R. J. Van Zee

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...

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 ...

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.

Hyperfine interaction and structure of a gallium arsenide cluster: Ga2As3

Vapor species produced by laser heating of gallium arsenide crystals were allowed to aggregate in a relatively high pressure of argon or krypton before condensation of the matrices at 4 K. Electron‐spin resonance (ESR) identified the spectrum of the S=1/2 Ga2As3 cluster via the 69,71Ga (I=3/2) and 75As (I=3/2) hyperfine structure as containing two equivalent Ga and three equivalent As nuclei. Its geometry is therefore trigonal bipyramid and its ground state probably 2A2‘, as obtained by Lou et al., from local‐spin‐density theory. The observed hyperfine parameters indicate that the unpaired spin is confined almost entirely to the two axial gallium atoms.

Transition‐metal diatomics: Mn2, Mn+2, CrMn

The Mn2 molecule, exhibiting antiferromagnetic exchange coupling between the 3d5 electrons on each atom, has been investigated further in cyclopropane matrices at temperatures ranging from 12 to 110 K. This allowed electron‐spin‐resonance spectra to be observed in the higher spin states S=4 and 5 and confirmed the previous deduction of van der Waals bonding with r≂3.4 A. In addition, the derived anisotropic parameters DS suggest that exchange striction (as previously observed in solids) is occurring such that r varies from about 3.2 to 3.6 A in the S=0 to S=5 states, with a corresponding variation in the exchange coupling constant J in these states. The removal of one electron to produce Mn+2 results in the formation of a single σ bond and high‐spin coupling among the remaining 11 unpaired electrons to yield a 12Σ ground state. In contrast, its isoelectronic counterpart CrMn is found to have a 4Σ ground state and multiple bonding, more in accord with expectations for a diatomic between Cr2 and Mn2.

Si2, SiH3, and HSiO molecules: ESR at 4 K

ESR spectra of 29Si2, 29SiH3, and H29SiO molecules in argon and/or neon matrices at 4 K have yielded the following new data: Si2 has a zero‐field splitting in its ground3Σ−g state of ‖D‖=2.53(1) cm−1 in an argon matrix, which extrapolates to a gas‐phase parameter of ‖λ‖=1.35(5) cm−1. The hyperfine interaction constant ‖A⊥(29Si)‖=40(2) MHz. SiH3 has g∥=2.0043(5), g⊥=2.0035(5), ‖Aiso(H)‖=25(3), ‖Aiso(29Si)‖=665(4), and ‖Adip(29Si)‖=166(3) MHz. Probable spin densities are 15% sSi+91% pSi−5% sH with a calculated bond angle of 113° in solid argon. HSiO has been observed for the first time. Its g components and spin distribution are very similar to those of the HCO radical. It is bent with a presumed 2A’ ground state, in accord with unpublished calculations of Bruna and Peyerimhoff and the recently published study of Frenking and Schaefer. The more stable isomer HOSi may have also been detected here.

MAGNETO-INFRARED SPECTRA OF THE SI2, GE2, AND SN2 MOLECULES IN RARE-GAS MATRICES

Absorption spectra of Si2, Ge2, and Sn2, observed between 50 and 5000 cm−1 in neon and argon matrices at 4 K, were assigned to the lowest 3Πu←X 3Σg− electronic transition. In all three cases, the Franck–Condon envelope of vibrational bands allowed T00, ωe’, ωe’xe’ to be determined. In the Si2 molecule [near Case (a)] T00=313 cm−1 and each vibrational band was split into Ω triplets (0,1,2) in the upper state. A Zeeman effect on these lines was detected in magnetic fields up to 4 T. As the spin–orbit coupling becomes larger in Ge2 and Sn2, the 1u←X0+g transition [case (c)] was observed, and the corresponding parameters in the upper state determined. For Sn2 the magnetic‐dipole 1g←X0+g transition was also detected and identified by the splitting and broadening of the band in magnetic fields up to 4 T. The results are discussed in the context of the ab initio calculations and other experimental results on these molecules.

ESR of Co, Rh, Ir trimers and diatomic ions

Abstract Co 3 and Rh 3 have been prepared in argon/krypton matrices and their electron-spin-resonance spectra measured at ≈ 2 K. Only the lowest Kramers doublet transitions were observed, indicating that the total spin S = 5 2 or 7 2 . The hyperfine intensity patterns and small splittings indicate in each case that the trimers have equilateral triangle structures (D 3h symmetry) with the unpaired spins essentially in non-bonding orbitals involving atomic dδ atomic orbitals. For Rh 3 these findings are in agreement with the theoretical calculations of Dai and Balasubramanian with the condition that the barrier to pseudorotation of the 6 A 1 ground state (C 2v symmetry) is so small as to allow complete fluxional behavior at 2 K. Ir 3 may also have been detected and if so, it is also of high spin. Laser vaporization produces Co + 2 (only in neon matrices) and Ir + 2 (in neon and argon matrices), and their ESR spectra show that each has a 6 Σ ground state.