ABSTRACT
The supposed importance of hydrogen bonds toward the origin of second harmonic generation (SHG) and electric conductivity in crystalline sucrose was investigated by IR (4000-10cm(-1)), INS (2000-10cm(-1), at 35K), polarized Raman (3600-50cm(-1)) spectra, and (1)H NMR second moment line records in the temperature range 450-80K. The temperature dependence of NIR (7000-5500cm(-1)) polarized spectra gave information about -CH2 motions complementary to NMR results concerning -CH2OH group rearrangements. The EPR spectra were applied to study the generation of radical ions by exposure to NIR radiation. Density functional theory quantum chemical calculations were performed to reproduce the vibrational spectra in order to complete as far as possible the assignments of bands observed by us and in the literature in sucrose crystals, and to throw more light on the possible reasons of sucrose electric conductivity and optical nonlinearity by the knowledge of theoretical values of dipole moments, polarizabilities, first order hyperpolarizabilities of sucrose molecule and clusters as well as ionization energy and electron affinity. The proton transfer in one specific hydrogen bond parallel to the helical axis b is proposed to be the most important in SHG and conductivity origin.
Subject(s)
Protons , Sucrose/chemistry , Crystallization , Electric Conductivity , Hydrogen Bonding , Quantum Theory , Spectrum Analysis/methods , TemperatureABSTRACT
FT-IR and Raman spectra of the orthorhombic and monoclinic N-benzyl-2-methyl-4-nitroaniline (BNA) single crystals, powders, and BNA solutions were measured in the 15-4000 cm(-1) range, and complete assignments of bands to normal vibrations are proposed. The assignments have been reinforced by density functional theory (DFT) calculations. Polarized FT-NIR spectra of the orthorhombic (010) BNA plate were measured for the overtones and combinations analysis and for the mechanical and electrical anharmonicity estimation. Resonance Raman spectra of the orthorhombic BNA polymorph together with the resonance excitation profiles of fundamental and lattice vibrations, compared with the band maxima in the measured UV-vis-NIR spectra of BNA, have revealed vibronic couplings with intramolecular and intermolecular charge transfers. The role of anharmonicity, vibronic couplings and intermolecular hydrogen bonds as well as of the N-benzyl substituent are discussed and the origin of second harmonic generation in the orthorhombic BNA crystal is proposed.
ABSTRACT
The Raman excitation profiles of solid 2-methyl-4-nitroaniline (MNA) reveal several band enhancements by intermolecular and intramolecular charge transfer states. Calculated excited- and ground-state molecular geometries and excited state distortions qualitatively determined from Raman spectra indicate multiple vibrations leading to MNA dissociation. Also, overtones and combination tones can generate charged polarons, as detected by electron paramagnetic resonance after the exposure to 980 and 1550 nm laser diodes. The MNA space group Ia (C(s)(4)) is nonchiral; however, the electronic circular dichroism (CD) spectra of solution, KBr pellet, and single crystal were recorded. The crystal chirality is elucidated by room-temperature dynamic disorder, possible helical superstructure along the [102] polar axis, and charged polarons presence. The CD spectra ab initio calculations for MNA neutral and negatively charged monomers, dimers, and trimers, lying along the helix, confirmed the chirality. The role of these findings toward efficient optical nonlinearity and electric conductivity failure is discussed.
ABSTRACT
Photoemission spectra of para-nitroaniline (p-NA) and ortho-methyl-para-nitroaniline (MNA) single crystals measured between 5 and 250K revealed that the MNA crystals emit phosphorescence and fluorescence in contrast with the p-NA crystals which have only fluorescence. It is assumed that the fluorescence of p-NA crystals at 5K originates from the doublet state of negatively charged polarons while the fluorescence of MNA crystals at 60K is due to trap states. The Stokes shift between absorption and emission onsets, measured at 5K, is much larger in the p-NA crystals than in the MNA and meta-nitroaniline (m-NA) crystals. This fact is rationalized by different geometrical changes caused by excitation.
