Chirped pulse Fourier transform microwave study of 2,2,2-trifluoroethyl formate

The rotational spectra of 2,2,2-trifluoroethyl formate and its three (13)C isotopologues have been measured with a molecular-beam-based, chirped-pulsed Fourier transform microwave spectrometer in combination with a conventional Balle-Flygare-type instrument up to 18 GHz. Although ab initio calculations predict the presence of two low-energy conformers (analogous to the trans and gauche forms of ethyl formate), the trans isomer was the only stable conformer observed. The r(s) geometry of the molecular main carbon frame was precisely derived based on a coplanar heavy-atom backbone of this conformer. ESPs of the two lowest energy conformers were calculated to obtain information about the role of through-space effects on their structures and relative stability.     

A microwave and quantum chemical study of cyclopropaneselenol

The microwave spectrum of cyclopropaneselenol, C 3H 5SeH, has been investigated in the 21.9-80 GHz frequency range. The microwave spectra of the ground vibrational state of five isotopologues of cyclopropaneselenol (C 3H 5 (82)SeH, C 3H 5 (80)SeH, C 3H 5 (78)SeH, C 3H 5 (77)SeH, and C 3H 5 (76)SeH) of one conformer, as well as the spectra of two vibrationally excited states of each of the C 3H 5 (80)SeH and C 3H 5 (78)SeH isotopologues of this rotamer, have been assigned. The H-C-Se-H chain of atoms is synclinal in this conformer, and there is no indication of further rotameric forms in the microwave spectrum. The b-type transitions of the ground vibrational state of the more abundant species C 3H 5 (80)SeH and C 3H 5 (78)SeH were split into two components, which is assumed to arise from tunneling of the proton of the selenol group between two equivalent synclinal potential wells. The tunneling frequencies were 0.693(55) MHz for C 3H 5 (80)SeH and 0.608(71) MHz for C 3H 5 (78)SeH. The microwave study has been augmented by high-level density functional and ab initio quantum chemical calculations, which indicate that the H-C-Se-H dihedral angle is approximately 75 degrees from synperiplanar (0 degrees).     

The gas-phase structure of alanine

The jet-cooled rotational spectrum of neutral alanine has been studied using laser-ablation molecular-beam Fourier transform microwave spectroscopy (LA-MB-FTMW). The spectra of the two most stable forms were observed in the frequency range 6-18 GHz for the parent, (15)N alanine, three single (13)C species, and four single D species. The (14)N nuclear quadrupole coupling hyperfine structures have been resolved, and their comparison with those calculated theoretically confirms unambiguously the conformer assignments. The independent structures of both conformers have been determined experimentally for the first time using r(s) and r(0) procedures. In both cases, the amino acid backbone is nonplanar. For the most stable conformer I, the COOH group adopts a cis configuration and an asymmetric bifurcated hydrogen bond is formed between the amino group and carbonyl oxygen (r(N-H(a)...O=C) = 2.70(2) A and r(N-H(b)...O=C) = 2.88(2) A). For conformer IIa, the COOH group adopts a trans configuration and is stabilized by a O-H...N hydrogen bond (r(O-H...N) = 1.96(2) A). The relative conformer abundances in the supersonic expansion have also been investigated.     

A microwave and quantum chemical study of the conformational properties of etheneselenocyanate (H(2)C=CHSeC[triple bond]N)

The structural and conformational properties of etheneselenocyanate (H2C=CHSeC[triple bond]N) have been explored by microwave spectroscopy and quantum chemical calculations performed at the MP2/aug-cc-pVTZ and B3LYP/aug-cc-pVTZ levels of theory. The spectra of two rotameric forms were assigned. The more stable form has a synperiplanar conformation, whereas the less stable form has an anticlinal conformation characterized by a C-C-Se-C dihedral angle of 163(3) degrees from the synperiplanar position (0 degrees). The synperiplanar form was found to be 4.5(4) kJ/mol more stable than the anticlinal form by relative intensity measurements performed on microwave transitions. The spectra of several isotopologues and two vibrationally excited states were assigned for the synperiplanar conformer. The anticlinal rotamer displays a complicated pattern of low-frequency vibrational states, which is assumed to reflect the existence of a small potential hump at the antiperiplanar (180 degrees) conformation. The predictions made in the MP2 and B3LYP calculations are in reasonably good agreement with the experimental results in some cases, whereas rather large differences are seen for other molecular properties.     

