A joint experimental-theoretical study has been carried out on electronic states of propadienylidene (H2CCC), using results from negative-ion photoelectron spectroscopy. In addition to the previously characterized

${\tilde{X}}^1A_1$
X̃1A1 electronic state, spectroscopic features are observed that belong to five additional states: the low-lying
${\tilde{a}}^3B_1$
ã3B1
and
${\tilde{b}}^3A_2$
b̃3A2
states, as well as two excited singlets,
${\tilde{A}}^1A_2$
Ã1A2
and
${\tilde{B}}^1B_1$
B̃1B1
, and a higher-lying triplet,
${\tilde{c}}^3A_1$
c̃3A1
. Term energies (T0, in cm−1) for the excited states obtained from the data are: 10 354±11 (
${\tilde{a}}^3B_1$
ã3B1
); 11 950±30 (
${\tilde{b}}^3A_2$
b̃3A2
); 20 943±11 (
${\tilde{c}}^3A_1$
c̃3A1
); and 13 677±11 (
${\tilde{A}}^1A_2$
Ã1A2
). Strong vibronic coupling affects the
${\tilde{A}}^1A_2$
Ã1A2
and
${\tilde{B}}^1B_1$
B̃1B1
states as well as
${\tilde{a}}^3B_1$
ã3B1
and
${\tilde{b}}^3A_2$
b̃3A2
and has profound effects on the spectrum. As a result, only a weak, broadened band is observed in the energy region where the origin of the
${\tilde{B}}^1B_1$
B̃1B1
state is expected. The assignments here are supported by high-level coupled-cluster calculations and spectral simulations based on a vibronic coupling Hamiltonian. A result of astrophysical interest is that the present study supports the idea that a broad absorption band found at 5450 Å by cavity ringdown spectroscopy (and coincident with a diffuse interstellar band) is carried by the
${\tilde{B}}^1B_1$
B̃1B1
state of H2CCC.

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While CCSD(T) has proven to be a very accurate (and affordable) treatment of electron correlation for “single reference” cases, a proper linear response approach for this method cannot be formulated. Hence, while we have used CCSD(T) for the
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X̃1A1
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Term energies (Te, in cm−1) calculated with the ANO0 basis (using the ANO1 geometries) at the CCSDT and CCSDTQ levels of theory are: 10 372 and 10 470 (
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ã3B1
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b̃3A2
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c̃3A1
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Ã1A2
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B̃1B1
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This is of course overly simplistic, as both q2 and q4 include some mixing with the “other” CC distance. A more complete picture is this: q2 is an out-of-phase combination of the two CC stretches, with the outer C2C3 distance dominating; q4 is in-phase, with the inner C1C2 distance playing the dominant role.
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The prominence of both features—as well as the absence of the bending fundamental (ν3) which would be between them—is easily explained by the geometry shifts that occur upon electron detachment from the anion (see Tables I and IV).
74.
The tail probably contains a minor contribution from a combination level, ν2 + 2ν6. The simulation, however, predicts this contribution to be much smaller than that from the 2ν5 level. Also, the eBE for the ν2 + 2ν6 level is significantly higher than that for the 2ν5 level.
75.
That the predicted ω6 for the
${\tilde{a}}^3B_1$
ã3B1
state is very much in line with the other electronic states (see Table IV) also argues against strong coupling involving this mode in the present case. In addition, and providing further support for the assignment, there is a pronounced difference between ω9 in the anion and the
${\tilde{b}}^3A_2$
b̃3A2
state, which suggests that it should have reasonable Franck-Condon activity for even-quantum transitions in the
${\tilde{a}}^3B_1$
ã3B1
state.
76.
In this regard, there is some additional reproducible signal in the SEVI spectrum at an eBE near 25 600 cm−1 that could plausibly be assigned to 4ν9. Although the magnitude of the anharmonicity here is substantial, its direction is consistent with vibronic coupling involving the higher-lying
${\tilde{b}}^3A_2$
b̃3A2
state, and its magnitude is actually quite similar to that obtained in the model Hamiltonian calculations.
77.
It should be noted that both the ν5 + ν6 and ν8 + ν9 combination levels (both of which have overall a1 vibrational symmetry) are expected to occur very near this region, and the possibility certainly exists that a Fermi resonance involving one or both of these combination levels and ν4 causes the apparent splitting of this peak. The vibronic simulation—which reproduces this region of the spectrum quite well—predicts that the ν8 + ν9 combination level will have appreciable intensity and will also be located just below the stronger ν4, just as observed. While modes q5 and q6 are not included in the simulations, this finding suggests that it is the ν8 + ν9 level that manifests itself in the fine structure of the SEVI spectrum and not ν5 + ν6. The absence of a ν5 + ν6 in the simulated spectrum is, of course, due to the fact that these modes were excluded from the simulations for the coupled triplet states. This was done because the harmonic frequencies of these two modes are quite similar in the anion,
${\tilde{a}}^3B_1$
ã3B1
, and
${\tilde{b}}^3A_2$
b̃3A2
triplet states. Nevertheless, a simulation was run that includes these modes, and essentially no intensity is observed for the combination level. Therefore, we are confident that ν8 + ν9 is the observed feature.
