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Matrix-Isolation and Quantum-Chemical Analysis of the C Conformer of XeF, XeOF and Their Acetonitrile Adducts 6
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Maxim Gawrilow, Helmut Beckers, Sebastian Riedel, and Lan Cheng J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b09902 • Publication Date (Web): 08 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017
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The Journal of Physical Chemistry
Matrix-Isolation and Quantum-Chemical Analysis of the C3v Conformer of XeF6, XeOF4 and Their Acetonitrile Adducts Maxim Gawrilow,† Helmut Beckers,∗,† Sebastian Riedel,∗,† and Lan Cheng∗,‡ †Institut für Chemie und Biochemie, Anorganische Chemie, Freie Universität Berlin, Fabeckstr. 34-36, 14195 Berlin, Germany ‡Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218, USA E-mail:
[email protected];
[email protected];
[email protected] 1
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Abstract A joint experimental-computational study of the molecular structure and vibrational spectra of the XeF6 molecule is reported. The vibrational frequencies, intensities and in particular the isotopic frequency shifts of the vibrational spectra for 136
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XeF6 and
XeF6 isotopologues recorded in the neon matrix agree very well with those obtained
from relativistic coupled-cluster calculations for XeF6 in the C3v structure, thereby strongly support the observation of the C3v conformer of the XeF6 molecule in the neon matrix. A C3v transition state connecting the C3v and Oh local minima is located computationally. The calculated barrier of 220 cm−1 between the C3v minima and the transition state corroborates the experimental observation of the C3v conformer and the absence of the Oh conformer in solid noble gas matrices. For comparison matrix-isolation spectra have also been recorded and analysed for the the
136
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XeOF4 and
XeOF4 isotopologues. The matrix-isolation complexation shifts obtained for the
XeF6 · NCCH3 relative to those of free matrix-isolated XeF6 and CH3CN are in good agreement with those reported for crystalline XeF6 · NCCH3.
Introduction Despite many experimental attempts to solve the molecular structure of the XeF6 molecule since the first report about its synthesis in 1962 1–3 some main questions remain unresolved. The pioneering experimental studies of electron diffraction 4,5 and photoelectron spectroscopy 6 concluded that the XeF6 molecule possesses a distorted octahedral structure, and the vibrational spectra of XeF6 were investigated by Claassen et al., 7 who assumed the presence of three electronic isomers. This assumption was, however, disputed in 1974 by Nielsen et al. 8 From the analysis of the electronic spectrum obtained with synchrotron radiation at DESY they concluded that the population of any potential low lying excited state has to be less than 1 % and that the structure has to be – if not a perfect octahedron – at least very close to Oh symmetry. This result is consistent with the observation that molecular 2
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The Journal of Physical Chemistry
beams of XeF6 and (definitely octahedral) SF6 behave identically in magnetic and electric fields. 9,10 Thus Code et al. and Falconer et al. concluded that XeF6 has neither any measurable electronical magnetic moment, nor any dipole moment. The missing or very low dipole moment might be the reason why attempts to record a rotational spectrum of XeF6 failed so far. 11 Thus, from the experimental point of view our knowledge about the structure of XeF6 remains limited. A more detailed summary of the numerous previous studies on this molecule is enclosed in the Supporting Information. It has been assumed that the XeF6 molecule is highly fluxional, and the molecule interconverts between the degenerate C3v structures rapidly. Therefore, the averaged electric dipole moment might be vanishing and consequently it is difficult to observe a pure rotational spectrum of the molecule. Alternatively, the C3v conformer might be a fairly stable equilibrium structure but accidentally has a very small value for the electric dipole moment. Computational and experimental studies that are capable of providing new information about the structure and energies of the C3v and Oh conformers as well as the potential energy surface relevant to interconversion between these conformers are thus valuable for better understanding the molecular structure of XeF6. We should mention that the XeF6 molecule has also been demonstrated to be a challenging system for quantum-chemical calculations. 12–16 The relative energy of the C3v and Oh conformers is dictated by a delicate balance of relativistic, electron-correlation, and basis-set effects. While self-consistent-field calculations using large basis sets clearly favor the C3v structure, relativistic and electron-correlation effects both favor the Oh structure. 13 Due to the major cancellation among these contributions, the energy difference between C3v and Oh conformers is very small (at the level of a few kJ mol−1 ). 13,15 Therefore, it is necessary to sufficiently account for electron-correlation, relativistic, and basis-set effects for accurate calculations of the XeF6 molecule. In this article we present a joint computational-experimental study of the structure and vibrational spectra of XeF6. The present computational study consists of an extension of our 3
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recent high-level relativistic coupled-cluster calculations, in which two different conformers of C3v and Oh symmetry were predicted as local minima. 16 In this work structural parameters, relative energies, vibrational spectra and isotopic frequency shifts of both conformers as well as the barrier of their mutual interconversion have been calculated at the CCSD(T) level of theory with scalar relativistic effects taken into account using the spin-free exact two-component theory in its one-electron variant (SFX2C-1e). The quantum-chemical calculations will be used to facilitate the interpretation of a matrix-isolation vibrational study of molecular XeF6. Prior to this work the Raman, infrared, visible, and ultraviolet spectra have been reported of XeF6 in the vapour and in Ar matrix isolation by Claassen et al. 7 From their vapourphase spectra the authors concluded that the XeF6 vapour consists of at least two or more isomeric forms, in which the presence of dimers or oligomers could be excluded. Their attempts to estimate the relative abundance of different isomers in the XeF6 vapour by a rapid matrix- isolation of XeF6 vapour, however, failed, as “no clear relationship emerged between sample conditions and resulting spectra”. Furthermore the assignment of their spectra was complicated due to complex band envelopes, band broadening and overlapping bands especially in the Xe F stretching region. Although vibrational bands observed from a solid Ar matrix isolated species at temperatures
