Rotational spectroscopy of natural products dates over 35 years when six different species including thujone were investigated.Z.Kisiel, A.C.Legon, JACS 100, 8166 (1978).evertheless, the technique of low-resolution microwave spectroscopy employed therein allowed determination of only a single conformational parameter. Advances in sensitivity and resolution possible with supersonic expansion techniques of rotational spectroscopy made possible much more detailed studies such that, for example, the structures of first camphor,Z.Kisiel, O.Desyatnyk, E.Bia kowska-Jaworska, L.Pszczó kowski, PCCP 5 820 (2003).nd then of multiple clusters of camphor with waterC.Pérez, A.Krin, A.L.Steber, J.C.López, Z.Kisiel, M.Schnell, J.Phys.Chem.Lett. 7 154 (2016).ere determined. We revisited the rotational spectrum of the well known thujone molecule by using the chirped pulse spectrometer in Hamburg. The spectrum of a single thujone sample was recorded with an admixture of ¹⁸O enriched water and was successively analysed using an array of techniques, including the AUTOFIT program,N.A.Seifert, I.A.Finneran, C.Perez, et al. J.Mol.Spectrosc. 312, 12 (2015).he AABS packageZ.Kisiel, L.Pszczó kowski, B.J.Drouin, et al. J.Mol.Spectrosc. 280, 134 (2012).nd the STRFIT program.Z.Kisiel, J.Mol.Spectrosc. 218, 58 (2003).e have, so far, been able to assign rotational transitions of α-thujone, β-thujone, another thujone isomer, fenchone, and several thujone-water clusters in the spectrum of this single sample. Natural abundance molecular populations were sufficient to determine precise heavy atom backbones of thujone and fenchone, and H₂¹⁸O enrichment delivered water molecule orientations in the hydrated clusters. An overview of these results will be presented.

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Footnotes:

Z.Kisiel, A.C.Legon, JACS 100, 8166 (1978).N Z.Kisiel, O.Desyatnyk, E.Bia kowska-Jaworska, L.Pszczó kowski, PCCP 5 820 (2003).a C.Pérez, A.Krin, A.L.Steber, J.C.López, Z.Kisiel, M.Schnell, J.Phys.Chem.Lett. 7 154 (2016).w N.A.Seifert, I.A.Finneran, C.Perez, et al. J.Mol.Spectrosc. 312, 12 (2015).t Z.Kisiel, L.Pszczó kowski, B.J.Drouin, et al. J.Mol.Spectrosc. 280, 134 (2012).a Z.Kisiel, J.Mol.Spectrosc. 218, 58 (2003).W

The non-planar molecular structure of the complex formed between the argon atom and cis-1,2-dichloroethylene is determined via analysis of its microwave spectrum. Spectra of the ³⁵Cl and ³⁷Cl isotopologues are observed in natural abundance and the nuclear quadrupole splitting due to the two chlorine nuclei is fully resolved. In addition, the complete quadrupole coupling tensor for the cis-1,2-dichloroethylene molecule, including the single non-zero off-diagonal element, has been determined. Unlike the argon-cis-1,2-difluoroethylene and the argon-vinyl chloride complexes, tunneling between the two equivalent non-planar configurations of argon-cis-1,2-dichloroethylene is not observed.

Chirped-pulse and resonant cavity Fourier-transform microwave spectroscopy have been used to investigate dimers of CO₂ with vinyl fluoride (VF), vinyl chloride (VCl) and vinyl bromide (VBr). For all three complexes, CO₂ is aligned adjacent to the X-C-H end (X = F, Cl, Br) of the ethylene subunit, with C-X…C and C-H…O contacts. For VF…CO₂, a second isomer is also observed, with CO₂ roughly parallel to the H-C=C-F side of VF; however, there is no spectroscopic indication that similar structures are present for VCl…CO₂ or VBr…CO₂. For vinyl fluoride…CO₂, a full structural analysis has previously been published,C. L. Christenholz, R. E. Dorris, R. A. Peebles, S. A. Peebles, J. Phys. Chem. A, 118, (2014), 8765-8772.hile for the Cl- and Br-containing species, insufficient data are presently available for complete structure determinations. However, structural information from ab initio calculations, ³⁵Cl/³⁷Cl and ⁷⁹Br/⁸¹Br isotopic substitution, and analysis of chlorine and bromine nuclear quadrupole coupling constants will be presented. In addition, for this series of dimers containing C-H…O contacts, further insight into the nature of the weak interactions may be obtained from Quantum Theory of Atoms in Molecules (QTAIM) and other ab initio analyses that are presently in progress.