Gas-phase Raman spectra of s-trans- and s-gauche-1,3-butadiene and their deuterated isotopologues

The gas-phase Raman spectra of 1,3-butadiene and its 2,3-d(2),1,1,4,4-d(4) and d(6) isotopologues have been recorded using intense (6 W) green laser excitation and sensitive CCD detection. Hundreds of bands have been observed and assigned for each isotopologue. These spectra provide the best data to date for the s-trans conformer and also provide the first direct observation of the gas-phase Raman bands of the s-gauche conformer. Spectra recorded at elevated temperatures up to 250 °C for the d(0) and d(6) species help confirm the assignment of bands for the gauche rotamer. DFT computations were utilized to complement the studies. For the most part, the observed gas-phase gauche bands are in good agreement with previous matrix isolation studies. A best set of frequencies are reported for the fundamentals of the gauche rotamer of the d(0) and d(6) species.     

Microwave spectrum and conformational composition of 2-chloroethylisocyanide

2-Chloroethylisocyanide (ClCH(2)CH(2)N≡C) has been synthesized, and its microwave spectrum has been investigated in the 20-97 GHz spectral region. The spectra of (35)Cl and (37)Cl isotopologues of two conformers have been assigned. The Cl-C-C-N chain of atoms is antiperiplanar in one of these rotamers and synclinal in the second. The energy difference between the two forms has been obtained from relative intensity measurements. It was found that the antiperiplanar conformer is favored over the synclinal form by 4.3(8) kJ/mol. Quantum chemical calculations at the CCSD/cc-pVTZ and B3LYP/cc-pVTZ levels of theory have been performed. Most, but not all, of the spectroscopic constants predicted in these calculations are in good agreement with their experimental counterparts. The theoretical calculations correctly predict that the 2-chloroethylisocyanide exists as a mixture of an antiperiplanar and a synclinal conformer, with the former about 3.5 kJ/mol more stable than the latter. Both methods of calculations find that the antiperiplanar rotamer has a symmetry plane. The dihedral angle formed by the Cl-C-C-N link of atoms of the synclinal form is 67° according to the CCSD calculations. It is estimated from a comparison with the experimental rotational constants that this dihedral angle is uncertain by ±3°.     

Relative energy and structural differences of axial and equatorial 1-fluoro-1-silacyclohexane

The rotational spectra of the main isotopomer, of the (29)Si and of all (13)C isotopologues of axial and equatorial forms of 1-fluoro-silacyclohexane have been measured by conventional (only main species) and molecular beam Fourier transform microwave spectroscopy. r(0) and partial r(s) structures are given separately for the two forms. The main structural differences are discussed. From dipole moments and relative intensity measurements, a slight preference (E(Eq) - E(Ax) = 42 +/- 24 cm(-1)) for the axial conformer was found. The rotational spectra of some, the most intense, vibrational satellites have also been measured. They belong to the ring-puckering motions.     

Microwave spectroscopy measurements of rotational spectra and DFT calculations for two distinct structural isomers of 1,1'-dimethylferrocene

Microwave spectra were obtained for two distinct structural isomers of 1,1'-dimethylferrocene, an eclipsed synperiplanar isomer (phi = 0 degrees, the E0 isomer), with A = 1176.9003(2) MHz, B = 898.3343(2) MHz, C = 668.7469(2) MHz, and an eclipsed synclinal isomer (phi = 72 degrees, the E72 isomer) with A = 1208.7117(14) MHz, B = 806.4101(12) MHz, and C = 718.7179(8) MHz. The b-dipole, asymmetric-top spectra of both structural isomers were measured in the frequency range of 5-12 GHz using a Flygare-Balle type of spectrometer. A very good fit to observed transitions, with small distortion constants, was obtained for the E0 conformer, indicating that this conformer is nearly rigid. The deviations obtained in a similar least-squares fit for the E72 confomer are significantly larger, indicating possible fluxional behavior for this conformer. In addition, 7 out of the 26 transitions observed for the E72 isomer conformer clearly exhibit very small splittings, giving further evidence for internal motion. DFT calculations for the different possible conformations of 1,1'- dimethylferrocene arising from rotation of one methyl cyclopentadienyl ligand relative to the other about the nominal C5 axis by an angle phi (dihedral angle) were performed using the B3PW91 functional. The calculations converged and were optimized for five structures on this torsional potential energy surface corresponding to different dihedral angles phi; three yielded energy minima, and two gave energy maxima, corresponding to transition states. The experimental results are in very good agreement with the results of the DFT calculations.     

