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The 1:1 benzofuran–formaldehyde complex has been chosen as model system for analyzing π→π* interactions in supramolecular organizations involving heteroaromatic rings and carbonyl groups. A joint “rotational spectroscopy–quantum chemistry” strategy unveiled the dominant role of π→π* interactions in tuning the intermolecular interactions of such adduct. The exploration of the intermolecular potential energy surface led to the identification of 14 low-energy minima, with 4 stacked isomers being more stable than those linked by hydrogen bond or lone-pair→π interactions. All energy minima are separated by loose transition states, thus suggesting an effective relaxation to the global minimum under the experimental conditions. This expectation has been confirmed by the experimental detection of only one species, which was unambiguously assigned owing to the computation of accurate spectroscopic parameters and the characterization of 11 isotopologues. The large number of isotopic species opened the way to the determination of the first semi-experimental equilibrium structure for a molecular complex of such a dimension.

We assigned the rotational spectra of the parent and the OD isotopologues of the intermolecular complex pyridine-tert-butyl alcohol. The rotational and 14 N quadrupole coupling constants are in agreement with a σ-type shape and a C s symmetry of the complex. The two subunits are held together by a "classical" O-H⋯N intermolecular hydrogen bond. Structural features of these hydrogen bonds are given and compared to those of similar molecular adducts. The O⋯N distance decreases by 4 mÅ upon deuteration of the hydroxyl group, denoting a marked reverse Ubbelohde effect of the O-H⋯N hydrogen bond.

The rotational spectra of the parent and eight isotopologues of the 1:1 complex formic acid – tert-butyl alcohol (FA-TBA) have been measured by pulsed jet Fourier transform microwave spectroscopy. The spectra have been observed in the supersonic expansion of a mixture of FA and TBA in Helium, differently with respect to the mixtures of FA with primary and secondary alcohols, which undergo the esterification reaction upon supersonic expansion. In the complex, the two subunits are linked to each other by two different O-H···O hydrogen bonds (HB) in which FA and TBA are alternate their roles of bond acceptor and donor. Upon H → D substitution of the corresponding O-H···O HB, a small Ubbelohde effect is observed.

The rotational spectrum of the 1:1 complex formic acid-isopropylformate (FA-IPF) has been first observed when trying to assign the pulsed jet Fourier transform microwave (FTMW) spectrum of the adduct formic acid-2-propanol, by expanding a binary mixture of HCOOH and 2-propanol in He. The strong FTMW spectrum of isopropylformate, formed by the esterification reaction, was observed instead. However, when HCOOH was in excess in the binary mixture, it was possible to observe and assign the rotational spectrum of FA-IPF. Later on a much intense spectrum of FA-IPF was obtained, when combining FA with IPF. Finally, the spectra of five isotopologues of the most stable isomer of formic acid?isopropylformate have been observed by means of rotational spectroscopy in supersonic expansion. Some of them, HCOOH-(CH 3 ) 2 CHOOCD and HCOOH-(CH 3 ) 2 CDOOCH have been synthesized in the MW cavity by using DCOOH or (CH 3 ) 2 CDOH as precursors in the esterification process. In the observed isomer of the complex, the two subunits are linked to each other by a standard O-H···O and a weak C-H···O hydrogen bond. The dissociation energy has been estimated to be 34.1 kJ·mol -1 .

The rotational spectra of several isotopologues of the 1:1 complex between the inhaled anesthetic isoflurane and water have been recorded and analyzed by using Fourier transform microwave spectroscopy. The rotational spectrum showed a single rotamer, corresponding to the configuration in which the most stable conformer of isolated isoflurane is linked to the water molecule through an almost linear C-H⋅⋅⋅O weak hydrogen bond. All transitions display a hyperfine structure due to the (35)Cl (or (37)Cl) nuclear quadrupole effects.

Formic acid and dimethyl ether are combined in a supersonic expansion to form a molecular adduct with the two subunits held together by a "classical" OH···O hydrogen bond and a bifurcated weak CH2···O hydrogen bond. The rotational spectra of the parent and of two (13)C isotopologues in natural abundance show that the complex has Cs symmetry, with the heavy atom symmetry planes of HCOOH and (CH3)2O perpendicular to each other.

