Structural and vibrational analysis of 3-methyl-4-nitropyridine-N-oxide: neutron and X-ray diffraction studies

3-Nitro-benzoic acid-3-methyl-4-nitro-pyridine 1-oxide (1/1)

The title adduct, C₇H₅NO₄·C₆H₆N₂O₃, forms part of an ongoing study of the design of non-centrosymmetric systems based on 3-methy-4-nitro­pyridine 1-oxide. The components of the adduct are linked by inter­molecular O—H⋯O hydrogen bonds. The rings of the two components are nearly planar, with a dihedral angle of 11.9 (2)° between the planes. The supra­molecular structure shows that mol­ecules of the title complex are linked into sheets by a combination of strong O—H⋯O and weak C—H⋯O hydrogen bonds.

Dipole Moment Enhancement in Molecular Crystals from X-ray Diffraction Data

Although reliable determination of the molecular dipole moment from experimental charge density analyses on molecular crystals is a challenging undertaking, these values are becoming increasingly common experimental results. We collate all known experimental determinations and use this database to identify broad trends in the dipole moment enhancements implied by these measurements as well as outliers for which enhancements are pronounced. Compelling evidence emerges that molecular dipole moments from X-ray diffraction data can provide a wealth of information on the change in the molecular charge distribution that results from crystal formation. Most importantly, these experiments are unrivalled in their potential to provide this information in such detail and deserve to be exploited to a much greater extent. The considerable number of experimental determinations now available has enabled us to pinpoint those studies that merit further attention, either because they point unequivocally to a considerable enhancement in the crystal (of 50 % or more), or because the experimental determinations suggest enhancements of 100 % or more--much larger than independent theoretical estimates. In both cases further detailed experimental and theoretical studies are indicated.

Theoretical investigation of the linear and second-order nonlinear susceptibilities of the 3-methyl-4-nitropyridine-1-oxyde (POM) crystal

The linear and nonlinear optical properties of the 3-methyl-4-nitropyridine-1-oxyde (POM) crystal have been evaluated using semiempirical quantum chemistry techniques. The scheme includes (i) the evaluation of the polarizability and first hyperpolarizability of increasingly large one-dimensional, two-dimensional, and three-dimensional clusters of POM, (ii) the use of the time-dependent Hartree-Fock approach to determine the static and dynamic responses in combination with semiempirical Austin model 1 Hamiltonian, (iii) the assessment, for the POM monomer and dimer, of the electron correlation effects using second-order Moller-Plesset perturbation theory with several basis sets containing polarization and diffuse functions, (iv) the assessment of the validity of the multiplicative scheme and its application to get effective polarizability and first hyperpolarizability of the POM unit cell in the crystal, (v) the use of a sum-over-states approach to attribute the first hyperpolarizability to a dominant charge-transfer excited state, and (vi) comparison with experimental data as well as with calculated values obtained using the oriented gas approximation.

Crystal structure and spectral considerations of 5-methyl-2-nitraminopyridine N-oxide

The crystal structure of 5-methyl-2-nitraminopyridine N-oxide (5M) has been determined and its electronic absorption spectra in the solution have been investigated. In the solid, two monomeric proton-transferred (PT) units of 5M are perpendicularly located in the crystal lattice. They are connected by two intermolecular hydrogen bonds of different strength (O-H···N 2.647(3) and 2.695(3) Å). In the gas phase the same relative arrangement of both 5M (PT) monomers has been found. As results from semiempirical PM3-CI calculations (12–14), the prototropic amino (H) [Formula: see text] imino (PT) equilibrium may occur in the solution. Assignment of experimental absorption bands to the calculated transition energies of both forms of 5M reveals that hydrogen bonded 5M (PT) dimers are dominant in cyclohexane while imino (PT) monomers seem to prevail in acetonitrile solution. The different character of electronic transitions and different relative arrangement of both monomers in amino (H) than in imino (PT) dimers are responsible for very low oscillator strengths and dipole moments of the former.Key words: crystal structure, methylated nitraminopyridine N-oxides, semiempirical PM3-CI computations, intermolecular hydrogen bonding.