W. Weltner

Si3Si7. Experimental and theoretical infrared spectra

Abstract Infrared absorption bands in neon, argon, and krypton matrices at 4 K were assigned to Si 3 , Si 4 , Si 6 , and Si 7 from isotopic splittings and from ab initio calculations of their vibrational frequencies and relative intensities. The agreement between experimental and calculated frequencies is excellent. Si 5 is apparently in too low a concentration to be observed.

Electron spin resonance of the C6, C8, and C10 molecules

Electron‐spin‐resonance (ESR) signals attributed to the linear C6, C8, and C10 molecules in their lowest 3Σ states, presumably their ground states, have been observed in solid neon and argon matrices at 4 K. There is evidence of two forms of the C10 molecule, perhaps indicating two slightly bent structural isomers. Laser vaporization of graphite and 13C‐enriched graphite produced a high proportion of these larger molecules. Hyperfine interaction in the 13Cn molecules was small and resolved only for C6, indicating cumulene‐type bonding with the unpaired spins in pπ orbitals, as in C4. The zero‐field‐splitting parameters ‖D‖ were found to be 0.363, 0.783, and 0.190 cm−1, respectively, in solid neon. The increase in ‖D‖ through C8 is attributed to a corresponding variation in the spin–orbit coupling with low‐lying states, principally the 1Σ+g, as the chains lengthen. Gross orbital spin populations and 1Σ+g– X 3Σ−g energy differences were obtained from Hartree–Fock calculations in order to interpret the hfs a...

ESR of Mn2 and Mn5 molecules in rare‐gas matricesa)

Mn2 and Mn5 have been isolated in argon, krypton, and xenon matrices and their X‐band ESR spectra observed at 4 and up to 70 K, depending upon the matrix. As predicted by Nesbet, the lowest state of Mn2 is 1Σ, and the two atoms are exchange coupled (antiferromagnetically). The temperature behavior of the ESR bands in the higher spin states (S=1,2,3) was approximately in accord with a Lande interval rule, and a value of J=−9±3 cm−1 was obtained from the S=2 intensity variations. Each fine structure line appears with a superimposed 11‐line hyperfine pattern with splitting one‐half that of isolated 55Mn atoms (30 G). The anisotropic exchange interactions fit the Judd–Owen relationship with De=−0.043(2) and Dc=−0.001(4) cm−1. Assuming De arises solely from magnetic dipole interaction, the interatomic distance in Mn2 is calculated to be 3.4 A. Mn5 appears in more concentrated matrices as a highly oriented axial molecule with its axis perpendicular to the flat sapphire substrate surface. The observed fine struc...

ESR of the CN Radical in Inert Matrices

CN has been trapped as a randomly oriented molecule in argon, krypton, and neon matrices at 4°K. The ESR spectra of 12C14N and 13C14N in their ground 2Σ+ states have been determined and yield the following magnetic parameters in solid argon: g‖ = 2.0015(5), g⊥ = 2.0003(5), A‖(14N) = + 18.2(4) MHz, A⊥ (14N) = − 28.0(3) MHz, A‖ (13C) = + 678(1) MHz, A⊥ (13C) = + 543.1(5) MHz. Weak forbidden ΔMI = ± 1 transitions are also observed. Upon slight warming, the sites are irreversibly changed so that CN thereafter exhibits hindered rotation, even at 4°K, and raising the temperature leads to the isotropic ESR spectrum of rotating CN. Variations in linewidths due to motional effects make it possible to confirm the relative signs of the A values. The g⊥ shift is shown to arise from small negative contributions from spin–orbit coupling with both the 2Πi and 2Πr excited states. Approximate spin densities in the molecule are qC(2s) = 0.19, qC(2p) = 0.48, qN(2p) = 0.32, qN(2s) = − 0.008. These values are compared with those obtained from a Hartree–Fock wavefunction provided by Cade.CN has been trapped as a randomly oriented molecule in argon, krypton, and neon matrices at 4°K. The ESR spectra of 12C14N and 13C14N in their ground 2Σ+ states have been determined and yield the following magnetic parameters in solid argon: g‖ = 2.0015(5), g⊥ = 2.0003(5), A‖(14N) = + 18.2(4) MHz, A⊥ (14N) = − 28.0(3) MHz, A‖ (13C) = + 678(1) MHz, A⊥ (13C) = + 543.1(5) MHz. Weak forbidden ΔMI = ± 1 transitions are also observed. Upon slight warming, the sites are irreversibly changed so that CN thereafter exhibits hindered rotation, even at 4°K, and raising the temperature leads to the isotropic ESR spectrum of rotating CN. Variations in linewidths due to motional effects make it possible to confirm the relative signs of the A values. The g⊥ shift is shown to arise from small negative contributions from spin–orbit coupling with both the 2Πi and 2Πr excited states. Approximate spin densities in the molecule are qC(2s) = 0.19, qC(2p) = 0.48, qN(2p) = 0.32, qN(2s) = − 0.008. These values are compared with th...

