Jiří Czernek

Dynamics of circular hydrogen bond array in calix[4]arene in a nonpolar solvent: A nuclear magnetic resonance study

Hydroxyl groups on the lower rim of calix[4]arene form a circular array of four equivalent hydrogen bonds. The rate constants of reversal of the array in the temperature range of 221-304 K were determined by means of the NMR measurements of quaternary (13)C nuclear spin transverse relaxation dependence on the effective radio frequency field. The flip-flop rate constants are in the range of 1.4 x 10(2)-4.2 x 10(4) s(-1), the activation enthalpy is 36.8 kJ/mol, the activation entropy is -36 J mol(-1) K(-1). This process was found uncorrelated with conformational transition cone-inverted cone, which is about thousand times slower. Molecular tumbling of calix[4]arene measured using (13)C spin relaxation was found isotropic with correlation times lying in the range of 0.1-3 ns and with the activation energy of 21 kJ/mol. In order to assess relaxation of (13)C aromatic nuclei, their principal components of chemical shift tensor were calculated using the density functional theory approach.

The multiconfigurational-reference internally contracted configuration interaction/complete basis set study of the excited states of the trifluoride anion F3(-).

The multiconfigurational-reference internally contracted configuration interaction (MRCI)/aug-cc-pVQZ-based computational protocol was employed to search for the minima of the potential energy surface of the low-lying singlet and triplet electronic states of the trifluoride anion F(3) (-) in the D(infinity(h)), C(infinity(v)), C(2v), and C(s) symmetry groups. The (3)B(2) bound state was located (r(e)=1.8777 A and theta=103.39 degrees ), which was predicted to lie less than 1 eV above the X (1)Sigma(g) (+) ground state (r(e)=1.7382 A) by both the MRCI and equation-of-motion coupled-cluster singles, doubles and triples approaches [the MRCI adiabatic excitation energy extrapolated to the complete basis set (CBS) limit was 0.91 eV]. The latter value is proposed as a reliable estimate of the singlet-triplet energy gap in F(3) (-). The vertical transitions from the X (1)Sigma(g) (+) state were analyzed in terms of the reorganization of electrons leading to the excited states and the corresponding MRCI/CBS excitation energies.

Flip–flop Motion of Circular Hydrogen Bond Array in Thiacalix[4]arene

Thiacalix[4]arene contains a circular array of four equivalent hydrogen bonds on its lower rim. The array undergoes a flip–flop motion between two possible directions. The rate of this motion in the temperature range 223–313 K is assessed by means of measurements of the nuclear spin relaxation. The values of the activation enthalpy (38.7 kJ mol− 1) and of the activation entropy ( − 15 J mol− 1 K− 1) were determined. In addition, correlation times of molecular tumbling have been determined in the same temperature range. The measured properties of thiacalix[4]arene are compared to those of the “classical” calix[4]arene in order to utilize them for fine tuning of the building blocks in supramolecular chemistry.

An MRCI investigation of the electronically excited states of difluorocarbene and its monovalent ions

The MRCI/CBS computational protocol was employed to describe the low-lying electronic states of CF2 in excellent agreement with experiment. Also of interest for modeling the fluorocarbon plasmas are the difluorocarbene ions, and the geometries, adiabatic excitation energies and orbital characteristics of their electronically excited states were established. Most of the transitions leading to the excited states were found to be monoelectronic and, for the dipole-allowed ones, the oscillator strengths and radiative lifetimes were predicted to aid in the spectroscopic studies.

The UCCSD(T)/CBS Description of the C2h Symmetric, Negatively Charged Dimer of SF2

A highly accurate potential energy curve of the C2h symmetric, negatively charged dimer of SF2 was obtained by estimating the complete basis set limits of the UCCSD(T) energies at the intermonomer separations ranging from 2.15 to 7.75 A. The results serve as a benchmark for assessing the quality of methods describing the energetics of open‐shell species. Thus, a remarkably good performance of the UMP2 method was observed.