Relating environmental effects and structures, IR, and NMR properties of hydrogen-bonded complexes: ClH : pyridine

MP2/aug'-cc-pVDZ potential surfaces for the hydrogen-bonded complex ClH:pyr idine have been generated without and with external electric fields. The ze ro-field, gas-phase structure of this complex is stabilized by a traditiona l Cl-H . . .N hydrogen bond. As the field strength increases, the equilibri um structure changes to that of a proton-shared hydrogen-bonded complex, wh ich is close to quasi-symmetric at a field of 0.0040 au, and then an ion-pa ir complex at higher fields. Anharmonic dimer- and proton-stretching freque ncies have been computed from each surface, and compared to experimental fr equencies in Ar and N-2 matrices. The computed results suggest that the hyd rogen bond in ClH:pyridine is on the traditional side of quasi-symmetric in an Ar matrix, and on the ion-pair side in an N-2 matrix. EOM-CCSD and MP2 calculations have been performed on the equilibrium structure at each field strength to obtain the Cl-35-N-15 spin-spin coupling constant across the h ydrogen bond, and the chemical shift of the hydrogen-bonded proton, respect ively. As a function of field strength, the Cl-N distance, the proton-stret ching frequency, and the Cl-N coupling constant exhibit extrema for the qua si-symmetric complex found at a field of 0.0040 au. These IR and NMR proper ties are fingerprints of hydrogen bond type from which the intermolecular d istance in a complex may be determined. The chemical shift of the hydrogen- bonded proton is also a maximum at a field of 0.0040 au, but it does not de crease dramatically at higher fields, and may not be as useful for structur e determination. Deuteration of HCl lowers the proton-stretching frequency, as expected. The two-dimensional anharmonic proton-stretching frequencies for ClD:pyridine, as a function of field strength, show the same pattern as the ClH:pyridine frequencies.