Laser vaporization of imidazole in the presence of an argon buffer gas has allowed the generation and isolation of two isomers of an imidazole monohydrate complex, denoted herein as imid···H2O and H2O···imid, within a gas sample undergoing supersonic expansion. Imidazole and water are respectively proton-accepting and proton-donating in imid···H2O, but these roles are reversed in the H2O···imid complex. Both isomers have been characterized by chirped-pulse Fourier transform microwave spectroscopy between 7.0 and 18.5 GHz. The ground-state rotational spectra of four isotopologues of imid···H2O and three isotopologues of H2O···imid have been measured. All spectra have been assigned and fitted to determine rotational (A 0, B 0, C 0), centrifugal distortion (D J , D JK ), and nuclear quadrupole coupling constants (χ aa (N1), [χ bb (N1) – χ cc (N1)], χ aa (N3), and [χ bb (N3) – χ cc (N3)]). Structural parameters (r 0 and r s) have been accurately determined from measured rotational constants for each isomer. The imid···H2O complex contains a nonlinear hydrogen bond (∠(O–Hb···N3) = 172.1(26)° in the experimentally determined, r 0 geometry) between the pyridinic nitrogen of imidazole and a hydrogen atom of H2O. The DFT calculations find that the H2O···imid complex also contains a nonlinear hydrogen bond between the oxygen atom of water and the hydrogen attached to the pyrrolic nitrogen of imidazole (∠(O···H1–N1) = 174.7°). Two states observed in the spectrum of H2O···imid, assigned as 0– and 0+ states, confirm that large amplitude motions occur on the time scale of the molecular rotation. Density functional theory has been performed to characterize these large amplitude motions.
The rotational spectra of N-, 2-, 4-, and 5-methylimidazole are reported and analyzed. Liquid N-methylimidazole was vaporized from a reservoir, and each of 2-, 4-, and 5-methylimidazole was laser-vaporized from a solid target prior to mixing with argon buffer gas and undergoing supersonic expansion from a pulsed nozzle. The spectra were recorded by chirped-pulse Fourier transform microwave spectroscopy in the 7.0–18.5 GHz frequency range. Rotational constants, A0, B0, and C0, centrifugal distortion constants, DJ, DJK, DK, d1, and d2, and nuclear quadrupole coupling constants of nitrogen atoms, χaa(N1), χbb(N1) − χcc(N1), χaa(N3), and χbb(N3) − χcc(N3), are determined from experimentally measured transition frequencies. Data recorded for isotopologues containing 13C or 15N are used to determine the rs coordinates of all heavy atoms in N-, 2-, and 4-methylimidazole. The results allow fitting of parameters in the Hamiltonian that describes internal rotation of the CH3 group about its local C3 axis. The V3 terms in the periodic potential energy functions that describe the internal rotation in N-, 2-, 4-, and 5-methylimidazole are determined to be 185.104(11), 122.7529(38), 317.20(14), and 386.001(19) cm−1, respectively, by the internal axis method. The experiments are supported by density functional theory calculations. Observed variations in barrier height are explained with reference to the symmetry of overlap between a π-like orbital on the CH3 group and π-orbitals on the aromatic ring.
Broadband microwave spectra have been recorded between 7.0 and 18.5 GHz for N-methylimidazole…H2O and 2-methylimidazole…H2O complexes. Each complex was generated by co-expansion of low concentrations of methylimidazole and H2O in...
General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/pure/userguides/explore-bristol-research/ebr-terms/ A chalcogen-bonded complex H3N⋯S=C=S formed by ammonia and carbon disulfide characterised by chirped-pulse, broadband microwave spectroscopy Abstract Ground-state rotational spectra were observed for ten symmetric-top isotopologues H3N⋯S=C=S, H3N⋯ 34 S=C=S, H3N⋯S=C= 34 S, H3N⋯S= 13 C=S, H3 15 N⋯S=C=S, H3 15 N⋯ 34 S=C=S, H3 15 N⋯S=C= 34 S, H3 15 N⋯S= 13 C=S, H3 15 N⋯ 33 S=C=S and H3 15 N⋯S=C= 33 S, the first five in their natural abundance in a mixture of ammonia and carbon disulphide in argon and the second group with enriched 15 NH3. The four asymmetric-rotor isotopomers H2DN⋯S=C=S, H2DN⋯ 34 S=C=S, H2DN⋯S=C= 34 S, and HD2N⋯S=C=S were investigated by using a sample composed of ND3 mixed with CS2. Rotational constants, centrifugal distortion constants and 33 S nuclear quadrupole coupling constants were determined from spectral analyses and were interpreted with the aid of models of the complex to determine its symmetry, geometry, one measure of the strength of the intermolecular binding, and information about the subunit dynamics. The complex has C3v symmetry, with nuclei in the order H3N⋯S=C=S, thereby establishing that the non-covalent interaction is a chalcogen bond involving the non-bonding electron pair of ammonia as the nucleophile and the axial region near one of the S atoms as the electrophile. The small intermolecular stretching force constant k = 3.97(5) N m -1 indicates a weak interaction and suggests the assumption of unperturbed component geometries on complex formation. A simple model used to account for the contribution of the subunit angular oscillations to the zero-point motion leads to the intermolecular bond length r(N⋯S) = 3.388(10) Å. * Corresponding authors: a.c.
Two isomers of a complex formed between thiazole and two water molecules, thi···(H2O)2, have been identified through Fourier transform microwave spectroscopy between 7.0 and 18.5 GHz. The complex was generated by co-expansion of a gas sample containing trace amounts of thiazole and water in an inert buffer gas. For each isomer, rotational constants, A0, B0 and C0, centrifugal distortion constants, DJ, DJK, d1, d2, and nuclear quadrupole coupling constants, and , have been determined through fitting of a rotational Hamiltonian to the frequencies of observed transitions. The molecular geometry, energy, and components of the dipole moment of each isomer have been calculated using Density Functional Theory (DFT). The experimental results for four isotopologues of isomer I allow accurate determinations of the atomic coordinates of the oxygen atoms by r0 and rs methods. Isomer II has been assigned as the carrier of an observed spectrum on the basis of very good agreement between DFT-calculated results and spectroscopic parameters (including A0, B0 and C0 rotational constants) determined by fitting to measured transition frequencies. Non-covalent interaction (NCI) and natural bond orbital (NBO) analyses reveal that two strong hydrogen bonding interactions are present within each of the identified isomers of thi···(H2O)2. The first of these binds an H2O to the nitrogen of thiazole (OH···N) and the second binds the two water molecules (OH···O). A third, weaker interaction binds the H2O sub-unit to the hydrogen atom that is attached to C2 (for isomer I) or C4 (for isomer II) of the thiazole ring (CH···O).
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