³⁵Cl and ³⁷Cl Magic-Angle Spinning NMR Spectroscopy in the Characterization of Inorganic Perchlorates

Abstract

35Cl quadrupole coupling constants (C(Q)), asymmetry parameters (eta(Q)), and isotropic chemical shifts (delta(iso)) have been determined for a series of inorganic perchlorates from (35)Cl magic-angle spinning (MAS) NMR spectra at 14.1 T. Illustrative (37)Cl MAS NMR spectra are obtained and analyzed for some of the samples. For perchlorate anions with quadrupolar couplings less than about 1 MHz, the (35)Cl/(37)Cl NMR parameters are most precisely determined from the full manifold of spinning sidebands observed for the satellite transitions while line-shape analysis of the central transition is employed for the somewhat larger quadrupolar couplings. The environments for the individual perchlorate anions are best characterized by the quadrupole coupling parameters (e.g., C(Q) ranges from 0.3 to 3.0 MHz), while the dispersion in the isotropic (35)Cl chemical shifts is small (1029 ppm < delta(iso) < 1049 ppm) for the perchlorates studied. Due to the variation in quadrupole coupling parameters, (35)Cl MAS NMR may conveniently be employed for identification of anhydrous and hydrated phases of perchlorates, in studies of phase transitions, hydration reactions, and the composition of mixed phases. The perchlorates studied include anhydrous KClO(4), RbClO(4), CsClO(4), (CH(3))(4)NClO(4), and the anhydrous and/or hydrated forms of LiClO(4), NaClO(4), Mg(ClO(4))(2), Ba(ClO(4))(2), and Cd(ClO(4))(2). The (35)Cl MAS NMR spectra of LiClO(4), Mg(ClO(4))(2), and Ba(ClO(4))(2), for which the crystal structures are unknown, reveal that each of these salts possesses a single perchlorate site in the asymmetric unit. The (35)Cl NMR data for Mg(ClO(4))(2) and Ba(ClO(4))(2) suggest that these two samples are isostructural. Relationships between the (35)Cl NMR parameters and crystal symmetries are discussed for the other perchlorates where crystal structure data have been reported.

High-Field Chlorine NMR Spectroscopy of Solid Organic Hydrochloride Salts: A Sensitive Probe of Hydrogen Bonding Environment

“…Conversely, the isotropic chemical shifts are fairly constant, all falling between 45 and 55 ppm. This small variation in the chemical shift is also found for perchlorate salts, 16 although the chemical shift ranges for the two classes of compounds are on the order of 1000 ppm apart. The variation in chemical shifts within either class of compounds, on the order of tens of ppm, represents less than 5% of the total known chemical shift range for chlorine, 1500 ppm.…”

“…Given the range of chlorine quadrupolar coupling constants determined using MAS NMR for chloride anions, 2 to 4 MHz, multiple-quantum MAS (MQMAS) experiments 63 on 35/37 Cl nuclei should be practical at high field strengths for identifying and characterizing nonequivalent chlorine sites in these types 16 (ii) 35/37 Cl NMR Spectra of Stationary Samples. While MAS spectra allow for the characterization of the chlorine EFG tensor, analysis of the chlorine NMR spectra of stationary samples has the potential to provide information concerning the chemical shift tensor, i.e., its principal components and its orientation with respect to the EFG tensor.…”

“…10 Perchlorate anions have been a popular focus of chlorine solidstate NMR studies due to the high local symmetry at chlorine. [11][12][13][14][15][16] Studies of inorganic chlorides have been reported; [17][18][19][20][21][22][23] in many cases, such as the alkali metal chlorides, the chlorine nuclear quadrupolar coupling constant is zero due to cubic symmetry. A limited number of organic or ammonium hydrochloride salts have been studied previously: NH 4 Cl, 18 C 4 H 9 NH 3 Cl, 24 and cocaine hydrochloride.…”

“…The most insightful chlorine solid-state NMR study to date is that of Skibsted and Jakobsen. 16 This study focuses on the chlorine NMR interaction tensors of the perchlorate anion in several inorganic perchlorates and establishes chlorine solid-state NMR as a viable and informative technique for the investigation of these types of materials. Their experiments show that the chlorine chemical shifts for a wide variety of perchlorate salts fall in the narrow range of ∼980 to 1005 ppm relative to NaCl(aq) and that the 35 Cl nuclear quadrupolar coupling constants range from 0.3 to 3.0 MHz.…”

Solid-state NMR spectroscopy of the quadrupolar halogens: chlorine-35/37, bromine-79/81, and iodine-127

“…2). 60 High-quality spectra of the central and satellite transitions under MAS conditions were simulated to extract very precise values of C Q and Á Q . Isotropic chemical shifts were also determined with high precision; however, no information on the anisotropy of the CS tensor was reported.…”