Molecular and Electronic Structure of C₅H₅N−SO₃:  Correlation of Ground State Physical Properties with Orbital Energy Gaps in Partially Bound Lewis Acid−Base Complexes

Abstract

The donor-acceptor complex C 5 H 5 N-SO 3 has been studied in the gas phase using rotational spectroscopy. The adduct has the expected geometry in which the nitrogen lone pair is directed along the C 3 axis of the SO 3 and the SO 3 undergoes free or nearly free internal rotation within the complex. The N-S bond length is 1.915(1) Å, and the NSO angle is 98.91(2) o , indicating that the formation of the dative bond is nearly, but not entirely, complete. Small but significant changes in the heavy-atom ring structure of the pyridine upon complexation are also measured by a series of 13 C substitution experiments. Analysis of the 14 N quadrupole coupling constants indicates a transfer of approximately 0.54 electrons away from the pyridine upon formation of the dative bond. In the series of complexes of SO 3 with HCN, CH 3 CN, H 3 N, C 5 H 5 N, and (CH 3 ) 3 N, electrontransfer values increase as the bond length decreases, and these changes are shown to accompany a gradual decrease in the energy gap between the lone pair orbital of the base and the LUMO of SO 3 . Tabulated values of hardness (η) and electronegativity (χ) give rough but irregular estimates of these energy gaps, and the best correlations are obtained with energies derived directly from ionization potentials for the pertinent orbitals. Binding energies (D e ) have also been determined for the following complexes at the MP2/aug-cc-pVTZ level of theory/basis set: (CH 3 ) 3 N-SO 3 (36.3 kcal/mol), C 5 H 5 N-SO 3 (25.5 kcal/mol), H 3 N-SO 3 (19.6 kcal/mol), CH 3 CN-SO 3 (9.0 kcal/mol), HCCCN-SO 3 (7.4 kcal/mol), and HCN-SO 3 (7.3 kcal/mol). These values also correlate with the energy gap between the donor orbital of the base and the acceptor orbital of the SO 3 .

Correlation of Dative Bond Length and Donor Proton Affinity in Adducts of SO3: A Good Predictor for HCCCN–SO3

“…3 is reminiscent of that previously observed in a correlation between bond length and donor-acceptor orbital energy gap, E, for complexes of SO 3 (7 ). These data are re-plotted from Ref.…”

“…Since additional physical properties such as charge transfer, angular distortion of the SO 3 , and induced dipole moment correlate with bond length (7,41), the proton affinity should be useful for correlating any of these quantities.…”

Microwave spectrum of (CH3)3CCN–SO3

“…Further work has demonstrated that the proton affinity of the donor, which serves as a readily available measure of its basicity, is also a useful parameter for correlating trends in physical properties [4]. In this note, we report the microwave spectrum of the complex (CH 3 ) 3 CCN-SO 3 and further demonstrate the utility of proton affinity for predicting the properties of partially bound systems. Table 1 lists several nitrogen donors for which complexes with SO 3 have been studied.…”

“…These so-called ''partially bound" systems allow for a unique view of the evolution between weak, van der Waals interactions and fully-formed chemical bonds. Complexes of the strong acid SO 3 with nitrogen donors have provided a convenient venue for these studies and we have shown that a variety of physical properties such as N-S bond length, out-of-plane distortion of the SO 3 , binding energy, and electron transfer can be correlated with the energy gap between the donor and acceptor orbitals involved in the interaction [3]. Further work has demonstrated that the proton affinity of the donor, which serves as a readily available measure of its basicity, is also a useful parameter for correlating trends in physical properties [4].…”

“…Ab initio calculations were also performed to determine the binding energy and optimized equilibrium geometry of the complex using second-order Møller-Plesset (MP2) perturbation theory [20] and the Dunning's correlation consistent aug-cc-pVTZ basis set [21]. This choice was made to ensure that the results would be directly comparable with those of the related complexes in Table 1, which were performed using the same basis set and level of theory [3]. Consistent with the experimental spectrum, the lowenergy structure of the complex was found to be a symmetric top with a nitrogen-sulfur distance of about 2.4 Å and indeed the computed rotational constant, B, was within about 1 MHz of the observed value.…”