Free Internal Rotation Research Articles

The muonium adduct to biacetyl - ab initio calculations and vibrational averaging

The room temperature muon‐electron hyperfine coupling constant of the 2‐muoxy‐3‐ketobut‐2‐yl radical formed by Mu addition to biacetyl (butane‐2,3‐dione) is the smallest thus far measured in any radical, with A\prime_μ\simeq 0.7 MHz (R.M. Macrae, C.J. Rhodes, K. Nishiyama and K. Nagamine, Chem. Phys. Lett, submitted). Calculations on this radical using the ROHF, UHF, and B/PW 91 formalisms are presented, with averaging performed over the C–O torsional mode using an expansion in free internal rotation eigenfunctions. UHF and B/PW 91 give similar predictions of the dependence of A\prime_μ on the torsional angle \gamma, but are strikingly different in their predictions of the torsional energetics. This difference is traced to spin contamination in the UHF wavefunction. UHF disagrees with both ROHF and B/PW 91 in its structural predictions, yielding excessive electron delocalisation into the central C–C bond. Vibrational corrections lead to a small |A\prime_μ| with a weak temperature‐dependence, and suggest that the radical may twist from the trans geometry of the parent compound into the energetically more favourable intramolecularly H‐bonded cis structure.

Rotational Spectra of CH3CCH–NH3, NCCCH–NH3, and NCCCH–OH2

Microwave spectra of NCCCH–NH3, CH3CCH–NH3, and NCCCH–OH2have been recorded using a pulsed-nozzle Fourier-transform microwave spectrometer. The complexes NCCCH–NH3and CH3CCH–NH3are found to have symmetric-top structures with the acetylenic proton hydrogen bonded to the nitrogen of the NH3. The data for CH3CCH–NH3are further consistent with free or nearly free internal rotation of the methyl top against the ammonia top. For NCCCH–OH2, the acetylenic proton is hydrogen bonded to the oxygen of the water. The complex has a dynamicalC2vstructure, as evidenced by the presence of two nuclear-spin modifications of the complex. The hydrogen bond lengths and hydrogen-bond stretching force constants are 2.212 Å and 10.8 N/m, 2.322 Å and 6.0 N/m, and 2.125 Å and 9.6 N/m for NCCCH–NH3, CH3CCH–NH3, and NCCCH–OH2, respectively. For the cyanoacetylene complexes, these bond lengths and force constants lie between the values for the related hydrogen cyanide and acetylene complexes of NH3and H2O. The NH3bending and weak-bond stretching force constants for CH3CCH–NH3are less than those found in NCCCH–NH3, NCH–NH3, and HCCH–NH3, suggesting that the hydrogen bonding interaction is particularly weak in CH3CCH–NH3. The weakness of this hydrogen bond is partially a consequence of the orientation of the monomer electric dipole moments in the complex. In CH3CCH–NH3the antialigned monomer dipole moments lead to a repulsive dipole–dipole interaction energy, while in NCH–NH3and NCCCH–NH3the aligned dipoles give an attraction interaction.

Spectroscopy and dynamics of rare gas–spherical top complexes. II. The infrared spectrum of the ν3 band of Ne–SiH4 ( j=1←0 and j=0←1 transitions)

The infrared spectrum of the rare gas–spherical top complex Ne–SiH4 has been recorded in a supersonic jet in the region of the SiH4 ν3 triply degenerate stretching vibration at ∼2189 cm−1. In contrast to the previously measured Ar–SiH4 spectrum which showed almost equal rotational spacings within each band (corresponding to transitions between different internal rotor states of SiH4 within the complex), the Ne–SiH4 spectrum is complex with no obvious regular band structure. However, by analogy with the Ar–SiH4 spectrum, four bands of the Ne–SiH4 have been assigned and analyzed in terms of Hamiltonians incorporating Coriolis interaction between the angular momentum of the SiH4 monomer unit and the overall end over end rotation of the complex. These bands correlate with the SiH4 R(0) (K=0←0, K=1←0) and P(1) (K=0←0, K=0←1) transitions. Derived rotational constants demonstrate that the neon–silane separation (∼4.13 Å in the ground vibrational state) is larger than expected by analogy with Ar–SiH4, indicative of nearly free internal rotation by the silane monomer unit in Ne–SiH4. The smaller anisotropy of Ne–SiH4 compared with Ar–SiH4 results in a new angular momentum coupling scheme. Transitions arising from 22Ne–SiH4 correlating to SiH4 R(0) have also been observed and fitted; the higher than anticipated intensities demonstrate a novel isotope enrichment effect in the supersonic jet which is discussed.

