (1924). Single crystals of (Ia) and (Ib) were grown by slow
evaporation of chloroform and acetone solutions of (I), respectively. The
powders prepared from the crystals of (Ia) and (Ib) were brownish-yellow and
orange, respectively. The UV-visible spectra were recorded on a Specord
M-40 spectrophotometer (Carl Zeiss, Jena). Spectroscopic data:
UV-visible [CCl4, λmax, nm (lg ε)]: 350 (4.26), 443 (3.02). The ab
initio calculations were performed with GAMESS (Schmidt et al.,
1993) using the 6–31G** basis set. Details of the calculations employing
crystal electrostatic potential at the AM1 and INDO levels were reported
elsewhere (Yatsenko & Paseshnichenko, 2000).
For both compounds, data collection: CAD-4 Software (Enraf-Nonius, 1989); cell refinement: CAD-4 Software; data reduction: PROFIT (Streltsov & Zavodnik, 1989); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997). Molecular graphics: ORTEP-3 (Farrugia, 1997) for (Ia); ORTEP-3 (Farrugia, 1997) and PLUTON92 (Spek, 1992) for (Ib). For both compounds, software used to prepare material for publication: PARST (Nardelli, 1983).
(Ia) 4-[(2-nitrophenyl)azo]phenol
top
Crystal data top
C12H9N3O3 | Z = 2 |
Mr = 243.22 | F(000) = 252 |
Triclinic, P1 | Dx = 1.470 Mg m−3 |
a = 7.114 (2) Å | Mo Kα radiation, λ = 0.71073 Å |
b = 7.380 (2) Å | Cell parameters from 23 reflections |
c = 10.849 (4) Å | θ = 14.0–15.8° |
α = 96.68 (2)° | µ = 0.11 mm−1 |
β = 97.49 (2)° | T = 293 K |
γ = 100.54 (2)° | Prism, yellow-brown |
V = 549.4 (3) Å3 | 0.44 × 0.25 × 0.18 mm |
Data collection top
Enraf-Nonius CAD-4 diffractometer | Rint = 0.000 |
Radiation source: fine-focus sealed tube | θmax = 27.0°, θmin = 1.9° |
Graphite monochromator | h = −9→8 |
non–profiled ω scans | k = −9→9 |
2395 measured reflections | l = 0→13 |
2395 independent reflections | 3 standard reflections every 80 min |
1599 reflections with I > 2σ(I) | intensity decay: none |
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)] = 0.038 | Hydrogen site location: difference Fourier map |
wR(F2) = 0.101 | All H-atom parameters refined |
S = 1.12 | w = 1/[σ2(Fo2) + (0.047P)2] where P = (Fo2 + 2Fc2)/3 |
2395 reflections | (Δ/σ)max = 0.001 |
199 parameters | Δρmax = 0.13 e Å−3 |
0 restraints | Δρmin = −0.13 e Å−3 |
Crystal data top
C12H9N3O3 | γ = 100.54 (2)° |
Mr = 243.22 | V = 549.4 (3) Å3 |
Triclinic, P1 | Z = 2 |
a = 7.114 (2) Å | Mo Kα radiation |
b = 7.380 (2) Å | µ = 0.11 mm−1 |
c = 10.849 (4) Å | T = 293 K |
α = 96.68 (2)° | 0.44 × 0.25 × 0.18 mm |
β = 97.49 (2)° | |
Data collection top
Enraf-Nonius CAD-4 diffractometer | Rint = 0.000 |
2395 measured reflections | 3 standard reflections every 80 min |
2395 independent reflections | intensity decay: none |
1599 reflections with I > 2σ(I) | |
Refinement top
R[F2 > 2σ(F2)] = 0.038 | 0 restraints |
wR(F2) = 0.101 | All H-atom parameters refined |
S = 1.12 | Δρmax = 0.13 e Å−3 |
2395 reflections | Δρmin = −0.13 e Å−3 |
199 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 >
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. |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
O1 | −0.11585 (15) | 0.09869 (18) | 0.11935 (9) | 0.0623 (3) | |
O2 | 0.42739 (15) | 0.09597 (17) | 0.77164 (10) | 0.0637 (4) | |
O3 | 0.53975 (17) | 0.2873 (2) | 0.93921 (11) | 0.0986 (6) | |
N1 | 0.55694 (16) | 0.26659 (17) | 0.46467 (10) | 0.0449 (3) | |
N2 | 0.54162 (15) | 0.26991 (16) | 0.57761 (10) | 0.0439 (3) | |
N3 | 0.55263 (17) | 0.2176 (2) | 0.83473 (11) | 0.0502 (3) | |
C1 | 0.04451 (19) | 0.1421 (2) | 0.20802 (12) | 0.0434 (3) | |
C2 | 0.0354 (2) | 0.1219 (2) | 0.33272 (13) | 0.0467 (4) | |
C3 | 0.2008 (2) | 0.1632 (2) | 0.41883 (13) | 0.0442 (3) | |
C4 | 0.37845 (19) | 0.22757 (18) | 0.38277 (12) | 0.0404 (3) | |
C5 | 0.3859 (2) | 0.2484 (2) | 0.25806 (13) | 0.0467 (4) | |
C6 | 0.2206 (2) | 0.2066 (2) | 0.17076 (13) | 0.0482 (4) | |