The rotational spectra of two isotopologues of chlorotrifluoroethylene-dimethyl ether show that the two constituent molecules are held together by a lone pair center dot center dot center dot pi interaction. The ether oxygen is linked to the (CF2) carbon atom, with a C-O distance of 2.908 angstrom.

The potential energy surface (PES) of the general anesthetic fluoroxene (2,2,2-trifluoroethyl vinyl ether) was probed in a supersonic jet expansion using broadband chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy and theoretical calculations. The PES is dominated by a single conformation, as other stable minima are shown to kinetically relax in the expansion to the global minimum. Consistently, the rotational spectrum reveals a single conformation. Fluoroxene adopts a CS heavy-atom planar skeleton structure in the gas phase, with a cis-trans conformation (cis for the CH2=CH-O-CH2- and trans for the =CH-O-CH2-CF3 part). The sensitivity of a recently-built CP-FTMW spectrometer at the UPV/EHU is demonstrated by the detection of five isotopologues of fluoroxene in natural abundance, corresponding to the (13)C and (18)O monosubstituted species. The rS and r0 structures were determined and are in good agreement with theoretical predictions using the MP2, B3LYP and M06-2X methods.

The pulsed supersonic jet expansion microwave spectra of the parent and all three 13C mono-substituted isotopologues of methyl trifluoroacetate have been measured in the 6.5–18 GHz range. All observed transitions are split into two component lines, due to the internal rotation of the methyl group. The corresponding barrier has been determined to be V 3 = 4.379(3) kJ/mol. Structural information has been obtained from the 12 available rotational constants.

The rotational spectra of four isotopologues of the 1:1 complex between chloromethane and water revealed the presence of only one rotamer in a pulsed jet expansion. The two subunits are linked through two weak hydrogen bonds, O-H⋅⋅⋅Cl (R(H⋅⋅⋅Cl) =2.638(2) Å) and C-H⋅⋅⋅O (R(H⋅⋅⋅O) =2.501(2) Å), forming a five-membered ring. All transitions display the hyperfine structure due to the (35)Cl (or (37)Cl) nuclear quadrupole effects. Dynamical features in the spectrum are caused by two large-amplitude motions. Each component line appears as an asymmetric doublet with a relative intensity ratio of 1:3. The splittings led to the determination of barrier to internal rotation of water around its symmetry axis, V(2) =320(10) cm(-1). Finally, an unexpected small value of the inertial defect (-0.96 uÅ(2) rather than -3.22 uÅ(2)) allowed the estimation of the barrier to the internal rotation of the CH(3) group, V(3) ≈8 cm(-1).

The rotational spectra of 2 isotopologues of the molecular adduct CH2F2–CH2ClF show that the two subunits are held together through two weak C–H F–C and one C–H Cl–C linkages, rather than through three weak C–H F–C connections. This suggests the weak Cl H interaction to be observed preferentially to a weak F H one. Structural information on these weak hydrogen bonds has been obtained. The dissociation energy has been estimated from centrifugal distortion to be 5.3 kJ mol1.

The rotational spectra of trifluoroanisole (trifluoromethoxybenzene, C6H5OCF3) and of its 13C and 18O isotopologues in natural abundance have been measured in a supersonic expansion with pulsed-jet Fourier transform microwave spectroscopy. The spectrum is consistent with a perpendicular conformation of the CF3 group with respect to the phenyl ring.

The rotational spectra of four conformers (At, Ag, Et, Eg) of the tertiary alcohol 1-methylcyclohexanol were assigned by pulsed jet Fourier transform microwave spectroscopy. The transitions of two gauche conformers were split in two separated component lines, but it was not possible - from the available measured transitions - to accurately determine their vibrational ΔE0+0- ground-state splittings, respectively. In addition, the rotational spectra of the four OD deuterated isotopologues were measured and assigned. For the gauche species of this isotopologue we were able to determine the tunneling splittings, ΔE0+0-(Ag, OD) = 15.581(5) GHz and ΔE0+0-(Eg, OD) = 18.17(3) GHz, respectively. From these splittings the inversion barriers for Ag and Eg were determined, by using a flexible model, to be B2(Ag) = 356(10) and B2(Eg) = 320(10) cm-1, respectively.

