The guanidinium ion

Properties of the guanidinium cation, GuH ⁺=C(NH ₂)3+, CAS registry 43531-41-5, were calculated from literature data that were critically examined. The following values are valid for T = 298.15 K and 100 kPa. For the isolated cation (in the ideal gas phase), the molar quantities are: enthalpy of formation, Δ _fH° = (462 ± 3) kJ·mol ^-1, enthalpy increment H°- H0 = 14.22 kJ·mol ^-1, constant pressure heat capacity CP = 77.89 J·K ^-1·mol ^-1, entropy S°= 264.54 J·K·mol ^-1, and Gibbs free energy function -(G°- H0)/T = 216.94 J·K ^-1·mol ^-1. Values for the ion derived from data in aqueous solutions are the radius, r = (0.21 ± 0.02) nm, the surface charge density σ = 0.286 C·m ^-2, and polarizability α=4.44×10 ^-30m ³. Standard molar values for the aqueous ion are: enthalpy of formation Δ _fH∞ = -(140 ± 7) kJ·mol ^-1, entropy S∞ = (201 ± 7) J·K ^-1·mol ^-1, heat capacity CP∞ = (103 ± 14) J·K ^-1·mol ^-1, volume V∞ = (46.3 ± 3.3) cm ³·mol ^-1, electrostriction volume Δ _electrV∞ ∼ 0, refraction RD∞ = 11.21 cm ³·mol ^-1, conductivity λ∞ ∼ 39.6 S·cm ²·mol ^-1, self-diffusion coefficient ^D∞=1.06×10 ^-9m ²·s ^-1, viscosity B-coefficient B _η = (0.058 ± 0.006) dm ³·mol ^-1, dielectric decrement d _r/dc = -10.0 dm ³·mol ^-1 (chloride salt), and surface tension decrement dγ/dc = (0.04 ± 0.20) dm ³·mol ^-1. Standard molar values derived from a combination of the data for the gaseous and aqueous ions are the enthalpy of hydration Δ _hydrH∞ = -(602 ± 8) kJ·mol ^-1, entropy of hydration Δ _hydrS∞ = -63.7 J·K ^-1·mol ^-1, and the structural entropy Δ _structS∞ = 83 J·K ^-1·mol ^-1.