C11 | 0.72440 (18) | 0.30347 (18) | 0.65783 (12) | 0.0381 (3) | |
C12 | 0.72971 (18) | 0.28328 (19) | 0.78406 (12) | 0.0388 (3) | |
C13 | 0.8999 (2) | 0.3224 (2) | 0.86705 (14) | 0.0468 (4) | |
C14 | 1.0711 (2) | 0.3820 (2) | 0.82508 (14) | 0.0502 (4) | |
C15 | 1.0703 (2) | 0.4035 (2) | 0.70105 (14) | 0.0519 (4) | |
C16 | 0.8998 (2) | 0.3654 (2) | 0.61888 (14) | 0.0479 (4) | |
H1 | −0.219 (3) | 0.042 (3) | 0.1592 (19) | 0.105 (7)* | |
H5 | 0.1982 (18) | 0.1473 (19) | 0.5012 (13) | 0.039 (4)* | |
H2 | −0.093 (2) | 0.075 (2) | 0.3550 (13) | 0.052 (4)* | |
H3 | 0.894 (2) | 0.377 (2) | 0.5325 (16) | 0.066 (5)* | |
H4 | 0.5128 (19) | 0.2895 (19) | 0.2342 (12) | 0.046 (4)* | |
H6 | 1.186 (2) | 0.448 (2) | 0.6670 (13) | 0.052 (4)* | |
H7 | 1.188 (2) | 0.409 (2) | 0.8824 (13) | 0.056 (4)* | |
H8 | 0.229 (2) | 0.221 (2) | 0.0833 (14) | 0.056 (4)* | |
H9 | 0.896 (2) | 0.307 (2) | 0.9439 (14) | 0.048 (4)* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
O1 | 0.0473 (6) | 0.0882 (9) | 0.0446 (6) | −0.0022 (6) | −0.0057 (5) | 0.0205 (6) |
O2 | 0.0448 (6) | 0.0746 (8) | 0.0666 (7) | −0.0050 (6) | 0.0093 (5) | 0.0158 (6) |
O3 | 0.0648 (8) | 0.1724 (16) | 0.0506 (7) | 0.0099 (9) | 0.0201 (6) | −0.0077 (8) |
N1 | 0.0435 (6) | 0.0514 (8) | 0.0369 (6) | 0.0055 (5) | 0.0008 (5) | 0.0066 (5) |
N2 | 0.0421 (6) | 0.0500 (8) | 0.0376 (6) | 0.0067 (6) | 0.0013 (5) | 0.0078 (5) |
N3 | 0.0388 (6) | 0.0738 (9) | 0.0412 (6) | 0.0146 (6) | 0.0065 (5) | 0.0148 (6) |
C1 | 0.0438 (7) | 0.0441 (8) | 0.0383 (7) | 0.0030 (6) | −0.0022 (6) | 0.0092 (6) |
C2 | 0.0425 (8) | 0.0533 (9) | 0.0436 (8) | 0.0017 (7) | 0.0074 (6) | 0.0155 (7) |
C3 | 0.0508 (8) | 0.0497 (9) | 0.0318 (7) | 0.0058 (7) | 0.0056 (6) | 0.0120 (6) |
C4 | 0.0419 (7) | 0.0401 (8) | 0.0366 (7) | 0.0056 (6) | 0.0016 (6) | 0.0038 (6) |
C5 | 0.0429 (8) | 0.0554 (9) | 0.0401 (7) | 0.0017 (7) | 0.0089 (6) | 0.0103 (7) |
C6 | 0.0535 (9) | 0.0565 (10) | 0.0329 (7) | 0.0040 (7) | 0.0062 (6) | 0.0112 (7) |
C11 | 0.0378 (7) | 0.0356 (7) | 0.0393 (7) | 0.0064 (6) | 0.0023 (5) | 0.0045 (6) |
C12 | 0.0366 (7) | 0.0416 (8) | 0.0393 (7) | 0.0096 (6) | 0.0058 (5) | 0.0081 (6) |
C13 | 0.0474 (8) | 0.0554 (10) | 0.0383 (8) | 0.0131 (7) | 0.0015 (6) | 0.0107 (7) |
C14 | 0.0380 (8) | 0.0560 (10) | 0.0516 (9) | 0.0046 (7) | −0.0037 (6) | 0.0074 (7) |
C15 | 0.0406 (8) | 0.0566 (10) | 0.0549 (9) | −0.0020 (7) | 0.0085 (7) | 0.0109 (8) |
C16 | 0.0484 (8) | 0.0518 (9) | 0.0407 (8) | 0.0010 (7) | 0.0054 (6) | 0.0113 (7) |
Geometric parameters (Å, º) top
O1—C1 | 1.3557 (15) | C4—C5 | 1.3860 (18) |
O1—H1 | 0.96 (2) | C5—C6 | 1.3724 (19) |
O2—N3 | 1.2140 (15) | C5—H4 | 0.976 (14) |
O3—N3 | 1.2110 (16) | C6—H8 | 0.976 (15) |
N1—N2 | 1.2420 (16) | C11—C16 | 1.3848 (19) |
N1—C4 | 1.4132 (16) | C11—C12 | 1.3911 (18) |
N2—C11 | 1.4305 (16) | C12—C13 | 1.3740 (18) |
N3—C12 | 1.4640 (18) | C13—C14 | 1.371 (2) |
C1—C6 | 1.385 (2) | C13—H9 | 0.858 (15) |
C1—C2 | 1.3867 (19) | C14—C15 | 1.372 (2) |
C2—C3 | 1.3658 (19) | C14—H7 | 0.947 (14) |
C2—H2 | 0.988 (15) | C15—C16 | 1.3716 (19) |
C3—C4 | 1.3888 (19) | C15—H6 | 0.966 (15) |
C3—H5 | 0.917 (13) | C16—H3 | 0.947 (16) |
| | | |
C1—O1—H1 | 106.6 (12) | C5—C6—C1 | 119.41 (13) |
N2—N1—C4 | 114.35 (12) | C5—C6—H8 | 119.8 (8) |
N1—N2—C11 | 113.09 (11) | C1—C6—H8 | 120.8 (8) |
O3—N3—O2 | 123.18 (13) | C16—C11—C12 | 116.83 (12) |
O3—N3—C12 | 117.67 (13) | C16—C11—N2 | 123.67 (12) |
O2—N3—C12 | 119.14 (12) | C12—C11—N2 | 119.40 (11) |
O1—C1—C6 | 118.12 (13) | C13—C12—C11 | 122.11 (12) |
O1—C1—C2 | 121.71 (13) | C13—C12—N3 | 116.68 (12) |
C6—C1—C2 | 120.17 (12) | C11—C12—N3 | 121.21 (11) |
C3—C2—C1 | 120.00 (13) | C14—C13—C12 | 119.52 (14) |
C3—C2—H2 | 122.6 (8) | C14—C13—H9 | 121.6 (10) |
C1—C2—H2 | 117.4 (8) | C12—C13—H9 | 118.9 (10) |
C2—C3—C4 | 120.46 (13) | C13—C14—C15 | 119.68 (14) |
C2—C3—H5 | 121.3 (8) | C13—C14—H7 | 119.4 (9) |
C4—C3—H5 | 118.3 (8) | C15—C14—H7 | 120.9 (9) |
