8 Electronic Properties of Transition Metal-Benzene Sandwich Clusters
333
The dehydrogenation channels may be characteristic of transition metal-Bz cluster
ions as they have also been found in the reaction of V n
+ with deuterated benzene
(C 6 D 6 ), where deuterium-poor clusters V n C 6 D k
+ (k = 0, 2, 4) were detected [112].
Nevertheless, we recently succeeded in the gas-phase synthesis of V n Bz m
− anions
without dehydrogenated species [113]. In the mass spectrum of Fig. 8.19b, the peaks
of V n Bz m
− are denoted by the notation (n, m) − , and the number above each peak
indicates the number of V atoms, n. V n Bz n + 1
− (n ≥ 2) and some other kinds of
anions such as V n Bz n–1
− (n ≥ 2) and V n Bz n
− (n ≥ 1) are the most abundant, though
the other peaks are assigned to V n Bz m
− combined with oxygen or water molecules.
We adopted a high-pressure Even-Lavie pulsed valve [114] to load He carrier gas at
a stagnation pressure of 50 bar, which is approximately five times higher than that
in the former experiments (∼10 bar at maximum). The Even-Lavie valve is known
to generate cold molecules whose rotational temperature is even lower than 1 K
[114]. It is thus reasonable that such high pressure promotes further cooling down
of V n Bz m
− anions to avoid unwanted dehydrogenation.
The joint anion PE and computational method was first employed for Bz-rich the
anionic series of V n Bz n + 1
− (n ≥ 2), henceforth (n, n + 1) − [113]. The question
in this study was whether V n Bz n + 1
− anions exhibit multiple-decker structures
with high-spin characteristics in common with the corresponding neutrals. The PE
spectra for (n, n + 1) − (n = 2–5), taken with various photon energies, are shown
in Fig. 8.20. The 532 nm spectra in Fig. 8.20a–d, having a better resolution at a
particular EBE than that with larger photon energy, exhibit a monotonic increase of
EA with increasing cluster size, as indicated by the vertical arrows. On the other
hand, the EAs of (n, n + 1) (n = 1–5) were calculated by means of the DFT,
where all the possible spin multiplicities of the neutrals and anions were assessed.
The EA of VBz 2 was calculated to be negative, in agreement with the work by
Judai et al. [60] Moreover, the calculated EAs at n = 2–5, marked by the vertical
lines in Fig. 8.20a–d, showed the best matches with the experimental EAs when the
lowest-energy V n Bz n + 1
− anions were featured by monotonically increasing spin
multiplicities denoted as 2S + 1 = n.
The DFT calculations also give optimized geometries, molecular orbital energy
levels, and HOMO and LUMO pictures of the neutrals and anions, as displayed in
Fig. 8.21a, b, respectively. For the neutrals, both the HOMOs and LUMOs belong
to the minority spin. At n = 1–3, the HOMOs are degenerate dδ orbitals, while the
LUMOs are nonbonding dσ orbitals that are mostly composed of the 3d z
2 orbitals
of the V atoms. At n = 4 and 5, on the other hand, the Jahn-Teller effect lowers the
D 6h symmetry of the multiple-decker structure, as described previously by Wang
et al. [93] This also removes the degeneracy of the dδ orbitals. Because of their
close energies, the dδ and dσ orbitals are hybridized, making the orbital pictures a
bit complicated. However, this minor effect does not change the persistent multipledecker formation with the ferromagnetic spin ordering even at larger n. For the
anions, the comparison between Fig. 8.21a, b reveals that, except mononuclear
singlet (1, 2) − , the (n, n + 1) − anions have ferromagnetic multiple-decker sandwich
structures similarly to the corresponding (n, n + 1) neutrals. The excess electron
occupies a dσ orbital for n = 1–3, a hybridized orbital consisting of dδ and dσ for
333
The dehydrogenation channels may be characteristic of transition metal-Bz cluster
ions as they have also been found in the reaction of V n
+ with deuterated benzene
(C 6 D 6 ), where deuterium-poor clusters V n C 6 D k
+ (k = 0, 2, 4) were detected [112].
Nevertheless, we recently succeeded in the gas-phase synthesis of V n Bz m
− anions
without dehydrogenated species [113]. In the mass spectrum of Fig. 8.19b, the peaks
of V n Bz m
− are denoted by the notation (n, m) − , and the number above each peak
indicates the number of V atoms, n. V n Bz n + 1
− (n ≥ 2) and some other kinds of
anions such as V n Bz n–1
− (n ≥ 2) and V n Bz n
− (n ≥ 1) are the most abundant, though
the other peaks are assigned to V n Bz m
− combined with oxygen or water molecules.
We adopted a high-pressure Even-Lavie pulsed valve [114] to load He carrier gas at
a stagnation pressure of 50 bar, which is approximately five times higher than that
in the former experiments (∼10 bar at maximum). The Even-Lavie valve is known
to generate cold molecules whose rotational temperature is even lower than 1 K
[114]. It is thus reasonable that such high pressure promotes further cooling down
of V n Bz m
− anions to avoid unwanted dehydrogenation.
The joint anion PE and computational method was first employed for Bz-rich the
anionic series of V n Bz n + 1
− (n ≥ 2), henceforth (n, n + 1) − [113]. The question
in this study was whether V n Bz n + 1
− anions exhibit multiple-decker structures
with high-spin characteristics in common with the corresponding neutrals. The PE
spectra for (n, n + 1) − (n = 2–5), taken with various photon energies, are shown
in Fig. 8.20. The 532 nm spectra in Fig. 8.20a–d, having a better resolution at a
particular EBE than that with larger photon energy, exhibit a monotonic increase of
EA with increasing cluster size, as indicated by the vertical arrows. On the other
hand, the EAs of (n, n + 1) (n = 1–5) were calculated by means of the DFT,
where all the possible spin multiplicities of the neutrals and anions were assessed.
The EA of VBz 2 was calculated to be negative, in agreement with the work by
Judai et al. [60] Moreover, the calculated EAs at n = 2–5, marked by the vertical
lines in Fig. 8.20a–d, showed the best matches with the experimental EAs when the
lowest-energy V n Bz n + 1
− anions were featured by monotonically increasing spin
multiplicities denoted as 2S + 1 = n.
The DFT calculations also give optimized geometries, molecular orbital energy
levels, and HOMO and LUMO pictures of the neutrals and anions, as displayed in
Fig. 8.21a, b, respectively. For the neutrals, both the HOMOs and LUMOs belong
to the minority spin. At n = 1–3, the HOMOs are degenerate dδ orbitals, while the
LUMOs are nonbonding dσ orbitals that are mostly composed of the 3d z
2 orbitals
of the V atoms. At n = 4 and 5, on the other hand, the Jahn-Teller effect lowers the
D 6h symmetry of the multiple-decker structure, as described previously by Wang
et al. [93] This also removes the degeneracy of the dδ orbitals. Because of their
close energies, the dδ and dσ orbitals are hybridized, making the orbital pictures a
bit complicated. However, this minor effect does not change the persistent multipledecker formation with the ferromagnetic spin ordering even at larger n. For the
anions, the comparison between Fig. 8.21a, b reveals that, except mononuclear
singlet (1, 2) − , the (n, n + 1) − anions have ferromagnetic multiple-decker sandwich
structures similarly to the corresponding (n, n + 1) neutrals. The excess electron
occupies a dσ orbital for n = 1–3, a hybridized orbital consisting of dδ and dσ for