Interactions between azines and alcohols: a rotational study of pyridine–tert-butyl alcohol

Lorenzo Spada ab, Iciar Uriarte c, Weixing Li *a, Luca Evangelisti a, Emilio J. Cocinero c and Walther Caminati *a
aDipartimento di Chimica “G. Ciamician” dell’Università, Via Selmi 2, I-40126 Bologna, Italy. E-mail: walther.caminati@unibo.it
bScuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
cDepartamento de Química Física, Facultad de Ciencia y Tecnología, Universidad del País Vasco, (UPV-EHU) and Biofisika Institute (CSIC, UPV-EHU), Apartado 644, E-48940 Bilbao, Spain

Received 14th July 2018 , Accepted 20th August 2018

First published on 20th August 2018


We assigned the rotational spectra of the parent and the OD isotopologues of the intermolecular complex pyridine–tert-butyl alcohol. The rotational and 14N quadrupole coupling constants are in agreement with a σ-type shape and a Cs 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.


Introduction

In the field of non-covalent interactions, hydrogen bonds (HBs) are by far the most important, playing a key role in many chemical and biological processes such as molecular recognition,1 catalysis2,3 or solvation,4 among others.

The nature of HBs involves forces with different origins (electrostatic and dispersion, those arising from charge transfer), resulting in directional interactions with dissociation energies in the range of 15–25 kJ mol−1.5 Microwave (rotational) spectroscopy is a very powerful technique for investigating HBs, since it allows for the structural elucidation of weakly bound intermolecular complexes with unparalleled accuracy.

Typical HBs such as O–H⋯O, N–H⋯O and O–H⋯N have been accurately characterized by rotational spectroscopy studies on intermolecular complexes of the kind “water–organic molecule”.6

In particular, much attention has been devoted to the microwave investigation of the HBs that are formed between water and heteroaromatic molecules.

The O–H⋯N HB has been found to link pyridine,7 pyrazine,8 pyridazine9 and pyrimidine10 to water, in contrast to the O–H⋯π interaction that stabilizes the complex benzene–water.11

In the case of a pyrrole–water adduct,12 a N–H⋯O HB is established, acting as a pivot for the internal rotation of water. This internal motion is also observed in pyrazine–water, where the Ubbelohde effect13 was invoked to explain the lowering of the V2 barrier upon deuteration in the HB.

The formation of the complex with water inverts the relative stabilities of the tautomeric forms in 2-hydroxypyridine/2-pyridone.14

The proton donor features of water have been proved also in ortho- and para-fluoro monosubstituted pyridine (2-fluoro-pyridine and 3-fluoro-pyridine) complexes.15,16

The geometrical parameters of the O–H⋯N HB, which occurs in combination with a weak C–H⋯O HB (WHB), are not significantly different between the two complexes, suggesting that these two single fluorine substitutions do not affect the proton acceptor properties of nitrogen.

Recently, the O–H⋯N HB has been characterized in a pyridine (PYR)–formic acid (FA)17 adduct, where it occurs together with a C–H⋯O WHB. The dissociation energy has been estimated to be about 40 kJ mol−1. This value is slightly larger than that expected for an O–H⋯N HB, probably because of a contribution from a partial proton transfer.

In this case, upon H → D substitution in the O–H⋯N HB, an increase of 7 mÅ of the O⋯N distance has been observed, similar in nature to the one found in dimers of carboxylic acids.18

This result was surprising since for single HBs, such as an O–H⋯O HB,19 a shortening of the O⋯O distance, upon HB deuteration, has generally been observed.

What are the characteristics of the O–H⋯N HB (N being part of a heteroaromatic molecule) when the proton donor is other than water or a carboxylic acid (for example, an alcohol)?

To answer this question, we decided to undertake the rotational characterization of the PYR–tert-butyl alcohol (TBA) adduct in order to have insights into:

(i) the competition between an O–H⋯N HB, which would give rise to a σ-type complex, and an O–H⋯π WHB, which would form a π-type adduct;

(ii) the degree of proton transfer with respect to the case of PYR–FA, and the relationship with the acidity of the protons;

(iii) the topology of the Ubbelohde effect that presumably will take place in the O–D⋯N HB species, compared to other HB proton donors, such as water or FA;

(iv) the binding energy.

Experimental

The intermolecular complex PYR–TBA has been generated at room temperature by flowing sequentially helium over TBA and PYR at the stagnation pressure of ∼0.4 MPa and expanding the mixture into a Fabry–Pérot cavity. The cavity is part of a COBRA-type20 pulsed supersonic-jet Fourier-transform microwave (FTMW) spectrometer,21 described elsewhere,22 and the expansion takes place through a solenoid valve (General Valve, Series 9, nozzle diameter 0.5 mm). The same procedure has been followed for the PYR–TBA(OD) adduct by using the hydroxyl deuterated species of TBA, which has been obtained by direct H → D exchange of TBA with D2O.

