Beam Fourier Transform Microwave Spectroscopy Research Articles

Alkaline and Alkaline-earth Cyanoacetylides: A Combined Theoretical and Rotational Spectroscopic Investigation.

The metallic cyanoacetylides LiC3N, NaC3N, MgC3N and CaC3N have been investigated by combined spectroscopy measurements and theoretical calculations. The theoretical calculations predict for the four species that the linear isomer with formula MCCCN (M= Li, Na, Mg and Ca) is the most stable one. We used the laser ablation molecular beam Fourier transform microwave spectroscopy to synthesize these species by the reaction of metal vapors, produced by laser ablation, and the 3-bromo-2-propynenitrile (BrCCCN). The pure rotational spectra were observed by Fourier transform microwave spectroscopy in the 2-18 GHz frequency region only for LiCCCN and NaCCCN, while no spectral signatures for MgCCCN and CaCCCN could be detected. Finally, we have searched for LiCCCN and NaCCCN species towards the carbon-rich evolved star IRC + 10216 but only upper limits to their abundances have been obtained.

Favored Conformations of Carbonyl Compounds: A Structural Study of n-Octanal.

We report on the molecular structures of the two most abundant conformers of n-octanal observed by molecular beam Fourier transform microwave spectroscopy. Next to limonene, which is the main component of citrus-oil, octanal and other n-alkyl aldehydes strongly enhance the typical fresh smell of lemon-oil. Due to the high flexibility of its n-alkyl chain and the high number of possible conformers, different semi-empirical methods (AM1, PM3, MMFF94) were used to sample the conformational space of octanal before performing more sophisticated quantum chemical calculations at the MP2 level of theory. This technique has previously been shown to be an ideal tool to characterize relevant odorant structures in fragrance chemistry. The structure of octanal and structurally related molecules is discussed in the context of the most abundant chain conformations and the potential use of the microwave validated structures for further studies in biological media.

Three isomers detected for the whisky lactone: 5-butyl-4-methyl tetrahydrofuran-2-one by Fourier transform microwave spectroscopy and quantum chemical calculations

The ground-state rotational spectra of the whisky lactone (WL): 5-butyl-4-methyl tetrahydrofuran-2-one were observed and analyzed by molecular beam Fourier transform microwave spectroscopy combined with quantum chemical calculations. We have detected three stereo-isomers: the trans-TTT form with a methyl CH3 group attached to C(4) in an equatorial position (eq) and a butyl C4H9 group to C(5) in an eq position, for which 110 b-type and 113 a-type transitions were assigned, the cis-TTT form with a CH3 to C(4) in an axial position (ax) and a C4H9 to C(5) in eq, for which 96 a-type, 101 b-type, and 45 c-type transitions were observed, and the cis-GTT form with a CH3 to C(4) in ax and a C4H9 to C(5) in eq, for which 158 a-type, 52 b-type, and 17 c-type transitions were observed, where TTT and GTT denote the conformations about the C(6)C(5), C(7)C(6), and C(8)C(7) bonds, with T and G designating trans and gauche, respectively. The rotational constants thus derived agree with the predictions made by quantum chemical calculations, MP2/6-311++G(d, p) within 1.2%. The trans-TTT form was calculated to be the most stable. The splittings due to internal rotation of the terminal methyl in the butyl group were observed for all the three stereo-isomers and were analyzed by the XIAM program to determine the threefold potential barrier V3 to be 966.4 (25), 978.8 (11), and 1098.7 (48) in cm−1 for the trans-TTT (eq, eq), the cis-TTT (ax, eq), and the cis-GTT (ax, eq) forms, respectively, to be compared with quantum chemically calculated values: 1055, 1055, and 1053 in cm−1.

