Quantum-chemical investigations of small molecular anions

Ab Initio Rovibrational Spectroscopy of the Acetylide Anion

In this work the rovibrational spectrum of the acetylide anion HCC- is investigated using high-level electronic structure methods and variational rovibrational calculations. Using a composite approach the potential energy surface and dipole surface is constructed from explicitly correlated coupled-cluster accounting for corrections due to core-valence correlation, scalar relativistic effects and higher-order excitation effects. Previous approaches for approximating the latter are critically evaluated. Employing the composite potential, accurate spectroscopic parameters determined from variational calculations are presented. In comparison to the few available reference data the present results show excellent agreement with ground state rotational constants within 0.005% of the experimental value. Intensities determined from the variational calculations suggest the bending fundamental transition ν2 around 510 cm-1 to be the best target for detection. The rather weak CD stretching fundamental ν1 in deuterated isotopologues show a second-order resonance with the (0,20,1) state and the consequences are discussed in some detail. The spectroscopic parameters and band intensities provided for a number of vibrational bands in isotopologues of the acetylide anion should facilitate future spectroscopic investigations.

First Laboratory Detection of N13CO- and Semiexperimental Equilibrium Structure of the NCO- Anion

The cyanate anion (NCO^–) is a species of considerable astrophysical relevance. It is widely believed to be embedded in interstellar ices present in young stellar objects but has not yet been detected in the dense gas of the interstellar medium. Here we report highly accurate laboratory measurements of the rotational spectrum of the N¹³CO^– isotopologue at submillimeter wavelengths along with the detection of three additional lines of the parent isotopologue up to 437.4 GHz. With this new data, the rotational spectrum of both isotopologues can be predicted to better 0.25 km s^–1 in equivalent radial velocity up to 1 THz, more than adequate for an astronomical search in any source. Moreover, a semiexperimental equilibrium structure of the anion is derived by combining the experimental ground-state rotational constants of the two isotopologues with theoretical vibrational corrections, obtained by using the coupled-cluster method with inclusion of single and double excitations and perturbative inclusion of triple excitations (CCSD(T)). The estimated accuracy of the two bond distances is on the order of 5 × 10^–4 Å: a comparison to the values obtained by geometry optimization with the CCSD(T) method and the use of a composite scheme, including additivity and basis-set extrapolation techniques, reveals that this theoretical procedure is very accurate.

Description of the Methylene Amidogene Radical and Its Anion with an Economical Treatment of Correlation Effects Using Density Functional Theory Orbitals

The ground and low-lying excited state electronic structural properties (such as equilibrium geometries, harmonic frequencies, excitation energies, barrier energy, and so on) of the methylene amidogene radical (H₂CN) and its anion (H₂CN^-) have been studied using the CASCI (complete active space configuration interaction) and SSMRPT (state-specific multireference Møller-Plesset perturbation theory) methods with density function theory (DFT) orbitals. Here, the span of the active orbitals have been obtained from Kohn-Sham DFT using B3LYP exchange-correlation functionals in the CASCI (DFT-CASCI) approximation to describe nondynamic correlation associated with electronic degeneracies. The DFT-SSMRPT protocol provides an attractive way to deal with both dynamical and nondynamical correlation effects in strongly correlated systems such as H₂CN and H₂CN^-. The present work clearly indicates that the electronic absorption band near 35,050 cm^-1 corresponds to the B̃²A₁ ← X̃²B₂ transition. DFT-SSMRPT findings are in close agreement with high-level theoretical estimates. It is concluded that the transition at 1725 cm^-1 could be due to the CN stretching of the trans-HCNH isomer which is originally assigned to the CN stretch of H₂CN in the experiment. The present results confirm most of the previous vibrational assignments. It is not possible to definitively assign a transition to the 35,600 cm^-1 band with the present estimations, suggesting further experiment is urgently called for.