The rotational transitions appear as doublets due to the Doppler effect being enhanced by the coaxial arrangement of the supersonic jet with respect to the resonator axis.

The rest frequency was calculated as the arithmetic mean of the frequencies of the two Doppler components. The estimated accuracy of the frequency measurements and the resolution are better than 5 kHz and 7 kHz, respectively.

Theoretical calculation

Two different kinds of complexes (σ-type or π-type) have been observed in the adducts of PYR with other molecules, depending on which interaction site is involved (the lone-pair of the nitrogen atom or a π system, respectively), as observed for example in the cases of PYR–metal23 and PYR–freon24 complexes. Therefore, we have optimized the geometries of the σ and π conformers for PYR–TBA at the MP2/6-311++G(d,p) level, using GAUSSIAN09,25 in order to calculate the spectroscopic parameters necessary for the assignment of the rotational spectrum, namely rotational and quadrupole coupling constants and electric dipole moment components. They are reported in Table 1. After zero point energy corrections, the σ-type complex (O–H⋯N HB) is calculated to be 950 cm−1 more stable than the π-type complex (one O–H⋯π and two C–H⋯π WHBs). Both isomers have Cs symmetry and are real minima according to the frequency calculations in the harmonic approximation. The dissociation energy (ED) of both isomers has also been obtained by taking into account the BSSE corrections.26
Table 1 MP2/6-311++G(d,p) calculated spectroscopic parameters of the σ- and π-type isomers of PYR–TBA
σ-Type π-Type

image file: c8cp04462g-u1.tif

image file: c8cp04462g-u2.tif

a Absolute energy = −480.674112Eh. b Absolute energy zero-point corrected = −480.447535Eh. c Dissociation energy.
A/MHz 2588 1830
B/MHz 451 594
C/MHz 420 592
|μa|, |μb|, |μc|/D 4.2, 0.0, 2.0 1.2, 0.0, 0.6
χ aa, χbbχcc, |χac|/MHz −4.47, −1.52, 1.11 2.73, 5.71, 2.25
ΔE,a ΔE0b/cm−1 0,a 0b 979, 949
E D /kJ mol−1 21.4 7.0


Rotational spectra

Guided by ab initio calculations, we scanned the frequency region where the strongest μa-type transitions of the σ-type isomer were expected. Around 8546.4 MHz, the typical three-component line pattern due to the presence of 14N was observed, corresponding to the 100,10 ← 90,9 transition. After this first assignment, many other μa-type lines up to J = 13 and Ka = 5, and several μc-type lines were measured. No μb-type transitions were detected in agreement with the Cs symmetry of the molecular adduct. Pickett's SPFIT program,27 within the S reduction and Ir representation, was used to fit the observed frequency lines. The results are reported in Table 2 together with those obtained for the O–D isotopologue following the same procedure. A comparison between the theoretical predictions (Table 1) and experimental values (Table 2) of the rotational constants confirms the assignment of the experimentally observed species to the σ-type isomer of PYR–TBA.
Table 2 Experimental rotational parameters for the PYR–TBA(O–H) and PYR–TBA(O–D) species of the σ-type complex
PYR–TBA(O–H) PYR–TBA(O–D)
a Error in parentheses in units of the last digit. b Fixed to the ab initio value. c Fixed to the PYR–TBA species value. d RMS error of the fit. e Number of component lines in the fit.
A/MHz 2564.1867(5)a 2554.7805(3)
B/MHz 444.5725(1) 444.5612(2)
C/MHz 415.0428(1) 415.2750(2)
χ aa (14N)/MHz −4.21(1) −4.210(9)
χ bbχcc (14N)/MHz −1.415(7) −1.416(7)
|χac| (14N)/MHz 1.3(4) 2.6(8)
χ aa (D)/MHz 0.21(1)
χ bbχcc (D)/MHz −0.04(1)
|χac| (D)/MHz [0.097]b
D J/kHz 0.1099(3) 0.1070(4)
D JK/kHz 0.270(3) 0.243(7)
D K/kHz 0.95(4) [0.95]c
d 1/Hz −3.6(2) −3.2(4)
d 2/Hz 6.83(7) 6.8(2)
σ /kHz 2.0 2.7
N 219 343


All μa-type lines belonging to the O–D isotopologue, which are expected at lower frequencies than those of the parent species within the rigid rotor model, were observed at higher frequencies, as shown in Fig. 1 for the 110,11 ← 100,10 transition. This behaviour is ascribable to the so-called “reverse Ubbelohde effect”, whose sizing is described in the section below.


image file: c8cp04462g-f1.tif
Fig. 1 The 110,11 ← 100,10 rotational transition for the PYR–TBA(O–H) (left) and PYR–TBA(O–D) (right) species of the σ-type complex showing the F1′, F′ ← F1′′, F′ component lines and the doubling by the Doppler effect.