Spectra and structure of silicon containing compounds. XXXII. Raman and infrared spectra,conformational stability,vibrational assignment and ab initio calculations of n-propylsilane-d0 and Si-d3

The infrared (3100-40 cm(-1)) and Raman (3100-20 cm(-1)) spectra of gaseous and solid n-propylsilane, CH(3)CH(2)CH(2)SiH(3) and the Si-d(3) isotopomer, CH(3)CH(2)CH(2)SiD(3), have been recorded. Additionally, the Raman spectra of the liquids have been recorded and qualitative depolarization values obtained. Both the anti and gauche conformers have been identified in the fluid phases but only the anti conformer remains in the solid. Variable temperature (-105 to -150 degrees C) studies of the infrared spectra of n-propylsilane dissolved in liquid krypton have been recorded and the enthalpy difference has been determined to be 220+/-22 cm(-1) (2.63+/-0.26 kJ mol(-1)) with the anti conformer the more stable form. A similar value of 234+/-23 cm(-1) (2.80+/-0.28 kJ mol(-1)) was obtained for deltaH for the Si-d(3) isotopomer. At ambient temperature it is estimated that there is 30+/-2% of the gauche conformer present. The potential function governing the conformation interchange has been estimated from the far infrared spectral data, the enthalpy difference, and the dihedral angle of the gauche conformer, which is compared to the one predicted from ab initio MP2/6-31G(d) calculations. The barriers to conformational interchange are: 942, 970 and 716 cm(-1) for the anti to gauche, gauche to gauche, and gauche to anti conformers, respectively. Relatively complete vibrational assignments are proposed for both the n-propylsilane-d(0) and Si-d(3) molecules based on the relative infrared and Raman spectral intensities, infrared band contours, depolarization ratios, and normal coordinate calculations. The geometrical parameters, harmonic force constants, vibrational frequencies, infrared intensities, Raman activities and depolarization ratios, and energy differences have been obtained for the anti and gauche conformers from ab initio MP2/6-31G(d) calculations. Structural parameters and energy differences have also been obtained utilizing the larger 6-311 + G(d,p) and 6-311 + G(2d,2p) basis sets. From the isolated Si-H stretching frequency from the Si-d(2) isotopomer the r(0) distances of 1.484 and 1.485 A have been determined for the SiH(s) and SiH(a) bonds, respectively, for the anti conformer, and 1.486 A for the SiH bond for the gauche conformer. Utilizing previously reported microwave rotational constants for the anti conformer and the determined SiH distances along with ab initio predicted parameters 'adjusted r(0)' parameters have been obtained for the anti conformer. The results are discussed and compared to those obtained for some similar molecules.     

Anion conformation of low-viscosity room-temperature ionic liquid 1-ethyl-3-methylimidazolium bis(fluorosulfonyl) imide

Anion conformation of a low-viscosity room-temperature ionic liquid 1-ethyl-3-methylimidazolium bis(fluorosulfonyl) imide (EMI+FSI-) has been studied by Raman spectra and theoretical DFT calculations. Three strong Raman bands were found at 293, 328, and 360 cm(-1), which are ascribed to the FSI- ion. These Raman bands show significant temperature dependence, implying that two FSI- conformers coexist in equilibrium. This is supported by theoretical calculations that the FSI- ion is present as either C2 (trans) or C1 (cis) conformer; the former gives the global minimum, and the latter has a higher SCF energy of about 4 kJ mol(-1). Full geometry optimizations followed by normal frequency analyses show that the observed bands at 293, 328, and 360 cm(-1) are ascribed to the C2 conformer. The corresponding vibrations at 305, 320, and 353 cm(-1) were extracted according to deconvolution of the observed Raman bands in the range280-400 cm(-1 )and are ascribed to the C1 conformer. The enthalpy DeltaH degrees of conformational change from C2 to C1 was experimentally evaluated to be ca. 4.5 kJ mol(-1), which is in good agreement with the predicted value by theoretical calculations. The bis(trifluoromethanesulfonyl) imide anion (TFSI-) shows a conformational equilibrium between C1 and C2 analogues (DeltaH degrees = 3.5 kJ mol(-1)). However, the profile of the potential energy surface of the conformational change for FSI- (the F-S-N-S dihedral angle) is significantly different from that for TFSI- (the C-S-N-S dihedral angle).     