A detailed analysis of the spin-crossover reaction of H2S binding to heme and the six-coordinated FeP(Im)-HS- porphyrin complex

The potential energy surfaces of the H₂S binding to iron-porphyrin (FeP) with the imidazole (Im) ligand via intersystem crossings are investigated by using density functional theory. The minimum energy intersystem crossing point (MEISCP) between the quintet and triplet states (MEISCP_TQ) for the Fe(II)P(Im)-H₂S complex is located at a Fe-S distance of 3.39 Å with only 1.1 kcal/mol above the quintet state minimum. The second spin-crossover point, where a change from the triplet to the singlet state occurs, comes at a much shorter Fe-S distance of 2.79 Å, and the MEISCP_ST is located at 3.7 kcal/mol above the triplet state minimum. The nature of the chemical bonding along the Fe-S reaction coordinate from the ground state singlet to the quintet state along the path to the separated species is analyzed. An inspection of the vibrational modes reveals that the largest contribution to the triplet-quintet transition around the quintet and triplet state minimum comes from the symmetric shrinking of the pyrrole units of the porphyrin ring, indicating that the related reaction coordinate plays a main role in the intersystem crossing. The fully optimized structures of the Fe(II)P(Im)-HS^- complex corresponding to three different spin multiplicities (M = 1, 3, 5) are characterized by a bent Fe-H-S conformation. The binding of the hydrosulfide anion to Fe(II)P(Im) in the quintet state induces a 0.2 Å displacement of the Fe atom out of the nitrogen porphyrin (N_pyr) plane. The fully optimized structure of the ground state of Fe(II)P(Im)-HS^- agrees well with experimental data for the corresponding heme models.

Ab initio characterization of size dependence of electronic spectra for linear anionic carbon clusters C(n) (-) (n = 4-17)

In this article, we determine the ground-state equilibrium geometries of the linear anionic carbon clusters C n- (n = 4-17) by means of the density functional theory B3LYP, CAM-B3LYP, and coupled cluster CCSD(T) calculations, as well as their electronic spectra obtained by the multireference second-order perturbation theory CASPT2 method. These studies indicate that these linear anions possess doublet ²∏(g) or ²∏(u) ground state, and the even-numbered clusters are generally acetylenic, whereas the odd-numbered ones are essentially cumulenic. The energy differences, electron affinities, and incremental binding energies of C n- chains all exhibit a notable tread of parity alternation, with n-even chains being more stable than n-odd ones. In addition, the predicted vertical excitation energies from the ground state to four low-lying excited states are in reasonably good agreement with the available experimental observations, and the calculations for the higher excited electronic transitions can provide accurate information for the experimentalists and spectroscopists. Interestingly, the absorption wavelengths of the 1²∏(u/g) ← X²∏(g/u) transitions of the n-even clusters show a nonlinear trend of exponential growth, whereas those of the n-odd counterparts are found to obey a linear relationship as a function of the chain size, as shown experimentally. Moreover, the absorption wavelengths of the transitions to the higher excited states of C n- series have the similar linear size dependence as well.

The rotational spectrum of CN−

The rotational spectrum of the molecular negative ion CN(-) has been detected in the laboratory at high resolution. The four lowest transitions were observed in a low pressure glow discharge through C(2)N(2) and N(2). Conclusive evidence for the identification was provided by well-resolved nitrogen quadrupole hyperfine structure in the lowest rotational transition, and a measurable Doppler shift owing to ion drift in the positive column of the discharge. Three spectroscopic constants (B, D, and eQq) reproduce the observed spectrum to within one part in 10(7) or better, allowing the entire rotational spectrum to be calculated well into the far IR to within 1 km s(-1) in equivalent radial velocity. CN(-) is an excellent candidate for astronomical detection, because the CN radical is observed in many galactic molecular sources, the electron binding energy of CN(-) is large, and calculations indicate CN(-) should be detectable in IRC+10216-the carbon star where C(6)H(-) has recently been observed. The fairly high concentration of CN(-) in the discharge implies that other molecular anions containing the nitrile group may be within reach.

A fully ab initio potential curve of near-spectroscopic quality for OH- ion: importance of connected quadruple excitations and scalar relativistic effects

A benchmark study has been carried out on the ground-state potential curve of the hydroxyl anion, OH-, including detailed calibration of both the l-particle and n-particle basis sets. The CCSD(T) basis set limit overestimates omega(e) by about 10 cm(-1), which is only remedied by inclusion of connected quadruple excitations in the coupled cluster expansion--or, equivalently, the inclusion of the 2pi orbitals in the active space of a multireference calculation. Upon inclusion of scalar relativistic effects (-3 cm(-1) on omega(e)), a potential curve of spectroscopic quality (sub-cm(-1) accuracy) is obtained. Our best computed EA(OH), 1.828 eV, agrees to three decimal places with the best available experimental value. Our best computed dissociation energies, D0(OH-) = 4.7796 eV and D0(OH) = 4.4124 eV, suggest that the experimental D0(OH) = 4.392 eV may possibly be about 0.02 eV too low.