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Charge-Transfer Complexes. NH3-03, NH3-SO2, and N( CH3)3-S021 Robert R. Lucchese, Kenneth Haber, and Henry F. Schaefer 111" Contribution from the Department of Chemistry and Materials and Molecular Research Division, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720. Received May 7, 1976
Abstract: The donor-acceptor complex between trimethylamine and sulfur dioxide is the strongest (in terms of its dissociation energy to N(CH3)3 plus SO>) such complex yet to be studied experimentally in the gas phase. A b initio self-consistent-field theory has been applied to this and two related complexes, N H 3 0 3 and NHjS02. Minimum basis sets were used for all three complexes, while for NH3S02 two considerably larger sets (double [and double {pips sulfur d functions) were employed. The equilibrium structure of these complexes was predicted by an investigation of many points on the respective potential energy surface. To a surprising degree, the 0 3 or SO2 molecule is found to lie in a plane nearly perpendicular to the amine C3 axis. Further, the central atom in 0 3 and SO2 is predicted to lie only slightly off the amine C3[ axis. With these guidelines, the prediction of further structures of this type essentially reduces a one-dimensional search for the central atom-N distance. Using a minimum basis set, the three predicted binding energies are 2.24, 5.00, and 4.06 kcal/mol. Using the double deg
Total energy, hartrees
2.88 3.54 2.86 2.86 3.60 2.73 2.10 2.70 (assumed) 2.86
0 (assumed) 0 (assumed) 0 (assumed) 8.4 0 (assumed) 0 (assumed) 8.2 8.2 (assumed) 0 (assumed)
86.9 0 (assumed) 90 (assumed) 86.8 0 (assummed) 90 (assumed) 95.2 95.2 (assumed) 90 (assumed)
-216.1411 3 -596.05886 -596.06444 -596.06456 -603.14801 -603.15842 -603.15921 -603.3 5 744 -7 11.79749
sentence because the NH3-SO2 system has not been studied experimentally. However, as pointed out earlier, the N(CH3)3-S02 complex has a large gas-phase binding energy, 9.1 f 0.4 kcal/mol. And our minimum basis comparisons suggest that NH3-SO' is somewhat more strongly bound than its trimethylamine counterpart. Hence a value of 10.4 kcal/mol for the NH3-SO2 dissociation energy appears quite plausible. I n the absence of a new geometry optimization, the addition of sulfur d functions reduces the double {binding energy to 9.3 kcal. An especially interesting feature of donor-acceptor complexes is the dipole moment of the complex, and this property is also tabulated in Table I. These predictions are best understood in light of the theoretical dipole moments for the isolated molecules N H 3 and S02. Using the minimum basis we find k(NH3) = 1.79 D (1.47 experimentally3') and p(SO2) = 1.72 D (1.63 from experiment30). The double