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Footnotes:

C. L. Christenholz, R. E. Dorris, R. A. Peebles, S. A. Peebles, J. Phys. Chem. A, 118, (2014), 8765-8772.w

The conformational behaviour of sorbitol and dulcitol has been investigated for the first time using a combination of chirped pulse Fourier transform microwave spectroscopy (CP-FTMW) coupled with a laser ablation (LA) source. The observed conformers have been found to be overstabilised by cooperative networks of intramolecular hydrogen bonds between vicinal hydroxyl groups stretching throughout the whole molecule. A common structural signature - involving hydroxyl groups in the H-bond - has been characterized and ascribed to the glucophore's AH and B sites in accordance with Shallenberger’s old proposal.R. S. Shallenberger, T. E. Acree, Nature, 1967, 216, 480-482^, R.S.Shallenberger, T.E.Acree, C.Y.Lee, Nature, 1969, 221, 555-556

The multiconformational landscape of the non-proteinogenic cyclic amino acid pipecolic acid has been explored in the gas phase. Solid pipecolic acid (m.p. 280^°C) was vaporized by laser ablation (LA) and expanded in a supersonic jet where the rotational spectra of seven conformers were obtained by broadband microwave spectroscopy (CP-FTMW). All conformers were conclusively identified by comparison of the experimental spectroscopic constants with those predicted theoretically. The relative stability of the conformers rests on a delicate balance of the different intramolecular hydrogen bonds established between the carboxylic and the amino groups.

Vaporization of solid homoserine and homocysteine by laser ablation in combination with Fourier transform microwave spectroscopy techniques made possible the detection of their most stable structures in a supersonic expansion. All detected conformers have been identified through their rotational and ¹⁴N quadrupole coupling constants. They show hydrogen bonds linking the amino and carboxylic group through N-H···O=C (type I) or N···H-O (type II) interactions. In some of them there are additional hydrogen bonds established between the amino group and the hydroxyl/thiol groups in the gamma position. Entropic effects related to the side chain have been found to be significant in determining the most populated conformations.

A dimer of urea and isocyanic acid has been generated and observed in the gas phase. The complex was generated by laser vaporisation of a rod target containing urea and copper in a 1:1 ratio, then cooled in a supersonic expansion. Six isotopologues of the complex have been characterised using a chirped pulse Fourier-transform microwave spectrometer in the frequency range 6.5-18.5 GHz. The spectra have been fitted to the Hamiltonian for an asymmetric rotor using PGOPHER. Data obtained from the ¹³C and ¹⁵N isotopologues confirms that all nitrogen atoms are close to the a intertial axis while the carbon atoms are not. A tentative structure will be presented.

An isolated, gas-phase dimer of imidazole is generated through laser vaporisation of a solid rod containing a 1:1 mixture of imidazole and copper in the presence of an argon buffer gas undergoing supersonic expansion. The complex is characterised through broadband rotational spectroscopy and is shown to have a twisted, hydrogen-bonded geometry. Calculations at the CCSD(T)(F12*)/cc-pVDZ-F12 level of theory confirm this to be the lowest-energy conformer of the imidazole dimer. The distance between the respective centres of mass of the imidazole monomer subunits is determined to be 5.2751(1) Å, and the twist angle γ describing rotation of one monomer with respect to the other about a line connecting the centres of mass of the monomers is determined to be 87.9(4)^o. Four out of six intermolecular parameters in the model geometry are precisely determined from the experimental rotational constants and are consistent with results calculated ab initio.

