Non covalent interactions stabilizing the chiral dimer of CH2ClF: a rotational study

Laura B. Favero a, Assimo Maris b, Sonia Melandri b, Paolo Ottaviani b and Walther Caminati *b
aIstituto per lo Studio dei Materiali Nanostrutturati (ISMN), Sezione di Bologna CNR, via Gobetti 101, I-40129 Bologna, Italy
bDipartimento di Chimica “G. Ciamician” dell’Università, Via Selmi 2, I-40126 Bologna, Italy. E-mail:

Received 9th October 2018 , Accepted 27th December 2018

First published on 27th December 2018

We report the rotational spectra of three isotopologues of the dimer of chlorofluoromethane, namely CH235ClF–CH235ClF, CH235ClF–CH237ClF and CH237ClF–CH235ClF. The assigned (most stable) conformer is chiral (C1 symmetry) and displays a network of two C–H⋯Cl–C and one C–H⋯F–C weak hydrogen bonds, combined with a Cl⋯F halogen bond. The hyperfine structures due to the quadrupolar effects of the two non-equivalent 35Cl (or 37Cl) atoms have been fully resolved, leading to an accurate determination of two sets of diagonal and of some mixed quadrupole coupling constants. Information on the rs positions of the two Cl atoms and on the structural parameters of the hydrogen bonds has been obtained. The dissociation energy of the complex has been estimated as 5.9 kJ mol−1.


According to the IUPAC definition of a hydrogen bond, a weak hydrogen bond (WHB) is still the subject of some controversies about its classification as a hydrogen bond.1 It is, however, considered as a molecular interaction playing an important role in chemistry, and a vast literature is dedicated to this topic.2

WHBs such as C–H···O, C–H⋯N and C–H···F–C play an important role in biological, atmospheric and supramolecular chemistry.3 Studies on such WHBs have been mainly performed using X-ray diffraction4 and IR spectroscopy in rare gas solutions.5 This kind of experimental information on WHBs, from solid state or solution investigations, is influenced by other intermolecular interactions which take place in condensed phases. In contrast, investigations on this kind of molecular system performed with high resolution spectroscopic techniques, especially pulsed jet Fourier transform microwave (FTMW) spectroscopy, provide precise data in an environment free from the interference of solvation and crystal lattice effects.6

Information obtained with this technique on the C–H⋯O WHB has come from the study of the dimer of dimethylether,7 or of the adducts of some ethers, ketones or aldehydes with some hydrogenated fluoro-Freons, together with the C–H⋯F linkages.8 The C–H⋯N interaction has been described in the rotational studies of the complexes of pyridine with mono-, di- and tri-fluoromethane.9

Halogenated hydrocarbons have sites which can act as weak proton donors or weak proton acceptors, leading to easy formation of their oligomers or hetero adducts, with the subunits held together by a network of WHBs. The aliphatic hydrogen atoms have, indeed, been found to act as proton donors, thanks to the electron withdrawing effect of the halogen atoms. Difluoromethane (CH2F2) can be regarded as the prototype for this kind of ambivalent molecule. Its oligomers, (CH2F2)n, with n = 2–4, have been recently characterized by FTMW. The rotational investigations of the dimer,10 trimer11 and tetramer12 of CH2F2 pointed out the existence of 3, 9, and 16 C–H⋯F–C WHBs, respectively. In the hetero adduct CH3F–CHF3, the two subunits are linked together by three weak C–H⋯F–C WHBs, while the two subunits rotate through low V3 barriers around their symmetry axes.13

Only a few adducts between Freon molecules containing halogen atoms other than fluorine have been investigated by rotational spectroscopy, CH2ClF⋯FHC[double bond, length as m-dash]CH2 and CH2ClF⋯CH2F2, which presents a combination of C–H⋯F–C and C–H⋯Cl–C WHBs,14 and CH2F2 with CH2Cl2, characterized by two equivalent C–H⋯Cl–C and one C–H⋯F–C WHBs.15 No complexes between Freons with two different heavy halogen atoms (Cl, Br, and I) have been investigated. This is because, unlike the F atom, for which the nuclear spin quantum number is I = 1/2, the other halogens have I = 3/2 or 5/2 and this results in very complicated quadrupole hyperfine structures of the rotational spectra of the halogenated multi-molecular systems. The analysis of the spectrum and the achievement of a correct fit are quite challenging when the two heavy halogen atoms in the complex are not equivalent to each other. From a spectroscopic point of view, the interpretation of the rotational spectrum of a system with multiple quadrupolar nuclei can be a very challenging task. For example, the rotational spectra of even simple molecules with two heavy halogen atoms have been reported only in a limited number of cases. Herein, we investigate the rotational spectrum of the dimer of CH2ClF (Freon 31), (CH2ClF)2, with the aim of determining the orientation of the subunits in the complex and ascertaining which non-covalent interactions are more favorable.