The rotational spectra of five isotopologues of the 1:1 complex trifluoroanisole-water have been investigated with pulsed jet Fourier transform microwave spectroscopy. The triple fluorination of the methyl group greatly affects the features of the rotational spectrum of the complex with water, with respect to those of the related complex anisole-water. The shape, the internal dynamics, and the isotopic effects turned out to be quite different from those of the anisole-water adduct ( Giuliano , B. M. ; Caminati , W. Angew. Chem., Int. Ed. 2005 , 44 , 603 - 606 ). However, as in anisole-water, water has the double role as a proton donor and proton acceptor, and it is linked to trifluoroanisole through two, O-H···O and C-H···O hydrogen bonds.

The rotational spectra of two isotopologues of a 1:1 difluoromethane-dichloromethane complex have been investigated by pulsed-jet Fourier-transform microwave spectroscopy. The assigned (most stable) isomer has C(s) symmetry and it displays a network of two C-H⋅⋅⋅Cl-C and one C-H⋅⋅⋅F-C weak hydrogen bonds, thus suggesting that the former interactions are stronger. The hyperfine structures owing to (35)Cl (or (37)Cl) quadrupolar effects have been fully resolved, thus leading to an accurate determination of the three diagonal (χ(gg); g=a, b, c) and the three mixed quadrupole coupling constants (χ(gg'); g, g'=a, b, c; g≠g'). Information on the structural parameters of the hydrogen bonds has been obtained. The dissociation energy of the complex has been estimated to be 7.6 kJ mol(-1).

The rotational spectra of four isotopologues of the 1:1 complex between formic acid and pyridine show that the two units are linked together through a “classical” (OH···N) and a weak (CH···O) hydrogen bond. The molecular system appears quite rigid and no effects of the internal motions have been observed in the spectrum. The dissociation energy obtained from the centrifugal distortion by applying an approximate model, 39.8 kJ/mol, is quite similar to the ab initio value, 41.7 kJ/mol. Its relatively high value suggests a small charge transfer to take place.

The hydration of 2-fluoropyridine with one water molecule has been studied with a pulsed jet Fourier transform microwave technique. The rotational spectra of five isotopologues of the 1:1 complex have revealed the formation of a single most stable isomer. The observed cluster is characterized by an O−H⋅⋅⋅N hydrogen bond and a C−H⋅⋅⋅O weak hydrogen bond, where water is coplanar to the aromatic ring and acts in a double role of proton donor and proton acceptor.

The pulsed-jet Fourier transform rotational spectra of 4 isotopologues have been recorded for the most stable conformation of the molecular cluster pyridine-CH3F. Two weak C-H···N and C-H···F hydrogen bonds link the two subunits of the complex. Structural information on the hydrogen bridges has been obtained. The internal rotation of the CH3F subunit around its symmetry axis splits all rotational transitions into two (A and E) well resolved component lines, leading to a V3 barrier height of 1.55(1) kJ mol(-1).

The rotational spectra of two conformers of 1,4-butanediol have been assigned by pulsed-jet Fourier-transform microwave spectroscopy. Both species display an intra-molecular OH⋯O hydrogen bond. For the most stable conformer, the rotational spectra of four isotopologues (OH–OH, OD–OH, OH–OD and OD–OD) have been measured. The μa-type transition of its two symmetric species, OH–OH and OD–OD, are split into two evenly spaced component lines separated by 6.292(1) and 0.0137(5) MHz, respectively. These splittings are due to the concerted internal rotation of the two OH (or OD) alcoholic groups, which connects two equivalent minima. A partial rs structure has been obtained.

The rotational spectra of four isotopologues of the adduct C2F3Cl-NH3 show that NH3 is bound to the partner molecule through a (N)lone-pair···π interaction. Ammonia is located in proximity to the C2 atom (the one linked to two fluorine atoms), with the C2···N distance = 2.987(2) Å. The nuclear hyperfine structure due to the quadrupole coupling effects of (35)Cl/(37)Cl and (14)N nuclei has been fully resolved. The (14)N quadrupole coupling constants allow estimating the effective orientation of NH3 in the complex.

The rotational spectra of five isotopologues of dimethyl sulfate have been assigned by pulsed-jet Fourier-transform microwave spectroscopy. The global minimum configuration has a C2 symmetry, with the two methyl groups nearly trans with respect to the two sulfonic oxygen atoms. The internal rotations of the two methyl groups split each transition into several components line, allowing for the determination of accurate values of the V3 barrier height to internal rotation and of the orientation of the methyl groups with respect to the principal axis system. The geometry of the molecular skeleton is fully determined.