Continuous slit-jet infrared spectrum of the CO–N2 complex

The weakly bound complex CO–N2 has been studied in the 4.7 μm infrared region of the CO stretching vibration using a continuous slit-jet supersonic expansion and a tunable diode laser spectrometer. A total of 152 lines were observed and assigned to four connected subbands with K=0←1, 0←0, 1←0, and 2←1, and to one unconnected subband with K=1←1. Analysis of these bands yielded K-state origins, rotational parameters, and centrifugal distortion parameters. The effective intermolecular separation for the complex in its ground state was found to be 4.025 Å, and predictions of rotational frequencies were made to aid in the search for CO–N2 microwave transitions. The spectra observed were surprisingly simple and well behaved, to the extent that they could virtually be ascribed to a (fictitious) complex of CO with a rare gas atom having a mass of 28 a.m.u. This simplicity may be explained by postulating that the N2 undergoes relatively free internal rotation in the complex. All but one of the observed bands involve levels which correlate with the rotationless J=0 state of ortho-N2. Further spectroscopic work in the infrared and microwave regions should be combined with theoretical studies in order to learn more about the orientational structure and intermolecular potential of this atmospherically relevant system.

Stabilizing Heterobimetallic Complexes Containing Unsupported Ti-M Bonds (M = Fe, Ru, Co): The Nature of Ti-M Donor-Acceptor Bonds.

The stabilization of unsupported Ti-M (M = Fe, Ru, Co) heterodinuclear complexes has been achieved by use of amidotitanium building blocks containing tripodal amido ligands. Salt metathesis of H(3)CC(CH(2)NSiMe(3))(3)TiX (1) and C(6)H(5)C(CH(2)NSiMe(3))(3)TiX (2) as well as HC{SiMe(2)N(4-CH(3)C(6)H(4))}(3)TiX (3) (X = Cl, a; Br, b) with K[M(CO)(2)Cp] (M = Fe, Ru) and Na[Co(CO)(3)(PR(3))] (R = Ph, Tol) gave the corresponding stable heterobimetallic complexes of which H(3)CC(CH(2)NSiMe(3))(3)Ti-M(CO)(2)Cp (M = Fe, 6; Ru, 7) and HC{SiMe(2)N(4-CH(3)C(6)H(4))}(3)Ti-M(CO)(2)Cp (M = Fe, 12; Ru, 13) have been characterized by X-ray crystallography. 6: monoclinic, P2(1)/n, a = 15.496(3) Å, b = 12.983(3) Å, c = 29.219(3) Å, beta = 104.52(2) degrees, Z = 8, V = 5690.71 Å(3), R = 0.070. 7: monoclinic, P2(1)/c, a = 12.977(3) Å, b = 12.084(3) Å, c = 18.217(3) Å, beta = 91.33(2) degrees, Z = 4, V = 2855.91 Å(3), R = 0.048. 12: monoclinic, I2/c, a = 24.660(4) Å, b = 15.452(3) Å, c = 20.631(4) Å, beta = 103.64(3) degrees, Z = 8, V = 7639.65 Å(3), R = 0.079. 13: monoclinic, I2/c, a = 24.473(3) Å, b = 15.417(3) Å, c = 20.783(4) Å, beta = 104.20(2) degrees, Z = 8, V = 7601.84 Å(3), R = 0.066. (1)H- and (13)C-NMR studies in solution indicate free internal rotation of the molecular fragments around the Ti-M bonds. Fenske-Hall calculations performed on the idealized system HC(CH(2)NH)(3)Ti-Fe(CO)(2)Cp (6x) have revealed a significant degree of pi-donor-acceptor interaction between the two metal fragments reinforcing the Ti-Fe sigma-bond. Due to the availability of energetically low-lying pi-acceptor orbitals at the Ti center this partial multiple bonding is more pronounced that in the tin analogue HC(CH(2)NH)(3)Sn-Fe(CO)(2)Cp (15x) in which an N-Sn sigma-orbital may act as pi-acceptor orbital.