Structure and internal dynamics of n-propyl acetate studied by microwave spectroscopy and quantum chemistry

The gas phase structure of n-propyl acetate was determined using molecular beam Fourier transform microwave spectroscopy from 2 to 40GHzsupplemented by quantum chemical calculations. The experimental spectrum revealed only one conformer with trans configuration and C1 symmetry. Torsional splittings occurred for each rotational transition due to the internal rotation of the acetyl methyl group with a barrier height of approximately 100cm−1. The XIAM and BELGI-C1 codes were applied to reproduce the spectrum within the measurement accuracy. This investigation on n-propyl acetate has accomplished our studies on saturated linear aliphatic acetates CH3COOCnH2n+1 (n=1–6).

Conformational analysis of tert-butyl acetate using a combination of microwave spectroscopy and quantum chemical calculations

tert-Butyl acetate was investigated using a combination of quantum chemical calculations and molecular beam Fourier transform microwave spectroscopy. The microwave spectrum was recorded in the frequency range from 8.00 to 15.75GHz. Due to its rather rigid frame, the molecule possesses only two conformers: one of Cs symmetry and one of C1 symmetry that appears as a pair of enantiomers. The Cs conformer is the most abundant in the supersonic jet and according to ab initio calculations at the MP2/6-311++G(d,p) level of theory it is 46kJ/mol lower in energy than the C1 conformer.Here, we report on the structure and dynamics of the most abundant conformer of tert-butyl acetate, for which a set of rotational and centrifugal distortion constants, as well as the barrier to internal rotation of the acetyl methyl group were determined with high accuracy. Splittings due to the internal rotation of the methyl group of up to 1.3GHz were observed in the spectrum. Using the programs XIAM and BELGI-Cs, we were able to determine a barrier height of about 113cm−1 and subsequently compare the molecular parameters obtained from these two codes.

Rotational Spectrum of Dichloromethane-Ne: Internal Dynamics and Cl Quadrupolar Hyperfine Effects.

The rotational spectra of three isotopologues, CH2(35)Cl2···(20)Ne, CH2(35)Cl(37)Cl···(20)Ne, and CH2(35)Cl2···(22)Ne, of the complex dichloromethane-neon have been assigned and measured by molecular beam Fourier transform microwave spectroscopy. The corresponding tunnelling splittings-due to the motion of Ne from above to below the ClCCl plane-have been determined as ΔE0+0- = 6.8900(5), 6.6630(4), and 6.3724(7) MHz, respectively. From these data the barrier to planarity has been obtained, B2 = 68.7 cm(-1). In addition, the structure and the (35)Cl (or (37)Cl) quadrupole coupling constants have been determined.

Acetyl Methyl Torsion in N-Ethylacetamide: A Challenge for Microwave Spectroscopy and Quantum Chemistry.

The gas-phase structures and parameters describing acetyl methyl torsion of N-ethylacetamide are determined with high accuracy, using a combination of molecular beam Fourier-transform microwave spectroscopy and quantum chemical calculations. Conformational studies at the MP2 level of theory yield four minima on the energy surface. The most energetically favorable conformer, which possesses C1 symmetry, is assigned. Due to the torsional barrier of 73.4782(1) cm(-1) of the acetyl methyl group, fine splitting up to 4.9 GHz is found in the spectrum. The conformational structure is not only confirmed by the rotational constants, but also by the orientation of the internal rotor. The (14) N quadrupole hyperfine splittings are analyzed and the deduced coupling constants are compared with the calculated values.

The microwave spectrum of allyl acetone

Allyl acetone was investigated using a combination of quantum chemical calculations and molecular beam Fourier transform microwave spectroscopy. The microwave spectrum was recorded in the frequency range from 10.0 to 14.0GHz. The most abundant conformer in the supersonic jet possesses a C1 symmetry and appears as a pair of enantiomers. The observed conformer does not correspond to the most abundant conformer observed for allyl acetate, which is structurally related to allyl acetone.Here, we report on its structure and dynamics, for which a set of highly accurate rotational and centrifugal distortion constants, as well as the barrier to internal rotation of the acetyl methyl group could be determined with high accuracy. Splittings due to the internal rotation of the methyl group of up to 360MHz were observed in the spectrum. Using the programs XIAM and BELGI-C1, a barrier height (V3) of about 225cm−1 as well as the other molecular parameters were determined. The results obtained from the different codes were subsequently compared to each other. Additionally, the experimental rotational constants were used to validate quantum chemical calculations at different levels of theory.