C2H radical: 13C hyperfine interaction and optical spectrum

C2H was prepared by photolysis of acetylene, 90% 13C‐substituted acetylene, or iodoacetylene and trapped in solid argon at 4°K. A complete analysis of all hyperfine‐interaction and g tensor components was made for the randomly oriented 2Σ molecules yielding the following parameters (in solid argon): g∥=g⊥=2.0025(5), A⊥(H)=41(1), A∥(H)=51(1), A⊥(13Cα)=863(1), A∥(13Cα) =980(1), A⊥(13Cβ)=139(1), A∥(13Cβ) =191(1) MHz. Approximate spin densities are compared with an extended basis INDO calculation of Morikawa and Kikuchi. Weak optical absorption spectra were observed at about 10 000 and 3300 A. The former is assigned to an A2∏i←X2Σ transition with (0,0,0) at 10 080 A (in solid argon) where the upper state C–C stretching frequency ν′3 =1560 cm−1. The assignment of the 3300 A system (s) is less definite, but it may involve a B2A′←X2Σ, bent ← linear transition since the observed bands cannot definitely be assigned to an expected B2Σ←X2Σ transition.

Hyperfine Interaction and Chemical Bonding in MgF, CaF, SrF, and BaF molecules

ESR spectra of MgH, CaH, SrH, and BaH molecules in their ground 2Σ+ states were measured while isolated in solid argon matrices at 4°K. The molecules were prepared by quenching metal and hydrogen atom beams in a stream of argon. g tensors and hyperfine structure (hfs) constants for interaction with H, and in the case of MgH, with 25Mg and D, were determined. For 25MgH, | Ψ (0) |2 at Mg and H nuclei were found to be 0.800(8) and 0.0663(2) a.u., respectively, in good agreement with 0.7635 and 0.03789 a.u. recently calculated by Chan and Davidson using a 953 CI wavefunction. It is evident that such elaborate calculations are necessary to obtain accurate theoretical spin densities. Hydrogen hf interaction within the group of molecules indicate less ionicity than in the corresponding diatomic fluorides although the spin density is still predominantly on the metal atoms. Metal p and d contributions are approximately derived from the observed Δg⊥ values and discussed in relation to the united‐atom model.

Properties of Sc3, Y3, and Sc13 molecules at low temperatures, as determined by ESR

Sc3 and Y3 molecules have been isolated in rare gas matrices at temperatures near 4 K. ESR spectra establish that the structure of Sc3 is an equilateral triangle at 4–30 K with a 2A′1 ground state. The possibility remains that Sc3 is a fluxional bent molecule with a very low barrier to pseudorotation. The 45Sc hyperfine splitting indicates that the unpaired electron has little s character and is delocalized in 3d orbitals on the three equivalent atoms. Y3, however, is not equilateral and is most probably a bent molecule at these temperatures with the spin again distributed over the 3d atomic orbitals, but in a 2B2 ground state. La3 was not observed and is therefore judged to be a linear orbitally degenerate molecule. Under special conditions, a cluster of exceptional stability Scx, where x≥9 is formed in neon matrices. From its uniqueness and from the hyperfine structure in its ESR spectrum, it is suggested that it may be Sc13 with an icosahedral structure. This places 12 equivalent atoms at the vertices ...

B atoms, B2 and H2BO molecules: ESR and optical spectra at 4 °K

Elemental boron has been vaporized and trapped in solid argon at 4–10 °K. The Douglas–Herzberg transition of B2 has been observed in absorption at 3300 A at 10 °K, indicating that the lower Σ state is the ground electronic state. However, B2 was not observed via ESR, which is interpreted as support for a 3Σ−g ground state with a zero‐field splitting greater than about 4 cm−1, rather than 5Σ−u favored by ab initio calculations. The ESP spectrum of B atoms in solid argon is also detected. It exhibits axial symmetry and almost complete quenching of the orbital angular momentum of the 2P1/2 free atom, with g∥=2.0014(5), g⊥=1.9645(30). The simple crystal field model has been applied in the interpretation of the g and hyperfine tensors and comparison made with Al and Ga in argon, as studied by Ammeter and Schlosnagle. The ESR spectrum of H2BO, appearing as an impurity, was observed for the first time, and its magnetic parameters determined.

ESR and Optical Spectroscopy of the AlO Molecule at 4°K; Observation of an Al Complex and Its Interaction with Krypton

The AlO molecule in its 2Σ ground state has been trapped in neon, argon, and krypton matrices at 4°K, and its ESR and optical spectra (infrared, visible, and uv) measured. In neon its magnetic parameters are g‖ = 2.0015(3), g ⊥ = 2.0004(3), A‖( 27Al) = 872(1) MHz, and A ⊥ ( 27Al) = 713(1) MHz. Δg⊥ is in accord with the magnitude and sign of the spin‐doubling constant, γ0, determined in the gas phase. An infrared band at 924 cm−1 in neon (917 cm−1 in argon) is assigned to AlO: the large shift from the gas value of 965 cm−1 indicates a highly ionic molecule. Besides the strong B 2Σ←X 2Σ absorption bands near 4500 A, a progression of weak bands have been observed between 5500 and 11 000 cm−1 and assigned to the A 2πi←X 2Σ transition of AlO. ESR spectra of a weakly bonded Al–X molecule (where X may be Al2O) are observed in concentrated matrices; its magnetic properties are found to be very dependent upon the matrix gas used. In krypton matrices, 83Kr hfs is also observed but disappears (reversibly) above abou...

The ground state of the Sc2 molecule

Abstract The X-band ESR spectrum of the 45Sc2 molecule was observed in neon and argon matrices at 4 K. Two fine-structure transitions with extensive 45Sc(I = 7 2 ) hyperfine structure were analyzed to establish that the ground state of Sc2 is 5Σ. The hyperfine parameters were shown to be in essential agreement with a sσ1 do1 dπ2 electronic configuration.