G. Wójcik

Photoluminescence and quantum chemical studies of electronic and optical properties of m-nitroaniline and m-nitrophenol crystals

Fluorescence, phosphorescence and their excitation spectra of single crystals of m-nitroaniline (m-NA) and of m-nitrophenol (m-NPh) were measured at 5 K. m-NA and m-NPh were also studied in an n-hexane Shpolskii matrix. Phosphorescence and phosphorescence decays were also collected at higher temperatures. On the basis of band position analysis and quantum chemical calculations the phosphorescence is assigned to the molecular emission whereas the fluorescence is supposed to originate from crystal defects—radical ions. Radical ions are photogenerated in both materials and in m-NA by crystal freezing below the glassy phase transition. Computed values of the first order hyperpolarizability, βvec, are larger for radical ions than for neutral molecules which suggests that radical ions are intermediates in the molecular mechanism of optical nonlinearity generation.

1H-NMR, dielectric and calorimetric studies of molecular motions in m-nitroaniline crystal

Abstract Spin-lattice relaxation time, T 1 , spin-lattice relaxation time in the rotating frame, T 1 ρ , and the second moment of the resonance line measurements at 80 MHz and over the 190–380 K temperature range are reported for protons in the optically nonlinear material m -nitroaniline ( m -NA). T 1 has also been measured for samples irradiated by low energy and low intensity radiation. The real and imaginary parts of electric permittivity as well as tan  δ have been recorded in the 80–380 K temperature range at frequencies ranging from 100 Hz to 1.0 MHz. DSC measurements have been performed in the 110–387 K temperature range. Two phase transitions have been found: A glassy to rotative transition at 160 K and a plastic to plastic transformation at 365 K. The main feature of the m -NA crystal is that its plasticity continues to grow as the temperature increases. The reorientations of phenyl rings, the –NH 2 group proton 180° jumps, the lattice distortions caused by anisotropic thermal expansion and the cooperative reorientations of big molecular aggregates are thought to be the reasons for phase transitions and for the subsequent intermolecular charge transfer. The results are discussed with respect to optical second harmonic generation and near-IR photochemical reaction found in the m -NA crystal.

Polymorphs of p-nitrophenol as studied by variable-temperature X-ray diffraction and calorimetry: comparison with m-nitrophenol

Crystal structures of two polymorphic forms of p-nitrophenol have been determined at several temperatures between 120 and 375 K. The thermal expansion tensor has been determined for both polymorphs. The rigid-body mean-square amplitudes of molecular translations and librations and the amplitudes of the internal torsions of the nitro group have been calculated at different temperatures. Differential scanning calorimetry was used to find the temperature and enthalpy of the polymorphic transformation. The results were compared with those recently obtained for m-nitrophenol polymorphs. Some conclusions concerning the polymorphism of p- and m-nitrophenols are presented.

Variable-temperature crystal structure studies of m-­nitroaniline

The crystal structure of m-nitroaniline has been examined at several temperatures over the 90–350 K range. Thermal evolution of the lattice parameters reveals a weak anomaly at 110 K and an important one at 300 K. The thermal expansion coefficients have been calculated at several temperatures and the principal axes cross-sections of the tensor were drawn. The lattice contraction along the b axis direction has been observed. The rigid-body analysis including an attached rigid group has provided the values of the translation and libration tensors at temperatures studied. The results indicate that m-nitroaniline undergoes a glass transition around 130 K arising from freezing molecular librations and translations. From above 340 K the growing plasticity of the m-nitroaniline crystal results in the loss of X-ray diffraction reflections. This is probably a second-order phase transition. It is coupled with a considerable increase in the nitro group torsion amplitude, but the NH⋯O hydrogen bonds are preserved. Analysis of the temperature evolution of short intermolecular distances enabled us to consider the occurrence of reorienting aggregates of hydrogen-bonded molecules in the high-temperature plastic phase.

Stereochemistry of terpene derivatives. Part 2: Synthesis of new chiral amino acids with potential neuroactivity

Abstract The syntheses and stereochemistry of two new amino acids obtained from the monoterpene ketones (−)- cis -caran- trans -4-one and (−)-menthone via appropriate lactams are presented. The configuration of all stereogenic centers is confirmed by X-ray crystallography.

