EXPERIMENTAL AND COMPUTATIONAL STUDIES ON AMINOGUANIDINE FREE-BASE, MONOCATION AND DICATION - PART III - PROTON AFFINITIES OF GUANIDINE, AMINOGUANIDINE AND GLYOXAL BIS(AMIDINOHYDRAZONE)
Jt. Koskinen, EXPERIMENTAL AND COMPUTATIONAL STUDIES ON AMINOGUANIDINE FREE-BASE, MONOCATION AND DICATION - PART III - PROTON AFFINITIES OF GUANIDINE, AMINOGUANIDINE AND GLYOXAL BIS(AMIDINOHYDRAZONE), Zeitschrift fur Naturforschung. B, A journal of chemical sciences, 53(3), 1998, pp. 386-392
The structures of glyoxal bis(amidinohydrazone) (GBG) free base and gl
yoxal bis(amidinohydrazonium) monocation and dication were calculated
quantum chemically by using the density functional hybrid method B3-LY
P with the standard basis set 6-31G(d). Proton affinities calculated f
rom these data are 246.4 kcal/mol for the free base and 176.0 kcal/mol
for the monocation. The proton affinities of guanidine free base (246
.2 kcal/mol), aminoguanidine free base (242.9 kcal/mol), aminoguanidin
ium monocation (88.6 kcal/mol) were calculated for reference. The B3-L
YP functional overestimates the proton affinities for all the species
studied. For example, for guanidine the proton affinity at the MP2/6-3
1G(d) level is 238.3 kcal/mol, the experimental reference value being
233 kcal/mol. However, from the B3-LYP values it can be concluded that
in the gas phase all the three bases are nearly equally basic. On the
other hand. it is known that in aqueous solution guanidine is a much
stronger base than aminoguanidine and glyoxal bis(amidinohpdrazone). T
he results are discussed from the point of view of molecular size, sha
pe and symmetry, and hydrogen bonding in solution.