For this species, a quadrupole hyperfine structure caused by the presence of one deuterium atom (nuclear spin I = 1) has been observed for the μc-type lines, in addition to the one originated by the 14N nucleus. Therefore, it was possible to fit the quadrupole coupling constants relative to both nuclei (except χac for D which was fixed to its ab initio value) as reported in Table 2, according to the coupling scheme: F1 = J + I(14N), F = F1 + I(D). No other lines corresponding to the other calculated isomer have been observed, either because of being too high in energy or because undergoing a plausible isomeric relaxation toward the most stable σ-type.28

Structure and partial proton transfer

A partial effective structure (r0) has been calculated by fitting the O⋯N distance (rO⋯N) and the α angle (see Fig. 2), while keeping all the other geometrical parameters fixed to their ab initio values, in order to reproduce the three rotational constants for each of the two species (parent and deuterated isotopologues).
image file: c8cp04462g-f2.tif
Fig. 2 PYR–TBA geometry showing the fitted parameters (rO⋯N and α) for the partial effective structure (r0), along with the principal axes of inertia.

Table 3 reports the results obtained, whose maximum discrepancy between the experimental and the calculated rotational constants was found to be less than 0.05% for the A rotational constant.

Table 3 Comparison between the rO⋯N distance and α angle as obtained from the effective (r0) and equilibrium (re, from ab initio calculations) structures
r 0 r e
a Error in parentheses in units of the last digit.
r O⋯N 2.976(2)a 2.906
α/deg. 175.8(2) 170.3


Additionally, we can derive information on the geometry of the complex and the O⋯H⋯N partial proton transfer from the analysis of the quadrupolar hyperfine structure.29 In this work, we have derived the full set of diagonal and off-diagonal quadrupole coupling constants, namely χaa, χbb, χcc and χac (note that, among the off-diagonal terms, only χac is non-vanishing due to the Cs symmetry). Therefore, it is possible to obtain the quadrupole coupling tensor in its own principal axis system (x, y, z) after matrix diagonalization and the values of χxx, χyy and χzz. These values allow for the comparison of the field gradient at the 14N nucleus of PYR–TBA and that of isolated PYR in the gas phase.30

Because of symmetry reasons, in the isolated pyridine the quadrupole coupling constants determined in the principal axes of inertia and the ones in the principal tensor at the 14N nucleus are related in the following manner: χaaχzz, χbbχxx and χccχyy.

In the case of PYR–TBA, the quadrupole coupling constants in the principal tensor (derived from diagonalization of the obtained χaa, χbb, χcc and χac values (see Table 2)) are reported in Table 4, where they are compared with those of isolated pyridine. The quadrupole asymmetry parameter values (η), representing the deviation from the axial symmetry of the quadrupole tensor around the z axis, are also reported.

Table 4 Comparison between the quadrupole coupling constants in the principal tensor at the 14N nucleus in PYR–TBA and PYR
PYR–TBA PYR
a Error in parentheses in units of the last digit. b Quadrupole asymmetry parameter: η = (χxxχyy)/χzz.
χ zz /MHz −4.5(1)a −4.908(3)
χ xx /MHz 1.396(6) 1.434(3)
χ yy /MHz 3.1(1) 3.474(3)
η 0.37(3) 0.42(1)


The quadrupole coupling constants (for example, χzz) are defined as follows: χzz = eQ(∂2V/∂z2)z=0 with e, Q and (∂2V/∂z2)z=0 being the elementary charge, the electric quadrupole moment of the 14N nucleus and the electric field gradient at the 14N nucleus, respectively. Therefore, the decrease in the order of 0.3–0.4 MHz of the values of χzz and χyy in going from PYR to PYR–TBA proves the reduction of the field gradient at the N nucleus along the z and y axes.

The reduction of these field gradients upon complexation suggests a possible partial proton transfer from the O–H of TBA to the nitrogen of PYR. However, this proton transfer is in a smaller degree than the one in HCOOH–PYR, for which a decrease of 0.6 MHz in χccχyy with respect to that of isolated pyridine was observed. This is in agreement with the pKa values of HCOOH(3.77) and TBA(17.0).

Table 5 collects the values of the angles between the z axis and the principal axes of inertia arising from the abovementioned diagonalization. Also in that table, we report the values of the same angles resulting from the r0 and re structures, for the sake of comparison. We observe that there is a good agreement between the two sets of values.