78.
The photodetachment cross section in the near-threshold region has been studied with a procedure similar to that employed by Brauman and co-workers (Ref. 88) as adopted by Krylov (Ref. 89). The Dyson orbitals for photodetachment to the A2 and B1 states were calculated with EOMIP-CCSD theory to evaluate the transition dipole matrix elements. Also, the plane wave function for the scattering electron was orthogonalized with the Dyson orbital as well as with those occupied anion molecular orbitals that have non-zero transition moment with the Dyson orbital. This model predicts significantly larger cross sections for detachment to the B1 state than to the A2 state in the near-threshold region.
79.
The stick spectrum and convoluted spectral profile make the assumption that the photodetachment cross sections for the two states are equal. However, when the results of the model calculation for the photodetachment cross section discussed in Ref. 78 are taken into account, agreement between the experiment and the spectral simulation improves.
80.
One piece of evidence that contributes to support the strong vibronic coupling between the
${\tilde{a}}^3B_1$
ã3B1
and
${\tilde{b}}^3A_2$
b̃3A2
states that is clearly central to the analysis of this part of the spectrum is the near absence of the clearly Franck-Condon active q2 mode (see Table IV) in the
${\tilde{a}}^3B_1$
ã3B1
state in the observed spectrum. From the ab initio calculations, one would expect to see ν2 at an eBE near 26 800 cm−1, which is in the general location of the “third” peak discussed in the main text. The stick spectrum suggests that the ν2 level is heavily split by the vibronic interaction with its intensity spread out over a range of energy that “takes away” what would otherwise be a conspicuous peak.
81.
While the peak at eBE = 28 160 cm−1 is in (nearly) the right place to be associated with ν2 of the
${\tilde{b}}^3A_2$
b̃3A2
state, the position is nevertheless about 100 cm−1 above where the ν2 level would be expected based on the ab initio calculations. Moreover, the simulation of the photodetachment spectrum for the excited singlet final states shows that the
${\tilde{A}}^1A_2$
Ã1A2
origin is expected to be strong, which is consistent with this assignment rather than the ν2 fundamental of the
${\tilde{b}}^3A_2$
b̃3A2
state.
82.
That is, the geometry difference in q2 is smaller between the anion and the
${\tilde{B}}^1B_1$
B̃1B1
state than between the
${\tilde{X}}^1A_1$
X̃1A1
and
${\tilde{B}}^1B_1$
B̃1B1
states.
83.
The empirical shift of the gap between the
${\tilde{A}}^1A_2$
Ã1A2
and
${\tilde{B}}^1B_1$
B̃1B1
states was made so that the “ν2” band becomes coincident with the experimental feature. In addition, the direction of the shift is consistent with the correction arising from quadruple excitations (see Table IV), the magnitudes being 800 (empirical shift) and 300 cm−1 (CCSDTQ-CCSDT), respectively. Note that the effect of basis set (see Table IV and Ref. 67) is greater for the
${\tilde{B}}^1B_1$
B̃1B1
state, so that one expects further augmentation to narrow the gap yet more. To further address this issue, energies of the
${\tilde{A}}^1A_2$
Ã1A2
and
${\tilde{B}}^1B_1$
B̃1B1
states were calculated at the CCSD level of theory with the cc-pVQZ and cc-pV5Z basis sets, and then extrapolated to the basis set limit. The difference between this extrapolated limit (at the CCSD level) and the corresponding ANO1 calculation (all of these values correspond to the frozen-core approximation) is −266 cm−1. Finally, the effect of the frozen-core treatment was investigated by calculating frozen-core and all-electron energies using the cc-pCVTZ basis set; this found that the all-electron gap between the
${\tilde{A}}^1A_2$
Ã1A2
and
${\tilde{B}}^1B_1$
B̃1B1
states is 60 cm−1 lower than the frozen core value. Taken together, the ANO1/CCSDT vertical gap of 3573 cm−1 between the
${\tilde{A}}^1A_2$
Ã1A2
and
${\tilde{B}}^1B_1$
B̃1B1
states should be reduced by 626 (=300 + 266 + 60) cm−1, a magnitude that is broadly consistent with that applied to the Hamiltonian to best reproduce the experimental spectrum in this region.
84.
Of course, the smallest member of this series is vinylidene (H2C=C:), but this species isomerizes to acetylene on a picosecond time scale [see
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Ervin
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W. C.
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1989
)] and is consequently not a realistic candidate for interstellar observation.
85.
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Walker
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