Microwave spectroscopy provides a unique opportunity to study model non-covalent interactions. Of particular interest is the hydrogen bonding of water, whose various molecular properties are influenced by both strong and weak intermolecular forces. More specifically, measuring the hydrogen bonded structures of water-alcohol dimers investigates both strong (OH ··· OH) and weak (CH ··· OH) hydrogen bond interactions. Recently, we have measured the pure rotational spectrum of the isopropanol-water dimer using chirped-pulse Fourier transform microwave spectroscopy (CP-FTMW) between 8-18 GHz. Here, we present the spectrum of this dimer and elaborate on the structure’s strong and weak hydrogen bonding.

The odorant dihydrocarveol (C₁₀H₁₈O) has been investigated in the gas phase using a 2-8 GHz chirped-pulse Fourier transform microwave spectrometer. Dihydrocarveol was purchased as a mixture of n-, iso-, neo-, and neoiso- isomers. The sample was placed in a bespoke heating nozzle at about 85^°C and seeded in Ne at 5 bar. Three conformers were observed and their rotational constants were determined. By comparing the experimental rotational constants with those calculated ab initio the three conformers were identified as belonging to n-dihydrocarveol. In all three conformers the isopropenyl group is in equatorial position with respect to the six-membered ring, and the OH group maintains the same configuration. The conformers differ in the orientation of the isopropenyl group.

Although significant advances in understanding the human olfactory system have taken place over the last two decades, detailed information on how the interactions between odorants and olfactory receptors occur at the molecular level is still lacking. To achieve a better understanding on the molecular mechanisms involved in olfaction, we are investigating several odorants and their interactions with mimics of amino acid residues in olfactory receptors. We present here the structural characterisation of fenchone (C₁₀H₁₆O) and its complexes with ethanol (to mimic the side chain of serine) using a 2-8 GHz chirped-pulse Fourier transform microwave spectrometer built at King’s College London. The rotational spectrum of the parent species and all the ¹³C and ¹⁸O isotopologues of fenchone was observed, and from the experimental rotational constants the substitution (r₀) and effective (r_s) structures of fenchone were determined. The rotational spectrum of fenchone-ethanol was observed by adding ethanol to the carrier gas and passing the mixture through a receptacle with fenchone. Several 1:1 complexes of fenchone-ethanol have been identified in the rotational spectrum. In all the complexes the ethanol molecule binds to the carbonyl group through an O-H· · · O hydrogen bond.

Many biological processes such as chemical recognition and protein folding are mainly controlled by the interplay between hydrogen bonds and dispersive forces. Broadband rotational spectroscopy studies of weakly bound complexes are able to accurately reveal the structures and internal dynamics of molecular clusters isolated in the gas phase. To investigate the influence of the interplay between different types of weak intermolecular interactions and how it controls the preferred active sites of an amphiphilic molecule, we are using camphor (C₁₀H₁₆O, 1,7,7-trimethylbicyclo[2.2.1]hepta-2-one) with different aliphatic alcohol systems. Camphor is a conformationally rigid bicyclic molecule endowed with considerable steric hindrance and has a single polar group (-C=O). The rotational spectrum of camphor and its structure has been previously reported [1] as well as multiple clusters with water [2]. In order to determine the structure of the camphor-alcohol complexes, we targeted low energy rotational transitions in the 2-8 GHz range under the isolated conditions of a molecular jet in the gas phase. The data obtained suggests that camphor forms one complex with methanol and two with ethanol, with differences in the intermolecular interaction in both complexes. With these results, we aim to study the shift in intermolecular interaction from hydrogen bonding to dispersion with the increase in the size of the aliphatic alcohol.
[1] Z. Kisiel, et al., Phys. Chem. Chem. Phys., 5 (2003), 820–826. [2] C. Pérez, et al, J. Phys. Chem. Lett., 7 (2016), 154–160.