95 Mo Magic Angle Spinning NMR at High Field: Improved Measurements and Structural Analysis of the Quadrupole Interaction in Monomolybdates and Isopolymolybdates Jean-Baptiste d’Espinose de Lacaillerie,* ,² Fabien Barberon, ² Konstantin V. Romanenko, ‡ Olga B. Lapina, ‡ Laurent Le Polle ` s, § Re ´ gis Gautier, § and Zhehong Gan | Laboratoire de Physique Quantique, UMR CNRS SIEN 7142, Ecole Supe ´ rieure de Physique et de Chimie Industrielles, 10 rue Vauquelin, 75231 Paris Cedex 05, France, BoreskoV Institute of Catalysis, Prospekt LaVrentiVa 5, NoVosibirsk 630090, Russia, Laboratoire de Chimie du Solide et Inorganique Mole ´ culaire, UMR CNRS 6511, Ecole Nationale Supe ´ rieure de Chimie de Rennes, Institut de Chimie de Rennes, Campus de Beaulieu, 35700 Rennes, France, and National High Magnetic Field Laboratory, 1800 East Paul Dirac DriVe, Tallahassee, Florida 32310 ReceiVed: April 15, 2005; In Final Form: May 27, 2005 In this study, 95 Mo quadrupole couplings in various molydbates were measured easily and accurately with magic angle spinning (MAS) NMR under a directing field of 19.6 T. The resonance frequency of 54 MHz was sufficiently high to remove acoustic ringing artifacts, and the spectra could be analyzed in the usual terms of chemical shift and quadrupolar line shapes. For monomolybdates and molybdite, the quadrupole coupling dominated the NMR response, and the quadrupole parameters could be measured with better accuracy than in previous lower field studies. Moreover, despite the low symmetry of the molybdenum coordination, the usefulness of such measurements to probe molybdenum environments was established by ab initio density functional theory (DFT) calculations of the electric field gradient from known structures. The experimental NMR data correlated perfectly with the refined structures. In isopolymolybdates, the resonances were shapeless and DFT calculations were impossible because of the large and low symmetry unit cells. Nevertheless, empirical but clear NMR signatures were obtained from the spinning sidebands analysis or the MQMAS spectra. This was possible for the first time thanks to the improved baseline and sensitivity at high fields. With the generalization of NMR spectrometers operating above 17 T, it was predicted that 95 Mo MAS NMR could evolve as a routine characterization tool for ill-defined structures such as supported molybdates in catalysis. Introduction The different structures and polyhedron arrangements in solid molybdates result in extensive distortions of the ideal MoO 4 2- tetrahedron and MoO 6 6- octahedron. 1 Consequently, inorganic molybdates are amenable to meaningful studies by spectro- scopies probing the local environment of molybdenum such as neutron diffraction, Fourier transform infrared, Raman, 2 Mo K edge XAS, 3,4 and molybdenum NMR. The latter, however, have remained scarcely used in the solid state for, due to the quadrupolar nature (spins 5/2) and low gyromagnetic ratio (γ) of its NMR active isotopes, namely, 95 Mo (-1.751 10 7 rad s -1 T -1 ) and 97 Mo (-1.788 10 7 rad s -1 T -1 ), 5 molybdenum NMR suffers from a reduced sensitivity (∝γ 7/4 ) and a low resonance frequency. Even at 11.7 T, the Larmor frequencies in the 32- 33 MHz range result in extensive baseline distortion by acoustic ringing. However, 95 Mo and 97 Mo are not entirely unfavorable to NMR. Their natural abundance (15.92 and 9.55%, respec- tively) is not worse than other commonly studied nuclei such as 29 Si. Their longitudinal relaxation time is not excessively long (in the range of seconds to hundreds of seconds for insulators). They figure in the upper half of the low γ range (defined as comprising nuclei resonating below 40 MHz at 9.4 T, that is below 15 N). Finally, the quadrupole moments of 95 Mo (-22mB) and 97 Mo (255mB) are comparable to the ones of 17 O (-25.58 mB) and 27 Al (146.6 mB), respectively. NMR of Mo- (VI) in the solid state is thus possible even under moderate fields if the sensitivity is enhanced using isotopic enrichment or specific pulse scheme such as sechinv, 6 while the dead time associated with acoustic ringing can be circumvented using a quadrupolar echo sequence or even QCPMG. 7 Indeed, consider- ing the importance of molybdate chemistry, the feasibility of molybdenum solid state NMR has been periodically investigated in the past within the fields of inorganic chemistry, 8-12 materials science, 13-15 and catalysis. 16-20 As high field (>17 T) spectrometers are now being serviced worldwide, the limitations associated with the low γ are expected to vanish, and it has been recognized that it is now time to revisit the potential of 95 Mo to identify the local structures of molybdates. 11 However, to our knowledge, apart from a study of Bryce and Wasylishen 21 of a piano stool complex at 18.8 T, no studies of Mo(VI) in the solid state by NMR at ultrahigh fields have been published to date. We thus report here on one-pulse, natural abundance 95 Mo solid state magic angle spinning (MAS) NMR spectra obtained at 54.2 MHz (19.6 T) for model bulk compounds of monomolyb- dates and isopolymolybdate salts. This was at an applied magnetic field (B 0 ) nearly triple what was used for similar studies in the ‘80s 16 and still nearly double what was available in the ‘90s. 17 The feasibility of multidimensional 95 Mo experi- ments is also demonstrated for the first time. Finally, the * To whom correspondence should be addressed. E-mail: Jean- Baptiste.dEspinose@espci.fr. ² Ecole Supe ´rieure de Physique et de Chimie Industrielles. ‡ Boreskov Institute of Catalysis. § Ecole Nationale Supe ´rieure de Chimie de Rennes. | National High Magnetic Field Laboratory. 14033 J. Phys. Chem. B 2005, 109, 14033-14042 10.1021/jp0519621 CCC: $30.25 © 2005 American Chemical Society Published on Web 07/02/2005