C5—C4—C3 | 119.11 (12) | C16—C15—C14 | 120.50 (14) |
C5—C4—N1 | 116.76 (12) | C16—C15—H6 | 116.4 (8) |
C3—C4—N1 | 124.07 (12) | C14—C15—H6 | 123.1 (8) |
C6—C5—C4 | 120.85 (13) | C15—C16—C11 | 121.35 (14) |
C6—C5—H4 | 121.4 (8) | C15—C16—H3 | 122.7 (9) |
C4—C5—H4 | 117.7 (8) | C11—C16—H3 | 115.9 (9) |
| | | |
C4—N1—N2—C11 | 178.01 (11) | C16—C11—C12—C13 | 0.0 (2) |
O1—C1—C2—C3 | 178.54 (14) | N2—C11—C12—C13 | −176.73 (13) |
C6—C1—C2—C3 | −0.9 (2) | C16—C11—C12—N3 | −179.63 (13) |
C1—C2—C3—C4 | 0.7 (2) | N2—C11—C12—N3 | 3.7 (2) |
C2—C3—C4—C5 | −0.4 (2) | O3—N3—C12—C13 | 38.1 (2) |
C2—C3—C4—N1 | −177.46 (14) | O2—N3—C12—C13 | −141.16 (14) |
N2—N1—C4—C5 | 171.01 (13) | O3—N3—C12—C11 | −142.27 (16) |
N2—N1—C4—C3 | −11.8 (2) | O2—N3—C12—C11 | 38.4 (2) |
C3—C4—C5—C6 | 0.1 (2) | C11—C12—C13—C14 | −0.5 (2) |
N1—C4—C5—C6 | 177.43 (14) | N3—C12—C13—C14 | 179.12 (14) |
C4—C5—C6—C1 | −0.3 (2) | C12—C13—C14—C15 | 0.6 (2) |
O1—C1—C6—C5 | −178.80 (14) | C13—C14—C15—C16 | −0.1 (3) |
C2—C1—C6—C5 | 0.7 (2) | C14—C15—C16—C11 | −0.4 (2) |
N1—N2—C11—C16 | 13.2 (2) | C12—C11—C16—C15 | 0.5 (2) |
N1—N2—C11—C12 | −170.40 (13) | N2—C11—C16—C15 | 177.03 (14) |
(Ib) 4-[(2-nitrophenyl)azo]phenol
top
Crystal data top
C12H9N3O3 | F(000) = 1008 |
Mr = 243.22 | Dx = 1.445 Mg m−3 |
Monoclinic, P21/n | Mo Kα radiation, λ = 0.71073 Å |
a = 3.823 (1) Å | Cell parameters from 22 reflections |
b = 23.014 (8) Å | θ = 11.2–12.8° |
c = 25.437 (9) Å | µ = 0.11 mm−1 |
β = 92.73 (3)° | T = 293 K |
V = 2235.5 (13) Å3 | Needle, orange |
Z = 8 | 0.39 × 0.12 × 0.07 mm |
Data collection top
Enraf-Nonius CAD-4 diffractometer | Rint = 0.046 |
Radiation source: fine-focus sealed tube | θmax = 25.0°, θmin = 1.2° |
Graphite monochromator | h = 0→4 |
non–profiled ω scans | k = 0→27 |
4574 measured reflections | l = −30→30 |
3954 independent reflections | 3 standard reflections every 80 min |
1544 reflections with I > 2σ(I) | intensity decay: none |
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)] = 0.066 | Hydrogen site location: difference Fourier map |
wR(F2) = 0.124 | All H-atom parameters refined |
S = 0.98 | w = 1/[σ2(Fo2) + (0.032P)2] where P = (Fo2 + 2Fc2)/3 |
3954 reflections | (Δ/σ)max < 0.001 |
397 parameters | Δρmax = 0.16 e Å−3 |
0 restraints | Δρmin = −0.15 e Å−3 |
Crystal data top
C12H9N3O3 | V = 2235.5 (13) Å3 |
Mr = 243.22 | Z = 8 |
Monoclinic, P21/n | Mo Kα radiation |
a = 3.823 (1) Å | µ = 0.11 mm−1 |
b = 23.014 (8) Å | T = 293 K |
c = 25.437 (9) Å | 0.39 × 0.12 × 0.07 mm |
β = 92.73 (3)° | |
Data collection top
Enraf-Nonius CAD-4 diffractometer | Rint = 0.046 |
4574 measured reflections | 3 standard reflections every 80 min |
3954 independent reflections | intensity decay: none |
1544 reflections with I > 2σ(I) | |
Refinement top
R[F2 > 2σ(F2)] = 0.066 | 0 restraints |
wR(F2) = 0.124 | All H-atom parameters refined |
S = 0.98 | Δρmax = 0.16 e Å−3 |
3954 reflections | Δρmin = −0.15 e Å−3 |
397 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 >
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. |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
O1 | 1.1222 (10) | −0.09985 (15) | 0.34897 (14) | 0.0613 (11) | |
O21 | 0.6705 (11) | 0.34469 (16) | 0.24393 (15) | 0.0680 (12) | |
O2 | 0.3855 (12) | 0.13177 (18) | 0.53379 (15) | 0.0966 (14) | |
O22 | 1.4789 (11) | 0.12091 (17) | 0.06884 (14) | 0.1001 (14) | |
O3 | 0.7184 (12) | 0.19259 (19) | 0.57452 (14) | 0.1106 (16) | |
O23 | 1.1828 (12) | 0.05988 (17) | 0.02288 (13) | 0.1121 (16) | |
N1 | 0.6712 (9) | 0.12359 (14) | 0.38991 (14) | 0.0470 (10) | |
N21 | 1.1606 (9) | 0.12226 (15) | 0.20954 (13) | 0.0470 (10) | |
N2 | 0.6877 (9) | 0.14127 (14) | 0.43656 (14) | 0.0463 (10) | |
N22 | 1.1487 (9) | 0.10482 (14) | 0.16238 (13) | 0.0462 (10) | |
N3 | 0.5367 (13) | 0.1782 (2) | 0.53684 (18) | 0.0685 (14) | |
N23 | 1.3434 (13) | 0.07355 (18) | 0.06278 (16) | 0.0666 (13) | |
