Force Constants and Dipole Moment Derivatives of Ammonia from Hartree-Fock Calculations

The complete force field of ammonia has been calculated from Hartree‐Fock wavefunctions by the force method. Six Gaussian basis sets of increasing sizes were used. Contrary to previous calculations which were either inaccurate numerically or used very limited basis sets, our results reproduce the essential features of the experimental force field, even for the smallest basis set. In our best calculation we obtained the following results (experimental values in parentheses): symmetry force constants in $\mathrm{mdyn}{\mathrm{/\AA :\hspace{0.17em}\; F}}_{11}{\text{=7.376(7.075 ± 0.13),\hspace{0.17em} F}}_{12}{\text{=0.471(0.78 ± 0.2),\hspace{0.17em} F}}_{22}{\text{=0.579(0.543 ± 0.02), \hspace{0.17em}F}}_{33}{\text{=7.382(7.038 ± 0.13),\hspace{0.17em} F}}_{34}{\text{=−0.180(−0.174 ± 0.04),\hspace{0.17em} F}}_{44}\text{=0.726(0.665 ± 0.01)}$ . Dipole moment derivatives in $\mathrm{D}{\mathrm{/\AA :\mu }}_1{\text{=−0.017(± 0.40),\hspace{0.17em}μ}}_2{\text{=−1.819(± 1.52),\hspace{0.17em}μ}}_3{\text{=+0.209(± 0.18),\hspace{0.17em}μ}}_4\text{=+0.439(± 0.34).}$ From our results the probable value of F₁₂ is estimated as 0.5 mdyn/Å. Dipole moment derivatives are more sensitive with respect to variations of the basis set, and only the sign of the two bending derivatives, μ₂ and μ₄ could be established with certainty.