The sequential bond dissociation energies (BDEs) for M+(NH3) x (x = 1−4) for M = Ti−Cu are determined by examining the collision-induced dissociation reactions with xenon in a guided ion beam mass spectrometer. In the cases of Fe, Co, Ni, and Cu, the BDE for the second ammonia molecule is determined to be greater than the BDE for the first ammonia molecule. For all metal ions but Ti+, the BDEs for the first two ammonia molecules are large in comparison to the BDEs for the third and fourth ammonia molecules. In general, the results of this study for the BDEs of the first and second ammonia molecules agree well with the results of previous experimental and theoretical studies. Previous studies are available only for M+(NH3)3 (M = V, Mn, Ni, and Cu) and M+(NH3)4 (M = V and Cu) complexes, such that this study provides the first determination of all other (NH3)2M+−NH3 and (NH3)3M+−NH3 BDEs. The trends in BDEs are discussed in terms of hybridization, dative interactions, and spin changes and compared to trends for other comprehensively studied ligands, H2O and CO.

The sequential bond energies for complexes of Mg + with CO, CO 2 , NH 3 , CH 4 , CH 3 OH, and C 6 H 6 are determined by collision-induced dissociation (CID) with xenon or argon in a guided ion beam tandem mass spectrometer. The kinetic energy dependence of the CID and ligand exchange cross sections are analyzed to yield 0 and 298 K bond energies for Mg + -L after accounting for the effects of multiple ion-molecule collisions, internal energy of the reactant ions, and dissociation lifetimes. Bond energies (in eV) to Mg + at 0 K are determined for L ) Ar (0.10 ( 0.07), Xe (0.32 ( 0.12), 1-2 CO (0.43 ( 0.06 and 0.40 ( 0.03), 1-3 CO 2 (0.60 ( 0.06, 0.50 ( 0.03, and 0.46 ( 0.06), 1-5 NH 3 (1.60 ( 0.12, 1.27 ( 0.07, 0.99 ( 0.09, 0.45 ( 0.11, and 0.58 ( 0.12), 1-2 CH 4 (0.29 ( 0.07 and 0.15 ( 0.07), 1-3 CH 3 OH (1.51 ( 0.07, 1.25 ( 0.07, and 0.95 ( 0.09), and one C 6 H 6 (1.39 ( 0.10 eV). As expected for largely electrostatic interactions, the sequential bond energies generally decrease monotonically with increasing number of ligands. These values are in good agreement with theoretical values in the literature and ab initio calculations performed here, but the agreement is mixed for comparison with results of photodissociation measurements. Qualitatively, geometries of these complexes are controlled by interactions of the ligands with the single polarized valence electron on Mg + .

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We describe a multireference configuration interaction method that takes advantage of local correlation methods in both the internal ͑originally occupied͒ and external ͑originally unoccupied or virtual͒ orbital spaces. In the internal space, implementation of local correlation is trivial and involves neglecting configurations having simultaneous excitations out of widely separated orbitals. In the external space, the method involves restricting the space of allowed correlating orbitals to those localized near the hole orbitals. Of course, this necessitates the use of localized virtual orbitals which in turn requires one to sacrifice the orthogonality of the virtual space. This complicates the formalism substantially, and we discuss the necessary changes to the traditional expressions in detail. The scaling of the method with system size, basis set size, and the average number of allowed virtual orbitals is explored. An examination of systems having up to 8 heavy atoms reveals that the computational costs of the method scales somewhere between the third and fourth power of the size of the system. Furthermore, this reduced scaling method is capable of recovering greater than 97% of the correlation energy. Additionally, we demonstrate that the method can produce smooth potential energy surfaces and recover bond dissociation energies in organic molecules at a fraction of the cost ͑уtenfold less expensive͒ while retaining accuracy. We go on to use this new reduced scaling approach to predict bond energies in several large organic molecules for which no experimental data are available.

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