C1 | 1.0228 (12) | −0.0444 (2) | 0.36164 (18) | 0.0483 (13) | |
C21 | 0.7855 (12) | 0.28936 (19) | 0.23324 (19) | 0.0451 (12) | |
C2 | 1.0744 (13) | −0.0219 (2) | 0.41173 (19) | 0.0496 (13) | |
C22 | 0.7464 (13) | 0.2652 (2) | 0.18338 (18) | 0.0527 (14) | |
C3 | 0.9648 (12) | 0.0335 (2) | 0.42221 (17) | 0.0477 (13) | |
C23 | 0.8646 (13) | 0.2107 (2) | 0.17472 (19) | 0.0512 (13) | |
C4 | 0.8006 (11) | 0.06676 (19) | 0.38299 (16) | 0.0440 (12) | |
C24 | 1.0205 (11) | 0.17883 (17) | 0.21531 (16) | 0.0403 (11) | |
C5 | 0.7531 (13) | 0.04426 (19) | 0.33286 (18) | 0.0471 (12) | |
C25 | 1.0572 (13) | 0.2024 (2) | 0.26531 (17) | 0.0481 (13) | |
C6 | 0.8604 (13) | −0.01182 (19) | 0.32249 (18) | 0.0490 (13) | |
C26 | 0.9412 (14) | 0.2579 (2) | 0.27327 (19) | 0.0526 (14) | |
C11 | 0.5622 (11) | 0.19913 (18) | 0.44222 (17) | 0.0413 (11) | |
C31 | 1.2864 (11) | 0.04798 (18) | 0.15629 (16) | 0.0411 (12) | |
C12 | 0.4996 (13) | 0.2188 (2) | 0.49205 (18) | 0.0488 (12) | |
C32 | 1.3711 (12) | 0.03183 (19) | 0.10622 (18) | 0.0452 (12) | |
C13 | 0.4036 (14) | 0.2753 (2) | 0.5024 (2) | 0.0623 (16) | |
C33 | 1.4829 (14) | −0.0229 (2) | 0.0940 (2) | 0.0601 (15) | |
C14 | 0.3603 (15) | 0.3129 (2) | 0.4610 (2) | 0.0664 (17) | |
C34 | 1.5147 (15) | −0.0631 (2) | 0.1336 (2) | 0.0667 (16) | |
C15 | 0.4207 (13) | 0.2947 (2) | 0.4110 (2) | 0.0574 (15) | |
C35 | 1.4299 (15) | −0.0495 (2) | 0.1837 (2) | 0.0637 (16) | |
C16 | 0.5177 (13) | 0.2379 (2) | 0.40111 (19) | 0.0504 (13) | |
C36 | 1.3137 (13) | 0.0064 (2) | 0.1953 (2) | 0.0523 (14) | |
H1 | 1.225 (14) | −0.115 (2) | 0.3728 (18) | 0.10 (2)* | |
H2 | 1.195 (9) | −0.0455 (14) | 0.4385 (12) | 0.037 (11)* | |
H3 | 0.994 (9) | 0.0495 (15) | 0.4582 (13) | 0.047 (12)* | |
H5 | 0.624 (10) | 0.0676 (15) | 0.3052 (13) | 0.046 (12)* | |
H6 | 0.840 (9) | −0.0306 (14) | 0.2876 (12) | 0.036 (11)* | |
H13 | 0.349 (11) | 0.2875 (17) | 0.5386 (15) | 0.067 (15)* | |
H14 | 0.315 (10) | 0.3522 (15) | 0.4696 (13) | 0.047 (13)* | |
H15 | 0.394 (10) | 0.3245 (16) | 0.3820 (14) | 0.054 (13)* | |
H16 | 0.586 (9) | 0.2260 (15) | 0.3670 (13) | 0.036 (11)* | |
H21 | 0.566 (11) | 0.3543 (18) | 0.2179 (14) | 0.047 (17)* | |
H22 | 0.646 (9) | 0.2901 (13) | 0.1545 (11) | 0.027 (10)* | |
H23 | 0.846 (9) | 0.1899 (15) | 0.1415 (13) | 0.040 (12)* | |
H25 | 1.146 (10) | 0.1783 (15) | 0.2939 (13) | 0.050 (13)* | |
H26 | 0.951 (10) | 0.2741 (15) | 0.3076 (14) | 0.054 (13)* | |
H33 | 1.561 (11) | −0.0299 (18) | 0.0593 (15) | 0.073 (17)* | |
H34 | 1.585 (11) | −0.1020 (17) | 0.1262 (15) | 0.064 (14)* | |
H35 | 1.433 (11) | −0.0768 (16) | 0.2123 (14) | 0.057 (14)* | |
H36 | 1.253 (11) | 0.0155 (18) | 0.2285 (15) | 0.065 (16)* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
O1 | 0.082 (3) | 0.039 (2) | 0.063 (3) | 0.0090 (19) | 0.003 (2) | −0.0044 (18) |
O21 | 0.090 (3) | 0.046 (2) | 0.067 (3) | 0.001 (2) | −0.004 (3) | −0.012 (2) |
O2 | 0.117 (4) | 0.087 (3) | 0.086 (3) | 0.001 (3) | −0.001 (3) | 0.024 (3) |
O22 | 0.137 (4) | 0.085 (3) | 0.078 (3) | −0.035 (3) | −0.001 (3) | 0.024 (2) |
O3 | 0.134 (4) | 0.136 (4) | 0.058 (2) | 0.027 (3) | −0.031 (3) | 0.000 (3) |
O23 | 0.180 (5) | 0.104 (3) | 0.049 (2) | 0.012 (3) | −0.027 (3) | −0.012 (2) |
N1 | 0.049 (3) | 0.041 (2) | 0.051 (2) | 0.000 (2) | 0.004 (2) | −0.0025 (19) |
N21 | 0.051 (3) | 0.048 (2) | 0.042 (2) | −0.001 (2) | 0.002 (2) | 0.0012 (19) |
N2 | 0.047 (3) | 0.047 (2) | 0.045 (2) | 0.001 (2) | 0.002 (2) | −0.0055 (19) |
N22 | 0.052 (3) | 0.045 (2) | 0.041 (2) | 0.003 (2) | −0.001 (2) | −0.0033 (19) |
N3 | 0.074 (4) | 0.078 (4) | 0.054 (3) | 0.024 (3) | 0.013 (3) | 0.000 (3) |
N23 | 0.097 (4) | 0.056 (3) | 0.047 (3) | 0.003 (3) | 0.011 (3) | −0.007 (3) |
C1 | 0.047 (3) | 0.044 (3) | 0.055 (3) | −0.006 (3) | 0.014 (3) | −0.009 (3) |
C21 | 0.042 (3) | 0.036 (3) | 0.058 (3) | −0.007 (2) | 0.006 (3) | −0.003 (2) |
C2 | 0.048 (3) | 0.046 (3) | 0.054 (3) | 0.009 (3) | −0.002 (3) | 0.005 (3) |
