Relating Environmental Effects and Structures, IR, and NMR Properties of Hydrogen-Bonded Complexes: ClH:Pyridine

Abstract

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 N2 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 N2 matrix. EOM-CCSD and MP2 calculations have been performed on the equilibrium structure at each field strength to obtain the 35Cl−15N 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.