Experimental section

Molecular clusters were generated under supersonic expansion, under conditions optimized for the formation of adducts. Details of a Fourier transform microwave spectrometer16 (COBRA-type17), which covers the range of 6.5–18 GHz, have been described previously.18

A gas mixture of ca. 1% of CH2ClF (commercial samples used without any further purification) in He at a stagnation pressure of ∼0.5 MPa was expanded through a solenoid valve (General Valve, Series 9, nozzle diameter 0.5 mm) into the Fabry–Pérot cavity. The spectral line positions were determined after Fourier transformation of the time-domain signal with 8k data points, recorded with 100 ns sample intervals. Each rotational transition appears as doublets due to the Doppler Effect. The line position is calculated as the arithmetic mean of the frequencies of the Doppler components. The estimated accuracy of the frequency measurements is better than 3 kHz. Lines separated by more than 7 kHz are resolvable.

Theoretical calculation

Two conformers (I and II), both stabilized by three WHBs, are, by chemical intuition and analogous to similar observed complexes, expected to be the most stable forms of the title complex. Their cage-like structures are shown in Table 1, together with the corresponding calculated spectroscopic parameters, useful for the investigation of the microwave spectrum. The corresponding geometries were optimized both at the MP2/6-311++G(d,p) and MP2/aug-cc-pVTZ levels of calculation, and the subsequent evaluation of the Hessian matrix confirmed that the found stationary points are minima.
Table 1 Theoretical shapes and spectroscopic parameters of the two most stable conformers of (CH2ClF)2: (a) MP2/aug-cc-pVTZ, (b) MP2(BSSE)/aug-cc-pVTZ, (c) MP2/6-311++G(d,p), (d) MP2(BSSE)/6-311++G(d,p)

image file: c8cp06288a-u1.tif

image file: c8cp06288a-u2.tif

(a) (b) (c) (d) (a) (b) (c) (d)
a L and R indicate the chlorine atom in the left or right part of the complex, as it appears in the drawings at the top of the table. b Absolute energy: −1197.333373Eh. c Absolute energy: −1197.332076Eh. d Absolute energy: −1197.014584Eh. e Absolute energy: −1197.011692Eh. f Absolute energy: −1197.269344Eh. g Absolute energy: −1197.268141Eh. h Absolute energy: −1196.950212Eh. i Absolute energy: −1196.947343Eh.
Rotational constants/MHz
A 2992 3019 3040 3096 4116 4076 4360 4292
B 918 885 885 781 765 748 751 672
C 893 868 856 773 713 691 699 631
First order centrifugal distortion constants/kHz
D J 0.80 0.86 1.04 1.41 0.48 0.81 0.58 0.88
D JK 2.74 2.88 2.62 0.77 3.27 −3.03 2.07 −2.32
D K 1.21 1.86 3.82 12.8 16.0 57.5 25.1 85.8
d 1 0.00 −0.01 0.02 −0.03 −0.03 −0.13 −0.05 −0.12
d 2 −0.04 −0.04 −0.05 0.00 0.00 −0.01 0.00 −0.05
Electric dipole moment components/D
μ a −2.2 −2.3 −2.6 −2.5 −1.9 −1.8 −1.7 −1.6
μ b −1.2 −1.1 −1.3 −1.1 −0.1 0.9 0.0 0.7
μ c 0.4 0.3 0.5 0.7 0.3 0.0 0.1 0.1
Quadrupole coupling constants of the La chlorine atom/MHz
χ aa 33.3 33.4 33.4 30.9 33.6 34.0 34.5 33.9
χ bbχcc −71.0 −71.7 −58.4 −60.7 −72.3 −83.6 −72.2 −80.1
χ ab 5.2 5.2 9.4 1.7 −4.9 −2.1 −8.9 −13.2
χ ac −5.6 −5.5 −8.7 −1.1 1.3 2.7 −0.1 −1.1
χ bc 32.6 32.2 40.4 41.9 −31.7 −24.2 −34.2 −28.8
Quadrupole coupling constants of the Ra chlorine atom/MHz
χ aa 29.6 29.5 31.4 32.8 −63.3 −57.2 −66.5 −63.3
χ bbχcc 13.1 15.7 −7.5 0.0 3.8 −1.7 4.2 1.9
χ ab −2.7 −3.2 −1.3 −8.6 −13.5 27.8 −13.0 22.1
χ ac 0.4 −0.6 1.8 13.3 7.2 −3.1 −0.5 −2.2
χ bc −49.2 −49.1 −51.2 −49.4 −1.1 0.6 −1.7 0.3
Equilibrium and zero point relative energies and dissociation energies/kJ mol−1
ΔEe 0b 0c 0.70 0.72 1.12 0.97 0d 0e
ΔE0 0f 0g 0.47 0.60 1.14 1.01 0h 0i
D e 17.0 14.0 17.0 8.7 16.3 13.0 17.7 9.4
D 0 15.0 11.8 14.6 6.9 13.8 10.8 15.0 7.5

