Nigel A. Young

The first combined in situ FTIR and EXAFS study of a matrix isolated molecule

Abstract The results of a combined matrix isolation IR and EXAFS study on methylene bromide (CH 2 Br 2 ) isolated in argon are described and it is demonstrated that data of acceptable quality from both techniques may be obtained from the same sample. The IR data are in excellent agreement with literature values and confirm that matrix isolated samples may be routinely obtained with the novel window arrangement employed. The EXAFS data lead to intramolecular bonded and non-bonded distances ( d (Brue5f8C)=1.95(2) A, d (Br··Br)=3.19(3) A) in agreement with microwave and electron diffraction data and additionally an interaction with the argon matrix is observed with d (Br⋯Ar) ≈ 3.75 A consistent with the sum of the van der Waals radii of Br and Ar.

ZINC CO-ORDINATION IN THE DNA-BINDING DOMAIN OF THE YEAST TRANSCRIPTIONAL ACTIVATOR PPR1

The structure of the native zinc form of the DNA binding domain in the yeast transcriptional activator PPR1 was investigated by extended X‐ray absorption fine structure (EXAFS). By carrying out the EXAFS measurements at 11k we were able to demonstrate explicitly the proximity of the two zinc ions ( ) and the presence of bridging cysteine ligands. The results show that the six cysteine residues co‐ordinate two zinc ions in a two‐metal ion cluster. PPR1 is the first member of this class of protein for which such information has been obtained.

Interatomic distances for some first row transition element dichlorides isolated in cryogenic matrices using x‐ray absorption fine structure spectroscopy

X‐ray absorption fine structure (XAFS) data for several 3d transition metal dichlorides isolated in nitrogen, argon, or methane matrices have been collected and analyzed. The bond lengths obtained are in reasonable agreement with those from vapor phase electron diffraction. The results are briefly discussed with reference to Badger’s rule extended to triatomics.

The characterisation of molecular boric acid by mass spectrometry and matrix isolation infrared spectroscopy

When crystalline orthoboric acid is heated in vacuo to ca. 40 °C, it vaporises to yield molecular H3BO3. The i.r. spectrum of this species isolated in low-temperature nitrogen matrices shows characteristic absorptions at 3 668.5 (E′), 1 426.2 (E′), 1 009.9 (E′), 675.0 (A″), 513.8 (A″), and 448.9 (E′) cm–1 consistent with C3h symmetry. These spectral assignments are supported by extensive isotope labelling, and by a partial normal co-ordinate analysis.

The importance of multiple scattering pathways involving the absorbing atom in the interpretation and analysis of metal K-edge XAFS data of co-ordination compounds

The presence of peaks at twice the metal–ligand distance (2R) in the Fourier transform of the Ni K-edge XAFS data of trans-[NiBr2(PEt3)2], and the absence of such peaks in the data of tetrahedral-[NiBr2(PPh3)2] confirm unambiguously that these 2R features are due to multiple scattering pathways involving the central metal atom in linear (or near linear) L–M–L units. Attention is drawn to the dangers of mis-interpreting these 2R features as due to solvation shells or lattice packing.

Characterisation of the oxo-anions of bromine BrOx–(x= 1–4) by infrared, Raman, nuclear magnetic resonance, and bromine K-edge extended X-ray absorption fine structure techniques

The bromine oxo-anions BrOx–(x= 1–4) have been studied by vibrational and multinuclear n.m.r. spectroscopy and bromine K-edge extended X-ray absorption fine structure (EXAFS) measurements. Raman and/or i.r. data for BrO– and BrO2– are presented, and conflicts in the literature for the latter resolved. Oxygen-17 n.m.r. data for the halogen oxo-anions have been recorded, and an increase in δ(17O) with formal oxidation state of the halogen noted. Coupling to the halogen nucleus has been observed for ClO4–, 1J(35/37Cl–17O)= 85, and for BrO4–, 1J(79/81Br–17O)= 420Hz, but could not be resolved for IO4–. The 81Br n.m.r. resonance of BrO4–(δ= 2 476) and the 127I resonance of IO4–(δ= 4 090 p.p.m.) are reported. Bromine K-edge EXAFS data are presented for all four ions in solution, and for all but BrO– in the solid state, and differences in bond lengths between solid state and solution found to be within experimental error. Bond lengths obtained are: BrO4– 1.61, BrO3– 1.65, BrO2– 1.72, and BrO– 1.81 A. This is the first bond length reported for the hypobromite ion.

