Volker Typke

On the molecular structure of X-CF3 molecules (X = Cl, Br, I)

Abstract The average molecular structures of X-CF 3 , molecules (X = Cl, Br, I) have been determined by combined use of electron diffraction and microwave data. Including results for X = H and X = F a strict correlation between C-F bond length and the electronegativity of the X atom is observed. This correlation can be nicely understood in terms of the electron distribution calculated in the CNDO/2 approximation. It is also observed that the change in the C-X bond length with substitution of the CF 3 -group by a CH 3 -group is strictly correlated with the electronegativity of the X atom.

Microwave spectra, centrifugal distortion constants, and rz structure of acetonitrile and its isotopic species

Abstract The ground-state microwave spectra of acetonitrile and acetonitrile-d3 and their 13C and 15N isotopic species have been remeasured between 8 and 240 GHz. The derived quartic centrifugal distorition constants are in very good agreement with the values calculated from the general harmonic force field. The rotational constants we have obtained allow us to calculate very accurate rs and rz structures. The rz structure is: r(CC), 1.4617 (6) A; r(CN), 1.1567 (6) A; r(CH), 1.0947 (24) A; ∡(CCH), 109.85° (10). This structure shows no significant difference from the parameters derived in an earlier electron diffraction study.

Utilization of simultaneous multiple substitutions in the determination of the rs structure

Abstract A new procedure is proposed to calculate the substitution structure of molecules. The procedure allows the inclusion of isotopic species with single or multiple substitutions of any type. It is equivalent to Kraitchmans method when the proper number of singly substituted isotopes is used. Some possible applications are demonstrated with the example of ethylene epoxide.

Centrifugal distortion analysis including P6-terms

Abstract A treatment of centrifugal distortion analysis up to P 6 -terms which is advantageous in several respects is presented. The considerations of van Eijck in the treatment of the P 4 -terms have been extended to the higher order P 6 -terms. A reduced Hamiltonian is proposed which is convergent in the proper representation also for nearly symmetric top molecules. The results of a centrifugal distortion analysis based on this Hamiltonian are given for dimethylsulfoxide (DMSO) and carbonyl fluoride (COF 2 ). The results for COF 2 show a remarkable improvement in the correlations of the fitted parameters and in the numerical stability compared to Carpenters analysis of the same experimental data.

Electron diffraction investigation of the molecular structure of cyclopropyl silane

Abstract The molecular structure of cyclopropyl silane (CPS) has been determined by gas phase electron diffraction. Among other parameters the bond distances (ra) are: C1C2 = 1.528(2) A, C2C3 = 1.490(4) A, SiC = 1.840(2) A, CH = 1.095(3) A. The angle between the ring plane and the bond SiC is 55.9(3)°. The introduction of a tilt of the silyl group is in agreement with the secondary effect of substituents. The role of the silicon 3d orbitals in the interpretation of the structural data of CPS is discussed. Our results support the interpretation of the SiC bond to silicon in terms of dπ conjugation. This causes an asymmetry in the structure of the ring. The (pd)π bonding concept is considered to be the sum of three different contributions: ionic, steric and dπ conjugation.

The molecular structure of carbonyl cyanide

Abstract The molecular structure of carbonyl cyanide has been investigated by means of gas phase electron diffraction. In the rga-representation of the molecular structure the C—CN chain is found to be slightly bent by 1.6° anti to the CO bond. However, from the calculation of the rav-structure by combined use of rotational constants and diffraction intensities the C—CN chain appears to be nearly linear. This comparison suggests a change δη⋍1.5° in the bond angle C—CN in going from the rga-structure to the rga-structure. The bond distances in carbonyl cyanide are consistent with the secondary environment effect found for a number of oxygen- and nitrogen-containing compounds.