In order to account for basis set superposition effects (BSSE), the same kind of computation was also performed using the counterpoise correction procedure.19 All calculations were performed by using the Gaussian09 suite of programs.20

The two isomers are very close in energy, but the MP2/aug-cc-pVTZ method indicates isomer I to be more stable (0.7 kJ mol−1), while the opposite is obtained with MP2/6-311++G(d,p) calculations (−1 kJ mol−1).

Considering the conformational space of (CH2ClF)2, besides the structures stabilized by three WHBs, additional structures stabilized by two halogen–hydrogen intermolecular bonds can be hypothesized. Seven non-equivalent arrangements are possible, as shown in Fig. 1. Their characterization was attempted by free geometry optimization. All runs converged to isomer I or II, suggesting that structures stabilized by two WHBs either lie on very shallow minima or are not minima at all. It can also be hypothesized that these forms are transition states or intermediate forms in the pathway connecting conformers I and II, but unfortunately, our attempts to characterize such a kind of pathway, looking for the corresponding transition states, were unsuccessful.

image file: c8cp06288a-f1.tif
Fig. 1 Sketches of the possible geometries of (CH2ClF)2 with two or three intermolecular contact points.

Rotational spectra

The presence of several abundant isotopologues (35Cl/35Cl, 35Cl/37Cl, 37Cl/35Cl, and 37Cl/37Cl, ratio 9/3/3/1) and of the two non-equivalent quadrupolar nuclei (35Cl and 37Cl) with a nuclear spin I = 3/2 and a relatively large nuclear electric quadrupole moment (Q) makes the spectrum quite dense and complicates the assignment.

The two isomers have very similar energies, and for both the μa dipole moment component is calculated to be the largest one. For this reason we scanned frequency regions where μa R-type bands of both conformers were expected to fall. We could easily observe one set of this kind of band, identifying at the beginning the intense K = 0, 1 transitions of the parent species. Each of them was split into several quadrupole component lines, as illustrated in Fig. 2 for the 918 ← 817 transition.

image file: c8cp06288a-f2.tif
Fig. 2 Recorded 91,8–81,7 transition of the observed conformer of (CH2ClF)2, showing the hyperfine structure due to the quadrupolar effects of the two – nonequivalent – 35Cl nuclei. Each line exhibits the Doppler doubling and is labeled as F1′′F′′ − F1F′, where F1 = J + I1, F = F1 + I2 and I1 and I2 are the quantum numbers of the two nuclear spins. F1 and F are half-integer values, but rounded to the upper integer in the figure.

After the first assignments, many additional μa-type transitions, with Jupper and Ka up to 10 and 3, respectively, have been measured. Then, some hundred MHz below each transition of the parent species, two weaker transitions were observed, belonging to the 35Cl/37Cl and 37Cl/35Cl isotopically mixed isotopologues.

The intensities of these transitions were about 1/3 of those of the parent species, in consistency with the natural relative abundance of the two Cl isotopes and the existence of two non equivalent 35Cl/37Cl isotopologues.

