Read the paper: "Structure and Bonding of the Sulfamic Acid Zwitterion: Microwave Spectrum of +H3N-SO3–," by M. Canagaratna, J. A. Phillips, H. Goodfriend, & K. R. Leopold, J. Am. Chem. Soc., 1996, 118, 5290-5295. Using VSEPR techniques, predict the structure of the sulfamic acid zwitterion, including estimates of all bond angles. How does your predicted structure compare to the observed structure? Compare the observed bond lengths to standard bond lengths, based on your predicted structure. For which bonds are the comparisons favorable and for which bonds are the comparisons poor?
Answer
The charge minimized Lewis structure )one of three equivalent resonance structures) is shown below:
Based on this Lewis structure, VSEPR predicts all angle around the N and S centers to be near 109o. Because of repulsions, the O-S-O angle could be expected to open up somewhat to perhaps 111-112o causing a reduction of the N-S-O angle to perhaps 106-107o. The H-N-H angles should remain near the ideal value, with perhaps a small reduction (to 108-108.5o) to accommodate the O atoms. A comparison of these predicted angles and the observed angles is given in the table below:
Angle |
Predicted |
Observed |
H-N-H |
108-108.5o |
108.8o |
N-S-O |
106-107o |
97.6o |
O-S-O |
111-112o |
118.9o |
The deviations between the predicted N-S-O and O-S-O angles is significant, indicating that the Lewis structure shown is not an ideal descriptionof the bonding in this molecule.
The bond length comparison reinforces this conclusion. A comparison of standard bond lengths (from the CRC Handbook) and the observed bond lengths points this out:
Bond |
Predicted |
Observed |
H-N |
1.00-1.05 Å |
1.03 Å |
S-O |
1.40-1.45 Å |
1.44 Å |
N-S |
1.6-1.7 Å |
1.96 Å |
The very long N-S bond shows that the bond between the N and S is poor and that the complex between the ammonia and the sulfur trioxide is only weakly bound. This is consistent with the nearly planar SO3 portion of the molecule and the small N-S-O angle.