Ab Initio Studies of Solvent Effects on Molecular Conformation and Vibrational Spectra of Diglycine Zwitterion

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The molecular conformation of diglycine zwitterion and neutral diglycine is determined by the ab initio calculations at the Hartree-Fock level using the basis sets 6-31+g(d), 6-31++g(d,p), and 6-31+g(d,p). The infrared and Raman spectra of the diglycine zwitterion are assigned using the ab initio force field and the vibrational spectra of its nine selectively C-, N,C-, and N-deuterated and $^{15}N$ isotopic species. The effect of solvation on the conformation and vibrational spectra of diglycine zwitterion in solvents of dielectric constant 2.2 and 78 has been investigated by the self-consistent reaction field theory. The inclusion of a solvent reaction field has a strong influence on the total energies, conformation, and vibrational spectra. The frequency shifts in the solvents support stabilization of the nondipolar amide structure in polar solvents, unlike in neutral amide molecules where the dipolar structure is stabilized in polar solvents. Ab initio calculations have also been made on neutral diglycine in the gas phase and in a solvent medium (\epsilon = 78.0) at the Hartree-Fock and B3LYP level using the basis sets 6-31+g(d) and 6-31++g(d,p) and for the zwitterion in the solvent medium (\epsilon = 78.0) at B3LYP level using the latter basis set.