Vibrational Spectroscopy of Cr+(NH3) n ( n = 1-6) Reveals Coordination and Hydrogen-Bonding Motifs.

Abstract

Vibrational spectra are obtained for Cr+(NH3)1-6 in the N-H stretching region (2950-3600 cm-1) using photofragment spectroscopy and complemented by calculations at the M11L/6-311++G(3df,3pd) level of theory. Because of the high bond dissociation energies of Cr+(NH3) and Cr+(NH3)2, their spectra are obtained via N2 tagging; the spectrum of Cr+(NH3) is also obtained by vibrationally mediated photodissociation. The spectra all show intense peaks near 3380 cm-1 due to the antisymmetric N-H stretch. Peaks due to the symmetric N-H stretch (∼3300 cm-1) are intense for n = 1-2, weak for n = 3, and not observed for n > 3. The spectrum of Cr+(NH3) and those of Cr+(NH3)(N2)2 and Cr+(NH3)2(N2) show two peaks near 3200 and 3225 cm-1 due to bend overtones. The spectra indicate that the coordination number of Cr+(NH3) n is 4. In the spectra of Cr+(NH3)5-6 intense, broad peaks appear in the 3080-3280 cm-1 region. Peaks at 3080-3180 cm-1 are due to one first-shell NH3 donating to a second-shell NH3; peaks at 3180-3280 cm-1 are produced by two first-shell NH3 donating to a second-shell NH3. The calculations indicate that the double-donor complexes are energetically favored, while single-donor complexes are entropically favored.