B. Bak

Microwave spectra of isotopic cyclobutenes: Molecular structure of cyclobutene

Abstract Microwave spectra of cyclobutene and four monosubstituted isotopic species have been recorded and analyzed, yielding a conventional r s -structure ( C 2v symmetry). The carbon ring is planar with valence angles 94.2° (at the double bond) and 85.8°. The C(1)-C(2) distance is 1.342 A, the C(2)-C(3) distance 1.517 A, and the C(3)-C(4) distance 1.566 A, all of these longer than any acyclic normals.

HCN and HNC dimers. A new and stable variant

Abstract A dimer of HCN, N-cyanoformimine or CH 2 NCN, hitherto only postulated as the repetitive unit of a polymer has been produced by pyrolysis (1200–1300 K) of dimethylcyanamide at low pressure (≈30 mtorr) and identified by simultaneous recording of its microwave spectrum (18.6–40.0 GHz). Later, the same dimer was detected in a sample of liquid HCN stored for 1–2 years at 277 K under exclusion of light. The new dimer is, therefore, stable under widely different conditions of pressure and temperature. The possibility of assigning the observed microwave spectrum to iminoacetonitrile or to cyclic azapropenylidenimine is discussed.

Rotational spectra of isotopic di-thioformic acids with a structure of trans HCSSH: Ab initio estimates of cis/trans structures, energies, dipole moments, and torsional barrier

Abstract Microwave transitions (18.6–40 GHz) of ground-level trans HC34S32SH, HC32S34SH, HC32S32S2H and of cis HC32S32S2H have been observed and assigned. Rotational and distortion constants have been calculated. These data with earlier data for trans/cis HC32S32SH are not sufficient to fix an rs or r0 structure for trans HCSSH. Instead, an “iterative” structure of trans HCSSH has been derived making full use of all data obtained for isotopic trans species. An ab initio estimate of the geometry of trans HCSSH agrees with the iterative structure within expectation. This also holds for the calculated energy difference ϵ(cis) - ϵ(trans) and for calculated dipole moment components μa along the a axis of least moment of inertia.

Production and microwave spectra of dithioformic acid, HCSSH

Abstract Hitherto unknown dithioformic acid, HCSSH, has been produced by pyrolysis (300°C) of methanetrithiol, HC(SH) 3 , and identified by the microwave spectrum (18–40 GHz) of its most abundant species H 12 C 32 S 32 SH. Rotational transitions of this species in its ground state have been assigned to a trans and a cis rotamer. Both rotamers are planar, the trans conformer being the more stable by ca. 350 cm −1 mole −1 . For each rotamer spectra of molecules in two vibrationally excited states have been assigned. Frequencies of transitions of importance for search for HCSSH in interstellar space are reported.

Formation, microwave spectrum and preliminary structure of selenoketene

Abstract Hitherto unknown selenoketene, CH 2 CSe, has been produced by pyrolysis of 1,2,3-selenodiazole and identified by observation of microwave absorption from CH 2 C&.dbnd; 80 Se and CH 2 C 78 Se. The CC and CSe bond lengths of selenoketene agree with bond lengths in ketene, CH 2 CO, and in carbonyl sulphide, OCS.

On the stability of NCNO and its dissociation into CN and NO radicals as treated by ab initio methods

Abstract Ab initio calculations have been performed in order to investigate the ground state potential energy surface of the gas phase reaction NC + NO ⇌ NCNO. Both restricted (RHF) and unrestricted (UHF) Hartree-Pock approximations and direct configuration interaction (CI) methods were used. Up to 24067 configurations were included in the CI expansion. The RHF solution was found to be unstable for all investigated geometries and appreciably lower energies were obtained in the UHF approximation. The CI expansion based on the RHF solution as the reference state and including all single and double replacements, as a result of this instability, also proved to give an inadequate description of the potential energy surface even at the equilibrium configuration. Such a wavefunction obviously fails to describe the system at large internuclear separations. A more correct description of the surface was obtained when a UHF wavefunction was used as the reference state.

On the structure of phenylisocyanide

Abstract Microwave spectra of phenylisocyanide, (I), 2-D-phenylisocyanide, 2,4-D 2 -phenylisocyanide, and 2,4,6-D 3 -phenylisocyanide were recorded and analyzed. Assuming the structural identity of the C 6 H(3)H(4)H(5) fragments of I and of phenylcyanide (II). A C(1)-N distance of 1.410 A and an distance of 1.142 A followed. The effects of changing some of the assumptions are discussed.

AB Initio approach to the cyanoform problem

Abstract Relative stabilities of suggested components of gaseous HC 4 N 3 have been calculated Ab Initio using a 7 s , 3 p basis set contracted to 4 s , 2 p . Molecular structures in accordance with experimental data for related molecules were assumed. The result is that cyanoform, HC(CN) 3 , dominates the gas phase under conditions of equilibrium. A competing species, “di-cyanoketenimine”, HNCC(CN) 2 , is ca. 10 kcal mol −1 higher in energy. According to (incomplete) ir data it may dominate the crystalline phase.

Experimental and ab initio evidence for preferred syn-conformation of thionyl amines, , X = H, CH3, and C6H5

Abstract The conformational syn—anti problem with respect to the bond in thionylamines, , has only partially been solved for X = H and CH 3 [1, 2]. By providing new experimental material for C 6 H 5 NSO and p -FC 6 H 4 NSO (vibrational spectra, low-resolution microwave spectra, dipole moment determinations) and by performing ab initio calculations for HNSO and CH 3 NSO it is shown that these two molecules are exclusively syn conformers The experimental data here presented strongly suggest the same conclusion for C 6 H 5 NSO. The result is consistent with a previous investigation by van Woerden and Bijl-Vlieger [3] comprising 13 mono- and polysubstituted sulfinylanilines.

The symmetry of azodicarbonitrile

Abstract The infrared and Raman spectra of azodicarbonitrile (I) made from cyanogen azide (II) were recorded under varied conditions of phase, temperature, concentration, and pressure and strongly indicate a C2h symmetry (trans-(I)). Cis-(I) was not shown positively either by these techniques or by microwave spectra. The fundamental vibration frequencies tentatively suggested for trans-(I) are (cm−1): v1 = 2176, v2 = 1422, v3 = 1002, v4= 741, v5 = 282 (Ag), v6 = 108 (Bg, v7 = 2204, v8 = 982, v9 = 904, v10 = 596 (Bu), v11 = 574, v12 = 133 (Au). Of these, v4 and v6 were not observed directly, but inferred from an analysis of a complex band in the infrared spectrum.