Matrix-isolation FT-IR spectra and theoretical study of dimethyl sulfate

The preferred conformations of dimethyl sulfate and their vibrational spectra were studied by matrix-isolation FT-IR spectroscopy and theoretical methods (DFT and MP2, with basis sets of different sizes, including the quadruple-zeta, aug-cc-pVQZ basis). Conformer GG (of C2 symmetry and exhibiting OSOC dihedral angles of 74.3 degrees ) was found to be the most stable conformer in both the gaseous phase and isolated in argon. Upon annealing of the matrix, the less stable observed conformer (GT; with C1 symmetry) quickly converts to the GG conformer, with the resulting species being embedded in a matrix-cage which corresponds to the most stable matrix-site for GG form. The highest energy TT conformer, which was assumed to be the most stable conformer in previous studies, is predicted by the calculations to have a relative energy of ca. 10 kJ mol-1 and was not observed in the spectra of the matrix-isolated compound.     

Free-jet rotational spectrum and tunneling motion of difluoromethane...xenon

The rotational spectra of three isotopologues of difluoromethane...xenon have been investigated by free-jet millimeter-wave absorption spectroscopy. Only mu(c)-type transitions have been observed, all of them evenly split due to the internal motion of Xe relative to the difluoromethane moiety. The vibrational splitting, 39.1(3) MHz, has been used to estimate the tunneling barrier, V(2) = 109 cm(-1). Information on the dissociation energy has been deduced from centrifugal distortion effects (E(B) = 1.8 kJ mol(-1)). The xenon atom lies in the sigma(v) symmetry plane of difluoromethane containing the hydrogen atoms, at an r(0) distance of 3.816 A from its center of mass (cm), and forms a Xe-cm-C r(0) angle of 118 degrees . The observed conformation is in agreement with the minimum found with a distributed polarizability model.     

A microwave spectroscopic and quantum chemical study of propa-1,2-dienyl selenocyanate (H(2)==C==CHSeC[triple bond]N) and cyclopropyl selenocyanate (C(3)H(5)SeC[triple bond]N)

The microwave spectra of propa-1,2-dienyl selenocyanate, H(2)C==C==CHSeC[triple bond]N, and cyclopropyl selenocyanate, C(3)H(5)SeC[triple bond]N, are reported. The spectra of the ground and two vibrationally excited states of the (80)Se isotopologue and the spectrum of the ground state of the (78)Se isotopologue were assigned for one rotameric form of H(2)C==C[double bond, length as m-dash]CHSeC[triple bond]N. This conformer is characterized by a C-C-Se-C dihedral angle of 129(5) degrees from synperiplanar (0 degrees ) and is shown to be the global minimum of H(2)C[double bond, length as m-dash]C[double bond, length as m-dash]CHSeC[triple bond]N. The spectra of the ground and of three vibrationally excited states of the (80)Se isotopologue, as well as of the ground state of the (78)Se isotopologue of one rotamer of C(3)H(5)SeC[triple bond]N were assigned. This conformer has a H-C-Se-C dihedral angle of 80(4) degrees from synperiplanar and is at least 3 kJ mol(-1) more stable than any other form of the molecule. The microwave study has been augmented by quantum chemical calculations at the B3LYP/6-311+ +G(3df,3pd) and MP2/6-311+ +G(3df,3pd) levels of theory.     

Analysis of the rotational structure in the high-resolution infrared spectrum of trans-hexatriene-1-13C1: a semiexperimental equilibrium structure for the C6 backbone of trans-hexatriene

trans-Hexatriene-1-(13)C(1) (tHTE-1-(13)C(1)) has been synthesized, and its high-resolution (0.0015 cm(-1)) infrared spectrum has been recorded. The rotational structure in the C-type bands for ν(26) at 1011 cm(-1) and ν(30) at 894 cm(-1) has been analyzed. To the 1458 ground state combination differences from these bands, ground state rotational constants were fitted to a Watson-type Hamiltonian to give A(0) = 0.8728202(9), B(0) = 0.0435868(4), and C(0) = 0.0415314(2) cm(-1). Upper state rotational constants for the ν(30) band were also fitted. Predictions of the ground state rotational constants for tHTE-1-(13)C(1) from a B3LYP/cc-pVTZ model with scale factors based on the normal species were in excellent agreement with observations. Similar good agreement was found between predicted and observed ground state rotational constants for the three (13)C(1) isotopologues of cis-hexatriene, as determined from microwave spectroscopy. Equilibrium rotational constants for tHTE and its three (13)C(1) isotopologues, of which two were predicted, were used to find a semiexperimental equilibrium structure for the C(6) backbone of tHTE. This structure shows increased structural effects of π-electron delocalization in comparison with butadiene and some differences from the cis isomer of HTE. Structures predicted with the MP2/cc-pVTZ model are also compared.     

