Site Symmetry Splitting Research Articles

Synthesis, Crystal Structure and Vibrational Spectra of Mg2[BN2]Br: A New Magnesium Nitridoborate Halide with Discrete [N–B–N]3– Groups

AbstractMg2[BN2]Br was prepared from stoichiometric mixtures of the respective binary components Mg3N2, MgBr2 and BN in sealed niobium ampoules placed in evacuated silica tubes at 1473 K. The monoclinic crystal structure (C2/c (no. 15); a = 9.1509(3) Å, b = 6.6209(3) Å, c = 6.4756(2) Å, β = 109.353(3)° and Z = 4) was determined from powder data. The crystal structure is characterized by cationic 32.4.3.4 nets of magnesium and anionic hexagonal closed packed 36 nets of six‐connected nodes occupied by [N–B–N]3– units and Br– anions. Both types of planar nets alternate along the [100] direction. The packing of such incoherent nets results in a monoclinic symmetry, which is a compromise for a structure built up from rod‐like [N–B–N]3– units and spherical bromine and magnesium atoms. Bond length and bond angle for the [N–B–N]3– unit are d(B–N) = 1.354(2) Å and ∠(N–B–N) = 177.1(9)°, respectively., Vibrational spectra were analyzed based on the D∞h symmetry of the relevant [N–B–N]3– moieties, considering the site symmetry splitting. The results are discussed in context with those of the known members of the magnesium dinitridoborate halide series, Mg2[BN2]Cl and Mg8[BN2]5I.

Mg2[BN2]Cl and Mg8[BN2]5I: Novel Magnesium Nitridoborate Halides — Syntheses, Crystal Structures, and Vibrational Spectra

AbstractThe title compounds have been synthesized at 1473 K from stoichiometric mixtures of the binary components Mg3N2, MgX2 (X = Cl, I) and BN in arc‐welded steel ampoules encapsulated in evacuated silica tubes. Mg2[BN2]Cl (1) and Mg8[BN2]5I (2) crystallize in the orthorhombic space groups Pbca (no. 61) and Imma (no. 74), respectively, with a = 6.6139(8)Å, b = 9.766(1)Å, c = 10.600(1)Å, Z = 8 for 1 and a = 13.535(3)Å, b = 9.350(2)Å, c = 11.194(2)Å, Z = 4 for 2. The crystal structures are characterized mainly by Mg6 trigonal prisms which are condensed to 3D frameworks in different ways. Part of the trigonal prisms are centered by the [N—B—N]3— anions and other voids in the framework by the X— anions. The magnesium environment around Cl— is a very distorted monocapped trigonal prism (CN = 6+1) and that of I— is a bicapped heptagonal prism (CN = 14+2). The bond lengths and bond angles for the relevant [BN2]3— anions are d(B—N) = 1.330 — 1.338Å, ∠N—B—N = 175.8° in 1 and d(B—N) = 1.330 — 1.339Å, ∠N—B—N = 176.8° — 178.0° in 2.The vibrational spectra of the title compounds have been recorded and interpreted based on the D∞h symmetry of the relevant [N—B—N]3— groups considering the site symmetry splitting.

Infrared photofragmentation spectra of size-selected Krn+SF6 cluster ions

Presented here are the results of an experimental study of the infrared photofragmentation patterns of size-selected Krn+SF6 cluster ions for n in the range 2⩽n⩽50. The ν3 (T1u) vibrational mode of the SF6 molecule in each cluster ion has been excited using a line-tunable CO2 infrared laser, and the subsequent fragmentation patterns were monitored for the loss of one and two krypton atoms and also for the loss of the SF6 chromophore. For ions containing more than four krypton atoms, the dominant fragmentation process observed was the loss of a single krypton atom, with the loss of SF6 being the least intense of the fragmentation channels for most clusters. For one ion, Kr3+SF6, significantly different behaviour was observed with the loss of SF6 being the dominant fragmentation route, which suggests a stable Kr3+ trimer. Fragmentation intensities were monitored as a function of laser wavelength in order to determine infrared absorption profiles for individual Krn+SF6 cluster ions. The observation of site-symmetry splittings of the triply degenerate ν3 mode and the relatively narrow linewidths suggest the presence of asymmetric, but ordered, structures. With only a maximum of three absorption features apparent for clusters consisting of less than thirty five krypton atoms, the number of structural isomers appears to be small. Comparisons are made with results previously obtained for the Arn+SF6 series of cluster ions (J. Chem. Phys., 1995, 103, 5177), and differences in spectra are attributed to the sensitivity of the SF6 molecule to its environment. Additional interpretation comes from comparisons with computer simulation studies on neutral clusters with the compositions ArnSF6 and KrnSF6 (J. Chem. Phys., 1991, 95, 6271). The appearance of SF6 loss as a minor fragmentation channel for clusters in the region of K12+SF6 is interpreted in terms of stacked structures.

