| Abstract
| - MP2/aug‘-cc-pVDZ potential surfaces for the hydrogen-bonded complex ClH:pyridine have been generatedwithout and with external electric fields. The zero-field, gas-phase structure of this complex is stabilized bya traditional Cl−H···N hydrogen bond. As the field strength increases, the equilibrium structure changes tothat 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 havebeen computed from each surface, and compared to experimental frequencies in Ar and N2 matrices. Thecomputed results suggest that the hydrogen bond in ClH:pyridine is on the traditional side of quasi-symmetricin an Ar matrix, and on the ion-pair side in an N2 matrix. EOM-CCSD and MP2 calculations have beenperformed on the equilibrium structure at each field strength to obtain the 35Cl−15N spin−spin couplingconstant across the hydrogen bond, and the chemical shift of the hydrogen-bonded proton, respectively. Asa function of field strength, the Cl−N distance, the proton-stretching frequency, and the Cl−N couplingconstant exhibit extrema for the quasi-symmetric complex found at a field of 0.0040 au. These IR and NMRproperties are fingerprints of hydrogen bond type from which the intermolecular distance in a complex maybe 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 anharmonicproton-stretching frequencies for ClD:pyridine, as a function of field strength, show the same pattern as theClH:pyridine frequencies.
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