A broader definition of symmetry number and its application to a kinetic model describing polyarene growth

A 1:1 mole ratio mixture of solid naphthalene and anthracene has been fed into a laminar flow drop tube furnace and pyrolyzed for a residence time of approximately 2 s at five different temperatures in the range of 1200–1500 K. Experimentally observed concentrations of individual biarenes and of larger polyarene species (up to 700 amu) and their condensation products are compared against those predicted by a kinetic model. The model, consisting of 2146 reactions between 169 species representing more than 5 million compounds, is constructed using existing literature values for rate parameters for five key types of reactions (hydrogen balance, hydrogen abstraction, condensation, aryl/arene reactions, and radical/radical combinations). However, there are two important differences: (1) steric hindrance to free internal rotation of polyarene species is considered for the first time in a detailed kinetic model and (2) a new technique using lumped compounds to represent entire polyarene series has been developed that makes use of a more general definition of a symmetry number. This definition allows for more accurate accounting of multiple isomers' contributions to statistical factors in the preexponential kinetic parameters and for a more theoretically based alternative to current molecular weight growth schemes that does not rely on empirical lumping parameters. The model accurately predicts relative mass yields and temperature trends of the biarenes, higher molecular weight growth products (up to at least 700 amu), and condensation products within the temperature range of the study. High molecular weight growth is found to be very sensitive to the parameter of steric hindrance mentioned previously. A threefold increase in the contribution to heat of formation of hindrance to internal rotation in the polyarenes results in at least a tenfold decrease in molecular weight growth.

Resonant ion-dip infrared spectroscopy of benzene–H2O and benzene–HOD

Resonant ion-dip infrared spectra of C6H6–H2O and C6H6–HOD have been recorded in the OH stretch fundamental region. The spectra provide further evidence for the unique, large-amplitude motions present in these π hydrogen-bonded complexes. In C6H6–H2O, transitions out of the lowest ortho (Π) and para (Σ) ground state levels are observed. A transition at 3634 cm−1 is assigned as an unresolved pair of parallel transitions (Σ→Σ and Π→Π) involving the symmetric stretch fundamental (at 3657 cm−1 in free H2O). In the antisymmetric stretch region, transitions at 3713, 3748, and 3774 cm−1 are assigned as Π→Σ, Σ→Π, and Π→Δ transitions, respectively. The spacing of the transitions is consistent with nearly free internal rotation of H2O about benzene’s sixfold axis in both ground and vibrationally excited states. The intensities of combination bands depends critically on the mixing of some local mode character into the symmetric and antisymmetric stretches at asymmetric positions of H2O on benzene. Surprisingly, in C6H6–HOD, five transitions are observed in the OH stretch region, all arising from the ground state zero point level. Even more unusual, the higher-energy combination bands are many times stronger than the OH stretch fundamental. The local mode OH stretch has components both parallel and perpendicular to benzene’s sixfold axis, leading to strong parallel and perpendicular transitions in the spectrum. A two-dimensional model involving free internal rotation and torsion of HOD in its plane is used to account for the qualitative appearance of the spectrum. The form of the OH(v=0) and OH(v=1) torsional potentials which reproduce the qualitative features of the spectrum are slightly asymmetric, double-minimum potentials with large-amplitude excursions for HOD over nearly 180°.

IR spectrum of HCF2CF2Br: hindered intramolecular vibrational energy redistribution

Abstract The CH stretch band of the anti isomer of HCF 2 CF 2 Br shows a Q branch of only 1 cm −1 of width and in the first overtone, it is the syn isomer which has a very narrow Q branch. Obviously, the hot bands starting, e.g., from the thermally populated states of the torsional vibration, practically coincide with the fundamental. Hence, the torsional frequency does not depend on the degree of CH excitation, and therefore neither does the torsional potential, although already a single CH quantum has more energy than the torsional barrier, above which there could be free internal rotation. A barrier height of 1590 cm −1 is deduced from the observed torsional frequency of 76 cm −1 . This is too high for tunnelling. We conclude that there is no direct conversion of the energy from the CH vibration to internal rotation, although higher order coupling can probably cause a slow redistribution. Similarly, we conclude that the flow of CH vibrational energy to the CBr stretch and other low frequency modes is hindered, too. We point out the similarity to van der Waals molecules, in which the low-frequency mode also has little coupling to the other vibrations. But the torsional-CH non-coupling is especially intuitive, because it infers an unchanged barrier height and therefore an unchanged tunnelling frequency. We also propose that generally the nondiagonal anharmonicity between high- and low-frequency modes is small (⪡1 cm −1 ).

