We apply many criteria to estimate the diradical character of the ground state singlets of several oxyallyl derivatives. This is carried out as the oxyallyl derivatives like squaraine and croconate dyes can be represented by both mesoionic and diradical formulas, the domination of which would characterize its lowest energy transition. One criterion applied is the singlet-triplet gap, which is known to be inversely proportional to the diradical character. Another criterion is the occupation number; this is determined for the symmetry broken state of the molecules in the unrestricted formalism, and the difference of occupation in the HOMO and LUMO is related to the diradical character. The diradical character of all of the croconates and few squaraines is estimated to be large. All of these have absorption above 750 nm and can be classified as near infrared (NIR) dyes, leading to the inference that NIR absorptions in these molecules are largely due to the dominance of the diradical character. To understand the reliability of the DFT methods for the absorption property predictions of these molecules, TD-DFT studies to calculate the vertical excitation energies have been carried out, using the B3LYP/ BLYP exchange correlation functionals and the LB94 asymptotic functional with and without the inclusion of solvent. The deviations, in both the squaraine series (average lower diradical character), are found to be systematic, and with the inclusion of the solvent in the calculation, the deviations decrease. The best least-squares fit with the experimentally observed values using B3LYP /6-311G(d, p) for the symmetric squaraines yields an R value of 0.92 and, for the unsymmetric squaraines, an R value of 0.936. With inclusion of the solvent, the R value is 0.96 for the symmetric squaraines and 0.961 for the unsymmetric squaraines, indicating that these DFT functionals with linear scaling may be used to study these systems. The croconate dyes, however, have larger deviation from the experimentally observed values in all of the functionals studied even after inclusion of the solvent effects. The deviations are also not systematic. The deviation with respect to the experiment in this case is attributed to the average larger diradical character in this series.

L-Tyrosine and iodinated L-tyrosines, i.e., 3-iodo-L-tyrosine and 3,5-diiodo-L-tyrosine, are successfully used as chiral references for the chiral discrimination of aliphatic, acidic, and aromatic amino acids. Chiral discrimination is achieved by investigating the collision-induced dissociation spectra of the trimeric complex [Cu(II)(ref)(2)(A) - H](+) ion generated by electro spraying the mixture of D- or L-analyte amino acid (A), chiral reference ligand (ref) and M(II)Cl(2) (M = Ni and Cu). The relative abundances of fragment ions resulted by the competitive loss of reference and analyte amino acids are considered for measuring the degree of chiral discrimination by applying the kinetic method. The chiral discrimination ability increases as the number of iodine atom increases on the aromatic ring of the reference and the discrimination is better with Cu when compared with Ni. A large chiral discrimination is obtained for aliphatic and aromatic amino acids using iodinated L-tyrosine as the reference. Computational studies on the different stabilities of the diastereomeric complexes also support the observed differences measured by the kinetic method. The suitability of the method in the measurement of enantiomeric excess over the range of 2% to 100% ee with relative error 0.28% to 1.6% is also demonstrated.

We determined the gas-phase acidities (⌬H acid ) of four deoxyribonucleosides, i.e., 2=-deoxyadenosine (dA), 2=-deoxyguanosine (dG), 2=-deoxycytidine (dC), and 2=-deoxythymidine (dT) by applying the extended kinetic method. The negatively charged proton-bound hetero-dimeric anions, [A Ϫ H Ϫ B]Ϫ of the deoxyribonucleosides (A) and reference compounds (B) were generated under electrospray ionization conditions. Collision-induced dissociation spectra ofϪ were recorded at four different collision energies using a triple quadrupole mass spectrometer. The abundance ratios of the individual monomeric product ions were used to determine the ⌬H acid of the deoxyribonucleosides. The obtained ⌬H acid value follows the order dA Ͼ dC Ͼ dT Ͼ dG. The ⌬G acid (298 K) values were determined by using ⌬G acid ϭ ⌬H acid -T⌬S acid where the ⌬H acid and ⌬S acid values were determined directly from the kinetic method plots. The ⌬H acid values were also predicted for the deoxyribonucleosides at the B3LYP/6-311ϩG**//B3LYP/6-311G** level of theory. The acidity trend obtained from the computational investigation shows good agreement with that obtained experimentally by the extended kinetic method. Theoretical calculations provided the most preferred deprotonation site as C5=-OH from sugar moiety in case of dA, and as ϪNH 2 (dC and dG) or ϪNH- . The determination of thermochemical properties of these molecules in the gas phase could be of importance for biological reasons because biological environment can be relatively nonpolar in nature. Moreover, the gasphase studies explore the reactivity of molecules and ions without solvent effects. To date, mass spectral studies have been focused towards analysis and structural characterization of deoxyribonucleosides [2][3][4][5][6][7][8][9][10]. While thermochemical properties of individual nucleobases have been well studied experimentally in the gas phase, the studies on nucleosides are limited [11][12][13][14][15][16].Xia et al.[11] performed extensive calculations on proton affinities (PAs) of nucleosides by the density functional approach. The PA of the four deoxyribonucleosides, i.e., 2=-deoxyadenosine (dA), 2=-deoxyguanosine (dG), 2=-deoxycytidine (dC), and 2=-deoxythymidine (dT) were reported to be in the order of dG Ͼ dC Ͼ dA Ͼ dT, and this order is similar to the PA order of the free bases (G Ͼ C Ͼ A Ͼ T). They concluded the protonation features of deoxyribonucleosides were less changed compared with the corresponding free bases. Donna et al. [12] determined the PA of dA and demonstrated the effective use of the kinetic method for the thermochemical measurement of multifunctional molecules like nucleosides and nucleobases. Besides the wide array of proton affinity studies on deoxyribonucleosides [13][14][15][16], there have been no experimental or theoretical reports on the gas-phase acidities (⌬H acid ) of deoxyribonucleosides. In this study, we report the first determination of ⌬H acid of dA, dG, dC, and dT through application of the extended kinetic method. We have also calculated the ...

A series of fluorenylethynylpyrene derivatives with large two-photon absorption cross-sections (≈250–2500 GM) and good fluorescence quantum yields (Φfl = 0.55–0.98) and with high two-photon brightness were synthesized.