Molecular Structure and Chirality Detection by Fourier Transform Microwave Spectroscopy

We describe a three-wave mixing experiment using time-separated microwave pulses to detect the enantiomer-specific emission signal of the chiral molecule using Fourier transform microwave (FTMW) spectroscopy. A chirped-pulse FTMW spectrometer operating in the 2–8 GHz frequency range is used to determine the heavy-atom substitution structure of solketal (2,2-dimethyl-1,3-dioxolan-4-yl-methanol) through analysis of the singly substituted ¹³C and ¹⁸O isotopologue rotational spectra in natural abundance. A second set of microwave horn antennas is added to the instrument design to permit three-wave mixing experiments where an enantiomer-specific phase of the signal is observed. Using samples of <i>R</i>-, <i>S</i>-, and racemic solketal, the properties of the three-wave mixing experiment are presented, including the measurement of the corresponding nutation curves to demonstrate the optimal pulse sequence

High-Resolution Electronic Spectroscopy of the Doorway States to Intramolecular Charge Transfer

Reported here are several of the ground, first, and second excited state structures and dipole moments of three benchmark intramolecular charge transfer (ICT) systems; 4-(1<i>H</i>-pyrrol-1-yl)­benzonitrile (PBN), 4,4′-dimethylaminobenzonitrile (DMABN), and 4-(1-pyrrolidinyl)­benzonitrile (PYRBN), isolated in the gas phase and probed by rotationally resolved spectroscopy in a molecular beam. The related molecules 1-phenylpyrrole (PP) and 4-aminobenzonitrile (ABN) also are discussed. We find that the S₁ electronic state is of B symmetry in all five molecules. In PBN, a second excited state (S₂) of A symmetry is found only ∼400 cm^–1 above the presumed origin of the S₁ state. The change in dipole moment upon excitation to the A state is measured to be Δμ ≈ 3.0 D, significantly smaller than the value predicted by theory and also smaller than that observed for the “anomalous” ICT band of PBN in solution. The B state dipole moments of DMABN and PYRBN are large, ∼10.6 D, slightly larger than those attributed to “normal” LE fluorescence in solution. In addition, we find the unsaturated donor molecules (PP, PBN) to be twisted in their ground states and to become more planar upon excitation, even in the A state, whereas the saturated donor molecules (ABN, DMABN, PYRBN), initially planar, either remain planar or become more twisted in their excited states. It thus appears that the model that is appropriate for describing ICT in these systems depends on the geometry of the ground state

Structure Determination of Strawberry Aldehyde by Broadband Microwave Spectroscopy: Conformational Stabilization by Dispersive Interactions

The rotational spectrum of ethyl 3-methyl-3-phenylglycidate (C₁₂H₁₄O₃, strawberry aldehyde) has been obtained with chirped-pulse Fourier transform microwave spectroscopy. The sample is a mixture of diastereomers, cis and trans, with different relative stereochemistry around the central epoxide. The spectra of five conformers of this molecule (two of cis and three of trans) have been assigned, and carbon backbone structures for the two most populated conformers (one of cis and one of trans) were determined from ¹³C isotopomers in natural abundance using the Kraitchman relations. Comparisons of experimentally determined structural data to ab initio calculations show that the B3LYP density functional fails to account adequately for a long-range dispersive interaction between the phenyl ring and the terminal ethyl group in <i>cis</i>-strawberry aldehyde. However, calculations performed using both MP2 and the M05-2X density functional are able to capture the effects of this interaction on the molecular geometry

Molecular Structure of Cyclopropyl (Isocyanato) Silane: A Combined Microwave Spectral and Theoretical Study

The molecular equilibrium structures of two conformers (cis and gauche) of C₃H₅–SiH₂–NCO have been deduced by a combination of microwave (MW) spectra at natural abundance including data from ¹³C and ^29,30Si isotopomers and ab initio calculations. The MW rotational constants (RCs) for the most abundant isotopes are cis: A = 4216.3617(64), B = 1225.76654(91), and C = 1037.31468(77) MHz and gauche: A = 4955.55(79), B = 1094.9276(81), and C = 942.7031(80) MHz. The symmetric quartic centrifugal distortion constants have been evaluated for the cis conformer, using the I^r representation for <i>C</i>_<i>S</i> symmetry. Only partial substitution structures (PSSs) could be derived from the spectra after inclusion of the above isotopic combinations at each center. Using the PSSs, the full structures were determined by ab initio calculation of the equilibrium structures using coupled-cluster singles and doubles with selected triples configuration calculations (CCSD­(T)); the two conformers have an energy difference of 228 cm^–1 (cis lower than gauche). The similarity of the calculated and MW RC results confirms the identities of the two compounds. The more interesting cis conformer has bond lengths C₂–Si₃, 1.9072(73), C₂–C₉ 1.464(22), and C₉–C₁₀ 1.4944(33) Å and angles Si₃–C₂–C₁₀ 119.4(12)° and C₉–C₂–C₁₀ 57.1(12)°, with similar results for the gauche conformer. The Si₃N₄C₅ angle is wide in the cis conformer (145°) and nearly linear in the gauche conformer (179°). New physical insights into the bonding of cis conformers of this type have led the identification of an attractive force between the relatively crowded cyclopropyl and isocyanato groups in the cis conformation. This is demonstrated by three methods: Comparing electronic charges (both AIMALL and Mulliken analyses) in the pair of conformers shows a relative shift of density between these groups in the cis compound. Comparison of the highest occupied molecular orbitals (HOMOs) shows major mixing of density, exemplified by HOMO-1 in these structural units for the cis conformer but which is absent for the gauche conformer. Finally, the nearly linear isocyanate moiety (and the molecular dipole moment) of the cis conformer points closely toward the connected C atom of the cyclopropyl ring, while the gauche conformer dipole moment is significantly different in direction and points toward the midpoint of the C₂Si₃ bond. Both the HCSiN torsional and Si–NC bending surfaces connecting these conformers were explored at the Møller–Plesset second-order perturbation theory level (MP2), which led to the exclusion of other conformers. The bending surface shows a very high amount of quartic potential function