All measured line frequencies were fitted with Pickett's SPFIT program21 by direct diagonalization of the Hamiltonian consisting of Watson's “S” reduced semirigid-rotor Hamiltonian22 in the Ir representation, implemented by the hyperfine Hamiltonian:

H = HR + HCD + HQ(1)
HR represents the rigid-rotor Hamiltonian and the centrifugal distortion contributions are represented by HCD, while HQ is the operator associated with the quadrupolar interaction of the 35Cl (or 37Cl) nuclei with the overall rotation. The coupling scheme F1 = J + I(Cl1), F = F1 + I(Cl2) has been adopted. The obtained spectroscopic constants are reported in Table 2.

Table 2 Experimental rotational parameters of the observed isomer of (CH2ClF)2
35Cl/35Cl 35Cl(L)/37Cl(R) 37Cl(L)/35Cl(R)
a Error in parentheses in units of the last digit. b D k is not determined by the fit. c The non diagonal quadrupole coupling constants χab and χac are not determined by the fit. d RMS error of the fit. e Number of component lines in the fit.
A/MHz 3014.53(2)a 2971.419(1) 2981.2(1)
B/MHz 867.7519(1) 859.3929(1) 854.4724(2)
C/MHz 848.5595(2) 840.3234(2) 834.4918(2)
D J/kHzb 0.8967(4) 0.8885(4) 0.8864(4)
D JK/kHz 4.68(2) 4.61(3) 4.42(3)
d 1/Hz −0.0154(5) −0.0063(5) −0.0191(6)
d 2/Hz −0.0447(5) −0.042(1) −0.041(2)
χ aa(L)/MHz 35.85(2) 35.83(2) 28.16(3)
χ bbχcc(L)/MHz −77.29(6) −63.23(5) −66.95(8)
χ bc(L)c/MHz 34.28(6) 40.85(2) 23.1(1)
χ aa(R)/MHz 31.64(2) 24.93(1) 31.66(3)
χ bbχcc(R)/MHz 2.82(8) −13.25(6) 15.19(8)
χ bc(R)c/MHz −52.71(6) −41.07(2) −52.2(1)
σ /kHz 3 3 3
N 333 268 266

The two mixed isotopologues 35Cl/37Cl are distinguished from each other by the labels (L) and (R), which indicate the chlorine atom in the left (L) or right (R) part of the complex, as it appears in the drawings reported in Table 1.

We did not succeed in measuring the transitions of the 37Cl/37Cl isotopologue since its abundance is only ∼10% of that of the parent species.

The comparison of the experimental spectroscopic parameters with the theoretical values for the two conformations of Tables 1 and 2 leads to a straightforward assignment of the observed spectrum to conformer I, the one stabilized by two C–H⋯Cl–C and one C–H⋯F–C WHBs.

We could not observe lines belonging to conformer II, despite the very small complexation energy difference. This could be due to the conformational relaxation to the most stable conformer upon supersonic expansion. It has, indeed, been shown that this kind of relaxation takes place easily when the interconversion barrier is smaller than 2kT.23

Structural information

The C1 configuration of the observed conformer of (CH2ClF)2 is shown in Fig. 3.
image file: c8cp06288a-f3.tif
Fig. 3 Sketch, atom numbering, and principal axes of the observed conformer of (CH2ClF)2.

In Tables 1 and 3, Cl7 and Cl2 have been labeled as Cl(L) and Cl(R), respectively.

Table 3 Substitution (rs), r0 and ab initio (re, MP2-(BSSE)/aug-cc-pVTZ) coordinates of the chlorine atoms in the observed isomer of (CH2ClF)2
a b c
a Error in parentheses is in units of the last digits.
Cl(L) r 0 −2.097(5)a 0.852(9) −0.50(1)
r s ±2.087(1) ±0.846(2) ±0.502(3)
r e −2.171 0.760 −0.558
Cl(R) r 0 1.524(6) −0.77(1) −0.822(8)
r s ±1.514(1) ±0.782(2) ±0.798(2)
r e 1.530 −0.871 −0.713

From the rotational constants of the two isotopologues, it is possible to calculate the substitution, rs, coordinates24 of the two Cl atoms in the principal axes of the parent species. A partial r0 structure was obtained by adjusting five structural parameters, while keeping the remaining parameters fixed to their ab initio values, in order to reproduce the nine experimental rotational constants. The obtained parameters are reported in Table 4 and compared to the ab initio values.