The trifluoroacetato (CF3COO–) and trifluoroacetate (CF3COO–) groups, their conformation from a database survey of crystal structures and ab initio MO calculations

Data for structures involving either of these groups have been drawn from the Cambridge Structural Database and divided into bidentate and monodentate categories. Statistical distributions of bond distances, selected bond and torsion angles from 414 groups contained in 194 structures yield the following mean values (with e.s.d.'s) : C-F = 1.29 (6), C-C = 1.53 (4), C-O (bidentate)= 1.24 (5), C-O- (monodentate) = 1.25 (4), C=O (monodentate)= 1.21(4) A. Angles in the CF group vary widely from the expected threefold symmetry (e.s.d. of distribution 8.8°). Torsion angles F-C-C-O for the bidentate groups range fairly evenly over all possible values, suggesting free internal rotation. Among monodentate groups all values of the torsional angles are possible but with a marked preference for a C-F to eclipse the carbonyl C=O bond. These observations show good agreement with the results of ab initio calculations using GAUSSIAN90. The calculated barrier to rotation for the bidentate (ion) form of the group is only 310 Jmol- 1 and for the monodentate form (as in the acid) 2.55 kJmol- 1 .

Ab initio studies of the complexes of benzene with carbon monoxide and formaldehyde

Benzene...carbon monoxide and benzene...formaldehyde complexes are studied using ab initio methods with the highest calculations at the MP4SDTQ/6–31+G**//MP2/6–31+G** level. The benzene...carbon monoxide dimer forms a π complex of Cs symmetry where the CO top is nearly parallel with the benzene plane. In the benzene...formaldehyde π complex the dimer is without any symmetry. In this arrangement a weak hydrogen bond is expected between the elements where benzene acts as the acceptor, while in a linear benzene...formaldehyde alignment benzene has been identified as a weak hydrogen bond donor to the carbonyl oxygen. Changes in the intramolecular geometric parameters upon dimerization are small. Interaction energy of the benzene...CO dimer seems to be underestimated compared to the experimental value. No experimental value has been found for the benzene...formaldehyde binding energy. The calculated value is more negative by 0.7 kcal/mol with reference to the benzene...CO dimer. Calculated intermolecular vibrational frequencies are in partial agreement with the experiment. The stretching frequency of the benzene...CO dimer is well reproduced, intermolecular bending and torsional frequencies are overestimated. The intramolecular vibrational frequencies for the monomers show over and underestimation in the high and low frequency ranges, respectively. Experimental results in the literature suggest an almost free internal rotation of the CO top above benzene. The calculated barrier to internal rotation is 0.01 kcal/mol in good agreement with the experimental value. Based on this theoretical value the model with the almost free internal rotation was supported. Analysis for the benzene...formaldehyde dimer suggests more hindered rotation, if at all, with a H2CO top.

Proton interchange tunneling and internal rotation in HSH–NH3

An electric-resonance optothermal spectrometer and phase-locked backward-wave oscillators are used to investigate the b type, ΔK=±1, Δm=0 spectrum of the hydrogen-bonded HSH--NH3 and H34SH--NH3 complexes near 300 GHz. The spectrum is characterized by nearly free internal rotation of the NH3 subunit against the H2S, as initially concluded from Stark-effect measurements by Herbine et al. [J. Chem. Phys. 93, 5485 (1990)]. Transitions are observed for the K=1←0, m=0, A symmetry and the K=0←±1 and K=±2←±1, m=±1, Km≳0, E-symmetry subbands. The transitions are split into doublets with a 3:1 relative intensity ratio indicative of tunneling interchange of the two H2S protons. The observed selection rules, symmetric ↔ antisymmetric in the tunneling state, indicate that the tunneling motion reverses the sign of the molecular electric dipole moment component along the b inertial axis. The most likely interchange motion consists of a partial internal rotation of the H2S unit about its c inertial axis, through a bifurcated, doubly hydrogen-bonded transition state. The proton interchange tunneling splittings of 859–864 MHz vary little between K and m states, indicating that the interchange motion is only weakly coupled to the internal rotation. The barrier to proton interchange is determined to be 510(3) cm−1, which can be compared to the ∼700 cm−1 barrier estimated from the 57 MHz tunneling splittings associated with the H2O proton interchange in the related HOH--NH3 complex. The observation of dissociation of HSH--NH3 following excitation of the NH3 umbrella mode with a line-tunable CO2 laser places an upper bound of 992 cm−1 on the hydrogen-bond zero-point dissociation energy. The band origin for the umbrella vibration of 992.5(10) cm−1 is blueshifted by 43 cm−1 from the hypothetical inversion-free band origin of uncomplexed NH3. Previous studies have shown that the HOH--NH3 binding energy is greater than 1021 cm−1.

