Kurt W. Sattelmeyer

On the photoelectron spectrum of p-benzoquinone

A high-resolution photoelectron spectrum of p-benzoquinone in the low energy (9.5–11.5 eV) region is reported and analyzed with the aid of simulations based on high-level ab initio calculations. The results generally support the notion that the two prominent spectral features in this region are each due to a pair of final ion states. The lower energy feature beginning near 10 eV is due to oxygen lone-pair ionizations, while that beginning near 11 eV comes from π electron removal. Contrary to previous interpretations of the spectrum, however, the results of this study indicate that the two π states are nearly degenerate, with the strongest peak in the photoelectron spectrum representing a convolution of the corresponding pair of 0–0 ionizations.

On the vertical excitation energy of cyclopentadiene.

The vertical excitation energy for the lowest valence pi-->pi(*) transition of cyclopentadiene is investigated. Using a combination of high-level theoretical methods and spectroscopic simulations, the vertical separation at the ground state geometry is estimated to be 5.43+/-0.05 eV. This value is intermediate between those calculated with coupled-cluster and multireference perturbation theory methods and is about 0.13 eV higher than the observed maximum in the absorption profile.

Binding energies of small lithium clusters (Lin) and hydrogenated lithium clusters (LinH)

Large coupled cluster computations utilizing the Dunning weighted correlation-consistent polarized core-valence (cc-pwCVXZ) hierarchy of basis sets have been conducted, resulting in a panoply of internally consistent geometries and atomization energies for small Li(n) and Li(n)H (n=1-4) clusters. In contrast to previous ab initio results, we predict a monotonic increase in atomization energies per atom with increasing cluster size for lithium clusters, in accordance with the historical Knudsen-effusion measurements of Wu. For hydrogenated lithium clusters, our results support previous theoretical work concerning the relatively low atomization energy per atom for Li(2)H compared to LiH and Li(3)H. The CCSD(T)/cc-pwCVQZ atomization energies for LiH, Li(2)H, Li(3)H, and the most stable isomer of Li(4)H, including zero-point energy corrections, are 55.7, 79.6, 113.0, and 130.6 kcal/mol, respectively. The latter results are not consistent with the most recent experiments of Wu.

Electronic Spectrum of Propadienylidene (H2C=C=C:) and its Relevance to the Diffuse Interstellar Bands

The electronic spectrum of singlet propadienylidene (H2C=C=C:) matrix isolated in neon at 6 K was measured. Propadienylidene is the only member of the homologous series of neutral carbon chain molecules H2C(=C)n to be observed by electronic spectroscopy. The spectrum displays three electronic transitions: two in the visible ( 1A2 ← 1A1 and 1B1 ← 1A1) and one in the UV ( 1A1 ← 1A1). The longest wavelength transition ( 1A2 ← 1A1) is symmetry forbidden but is weakly seen in the spectrum via vibronic coupling with the 1B1 state. The other two transitions exhibit well-resolved vibrational progressions. The high density of absorptions in the visible region confirms earlier suggestions that the H2C(=C)n family of cumulene carbenes merits careful consideration as potential carriers of diffuse interstellar bands.

A comparison of excited state properties for iterative approximate triples linear response coupled cluster methods

Abstract A computational study of the potential energy curves of the 1 Π state of BH, 1 Π state of CH + , 1 Σ u and 1 Π u states of C 2 , 1 Π state of CO, and 1 Π g and 1 Σ − u states of N 2 is carried out with the CC3 and CCSDT-3 corrections to EOMEE-CCSD. Good agreement in structure, vibrational frequencies, and excitation energies of these iterative triples-corrected methods with respect to experiment is found for most of these examples. However, deficiencies in the approximate treatment of triples is evident for BH and CH + .

The ν5 vibrational frequency of the vinyl radical: Conflict between theory and experiment

A balanced theoretical treatment of the X 2A′ state of the vinyl radical is performed via application of equation-of-motion coupled cluster theory for ionized states from the anion. Good agreement with experiment is generally found. The only exception is the ν5 vibrational mode, which seems to have been misassigned experimentally.

The global minimum structure of SiC3: The controversy continues

The linear triplet structure of SiC3 (with terminal Si) has been investigated using coupled cluster methods, including a perturbative treatment of triple excitations, and Dunning’s correlation-consistent polarized core-valence quadrupole zeta basis set (CCSD(T)/cc-pCVQZ). Based on the demonstrated convergence with respect to correlation and the large basis set employed, this triplet isomer appears to be energetically less stable than the two ring isomers, which have either a C–C (2s) or a Si–C (3s) transannular bond, by 7.5 and 1.3 kcal /mole, respectively.The linear triplet structure of SiC3 (with terminal Si) has been investigated using coupled cluster methods, including a perturbative treatment of triple excitations, and Dunning’s correlation-consistent polarized core-valence quadrupole zeta basis set (CCSD(T)/cc-pCVQZ). Based on the demonstrated convergence with respect to correlation and the large basis set employed, this triplet isomer appears to be energetically less stable than the two ring isomers, which have either a C–C (2s) or a Si–C (3s) transannular bond, by 7.5 and 1.3 kcal /mole, respectively.

Characterization of the three lowest-lying singlet electronic states of AlOH

Two linear (1Σ+ and 1Π) and three bent (1 1A′, 2 1A′, and 1 1A″) lowest-lying electronic singlet states of AlOH have been systematically investigated employing ab initio self-consistent-field, configuration interaction with single and double excitations, coupled cluster with single and double excitations (CCSD), CCSD with perturbative triple excitations [CCSD(T)], and CCSD with iterative partial triple excitations (CCSDT-3 and CC3) quantum mechanical methods with basis sets up to augmented correlation consistent polarized valence quadruple zeta (aug-cc-pVQZ). The linear 1Σ+ state is found to be a remarkably low-energy transition state between the two equivalent bent 1 1A′ structures, while the linear 1Π state is a second-order saddle point, which leads to the bent 2 1A′ and 1 1A″ states. The bent ground (X 1A′) state of AlOH is predicted to have a bond angle of 157° at the aug-cc-pVQZ CC3 level of theory and is classified as a quasilinear molecule, confirming previous studies. Employing the equation-of-m...

Elusive electron affinity of ClF

Highly correlated methods were used to obtain the optimized bond lengths and vibrational frequencies of ClF and ClF−. With convergent quantum mechanical methods, the anion is much more difficult to treat than neutral ClF. Adiabatic electron affinities (EAad), vertical electron affinities, and vertical detachment energies have been evaluated and compared to the controversial experimental values reported in the literature. Our best prediction for the zero-point vibrationally corrected EAad is 2.25±0.1 eV.

The equilibrium structure of the ammonium radical Rydberg ground state

The equilibrium structure of the ammonium radical is determined by an extrapolation procedure of large [up to CCSD(T)/aug-cc-pV5Z] coupled cluster calculations and analysis of experimental rotational constants corrected for effects of rotation–vibration interaction. These approaches yield re values of 1.0367 and 1.0363 A, respectively. The small difference serves as a valuable internal consistency check and suggests that re in NH4 lies in the range 1.0365±0.0005 A.