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DFT calculations vs. experiment for UV/vis spectroscopy: aryl guanidines and aryl guanidinium salts

The hydrogen bond is a ubiquitous phenomenon in nature which is of great importance in functioning of living organisms and design of technologically interesting materials. Recently, an overview of a new discoveries in this research field has been published encompassing usage of hydrogen bonds from an anion binding to a construction of large polymeric structures. To achieve an efficient binding of the anions, a various proton donating functional groups were used of which an guanidine functionality is especially interesting due to its acid/base properties[2]. Namely, specific Y-shaped structure of its protonated form allows efficient delocalization of the electrons which results in the high stability of its protonated form and therefore its unusually high basicity. It can be easily protonated and in this form, anion binding is assisted by coulombic charge-charge attractive forces, and therefore, we can expect that guanidines bind anions significantly stronger than akin urea, thioureas or amides. Following our interest in the guanidine chemistry in the ground and excited states, the UV/vis spectra of different guanidines (phenyl, napthyl, antracenyl, quinolinyl, antraquinonyl and cumarinyl) in acetonitrile have been simulated by TD-DFT calculations using the hybrid long range corrected CAM-B3LYP functional. Solvent effects were added by using polarizabile continuum model (PCM). Special emphasize was put to the effect of protonation on the excitation energies and absorption intensities. Also, the effect of complexation and specific interactions through hydrogen bonds with different anions (F-, NO3-, CF3SO3-, CH3COO- and HCOO-) were examined. It was shown that the protonation of guanidine subunit shifts the low energy absorption bands toward higher energies (hypsochromic shift). The shift is reduced upon complexation with anions. In the phenylguanidine, lambda_max of salts are clearly in a correlation to the anion basicity and strength of H-bonding. Observed changes are decreased upon increase of chromophoric size. To check the correctness of theoretical predictions, UV/vis spectra of available guanidine derivatives and their salts were measured and compared with the calculated data.

time dependent density functional theory; guanidinium functional group; UV/vis spectroscopy; protonation; anion complexation

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