Bond dissociation energy and electronic spectroscopy of Cr+(NH3) and its isotopomers

Abstract

The electronic spectra of Cr+(NH3), Cr+(ND3), and Cr+(15NH3) have been measured from 14 200 to 17 400 cm−1 using photodissociation spectroscopy. Transitions are predominantly observed from the 6A1 ground state, in which the Cr+ has a 3d5 electronic configuration, to the B̃ 6E (Π) state (3d44s). There is extensive vibronic structure in the spectrum due to a long progression in the Cr–N stretch and transitions to all six spin-orbit levels in the upper state. The spin-orbit splitting in the excited state is observed to be Aso′ = 39 cm−1. For the lowest spin-orbit level, the Cr–N stretching frequency in the excited state is 343 cm−1, with an anharmonicity of 4.2 cm−1. The 6E (Π) origin is predicted to lie at T0 = 14 697 cm−1. The first peak observed is due to v′ = 1, so the observed photodissociation onset is thermodynamic rather than spectroscopic, giving D0(Cr+–NH3) = 14 830 ± 100 cm−1 (177.4 ± 1.2 kJ/mol) and D0(Cr+–ND3) = 15 040 ± 30 cm−1 (179.9 ± 0.4 kJ/mol). The 6E (Π) state of Cr+(NH3) is ∼2740 cm−1 less strongly bound than the ground state, and the Cr–N bond length increases by 0.23 ± 0.03 Å upon electronic excitation. Calculations at the time-dependent density functional theory (M06) and equations of motion coupled cluster, with single and double excitations (EOM-CCSD) level fairly accurately predict the energy and vibrational frequency of the excited state. Multi-reference configuration interaction calculations show how the spin-orbit states of Cr+(NH3) evolve into those of Cr+ + NH3.

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

“…Sequential M + (NH 3 ) n −1 –NH 3 BDEs have been measured using CID by Marinelli and Squires (MS) for n = 1,2 and by Walter and Armentrout (WA) for n = 1–4. Our group recently measured the BDE of Cr + (NH 3 ) to be 177.4 ± 1.2 kJ/mol based on its photodissociation onset . This value refines the WA measurement of 183 ± 10 kJ/mol and is just outside the error bars of the MS value, 157 ± 19 kJ/mol …”

“… a Calculations are at 0 K and use the M11L functional with the 6-311++G­(3df,3pd) basis set. b Result from electronic spectroscopy of Cr + (NH 3 ) at 0 K c CID results from Walter and Armentrout at 298 K. , d CID results from Marinelli and Squires at 298 K …”

“…Typically, this occurs because the electronic spectrum has well-resolved peaks as in our VMP studies of V + (OCO) , and Co + (H 2 O) . In the case of Cr + (NH 3 ), we also take advantage of the fact that the photodissociation onset is thermodynamic, rather than spectroscopic, and set the visible laser to 14 670 cm –1 , which is about 200 cm –1 below the one-photon dissociation onset …”

“…Studies of metal ion interactions with ammonia thus complement those on weaker-field, π-donating ligands such as water. The desire to understand M + –NH 3 interactions has spurred numerous computational studies − and measurements of M + –NH 3 bond strengths via collision-induced dissociation (CID) − and photodissociation . These studies find that the bond dissociation energies (BDEs) of first-row transition-metal cations with NH 3 are around 200 kJ/mol, , about twice as large as those with H 2 O.…”

Spectroscopy and photochemistry of copper nitrate clusters

“…Ultraviolet/visible/near-infrared (UV/vis/NIR) photodissociation spectroscopy is a very powerful tool to characterize the complex electronic structure of transition metal complexes and clusters in the gas phase. [44][45][46][47][48][49][50][51][52][53][54] We recently investigated photochemical hydrogen evolution from hydrated magnesium, 55,56 the effect of salt environments on the reactions and photolysis of organic substances, [57][58][59] as well as the evolution of the hydration environment of a single electron 60 or a carbon dioxide radical anion 61 using action spectroscopy.…”