The effects of methyl internal rotation and (14)N quadrupole coupling in the microwave spectra of two conformers of N,N-diethylacetamide.

The gas phase structures and internal dynamics of N,N-diethylacetamide were determined with very high accuracy using a combination of molecular beam Fourier-transform microwave spectroscopy and quantum chemical calculations at high levels. Conformational studies yielded five stable conformers with C1 symmetry. The two most energetically favorable conformers, conformer I and II, could be found in the experimental spectrum. For both conformers, quadrupole hyperfine splittings of the (14)N nucleus and torsional fine splittings due to the internal rotation of the acetyl methyl group occurred in the same order of magnitude and were fully assigned. The rotational constants, centrifugal distortion constants as well as the quadrupole coupling constants of the (14)N nucleus were determined and fitted to experimental accuracy. The V3 potentials were found to be 517.04(13) cm(-1) and 619.48(91) cm(-1) for conformer I and II, respectively, and compared to the V3 potentials found in other acetamides. Highly accurate CCSD(T) and DMC calculations were carried out for calculating the barriers to internal rotation in comparison with the experimentally deduced V3 values.

Rotational spectrum of phenylglycinol

Solid samples of phenylglycinol were vaporized by laser ablation and investigated through rotational spectroscopy in a supersonic expansion using two different techniques: chirped pulse Fourier transform microwave spectroscopy and narrow band molecular beam Fourier transform microwave spectroscopy. One conformer, bearing an O–H···N and an N–H···π intramolecular hydrogen bonds, could be successfully identified by comparison of the experimental rotational and 14N nuclear quadruple coupling constants with those predicted theoretically.

Generation and structural characterization of aluminum cyanoacetylide.

Combined spectroscopy measurements and theoretical calculations bring to light a first investigation of a metallic cyanoacetylide, AlC3N, using laser ablation molecular beam Fourier transform microwave spectroscopy. This molecule was synthesized in a supersonic expansion by the reaction of aluminum vapour with C3N, produced from solid aluminum rods and BrCCCN in a newly constructed ablation-heating nozzle device. A set of accurate rotational and (27)Al and (14)N nuclear quadrupole coupling constants have been determined from the analysis of the rotational spectrum and compared with those predicted in a high-level ab initio study, conducting to the assignment of the observed species to linear AlCCCN. We have searched for this species towards the carbon-rich evolved star IRC + 10216 but only an upper limit to its abundance has been obtained.

Pyridine-CF4: A Molecule with a Rotating Cap

The rotational spectrum of pyridine-CF4 has been investigated by molecular beam Fourier transform microwave spectroscopy in a supersonic expansion. The CF4 moiety is located as a cap over the pyridine nitrogen, and the two parts are freely rotating with respect to each other. For this reason, in a first approximation, in the m = 0 state only the pyridine ring is rotating along the a-axis, and the value of rotational constant A' is nearly the same of the constant A of isolated pyridine. The N···CCF4 distance is 3.372(1) Å. The dissociation energy has been estimated, from the centrifugal distortion, to be ca. 10 kJ/mol.

Sulfur-Containing Flavors: Gas Phase Structures of Dihydro-2-methyl-3-thiophenone

Dihydro-2-methyl-3-thiophenone was investigated using a combination of quantum chemical calculations and molecular beam Fourier transform microwave spectroscopy. The substance is present in coffee, roasted peanuts, and whiskey. The microwave spectrum was recorded under molecular beam conditions in the frequency range from 9 to 14 GHz. We report on the two main conformers of dihydro-2-methyl-3-thiophenone, for which highly accurate rotational constants and centrifugal distortion constants were obtained. No splittings due to internal rotation of the methyl group could be observed in the microwave spectrum. This is in agreement with the theoretical predictions of the barrier heights, which have been determined to be more than 1000 cm(-1) at the MP2/6-311++G(d,p) level of theory. In addition to the most abundant (32)S-isotopologue of the main conformer, also the (34)S-isotopologue was assigned, which occurs with a natural abundance of about 4%. Using the experimental rotational constants, different quantum chemical calculations were validated for the two observed conformers. To complete the theoretical investigation of dihydro-2-methyl-3-thiophenone, different transition states were optimized to understand the intramolecular conversion between the two conformers at the MP2/6-311++G(d,p) level. The transition states were optimized using the Berny algorithm.