Vibrational predissociation dynamics and internal rotation in aromatic van der Waals complexes

The Vibrational Predissociation (VP) lifetimes and dissociation channels are reported for the van der Waals complexes p-DiFluoroBenzene-Ar (pDFB-Ar), pDFB-N2 and p-FluoroToluene-Ar (pFT-Ar). The dissociation occurs on theS1 potential energy surface after excitation of an S1 ring breathing level with evib ≈ 800 cm−1. Trends are observed in the lifetimes and dissociation channels as the experiments move among the complexes. The VP characteristics of pFT-Ar are most distinctive. As opposed to all other aromatic complexes, pFT-Ar has little VP channel selectivity. These complexes are members of a set chosen to explore the VP vibrational dynamics by systematic changes of complex level structure. The change from pDFB-Ar to pDFB-N2 introduces nearly free internal rotation in the complex partner and an additional van der Waals mode. pFT-Ar introduces a nearly free internal rotation attached directly to the ring. We discuss how these changes may be related to the observed VP dynamics.

Calculated rotational spectrum of Ar...CO from an ab initio potential energy surface: A very floppy van der Waals molecule

Using the ab initio potential of Shin et al. (to be published), we have calculated the bound states and infrared absorption spectrum of the van der Waals complex Ar...CO. The results show that Ar...CO cannot be treated as a quasirigid rotor, nor as a molecule with a free internal rotor. In particular, a transition to the first excited van der Waals bending level is predicted to be present in the spectrum, and its frequency varies with Ω (the projection quantum number of the total angular momentum onto the intermolecular axis going from the center of mass of CO to the Ar atom). It is also shown that, although the spectrum cannot be analyzed by the use of a rigid rotor model, rotational ‘‘constants’’ can still be defined for each value of Ω. This is consistent with the available experimental data and the predicted bending excitation can account for unassigned transitions in the infrared spectrum of this complex. Finally, a sensitivity analysis of the calculated spectrum with respect to the potential anisotropy has been performed.

Vibrational spectra of zinc, cadmium and mercury dimethyl species

Abstract Infrared and Raman spectra have been obtained in the gas phase for the species M( 12 CH 3 ) 2 , M( 13 CH 3 ) 2 , M( 12 CD 3 ) 2 and M( 13 CD 3 ) 2 , where M  Zn, Cd and Hg. All but one of the skeletal bending modes are observed directly in the infrared. 13 C frequency shifts confirm the assignment of a number of combination and overtone bands, including the 1300 cm −1 bands of Zn(CH 3 ) 2 and Cd(CH 3 ) 2 which arise from ν 10 + ν 14 . Infrared spectra in the 1400–1500 cm −1 region are difficult to interpret but the evidence points to both the δ as CH 3 modes lying near 1445 cm −1 . A number of Fermi resonances affecting δ s CD 3 levels are identified in the d 6 species. Some features of the ν as CH 3 bands can be explained in terms of existing free internal rotor theory. Observed values of 13 C frequency shifts are generally in good agreement with those from previous scaled ab initio calculations. There is an urgent need for high resolution studies of jet-cooled samples.

The partition function of free internal rotation. Combined direct and transformed expressions numerical study

A simple and exact double formula expression is proposed to evaluate the partitiion function of free internal rotation. This expression has the correct behaviour for both temperature limits and gives accurate numerical values for all thermodynamic functions.

Spectra and structure of silicon compounds. XVII Raman and infrared spectra and vibrational assignment for hexamethyldisilane and ab initio calculations for disilane and hexamethyldisilane

The IR spectra of gaseous and solid hexamethyldisilane between 4000 and 25 cm −1 and the far-IR spectrum of the liquids from 450 to 25 cm −1 have been recorded. The Raman spectra have been recorded from 3500 to 10 cm −1 for all three physical phases. Assisted by ab initio calculations, the vibrational spectrum of hexamethyldisilane has been assigned under D 3 d symmetry and the results of a normal coordinate analysis are discussed. No spectral features indicative of free internal rotation have been observed. Gradient ab initio calculations have been carried out for the disilane and hexamethyldisilane molecules using different types of basis sets. The structural parameters, rotational constants, unscaled and scaled frequencies and harmonic force constants have been reported for both disilane and hexamethyldisilane.