Is nickel dibromide bent in a nitrogen matrix? A combined simultaneous Fourier-transform infrared and X-ray absorption fine structure matrix isolation study

Combined Fourier-transform (FT) IR and X-ray absorption fine structure (XAFS) studies of NiBr2 isolated in a nitrogen matrix have shown that it is non-linear (125° FTIR, 145° XAFS), with the shortest Ni–Nmatrix interaction at 2.61 A, compared to a linear geometry in a methane matrix.

The importance of multiple scattering pathways through the central atom in the analysis of metal K-edge XAFS data of coordination complexes.

Whilst the presence of multiple scattering in compounds containing linear ligands (such as CO or CN-) and/or ring systems (such as imidazole or histidine) has long been recognised and used to obtain important structural data, the multiple scattering pathways through the central atom such as those shown below have not received so much attention. We have previously observed, and cautiously identified peaks at twice the metalligand distance (2R) in the FTs of metal K-edge XAFS to be due to such pathways (Young, 1996a, 1996b), but have not been able to fit them using EXCURV92. These type of multiple scattering paths are implemented within the FEFF codes and subsequent versions of EXCURVE. Whilst a number of workers have identified such features in their data (ODay et al., 1994; Hudson et al., 1996; DiazMoreno et al., 1998; Sakane et aL 1998), our belief is that the majority of XAFS users are not sufficiently aware of these important pathways, or how they can be used to obtain (tJ

X-Ray absorption fine structure study of the structural changes accompanying spin cross-over in the acetone solvates of [Fe(dppen)2X2][dppen =cis-1,2-bis(diphenylphosphino)ethylene; X = Cl or Br]

Iron and bromine K-edge X-ray absorption fine structure (XAFS) have been used to determine the changes in the local iron environment in [Fe(dppen)2Br2]·2Me2CO on going through the spin cross-over transition at 160–185 K. The mean Fe–P bond length decreases from the high- to the low-spin isomer by 0.29 A[2.60(3) to 2.31(3)A], whereas the mean Fe–Br bond length decreases by only ca. 0.02 A[2.51(3) to 2.48(3), iron K-edge; 2.48(3) to 2.47(3)A, bromine K-edge]. Iron K-edge XAFS for [Fe(dppen)2Cl2]·2Me2CO showed a reduction of 0.28 A in the mean Fe–P distance [2.55(3) to 2.27(3)A] and a reduction of 0.02 A in the mean Fe–Cl bond length [2.33(3) to 2.31(3)A] on going from the high- to the low-spin form, in good agreement with literature values, the spin transition being observed at 220–240 K.

Notes. Structure of chromyl chloride (CrO2Cl2) in a nitrogen matrix and in the solid state as determined by X-ray absorption spectroscopy

Chromium K-edge extended X-ray absorption fine structure (EXAFS) data for 1% chromyl chloride (CrO2Cl2) in a nitrogen matrix at 10 K have yielded structural data for this molecule, d(Cr–O)= 1.58(2)A and d(Cr–Cl)= 2.10(3)A, in good agreement with the vapour-phase electron-diffraction values, confirming the reliability of data obtained from matrix-isolation EXAFS. The EXAFS data from chromyl chloride as a cold solid at 10 and 130 K represent the first solid-state structural determination for this molecule and the refined parameters are essentially identical to those from the matrix sample but with better signal-to-noise statistics.