Davydov and site symmetry splitting in polarized infrared spectra of ordered monolayers. A quantitative theoretical analysis of vibrational frequencies and integrated absorptions for CO 2 on NaCl(100)

In the polarized infrared (IR) spectra of ordered physisorbates on insulator single crystal surfaces we observed absorption multiplets, which were assigned to correlation field or Davydov splittings, in some cases combined with site symmetry splittings. In our analysis of the infrared absorptions, based on the classical theory of dynamic dipole coupling, a two-layer model is used. The electric field of the incident light at the surface is dependent on the refractive index of the substrate and the angles of incidence and polarization, as manifested in the Fresnel equations. Analytical expressions are presented for the vibrational frequencies and the s- and p-polarized integrated absorptions of the correlation field and site symmetry multiplets for unit cells with two molecules and the two-dimensional space group pg. Many adsorbates belong to this class. The expressions presented are useful in determining the orientational order of adsorbates. As an example the monolayer CO 2 on NaCl(100) is treated, and the calculated data are compared with the measured spectra.

Raman spectra of the permanganate ion in the solid state with excitation in the near infrared

AbstractBy means of a conventional Raman spectrometer that had been carefully designed for near‐infrared Raman excitation, it was possible to record highly resolved Raman spectra of solid polycrystalline potassium and barium permanganate. The spectra of these deeply coloured materials are of high quality and comparable to those reported hitherto, obtained using Fourier transform Raman techniques and near‐infrared excitation. Full site symmetry splitting and partial factor group splitting of the bands of the permanganate ion could be observed.

Hindered rotation and site structure of methane trapped in rare gas solids

High resolution infrared absorption spectra have been recorded for the ν4 vibrational mode of CH4 trapped in rare gas solids. In argon, krypton, and xenon hosts two dominant sites are observed which we call site R and site A,B. For both sites we believe that the CH4 molecule replaces one host atom. The fine structure of site R at a temperature of 5 K is that predicted for a tetrahedral molecule rotating in an octahedral field; singlet peaks for P(1), Q(1), R(0), and a doublet for the R(1) transitions. The barrier for this rotation is not clearly defined. Site A,B shows site symmetry splitting and orientational ordering suggestive of C3v symmetry. Rotational structure is also associated with site A,B; it is not clear whether the hindered rotation for this site takes place about only the C3 axis or about axes perpendicular to C3 as well. From annealing studies and the effects of deposition temperature we conclude that site R is for CH4 molecules trapped in the normal cubic-close-packed environment while site A,B is for CH4 trapped in hexagonal-close-packed pockets induced by stacking faults formed during deposition. Temporal changes of peak intensities while the matrices stand at 5 K are indicative of a slow relaxation of nuclear spin from a triplet state (J=1) to a quintet state (J=0). For site A,B the triplet to quintet spin relaxation is considerably faster than for site R; for example, for a matrix of CH4 in krypton (1/10 000) at 5 K the spin relaxation half-lives are 87 minutes and 26 minutes for sites R and A,B, respectively.

ChemInform Abstract: ISOTOPE SHIFTS AND FORCE FIELD FOR CARBON TETRACHLORIDE IN A KRYPTON MATRIX

The high-resolution infrared ir spectrum of the Nu/sub 3/ and Nu/sub 1/ + Nu/sub 4/ modes of carbon tetrachloride of natural isotopic abundance in a krypton matrix displays a multitude of absorption peaks due to the several isotopic species and the presence of two dominant sites, one of which shows site symmetry splitting. We have assigned all of the observed transitions and from the isotope shifts have estimated a quadratic force field for each site (one assumed to have T/sub d/ symmetry and one assumed to have C/sub 3Nu/ symmetry). The difference in force fields is slight for the two sites. The results are in agreement with a Urey-Bradley force field estimated earlier without the benefit of isotope shifts. 12 references, 2 figures, 6 tables.

Orientational ordering and site structure of SiF4 trapped in rare gas solids

The infrared spectrum of the ν3 mode of SiF4 shows only one site with marked site-symmetry splitting accompanied by pronounced (>95%) orientational ordering. The results indicate a site symmetry of C3v with the SiF4 molecule occupying a four-atom substitutional site with a cubic-close-packed environment. In krypton and xenon matrices, however, SiF4 occupies a three-atom substitutional site. The fact that SiF4 shows only one dominant site when present in argon matrices of UF6 indicates that one of the two UF6 sites has a local hexagonal close packed environment induced by short range stacking faults.