Table 5 Angles between the z axis at the 14N nucleus and the principal axes of inertia of the complex arising from quadrupole hyperfine structure diagonalization (Q), both for the effective (r0) and equilibrium (re) structures
Q r 0 r e
a The angle θzb is 90° because of the Cs symmetry of the complex. b Error in parentheses in units of the last digit.
θ za 10(3)b 11.7 8.4
θ zc 80(3) 78.3 81.6


Reverse Ubbelohde effect

The experimental value of the variation of the planar moment of inertia along the a-axis (Paa = Σimiai2) upon H → D substitution (namely, Paa(PYR–TBA(O–D))Paa(PYR–TBA(O–H))) is −0.689 mÅ2, whereas the calculated value from the r0 structure is +0.784 mÅ2. The so-called “reverse Ubbelohde effect” has to be invoked to explain this discrepancy.

In fact, in order to reproduce the aforementioned difference, a shrinkage of 4 mÅ of the O⋯N distance has to be taken into the account. This result is similar to the case of the TBA dimer (Δr(O⋯O) = −7 mÅ)31 TBA-1,4-dioxane (Δr(O⋯O) = −6 mÅ),32 but opposite in nature with respect to the case of PYR–HCOOH (Δr(O⋯N) = +7 mÅ)17 upon H → D substitution.

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:33
 
ks = 16π4(μRCM)2[4B4 + 4C4 − (BC)2(B + C)2]/(hDJ),(1)
where μ is the pseudo-diatomic reduced mass and RCM (=4.93 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 = 5.8 N m−1, for which the corresponding stretching frequency, in the harmonic approximation, is 51 cm−1.

Assuming a Lennard-Jones type potential the dissociation energy (ED) can be estimated as:34

 
ED = 1/72ksRCM2(2)

We have estimated ED ∼ 12 kJ mol−1, which is quite different (presumably because of zero-point energy contribution) from the ab initio value of 21.4 kJ mol−1 given in Table 1, but similar to the value, estimated in the same way, of the O–H⋯N linked TBA–NH3 adduct (10 kJ mol−1).35

Conclusions

The rotational investigation of two isotopologues of PYR–TBA allowed for the identification of a σ-type adduct, stabilized by an O–H⋯N HB. The H → D replacement in the O–H⋯N HB causes a shortening of the O⋯N distance by 4 mÅ, similar to the one found in the O–H⋯O HB for the TBA dimer31 and TBA-1,4-dioxane,32 a phenomenon called reverse Ubbelohde effect.

The dissociation energy of the complex has been estimated to be about 12 kJ mol−1, a value similar in magnitude to that of the TBA–NH3 adduct (10 kJ mol−1).35

The analysis of the quadrupole hyperfine structure suggests that a partial proton transfer takes place, but to a lesser extent than the PYR–FA complex.17

By comparing the geometrical parameters (r0 structure) of the O–H⋯N HB and the estimated dissociation energies of heteroaromatic complexes with different HB proton donors (see Table 6), we can infer that, as the acidity of the proton donor (in water) increases (TBA (pKa = 17.0), water (pKa = 15.7), and FA (pKa = 3.77)), the dissociation energy is also increased. The HB distances in TBA– and water–heteroaromatic complexes are in the range of 1.94–2.04 Å, that is, ∼0.3 Å longer than that in FA–PYR which involves the strongest proton donor and the strongest acid acceptor in Table 6. For the latter complex, the O–H⋯N angle is almost collinear, while shifts of up to 23° from 180° are observed for the other adducts.

Table 6 Comparison between different HB proton donors and different heteroaromatic proton acceptors, concerning HB geometrical parameters and dissociation energies
HB proton donor Heteroaromatic r (O)H⋯N ∠O–H⋯N/° E D/kJ mol−1 Ref.
TBA Pyridine 1.998(2) 175.8(1) 11.7 This work
Water Pyrazine 1.94 152 8
Water Pyrimidine 1.98 167 21.4 10
Water Pyridazine 2.04 166 ∼20 9
Water 2-Fluoropyridine 1.988(5) 158(4) 15
Water 3-Fluoropyridine 1.9961(5) 156.8(1) 30.5 16
FA Pyridine 1.661(1) 175.3(1) 39.8 17


Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the financial support from the Italian MIUR (PRIN, project 2010ERFKXL_001) and the University of Bologna (RFO). W. L. thanks the China Scholarship Council (CSC) for scholarships (grant no. 201406750002). L. E. was supported by a Marie Curie fellowship (PIOF-GA-2012-328405). I. U. acknowledges an FPU contract from the MECD and thanks the group at the University of Bologna for their kind hospitality. We acknowledge the CINECA award under the ISCRA initiative for the availability of high performance computing resources and support. E. J. C. and I. U. thank the MINECO (CTQ2017-89150-R) and the UPV/EHU (UFI11/23 and PPG17/10) for funds.

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