C22 | 0.066 (4) | 0.045 (3) | 0.045 (3) | 0.002 (3) | −0.009 (3) | 0.006 (3) |
C3 | 0.052 (3) | 0.053 (3) | 0.039 (3) | −0.001 (3) | 0.000 (3) | −0.008 (3) |
C23 | 0.063 (4) | 0.047 (3) | 0.044 (3) | 0.005 (3) | 0.005 (3) | −0.001 (3) |
C4 | 0.043 (3) | 0.042 (3) | 0.048 (3) | 0.002 (2) | 0.012 (3) | 0.000 (2) |
C24 | 0.043 (3) | 0.038 (3) | 0.040 (3) | −0.005 (2) | 0.007 (2) | −0.002 (2) |
C5 | 0.052 (3) | 0.043 (3) | 0.047 (3) | −0.001 (3) | 0.002 (3) | 0.001 (2) |
C25 | 0.057 (3) | 0.047 (3) | 0.040 (3) | −0.005 (3) | 0.000 (3) | 0.004 (2) |
C6 | 0.060 (4) | 0.047 (3) | 0.040 (3) | −0.005 (3) | 0.004 (3) | −0.009 (2) |
C26 | 0.066 (4) | 0.051 (3) | 0.041 (3) | −0.003 (3) | 0.002 (3) | −0.002 (3) |
C11 | 0.037 (3) | 0.040 (3) | 0.047 (3) | 0.000 (2) | −0.001 (2) | −0.006 (2) |
C31 | 0.038 (3) | 0.043 (3) | 0.043 (3) | −0.004 (2) | 0.003 (2) | −0.005 (2) |
C12 | 0.047 (3) | 0.058 (3) | 0.041 (3) | 0.008 (3) | −0.007 (2) | 0.000 (3) |
C32 | 0.048 (3) | 0.039 (3) | 0.049 (3) | 0.003 (3) | 0.003 (2) | 0.001 (2) |
C13 | 0.068 (4) | 0.065 (4) | 0.054 (4) | 0.013 (3) | 0.005 (3) | −0.019 (3) |
C33 | 0.052 (4) | 0.063 (4) | 0.066 (4) | −0.004 (3) | 0.010 (3) | −0.009 (3) |
C14 | 0.082 (4) | 0.044 (3) | 0.073 (4) | 0.005 (3) | −0.003 (4) | −0.014 (3) |
C34 | 0.065 (4) | 0.044 (3) | 0.091 (5) | 0.005 (3) | 0.000 (4) | −0.018 (3) |
C15 | 0.064 (4) | 0.041 (3) | 0.066 (4) | 0.000 (3) | −0.005 (3) | 0.007 (3) |
C35 | 0.064 (4) | 0.044 (3) | 0.082 (4) | 0.001 (3) | −0.005 (4) | 0.012 (3) |
C16 | 0.054 (3) | 0.048 (3) | 0.049 (3) | −0.001 (3) | 0.000 (3) | 0.002 (3) |
C36 | 0.052 (4) | 0.056 (3) | 0.048 (3) | −0.004 (3) | 0.002 (3) | 0.003 (3) |
Geometric parameters (Å, º) top
O1—C1 | 1.373 (5) | C4—C5 | 1.380 (5) |
O1—H1 | 0.79 (5) | C24—C25 | 1.383 (5) |
O21—C21 | 1.378 (5) | C5—C6 | 1.384 (6) |
O21—H21 | 0.79 (4) | C5—H5 | 1.00 (3) |
O2—N3 | 1.216 (5) | C25—C26 | 1.372 (6) |
O22—N23 | 1.213 (5) | C25—H25 | 0.96 (3) |
O3—N3 | 1.203 (5) | C6—H6 | 0.99 (3) |
O23—N23 | 1.203 (5) | C26—H26 | 0.95 (3) |
N1—N2 | 1.253 (4) | C11—C12 | 1.377 (5) |
N1—C4 | 1.412 (5) | C11—C16 | 1.379 (5) |
N21—N22 | 1.264 (4) | C31—C36 | 1.379 (5) |
N21—C24 | 1.418 (5) | C31—C32 | 1.380 (5) |
N2—C11 | 1.425 (5) | C12—C13 | 1.380 (6) |
N22—C31 | 1.421 (5) | C32—C33 | 1.370 (6) |
N3—C12 | 1.474 (6) | C13—C14 | 1.365 (6) |
N23—C32 | 1.464 (5) | C13—H13 | 0.99 (4) |
C1—C6 | 1.372 (6) | C33—C34 | 1.367 (7) |
C1—C2 | 1.381 (6) | C33—H33 | 0.96 (4) |
C21—C26 | 1.363 (6) | C14—C15 | 1.370 (6) |
C21—C22 | 1.386 (6) | C14—H14 | 0.95 (3) |
C2—C3 | 1.373 (6) | C34—C35 | 1.368 (7) |
C2—H2 | 0.97 (3) | C34—H34 | 0.96 (4) |
C22—C23 | 1.354 (6) | C15—C16 | 1.384 (6) |
C22—H22 | 0.99 (3) | C15—H15 | 1.01 (3) |
C3—C4 | 1.383 (5) | C35—C36 | 1.398 (6) |
C3—H3 | 0.99 (3) | C35—H35 | 0.96 (4) |
C23—C24 | 1.379 (5) | C16—H16 | 0.96 (3) |
C23—H23 | 0.97 (3) | C36—H36 | 0.91 (4) |
| | | |
C1—O1—H1 | 112 (4) | C1—C6—C5 | 120.0 (4) |
C21—O21—H21 | 104 (3) | C1—C6—H6 | 115 (2) |
N2—N1—C4 | 114.6 (4) | C5—C6—H6 | 125 (2) |
N22—N21—C24 | 113.1 (4) | C21—C26—C25 | 121.2 (5) |
N1—N2—C11 | 113.4 (4) | C21—C26—H26 | 118 (2) |
N21—N22—C31 | 113.5 (4) | C25—C26—H26 | 120 (2) |
O3—N3—O2 | 123.2 (5) | C12—C11—C16 | 117.7 (4) |
O3—N3—C12 | 118.2 (5) | C12—C11—N2 | 118.3 (4) |
O2—N3—C12 | 118.6 (5) | C16—C11—N2 | 123.9 (4) |
O23—N23—O22 | 122.7 (5) | C36—C31—C32 | 117.7 (4) |
O23—N23—C32 | 118.8 (4) | C36—C31—N22 | 125.1 (4) |
O22—N23—C32 | 118.6 (4) | C32—C31—N22 | 116.9 (4) |
C6—C1—O1 | 117.3 (4) | C11—C12—C13 | 123.0 (5) |
C6—C1—C2 | 120.3 (4) | C11—C12—N3 | 119.3 (4) |
O1—C1—C2 | 122.4 (5) | C13—C12—N3 | 117.8 (5) |
C26—C21—O21 | 118.4 (5) | C33—C32—C31 | 123.1 (5) |
C26—C21—C22 | 119.7 (5) | C33—C32—N23 | 116.4 (5) |
O21—C21—C22 | 122.0 (5) | C31—C32—N23 | 120.5 (4) |
C3—C2—C1 | 119.8 (5) | C14—C13—C12 | 118.2 (5) |
C3—C2—H2 | 122 (2) | C14—C13—H13 | 121 (2) |
