Rainer Oswald

Ab initio spectroscopic constants and the equilibrium geometry of HCCF

Abstract An accurate equilibrium geometry has been obtained for HCCF by combination of experimental and ab initio data at the CCSD (T) level: r e (CH) = 1.0591 A, R 1e (CC) = 1.1961 A and R 2e (CF) = 1.2765 A. Calculated and experimental spectroscopic constants for various isotopomers of monofluoroacetylene compare well with each other and many predictions are made. The performance of CCSD (T) for the equilibrium geometries of ten different molecules is examined and excellent agreement with the best available equilibrium structures is found.

Spectroscopic and theoretical characterization of linear centrosymmetric N≡N⋅⋅H+⋅⋅N≡N

The first high resolution infrared spectrum of the ionic complex N2⋅⋅H+⋅⋅N2 and its deuterated derivative is reported. The spectra were obtained in direct absorption in a supersonic slit nozzle plasma. The observed rovibrational transitions were assigned to the ν3 antisymmetric NN stretching vibration and the spectrum is consistent with a linear centrosymmetric equilibrium structure. The band origin is found at 2352.2364(6) cm−1 and the ground state rotational constant is determined as B″=0.081 809(14) cm−1. The assignment is supported by ab initio calculations including electron correlation effects. The best estimate for the equilibrium structure is Re (NN)=1.095 A and re (N⋅⋅H)=1.277 A. The transition moment of the ν3 band of N2⋅⋅H+⋅⋅N2 is predicted to be 0.21 D, an order of magnitude larger than for the NN stretching vibration of HN2+. The equilibrium dissociation energy De for fragmentation into N2 and HN2+ is calculated to be ∼5900 cm−1.

Coupled cluster calculations for HC7N, HC7NH+ and C7N, molecules of interest to astrochemistry

Large-scale coupled cluster calculations were carried out for linear HC7N, HC7NH+ and C7N (2Π and 2Σ states). Accurate equilibrium geometries (bond lengths accurate to better than 0·001 a) are established. The ground-state rotational constant of HC7 NH+ is predicted to be B 0= 553·8 ±0·5 MHz. Owing to its large equilibrium dipole moment of 6·35 D it may be a suitable candidate for forthcoming experimental investigation by microwave spectroscopy or radio astronomy. Various predictions are made for HC7N and HC7NH+ in order to facilitate their detection by means of infrared spectroscopy. Partially restricted open-shell coupled cluster calculations including connected triple substitutions yield the 2Π and 2Σ states of C7N to be very close in energy and substantial rovibronic interaction is expected.

Preparation of Ti(IV) fluoride N-heterocyclic carbene complexes.

1,3,4,5-Tetramethylimidazol-2-ylidene (L(Me)) and 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (L(iPr )) readily form complexes of trans-TiF4(L(Me))2 (1) and of trans-TiF4(L(iPr))2 (4) with TiF4 in THF, respectively. Complex 1 has been used as a precursor for preparing the Ti(IV) fluoride carbene complexes [{TiF2(L(Me))(NEt 2)}2(mu-F)2] (2) and (TiF4(L(Me))2)(NacNacLi) (3) (NacNac = HC(CMeN(2,6- iPr2C6H3))2). Complex 2 was prepared from the reaction of 1-3 equiv of 1 and 1 equiv of Ti(NEt2)4 or by reacting TiF4 with Ti(NEt2)4 and L(Me) in toluene. Complex 3 has been prepared from 1 and NacNacLi in toluene. Reaction of 1 and AlMe3 in toluene results in ligand transfer and formation of AlMe3(L(Me)). Complex 4 is unstable in solution at room temperature and degrades with formation of [HL(iPr)][TiF5(L(iPr))] (5). Complexes 1, 2.2CH2Cl2, 4, and 5 were characterized by single crystal X-ray structural analysis, elemental analysis, IR and NMR spectroscopy, and mass spectrometry. The relative basicities of L(Me), L (iPr), and the donor ligands THF, pyridine, DMSO, and H2O as well as [Cl](-) and [F](-) toward the Ti(IV) pentafluoride anion were established by NMR and confirmed by density functional theory (DFT) calculations. L(Me) and L(iPr ) are more basic than the mentioned molecular donors and more basic than chloride, however less basic than fluoride.

Quantum-chemical investigations of small molecular anions

Abstract Dedicated to Dr Gerhard Herzberg on the occasion of his 90th birthday Recent large-scale ab initio calculations for small negative molecular ions are reviewed. Accurate equilibrium geometries are established for several species like NH2−, HCC−, NO2 −, CH2N− C5 − and C6 −. Predictions are made for various spectroscopic properties like vibrational frequencies, rotational constants and infrared intensities. The effects of a shallow energy minimum in the T-shaped configuration on the rovibrational term energies of HCC − are investigated. The calculated vibrational structures of the photoelectron spectra of SiH3 − and CH2N − are in very good agreement with the experiment. The present calculations support the assignment of an absorption observed at 608 nm in a neon matrix to the [Ctilde]2Πg ← [Xtilde]2Πu transition of C6 −. Electron affinities are obtained with an accuracy of 0-05 eV or better. Spectroscopic properties of intermediates in simple SN2 reactions are calculated.

