Conformation of chiral molecules: the rotational spectrum of 2-chloropropionic acid

Reported here are measurements of the rotational spectra of two isotopologues (35Cl and 37Cl) of one of the simplest chiral carboxylic acids, 2-chloro-propionic acid. The spectra have been obtained in a supersonic expansion, using free-jet millimeter-wave direct-absorption as well as pulsed-jet FT microwave Spectroscopies. A single conformer has been detected, characterized by a methyl group nearly cis with respect to the carbonylic oxygen. Within the realm of chiral recognition, the relative simplicity of the observed spectrum should allow for the detection of the microwave spectrum of the polar homo-dimer

N-Methyl Inversion and Structure of Six-Membered Heterocyclic Rings: Rotational Spectrum of 1-Methyl-4-piperidone

The conformational and structural properties of the six-membered heterocyclic ring of 1-methyl-4-piperidone have been observed in a jet-cooled supersonic expansion using Fourier transform microwave spectroscopy (FT-MW). The rotational spectrum evidenced two different conformations originated by nitrogen inversion, with the N-methyl group in either equatorial (most stable) or axial position. Additional observation of the rotational spectra for all possible carbon, nitrogen, and oxygen monosubstituted species (4 13C, 15N, 18O) in natural abundance allowed us to determine substitution (rs) and effective structures (r0) for the equatorial conformer. Additional ab initio and DFT calculations provided comparative rotational parameters, structural data, conformational energies, and the axial equatorial interconversion barrier. The structural data were compared with the related azabicycle of tropinone, revealing the molecular changes and structural relaxation associated with the presence of the two-carbon bridge in the latter molecule

The Conformational Landscape of Nicotinoids: Solving the Conformational Disparity of Anabasine

The conformational landscape of the alkaloid anabasine (neonicotine) has been investigated using rotational spectroscopy and ab initio calculations. The results allow a detailed comparison of the structural properties of the prototype piperidinic and pyrrolidinic nicotinoids (anabasine vs. nicotine). Anabasine adopts two most stable conformations in isolation conditions, for which we determined accurate rotational and nuclear quadrupole coupling parameters. The preferred conformations are characterized by an equatorial pyridine moiety and additional N-H equatorial stereochemistry at the piperidine ring (Eq-Eq). The two rings of anabasine are close to a bisecting arrangement, with the observed conformations differing in a ca. 180\uba rotation of the pyridine subunit, denoted either Syn or Anti. The preference of anabasine for the Eq-Eq-Syn conformation has been established by relative intensity measurements (Syn/Anti~5(2)). The conformational preferences of free anabasine are directed by a weak N\u2022\u2022\u2022H-C hydrogen bond interaction between the nitrogen lone pair at piperidine and the closest C-H bond in pyridine, with N\u2022\u2022\u2022H distances ranging from 2.686 (Syn) to 2.667 \uc5 (Anti). Supporting ab initio calculations using MP2 and the recent M05-2X density functional are provided, evaluating the predictive performance of both methods

How Trifluoroacetone Interacts with Water

The rotational spectra of five isotopologues of the molecular adduct 1,1,1-trifluoroacetonewater have been assigned using pulsed-jet Fourier-transform microwave spectroscopy. All rotational transitions appear as doublets, due to the internal rotation of the methyl group. Analysis of the tunneling splittings allows one to determine accurately the height of the 3-fold barrier to internal rotation of the methyl group and its orientation, leading to V3 = 3.29 kJ 3 mol1 and \u2014(a,i) = 67.5, respectively. The water molecule is linked to the keton molecule on the side of the methyl group through a OH3 3 3O hydrogen bond and a CH3 3 3O intermolecular contact, lying in the effective plane of symmetry of the complex

Interplay of Phenol and Isopropyl Isomerism in Propofol from Broadband Chirped-Pulse Microwave Spectroscopy

The conformational equilibrium of the general anaesthetic propofol (2,6-di-isopropylphenol) has been studied in a supersonic expansion using broadband chirped-pulse microwave spectroscopy. Three conformers originated by the combined internal rotation of the hydroxyl and the two isopropyl groups have been detected in the jet-cooled rotational spectrum. The most stable conformer exhibits tunnelling splittings associated to the internal rotation of the hydroxyl group, from which we determined the torsional potential and barrier heights (905 and 940 cm-1). A carbon backbone structure derived from spectral assignments of all twelve 13C monosubtituted isotopologues of this conformer in natural abundance confirmed a plane-symmetric gauche orientation of the two isopropyl groups (Gg). In the other two detected conformers (EG and GE), one of the isopropyl groups is eclipsed respect to the ring plane while the other is gauche, differing in the orientation of the hydroxyl group. Supporting ab initio calculations provided information on the potential energy surface and molecular properties of the title compound