C1—C2—H2 | 119 (2) | C12—C13—H13 | 121 (2) |
C23—C22—C21 | 119.9 (5) | C34—C33—C32 | 118.2 (5) |
C23—C22—H22 | 122.3 (18) | C34—C33—H33 | 123 (3) |
C21—C22—H22 | 117.7 (18) | C32—C33—H33 | 118 (3) |
C2—C3—C4 | 120.4 (4) | C13—C14—C15 | 120.2 (5) |
C2—C3—H3 | 120 (2) | C13—C14—H14 | 116 (2) |
C4—C3—H3 | 119 (2) | C15—C14—H14 | 123 (2) |
C22—C23—C24 | 120.4 (5) | C33—C34—C35 | 121.0 (5) |
C22—C23—H23 | 126 (2) | C33—C34—H34 | 120 (2) |
C24—C23—H23 | 114 (2) | C35—C34—H34 | 119 (2) |
C5—C4—C3 | 119.6 (4) | C14—C15—C16 | 121.0 (5) |
C5—C4—N1 | 115.5 (4) | C14—C15—H15 | 117 (2) |
C3—C4—N1 | 124.9 (4) | C16—C15—H15 | 122 (2) |
C23—C24—C25 | 120.1 (4) | C34—C35—C36 | 119.9 (5) |
C23—C24—N21 | 124.3 (4) | C34—C35—H35 | 124 (2) |
C25—C24—N21 | 115.6 (4) | C36—C35—H35 | 116 (2) |
C4—C5—C6 | 119.9 (5) | C11—C16—C15 | 119.8 (5) |
C4—C5—H5 | 119 (2) | C11—C16—H16 | 118 (2) |
C6—C5—H5 | 121 (2) | C15—C16—H16 | 121 (2) |
C26—C25—C24 | 118.8 (5) | C31—C36—C35 | 120.0 (5) |
C26—C25—H25 | 122 (2) | C31—C36—H36 | 120 (3) |
C24—C25—H25 | 119 (2) | C35—C36—H36 | 120 (3) |
| | | |
C4—N1—N2—C11 | 178.8 (4) | C16—C11—C12—C13 | −1.5 (8) |
C24—N21—N22—C31 | −179.6 (4) | N2—C11—C12—C13 | 174.9 (5) |
C6—C1—C2—C3 | −0.5 (7) | C16—C11—C12—N3 | 178.6 (4) |
O1—C1—C2—C3 | −179.1 (4) | N2—C11—C12—N3 | −4.9 (7) |
C26—C21—C22—C23 | −0.3 (7) | O3—N3—C12—C11 | 128.0 (5) |
O21—C21—C22—C23 | 179.8 (5) | O2—N3—C12—C11 | −51.3 (7) |
C1—C2—C3—C4 | 0.5 (7) | O3—N3—C12—C13 | −51.8 (7) |
C21—C22—C23—C24 | 0.7 (8) | O2—N3—C12—C13 | 128.9 (6) |
C2—C3—C4—C5 | −1.2 (7) | C36—C31—C32—C33 | −0.7 (7) |
C2—C3—C4—N1 | 178.5 (4) | N22—C31—C32—C33 | −175.3 (5) |
N2—N1—C4—C5 | 173.8 (4) | C36—C31—C32—N23 | 179.2 (4) |
N2—N1—C4—C3 | −5.9 (6) | N22—C31—C32—N23 | 4.5 (6) |
C22—C23—C24—C25 | 0.0 (7) | O23—N23—C32—C33 | 51.1 (7) |
C22—C23—C24—N21 | −178.1 (5) | O22—N23—C32—C33 | −130.0 (5) |
N22—N21—C24—C23 | 4.2 (6) | O23—N23—C32—C31 | −128.7 (5) |
N22—N21—C24—C25 | −174.0 (4) | O22—N23—C32—C31 | 50.1 (7) |
C3—C4—C5—C6 | 1.9 (7) | C11—C12—C13—C14 | 1.7 (8) |
N1—C4—C5—C6 | −177.9 (4) | N3—C12—C13—C14 | −178.5 (5) |
C23—C24—C25—C26 | −1.0 (7) | C31—C32—C33—C34 | −0.6 (8) |
N21—C24—C25—C26 | 177.2 (4) | N23—C32—C33—C34 | 179.6 (5) |
O1—C1—C6—C5 | 179.8 (4) | C12—C13—C14—C15 | −1.7 (9) |
C2—C1—C6—C5 | 1.1 (7) | C32—C33—C34—C35 | 1.4 (8) |
C4—C5—C6—C1 | −1.9 (7) | C13—C14—C15—C16 | 1.6 (9) |
O21—C21—C26—C25 | 179.2 (4) | C33—C34—C35—C36 | −1.0 (9) |
C22—C21—C26—C25 | −0.7 (7) | C12—C11—C16—C15 | 1.4 (7) |
C24—C25—C26—C21 | 1.4 (7) | N2—C11—C16—C15 | −174.9 (4) |
N1—N2—C11—C12 | 166.8 (4) | C14—C15—C16—C11 | −1.4 (8) |
N1—N2—C11—C16 | −17.0 (6) | C32—C31—C36—C35 | 1.1 (7) |
N21—N22—C31—C36 | 22.8 (6) | N22—C31—C36—C35 | 175.3 (5) |
N21—N22—C31—C32 | −162.9 (4) | C34—C35—C36—C31 | −0.4 (8) |
Experimental details
| (Ia) | (Ib) |
Crystal data |
Chemical formula | C12H9N3O3 | C12H9N3O3 |
Mr | 243.22 | 243.22 |
Crystal system, space group | Triclinic, P1 | Monoclinic, P21/n |
Temperature (K) | 293 | 293 |
a, b, c (Å) | 7.114 (2), 7.380 (2), 10.849 (4) | 3.823 (1), 23.014 (8), 25.437 (9) |
α, β, γ (°) | 96.68 (2), 97.49 (2), 100.54 (2) | 90, 92.73 (3), 90 |
V (Å3) | 549.4 (3) | 2235.5 (13) |
Z | 2 | 8 |
Radiation type | Mo Kα | Mo Kα |
µ (mm−1) | 0.11 | 0.11 |
Crystal size (mm) | 0.44 × 0.25 × 0.18 | 0.39 × 0.12 × 0.07 |
|
Data collection |
Diffractometer | Enraf-Nonius CAD-4 diffractometer | Enraf-Nonius CAD-4 diffractometer |
Absorption correction | – | – |
No. of measured, independent and observed [I > 2σ(I)] reflections | 2395, 2395, 1599 | 4574, 3954, 1544 |
Rint | 0.000 | 0.046 |
(sin θ/λ)max (Å−1) | 0.638 | 0.594 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.038, 0.101, 1.12 | 0.066, 0.124, 0.98 |
No. of reflections | 2395 | 3954 |
No. of parameters | 199 | 397 |
H-atom treatment | All H-atom parameters refined | All H-atom parameters refined |