Vibrational analysis of n-butyl, isobutyl, sec-butyl and tert-butyl nitrite

Raman and infrared spectra of n-butyl, isobutyl, sec-butyl and tert-butyl nitrite are reported. Density functional theory and M?eller-Plesset calculations with 6-31G* and 6-311G* basis sets were used to determine ground state molecular geometries and vibrational frequencies of these compounds. Calculations and spectral data of these molecules were used to perform partial vibrational mode assignments for the observed transitions. In agreement with previous investigations of alkyl nitrites, cis and trans rotational conformers of n-butyl, isobutyl and sec-butyl nitrite were observed in the gas phase infrared spectra and the condensed phase Raman and infrared spectra. Among the several predicted geometries of these compounds, the cis-trans geometry (cis with respect to the C-O-N=O dihedral angle and trans with respect to the C-C-O-N dihedral) was calculated to be the most stable conformer of n-butyl and isobutyl nitrite, while the cis-gauche conformer was found to be the most stable geometry of sec-butyl nitrite. The cis-type structures of these three molecules are favored due to formation of a pseudo hydrogen bond between the nitrite group and the alpha-carbon hydrogen atoms. Hindrance with the alkyl moiety, however, causes the trans conformer (trans with respect to the C-O-N=O dihedral) of tert-butyl nitrite to lie lower than its cis conformer, a result that was supported by our spectroscopic measurements. The characteristic N=O stretch frequency for the trans conformers of all the compounds examined was observed to decrease with increasing branching at the alpha-carbon, while the same mode for the cis conformers shows no change among the primary and secondary nitrites. Evidence is also provided that suggests that the relative number of cis conformers to trans conformers decreases as the alpha-carbon branching increases.     

Microwave spectrum, conformation and intramolecular hydrogen bonding of 2,2,2-trifluoroethanethiol (CF3CH2SH)

The microwave spectrum of 2,2,2-trifluoroethanethiol, CF3CH2SH, and of one deuterated species, CF3CH2SD, has been investigated in the 7-80 GHz spectral interval. The microwave spectra of the ground and three vibrationally excited states belonging to three different normal modes of one conformer were assigned for the parent species, and the vibrational frequencies of these fundamentals were determined by relative intensity measurements. Only the ground vibrational state was assigned for the deuterated species. The identified form has a synclinal arrangement for the H-S-C-C chain of atoms and the corresponding dihedral angle is 68(5) degrees from synperiplanar (0 degrees). A weak intramolecular hydrogen bond formed between the thiol (SH) group and one of the fluorine atoms is stabilizing this conformer. There is no evidence in the microwave spectrum for the H-S-C-C antiperiplanar form. The hydrogen atom of the thiol group should have the ability to tunnel between two equivalent synclinal potential wells, but no splittings of spectral lines due to tunneling were observed. The microwave work was augmented by quantum chemical calculations at the B3LYP/aug-cc-pVTZ and MP2/aug-cc-pVTZ levels of theory.     

Microwave and vibrational spectra, ab initio calculations, conformational stabilities and assignments of the fundamentals of the Cs conformer of n-butylgermane

The FT-microwave spectrum of n-butylgermane, CH₃CH₂CH₂CH₂GeH₃ has been investigated from 4000 to 18,000 MHz and the microwave spectra have been observed for all of the five naturally occurring germanium isotopologues for the anti-anti (aa) conformer. The dipole moment for the ⁷⁴Ge containing species has been measured, giving a total dipole moment of 0.881 (26) D. In addition, the vibrational spectrum of n-butylgermane is described. Modestly complete assignments are made for the aa conformer. The relative stabilities of the five conformers are calculated, and the anti-anti (aa) conformer is found to be the most stable in all calculations done. This conclusion is confirmed by the infrared and Raman spectrum of the annealed crystal. The dipole moments of all conformers are calculated to be approximately equal and less than 1 D, ranging from approximately 0.8 to 0.9 D.     

