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:pyridine have been generated without and with external electric fields. The zero-field, gas-phase structure of this complex is stabilized by a traditional Cl-H . . .N hydrogen bond. As the field strength increases, the equilibrium structure changes to that of a proton-shared hydrogen-bonded complex, which is close to quasi-symmetric at a field of 0.0040 au, and then an ion-pair complex at higher fields. Anharmonic dimer- and proton-stretching frequencies have been computed from each surface, and compared to experimental frequencies in Ar and N(2) matrices. The computed results suggest that the hydrogen 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 (35)Cl-(15)N spin-spin coupling constant across the hydrogen bond, and the chemical shift of the hydrogen-bonded proton, respectively. As a function of field strength, the Cl-N distance, the proton-stretching frequency, and the Cl-N coupling constant exhibit extrema for the quasi-symmetric complex found at a field of 0.0040 au. These IR and NMR properties are fingerprints of hydrogen bond type from which the intermolecular distance 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 decrease dramatically at higher fields, and may not be as useful for structure 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.