Protic Group Tunnelling in the 1:1 Complexes of Dimethylether with HF, HCl and H2O

The rotational spectra of the molecular adducts concerning dimethylether (DME) show interesting features such as: (i) three weak C-H 7 7 7O improper H-bonds in the DME-DME dimer [1] and, (ii) the tunnelling splittings of Ar and Ne in the 1:1 complexes with DME [2,3]. Here we report the results of the studies on the interactions of DME with strong proton donors such as H2O, HF and HCl. All of them tunnel between the two lone pairs of the O atom of DME, generating huge inversion splittings in the rotational spectra. From them it has been possible to determine the rates and pathways of the inversion processes. . Millimeter wave absorption free jet and molecular beam Fourier transform microwave spectroscopy techniques have been used for the experiments.DME 7 7 7HCl is shown on the figure to the right. HCl is tunnelling from above to below the COC plane with a frequency of 8215.7 MHz. [1] \u2013 Y. Tatamitani, B. Liu, J. Shimada, T. Ogata, P. Ottaviani, A. Maris, W. Caminati and J. L. Alonso, J. Am. Chem. Soc., 124 (2002) 2739. [2] \u2013 P. Ottaviani, A. Maris, W. Caminati, Y. Tatamitani, Y. Suzuki, T. Ogata and J. L. Alonso, Chem. Phys. Letters, 361 (2002) 341. [3] \u2013 A. Maris and W. Caminati, J. Chem. Phys., 118 (2003) 1649

Features of weak C-H\ub7\ub7\ub7O and C-H\ub7\ub7\ub7F intermolecular hydrogen bonds by Fourier transform microwave spectroscopy

So much scientific interest has been dedicated during the last years to the Weak Hydrogen Bond (WHB), a non covalent interaction which manifests itself in myriad ways in Structural Chemistry and Biology, that this topic deserved recently the publication of a book [1]. Only a few rotationally resolved spectroscopic investigations, which detail specific or local interactions, are available, however, on WHB. The three C-H 7 7 7F-C WHB interactions of the difluoromethane dimer [2] and the three C-H\ub7\ub7\ub7O interactions of dimethylether dimer [3] have been characterized from their pure rotational spectra. Each of these interactions has been estimated to have an energy of about 2 kJ/mol and a little blue shift of the C-H stretching Here we report the results of the investigations of the rotational spectra of some heterodimers held together through three WHB\u2019s. 1) Oxirane-trifluoromethane. The investigation of the parent and of its two 13C species has been used to establish a Cs geometry, with the two moieties bound by one C-H 7 7 7O and two C-H 7 7 7F-C hydrogen bonds. An overall bonding energy of about 6.7 kJ/mol has been derived from the centrifugal distortion analysis. The lengths of the C-H 7 7 7O and C-H 7 7 7F hydrogen bonds, r(O 7 7 7H) and r(F 7 7 7H), are 2.37 and 2.68 \uc5, respectively. The C-H\ub7\ub7\ub7F-C interactions are at the origin of the HCF3 internal rotation motion barrier of 0.55(1) kJ/mol giving rise to the A-E splittings observed in the rotational spectra. 2) 1,4-dioxane-trifluoromethane. The FT-MW spectrum of the Cs conformation with CHF3 axial with respect to 1,3-dioxane has been assigned. It is stabilized by one C-H 7 7 7O and two C-H 7 7 7F weak hydrogen bonds. 3) cyclobutanone-trifluoromethane. This is the first rotationally resolved investigation of a complex with a C-H 7 7 7O interaction involving one carbonylic oxygen. This bridge, together with two C-H 7 7 7F weak hydrogen bonds, stabilizes the configuration shown aside. [1] - \u201cThe weak hydrogen bond in structural chemistry and biology\u201d IUCr Monographs on crystallography, Vol. IX (G.R.Desiraju and T.Steiner Ed.s) Oxford University Press, (2001). [2] - (a) Caminati, W.; Melandri, S.; Moreschini, P.; Favero, P.G.; Angew. Chem. Int. Ed., 1999, 38, 2924; (b) Blanco, S.; L\uf3pez, J. C.; Lesarri, A.; Alonso, J. L.; J. Mol. Struct. 2002, 612, 255. [3] - Y. Tatamitani, B. Liu, J. Shimada, T. Ogata, P. Ottaviani, A. Maris, W. Caminati and J. L. Alonso, J. Am. Chem. Soc., 124 (2002) 2739