The ν1 and ν2 bands of Ar⋯HN2+: A joint theoretical/experimental study

A combined theoretical and experimental study of the ν1 and ν2 stretching vibrations of Ar⋯HN2+ is presented. These correlate asymptotically with the almost local NH and NN stretching vibrations of free HN2+, but undergo mode mixing and are strongly influenced by vibrational anharmonicity in the complex. The first observation of the strong ν2 band by means of diode laser absorption spectroscopy in a supersonic planar plasma is reported. Its band origin is found at 2041.1802(3) cm−1. New results are presented for the ν1 band with origin at 2505.5000(2) cm−1. The ground-state rotational constant is determined from both band systems as 0.080 868(6) cm−1.

Carbon chains of type C2n+1N− (n=2–6): A theoretical study of potential interstellar anions

Linear anions of type C(2n+1)N(-) (n=2-6), which are expected to be good candidates for experimental investigation by microwave spectroscopy and radio astronomy, were studied by means of the coupled cluster variant CCSD(T). Making use of corrections taken over from HC(3)NC(3)N(-) and HC(5)N, accurate equilibrium structures ( approximately 0.0005 A accuracy in bond lengths) have been established for all five anions. The electric dipole moments increase strongly with increasing chain length. For C(13)N(-), a very large equilibrium dipole moment of 16.53 D (with respect to center-of-mass coordinate system, negative end of dipole at terminal carbon site) is predicted. The lowest vertical detachment energies, leading to (2)Sigma states of the radicals for C(3)N(-) and C(5)N(-) and to (2)Pi states in the case of the larger anions, are calculated to lie in the range of 4.40-4.63 eV. The ground-state rotational and quartic centrifugal distortion constants of C(5)N(-) are predicted to be 1389.4 MHz and 33.8 Hz, respectively. All anions studied appear to be fairly normal semirigid linear molecules. Throughout, good agreement with available matrix isolation IR spectroscopic data is obtained and many predictions of spectroscopic properties are made.

FHF- isotopologues: highly anharmonic hydrogen-bonded systems with strong Coriolis interaction.

Explicitly correlated coupled cluster theory at the CCSD(T*)-F12b level in conjunction with the aug-cc-pV5Z basis set has been used in the calculation of three-dimensional potential energy and dipole moment surfaces for the bifluoride ion (FHF(-)). An empirically corrected analytical potential energy function (PEF) was obtained by fit to four pieces of accurate spectroscopic information. That PEF was used in variational calculations of energies and wave functions for a variety of rovibrational states of the isotopologues FHF(-), FDF(-), and FTF(-). Excellent agreement with available data from IR laser diode spectroscopy is observed and many predictions are being made. Unusual isotope effects among the spectroscopic constants and unusual features of the calculated line spectra are discussed.

Theoretical investigations of free radicals and negative molecular ions and their calculated photoelectron spectra

Abstract Large-scale coupled cluster or coupled electron pair calculations have been carried out for the radicals H 2 CCCH, CF 3 and CCl 2 F, the anions SiC 4 − and Si 2 C 5 − as well as the corresponding (N−1) electron systems. Accurate equilibrium structures are established in all cases. For the adiabatic ionization potential (IP ad ) of CF 3 , the calculations strongly favour the higher experimental value (9.05±0.01 eV). The photoelectron (PE) spectrum of CCl 2 F is predicted to have a very complex vibrational structure; IP ad (CCl 2 F) is predicted to be 8.22 eV. The PE spectrum of SiC 4 − shows some excitation in the ν 4 and ν 1 stretching vibrational modes and the PE spectrum of Si 2 C 5 − is dominated by the adiabatic peak.

STRONG THEORETICAL SUPPORT FOR THE ASSIGNMENT OF B11244 TO l-C3H+

Highly accurate quantum chemical calculations beyond CCSD(T) have been used to study the molecular cation l-C3H+ which is the carrier of harmonically related radio lines observed in the Horsehead photodissociation region and toward Sgr B2(N). Excellent agreement with spectroscopic and radioastronomical measurements is obtained for the rotational constant, with the calculated value of B 0 = 11246.4 MHz only 1.5 MHz or 0.01% above the experimental value. The unusually large ratio of centrifugal distortion constants D 0(exp.)/De(theor.) = 1.80 is attributed to the shallow CCC bending potential of l-C3H+ and is quantitatively reproduced by variational calculations within a pseudo-triatomic model. A comparative study of centrifugal distortion constants in a series of four linear interstellar molecules (C3N–, C3O, l-C3H+, and C3) is made and some general conclusions are drawn.