The microwave spectrum of cyclopropane · ammonia. A novel structure for cyclopropane complexes

The microwave spectrum of the cyclopropane·ammonia (C 3H 6· 14NH 3) complex has been observed using a pulsed nozzle, Fourier-transform microwave spectrometer. The spectrum is characteristic of a symmetric top, B 0=2668.7161(4), with free internal rotation of the NH 3 subunit. The spectra of the C 3H 6· 15NH 3, C 3D 6· 15NH 3, and C 3H 6· 14ND 3 isotopomers were also measured. This gives a structure in which the nitrogen of the ammonia interacts with the top of the cyclopropane ring, resulting in a stacked structure with R c.m.=3.657(3). The quadrupole coupling constant of the nitrogen nucleus is eQq=−2.509(2) MHz.

Far infrared vibration-rotation-tunneling spectroscopy and internal dynamics of methane–water: A prototypical hydrophobic system

Thirteen vibration-rotation-tunneling (VRT) bands of the CH4–H2O complex have been measured in the range from 18 to 35.5 cm−1 using tunable far infrared laser spectroscopy. The ground state has an average center of mass separation of 3.70 Å and a stretching force constant of 1.52 N/m, indicating that this complex is more strongly bound than Ar–H2O. The eigenvalue spectrum has been calculated with a variational procedure using a spherical expansion of a site–site ab initio intermolecular potential energy surface [J. Chem. Phys. 93, 7808 (1991)]. The computed eigenvalues exhibit a similar pattern to the observed spectra but are not in quantitative agreement. These observations suggest that both monomers undergo nearly free internal rotation within the complex.

The effect of internal rotation on the methyl CH-stretching overtone spectra of toluene and the xylenes

The structure of methyl CH-stretching overtone bands in the vibrational spectra of methylbenzenes was investigated theoretically. The anharmonic CH-stretching vibration, described by a Morse potential, was represented in terms of a harmonic basis while hindered internal rotation of the methyl group was represented by a rigid rotor attached to an infinitely massive frame. Relatively weak coupling between the anharmonic CH vibration and the hindered internal rotation is sufficient to shift the positions of rovibrational lines from a PQR-like rotational contour to patterns similar to those observed experimentally. For high rotational barriers, as in o-xylene, the rovibrational transitions form two bands associated with conformationally nonequivalent CH-bonds, consistent with the conformational preference established by microwave spectroscopy and molecular orbital calculations. For nearly free internal rotation, as in toluene, m-xylene and p-xylene, a prominent middle band is also present. This ‘‘free rotor’’ band corresponds to rotational transitions between states high above the barrier and disappears as the barrier height increases. The outer bands correspond to transitions for which either the initial or the final state is below or near the barrier height in energy. Contrary to earlier suggestions, the band structure is not indicative of the conformational preference of the methyl group in toluene. In fact, the calculated spectra of nearly free internal rotors are insensitive to this preference.

Is protonated ammonium fluoride an ion-neutral complex in the gas phase?

The two-dimensional rotation of the ammonium ion within NH4+-.FH is compared with hindered internal rotations in ethane. Both have bound levels below their internal rotation barriers as well as higher lying free rotor levels. Rotation in NH4+.-FH is the large-amplitude limit of bending vibrations, whereas free rotations in ethane are the large-amplitude limits of torsional oscillations. The accuracy of ab initio computations is calibrated using another tetrahedral-diatomic complex, CH4-.FH, for which MP4(SDTQ) calculations using the 6-3 1 1G** basis set reproduce experimentally determined spectroscopic properties. The calculations predict that the internal rotation barrier for NH4+-.FH is 13 kJ mol-', much lower than the calculated dissociation energy, DO = 48 kJ mol-'. From the quantization of the two-dimensional rotation, statistical mechanics predicts that NH4+--FH exists as an ion-neutral complex for a significant fraction of the time at temperatures above 300 K. We therefore propose that it represents a paradigm for ionic bonding between polyatomic partners in the gas phase. Conformationally mobile molecules are widely viewed as being highly fluxional. Nearly 120 years ago, Le Bel and van't Hoff proposed that free rotation occurs around single bonds. This description is satisfactory for many purposes, although most chemists are well aware that even the simplest example, ethane, has a 12 kJ mol-' barrier to internal rotation.' An ethane molecule has four bound levels corresponding to torsional oscillations. The energy gap between the zero point and the highest bound torsional level is 9 kJ mol-'. The next level, which is approximately 10 kJ mol-' above the zero point, corresponds to free internal rotation of one methyl with respect to the other. At sufficiently elevated temperatures, ethane behaves as though the methyl groups turn