Accuracy of quantum chemistry structures of chiral tag complexes and the assignment of absolute configuration.

The absolute configuration of a molecule can be established by analysis of molecular rotational spectra of the analyte complexed with a small chiral molecule of known configuration. This approach of converting the analyte enantiomers, with identical rotational spectra, into diastereomers that can be distinguished spectroscopically is analogous to chiral derivatization in nuclear magnetic resonance (NMR) spectroscopy. For the rotational chiral tag method, the derivatization uses noncovalent interactions to install the new chiral center and avoids complications due to possible racemization of the analyte when covalent chemistry is used. The practical success of this method rests on the ability to attribute assigned rotational spectra to specific geometries of the diastereomeric homochiral and heterochiral tag complexes formed in the pulsed jet expansion that is used to introduce samples into the microwave spectrometer. The assignment of a molecular structure to an experimental rotational spectrum uses quantum chemistry equilibrium geometries to provide theoretical estimates of the spectrum parameters that characterize the rotational spectrum. This work reports the results of a high-sensitivity rotational spectroscopy study of the complexes formed between (3)-butyn-2-ol and verbenone. The rotational spectra of four homochiral and four heterochiral complexes are assigned. In addition, the 14 distinct, singly-substituted 13C isotopomer spectra of five of these species are assigned in natural abundance. Analysis of these spectra provides direct structural characterization of the complexes through determination of the carbon atom position coordinates. This data set is used to benchmark quantum chemistry calculations of candidate equilibrium geometries of the chiral tag complexes. The quantum chemistry calculations are limited to methods commonly used in the field of rotational spectroscopy. It is shown that the accuracy of the structures from quantum chemistry provides a high-confidence assignment of cluster geometries to the observed spectra. As a result, a high-confidence determination of the analyte (verbenone) absolute configuration is achieved.

The molecular structure and curious motions in 1,1-difluorosilacyclopent-3-ene and silacyclopent-3-ene as determined by microwave spectroscopy and quantum chemical calculations.

The molecules 1,1-difluorosilacyclopent-3-ene (3SiCPF2) and silacyclopent-3-ene (3SiCP) have been synthesized and studied using chirped pulse, Fourier transform microwave (CP-FTMW) spectroscopy. For 3SiCP this is the first ever microwave study of the molecule and, for 3SiCPF2, the spectra reported in this work have been combined with that of previous work in a global fit. The spectra of each contain splitting which has been fit using a Hamiltonian consisting of semirigid and Coriolis coupling parameters. A refit of the original 3SiCPF2 work was also carried out. All fits and approaches are reported. Analyses of the spectra provide evidence that the molecule is planar which is in agreement with the high-level calculations, but the source of the splitting in the spectra has not been determined.

Theoretical Calculations, Microwave Spectroscopy, and Ring-Puckering Vibrations of 1,1-Dihalosilacyclopent-2-enes.

High-level theoretical CCSD/cc-pVTZ computations have been carried out to calculate the structures and ring-puckering potential energy functions (PEFs) for 1,1-difluorosilacyclopent-2-ene (2SiCPF2) and 1,1-dichlorosilacyclopent-2-ene (2SiCPCl2). The structure and PEF for 1,1-dibromosilacyclopent-2-ene (2SiCPBr2) were obtained by ab initio MP2/cc-pVTZ computations. The parent silacyclopent-2-ene (2SiCP) is puckered with a 49 cm-1 barrier to planarity, 2SiCPF2 has a planar ring system, 2SiCPCl2 has a calculated tiny 4 cm-1 barrier but is essentially planar, and the dibromide has a calculated barrier of 36 cm-1. Microwave spectra of seven isotopic species of 2SiCPF2 were recorded on a chirped pulse, Fourier transform microwave (CP-FTMW) spectrometer in the 6-18 GHz region. The a-type and b-type transitions were observed. The rotational constants and three quartic centrifugal distortion constants were determined for the parent, 29Si, 30Si, and all singly substituted 13C isotopologues in natural abundance. This allowed for the determination of the heavy-atom structure of the ring and showed the ring to be planar. The experimentally determined rotational constants and geometrical parameters agree very well with the theoretical values and confirm the planarity of the five-membered ring. A comparison of the PEFs for the silane and the three dihalides shows the silane to have the stiffest puckering motion and the dibromide to be the least rigid.

The Conformational Landscape, Internal Rotation, and Structure of 1,3,5-Trisilapentane using Broadband Rotational Spectroscopy and Quantum Chemical Calculations.

The rotational spectrum of 1,3,5-trisilapentane was observed on a chirped-pulse Fourier transform microwave spectrometer and is reported. During assignment, multiple conformations of the molecule were identified in the molecular beam. Prior quantum-chemical calculations performed on the molecule show that the identified spectra correspond to the lowest three calculated energetic structures. These structures are of C2 (Conf.1), C2v (Conf.2), and C1 (Conf.3) symmetry, with relative energy ordering of Conf.1 < Conf.3 < Conf.2, which is in stark contrast to n-pentane and all known silicon-substituted n-pentane derivatives. This is found to most likely arise from the elongation of the Si-C bond and the size of the silicon atoms providing for the C2 and C1 structures relieving steric hindrance in comparison to that of the C2v. In the C2v and C1 conformers, splitting in the spectra due to internal rotation of the -SiH3 end groups of 1,3,5-trisilapentane was observed and determined. The C2v equivalent V3 values are 368.46(33) cm-1, and the C1V3 values are 347.78(21) and 360.18(88) cm-1, respectively. These barriers are compared to similar species in order to help verify their veracity and are determined to be accurate based on similar molecular silyl rotors.

Microwave Spectra and Structure of Ar-1,3-Difluorobenzene.

The microwave spectrum of the dimer Ar-1,3-difluorobenzene from 2 to 18 GHz is reported. The spectrum has been observed using a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer that has recently been expanded to include the 2-6 GHz region of the electromagnetic spectrum. Details of this upgraded spectrometer are reported. Eighty-seven transitions were observed for the parent dimer spectrum, which was adequately fit to a semirigid rotational Hamiltonian consisting of A, B, and C as well as four quartic centrifugal distortion constants. Observations of 13C species in natural abundance were aided by utilizing smaller chirp ranges of 7-9 and 9-11 GHz for 1.9 million and 3.73 million averages, respectively. Assignment of 13C isotopologues allowed for determination of the Kraitchman coordinates of the carbon atoms as well as inertial fits of the complex. The quantum-chemical structure predicts an Ar to monomer center of mass distance of 3.48 Å, compared with 3.564(1) Å determined from experimental structural analysis. This new study indicates that in fluorinated benzene-Ar dimers, when the fluorines are separated by more carbon atoms, the Ar-ring center distance decreases.