The force field and molecular structure of dimethylsulfoxide from spectroscopic and gas electron diffraction data and ab initio calculations

Abstract Scale factors on the ab initio force field of dimethyl sulfoxide (DMSO) produced by the HF/6-31G ∗∗ method have been fitted using vibrational frequencies of the isotopomers DMSO-h 6 and DMSO-d 6 as well as centrifugal distortion constants of a total of nine isotopomers. As indicated by the ab initio calculated sequences of the methyl deformation and methyl rocking vibrations, the attribution to the symmetry species A′ and A″ have been reversed compared to previous work. Then fitting 12 scale factors, the experimental vibrational frequencies are reproduced to 0.6%. Subsequently, the molecular structure of gaseous DMSO has been investigated by a combined analysis of electron diffraction data, rotational constants and ab initio calculations. The fitted quadratic force field was used to calculate the mean amplitudes of vibration, to obtain the necessary harmonic corrections for the conversion of B 0 rotational constants to B z , and to determine the isotopic differences in bond lengths. The r av structural parameters (uncertainties in parenthesis are 3σ) we obtained from the present analysis are: S O =1.4837(11) A , C – S =1.8071(8) A , C – H =1.0934(23) A , ∠C–S–C=96.56(6)°, ∠(C–S–C) plane and S O bond=115.43(5)°. The geometry of DMSO has been optimized using the methods: HF/6-31G(p,d), DFT/B3LYP/6-311++G(3df,3pd), DFT/B3PW91/6-311++G(3df,3pd), MP2/6-31G(d,p), MP2/6-311++G(3df,3pd) and QCISD(T)/6-311++G(2d). The structural parameters predicted at these levels of theory have been discussed and compared with the experimental results. The values for the geometrical parameters provided by the DFT/B3PW91/6-311++G(3df,3pd) method are in best agreement with the experimental results. The importance of the inclusion of the d-polarization functions on sulfur has also been investigated. Our results support earlier observations emphasizing the needlessness of the hypervalency concept for describing the bonding properties in DMSO because the S O bond is ionic and thus the octet rule is not violated.

THE RS STRUCTURE OF DMSO, REVISITED

Abstract The substitution structure of DMSO has been reinvestigated using various sets of experimental rotational constants. It is found that the exclusive deuteration of atom H I induces an as yet unidentified contribution to the rotational constants making the r s coordinates of this atom calculated from these rotational constants inconsistent with the experimental rotational constants of the triply and sixfold deuterated isotopomers. However, the complete substitution structure can be derived consistently from the available rotational constants excluding the isotopomer deuterated at position I. This results in a CH I bond length of 1.096(4) A. The substitution structure now compares well with the results of ab initio calculations.

The ground-state rotational spectrum of cyclopropyl silane

Abstract The rotational spectrum of cyclopropyl silane has been recorded in the region 9.0–35.0 GHz. Eighty-eight transitions of the ground vibrational state were measured and analyzed to give rotational constants and centrifugal distortion constants. The dipole moment was determined to be μ a = 0.847(17)D, μ c = 0.273(10)D, and μ tot = 0.890(18)D. The rotational constants are consistent with a shortening of the C C bond length opposite to the silyl group. Since no splittings due to internal rotation were observed, a lower limit for the hindering potential of the internal rotation of the silyl group is V 3 ≥ 1950cal/mole.

The gas-phase molecular structure of cyclopropyltrifluorosilane as studied by electron diffraction and ab initio calculations

Abstract The gas-phase molecular structure of cyclopropyltrifluorosilane was investigated by electron diffraction and ab initio calculations. The major bond distances ( r a ) and the valence angles ( r α ) with uncertainties (3σ) obtained from diffraction data are as follows: C 1 C 2 = 1.522(8) A, CSi = 1.807(2) A, SiF = 1.572(3) A, CH = 1.104(3) A, SiC(ring plane) = 124.5(9)°, SiCH = 116(4)°, CSiF 6  112.3(10)°, CSiF 7 = 111.6(10)°. The difference ΔCC = (C 1 C 2 )  (C 2 C 3 ) = 0.032 A obtained from ab initio calculations was treated as a constant parameter in the analysis of the diffraction data. The present study has shown that the fluorination of the silyl group leads to a significant shortening of the SiC bond by about 0.03 A in comparison to cyclopropylsilane. The analysis of the diffraction data was performed with the r α -representation of the molecular structure applying a dynamical model, which treats the torsional vibration of the SiF 3 group as large amplitude motion. The required vibrational mean square amplitudes and corrections r a — r α as well as their torsional dependence have been calculated from a normal coordinate analysis on the basis of the ab initio force field (6–31G*).