Table 4 Partial r0 and re (MP2(BSSE)/aug-cc-pVTZ) structures of the observed isomer of (CH2ClF)2
r 0 r e
a Uncertainties (in parentheses) are expressed in units of the last digit.
Fitted parameters
R Cl7H4 3.014(8)a 3.003
Cl7H4C1/° 131.2(2) 130.7
Cl7H4–C1F3 166.9(8) 164.6
C6Cl7H4 87.4(6) 85.0
F8C6–Cl7H4 36(2) 42
Derived parameters
R F8H5 2.713(8) 2.712
R Cl2H10 3.216(8) 2.963

From this partial r0 structure, the lengths of the three WHBs have been derived and reported in Table 4. The full ab initio geometry is available in the ESI.

Table 5 Theoretical bond energy values (QTAIM) and intermolecular distances (MP2-(BSSE)/aug-cc-pVTZ) in (CH2ClF)2. L and R indicate the atoms in the left or right part of the complexes, as it appears in the drawings at the top of Table 1
Rotamer L⋯R E bond/kJ mol−1 d
I Cl⋯H 3.6 3.00
I H⋯Cl 4.5 2.96
I F⋯H 5.2 2.71
I F⋯Cl 3.8 3.49
II Cl⋯H 3.2 3.06
II H⋯F 7.0 2.52
II F⋯H 4.9 2.82
II Cl⋯F 3.58

Dissociation energy

Within the pseudo-diatomic approximation, the dissociation energy of the molecular adduct can be evaluated from the estimation of the force constant (ks) relative to the stretching motion connecting the centres of mass of the two subunits, if such motion takes place almost parallel to the a-axis, so that:25
ks = 16π4(μRCM)2[4B4 + 4C4 − (BC)2(B + C)2]/(hDJ),(2)
where μ is the pseudo-diatomic reduced mass and RCM (=3.74 Å) is the distance between the centres of mass of the two subunits. B, C and DJ are the experimental spectroscopic constants reported in Table 2. We obtained the value ks = 5.1 N m−1, for which the corresponding stretching frequency, in the harmonic approximation, is 50 cm−1.

Assuming a Lennard-Jones type potential the dissociation energy (ED) can be estimated as:26

ED = 1/72ksRCM2(3)

We have estimated ED = 5.9 kJ mol−1, which is similar to the values obtained for CH2ClF⋯CH2F2 (ED = 5.3 kJ mol−1),14 (CH2F2)2 (ED =6.6 kJ mol−1)10 and CH2Cl2⋯CH2F2 (ED = 7.6 kJ mol−1)15 within the uncertainty of 20% that is expected for the ED values. So, this value is comparable with the results of the MP2(BSSE)/6-311++G(d,p) method (see Table 1).

Atoms in molecule analysis

After the assignment of the spectrum, we believed the presence of two C–H⋯Cl–C and one C–H⋯F–C WHBs in the observed isomer to be indicative of a higher strength of the former one. However, following the suggestions of one referee, we applied to our system the quantum theory of atoms in molecules (QTAIM),27 obtaining interesting results. This theory allows correlation of the topological properties of electron density function, ρ(r), with elements of molecular structures: the localization of maxima ρ(r) values enables the identification of atomic positions, whereas chemical bonds are defined as saddle points between the maxima. These saddle points, called Bond Critical Points (BCP), represent the minimum along the bonding direction and the maximum in all others.

Since both covalent and non-covalent bonds can be identified, using the Multiwfn software,28 we applied QTAIM to the ab initio ρ(r) values achieved at the MP2(BSSE)/aug-cc-pVTZ level of calculation, for both rotamers. With regard to rotamer I, four BCPs were obtained: one for each of the hydrogen–halogen interactions and one between the fluorine and chlorine atoms. Three BCPs were found for rotamer II, corresponding to two hydrogen–halogen interactions and a carbon–fluorine interaction. All these are shown graphically in Fig. 4.

image file: c8cp06288a-f4.tif
Fig. 4 Non covalent interactions in the two more stable isomers of (CH2ClF)2, according to the QTAIM theory.