Conformers of β-aminoisobutyric acid probed by jet-cooled microwave and matrix isolation infrared spectroscopic techniques.

β-aminoisobutyric acid (BAIBA) has been studied in isolation conditions: in the gas phase and trapped into a cryogenic N2 matrix. A solid sample of the compound was vaporized by laser ablation and investigated through their rotational spectra in a supersonic expansion using two different spectroscopic techniques: broadband chirped pulse Fourier transform microwave spectroscopy and conventional molecular beam Fourier transform microwave spectroscopy. Four conformers with structures of two types could be successfully identified by comparison of the experimental rotational and (14)N nuclear quadruple coupling constants with those predicted theoretically: type A, bearing an OH⋯N intramolecular hydrogen bond and its carboxylic group in the trans geometry (H-O-C=O dihedral ∼180°), and type B, having an NH⋯O bond and the cis arrangement of the carboxylic group. These two types of conformers could also be trapped from the gas phase into a cryogenic N2 matrix and probed by Fourier transform infrared (IR) spectroscopy. In situ irradiation of BAIBA isolated in N2 matrix of type B conformers using near-IR radiation tuned at the frequency of the O-H stretching 1st overtone (∼6930 cm(-1)) of these forms allowed to selectively convert them into type A conformers and into a new type of conformers of higher energy (type D) bearing an NH⋯O=C bond and a O-H "free" trans carboxylic group.

The microwave spectrum of n-butyl acetate

We recorded the rotational spectrum of n-butyl acetate (CH3-COO-C4H9) using molecular beam Fourier transform microwave spectroscopy. The rotational constants and the centrifugal distortion constants of the most abundant and of the two less abundant conformers were determined. The main conformer with C1 symmetry has a strong spectrum. The other two conformers have Cs and C1 symmetries. Their intensities are considerably weaker. The quantum chemical calculations of specific conformers were carried out at the MP2/6-311++G(d,p) level. Harmonic frequency calculations were done to exclude imaginary frequencies. Furthermore, calculations at different levels of theory were performed for the main conformer. To analyze the internal rotation of the acetyl methyl groups the codes XIAM and BELGI were used. The molecular structures of the three conformers were determined and the values of the experimental rotational constants were compared with those obtained by ab initio methods. For all conformers torsional barriers of approximately 100cm−1 were found.

All Five Forms of Cytosine Revealed in the Gas Phase

Give me five! All five tautomers and conformers of cytosine were characterized in the gas phase by laser ablation molecular beam Fourier transform microwave spectroscopy. The spectra were assigned unambiguously on the basis of the hyperfine structure due to the three 14N nuclei (see picture; N blue, O red). The relative energies of the identified species were estimated from the relative intensities of the spectra.

Conformational Landscape of Diisopropyl Ketone: Quantum Chemical Calculations Validated by Microwave Spectroscopy

We report on the gas-phase structure of the most abundant conformer of diisopropyl ketone, (CH(3))(2)HC-CO-CH(CH(3))(2), as observed by molecular beam Fourier transform microwave spectroscopy. The gas-phase structures of five conformers of diisopropyl ketone were optimized using ab initio calculations at the MP2/6-311++G(d,p) level of theory. The natures of the stationary points were verified using harmonic frequency calculations. The only conformer observed in the supersonic jet possesses C(2) symmetry and appears as an enantiomeric pair. From the microwave spectrum, a set of three highly accurate rotational constants, five centrifugal distortion constants, and three sextic centrifugal distortion constants were determined. The structure of the observed conformer was optimized again at different levels of theory using the HF, MP2, and B3LYP methods. The theoretical constants of the C(2) conformer were subsequently validated using the experimental constants. To understand the transitions of one conformer to the others, the isopropyl groups were rotated against each other. The resulting two-dimensional potential energy surface shows nicely the symmetry of the conformational landscape and also indicates the enantiomeric pairs of the conformers. The barriers to internal rotation of the methyl groups were determined to be 1052 and 905 cm(-1) at the MP2/6-311++G(d,p) and the B3LYP/6-311++G(d,p) levels, respectively. In agreement with the theoretical predictions, no internal rotation patterns could be observed in the microwave spectrum.