Single-conformation ultraviolet and infrared spectroscopy of model synthetic foldamers: beta-peptides Ac-beta3-hPhe-NHMe and Ac-beta3-hTyr-NHMe

The conformational preferences and infrared and ultraviolet spectral signatures of two model beta-peptides, Ac-beta3-hPhe-NHMe (1) and Ac-beta3-hTyr-NHMe (2), have been explored under jet-cooled, isolated molecule conditions. The mass-resolved, resonant two-photon ionization spectra of the two molecules were recorded in the region of the S0-S1 origin of the phenyl or phenol ring substituents, respectively. UV-UV hole-burning spectroscopy was used to determine that two conformations of 1 are present, with the transitions due to conformer A, with S0-S1 origin at 34431 cm(-1), being almost 20 times larger than those due to conformer B, with S0-S1 origin at 34404 cm(-1). Only one conformation of 2 was observed. Resonant ion-dip infrared spectroscopy provided single-conformation infrared spectra in the 3300-3700 cm(-1) region. The spectra of conformer A of both molecules have H-bonded and free amide NH stretch infrared transitions at 3400 and 3488 cm(-1), respectively, while conformer B of 1 possesses bands at 3417 and 3454 cm(-1). For comparison with experiment, full optimizations of all low-lying minima of 1 were carried out at the DFT B3LYP/6-31+G* and RIMP2/aug-cc-pVDZ levels of theory, and single point MP2/6-31+G* calculations at the DFT geometries. On the basis of the comparison with previous studies in solution and the calculated results, conformer A of 1 and 2 were assigned to a C6 conformer, while conformer B of 1 was assigned to a unique C8 structure with a weak intramolecular H-bond. The reasons for the preference for C6 over C8 structures and the presence of only two conformations in the jet-cooled spectrum are discussed in light of the predictions from calculations.     

Spectra and structure of silicon-containing compounds. XXX. Raman and infrared spectra,conformational stability,vibrational assignment of chloromethyl silyl dichloride

The Raman spectra (3200-30 cm(-1)) of liquid and solid, and infrared spectra of gaseous and solid chloromethyl silyl dichloride, ClCH2SiHCl2, have been recorded. Variable temperature (-105 to -150 degrees C) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out. From these data, the enthalpy difference was determined to be 363 +/- 40 cm(-1) (4.34 +/- 0.48 kJ mol(-1)), with the more stable form being the gauche conformer, which is consistent with the prediction from ab initio calculations at both the Hartree-Fock level and with full electron correlation by the perturbation method to second order. It is estimated that 92% of the sample is in the gauche form at ambient temperature. A complete vibrational assignment is proposed for the gauche conformer and several of fundamentals of the trans conformer based on infrared band contours, relative intensities, depolarization values, and group frequencies, which is supported by normal coordinate calculations utilizing the force constants from the ab initio MP2/6-31G(d) calculations. The r0 SiH bond distances of 1.476 and 1.472 A have been obtained for the trans and gauche conformers, respectively, from the silicon-hydrogen stretching frequencies. The optimized geometries have also been obtained from ab initio calculations utilizing several different basis sets with full electron correlation by the perturbation method up to MP2/6-311 + G(2d,2p). The results are discussed and compared to some corresponding results for several related molecules.     

High‐Resolution Rotational Spectroscopy Study of the Smallest Sugar Dimer: Interplay of Hydrogen Bonds in the Glycolaldehyde Dimer

Molecular recognition of carbohydrates plays an important role in nature. The aggregation of the smallest sugar, glycolaldehyde, was studied in a conformer‐selective manner using high‐resolution rotational spectroscopy. Two different dimer structures were observed. The most stable conformer reveals C₂‐symmetry by forming two intermolecular hydrogen bonds, giving up the strong intramolecular hydrogen bonds of the monomers and thus showing high hydrogen bond selectivity. By analyzing the spectra of the ¹³C and ¹⁸O isotopologues of the dimer in natural abundance, we could precisely determine the heavy backbone structure of the dimer. Comparison to the monomer structure and the complex with water provides insight into intermolecular interactions. Despite hydrogen bonding being the dominant interaction, precise predictions from quantum‐chemical calculations highly rely on the consideration of dispersion.