The Phase Transition of m-Nitrophenol

Abstract The monocrystals of two polymorphic phases of m-nitrophenol were studied by the DSC method, and near IR (NIR) and far IR (FIR) spectroscopy. We found the phase transition of the first order 14 degrees below the melting point. The temperatures, enthalpies and entropies of the phase transition and the melting are presented. The polarized NIR spectra in the region of the overtones of the OH…O stretching vibrations for the two phases are presented. The polarized FIR spectra of the two phases in the room and liquid nitrogen temperature from 0 to 200 cm−1 are presented.

Electron-density study of m-nitrophenol in the orthorhombic structure

m-Nitrophenol (m-NPH) occurs at room temperature in two solid modifications : monoclinic P2 1 /n and orthorhombic P2 1 2 1 2 1 [Pandarese, Ungaretti & Coda (1975). Acta Cryst. B31, 2671-2675]. The thermal vibrations and electronic-density distribution of the molecule in the orthorhombic structure have been analyzed in terms of Stewarts rigid pseudoatom model, using restricted Slater radial functions and angular multipole terms extending to actapoles for C, N and O and dipoles for H pseudoatoms. The net atomic charge and the in-crystal molecular dipole moment have been determined in order to understand the nature of the inter- and intramolecular charge transfer. The analysis suggests that aspherical pseudoatoms are essential for modeling the charge distribution in a noncentrosymmetric crystal. Careful consideration must also be given to the treatment of the H atoms, in the absence of complementary neutron diffraction data.

ESR studies of near infrared induced unpaired spins in two polymorphs of meta-nitrophenol

Abstract The ESR spectra in the X-band region of two polymorphs of m-nitrophenol: the noncentrosymmetrical and optically nonlinear orthorhombic form and the monoclinic one were measured after the previous irradiation with the near IR beams corresponding to the overtones of the OH and CH stretching vibrations and the following heating to about 340K. The evolution of the strong signals as a function of time and temperature suggests that photochemical reactions occur in the studied crystals. The possible interpretation of the ESR signals is presented.

The Inter- and Intramolecular Charge Transfer Along the Polymeric Chain of Hydrogen-Bonded Molecules in Two Crystal Forms of m-Nitrophenol

Abstract The crystal structure of the orthorhombic polymorph of m-nitrophenol (mNPh) is presented and compared with the known structure of the monoclinic one. The inter- and intramolecular interactions previously studied by means of vibrational spectros-copy (IR, Raman) are discussed in terms of the polymeric character of both structures, in which the charge transfer interactions within the molecular chains contrast with the Van der Waals interactions between the chains. The structural instability of the ortho-rhombic phase is also discussed in terms of different charge transfer along the polymeric chains in the two structures and the considerable anharmonism of the interchain potential in the ortho-rhombic crystal.

Thermally induced rearrangement of hydrogen-bonded helices in solid 4-isopropylphenol as studied by calorimetric, proton NMR, dielectric and near IR spectroscopic methods

Abstract Calorimetric, dielectric and Fourier transform near infrared (IR) spectroscopic methods were used to study molecular dynamics and structural transition in solid 4-isopropylphenol (4IP) above room temperature. Pulse proton nuclear magnetic resonance (NMR) measurements were performed in the 100–340 K temperature range. A phase transition was found at 331.5 K, 1.5 K below the melting point. Energetically inequivalent methyl groups reorientations were observed in differently prepared samples and this suggested that a high-temperature polymorph occurs below the transition point as a metastable phase. Dielectric relaxation measurements showed an electric conductivity similar in value to that in water. This was detected as a pronounced contribution to the imaginary part of dielectric permittivity at temperatures higher than 310 K. Near IR spectra revealed that hydrogen bondings are stronger in the high-temperature phase than in the room-temperature-stable one. We propose that thermally induced molecular rearrangements enable proton transfer in hydrogen bonds (HBs) and this stimulates protonic conduction.