Solvent Polarity-Induced Conformational Unlocking of Asparagine

Classical and Car-Parrinello molecular dynamics simulations are performed to study the solvent effect on the conformational distribution of asparagine. Conformational populations obtained from the simulations in gas phase and in nonpolar chloroform solvent are in agreement with the most probable single conformation of asparagine in the gas phase measured in recent laser ablation with molecular beam Fourier transform microwave spectroscopy experiments. We rationalize that intramolecular hydrogen bonding and dipole-dipole interactions between carbonyl groups dictate such a conformational locking to a single asparagine conformer. The solvent polarity induced interlocking or intermolecular hydrogen bonding with water solvent molecules destabilizes the (NH···O═C) bonding between side chain and terminal groups of asparagine, while not essentially affecting the (NH···O═C) intramolecular hydrogen bondings within the side chain nor within the terminal groups. Such a conformational unlocking or cage effect is observed in asparagine within aqueous solution. We observed a spontaneous conversion of neutral to zwitterionic isomer of asparagine in aqueous solution, which is in agreement with interpretation of Raman spectroscopy results. Using Møller-Plesset second order perturbation theory, we show that a tautomeric shift from neutral to zwitterionic occurs on asparagine in between DMSO and water solvents. The ramification of these findings for the conformational character of asparagine is briefly discussed.

Conformational Analysis of Green Apple Flavour: The Gas‐Phase Structure of Ethyl Valerate Validated by Microwave Spectroscopy

We report on the microwave spectrum of ethyl valerate, C(4)H(9)-COO-C(2)H(5), observed by molecular beam Fourier transform microwave spectroscopy (MB-FTMW). Highly accurate rotational and centrifugal distortion constants of the two most abundant conformers were determined. Their structure was investigated by comparison of the experimental rotational constants with those obtained by quantum chemical calculations. MP2/ 6-311++G(d,p) calculations failed to give a proper description of the observed conformers in the supersonic jet. Therefore, also other basis sets were tried using Hartree-Fock, second-order Møller-Plesset (MP2), and B3LYP methods to predict the observed gas-phase structures of the molecule. The quantum chemical results guided the conformer assignment of the rotational constants, obtained from the microwave experiment. One of the two observed conformers has C(s) symmetry, while the most abundant conformer has C(1) symmetry. The main conformer possesses a strong spectrum with high intensities. Additionally, harmonic frequency calculations at different levels of theory where carried out and a low lying vibration of the entire n-butyl group against the rest of the molecule was identified. The symbiotic interplay of microwave gas-phase investigations and quantum chemical calculations becomes evident in our results.

High-Resolution Rotational Spectroscopy of a Cyclic Ether.

The conformational landscape of crown ethers has constituted a central topic in the development of host-guest supramolecular chemistry. We report a high-resolution rotational study of a crown ether, 1,4,7,10,13-pentaoxacyclopentadecane (15-crown-5), by means of molecular beam Fourier transform microwave spectroscopy. The considerable size and the broad range of conformations allowed by the flexibility of the cyclic backbone of this ether pose important challenges to spectroscopy approaches. In this investigation, three stable rotamers of the 15-crown-5 ether have been identified and characterized through their rotational constants and centrifugal distortion coefficients. Ab initio quantum calculations at the MP2 level predict these conformers as the most stable ones for the title system and reproduce accurately their distinct structural features. The results pave the ground for an extensive survey of the conformational landscape of the 15-crown-5 and related cyclic ethers in the near term.