Δρmax, Δρmin (e Å−3) | 0.13, −0.13 | 0.16, −0.15 |
Selected geometric parameters (Å, °) for (Ia) and (Ib). top | | (Ia) | (Ib) | |
| | | Molecule 1 | Molecule 2 |
C4-N1-N2-C11 | | 178.0 (1) | 178.8 (4) | -179.6 (4) |
O2···N2 | | 2.726 (2) | 2.788 (5) | 2.771 (5) |
Interplanar angles: | | | | |
C1-C6/C4,N1,N2,C11 | | 11.12 (9) | 6.7 (3) | 5.8 (3) |
C4,N1,N2,C11/C11-C16 | | 10.93 (9) | 15.7 (3) | 19.9 (3) |
C1-C6/C11-C16 | | 1.73 (9) | 22.0 (3) | 25.2 (3) |
N3,O2,O3/C11-C16 | | 38.44 (6) | 51.6 (2) | 51.0 (2) |
Hydrogen-bonding geometry (Å, °) for (Ia) and (Ib). top | D-H···A | | D-H | H···A | D···A | D-H···A |
(Ia) | O1-H1···O2i | | 0.96 (2) | 1.93 (2) | 2.879 (2) | 174 (2) |
(Ib) | O1-H1···O3ii | | 0.79 (5) | 2.23 (5) | 2.933 (5) | 148 (5) |
(Ib) | O21-H21···O1iii | | 0.79 (4) | 2.10 (4) | 2.866 (5) | 164 (4) |
Symmetry codes: (i) -x, -y, 1 - z; (ii) 2 - x, -y, 1 - z;
(iii) 3/2 - x, 1/2 + y, 1/2 - z. |
The first examples of differently coloured crystalline forms of organic compounds were reported 94 years ago (Hantzsch, 1907). This phenomenon was originally called chromoisomerism, but recently the more appropriate term crystallochromy has been introduced (Klebe et al., 1989). The current version of the Cambridge Structural Database (CSD; Allen & Kennard, 1993) contains more than 50 families of differently coloured polymorphs and pseudopolymorphs, but for only a few of them have the solid-state spectra been reported and the distinctions in colour rationalized.
These distinctions may be of various natures. Firstly, the molecules in the crystals may exist as different tautomeric forms or may adopt different conformations. In this case, the difference in colour can be interpreted at the level of the calculated electronic spectra of these tautomers or conformers. Secondly, the shifts of the absorption bands may arise from the perturbation of molecular orbitals (MOs) under the effect of the crystal environment. This effect is closely related in its nature to solvatochromism and can be modelled via incorporation of the external electrostatic potential into the Hamiltonian of a molecule (Csikós et al., 1999; Yatsenko & Paseshnichenko, 2000). Thirdly, the absorption bands may be shifted and split, due to the collective interactions within the crystals upon excitation. These effects can be considered at the level of the exciton-polariton approach (Philpott, 1971) and they are very large for strongly allowed transitions in some organic dyes, but are shadowed by other effects for moderately absorbing crystals. Fourthly, the bands corresponding to the intermolecular charge-transfer (CT) interaction can appear not only in the crystals with interlaced donor and acceptor molecules, but also in homomolecular crystals too (Sebastian et al., 1981). If the intermolecular π interactions are strong, the excitonic and CT states mix and should be considered together within the general theory (Hoffmann et al., 2000). In order to extend the understanding of these various factors, we have studied 4-[(2-nitrophenyl)azo]phenol, (I), and present its crystal structure here. \sch
As shown in Fig. 1, the electronic spectrum of (I) in CCl4 solution is typical of azophenols (Okawara et al., 1988). It contains a medium-strong absorption band in the near UV and a weak maximum in the visible region, attributed to the π → π* and the n → π* excitations, respectively. Fig. 1 indicates that the polymorphs (Ia) and (Ib) differ in colour due to the different intensity of the nπ* band in their solid-state reflection spectra, because the red and yellow hues of coloured materials arise from the absorbance at 20000 and above 23000 cm-1, respectively (Griffiths, 1976). The yellow and orange forms of 1,1'-dinitro-3,3'-azo-1,2,4-triazole (Cromer et al., 1988) provide another example of polymorphs whose colour arises from the n → π* excitation.
