Antol Ivana, Glasovac Zoran, Crespo-Otero Rachel, Barbatti Mario
Division of Organic Chemistry and Biochemistry, Ruđer Bošković Institute, P.O. Box 180, HR-10002 Zagreb, Croatia.
Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany.
J Chem Phys. 2014 Aug 21;141(7):074307. doi: 10.1063/1.4892569.
Diverse ab initio and density-functional-theory methods were used to investigate geometries, energies, and electronic absorption spectra of guanidine and its protonated form, as well as their photo-deactivation processes. It was shown that the guanidine is a weakly absorbing species with the excitation spectrum consisting mostly of transitions to the Rydberg excited states and one valence n-π4 state. The lowest energy band has a maximum at ca. 6.9 eV (∼180 nm). The protonation of guanidine affects its excitation spectrum substantially. A major shift of the Rydberg states to higher energies is clearly visible and strongly absorbing transitions from the ground state to the π3-π4 and π2-π4 states appears at 7.8 eV (∼160 nm). Three low-lying conical intersections (two for guanidine and one for protonated guanidine) between the ground state and the first excited singlet state were located. They are accessible from the Franck-Condon region through amino N-H stretching and out-of-plane deformations in guanidine and protonated guanidine, respectively. The relaxation of the π3-3s Rydberg state via amino N-H bond stretching was hindered by a barrier. The nondissociated conical intersection in protonated guanidine mediates the radiationless deactivation of the compound after excitation into the π3-π4 state. This fact is detrimental for the photostability of guanidine, since its conjugate acid is stable in aqueous solution over a wide pH range and in protein environment, where guanidinium moiety in arginine is expected to be in a protonated form.
采用多种从头算和密度泛函理论方法研究了胍及其质子化形式的几何结构、能量和电子吸收光谱，以及它们的光失活过程。结果表明，胍是一种弱吸收物种，其激发光谱主要由向里德堡激发态和一个价态n-π4态的跃迁组成。最低能带的最大值约为6.9 eV（180 nm）。胍的质子化对其激发光谱有显著影响。里德堡态向更高能量的明显位移清晰可见，并且从基态到π3-π4和π2-π4态的强吸收跃迁出现在7.8 eV（160 nm）处。在基态和第一激发单重态之间找到了三个低位锥形交叉点（胍有两个，质子化胍有一个）。它们分别通过胍和质子化胍中的氨基N-H伸缩和平面外变形从弗兰克-康登区域可达。通过氨基N-H键伸缩对π3-3s里德堡态的弛豫受到一个势垒的阻碍。质子化胍中未离解的锥形交叉点介导了化合物在激发到π3-π4态后的无辐射失活。这一事实对胍的光稳定性不利，因为其共轭酸在很宽的pH范围内的水溶液和蛋白质环境中都是稳定的，在蛋白质环境中，精氨酸中的胍基部分预计处于质子化形式。