Isotope shifts and force field for carbon tetrachloride in a krypton matrix

The high-resolution infrared ir spectrum of the Nu/sub 3/ and Nu/sub 1/ + Nu/sub 4/ modes of carbon tetrachloride of natural isotopic abundance in a krypton matrix displays a multitude of absorption peaks due to the several isotopic species and the presence of two dominant sites, one of which shows site symmetry splitting. We have assigned all of the observed transitions and from the isotope shifts have estimated a quadratic force field for each site (one assumed to have T/sub d/ symmetry and one assumed to have C/sub 3Nu/ symmetry). The difference in force fields is slight for the two sites. The results are in agreement with a Urey-Bradley force field estimated earlier without the benefit of isotope shifts. 12 references, 2 figures, 6 tables.

Orientational ordering and site structure for CCl4 in a krypton matrix

Infrared studies of the ν3 and ν1+ν4 modes of CCl4 trapped in krypton matrices show that the CCl4 molecules adopt two sites, one of which preserves the triple degeneracy while the other shows site symmetry splitting and a high degree of orientational ordering. At deposition temperatures above 35 K the orientational ordering of the low symmetry site is diminished as is the relative concentration of this site compared to the other. The results suggest that both cubic-close-packed and hexagonal-close-packed environments exist in these matrices. Significant differences in the ν3 vs ν1+ν4 line widths have been observed and attributed to rapid V–V energy transfer from the higher to the lower of these Fermi coupled modes.

Matrix–molecule interactions, dynamics, and exchange phenomena in low temperature matrices: SF6 in argon and krypton

High resolution infrared studies of argon/SF6 and krypton/SF6 matrices at 10 000/1 show an unexpected amount of structure; 10–12 peaks are observed in argon and six or more peaks in krypton matrices for the n3 mode. Studies in which the deposition temperatures are varied, in conjunction with the structure observed for combination modes and temperature reversible spectral changes, allowed differentiation between multiple trapping sites and site symmetry splittings. We conclude that there are at least six different sites for monomeric SF6 in argon and at least three sites in krypton. Two sites in argon and two in krypton exhibit site symmetry splittings. Temperature dependence studies have shown striking spectral changes wherin for three of these sites (one in argon and two in krypton) the peaks, due to site symmetry splitting, coalesce as the temperature is raised. The temperature dependence can be explained in terms of exchange dephasing involving two high energy fundamentals and a local phonon mode. The dynamics associated with motional collapse of the high enery fundamentals corresponds to rapid site-to-site exchange on a less than nanosecond time scale resulting in higher effective site symmetry as observed by infrared. Unusual 32S–34S isotope effects have also been observed for the spectra of these low temperature matrices.

High resolution infrared studies of dynamics in low temperature matrics: Vibrational dephasing for SeF6 in noble gas solids

High resolution infrared absorption spectra for the y3 mode of SeF6 in low temperature noble gas matrices have been observed. A remarkable amount of structure is observed leading to the conclusion that multiple trapping sites as well as site symmetry splittings, similar to those observed previously for SF6, are common. Temperature dependent studies give a rich display of dynamics arising from host–guest interactions. These give evidence for dynamic exchange among equivalent low symmetry sites within a given trapping cage. This exchange is evidence for a dephasing of the components of y3 by a local site phonon mode in resonance with part of the phonon bath. Unexpected matrix shifts and intensity changes with temperature have been observed. Accurate isotope shifts, especially in a neon matrix, are shown to be useful for calculation of potential constants. A discussion of host–guest interactions, dephasing, and dynamic site exchange in these matrices is given.

The magnetically induced circular emission spectrum of Cr(III) in guanidinium aluminium sulphate

The magnetically induced, circular emission spectrum (MICE) of Cr(III) in hexagonal guanidinium aluminium sulphate hexahydrate has been measured together with the axial emission spectrum. Four lines are observed in the 2E region suggesting that these are the result of trigonal field and site symmetry splittings since each component exhibits an A term type dispersion. The MICE spectrum enables several of the vibrational features to be identified.

Raman spectra of phosphorus pentabromide, phosphorus heptabromide and caesium tribromide

AbstractThe Raman spectra of solid powders of phosphorus pentabromide, phosphorus heptabromide and caesium tribromide are reported. The spectra are explained in terms of site symmetry splitting and factor group splitting.

The vibrational spectra of sodium, potassium and ammonium fluoroborates

The infrared spectra (500–4000 cm−1) of NaBF4, KBF4 and NH4BF4 with natural boron abundance and enriched to 96.4% 11B were obtained at sample temperatures between 30 and 700°K. Solid solution effects, very strong in KBr, were avoided by using NaCl pressed pellets. Raman shifts were measured with a laser Raman spectrometer. The expected site symmetry splittings, isotopic frequency shifts and comparisons with spectra of solid solutions in alkali halides have been utilized in making the vibrational assignments of the bands