Exploring the Rich Potential Energy Surface of (H₂O)₁₁ and Its Physical Implications

The rich potential energy surface of the water undecamer (H₂O)₁₁ was explored with a basin hopping algorithm using a TIP4P potential and other methods followed by extensive ab initio MP2 minimizations and CCSD­(T) corrections. This protocol yielded 17, 66, and 125 distinct isomers within 0.5, 1.0, and 2.0 kcal mol^–1 of the complete basis set CCSD­(T) global minimum, respectively. These isomers were categorized into 15 different families based on their oxygen framework and hydrogen bonding topology. Determination of the global minimum proved challenging because of the presence of many nearly isoenergetic isomers. The predicted global minimum varied among ab initio methods, density functionals, and model potentials, and it was sensitive to the choice of energy extrapolation schemes, higher-order CCSD­(T) corrections, and inclusion of zero-point vibrational energy. The presence of a large number of nearly degenerate structures and the isomerization between them has manifested itself in the anomalous broadening of the heat capacity curve of the undecamer in simulations around the melting region

The rotational spectrum and complete heavy atom structure of the chiral molecule verbenone

As the first step of a two-part chiral tagging experiment, the spectrum and subsequent isotopologue analysis on the heavy atoms of (1S)-(-)-Verbenone is presented. The spectrum has been recorded up to 69. GHz on three spectrometers, one CP-FTMW spectrometer from the University of Virginia functional from 2 to 8. GHz, a CP-FTMW spectrometer operational in the 6-18. GHz range located at the Missouri University of Science and Technology, and a Stark-modulated spectrometer operational from 48 to 72. GHz. 1250 transitions have been assigned to the parent and isotopologues for the predominantly b-type spectrum. Rotational constants and quartic centrifugal distortion constants have been determined for the parent species while for the 11 isotopologues only rotational constants have been determined. A Kraitchman analysis has been performed and the resulting coordinates are reported. The experimental heavy-atom structure has been compared to previously studied bicyclic terpenes and the computational structure and is found to be in excellent agreement with both, showing reliability of the theoretical approaches needed for the future chiral tagging work

Molecular Structure of Methyldifluoroisocyanato Silane: A Combined Microwave Spectral and Theoretical Study

The structure of methyldifluoroisocyanato silane, MeF₂SiNCO (<b>2</b>), has been studied by molecular rotational spectroscopy. The rotational spectrum has a complicated structure from ¹⁴N nuclear quadrupole coupling and internal rotation of the methyl group. Cavity Fourier-transform microwave spectroscopy measurements were important for providing high spectral resolution to analyze the quadrupole and internal rotation fine structure. Broadband chirped-pulse Fourier-transform microwave spectroscopy was used to achieve high measurement sensitivity making it possible to observe the lower abundance C, N, O, and Si isotopologues in natural abundance for structure determination. Analysis of the microwave spectrum of the most abundant isotopomer of MeF₂SiNCO (<b>2</b>) yields the rotational constants: <i>A</i> = 3827.347(7), <i>B</i> = 1264.5067(14), and <i>C</i> = 1240.6182(11) MHz. The spectrum has been analyzed in the <i>I</i>^r representation for <i>C_s</i> symmetry, with inclusion of the 3-fold rotor (<i>V</i>₃ = 446(50) cm^–1). A partial substitution structure was obtained for the C, Si, N, and O atoms. The analysis was assisted by calculations of the equilibrium structure, using a 6-311++G (3df, 3pd) basis set, with calculations at each of the B3LYP, MP2, and CCSD­(T) levels. The calculated and experimental rotational constants are only consistent with a <i>trans</i>-orientation at each of the HCSiN, CSiNC, and SiNCO centers; there is relatively close agreement between the experimental and the theoretical structures, especially at the CCSD­(T) level. In addition, the observed low value for the ¹⁴N quadrupole coupling term (χ_<i>bb</i> – χ_<i>cc</i>) implies a wide SiNC angle, which is consistent with the calculated values: 165.3° (B3LYP), 157.6° (MP2), and 157.4° (CCSD­(T)). The skeletal bending potential is discussed