RESUMEN
Quinacridone, a red pigment, is prone to aggregation, which results in undesirable color changes. Cellulose nanofibers (NFs) have been reported to adsorb quinacridone and suppress its aggregation. In this study, we investigated the potential of chitin and chitosan NFs which possess acetoamide and amino groups, as a quinacridone dispersant. Chitosan NFs, obtained by fibrillation using high-pressure homogenizer, adsorbed more quinacridone than cellulose NFs. SEM observations showed that chitosan NFs inhibited the aggregation of quinacridone, but chitin NFs did not. NMR analysis suggested the hydrogen bonding between chitosan NFs and quinacridone induced by the amino groups. The results indicated that the amino groups more facilitated the intermolecular interactions between NFs and quinacridone than the hydroxyl groups whereas the acetamide groups hindered them. Color measurements showed that the redness of quinacridone improved when cellulose or chitosan NFs were added. Chitosan NFs were found to be a novel candidate for quinacridone dispersants.
RESUMEN
The structure of the calix[4]arene(C4A)-Ar(n) complexes has been investigated by laser induced fluorescence spectroscopy, mass-selected resonant two-color two-photon ionization (2C-R2PI) spectroscopy, fragment detected IR photodissociation (FDIRPD) spectroscopy, and high level first principles electronic structure calculations at the MP2 and CCSD(T) levels of theory. C4A has a very high ability to form van der Waals complexes with rare gas atoms. For the C4A-Ar dimer two isomers are observed. A major species shows a 45 cm(-1) red-shift of its band origin with respect to the monomer, while that of a minor species is 60 cm(-1). The binding energy of the major species is determined to be in the range of 350-2250 cm(-1) from 2C-R2PI spectroscopy and FDIRPD spectroscopy. Two isomers are also identified in the quantum chemical calculations, depending on whether the Ar atom resides inside (endo) or outside (exo) the C4A. We propose a scheme to derive CCSD(T)/Complete Basis Set (CBS) quality binding energies for the C4A-Ar complex based on CCSD(T) calculations with smaller basis sets and the ratio of CCSD(T)/MP2 energies for the smaller model systems benzene-Ar and phenol-Ar, for which the CCSD(T) level of theory converges to the experimentally determined binding energies. Our best computed estimates for the binding energies of the C4A-Ar endo- and endo-complexes at the CCSD(T)/CBS level of theory are 1560 cm(-1) and 510 cm(-1), respectively. For the C4A-Ar(2) trimer the calculations support the existence of two nearly isoenergetic isomers: one is the {2 : 0} endo-complex, in which the Ar(2) dimer is encapsulated inside the C4A cavity, and the other is the {1 : 1} endo-exo-complex, in which one Ar resides inside and the other outside the C4A cavity. However, the experimental evidence strongly suggests that the observed species is the {2 : 0} endo-complex. The endo structural motif is also suggested for the larger C4A-Ar(n) complexes because of the observed systematic red-shifts of the complexes with the number of bound Ar atoms suggesting that the Ar(n) complex is encapsulated inside the C4A cavity. The formation of the endo-complex structures is attributed to the anisotropy of the interaction with C4A during the complex formation in the expansion region.
RESUMEN
The structure of the calix[4]arene(C4A)-(H(2)O) cluster formed in a supersonic beam has been investigated by mass-selected resonant two-photon ionization (R2PI) spectroscopy, IR-UV double resonance spectroscopy, IR photodissociation (IRPD) spectroscopy and by high-level quantum chemical calculations. The IR-UV double resonance spectrum of C4A-(H(2)O) exhibits a broad and strong hydrogen-bonded OH stretching band at 3160 cm(-1) and a weak asymmetric OH stretching band at 3700 cm(-1). The IRPD measurement of the cluster produced a value of 3140 cm(-1) for the C4A-(H(2)O) --> C4A + H(2)O dissociation energy. High-level electronic structure calculations at the MP2 level of theory with basis sets up to quadruple-zeta quality suggest that the endo-isomer (water inside the C4A cavity) is approximately 1100 cm(-1) more stable than the exo-isomer (water hydrogen bonded to the rim of C4A). The endo-isomer has a best-computed (at the MP2/aug-cc-pVQZ level) value of 3127 cm(-1) for the binding energy, just approximately 15 cm(-1) shy of the experimentally determined threshold and an IR spectrum in excellent agreement with the experimentally observed one. In contrast, the B3LYP density functional fails to even predict a stable structure for the endo-isomer demonstrating the inability of that level of theory to describe the delicate balance between structures exhibiting cumulative OH-pi H-bonding and dipole-dipole interactions (endo-isomer) when compared to the ones emanating from maximizing the cooperative effects associated with the formation of hydrogen bonded homodromic networks (exo-isomer). The comparison of the experimental results with the ones from high-level electronic structure calculations therefore unambiguously assign the endo-isomer as the global minimum of the C4A-(H(2)O) cluster, world's smallest cup of water.