Peter Sebald

Spectroscopic properties of CS and HCS+ from ab initio calculations

Abstract Spectroscopic properties have been calculated for CS and HCS + by SCF, CI, and CEPA. The calculated dipole moments of 12 C 32 S in the lowest two vibrational states showed excellent agreement with experiment, and the integrated molar IR intensity for the 1 ← 0 transition was predicted to be 6254 cm 2 mol −1 . Equilibrium bond lengths of r e = 1.080 A and R e = 1.475 A were predicted for HCS + . The band origins of the fundamentals of H 12 C 32 S + and D 12 C 32 S + (in parentheses) should occur close to 3154 (2399), 736 (573), and 1410 (1344), all values being given in cm −1 . Intensities of the stretching vibrations of H 12 C 32 S + and D 12 C 32 S + were reported. The proton affinity of CS at T = 0 K was calculated to be 793 kJ mol −1 , which is in unsatisfactory agreement with McAllisters experimental value. The heat of formation of HCS + was predicted to be Δ H f ,0 0 = 1009 kJ mol −1 , and heats of reactions of several reactions producing HCS + were reported.

Calculated spectroscopic properties for NCCN, CNCN, CNNC AND HNCCN

Abstract Ab initio calculations employing the coupled electron pair approximation (CEPA) have been carried out for NCCN, CNCN, CNNC and HNCCN+. CNCN and CNNC are less stable than NCCN by 102 and 302 kJ mol−1, respectively, and various spectroscopic properties are predicted for these species. The most intense IR absorption of HNCCN+ is provided by the ν1 band with origin predicted at 3445 ± 5 cm−1 (intensity: 898 km mol−1). The calculated proton affinity of NCCN at 298 K is 657 kJ mol−1, in reasonable agreement with a recent experimental value of 674 ± 8 kJ mol−1.

A theoretical investigation of C5

Abstract Large-scale CEPA-1 calculations have been carried out for linear C 5 , a molecule of substantial interest to combustion processes and astrochemistry. The equilibrium bond lengths are predicted to be 1.289 A (outer CC bond) and 1.283 A (inner CC bond), with an accuracy of 0.002 A. The calculated ν 3 band origins of 2161 cm −1 (105 CGTO basis) and 2137 cm −1 (150 CGTO basis) are in good agreement with the experimental value of 2169 cm −1 . This band has an extremely large transition moment of 0.74 D. The less intense stretching fundamental ν 4 (μ=0.18 D) is predicted to occur at 1478 ± 10 cm −1 . Predictions for the totally symmetric stretching and the bending vibrational frequencies (in cm −1 ) are 2008 (1σ g + ), 792 (2σ g + ), 570 (1π u ), 209 (1π g ) and 119 (2π u ).

Vibrational frequencies from anharmonic ab initio/empirical potential energy functions: Stretching vibrations of hydroisocyanic acid, phosphaethyne, isocyanoacetylene, and phosphabutadiyne

Abstract Anharmonic potential energy functions for the stretching vibrations of HNC, HCP, HCP + , HC 2 NC, and HC 3 P have been constructed from ab initio calculations and little experimental information. Stretching vibrational frequencies are calculated by a variational method employing an approximate vibrational Hamiltonian which neglects the anharmonic coupling between stretching and bending modes. Equilibrium geometries are estimated for HC 2 NC and HC 3 P and quartic and sextic centrifugal distortion constants have been calculated.

The millimeter-wave spectra of FN2+ and FCO+ and ab initio calculations for FCN, FCO+, FN2+, and FNC

Abstract The millimeter-wave spectra of FN 2 + and FCO + were obtained in a liquid-nitrogen-cooled discharge of a mixture of F 2 and N 2 or CO. Fifteen and thirteen lines were measured for FN 2 + and FCO + , respectively, from the positions of which rotational constants of 11 154.83371(70) and 10 768.6958(11) MHz were derived. Accompanying ab initio calculations predict ground state dipole moments of 0.556 and −0.269 D for the two cations. An unusually large wavenumber of 2456 cm −1 is obtained for ν 3 , the pseudoasymmetric stretching vibration of FCO + . The ground state rotational constant and the electric dipole moment of isoelectronic FNC, which is less stable than FCN by 285 kJ mol −1 , are predicted to be 10 881(50) MHz and −1.583(30) D, respectively.

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.

The equilibrium geometry of HOSi

Abstract The equilibrium bond lengths of linear HOSi+ have been calculated by combination of experimental and ab initio data to be r e = 0.9616(10) A and R e = 1.5302(3) A , where somewhat conservative error estimates are given in parentheses. Rather unusual for a linear triatomic molecule, the vibration-rotation coupling constant α2 was found to be positive for HOSi+, whereas deuterium substitution leads to the common negative sign. The bending vibrational wavenumber of HOSi+ is predicted to be 373 cm−1, with an uncertainty of ca. 10 cm−1.

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.

Accurate bond dissociation energies (D 0) for FHF− isotopologues

Accurate bond dissociation energies (D 0) are determined for three isotopologues of the bifluoride ion (FHF−). While the zero-point vibrational contributions are taken from our previous work (P. Sebald, A. Bargholz, R. Oswald, C. Stein, P. Botschwina, J. Phys. Chem. A, DOI: 10.1021/jp3123677), the equilibrium dissociation energy (D e ) of the reaction was obtained by a composite method including frozen-core (fc) CCSD(T) calculations with basis sets up to cardinal number n = 7 followed by extrapolation to the complete basis set limit. Smaller terms beyond fc-CCSD(T) cancel each other almost completely. The D 0 values of FHF−, FDF−, and FTF− are predicted to be 15,176, 15,191, and 15,198 cm−1, respectively, with an uncertainty of ca. 15 cm−1.

A theoretical investigation of H2C4H+ and the proton affinity of HC4H

Abstract A six-dimensional anharmonic potential and electric-dipole-moment surface have been calculated for H 2 C 4 H + , an ion of interest to combustion processes and astrochemistry, using CEPA-1 and a basis set of 102 contracted Gaussian-type orbitals. Vibrational frequencies and IR intensities are calculated variationally from an approximate vibrational Hamiltonian. The ν 3 band of H 2 C 4 H + , with predicted origin of 2010±10 cm −1 , has a large IR intensity and should be suitable for a forthcoming diode-laser investigation. The equilibrium dipole moment is as small as −0.085 D and H 2 C 4 H + is thus no good candidate for radioastronomy. The proton affinity of HC 4 H is calculated to be 741 kJ mol −1 , in reasonable agreement with an experimental value of 753±4 kJ mol −1 .