Resonance raman studies of guanidinium and substituted guanidinium ions

Resonance Raman spectra of aqueous solutions of guanidinium, methylguanidinium, 1,1-dimethylguanidinium, ethylguanidinium, and L-arginine salts are reported. The results are interpreted in terms of excited electronic state symmetries and geometry changes with the help of semiempirical INDO/S calculations. The low-lying E′(ππ*) excited state of guanidinium exhibits a linear Jahn-Teller distortion along the e′ CN₃ stretching nuclear coordinate. The resonance Raman spectra also indicate a significant force constant change, possibly even a double minimum potential, along the CN₃ umbrella coordinate in the excited electronic state. The π → π* transitions of guanidinium and of all of the substituted guanidinium ions involve charge transfer between the nitrogen atoms and the central carbon atom. In the ground state, the positive charge is shared by the carbon atom and all three of the nitrogen atoms. In the excited electronic state, the carbon atom has a net negative charge. The resonance Raman spectra of all of the substituted guanidinium ions are characterized primarily by a strong enhancement in the intensity of a band located at approximately 1650 cm^-1 (1620 cm^-1 in the deuterated species). This band is assigned to the fundamental vibrational transition in the asymmetric CN³ stretching coordinate involving the symmetric C-N stretching of the unsubstituted nitrogen atoms.