Knowledge Bank

The microwave spectrum of $H3N-SO3$ has been observed by pulsed-nozzle Fourier transform microwave spectroscopy. The N-S distance is $1.971(1)\backslash AA$ and the NSO angle is $98.0(1)\circ$. The complex is the gas phase form of the $NH3+SO3$ zwitterion which comprises the crystalline form of sulfamic acid. However, the structure of the complex changes dramatically upon formation of the solid: The N-S bond length in the gas phase is $0.207(21)\backslash AA$ longer than that in the crystal, and the NSO angle is $5.2(6)\circ$ smaller. These results are in reasonable agreement with recent predictions by self consistent reaction field $theory1$. Also the gas phase system appears to be intermediate in character between a weakly bound complex and a chemically bonded species. For example, the $N-S$ bond length, while $0.2\backslash AA$ longer in the gas phase than in the crystal is still about $0.6\backslash AA$ shorter than that observed in the gas phase complex $HCN-SO3$. Similarly, the $8\circ$ out-of-plane distortion of the $SO3$ in the gaseous adduct lies between the $0\circ$ value expected for a weakly bound system and the $13\circ$ distortion found in solid sulfamic acid. The centrifugal distortion constant is somewhat small, indicating a large stretching force constant for the N-S bond, and analysis of the nitrogen quadrupole coupling constant indicates about a quarter of an electron transferred in forming the dative bond. The general connection between partial bonding in the gas phase and large gas-solid structure differences will be discussed.