A rough estimation of the bond energy Ebond is given by one half of the potential energy density V(r) at the corresponding BCP: Ebond = 1/2V(rBCP).29 The values achieved for rotamers I and II are reported in Table 5 with the theoretical bond distances. The Cl⋯H bonds appear weaker than the F⋯H ones, thus rotamer II should be more stable than rotamer I, but the presence of an additional halogen–halogen interaction stabilizes rotamer I more than rotamer II.

In our previous investigations of dimers of Freons, we noted that the strength of halogen bonds was overwhelming compared to that of WHBs when one of the constituting Freons was fully halogenated. This happens in CClF3⋯CH3F, where the linking interaction is a Cl⋯F halogen bond.30 However, it seems that the QTAIM method can point out when halogen bonds begin to contribute to the stability of a Freon dimer upon increasing the halogenation degree of the constituting Freons.


The microwave spectrum of (CH2ClF)2 represents an unprecedented investigation of an intermolecular complex with two non equivalent Cl atoms by rotational spectroscopy. This cluster consists, indeed, of the combination of two molecules of the same monomer, the different orientations of which make the two – heavy quadrupolar nuclei (35Cl and 37Cl, with high nuclear spin quantum numbers and large electric nuclear quadrupole moments) – non equivalent to each other. The consequent complex hyperfine structure of each transition has been successfully analyzed and interpreted in terms of ten quadrupole coupling parameters.

The complex does not have any element of molecular symmetry, thus it is a chiral species. Although one should expect the same concentration for the two enantiomers, it could be interesting to measure experimentally their relative concentration, since the formation of the complex upon supersonic expansion is a complicated process, and it could favor the formation of an even slightly more stable form. A considerable increase of the population of slightly more stable isotopologues of molecular complexes upon supersonic expansion has been often observed, for example in molecular complexes involving neon. In the case of the trimer pyridine–Ne2, it turns out that under the jet conditions the ground-state population of the Py–22Ne22Ne isotopologue increases by a factor of ≈20 with respect to that of the normal (Py–20Ne20Ne) species.31 In this example, the mass increase for the heavier isotopologue results in a lower zero-point energy. Then, the enrichment of slightly more stable species can be explained by the thermal quasi-equilibrium reached as a result of repeated dissociation and re-formation of adducts in the low-temperature molecular expansion.

The detection of conformer I, where the two subunits are linked to each other by two C–H⋯Cl–C and one C–H⋯F–C WHBs, rather than conformer II, with two C–H⋯F–C and one C–H⋯Cl–C interactions, suggests that C–H⋯Cl–C is a linkage slightly stronger than C–H⋯F–C for the investigated dimer. This confirms what was already determined in the case of CH2Cl2⋯CH2F2,15 but not for the adduct 1-chloro-1-fluoroethylene-acetylene.32 The effects of C–X⋯H bond angles or C–X bond lengths, which differ in the various adducts, could play a role in this preference. Moreover we found that besides hydrogen–halogen interactions, also halogen–halogen stabilizing interactions have to be considered to explain these apparently conflicting results. Indeed, a Cl⋯F contact can take place both in CH2Cl2⋯CH2F2 and CH2FCl⋯CH2FCl species, whereas it is not feasible in CHFClCH2⋯HCCH.

With a pseudo diatomic approximation, which considers the two subunits to be rigid in the angular coordinates, the dissociation energy of this complex has been estimated, ED = 5.9 kJ mol−1. This value is similar to those obtained with the same method for a similar dimer involving Freons. It is not very different from the lowest theoretical value of Table 1 (6.9 kJ mol−1, MP2(BSSE)/6-311++G(d,p)).

Conflicts of interest

There are no conflicts to declare.


We gratefully acknowledge the financial support of the Italian MIUR (PRIN, project 2010ERFKXL_001) and the University of Bologna (RFO).

Notes and references

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  3. See, for example, (a) J.-M. Lehn, Angew. Chem., Int. Ed. Engl., 1988, 27, 89 CrossRef; (b) J.-M. Lehn, Angew. Chem., Int. Ed. Engl., 1990, 29, 1304 CrossRef.
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Electronic supplementary information (ESI) available: Completion of ref. 20 and tables listing transition frequencies and ab initio geometries. See DOI: 10.1039/c8cp06288a

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