The bond lengths and angles in both structures are within the normally expected ranges. The molecular conformations in (Ia) and (Ib) differ: in (Ia), the two rings attached to the azo linkage are twisted to the same side and are thus essentially coplanar, whereas in (Ib), the rings are rotated opposite to each other (Table 1). The two independent molecules in (Ib) form pseudo-centrosymmetric pairs and adopt essentially the same conformation. The twist of the nitro group out of the plane of the phenyl ring results from the compromise between the N···O repulsion and the π delocalization. Overall, the molecule of (Ia) is slightly flattened with respect to (Ib).
The molecules in (Ia) form stacks along [010]. The shortest intermolecular C···C distances are 3.536 (3) Å and neighbouring molecules within the stack are related by inversion centres. In (Ib), neighbouring molecules are related by the [100] translation, and the shortest C···C distances within the stack are 3.478 (6) Å. The molecules are linked via hydrogen bonds, to form dimers in (Ia) and tetramers in (Ib) (Fig. 4 and Table 2), and the two independent molecules in (Ib) are non-equivalent in the hydrogen-bonding pattern.
In order to compare the molecular electronic structures in (Ia) and (Ib), and to determine the effect of crystal packing, we have carried out semi-empirical calculations at the AM1 level (Dewar et al., 1985), with the experimental molecular geometries used as input for the calculations. The molecular dipole moment in (I) is determined by the contributions of the nitro group and the hydroxy oxygen lone pair, and it is nearly perpendicular to the long axis of the molecule. Intermolecular interactions therefore cause only a subtle increase (0.03–0.05 e) in the intramolecular CT from the phenolic moiety to the nitro-substituted ring.
Under the effect of the crystal electrostatic potential, the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) decreases from 7.79 to 7.61 eV in (Ia) and from 7.91 to 7.58 eV in (Ib), but in (Ib) LUMO and HOMO are localized on molecules 1 and 2, respectively. Such non-equivalence is caused by the hydrogen bonding: since the orbitals of the hydroxy O atom make a significant contribution to HOMO, this MO can be stabilized or destabilized by hydrogen bonds formed via the H or O atom of this group, respectively. Had the hydrogen bonds been formed via the azo N atoms, as in the structure of 4-(phenyldiazenyl)naphthalen-1-amine (Yatsenko et al., 2001), their impact on the molecular electronic structure would have been much more pronounced, because the azo linkage makes a much larger contribution to LUMO than the nitro group.
The intensity of the n → π* transition increases (as what is varied?) in the spectra of aromatic azo compounds, due to the interaction between the azo nitrogen lone pairs and the aromatic π systems, which is assisted by the deviation of a molecule from planarity (Griffiths, 1976). We have studied the effect of the conformation? of (I) on the electronic spectra at the INDO-CISD level. In its original parameterization (Dick & Nickel, 1983), this method strongly underestimates the energy and intensity of the n → π* transition. For (I), the calculations predict a gap of 14000 cm-1 between the nπ* and ππ* excited states, whereas the experimentally determined value is about 6000 cm-1. Similarly, the observed oscillator strength estimated according to Guillaumont & Nakamura (2000) is 0.018, versus 0.0009 obtained from the calculations (units?). The structures of the molecular orbitals calculated using INDO-CISD differ from those obtained in the ab initio calculations. Since INDO overestimates the energy of the MO, which is mainly composed of the nitrogen `lone-pair' atomic orbitals, by ca 1.4 eV, this orbital is HOMO-1 in the INDO calculations, but only HOMO-4 according to the ab initio results. Following Ridley & Zerner (1973), we have introduced into the INDO-CISD scheme an additional empirical parameter fσ (equal to 1.2) for scaling up the σ-σ overlap integrals. Besides this, we have increased both core integrals for the N atom by 9%. With these modifications, the INDO-calculated MOs match quite well with those obtained in the ab initio calculations, and the calculated nπ*-ππ* gap and the oscillator strength for the n → π* transition are in much better agreement with the experimental data.
Calculations on isolated molecules showed that the difference in molecular geometry between (Ia) and (Ib) is not enough to explain the observed spectral distinctions of these polymorphs. The n → π* oscillator strength in (Ia) was even calculated to be slightly larger than in (Ib) [0.007 in (Ia) versus 0.004 and 0.006 in the two molecules of (Ib)]. With the crystal electrostatic potential applied, INDO-CISD predicts a bathochromic shift of 700–800 cm-1 on the π → π* transition and a hypsochromic shift of 200–300 cm-1 on the n → π* transition, accompanied by a 30% increase in the oscillator strength for both (Ia) and (Ib). As with the π → π* transition, these results are in line with the observed effects: the positions of the corresponding absorption maxima in (Ia) and (Ib) are red-shifted by 900–1000 cm-1 with respect to the spectrum of a CCl4 solution of (I). However, neither the difference in molecular geometry nor the intermolecular electrostatic interactions allow explanation of the 1500 cm-1 red shift of the nπ* band on transfer from solution to solid (Ia) and (Ib), or of the increase in intensity of this band in (Ib) with respect to (Ia). Qualitatively, these effects can be explained by the mixing of the nπ* excited state with intermolecular CT excitations. The INDO-CISD calculations on the inversion- and translation-related molecular dimers modelling the stacking in (Ia) and (Ib) partially confirm this supposition: in translation-related dimers, the n → π* excitation is accompanied by some intermolecular CT (0.0003–0.0006 e) and the oscillator strength for this transition increases by 50% with respect to the isolated molecule, whereas for inversion-related dimers, the oscillator strength for this excitation decreases twice (? to half?). However, these calculations do not explain the observed red shift of the nπ* absorption band, probably due to the fact that ZDO-based methods (define ZDO) are not very well suited for reproducing intermolecular interactions.