Friday, July 20, 2012

A Paramagnetic Bonding Mechanism for Diatomics in Strong Magnetic Fields

Kai K. Lange, E. I. Tellgren, M. R. Hoffmann and T. Helgaker Science 2012, 337, 327 (Paywall)

A new bonding mechanism
As chemists we are familiar with two types of strong bonds occurring between atoms, covalent and ionic. This paper shows that when very strong magnetic fields (of the order of 105 T) are applied, a third bonding mechanism arrises. Helgaker and co-workers term this perpendicular paramagnetic bonding.


Binding triplet H2
Whilst previous Hartree-Fock calculations have shown that the lowest triplet state of H2 becomes bound in strong magnetic fields, this investigation uses the recently developed LONDON code to demonstrate the same phenomena at the Full-CI level. Similar calculations (again, with a very strong magnetic field) on the triplet state of He2 show a considerable strengthening of the interaction between the constituent atoms.

The nature of this bonding
By examining the behaviour of the molecular orbitals under different orientations relative to the external magnetic field, the authors note a stabilisation of antibonding orbitals in the perpendicular orientation, leading to a new type of bonding interaction. Although the potential of a new chemical bonding mechanism is undoubtably exciting, the magnetic fields required are beyond those that can be currently generated in a lab. However, such fields are present on some stellar objects and the findings of this paper are likely to aid in the spectroscopy of such bodies.

2 comments:

  1. Comment on: A Paramagnetic Bonding Mechanism in Strong Magnetic Fields.

    The exciting prediction that bound triplet states can exist in strong magnetic fields prompted us to comment here that this type of bonding that involves only spin-up electrons and no electron pairing exists also in the absence of magnetic fields [1-3]. Thus, high-spin clusters of alkali and coinage metals, (n+1)Mn (M=Li, Na, Cu, Ag, Au) exhibit this type of bonding, which has recently been termed, no-pair ferromagnetic (NPFM) bonding, since it does not involve even a single electron-pair to sustain the bonding [2]. Some of these clusters, specifically the dimers and trimers have been probed experimentally [3].
    As we have shown recently,[1c,d;2] for these clusters, even in the absence of any electron pairing, still the bonding energy per atom, De/n, exhibits a strongly non-additive behavior; it grows rapidly with the cluster size and converges to values as large as 16-19 kcal mol-1 for 11Au10 and 11Cu10. A valence bond (VB) analysis shows that this NPFM-bonding arises from bound triplet electron pairs that spread over all the close neighbors of a given atom in the clusters, thus stabilizing the ferromagnetic clusters by resonance energy [4]. The VB model demonstrates that a weak interaction in the dimer can become a remarkably strong binding force that holds together mono-valent atoms without even a single electron pair and with high magnetic moments. This type of the bonding is not something exotic under the standard Earth conditions.
    There are in fact many exciting news in bonding, other than the traditional covalent and ionic bond families [5,6]

    (1) (a) M.H. McAdon, W.A. Goddard, III. J. Phys. Chem. 1988, 92, 1352. (b) M.N. Glukhovtsev, P.v,R. Schleyer, Isr. J. Chem. 1993, 33, 455. (c) D. Danovich, W. Wu, S. Shaik, “No-Pair Bonding in the High-Spin state 3Sigmau+ of Li2. ” J. Am. Chem. Soc. 1999, 121, 3165. (d) S. P. de Visser, D. Danovich, W, Wu, S. Shaik, “Ferromagnetic Bonding: Properties of High-Spin Lithium Clusters (n+1)Lin (n=1-12) Devoid of Electron Pairs” J. Phys. Chem. A 2002, 106, 4961.
    (2) D. Danovich, S. Shaik “Bound Triplet Pairs in the Highest Spin States of Coinage Metal Clusters” J. Comp. Theor. Chem. 2010, 6, 1479.
    (3) (a) V.E. Bondybey, J. Chem. Phys. 1982, 77, 3771. (b) J. Higgins, W.E. Ernst, C. Callegari, J. Reho, K.K. Lehmann, G. Scoles, Phys. Rev. Lett. 1996, 77, 4532.
    (4) The resonance energy is provided by the mixing of the local ionic configurations, 3M(||)–M+ and M+ 3M(||)–, and the various excited covalent configurations (involving pz and dz2 atomic orbitals) into the fundamental covalent structure 3(M||M) having the s1s1 electronic configuration.
    (5) S. Shaik, “The Lewis Legacy: The Chemical Bond - A Territory and Heartland of Chemistry” J. Comput. Chem. 2007, 28, 51.
    (6) See for example, charge-shift bonding in: S. Shaik, D. Danovich, W. Wu, P.C. Hiberty, "Charge-shift Bonding and its Manifestations in Chemistry" Nature Chem. 2009, 1, 443.

    Contributed by Sason Shaik and David Danovich

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